KINETIC THEORIES 
OF 
GRAVITATION. 
BY 
WILLIAM B. TAYLOR. 
FROM THE SMITHSONIAN REPORT FOR 1876. 
WASHINGTON : 
GOVERNMENT PRINTING OFFICE. 
1877. KINETIC THEORIES 
OF 
GRAVITATION. 
BY 
WILLIAM B. TAYLOR. 
FROM THE SMITHSONIAN REPORT FOR 1876. 
WASHINGTON : 
GOVERNMENT PRINTING OFFICE. 
1877. KINETIC THEORIES OF GRAVITATION. 
William B. Taylor, of Washington. 
"Non fingendum, aut excogitandum, sed inveniendum quid Natura faciat aut ferat."-Bacon. 
"Causas return naturalium non plures admitti debere, quam qu<c et verce sint, et oarum phfenome- 
nis explicandia sufficiant."-Newton. 
Ever since the grand demonstration by Newton in 1682, that the moon 
is a falling body, observing precisely the same law of decline from a 
rectilinear path as the cannon-ball, and that it is therefore under the 
dominion of the same force, an eager and unceasing desire has been 
manifested to discover an antecedent or origin of this universal tendency 
of matter. 
Even before this date, or in 1671, the ingenious Dr. liobert Hooke 
had endeavored to trace the cause of gravitative fall to the external ac- 
tion of waves in a surrounding medium. He appears to have been led 
to this reflection by observing that small bodies floating on the surface 
of agitated water collected toward the center of disturbance or the'ori- 
gin of the waves.* 
Newton himself, as is well known, speculated on this subject, and 
some years before arriving at his great generalization, he threw out a 
suggestion as to the cause of terrestrial gravity in a letter to Mr. Boyle. 
As connected with this speculation, it may be well to recur to Newton's 
still earlier statement of his conceptions in regard to the nature and 
action of the ait her. In a letter to Mr. Henry Oldenburg, secretary of 
the Koyal Society of Loudon, in January, 1675-6, he thus unfolds the 
hypothesis: 
"First, it is to be supposed therein that there is an retherial medium,, 
much of the same constitution with air, but far rarer, subtiler, and more 
strongly elastic. But it is not to be supposed that this medium is of 
one uniform matter, but composed partly of the main phlegmatic body 
of the aether, partly of other various aetherial spirits, much after the 
manner that air is compounded of the phlegmatic body of air intermixed 
with various vapors and exhalations; for the electric and magnetic 
effluvia and the gravitating principle seem to argue such variety. Per- 
haps the whole frame of nature may be nothing but various contextures 
of some certain aetherial spirits or vapors, condensed as it were by pre- 
* Posthumous Works of Robert Hooke, edited by Richard Waller. London, 1705, pp. 
xiv, and 184. 2 
KINETIC THEORIES OF GRAVITATION. 
cipitation, much after the manner that vapors are condensed into water. 
. . . . Tints perhaps may all things be originated from aether. . . 
"In the second place, it is to be supposed that the a?ther is a vibrat- 
ing medium like air, only the vibrations far more swift and minute; 
those of air made by a man's ordinary voice succeeding one another at 
more than half a foot or a foot distance, but those of aether at a less 
distance than the hundred-thousandth of an inch. And as in air the 
vibrations are some larger than others, but yet all equally swift, (for in 
a ring of bells the sound of every tone is heard at two or three miles 
distance in the same order that the bells are struck,) so I suppose the 
aetherial vibrations differ in bigness but not in swiftness."* 
Newton had in 1072 controverted the supposed opposition of his views 
to the action of the aether by answering: " The objector's hypothesis 
as to the fundamental part of it is not against me. That fundamental 
supposition is, 'That the parts of bodies when briskly agitated do excite 
vibrations in the aether, which are propagated every way from those 
bodies in straight lines, and cause a sensation of light by beating and 
dashing against the bottom of the eye; something after the manner 
that vibrations of the air cause a sensation of sound by beating against 
the organ of hearing.' Now the most free and natural application of 
this hypothesis to the solution of phenomena 1 take to be this: That* 
the agitated parts of bodies, according to their several sizes, figures, 
and motions, do excite vibrations in the sether of various depths or 
bignesses, which being promiscuously propagated through that medium 
to our eyes, effect in us a sensation of light of a white color; but if by 
any means those of unequal bigness be separated from one another, the 
largest beget a sensation of a red color, the least or shortest of a deep 
violet, and the intermediate ones of intermediate colors, much after the 
manner that bodies, according to their several sizes, shapes, and motions, 
excite vibrations in the air of various bignesses, which according to 
those bignesses make several tones in sound; that the largest vibra- 
tions are best able to overcome the resistance of a refracting superficies, 
and so break through it with the least refraction ; whence the vibrations 
of several bignesses, that is the rays of several colors which are blended 
together in light, must be parted from one another by refraction, and so 
cause the phenomena of prisms and other refracting substances; and 
that it depends on the thickness of a thin transparent plate or bubble 
whether a vibration shall be reflected at its further superficies or trans- 
mitted; so that, according to the number of vibrations interceding the 
two superficies, they may be reflected or transmitted for many succes- 
sive thicknesses. And since the vibrations which make blue and violet 
are supposed shorter than those which make red and yellow, they 
must be reflected at a less thickness of the plate, which is sufficient to 
explicate all the ordinary phenomena of those plates or bubbles, and 
* History of the Royal Society of London, by Thomas Birch, 1757, 4 vols. quarto, 
vol. iii, pp. 249-251. KINETIC THEORIES OF GRAVITATION. 
3 
also of all natural bodies, whose parts are like so many fragments of 
such plates. These seem to be most plain, genuine, and necessary con- 
ditions of this hypothesis. And they agree so justly with my theory, 
that if the animadversor think fit to apply them, he need not on that 
account apprehend a divorce from it."* 
This passage is interesting as being the earliest presentation of a 
theory of color, now universally adopted. The same views were re- 
peated as a suggestion, some forty-five years later, in the second edition 
of his treatise on " Optics."! 
In his "Letter to the Hon. Mr. Boyle," dated February 28, 1678-9, 
(about six years later,) Newton, after proposing as an explanation of the 
phenomena of cohesion, chemical affinity, &c., the " supposition " that 
an exceedingly elastic subtile mtherial substance is diffused through all 
places and bodies, but much rarer within and near gross bodies than 
beyond them, adds toward the conclusion of his letter: "I shall set 
down one conjecture more, which came into my mind now as I was 
writing this letter: it is about the cause of gravity. For this end I 
will suppose aether to consist of parts differing from one another in sub- 
tilty by indefinite degrees, ... in such a manner that from the 
top of the air to the surface of the earth, and again from the surface of 
the earth to the center thereof, the aether is insensibly finer and finer. 
Imagine now any body suspended in the air or lying on the earth, and 
the aether being by the hypothesis grosser in the pores which are in the 
upper parts of the body than in those which are in the lower parts, 
and that grosser aether being less apt to be lodged in those pores than 
the finer aether below, it will endeavor to get out, and give yay to the 
finer aether below, which cannot be without the bodies descending to 
make room above for it to go into. From this supposed gradual sub- 
tilty of the parts of the aether, some things above might be further illus- 
trated and made more intelligible. . . . For my own part, I have 
so little fancy to things of this nature, that had not your encourage- 
ment moved me to it, I should never I think have thus far set pen to 
paper about them." J It will be seen from the above that Newton had 
not at this time (only three years before the crowning epoch of his life) 
extended his conception of " gravity" to the outlying universe. 
Fourteen years later-a decade after his culminating work-this topic 
was again incidentally touched upon by Newton in four letters ad- 
dressed to Doctor Bentley, "containing some arguments in proof of a 
Deity." In his second letter, dated January 17, 1692-3, he says in re- 
ply to one from Bentley : " You sometimes speak of gravity as essential 
and inherent to matter. Pray do not ascribe that notion to me, for the 
* Philosophical Transactions of the Royal Society ; November 18, 1672. No. 88, vol. 
vii, p. 5088. 
+ Newton's Optics. Second edition, 1717. Book iii, appendix. Qaery 13. 
t The Works of Isaac Newton, edited by Samuel Horsley : In 5 vols., quarto. Vol. iv, 
pp. 385-394. 4 
KINETIC THEORIES OF GRAVITATION. 
cause of gravity is what I do not pretend to know, and therefore would 
take more time to consider of it."* 
In his third letter, dated February 25, 1692-3, he expresses himself 
somewhat less guardedly thus: "It is inconceivable that inanimate 
brute matter should, without the mediation of something else which is 
not material, operate upon and affect other matter, without mutual con- 
tact, as it must do if gravitation in the sense of Epicurus be essential 
and inherent in it. And this is one reason why I desired you would not 
ascribe ' innate gravity ' to me. That gravity should be innate, inher- 
ent, and essential to matter, so that one body may act upon another at 
a distance, through a vacuum, without the mediation of anything else, 
by and through which their action and force may be conveyed from one 
to another, is to me so great an absurdity, that I believe no man who 
has in philosophical matters a competent faculty of thinking can ever 
fall into it. Gravity must be caused by an agent acting constantly ac- 
cording to certain laws; but whether this agent be material or imma- 
terial, I have left to the consideration of my readers."! 
At the conclusion of the third book of his Principia, Newton remarks: 
" Hitherto 1 have not been able to discover the cause of those properties 
of gravity from phenomena, and I frame no hypothesis ; for whatever is 
not deduced from the phenomena is to be called an hypothesis. . . . 
To us it is enough that gravity does really exist, and act according to 
the laws which we have explained." 
Still twenty-five years later than the date of these oft-quoted Bentley 
letters, Newton again recurred to the subject in an appendix to the 
second edition of his " Optics," published in 1717. After suggesting 
that the chromatic dispersion of luminous rays by refraction might be 
due to varying wave lengths of an all-pervading " mtherial medium," 
(as previously referred to,) he asks : " Is not this medium much rarer 
within the dense bodies of the sun, stars, planets, and comets, than in 
the empty celestial spaces between them ? And in passing from them 
to greater distances, doth it not grow denser and denser perpetually, 
and thereby cause the gravity of those great bodies toward one another, 
and of their parts toward bodies; every body endeavoring to go from 
the denser parts of the medium toward the rarer . And though 
this increase of density may at great distances be exceeding slow, yet 
if the elastic force of this medium be exceeding great, it may suffice to 
impel bodies from the denser parts of the medium toward the rarer, 
with all that power which we call gravity." J 
The intellectual spirit of the age in which "gravitation" was estab- 
lished was one of strong reaction from the previous metaphysical sway 
of "occult qualities;" and that the above crude suggestion (perhaps 
offered too much in deference to that spirit) by no means satisfied the 
judgment of Newton, is shown by his subsequent inclination to dispense 
* Wo.ks, edited by Horsiey, vol. iv, p. 437. 
t Works, ut supr., vol. iv, p. 438. 
t Optics, book iii, appendix. Query 21. KINETIC THEORIES OF GRAVITATION. 
5 
altogether with a medium which apparently must tend to retard the 
planetary movements, and which he thought insufficient to account for 
the ordinary behavior of the luminous ray. He concludes that as "there 
is no evidence for its existence, therefore it ought to be rejected. And 
if it be rejected, the hypotheses that light consists in pression, or motion, 
propagated through such a medium, are rejected with it."* This appears 
to have been the turning-point in the suspended balance of his judgment, 
determining bis choice between the alternative conceptions of emission 
and of undulation. 
Afterward, as if driven back from every assault to the only retreat, 
which in earlier years be had stigmatized as "so great an absurdity" 
that no competent thinker could " ever fall into it," he despairingly 
asks: " Have not the small particles of bodies certain powers, virtues, 
or forces, by which they act at a distance? .... What I call 
'attraction' may be performed by impulse, or by some other means un- 
known to me. I use that word here to signify only in general any force 
by which bodies tend toward one another, whatsoever be the cause."! 
And beyond this point, no human research has since been able to pene- 
trate. 
This last and presumably deliberate judgment of Newton is a quarter 
of a century later than the inconsiderate utterances of his third "Bent- 
ley letter," which have been so eagerly seized upon by every speculative 
writer intent on propounding new theories of the universe. 
The thoughtful philosopher Doctor Young, about a century later, 
commenting on Newton's suggestion of an aetherial medium-rarer 
toward and within dense bodies,-with great ingenuity remarks: "The 
effects of gravitation might be produced by a medium thus constituted, 
if its particles were repelled by all material substances with a force 
decreasing like other repulsive forces, simply as the distances increase. 
Its density would then be everywhere such as to produce the appear- 
ance of an attraction varying like that of gravitation. Such an setherial 
medium would therefore have the advantage of simplicity in the origi- 
nal law of its action, since the repulsive force which is known to belong 
to all matter would be sufficient, when thus modified, to account for the 
principal phenomena of attraction. 
" It may be questioned whether a medium capable of producing the 
effects of gravitation in this manner would also be equally susceptible of 
those modifications which we have supposed to be necessary for the 
transmission of light. In either case it must be supposed to pass 
through the apparent substance of all material bodies with the most 
perfect freedom, and there would therefore be no occasion to apprehend 
any difficulty from a retardation of the celestial motions, the ultimate 
impenetrable particles of matter being perhaps scattered as thinly 
through its external form as the stars are scattered in a nebula, which 
has still the distant appearance of a uniform light and of a continuous 
*Loco citat. Query 28. 
\ Loco citat. Query 31. 6 
KINETIC THEORIES OF GRAVITATION. 
surface; and there seems no reason to doubt the possibility of the propa- 
gation of an undulation through the Newtonian medium with the actual 
velocity of light. It must be remembered that the difference of its 
pressure is not to be estimated from the actual bulk of the earth or any 
planet alone, but from the effect of the sphere of repulsion of which 
that planet is the center; and we may then deduce the force of gravi- 
tation from a medium of no very enormous elasticity. 
" We shall hereafter find that a similar combination of a simple press- 
ure with a variable repulsion is also observable in the force of cohesion ; 
and supposing two particles of matter (floating in such an elastic me- 
dium capable of producing gravitation) to approach each other, their 
mutual attraction would at once be changed from gravitation to cohe- 
sion upon the exclusion of the portion of the medium intervening be- 
tween them. This supposition is however, directly opposite to that 
which assigns to the elastic medium the power of passing freely through 
all the interstices of the ultimate atoms of matter, since it could never 
pass between two atoms cohering in this manner. We cannot therefore 
at present attempt to assert the identity of the forces of gravitation and 
cohesion so strongly as this theory would allow us to do if it could be 
established." * 
In his succeeding lecture " On Cohesion," Dr. Young adds at its con- 
clusion: " With respect to the ultimate agent by which the effects of 
cohesion are produced, if it is allowable to seek for any other agent than 
a fundamental property of matter, it has already been observed that 
appearances extremely similar might be derived from the pressure of a 
universal medium of great elasticity ; and we see some effects so nearly 
resembling them, which are unquestionably produced by the pressure 
of the atmosphere, that one can scarcely avoid suspecting that there 
must be some analogy in the causes. Two plates of metal which cohere 
enough to support each other in the open air will often separate in a 
vacuumBut all suppositions founded on these analogies 
must be considered as merely conjectural; and our knowledge of every 
thing which relates to the intimate constitution of matter, partly from 
the intricacy of the subject, and partly for want of sufficient experi- 
ments, is at present in a state of great uncertainty and imperfection."! 
Very curiously, this ingenious scheme of universal repulsion leaves no 
room for that self-repulsion of matter exhibited in the phenomena of 
elasticity. That Young did not regard these speculations as reposing 
on a very firm basis is shown by his memoir " On the Theory of Light 
and Colors," in which the fourth "hypothesis" assumes the aether to be 
denser within transparent bodies, and for a small distance around them, 
than in the spaces beyond such bodies. | 
eu Natural Philosophy. 1807, 2 vols. quarto. Leet, xlix, vol. i, pp. 616, 
617. 
t Loco citat. Lecture 50, p. 630. 
t Philosophical Transactions of the Royal Society, 1802, vol. xcii, p. 21, and Young's 
Lectures on Natural Philosophy, vol. ii, p. 618. KINETIC THEORIES OF GRAVITATION. 
7 
Conditions of the Problem. 
It is well to bear in mind that every hypothesis directed to the expli- 
cation of gravity, is required in limine to give a satisfactory account of 
the following six characteristics-of this mysterious influence: 
1st. Its direction is radial toward the acting mass, or rectilinear-in- 
definitely. This rectilinear traction is incapable of deflection by any 
intermediate force. It suffers neither disturbance nor interference from 
any multiplication of similar lines of action, and admits neither of re- 
flection, refraction, nor of composition. 
2d, Its quantity is exactly proportional to the acting mass-indefi- 
nitely. Corollary: hence, 
2d b. Its integrity of action is complete with every accumulation of 
additional demand-indefinitely; that is to say, no multiplication of 
duty in the slightest degree impairs its previous tensions. 
3d. Its intensity is diminished by recession, in proportion to the square 
of the distance through which it acts-indefinitely; in a manner some- 
what analogous to-but (as modified by the second condition) radically 
different from-the action of light. 
4th. Its time of action is instantaneous throughout all ascertained dis- 
tances, and therefore presumably-indefinitely. Corollary: hence, 
4th b. Its rate of action (if the expression may be tolerated) is precisely 
the same on bodies at all velocities-indefinitely. It no more lags on a 
comet approaching the sun at the inconceivable speed of two hundred 
miles in one second than on a body at the lowest rate of motion, or than 
on the same comet receding from the sun at the same velocity. 
5th. Its quality is invariable under all circumstances-indefinitely. 
It is entirely unaffected by the interposition of any material screen, 
whatever its character or extent; or in other words, it can neither be 
checked by any insulator nor retarded by any obstruction. 
6th. Its energy is unchangeable in time, certainly for the past two 
thousand years; presumably-indefinitely. Corollary: hence, 
6th b. Its activity is incessant and inexhaustible-indefinitely; the 
ceaseless fall of planets from their tangential impulses involving no 
dynamic expenditure in the sun or in other known matter. 
It is scarcely necessary to add, as the necessary outcome of the latter 
propositions, that gravitation is a property immutable and inconvertible. 
As in the 1st proposition, two terminal elements (m' and m"} are neces- 
sarily assumed for determining the direction and measure of the radial 
straight line of action ; and as in the 2d proposition, " the acting mass'7 
(m) is the product of these two elements, (m/.m//,)-the action being 
reciprocal; so in the 3d proposition, the measure of the diminution of 
intensity (d2) has reference to the same two elements, between whose 
dynamic centers the value of the distance d is taken. And the expres- 
V III 
sion for these propositions considered collectively is -~-z- as the measure 
of the combined quantity and intensity of the traction between the two 
given elements. If we regard m" as incomparably smaller than m', (as 8 
KINETIC - THEORIES OF GRAVITATION. 
for example, a one-pound spherical iron shot thrown to a distance from 
our terrestrial globe,) its mass may be entirely neglected as a vanishing 
quantity, and we have the simpler expression - as indicating the amount 
of action exercised by our earth upon such a ball. 
No hypothesis failing to embrace each of these six requirements de- 
serves consideration ; and any hypothesis fully covering them all, might 
be expected to account equally for the quite incomparable actions of 
elasticity, magnetism, affinity, and cohesion, before being entitled to 
acceptance as a just or comprehensive theory of molecular force. 
As the projectors of kinetic systems of gravitation have almost invaria- 
bly quite ignored the fourth of the above conditions, it is worth while 
here to dwell somewhat upon this point. Swift as the earth's orbital 
motion is, (upward of 18 miles in one second,) the velocity of light is 
about ten thousand times greater, being 185,000 miles per second. And 
yet the composition of these two velocities gives a displacement or 
''aberration" of the heavenly bodies, as seen from our earth, of about 
20" of angle for the observed direction of the visual ray. A luminous 
impulse emanating from the sun requires about 8£ minutes to reach the 
earth. Were the gravitative influence supposed to be so much swifter 
than light as to require but a single minute to pass through this dis- 
tance, there would still be a corresponding gravity "aberration" of 2.4" 
of angle. The effect of this slight obliquity of traction would be an 
acceleration of the earth's orbital velocity which would become measur- 
able in a single year. 
This is a subject which has been very fully and carefully investigated 
by astronomers; and the illustrious Laplace, when he found an unex- 
plained minute acceleration in the moon's orbit, threw out the sugges- 
tion that if the velocity of transmission of gravitation did not exceed 
eight million times that of light, it would satisfactorily explain the 
lunar anomaly. It is scarcely necessary to say that when he subse- 
quently discovered the secular diminution of eccentricity in the earth's 
orbit, at present continuing, (though slowly reaching its minimum,*) he 
recognized the true cause of the moon's irregularity, which no longer 
permitted even the unimaginable limit of possible velocity he had pro- 
visionally assigned for gravitative action. 
Arago has remarked : "Now if we apply to the perturbation the maxi- 
mum value which the observations allow, when they have been corrected 
for the known acceleration due to the variation of the eccentricity of 
the terrestrial orbit, we find the velocity of the attractive force to 
amount to fifty millions of times the velocity of light."! 
* The minimum eccentricity will be reached in about one '•precession" period, or 
25,000 years hence. 
t Popular Astronomy, book xxiii, chap. 27, vol. ii, p. 469 of the English edition. To 
represent the real meaning of this velocity, it may be put into the equivalent form, 
that if gravity occupied the one hundred-thousandth of a second in passing from the 
sun to the earth, it would be detected. Or, the time required to reach us from the 
nearest star (distant in light-travel about three years) would not exceed two seconds. KINETIC THEORIES OF GRAVITATION. 
9 
If it is possible to represent in such terms the lowest assignable limit of 
transition, it is because we are furnished with a test of planetary move- 
ment of most marvelous delicacy in the record of eclipses occurring at 
a particular locality 2,000 years ago;-fixing the relation of annual revo- 
lution to diurnal rotation with an almost absolute precision. Sir John 
Herschel remarks: "From such comparisons Laplace has concluded 
that the sidereal day has not changed by so much as one-hundredth of a 
second since the time of Hipparchus!"* This implies the absence of 
even an infinitesimal "aberration" of the gravity radiant, or the negation 
of any assignable interval for its full and complete action. Hence the 
fourth category above stated. 
The same consideration serves to show that the energy of gravity has 
undergone no abatement or change during the lapse of two thousand 
years. Hence the sixth category. 
It is but just however, to notice here that a minute outstanding 
anomaly of the moon, detected in recent years, and still unexplained, 
detracts somewhat from the accuracy of the above infinitesimal measure; 
though it does not impair the value of the general argument, livery 
investigation, every calculation, of the astronomer, assumes the action 
of gravity to be for all distances,-absolutely instantaneous. 
Villemot. 1707. 
Philippe Villemot, a French doctor of theology, and a distinguished 
mathematician, published at Lyons in 1707 an astronomical treatise, 
entitled Nowreaw ou Nouvelle Explication du Mouvement des 
Planetes, in which, referring the movements of the planets to Cartesian 
vortices, he announced the theory that their gravitation is occasioned 
by a difference of pressure, on their outer and inner faces, of the fluid 
constituting the solar vortex, owing to an increase of its density out- 
ward from the sun. The general conception is obviously somewhat 
similar to the speculation cursorily hazarded by Newton in 1679, and 
again recurred to by him (though only transiently) in 1717, or ten years 
later than the above publication by Villemot. 
The details of this system cannot here be given, from want of access 
to his work. The Nouveau Systemc, however, appears to have been very 
favorably received by the author's contemporaries. 
Bernouilli. 1734. 
It is now nearly a century and a half since the elder John Bernouilli, 
of Switzerland, the illustrious mathematician, (professor at the univer- 
sities of Groningen and afterward of Basel,) imagined a method of ac- 
counting for the action of gravitation by centripetal impacts from with- 
out. Still retaining his early prepossessions in favor of the philosophy 
of Descartes, he devised a very curious combination of setherial vortices 
and Newtonian emissions. This eclectic hypothesis was promulgated 
* Outlines of Astronomy, cbap. xviii, sec. 908. 10 
KINETIC THEORIES OF GRAVITATION. 
in a competitive memoir on the cause of " The Mutual Inclination of 
the Planetary Orbits," which obtained the prize of the French Academy 
of Sciences in 1731. This treatise is divided into four parts, the first 
three of which are occupied with his exposition of the cause of gravita- 
tion, and the fourth with the main question proposed.* 
Referring to the respective systems of Descartes and Newton, Bernou- 
lli! finds in each u insurmountable difficulties," hence " a just mean be- 
tween the two appears the safer course. . . . The gravitation of 
the planets toward the center of the sun, and the weight of bodies 
toward the center of the earth, are not caused either by the attraction 
of Newton, or by the rotary force of the vortex medium of Descartes, 
but by the immediate impulsion of a substance which under the form 
of what I call a ' central torrent,' is continually thrown from the whole 
circumference of the vortex to its center, arid consequently impresses on 
all bodies encountered by it in its path the same tendency toward the 
center of the vortex. . . . And all that Newton has derived from 
his ' attractions' are by my theory, derived from the impulsions of the 
central torrent." f 
" According to my system, two kinds of matter are conceived as occupy- 
ing planetary space, and also two principal movements in the celestial vor- 
tex. One of these materials I conceive as perfectly fluid, or I would say, 
actually divisible without limit; that is, it is not composed of elementary 
corpuscles, as ordinary fluids are conceived, which according the num- 
ber and size of their constituent particles, present more or less sensible 
resistance to bodies moving in them, but being perfectly uniform and 
without structure, is also without resistance." This matter is called the 
primal element; which was employed by the Creator in forming the cor- 
puscles of sensible matter, definite small portions being compacted 
together into the coherent molecules of matter of the second element. 
" Matter of the primal element, being perfectly fluid without coher- 
ence, presents no resistance to bodies moving within it; for the resistance 
of fluids comes only from the inertia of the molecules of which they are 
This primal element, being without constituent parts and 
without inertia, is as the author states, the same in effect as a perfect 
vacuum. 
" The celestial vortex is composed in great part of the primal element, 
in which is mingled however, a considerable portion of the second ele- 
ment." According to Bernouilli's view, " the rotation of this vortex is not 
so rapid as to carry the planets around the sun, as Descartes assumed." 
In fact, its rotary velocity is in a subsequent portion of the memoir 
stated to be so low as to amount to only about one two hundred and 
thirtieth of the orbital velocity of the planets, an approximation to the 
* Published in the Pieces de Prix de VAcademic de Paris, tom. v, and included in his 
collected works under the title Essai d'une Nouvelle Physique Celeste. 
t Johanuis Bernoullii, Opera Omnia, 4 vols. 4to. Lausanne and Geneva, 1742; vol. iii, 
sec. viii, pp. 270,271. 
$ Loco citat., sections x and xvi, pp. 273, 276. KINETIC THEORIES OF GRAVITATION. 
11 
Keplerian third law being however maintained. Thus the vortex sur- 
vived its primary purpose and function. The small centrifugal motion 
of the matter of the second element contained in the celestial vortex is 
sufficient to make it denser at the outer than at the inner portion, though 
leaving it still too rare to sensibly affect the motions of the outer 
planets.* 
The primal element collected and condensed at the center of each vor- 
tex forms a star or sun, though neither thecause nor the manner of such 
condensation is very clearly unfolded. This central mass agitated with 
intestine motions of extreme violence in ail directions, as in a boiling 
caldron, produces an intense light and heat, while detached particles at 
the surface, continually more and more broken up by the frequency and 
impetuosity of their collisions, acquire a subtilty transcending all con- 
ception of the imagination, and are finally thrown off in all directions 
with explosive force to immense distances, and with the velocity known 
as that of radiant light. These infinitely small particles, forming the 
luminous "effluvias" incessantly springing from the sun in radial lines, 
pass without obstruction through the pores or interstices of the grosser 
matter forming the planets, though losing at their surface their luminif- 
erous property. 
Beyond the orbit of Saturn they encounter more and more of a similar 
material collected at the outer confines of the vortex; and, while a large 
number pass into adjacent vortices and continue their course unimpeded, 
a large number sutler collision by direct impacts from the similar radia- 
tions from these vortices. Being inelastic they are mutually arrested, 
and form molecules of various size, according to the number of such col- 
lisions suffered, until impelled by the resultant of these impacts, they 
either pass into adjoining vortices or back into their own in the form of 
material molecules of the second element, with correspondingly reduced 
velocities, t 
" In this manner we conceive there must continually descend from the 
heavens a copious and impetuous rain of pellets, driven inward by the 
shocks of molecules from surrounding vortices." This converging or cen- 
tripetal shower of pellets, called the " central torrent," perpetually de- 
luging the sun, forms the compensation and nourishment for its cease- 
less waste, as the evaporated water condensing into rain-drops replen- 
ishes the constant waste of the sea. These minute balls, having only the 
amount and direction of motion imparted to them at the boundaries of 
the vortex, are yet supposed to fall with mathematical precision toward 
the center of the sun without ever touching eachother as they approach. 
Notwithstanding the efforts expended by the author in attempting to 
establish this necessity, this centripetal directness undoubtedly remains 
physically the weak point of his hypothesis. 
As the inter-vortical molecules acquire different magnitudes before 
* Loco citat., sections xviii, xix, pp. 278, 279. 
t Loco citat., sec. xxviii-xxxvi. 12 
KINETIC THEORIES OF GRAVITATION. 
being finally driven toward the center'of any system, they will also have 
differing rates of descent, "their velocities being inversely as their 
masses, so that if one should become a thousand times larger and slower 
than the impelling particle of light, it would still have a velocity equal 
to the diameter of the earth in one minute;" and the average speed 
would be much greater than this, though always much less than the 
velocity of light. " The central torrent with such a velocity would ex- 
pend its force on any body which it encountered ; and this is precisely 
the gravitation of the planets toward the sun." And as the various 
sizes of the minute balls would permit them to penetrate the pores of 
gross matter to varying depths before being arrested, their impulses 
would be distributed through the mass instead of being entirely expended 
on the surface.* 
The particles of light radiated by very distant stars, having to run the 
gauntlet of all the intermediate stellar vortices, might be supposed to 
be very much obstructed and reduced in number, if not in some cases 
entirely suppressed. 
Each planet and satellite has its subordinate vortex, in which the 
same play of impalpable effluvia and returning torrent is carried on ; 
and although this is treated as a very obvious corollary of the system, 
it is one somewhat difficult to fully formulate or realize. Whether two 
independent masses of lead or iron also attract each other impulsively 
by virtue of their own special vortices, with atomized radiations and 
resulting central torrents, is not so definitely made out. 
Such then is the primum mobile of the planetary gravitations; and 
with an admirable complacency, Bernouilli contrasts the system of 
Newton with his own, in which the elementary particles of matter, having 
no pores, receive the gravitative impulse necessarily on their surfaces. 
"Now if it were the essential nature of bodies to 'attract' each other, 
it is evident that elementary particles would gravitate in proportion to 
their solidity, and not in the ratio of their surface, and that .... 
consequently their attraction should diminish in the ratio of the cubes 
of the distance, instead of as the squares. . . . What then becomes 
of the system of id. Newton, when its very foundation is tumbled into 
ruins'? I am surprised that not one of the partisans of this hypothesis 
has perceived this incongruity, in attributing attraction as an essential 
quality, not only to large masses, but even to the elementary particles 
destitute of pores I " f 
It is scarcely necessary to criticise this wonderful system of "Celestial 
Physics." The condensation of the impalpable atoms of caloric, without 
adhesions and without attractions, (and seemingly without inertia,) into 
the dynamic gravific molecules of the "central torrent," is a phenomenon 
certainly as recondite as the gravitation these molecules are summoned 
to impel. It is sufficient to say that the Nouvelle Physique satisfies no 
single condition of the six formerly indicated as essential pre-requisites. 
* Loco citat., sec. xxxvii-xl. 
t Loco citat., sec. xlii, p. 299. KINETIC THEORIES OF GRAVITATION. 
13 
It may be added, that in the fourth book of this labored treatise, the 
inclination of the ecliptic plane to that of the solar equator and vortex, 
is supposed to be due to a deviation or drift resulting from the oblong 
or prolate form of the revolving spheroidal earth."* 
That such an essay should have been crowned by the Academy (the 
prize however being shared between John Bernouilli and his son Daniel 
as an independent competitor) appears in our day, notwithstanding the 
mathematical pre-eminence of its author, a somewhat unfavorable illus- 
tration of the scientific character of the age. Bernouilli has however 
left us the statement of an elementary truth, which may appropriately 
close this notice : " In physics we should banish the practice of explain- 
ing phenomena by chimerical principles more obscure than those pre- 
sented for investigation."! 
Lesage. 1750. 
Some fifteen years later, another bold scheme of universal impact or 
pressure, designed to explain and supersede " attraction," was conceived 
by Georges-Louis Lesage, a French-Swiss physicist and mathematician. 
By means of an infinite number of " ultramundane corpuscles" of trans- 
cendent minuteness and velocity, traversing space in straight lines in 
all directions, atoms and masses of matter are impelled together differ- 
entially in the lines of their reciprocal mechanical shadows, or in the 
direction in which the rectilinear impulses of the " corpuscles" are un- 
counteracted by opposing ones, from the intervention of other atoms or 
masses. 
To quote Arago's exposition of the theory, "A single body placed in 
the midst of such an ocean of moving corpuscles would remain at rest, 
since it would be equally impelled in every direction. On the other 
hand, two bodies ought to advance toward each other, since they would 
form a mutual screen, as their opposed surfaces would no longer be hit 
in the direction of the line joining them by the ultramundane corpus- 
cles, and there would then exist currents, the effect of which would no 
longer be neutralized by opposite currents. Moreover, it will be readily 
seen that two bodies plunged into such ' gravitation fluid ' would tend 
to approach each other with a force varying inversely as the square of 
the distance."! 
Although this scheme presents merely the exchange of one incompre- 
hensible agent for another, it is perhaps one of the most ingenious 
attempts ever made to substitute the conception of primaeval motion 
for that of static tension. 
Lesage was only twenty-three years old, when in 1747 he first devised 
* The measurements of a meridional arc of the earth by James Casini, not long pre- 
viously, had brought out the curious result that the polar axis of the earth is its long- 
est diameter.-Traiid de la Grandeur et de la Figure de la Terre. Paris, 1720. 
t Opera: loco citat., sec. xxxii, p. 288. 
t Popular Astronomy, book xxiii, chap. 27, vol. ii, p. 4G8. 14 
KINETIC THEORIES OF GRAVITATION. 
his system of nature; and it is related in his biography that in the 
enthusiasm of his supposed discovery of so august a secret, he cried out, 
in the words of the Syracusian Sage, "eureka! eureka!n and though 
late at night, he immediately wrote to his father, under date of January 
15, 1747, " Euf)rtxa! evpyxa! Never have I felt such satisfaction as at 
this moment, in which 1 have just succeeded in explaining completely, 
by the simple laws of rectilinear movement, the principle of universal 
gravitation! " 
His first production (written in unsuccessful competition for a prize 
of the Academy) was an Essai sur Vorigine des forces mortes, in 1749. 
This memoir was principally occupied with his mechanical basis of grav- 
itation. Lesage wrote much, and published little. A memoir by him 
entitled Essai de Chimie Mecanique, which explained the phenomena of 
elective affinities by currents of ultramundane corpuscles of unequal 
size, was crowned by the Academy of Rouen in 1758. Another essay 
by him entitled Loi qui comprend toutes les Attractions et Repulsions, was 
published in the Journal des /Scrunfs, for April, 17G4. Eighteen years 
later, he wrote a dissertation entitled Lucrece Neutonien, more fully 
developing his system, and comprising a response to the objections 
which had been urged against it. This treatise was published in the 
Memoires de V Academic de Berlin, for 1782. He also left a Traite des 
Corpuscles ultramondaines, alluded to with high praise by Prevost in his 
account of Lesage's life and works, but which appears never to have 
been published. 
For more than fifty years did Lesage, with unwavering faith, proclaim 
his doctrine of what he called the " gravific fluid," and urge upon his 
contemporaries its adoption; but without success. The scheme has been 
rejected by intelligent physicists and astronomers as valueless in deal- 
ing with the complex facts of nature. 
Of the six requirements heretofore specified, it will be found to satisfy 
but two,-the first and the third. So far from fulfilling for example 
the second condition, (the ratio of mass,) on which Lesage himself most 
confidently expatiated, it can apparently give no true account of the 
behavior of a series of atoms placed in a line between two outer ones. 
The author supposed that he had covered the ground by the assump- 
tion that material atoms are so exceeding small in comparison with their 
interspaces that but few of the flying " corpuscles" will encounter the 
atoms. Professor Tait, of the University of Edinburgh, has remarked : 
" It is necessary also to suppose that particles and masses of matter 
have a cage-like form, so that enormously more corpuscles pass through 
them than impinge upon them ; else the gravitation action between two 
bodies would not be as the product of their masses."* While this sup- 
position fails notably to give a satisfactory mathematical representation 
of the observed facts, (on any assignable ratio of impact to percolation,) 
it is of course quite inadmissible with respect to atoms themselves. In- 
* Lectures on Recent Advances in Physical Science, London, 1876, Leet, xii, p. 300. KINETIC THEORIES OF GRAVITATION. 
15 
deed, if the atoms of matter are porous or penetrable to the " ultramun- 
dane corpuscles," the third condition will remain unsatisfied. 
This corpuscular system of course entirely ignores the fourth condi- 
tion of the problem, and its fundamental postulate stands in direct op- 
position to the fifth condition. It is certainly impossible, on any quan- 
titative assumption or numerical estimate whatever, to represent by 
this scheme the earth's residual gravitation toward the sun during an 
eclipse of the moon. 
Professor J. Clerk Maxwell, discussing the theory of Lesage, observes 
that if the number of ultramundane corpuscles arrested by our earth is 
by supposition much less than the number arrested by the sun, " the 
proportion of those which are stopped by a small body, say a one-pound 
shot, must be smaller still in an enormous degree, because its thickness 
is exceedingly small compared with that of the earth. Now the weight 
of the ball, or its tendency toward the earth, is produced according to 
this theory, by the excess of the impacts of the corpuscles which come 
from above, over the impacts of those which come from below and have 
passed through the earth. Either of these quantities is an exceedingly 
small fraction of the momentum of the whole number of corpuscles 
which pass through the ball in a second, and their difference is a small 
fraction of either, and yet it is equivalent to the weight of a pound. 
. . . . Now the energy of a moving system is half the product of its 
momentum into its velocity. Hence the energy of the corpuscles which 
by their impacts on the ball during one second, urge it toward the earth, 
must be a number of foot-pounds equal to the number of feet over which 
a corpuscle travels in a second, that is to say, not less than thousands 
of millions. But this is only a small fraction of the energy of all the 
impacts which the atoms of the ball receive from the innumerable streams 
of corpuscles which fall upon it in all directions. Hence the rate at which 
the energy of the corpuscles is spent in order to maintain the gravitating 
property of a single pound is at least millions of millions of foot-pounds 
per second. What becomes of this enormous quantity of energy ? . . . . 
The explanation of gravitation falls to the ground if the corpuscles are 
like perfectly elastic spheres, and rebound with a velocity of separation 
equal to that of approach. If on the other hand they rebound with a 
smaller velocity, the effect of attraction between the bodies will no 
doubt be produced; but then we have to find what becomes of the 
energy which the molecules have brought with them but have not car- 
ried away. If any appreciable fraction of this energy is communicated 
to the body in the form of heat, the amount of heat so generated would 
in a few seconds raise it, and in like manner the whole material universe, 
to a white heat."* 
Hence the energy expended by the ultramundane corpuscles in giv- 
ing motion to material masses must be so much abstracted from their 
aggregate store of velocity; and from the constantly-increasing num- 
•Encyclopaedia Britannica, ninth edition, 1S75, article " Atom," vol. iii, pp. 46,47. 16 
KINETIC THEORIES OF GRAVITATION. 
ber of such corpuscles which must thus be more or less uspent" in ful- 
filling their appointed function, it follows that the total activity of bom- 
bardment on matter cannot be as vigorous now as it was a million years 
ago, and must be still less vigorous a million years hence ; all which is 
contrary to the unchangeable continuity of gravity affirmed by our sixth 
condition. 
As has been well remarked by an able anonymous writer in the North 
British Review, " The attraction of gravitation is not as the surface of 
the bodies, but as their mass. Lesage had therefore to suppose his solid 
bodies not solid, but excessively porous, built up of molecules like cages, 
so that an infinite number of atoms went through and through them, 
allowing the last layer of the sun or earth to be struck by just as many 
atoms as the first, otherwise clearly the back part of the sun and earth 
would gravitate more strongly than the front or nearer sides, which 
would be struck only by the siftings of the previous layers of matter. 
This notion involves a prodigious quantity of material in the shape of 
flying atoms, where we perceive no gross matter, but very little mate- 
rial in solid bodies, where we do find gross matter; and it further re- 
quires that the accumulation of atoms which strike the solid bodies 
perpetually should be insensible."* 
Not only does the " gravific fluid " utterly fail to give an approximate 
representation of the actual conditions of the planetary movements, but 
as must be evident, it will not permit the continued existence of any 
such movements. A mass moving in-free space in any direction except- 
ing directly toward a similar mass, must receive a more active shower 
of corpuscles in its front than in its rear, and must thus be retarded by 
a differential of energy directly proportioned to its velocity. Every 
planet must accordingly encounter a tangential resistance to its orbital 
motion, proportional to its own gravitation and to its velocity. 
As illustrative of the different estimates of this hypothesis formed by 
distinguished men, the following citations may be permitted. M. Pierre 
Prevost, professor of philosophy and general, physics in the University 
of Geneva, published two years after the death of Lesage, an account 
of his writings, in which, after a sketch of his corpuscular hypothesis, 
he remarks, "I pause at the foot of this majestic edifice with a senti- 
ment of hope; persuaded that the labors of the founder will ijot be 
suffered to perish, and that men of genius will share with me the ad- 
miration it has inspired."! And Professor Tait regards it as "the only 
plausible answer to this [great problem] which has yet been pro- 
pounded."]: Sir John Herschel, on the other hand, has remarked, " The 
hypothesis of Lesage which assumes that every point of space is pene- 
trated at every instant of time by material particles sui generis, moving 
in right lines in every possible direction, and impinging upon the ma- 
* North British Review, March, 1868, vol. xlviii, p. 126 of American edition, 
t Notice de la Vie et des Merits de G-.-L. Le Sage, published at Geneva in 1805. 
t Lectures on Physical Science, loco citat., p. 290. KINETIC THEORIES OF GRAVITATION. 
17 
ferial atoms of bodies, as a mode of accounting for gravitation, is too 
grotesque to need serious consideration ; and besides will render no ac- 
count of the phenomenon of elasticity."* 
As an interesting illustration of Lesage's range of intellectual activity, 
it may be mentioned that to him belongs the credit of having devised, 
constructed, and operated, in his native city, Geneva, in 1774, the first 
working electric telegraph.! His system consisted in the employment 
of an insulated wire for each letter, terminating in an electroscope at the 
receiving-station. He also wrote a Dissertation sur Velectricite applique 
a la Transmission des < Nouvelles:-the first treatise on the electric tele- 
graph. 
Euler. 1760. 
Leonard Euler, the eminent Swiss mathematician and philosopher, 
(a pupil of Bernouilli previously* referred to,) entertained an indefinite 
impression that the aetherial medium is in some way a connecting link 
between the celestial bodies, inducing that mutual tendency to approach 
commonly called 11 attraction." Only some dozen years later than the 
date of Lesage's first conception, he briefly discussed the subject in his 
celebrated " Letters" commencing in 1760. 
He thus comments on the action of gravity : " Supposing a hole made 
in the earth through its center; it is clear that a body at the very center 
must entirely lose its gravity, as it could no longer move in any direc- 
tion whatever, all those of gravity tending continually toward the cen- 
ter of the earth. Since then a body has no longer gravity at the center 
of the earth, it will follow that in descending to this center, its gravity 
will be gradually diminished; and we accordingly conclude that a body 
penetrating into the bowels of the earth loses its gravity in proportion 
as it approaches the center. It is evident then that neither the intensity 
nor the direction of gravity is a consequence from the nature of any 
body, as not only its intensity is variable, but likewise its direction, 
w'hich, on passing to the antipodes, becomes quite contrary."! 
After some further exposition of the effects of gravitation, as observed 
in the courses of the planets, Euler indulges in some speculation on 
the probable nature of this influence. " But in attempting to dive into 
the mysteries of nature, it is of importance to know if the heavenly 
bodies act upon each other by impulsion or by attraction ; if a certain 
subtile invisible matter impels them toward each other ; or if they are 
* Fortnightly Review, July 1, 1865, vol. i, p. 438. 
t"The earliest attempt to apply frictional electricity to telegraphy seems to have 
been made by Lesage, of Geneva, who, in 1774. constructed a telegraph consisting of 
twenty-four insulated wires." (George B. Prescott, Electricity and the Electric Tele- 
graph, 8vo, N. Y., 1877, chap, xxix, p. 414.) 
t Letters a une Princesses d'Allemagne, Let. 50, 30th August, 1760. This work, since 
so popular, was republished in England, " Letters on different subjects in Physics and 
Philosophy, addressed to a German Princess." Translated from the French by Henry 
Hunter, 2 vols. 8vo, London, 1802. 
2 18 
KINETIC THEORIES OF GRAVITATION. 
endowed with a secret or occult quality, by which they are mutually 
attracted. On this question philosophers are divided. Some are of 
opinion that this phenomenon is analogous to an impulsion; others 
maintain with Newton, and the English in general, that it consists iu 
attraction."* 
"To avoid all confusion which might result from this mode of ex- 
pression, it ought rather to be said that bodies move as if they mutually 
attracted each other. ■ This would not decide whether the powers which 
act ou bodies reside in the bodies themselves or out of them ; and this 
manner of speaking might thus suit both parties. Let us confine our- 
selves to the bodies which we meet with ou the surface of the earth. 
Every one readily admits that all these would fall downward, unless 
they were supported. Now the question turns on the real cause of this 
fall. Some say that it is the earth which attracts these bodies, by an 
inherent power natural to it; others that it is the aether, or some other 
subtile or invisible matter, which impels the body downward, so that the 
effect is nevertheless the same in both cases. 
" This last opinion is most satisfactory to those who are fond of clear 
principles in philosophy, as they do not see how two bodies at a distance 
can act upon each other if there be nothing between them 
Let us suppose that before the creation of the world, God had created 
only two bodies, at a distance from each other ; that absolutely nothing 
existed outside of them, and that they were in a state of rest; would it 
be possible for the one to approach the other, or for them to have a pro- 
pensity to approach ? How could the one feel the other, at a distance ? 
Whence could arise the desire of approaching? These are perplexing 
questions. But if you suppose that the intermediate space is filled with 
a subtile matter, we can comprehend at once that this matter may act 
upon the bodies by impelling them. The effect would be the same as 
if they possessed a power of mutual attraction. Now as we know that 
the whole space which separates the heavenly bodies is filled with a sub- 
tile matter called aether, it seems more reasonable to ascribe the mutual 
attraction of bodies to an action which the aether exercises upon them, 
though its manner of acting may be unknown to us, than to have re- 
course to an unintelligible propertyAs the idea of all 
occult qualities is now banished from philosophy, attraction ought not 
to be considered in this sense." f 
It does not appear howT so vague and inexplicable a suppositiou is 
calculated to commend itself to those who are fond of clear principles 
in philosophy." In his anxiety to avoid an "occult quality" in matter, 
this learned writer seems quite unconscious of the fact that by investing 
his aether with an "unknown manner of acting," he is just as fatally "hav- 
ing recourse to an unintelligible property." Certainly, just as " perplex- 
*Letters, &c. Let. 54, 7th September, 1760. 
t Letters, &c. Let. 68, 18th October, 1760. KINETIC THEORIES OF GRAVITATION. 
19 
ing questions" are suggested by the hypothesis of aether pressure, as by 
the hypothesis of an original "propensity to approach." 
The speculation however, is too indeterminate to admit of precise 
criticism ; and is noteworthy only from the eminence of its proposer. 
It is a little remarkable that Euler, although in correspondence with 
Lesage, makes no allusion to his hypothesis. 
Herapath. 1816. 
Passing over a few names of less note in this connection, (Berthier, 
Churcol, and others,) we find a somewhat more developed attempt at 
unveiling the standing enigma, presented in the writings of John Her- 
apath, of Bristol, England. In a preliminary essay, published in Thom- 
son's Annals of Philosophy, u On the Physical Properties of Gases," he 
announced the hypothesis of "one cause for heat, light, gravitation, 
electricity, cohesion, serial repulsion, &c., from which all these flow, 
and are easily deducible; and their effects may be computed by mathe- 
matical induction, [deduction ?] It shows us that gravitation, cohesion, 
and affinity, are but the same thing under different modifications; that 
the differences of the two latter arise from a difference in the figures 
and sizes only of the particles; that attraction and repulsion are not 
properties of matter."* This hypothesis thus briefly stated by its au- 
thor, at the age of twenty-three years, does not appear to have been fur- 
ther publicly elaborated for five years. 
In 1821, Herapath contributed to the same journal a memoir entitled 
"A Mathematical Inquiry into the Causes, Laws, and principal Phenom- 
ena of Heat, Gases, Gravitation, &c.," memorable as presenting by far 
the fullest and clearest exposition of the mechanical theory of heat and 
of gases that had at that time been propounded. He states that about 
ten years earlier, while engaged in investigating an anomaly found in 
his calculations of lunar equation, his attention had been accidentally 
directed to Newton's suggestions as to the cause of gravitation; and he 
proceeds: 
"If gravitation depends upon the action of an elastic medium such as 
Newton supposed, which grows rarer and rarer as you approach the 
dense bodies of the sun and planets, there ought to be some reason for 
this variation of density; and as Newton has not, as far as I could per- 
ceive, given any, I began to consider what it might be. And after some 
little thinking, it occurred to me that if this medium be of the same 
nature as our atmosphere and other gaseous bodies, that is, if it be 
capable of being expanded by heat and contracted by cold, then the 
sun being a very hot body, and the heat being so much the greater the 
nearer we are to him, the density of the medium ought therefore to de- 
crease with a decreasing and increase with an increasing distance, 
the same as Newton would have it. And because we find by experience 
that dense solid bodies receive heat more strongly than much rarer 
* Annals of Philosophy, July, 1816, vol. viii, pp. 58, 59. 20 
KINETIC THEORIES OF GRAVITATION. 
ones, particularly than gases, the dense bodies of the planets being 
heated by the solar rays as well as by the medium about them, ought 
it appeared to me, to be hotter than this medium, and consequently 
ought to produce the same effects on the medium as the sun, though 
not in so great a degree. Therefore if as Newton imagines, the parti- 
cles of the planets be impelled toward the sun by the inequality of the 
pressure on their further and nearer sides, the denser parts of the me- 
dium pressing more forcibly than the rarer, the same reason will like- 
wise hold good why bodies should be impelled toward the planets and 
other material parts of the system." * 
After speaking of the discouragement resulting from his unsuccessful 
attempts at arriving at the mathematical laws of heat, he proceeds: 
" Yet sometimes when my thoughts were involuntarily turned this way, 
the idea that two inanimate bodies could act on each other at a distance 
without some other means than that of a mere tendency or inclination 
in them to approach, would appear so strongly unphilosophical, and the 
apparent coincidence of several phenomena with conclusions I had 
drawn from my notions of gravitation so very seductive, that I could 
not avoid thinking the views I had taken were tolerably correct; and 
that there was only wanting the direction of some happy idea, which 
patient perseverence might possibly attain, to set the whole in a clear 
and irrefragable light. Thus between hope and despair, between un- 
ceasing attempts and mortifying failures, I continued until May, 1814, 
at which time my ideas of heat underwent a complete revolution. Pre- 
vious to this time I had conceived heat to be the effect of an elastic 
fluid, and on this supposition, had repeatedly attempted to reduce its 
laws to mathematical calculation ; but uniform disappointment at length 
induced me to give this hypothesis a careful investigation, by compar- 
ing it with general and particular phenomena. The result of this in- 
vestigation convinced me that heat could not be the consequence of an 
elastic fluid. . . . After I had revolved the subject a few times in 
my mind, it struck me that if gases instead of having their particles 
endued with repulsive forces, subject to so curious a limitation as New- 
ton proposed, were made up of particles or atoms mutually impinging 
on one another and the sides of the vessel containing them, such a con- 
stitution of aeriform bodies would not only be more simple than repulsive 
powers, but as far as I could perceive, would be consistent with phe- 
nomena in other respects, and would admit of an easy application of 
the theory of heat by intestine motion. Such bodies I.easily saw pos- 
sessed several of the properties of gases; for instance, they would ex- 
pand, and if the particles be vastly small, contract almost indefinitely; 
their elastic force would increase by an increase of motion or tempera- 
ture, and diminish by a diminution; they would conceive heat rapidly, 
and conduct it slowly; would generate heat by sudden compression, 
* Annals of Philosophy, 1821, vol. xvii, or of new series, vol. i, p. 276. KINETIC THEORIES OF GRAVITATION. 
21 
and destroy it by sudden rarefaction; and any two, having ever so 
small a communication, would quickly and equally intermix."* 
Fanciful as are the considerations which led Herapath to this conclu- 
sion, it may be doubted whether a better statement of the dynamic 
theory of heat, and the modern view of gaseous temperature, has been 
published in the last half century. Certainly none can be found pre- 
ceding it. The scientific world was not then however prepared by a 
sufficient induction to fully appreciate this theory. 
These views of thermogenetic gravitation were amplified by their 
author at a later period, and included in an elaborate and excellent 
treatise ou the general principles of physics, published in 1847, in which 
work they form the concluding portion, or book iv, comprising four 
sections.! 
Herapath saw very clearly that a theory of molecular collision cannot 
dispense with resilient impacts; but he announced the startling paradox 
that atoms "perfectly bard" would on striking each other, rebound just 
as though they were elastic. This very difficult thesis is discussed at 
some length (though certainly not convincingly) in his general work,! 
in a chapter on " the collision of hard bodies." The conception of a re- 
pellant propensity in the atoms is of course, excluded by the very spirit 
of the hypothesis. " Only two properties to matter are assumed, namely, 
inertia and absolute hardness. . . . Our theory deprives the par- 
ticles of repulsion, or of any active properties, and merely assumes that 
airs are composed of small particles moving about in all possible direc- 
tions, and keeping up their state as airs by their mutual collisions and 
reflections from one another and the sides of the containing vessels. 
From this simple property, and that of heat consisting in corpuscular 
motion, the whole known laws of gases are deduced with mathematical 
rigor." § Unfortunately " two properties" are wholly insufficient either 
to set or to keep a system of molecules in motion. Matter thus consti- 
tuted, (with only " two properties,") with any amount of motion super- 
imposed, could never make a cosmos. The " stubborn fact" of elasticity 
has indeed been the insuperable obstacle and embarrassment of all 
kinetic schemes of molecular physics. 
" By extending the principles to find the temperatures of the planets, 
we arrive at an interesting conclusion, namely : supposing them to be 
all of the density of our earth, we bring out very nearly the amount of 
gravitation toward each of them which is actually found to exist. 
Mercury is not included, as our knowledge about him is uncertain." 
(Introduction, p. xxv.) Mercury however is excluded, because on the 
assumption that the absolute temperatures of the planets are inversely 
as their distances from the sun, the temperature of. this inner planet is 
* Loco citat.,p. 278. 
t Mathematical Physics. By John Herapath. 2 vols. 8vo. London, 1847. 
t Math. Phys., vol. i, pp. 106-137. Huyghens and Wren had both (a century and a 
half earlier) maintained the same doctrine. 
§ Math. Phys., Introduction, pp. xvii, xviii. 22 
KINETIC THEORIES OF GRAVITATION. 
found to be too high to satisfy the conditions of the calculation.* If 
" the amount of gravitation toward each " planet is at all indicated by 
the relative distance-periods of their satellites, it is very clear that they 
cannot have the same density. 
It might be expected that with the range of temperatures at our com- 
mand, the influence of heat on attraction could be subjected to the test 
of direct experiment. It is admitted that " We have no distinct evi- 
dence of attraction being either augmented or lessened by heat."t 
The radical defect of this ingenious application of the differential of 
heat-motion as the impelling force of gravity lies in the fallacy that 
any pressure-differences would, under the circumstances, result from 
temperature-differences. Our author says: " In Newton's day the 
notion of a fluid which had no visible tendency to one part of space 
more than to another, keeping up an equilibrium with itself, and yet 
able to press heavier on one side of a body within it than on the other, 
was quite enough to gain incredulityNor is it easy to perceive how 
the notion is made more credible in our day. The rarefaction of a free 
gas by heat is the direct effect of its increased elastic tension or pressure, 
and the two are proportional. In other words, if upon the planetary 
hemisphere exposed to the sun there were fewer impacts of gaseous 
molecules in a unit of time than on the outer or night hemisphere, 
these impacts would have a correspondingly higher velocity, so that 
the whole moment of impulse (or pressure) on the two sides would be 
precisely equal. 
It is doubtful whether this hypothesis (even supposing it operative) 
could really satisfy any of the six conditions heretofore propounded. 
With regard to the second postulate, it is evident that the mass of the 
attracting body cannot determine the quantity of attractive action, if 
heat be the efficient cause. This is very frankly conceded by Herapath, 
who says of the mass ratio : " This law has been proved experimentally 
by Sir Isaac Newton ; but though this be true, the converse case does 
not according to our principles hold good, namely that the attractive 
forces of bodies are directly proportional to their quantities of matter. 
Our principles do not therefore corroborate Newton's third law of 
motion, respecting the equality of action and reaction in attracting 
forces; for by our theory, a body might by the agency of the fluid 
medium, be impelled toward another, without any reciprocal action; 
which is by no means surprising if we consider attraction not to be an 
inherent or essential property of matter, but merely the action of a 
third body."§ The sufficient answer to all which is, that not only is it 
unconfirmed by any experimental research, but all experience contra- 
dicts the assumption. 
* Math. Phys., vol. ii, p. 318. 
t Math. Phys., Introduction, p. xv. 
t Loco citat., Introduction, p. xxxvi. 
§ Annah of Philosophy, new series, vol. i, p. 411; and Math. Pbys., vol. i, p. 9. KINETIC THEORIES OF GRAVITATION. 
23 
The force of the objection contained in our fourth condition-precedent 
is thus courageously confronted and defied : " It might be conceived 
that the attraction would be less on a body moving toward the central 
body, and greater ou one moving from it, which is contrary to what we 
find by experience. Though regarded mathematically, such an infer- 
ence would be strictly true, yet since the difference between the forces 
will depend on the activity of the medium, and since this activity will 
be increased in proportion to the tenuity of the parts of the medium, it 
is evident that the setherial atoms may be so small, and the activity of 
the medium consequently so great, that the swiftest motions we know 
of could produce no sensible difference in the vigor of its action." And 
with a marvelous boldness of assumption he adds: " We may hence 
fairly conclude that there might be a fluid medium pervading the 
heavens, and all bodies, of such activity that uo sensible difference 
could be observed in the intensity of its action on bodies in a state of 
quiescence, or moving with a velocity not only six million, but several 
million million times greater than that of light I" * 
Guyot. 1832. 
Another ingenious attack upon the " Problem," in a somewhat similar 
though really distinct direction, was made by Dr. Jules Guyot-a 
French physician,-in a very original and suggestive treatise entitled 
Elements de Physique Generate, 8vo, published in Paris in 1832, at 
about the age of twenty-seven years. This writer seems to have had, 
even at this early date, a general idea of correlating the physical forces. 
Assuming with other physicists, two kinds of matter,-one sensible in 
the gaseous liquid and solid forms, the other insensible, in the " simple 
elementary and atomic" condition of the setherial medium, filling all space 
under a state of constant and enormous pressure, and infinitely more 
subtile than any particle of combinable matter,-and assuming also two 
kinds of movement, one of translation and the other of vibration, Guyot 
maintained that these two forms of motion are reciprocally comple- 
mentary and convertible, their sum being constant. So that motion is 
essential to matter, is equally indestructible, and is directly propor- 
tional to its mass. He held that light, heat, sound, and the excitations 
of smell and taste, are all the results of molecular motion or vibration, 
as are also the agencies of electricity, magnetism, and gravitation. 
In a work on the pressures resulting from serial motions, published 
in 1835, Guyot records some curious experiments on the influence of 
vibratory sounding bodies in causing light objects to approach them;- 
experiments which immediately recall the suggestions made by Dr. Kob- 
ert Hooke more than a century and a half earlier, and which would cer- 
tainly have delighted that philosopher. Disks of pith and of paper, 
delicately suspended near a vibrating tuning-fork, were observed to be 
* Annals of Philosophy, new series, vol. i, p. 410. 24 
KINETIC THEORIES OF GRAVITATION. 
attracted as it were, to the origin of vibration, with a range of influ- 
ence approximately proportional to the area of the disk. The same 
phenomenon was observed with the employment of a bell, when caused 
to sound by drawing a bow across its edge, excepting that at the nodes 
of oscillation no ''attraction" was exhibited.* 
In an essay entitled "A synthetic Glance at the Forms and Forces of 
Matter," published in 1861, the same author, recapitulating his views 
and observations of 1832 and 1835, and still maintaining that all the 
properties of bodies are derivatives of their translatory or vibratory 
movements, and that theequilibrium and thephenomenaof the world exist 
only under the condition of constant pressure of the incoercible aether 
upon coercible matter, and the reaction of the latter upon the former, 
argues that, " if it be shown that the vibration of the atoms of bodies 
may and actually does cause a rarefaction in the sphere of activity of 
each of the atoms," this constitutes a proof that " the approximation of 
the atoms of bodies of ponderable matter is due to the rarefaction of 
the imponderable fluid, and consequently to the diminution cf its press- 
ure in the space between the atoms of the same body;" and hence that 
" we are compelled to admit that attraction is a mechanical force, con- 
sisting, first, of the rarefaction of the aether between molecules, masses, 
or the heavenly bodies, resulting from the ceaseless vibration of the 
atoms of ponderable matter, and secondly, of the reaction from the 
exterior pressure of the tether upon the same, resulting from the general 
pressure of the imponderable universal medium which constitutes the 
mother-liquor of the world."! 
In this article the writer brings out very distinctly an idea first sug- 
gested by New ton, and which has recently been fermenting, so to speak, 
in the minds of various speculative writers, to wit, that matter as ex- 
perimentally cognizable by our senses-having for its lowest constituent 
unit the compounded molecule of uniform structure for each element- 
ary substance, the indivisible " atom " of the chemist-has been by 
some mysterious process evolved in the indefinite past from the struc- 
tureless impalpable aether filling immensity. That the ultimate mole- 
cule of matter as known to us is a highly complex or organized cosmos, 
appears to be sufficiently demonstrated by the definite multiple peri- 
odicities exhibited by gaseous spectra. If the sympathetic responses 
*Des mouvements de I'air et des pressions de I'air en mouvement. (8vo, Paris, 1835.1 
Sir Henry C. Englefield, in 1773, observed at Brussels, that during the ringing of a 
large church bell, (weighing 16,000 pounds,) the serial vibrations affected a mercurial 
barometer, which was placed experimentally about seven feet below it, by raising 
the column about one-hundredth of an inch. (Journal of R. I., vol. i, p. 157.) On 
which Dr. Young offers the following suggestion : " It is easy to suppose that the law 
of the bell's vibration was in this experiment such that the air advanced toward the 
barometer with a greater velocity than it receded, although for a shorter time; and 
that hence the whole effect was the same as if the mean pressure of the air had been 
increased." (Lectures on Natural Philosophy, 1807, 2 vols. quarto, vol. ii, p. 270.) 
i Presse Scientifique, 1861, vol. iii, p. 130. KINETIC THEORIES OF GRAVITATION. 
25 
of the aether indicate a community of nature with the absolute atoms of 
the molecule, whose tremblings they transmit, we are confronted with 
the paradox, that while in the molecule these atoms are so firmly bound 
together that no known forces have ever been able to divorce them, in 
their isolated or discrete state constituting " the mother-liquor of the 
world," their repulsions are so intense that no known forces have ever 
been able to unite them. 
It is impossible not to be struck with the originality of speculation 
and the ingenuity of experimentation by'which, as Dr. Guyot believes, 
he has solved the great problem of energy or dynamic. But it is appa- 
rent at a glance that his system is at variance with every feature of the 
actual phenomenon of gravitation, and fails to represent any of the six 
conditions-precedent, perhaps excepting the first. If the resultant mo- 
tion of translation from an acoustic vibration of air (or from a thermal 
vibration of tether) may be supposed to occur in a right line, it is not 
established that it can so continue for any considerable distance; and 
the last five conditions are each and all directly incompatible with the 
assumption. 
Perhaps the most palpable fault of the scheme however, is the vio- 
lence done by it to the established law of the conservation of energy, 
while being proclaimed apparently in the interest of that law. A 
vibrating molecule is supposed to impress its motion upon an investing 
medium, without parting with any of its original vis viva ; or in other 
words, it is miraculously endowed with an inexhaustible fund of dynamic 
action, and its motion though constantly expended in "work" per- 
formed, yet requires no regeneration. 
Or on the other hand, if gravitation depend upon the vibrations 
transmitted from the active molecule as a center to the surrounding 
aether, this "attractive" action must decline with the expenditure of 
the vibratory energy, contrary to the observed fact as summed up in 
the sixth proposition. 
Faraday. 1844. 
Although the views announced by Professor Michael Faraday on the 
subject of gravitation were undoubtedly' very vague, he must be classed 
with the kinetic theorists; and the very influence necessarily attaching 
to his well-earned reputation as an investigator and experimental physi- 
cist, renders a full discussion and a free criticism of his published reflec- 
tions all the more imperative in the interests of scientific truth. 
In "A Speculation on the Nature of Matter," dated January 25,1844, 
Faraday remarks: "The safest coarse appears to be to assume as little 
as possible ; and in that respect, the atoms of Boscovich appear to me 
to have a great advantage over the usual notionA mind 
just entering on the subject may consider it difficult to think of the 
powers of matter independent of a separate something to be called the 
matter, but it is certainly far more difficult, and indeed impossible, to 
think of or imagine that matter, independent of the powers. Now the 26 
KINETIC THEORIES OF GRAVITATION. 
powers we know and recognize in every phenomenon of the creation ; 
the abstract matter in none; why then assume the existence of that of 
which we are ignorant, which we cannot conceive, and for which there 
is no philosophical necessity ? .... Doubtless the centers of force 
vary in their distance one from another, but that which is truly the 
matter of one atom touches the matter of its neighbors. Hence matter 
will be contfi/mo&s throughout, and in considering a mass of it, we have 
not to suppose a distinction between its atoms and any intervening 
space. The powers around the centers give these centers the properties 
of atoms of matter; and these powers again, when many centers by 
their conjoint forces are grouped into a mass, give to every part of that 
mass the properties of matterThe view now stated of the 
constitution of matter would seem to involve necessarily the conclu- 
sion that matter fills all space, or at least all space to which gravitation 
extends ; for gravitation is a property of matter dependent on a certain 
force, and it is this force which constitutes matter. In that view, mat- 
ter is not merely mutually penetrable, but each atom extends, so to say, 
throughout the whole of the solar system, yet always retaining its own 
centre of force."* 
This result of "assuming as little as possible" thus appears to com- 
mence with the Berkeleyan negation of matter, only to conclude that it 
is omnipresent. When it is inferred however, that every atom sepa- 
rately includes every other atom, it is obviously only influence that is 
conceived of, and not matter at all in any intelligible sense. If we call 
this multitudinous infinitely-extended and mutually inclusive influence 
" matter," there still remains the inexorable necessity of designating by 
some distinctive title that other form of influence inclosed within the 
visible tangible surfaces bounding those appearances which are charac- 
terized by inertia, which are accurately measurable in mass, and which 
are tire objects of all our direct observation and experiment. Neither in 
formula, nor in idea, therefore,-neither in nominalism, nor in realism,- 
are we advanced a particle by such speculations. 
In a memoir " On the Possible Relation of Gravity to Electricity," 
read before the Royal Society, November 28, 1850, Faraday remarks: 
" The long and constant persuasion that all the forces of nature are 
mutually dependent, having one common origin, or rather being differ- 
ent manifestations of one fundamental power, has made me often think 
upon the possibility of establishing by experiment, a connection between 
gravity and electricity, and so introducing the former into the group, 
the chain of which (including magnetism, chemical force, and heat,) 
binds so many and such varied exhibitions of force together by common 
relations." He then records experiments with a tubular helix of cov- 
ered copper wire of considerable length, and having its extremities 
connected with long covered wires which were brought to a very sensi- 
tive galvanometer, the said coil or helix being allowed to fall about 
* L. E. D. Philosophical Magazine, 1844, vol. xxiv, pp. 140-143. KINETIC THEORIES OF GRAVITATION. 
27 
thirty-six feet. No indications however, were perceived in the needle.* 
Experiments with solid cylinders of copper, iron, glass, &c., secured 
within the helix were successively made without result. Similar cyl- 
inders were then dropped through u fixed helix, and also reciprocating 
motion by mechanical devices was tried, but equally without any effect 
on the galvanometer needle. Faraday concludes, " Here end my trials 
for the present. The results are negative. They do not shake my 
strong feeling of the existence of a relation between gravity and elec- 
tricity, though they give no proof that such a relation exists."! These 
experiments were skillfully devised to detect a correlation between the 
two, if any such existed. Were gravity either a form or a resultant of 
molecular motion we should certainly expect to find evidence of an 
expenditure of such motion, proportioned to the energy of the "fall." 
Several years later, in a memoir "On the Conservation of Force," 
Professor Faraday thus states the result of his further meditations on the 
"attractive" theme of gravitation: "I believe I represent the received 
idea of the gravitating force aright in saying that it is a simple attractive 
force exerted between any two or all the particles or masses of matter 
at every sensible distance, but with a strength varying inversely as the 
square of the distance. The usual idea of the force implies direct action 
at a distance; and such a view appears to present little difficulty 
except to Newton, and a few, including myself, who in that respect may 
be of like mind with him.| This idea of gravity appears to me to 
ignore entirely the principle of the conservation of force: and by the 
terms of its definition, if taken in an absolute sense, kvarying inversely 
as the square of the distance,' to be in direct opposition to it."§ 
This singular misconception of his theme, which underlies all his 
subsequent reasoning, may be briefly rebutted by the simple averment 
that the conservation of force has no relation whatever to the laic of 
force, and can have no relation to it. All that the established doctrine 
affirms, is that be the law what it may, " conservation"' demands that 
none of the resultant effects shall vanish, and that the action of the law 
shall be absolutely the same in the same conditions. In the case of a 
dynamic radiation-indeed, through a perfectly elastic medium,-eon- 
* It is evident that whether the earth be contemplated as an electrically-charged 
globe or as a permanent magnet, the delicate experiments of Faraday, above described, 
would necessarily give indications thereof in the galvanometer : and it is an interesting 
illustration of the scientific conscientiousness of the experimenter, to observe with 
what caution these collateral results were eliminated. 
f Philosophical Transactions Roy. Soc., 1851, vol. 141, pp. 1-6. A Mr. Zalewski pre- 
sented to the French Academy of Sciences, (April 22, and August 19, 1850, and again 
July 5 and 19, 1852,) memoirs " On Electricity as the Cause of the Effects attributed to 
Universal Gravitation." {Compt.es liendus, 1850, vol. xxx, p. 485; vol. xxxi, p. 255; 
and for 1852, vol. xxxv, pp. 49 and 95.) "Faraday's insight was so profound, that we 
cannot assert that something may not yet be discovered by such experiments, but it 
will assuredly not be conservation of force." Professor Tait's Lecture on " Force," 
Nature, 21st September, 1876, vol. xiv, p. 462. 
t Referring, of course, to the " third Bentley letter." 
§L. E. D. Phil. Mag., 1857, vol. xiii, p. 228. 28 
KINETIC THEORIES OF GRAVITATION. 
serration requires that all the successive spheres described by increas- 
ing radii of action shall represent precisely the same amount of energy, 
which is the expression of " inverse squares." But in the case of a 
primitive force which is not radiation, (as in gravity, elasticity, cohesion, 
or affinity,) the law of increment or decrement with distance may have 
any mathematical value, and may be entirely different and incommen- 
surable with every variety of force. 
Unfortunately the human mind has been gifted with no insights or 
intuitions which can determine the a priori certainty of a single fact of 
natural law. After twenty-five centuries of vainly-struggling specula- 
tion, the true law of one kind of force was laboriously ascertained only 
two little centuries ago. And this result is justly regarded as the most 
brilliant achievement of the highest human intellect. Did experience 
teach us that the law of gravity was one of simple decrease of intensity 
directly with the distance, (in which case the periodic times of the plan- 
ets would be directly as their distances and their orbital velocities the 
same at all distances,) or did it teach us that its energy was precisely 
the same at all distances, as Faraday thinks to be the true desideratum, 
(in which case the periodic times as well as the orbital velocities would 
be as the square roots of the distances,) or did it teach us that its inten- 
sity increased directly as the distance, as by an elastic bond,* (in which 
case the periods of revolution would be the same for all distances, and 
the orbital velocities therefore, proportional to the distance,) in each 
and every case it would still be unalterably true that the energy ex- 
pended in separating two bodies would be exactly equal to the energy 
given out in their return to the antecedent position. And this is what 
is meant by the " conservation of force." 
Probably no generalization of science has been the occasion of more 
misapprehension and confusion than this of " conservation." Properly 
speaking, " Force " is not conserved at all! It is the offspring of Force, or 
"work" that is really conserved. As words necessarily folloic thought 
practically no less than genetically, (and sometimes longo intcrvallo,') it 
results that with the increasing specializations of scientific conception, 
many words continue to retain their more primitive or "comprehensive 
type"cf meaning, without originating the required varieties or differ- 
entiations of expression ; and such has been the case with the very 
useful word "force;" which is employed sometimes in its more general- 
ized sense, as including any stress or action whatever; sometimes as 
limited to quantity of motion ; sometimes as synonymous with energy, 
(in which sense alone is "conservation" applicable to it;) sometimes as 
expressing " the mere rate of conversion or transference of energy per 
unit length of that motion," (with a strong suspicion that " there is 
probably no such thing as force at all; ")+ and sometimes as signifying 
* There is reason to believe that this is actually the law of the atomic orbits. 
t Lecture on " Force," by Professor Tait, of Edinburgh. Nature, 21st September, 
1876, vol. xiv, pp. 459, 463. It is certain that Newton did not employ the word Fis in 
any such restricted sense, as the learned professor would imply. KINETIC THEORIES OF GRAVITATION. 
29 
primitive innate tension, exclusive of all motion, although the parent of 
all motion. So that while one would limit the word to designate a 
purely kinetic condition of matter, another would limit it on the opposite 
side to designate a purely static quality in matter. 
Elasticity is a natural force, having always an entirely different space- 
potential from gravity, and yet is equally removed in every case from 
that ratio of uniformity supposed to be the true representation of con- 
servation. In the case of tensile elasticity, (as of a rubber band or of a 
long spiral spring,) the tension increases directly with the distance of 
elongation. 
Professor Faraday thus proceeds to illustrate the difficulty he finds 
in the ordinary definition of gravity: " Assume two particles of matter, 
A and B, in free space. . . . Then at the distance of 10 the force 
may be estimated at 1, whilst at the distance of 1, i. e., one-tenth of the 
former, the force will be 100; and if we suppose an elastic spring to be 
introduced between the two as a measure of the attractive force, the 
power compressing it will be a hundred times as much in the latter case 
as in the former. But from whence can this enormous increase of the 
power come?" The answer is, that this increase of "power" comes 
from either particle being so much nearer the source of the influence. 
Why this increase should be just one hundred-fold in the case supposed, 
the present state of science does not furnish any. explanation. The re- 
sult is accepted simply as a very rigorously verified " fact." 
"Suppose the two particles A and B removed back to the greater 
distance of 10, then the force of attraction would be only a hundredth 
part of that they previously possessed ; this, according to the statement 
that the force varies inversely as the square of the distance, would 
double the strangeness of the above results; it would be an annihilation 
of force." Here again, the law of intensity, as a function of distance, 
is confounded with absolute quantity in the agent. Such a confusion 
could hardly have occurred in discussing the action of a permanent 
magnet. The actually existing gravity decrement no more involves 
any "annihilation of force," than would an equality of ratio irrespective 
of distance involve a creation of force, were it found in any case to be 
true. So far from there being any destruction or loss of force in the 
crucial case supposed, the doctrine of "conservation" teaches us that 
the separation of the two particles could be effected only by the expendi- 
ture of an adequate amount of energy, and that at their greater dis- 
tance of 10, these particles would possess a potential of position precisely 
equivalent thereto. 
Faraday continues: "According to the definition, the force depends 
upon both particles; and if the particle A or B were by itself, it could 
not gravitate, i. e. it could have no attraction, no force of gravity. . . . 
As t he particles can be separated, we can easily conceive of the particle 
B being removed to an infinite distance from A, and then the power in 
A will be infinitely diminished. Such removal of B will be as if it were 30 
KINETIC THEORIES OF GRAVITATION. 
annihilated in regard to A, and the force in A will be annihilated at 
the same time."* Although it is certainly true that when B is removed 
to an infinite distance from A, the power of A upon B will be infinitely 
diminished, it is not a sound inference that "the power in A will be 
infinitely diminished." The same inaccuracy occurs in the assumption 
that if an isolated particle "could not gravitate" it could have "noforce 
of gravity." This is but another expression of the not unusual sophism 
that force has no existence unless in active exercise. 
Varying bis illustration to attack the problem of mass, Professor 
Faraday thus further unfolds his difficulties: "The particle A will 
attract the particle B at the distance of a mile with a certain degree of 
force; it will attract the particle C at the same distance of a mile, with 
a power equal to that by which it attracts B. If myriads of like par- 
ticles be placed at the given distance of a mile, A will attract each with 
equal force. . . . How are we to conceive of this force growing up 
in A to a million-fold or more ? And if the surrounding particles be 
then removed, of its diminution in an equal degree ? Or how are wTe 
to look upon the power raised up in all these outer particles by the 
action of A on them, or by their action one on another, without admit- 
ting (according to the limited definition of gravitation) the facile genera 
tion and annihilation of force ?" The substance of this enigma is com- 
prised in the corollary to our second proposition. Striking out the 
fallacious expression "of this force groicing up in A," which has already 
been sufficiently criticised, surely the case as stated, is a very good 
illustration of " conservation." The hypothetical generation and anni- 
hilation of the distant particles surrounding A are just as "facile" as 
the hypothetical " generation and annihilation of force " exercised by 
them; but no whit more so. As if one should say, imagine the clock 
wound up, and it will run a week. The equation is correct only on con- 
dition that both the terms are equally real or equally imaginary. 
Inasmuch as the accepted definition of gravitative force (deemed by 
Faraday so objectionable) is merely the summation of an oversvheltning 
induction derived from a ceaseless observation, the question naturally 
arises, to what point are the difficulties imagined by him supposed to 
tend ? " The principle of the conservation of force would lead us to 
assume that when A and B attract each other less because of increas- 
ing distance, then some other exertion of power, either within or with- 
out them, is proportionately growing up. And again that when their 
distance is diminished, as from 10 to 1, the power of attraction, now in- 
creased a hundred-fold, has been produced out of some other form of 
power which has been equivalently reduced."! Were gravity merely a 
dynamic energy, generated in time and space by an anterior and exte- 
rior force, the inference would undoubtedly be correct. Conversely, the 
utter falsity of the inference, as established by all experience, in which 
* L. E. D. Phil. Mag. 1857, vol. xiii, pp. 228, 229. 
t Loco citat., pp. 230, 231. KINETIC THEORIES OF GRAVITATION. 
31 
experience, as, a question of fact, the keenest of experimental investi- 
gators, Faraday himself, has been able to detect no flaw, the utter 
falsity of the inference may be taken as conclusive against the premiss. 
Gravity is thereby proved to be a static tension,-incessant, inconverti- 
ble, inexhaustible; as affirmed by our fifth and sixth propositions. 
Whatever a priori conceptions may be indulged therefore, as to the 
natural fitness of a central force having the same tension at all dis- 
tances, it has been definitely established by two centuries of continu- 
ous and irreversible demonstration, that gravity is not such a force. 
And this announcement is the subject of our third proposition. 
"It will not be imagined for a moment," says Faraday, "that I am 
opposed to what may be called the laic of gravitative action; that is, the 
law by which all the known effects of gravity are governed. What I 
am considering is the definition of the force of gravitation. . . . 
That the totality of a force can be employed according to that law I do 
not believe, either in relation to gravitation, or electricity, or magnetism, 
or any other supposed form of power."* But the most refined and varied 
observations (even when conducted by a Faraday) have failed to detect 
any such supposed residuum of effect, and have substantiated as one of 
the largest results of our present knowledge the received formula as 
expressing the "totality" of the force recognized as gravity. Our 
"beliefs" should always be based upon, and conform to, the observed 
order of nature. "The safest course appears to be to assume as little 
as possible." 
Faraday thus sums up his own impressions: "For my own part, many 
considerations urge ray mind toward the idea of a cause of gravity which 
is not resident in the particles of matter merely, but constantly in them 
and all space." (p. 231.) " I would much rather incline to believe that 
bodies affecting each other by gravitation act by lines of force of definite 
amount, or by an aether pervading all parts of space, than admit that the 
conservation of force could be dispensed with." (238.) Fortunately, 
the alternative presented possesses no relation of its terms. The un- 
qualified assertion of " conservation" has no bearing whatever on either 
" lines of force" or the supposed action of " an aether;" and a choice is 
therefore quite unnecessary.! 
On no subject, perhaps, have the distinguished author's ideas been 
more vague and intangible than on the favorite one of " lines of force." 
After exhibiting the familiar magnetic curves or chains of iron-filings as 
atypical phenomenon, he says : "The term line of force, as defined above, 
is restricted to mean no more than the condition of the force in a given 
place as to strength and direction; and not to include any idea of the 
nature of the physical cause of the phenomena. At the same time, if 
* Loco citat., p. 233. 
t An excellent review and criticism of Professor Faraday's Memoir on Gravitation, 
by Professor Briicke, of Vienna, was published in theL. E. D., Phil. Mag., 1858, vol. xv, 
p. 81. 32 
KINETIC THEORIES OF GRAVITATION. 
reason should arise to think that the physical condition of the force par- 
takes generally of the nature of a current or of a ray, a view which the 
author inclines to, he sees no objection in the term."* 
"In the action of gravity, for instance, the line of force is a straight 
line, as far as we can test it by the resultant phenomena. It cannot be 
deflected or even affected in its course. Neither is the action in one line 
at all influenced, either in direction or amount, by a like action in another 
line.'T This is the affirmation made by our first proposition. 
Faraday continues: "There is one question in relation to gravity, 
which, if we could ascertain or touch it, would greatly enlighten us. It 
is, whether gravitation requires time. If it did, it would show unde- 
niably, that a physical agency existed in the course of the line of force. 
It seems equally impossible to prove or disprove this point, since there 
is no capability of suspending, changing, or annihilating the power, or 
annihilating the matter in which the power resides."$ Some six years 
before the date of this latter paper, Professor Faraday, in "Thoughts on 
Ray-vibrations," had suggested more doubtingly, the same inquiry: " I 
am not aware whether there are any data by which it has been or could 
be ascertained whether such a power as gravitation acts without occu- 
pying time ."§ 
This query finds its answer in our fourth proposition. The writer was 
evidently not aware that it had been definitely settled by the astrono- 
mers, and with a delicacy of precision infinitely beyond the reach of 
any direct or instrumental research; and not being a mathematician, 
he very naturally supposed the problem insoluble. Those not trained 
in the higher operations of the science of " necessary conclusions," have 
no conception of the resources of mathematical investigation applied to 
judicious comparisons of accurate observations. And just here the 
reminder may be permitted, that did the influence of gravitation occupy 
the millionth part of a second in traversing the distance of a million 
miles, the astronomer's analysis would easily detect it. This would 
represent only one-ninth of the velocity estimated by Laplace and Arago, 
as previously stated. 
* L. E. D., Phil. Mag., 1852, vol. iii, p. 67. Dr. P. M. Roget showed in 1831, by a very 
neat geometrical demonstration, that these so-called "lines of force" in the magnetic 
field, are simply the tangential resultants of the directive action by the two magnetic 
poles exerted in straight or radial lines with a ratio of diminished intensity as the 
square of the distance from either pole, on the minute iron particles regarded as 
needles. (Journal of the Royal Institution, February, 1831, vol. i, pp. 311-318; and 
also a treatise on "Magnetism" by the same author, in vol. ii of the "Library of 
Useful Knowledge," chap, ii, sect. 3, pp. 19-21.) M. Ch. Cellerier has also, by an 
analjtical discussion of the "magnetic curves," established the same conclusion 
mathematically. (A Treatise on Electricity, by Aug. De La Rive, Londou, 2 vols., 8vo, 
1853. part iii, chap. i. Note D, vol. i, pp. 542-544.) 
t Phil. Mag., 1852, vol. iii, p. 403. 
t Ibidem, p. 403. 
§ Phil. Mag., May, 1846, vol. xxviii, p. 349. KINETIC THEORIES OF GRAVITATION. 
33 
Seguin. 1848. 
Marc Seguin, a French engineer, distinguished as having affirmed in 
1839, from a study of the locomotive-engine, the correlation between 
heat and " work," and as having estimated the " mechanical equivalent " 
of heat, labored for many years to establish the unity of the natural 
forces. It is interesting to observe that like Herapath, he commenced 
his studies in molecular physics with an inquiry into the nature of heat; 
and like him, was led to discard entirely the generally-received theory 
of a material caloric in favor of the kinetic hypothesis now universally 
adopted. 
Nearly a quarter of a century before publishing his views on molecular 
forces generally, Seguin presented to Sir John Herschel a very original 
and suggestive communication on the probable nature of heat, which 
was published by the latter in the Philosophical Journal of Edinburgh 
for 1824. The writer infers from the compressibility of all known sub- 
stances that .their constituent molecules must be at a great relative dis- 
tance from each other; and from the characteristic odor of most solids, 
that the densest and hardest substances are subject to the escape of 
their surface molecules, or in other words, "are capable of being evap- 
orated." From the infinitesimal size of these escaping molecules, they 
of course elude all known methods of comparison or mechanical appre- 
ciation. 
"In order to assign to them the condition either of a solid, a liquid, 
or a gas, it is necessary to suppose the existence and the combination of 
two forces which are sometimes o equilibria, and sometimes predominate 
the one over the other. We shall admit then the supposition that these 
two forces may be the same as those which regulate the planetary sys- 
tem, and that the molecules of bodies are subject to circulate round one 
another, so that each body, though it appears at rest, has really a cer- 
tain quantity of motion, whose measure will be a function of the mass 
and the velocity of the molecules in motion. Upon these suppositions 
it is obvious that during the impact of two bodies, all the quantity of 
motion which is not employed in giving the body which is struck a 
motion of translation, will go to augment the quantity of interior motion 
which it possesses; and if this motion takes place in circles or ellipses, 
the parts will recede from the center of attraction, and the body will 
increase in volume. In this state it will have a tendency to transmit 
the excess of motion which it possesses to bodies which are near it, or 
to parts which it will emit in greater number in following the same law. 
If the quantity of motion is so great that the attraction of the molecules 
can no longer be in equilibria with their angular velocities, the body 
will remain in the gaseous state till it has transmitted to other bodies, 
the excess of velocity which it possesses."* 
This is a very neat and perspicuous presentation of the dynamical 
* The Edinburgh Philosophical Journal, October, 1824, vol. x, pp. 280-282. 
3 34 
KINETIC THEORIES OF GRAVITATION. 
theory of temperature, expansion, conduction, evaporation, and the 
transformation and conservation of energy ; and although three years 
later than Herapath's remarkable announcement of the theory of gaseous 
temperature, is doubtless an independent and original discovery ; for 
such it is entitled to be called. There is now little question that while 
the molecular excursions in gases take place in straight lines or in hy- 
perbolic trajectories, the atomic motions within the molecule (whose 
marvelous regularity of periodicity is attested by the fixed refrangibili- 
ties of the spectrum) are really described in elliptic orbits, as Seguin 
had so early preconceived. 
The writer proceeds to apply this hypothesis to a variety of appar- 
ently unconnected phenomena, as to the sudden development of motion 
in the fracture of a " Prince Rupert's Drop" or unannealed glass tear; 
to the action of the steam-engine, in which a large amount of molecular 
orbital motion in the vaporized water is transformed into.the rectilinear 
or translatory motion of the piston; for " if, as we suppose, an angular 
motion has been changed into a rectilineal motion or into a motion of trans 
lation, we should find after the effect only the quantity of motion which 
has not been employed in producing the useful effect." He shows that 
the same theory explains satisfactorily the great degree of refrigeration- 
observed in the higher regions of our atmosphere, while by the material 
theory of caloric the upper regions should be the hottest; and he main- 
tains that even " the motion produced by organized bodies may be ex- 
plained in the same manner as the steam-engine." This is certainly a 
very remarkable prevision of the correlation between the physical and 
the organic forces. 
It was not till 1848 that Seguin commenced a series of memoirs, read 
before the French Academy of Sciences, on the nature of the molecu- 
lar forces, but dealing mainly with cohesion regarded as a phase of 
gravitative action. A theory of mutual impacts and reactions between 
the molecules of matter and the atoms of the aether was proposed but 
not very clearly presented.* With a communication, made October 22, 
1849, the author submitted the results of experiments showing actions 
"very analogous, if not identical in their effects, with that of gravita- 
tion." The apparatus exhibited consisted essentially of a magnet at- 
tached to a pendulum which produced motion in small iron bullets sus- 
pended a short distance therefrom. 
In an editorial resume of Seguin's work on " Molecular Physics," in 
Abbe Moigno's Cosmos, in 1852, the Abbe, after alluding to Newton's 
speculations, affirms with characteristic confidence and earnestness: 
" If there is anything certain in the world, it is that the molecules of 
bodies and bodies themselves are not really self-attractive; it is that 
attraction is not an intrinsic but only a developed force; it is that not- 
* Comptes Itendus, September 25, 1848, vol. xxvii, pp. 314-318; January 22, 1849, vol. 
xxviii, pp. 97-101 ; October 22, 1849, vol. xxix, pp. 425-430; January 19, 1852, vol. 
xxxiv, pp. 85-89; November 7, 1853, vol. xxxvii, pp. 703-708. 35 
KINETIC THEORIES OF GRAVITATION. 
withstanding everything occurs as though bodies mutually attracted 
each other, it is incontestably true that bodies do not so attract. New- 
ton, as Euler,-as every philosopher worthy of the name,-has seen in 
nature but two things, inertia, and motion originally impressed by a 
free Will, the first and infinite Mover. And it is with these two great 
facts of inertia and movement that advancing science shall ultimately 
explain all the phenomena of the physical world. Already courageous 
thinkers have endeavored to explain by inertia and motion the great, 
the capital fact of universal attraction, but these explanations are neither 
so distinctly formulated nor so plausible as to enable us to give a correct 
idea of them." Abbe Moigno, as Seguin's interpreter, proceeds: " The 
secret of cohesion has been pursued by one of our most illustrious com- 
patriots, M. Seguin, senior, for the last twenty years, and he has cer- 
tainly discovered it. It consists most essentially, as we shall proceed 
to show, in the incontestable fact that the molecules of bodies exceed 
in number and minuteness anything that could have been imagined."* 
This theory of cohesion is then set forth at some length, the funda- 
mental assumption being that there are two classes of dynamic monads 
occupying the universe, the one in a state of relative repose, exhibiting 
the various phenomena of attraction, and commonly called the ponder- 
able elements, and the other class entirely free or independent, (improp- 
erly called imponderable elements,) actuated with extreme velocities of 
translation, of rotation, and of vibration, continually traversing the 
systems of ponderable monads in all directions.! 
Although the admiring editor avows himself a pupil of Seguin, it 
is doubtful whether he has cautiously followed him, in so enthusiasti- 
cally proclaiming his development of " a vast theory from the admission 
of but a single principle in the universe,-the attraction of two monads 
in the inverse ratio of the distance squared, without recourse to any 
hypothetical force of mysterious attractions or of impossible repulsions." 
In 1858, Seguin published in the Cosmos a somewhat elaborate essay 
"On the Origin and Propagation of Force," in which he seems to have 
abandoned a kinetic theory of gravitation. It is true that he there 
holds: "Matter is inert; that is to say, it does not harbor in itself the 
power to put itself into movement, and still less a fortiori to communi- 
cate it, since a thing to be transmitted must first exist."| And it is 
also true that he repeatedly speaks of " the great principle of the inde- 
finite conservation of motion " as being " the foundation of all me- 
and regards "the possibility of the destruction of motion 
as equivalent to " the annihilation of force,"|| which is the very shibboleth 
of kinetic theorists; and further that he disputes Poisson's proposition 
that two spheres of equal mass and velocity, devoid of elasticity, if 
* Cosmos, November 14, 1852, vol. i, pp. 693, 694. 
t Cosmos, vol. ii, pp. 371-382, and pp. 625-632. 
t Cosmos, October 15, 1858, vol. xiii, p. 485. 
§ Ibidem, pp. 503, 505, 515, 518, 527. 
|| Ibidem, p. 509. 36 
KINETIC THEORIES OF GRAVITATION. 
directly meeting, would have their motion destroyed, and be reduced 
to rest; maintaining that " the idea of the possibility of the destruction 
of force and of the complete disappearance of motion has always been 
insuperably repugnant to sound and careful thinkers, who have made 
this question a subject of study."* 
Notwithstanding all which, he says, in regard to the uniform tendency 
of a material system to its center of gravity, " we are thus led to con- 
sider attraction as a first cause, emanating directly from the Divine Will 
in the creation of matter. Doubtless it is not impossible that it may 
hereafter be discovered that attraction in its turn is only a consequence 
of a more general law, comprehending in itself more implicitly the 
means of explaining the effects attributed to attraction. . . . But 
as these considerations are purely metaphysical, since observation cannot 
reach beyond the established fact that two confronting bodies gravitate 
toward each other by virtue of a force to which is given the name of 
attraction, it appears to me wiser not to advance further to penetrate a 
mystery which nothing within our knowledge as yet appears able to 
explain. Let us then consider matter as existing from the beginning 
uniformly in space, and attraction as an essential property with which 
it is endowed, by virtue of which the different parts or molecules com- 
posing it possess in themselves the power of mutual attraction."! 
So explicit a statement would seem quite sufficient to prove that Abbe 
Moigno has in his zeal transcended the doctrines of the one whom he had 
effusively recognized as his teacher; and that whatever may have been 
the earlier views of Seguin as to the origin of gravitation, be can no 
longer be numbered with those who conceive it to be " a mode of mo- 
tion." 
Boucheporn. 1849. 
M. F. de Boucheporn read a memoir to the French Academy of Sci- 
ences, July 30, 1849, entitled " Researches on physical laws considered 
as consequences of the only essential properties of matter, impenetra- 
bility and inertia; " the object proposed being to show the considera- 
tions leading to the conclusion that all physical law rests simply on 
these necessary attributes of matter, " without the supposition of any 
force.'1'1 
He commences with the general recognition, "It is an idea quite old 
in science that the movements of the heavenly bodies may be explained 
by an external impulsion or by the action of a universal fluid. This 
was the earliest idea of French philosophy, being that of Descartes, 
and even Newton himself had thought of connecting with it the great 
law of gravitation." 
Boucheporn proceeds to cite some of the principles and results devel- 
oped. " 1st. The intensity of an impulse propagated in the aetherial 
* Cosmos. October 15, 1858, vol. xiii, p. 508. 
i Ibidem, pp. 486, 487. KINETIC THEORIES OF GRAVITATION. 
37 
medium follows the law of the inverse square of the distance from the 
center of disturbance. 2d. The resistance of the aether does not sensr 
bly affect the velocity of a body when this is sufficiently less than that 
of aetherial propagation ; but this resistance becomes a uniform pressure 
on the entire surface of a body, (supposed spherical,) and even deter- 
mines its sphericity. 3d. Taking as unity the density of the fluid, the 
quantity of motion impressed by a body on the aether is equal to its 
volume multiplied by the square of its velocity ; which is also the meas- 
ure of the total pressure on the surface of a body. 4th. Propagated to 
the interior of the heavenly bodies, the pressure would produce the 
effect that all layers of equal thickness will inclose the same quantity of 
matter, and that the mean density is three times that of the surface. 
This kind of homogeneity would not be affected by the action of heat. 
In short, from such great internal condensation, it may be conjectured 
that the heavenly bodies are almost entirely impermeable by the .ether, 
as will shortly appear from an astronomical law. 5th. As to attraction; 
the displacement of the tether by the movement of a body A, will pro- 
duce in all parts of the fluid a sort of aspiration toward the point being 
left by its center; any other body B receiving these aspiring waves on 
its nearer hemisphere will have lost all or a part of its own pressure; 
and the half pressure (volume multiplied by the squared velocity) which 
acts on the opposite hemisphere, no longer being counterbalanced, will 
give an impulse to the body B in the direction of A. Such would be 
the principle of attraction. ..." 
The writer finds a verification of his principles in the relation existing 
between the respective masses of the planets and the product of their 
volumes by the square of their velocities, omitting the cases of Uranus 
and Neptune. Also by determining the velocity of an attracting body 
from that of its satellite, knowing only the ratio of the radius to the dis- 
tance; and lastly, by determining the amount of fall of heavy bodies 
from the angular velocity of the earth, irrespective of its mass!* 
Lame. 1852. 
Gabriel Lame, a distinguished French geometer, and author of a very 
learned and valuable work on the laws of elasticity, embracing a pro- 
found mathematical discussion of the theory of vibrations in almost all 
its scientific aspects, has incidentally alluded tp gravitation in such a 
manner as to deserve a notice here. 
Of his more immediate theme he forcibly remarks: "Elasticity is thereat 
origin or indispensable intermediary of all the more important physical 
phenomena of the universe. . . . In a word, the function of elasticity 
in nature is at least as important as that of universal gravitation. 
Indeed gravitation and elasticity should be considered as effects of the 
same cause, which correlate or connect all the material parts of the uni. 
* Comptes llendus, July 30, 1849, vol. xxix, pp. 108-112. The author embodied his 
views in a work entitled Principe Generate de la Philosophic Naturelie, 8vo, Paris, 1853. 38 
KINETIC THEORIES OF GRAVITATION. 
verse; the first asserting this relation through immense distances, the 
second exhibiting it only in very small spaces."* In what way these 
two great master-forces of nature, seemingly so unlike, and even antag- 
onistic to each other, may possibly be connected in action or in principle, 
is nowhere suggested ; but the character of the author forbids the sup- 
position that the remark was hastily ventured, or conceived without 
sober reflection. 
No further reference however, to the subject of gravitation occurs in the 
work, till toward its close. In the last " Lesson," Lame shows the neces- 
sity for admitting a pervading aether. And considering the question 
whether ponderable matter is really the medium which vibrates and 
transmits light in transparent crystals, he decides: " There can no 
longer exist a doubt on this question ; for it clearly results from our 
analysis that ponderable matter alone is incapable of producing pro- 
gressive waves which will explain the optical phenomena of birefractive 
bodies, or which could have led to the discovery of most of these phe- 
nomena. Luminous waves then are produced and propagated in trans- 
parent bodies by the vibrations of an imponderable fluid, which is no 
other than the tether." He determines analy tically two systems of undu- 
lation in the tether, of differing velocities ; one system radial, or normal 
to the ellipsoidal surface of the wave, affecting the dilatation or conden- 
sation of the medium, and not concerned in optical phenomena; and the 
other system transverse to this in two sets, or in the direction of two 
tangents to the ellipsoidal wave, representing the phases of polarized 
light.t 
Lame concludes his Lessons with some reflections on the internal con- 
stitution of solid bodies. " It seems highly probable that the progress 
of general physics will conduct one day to a principle analogous to that 
of universal attraction, of which this itself shall prove only a corollary, 
and which may serve as the basis of a rational theory comprehending 
both mechanics-the celestial and the terrestrial. But to presuppose 
this unknown principle, or to infer the whole from one of its parts, is to 
retard-it may be for a long time-the epoch of its discovery." And 
speaking of the great desideratum, a rational science of molecular 
mechanics, he asks: "Is this an enigma forever insoluble? To this 
question must be answered yes, if the existence of ponderable matter 
only is to be admitted;-no, if we admit also the existence of the 
mther."t 
"Since then the existence of the mtherial fluid is incontestably demon- 
strated by the propagation of light through celestial spaces,-by the 
explanation (as simple as complete) of t'he condition of diffraction in the 
theory of waves,-and as has been seen,-by the laws of double refrac- 
tion, which prove with no less certitude the existence of an mtber within 
* Lemons sur la Theorie Mathematique de VJLlasticite des Corps Solldes. 8vo, Paris, 1852. 
Lesson i, p. 2. 
t Loco citat., Less, xxiv, sec. 131, pp. 327, 328. 
t Loco citat., sec. 134, pp. 332, 333. KINETIC THEORIES OF GRAVITATION. 
39 
transparent bodies, we know that ponderable matter is not alone in 
the universe; its particles swim as it were in a kind of fluid medium. 
If this fluid be not the unique cause of all the observed facts, it must at 
least modify them, diffuse their action, and complicate their laws. It is 
then no longer possible to attain a rational and complete explanation of 
the phenomena of physical nature, without recognizing the intervention 
of this agent, whose presence is so inevitable. It is scarcely to be 
doubted that in this intervention, sagaciously investigated, will be found 
the secret or the true cause of the effects which are attributed to heat, 
to electricity, to magnetism, to universal attraction, to cohesion, to 
chemical affinities ; for all these mysterious aud incomprehensible agen- 
cies are at bottom but co-ordinating hypotheses,-useful without doubt 
to our existing ignorance, but which the progress of true science will 
complete by dethroning."* 
These passages are less notable for any precise hypothesis as to the 
cause of gravitation than for their earnest unformulated faith in the 
mechanical agencies of the aether as the fountain head of all force. 
A very striking illustration of the author's realizing sense of the 
aetherial presence occurs in a memoir communicated by him to the 
Academy of Sciences about ten years before this time, or in 1842; in 
which, discussing the difference between the determiuation by Gay- 
Lussac of the co-efficient of gaseous dilatation, and that made by Rud- 
berg and verified by Regnault twenty-five years later, Lame made the 
somewhat startling announcement that the observed difference indicated 
an increasing mther-pressure on terrestrial matter 1 " The difference 
between these results is explained by admitting that the pressure of the 
aether has undergone on the earth in a quarter of a century an aug- 
mentation equal to a pressure of eight or nine tenths of a millimetre of 
the mercurial column."! 
Waterston. 1858. 
In an essay "On the integral of gravitation, and its consequents with 
reference to the measure and transfer, or communication of force, by J. 
J. Waterston," of Edinburgh, published in the Philosophical Magazine, 
the writer commences with the general consideration : "Modern ideas 
with relation to heat and the active condition of the molecular element 
naturally incline us to estimate every force with regard to its work-pro- 
ducing capacity. In the following paper I have considered gravitation 
under this aspect, and in doing so, have been led to discuss some points 
relating to dynamical sequence in the abstract." 
After referring to the fact that neither Newton nor Laplace recog- 
nized the principle of the conservation of force in their grand researches, 
Waterston continues: "Even at the present day, mathematicians have 
been so long accustomed to and brought up in the statical method of 
* Loco dial., sec. 134, pp. 334, 335. 
+ Comptes Rendus, January 3, 1842, vol. xiv, p. 37. 40 
KINETIC THEORIES OF GRAVITATION. 
treating molecular physics initiated! by those great leaders, that the 
inefficiency and inconsistency of this mode of inquiry with the mechani- 
cal theory of heat seems as yet not to be fully appreciated by some even 
of the most zealous upholders of that theory. With this impression, it 
seems desirable that every effort should be made to arrive at a clear 
understanding of fundamental points, and the principle of physical 
causation, which the mechanical theory supplies."* 
As this is the usual point of departure with kinematists, {not " brought 
up in the statical method,") it may be well here to affirm with some 
positiveness that without the ceaseless co-operation of two antagoniz- 
ing or reciprocating statical tensions, a " mechanical theory of heat" is 
rationally impossible. Matter possessing only inertia and motion (whose 
product is momentum) would very speedily arrive at a state of stable 
and inert equilibrium, without having ever exhibited a single phenome- 
non of force, and without the possibility of any dynamic potential. 
An abstract system of kinematics, under any geometrical arrangement 
or conception, is indeed quite irreconcilable with the actual system of 
dynamics found in nature and subject to our observation. All gases 
would under the operation of the first law of motion, tend to infinite 
and equable diffusion ; and liquids and solids would quickly follow in 
their wake. Heat, whether considered as a vibration or a revolution, 
(or preferably as both a rectilinear and an orbital movement,) could of 
course have no existence, since there could be neither recoil nor con- 
straining bond; and the very first step toward an oscillation would 
also be the last one. Even the principle itself of "conservation of 
force" is absolutely dependent on the existence of primordial static po- 
tentiality. So much for a " clear understanding of fundamental points." 
" Section 1. The integral of gravitation is a function of space. Sup- 
pose a central homogeneous globe to augment in bulk by the descent 
of similar matter from an infinite distance in radial directions all around; 
each descending element, on arriving at the surface of the globe, pre- 
sents itself charged with a certain amount of mechanical force equiva- 
lent to the square velocity with which it impinges. If we confine our 
attention to the centripetal influence of the original central globe only, 
the square velocity of the descending element diminishes in the inverse 
ratio of the radius of th e augmenting globular mass ; for it is upon the 
surface of this that impact takes place, and the matter that has been 
added to the original globe is assumed not to augment the centripetal 
force acting upon the descending matter"! This established ratio of 
final velocity does not appear in any way to substantiate the general 
proposition ; nor is it seen to receive support from any of the succeed- 
ing illustrations. Gravitation,as a fact of observation, is always found 
to be rigorously relative to two posited elements ; and its integral is by 
such observation a function of the distance between these elements. If 
* L. E. D. Phil. Mag., May, 1858, vol. xv, p. 329. 
t Loco citat., p. 330. KINETIC THEORIES OF GRAVITATION. 
41 
the distance is infinite, the integral is negatively infinite. But how it 
is a function of " space" in any more general sense, is certainly not ob- 
vious. As Professor Briicke, of Vienna, has forcibly said in his response 
to Faraday : " So far as my consciousness reaches, so far as I am capa- 
ble of distinguishing true from false, and like from unlike, all known 
facts are brought into complete harmony with our laws of thought, 
when we suppose forces as the causes of phenomena to reside in the 
masses, the spaces between these masses being traversed by the forces. 
If the forces could be imagined as existing in space, it must also be 
conceivable that matter may be annihilated without changing the sum 
of the forces, and this, at least by me, is not conceivable."* 
" Section 2. The force-generating faculty exists in space, and is di- 
rected centripetally. [?] This is proved by the following considerations. 
The integral force-producing power of any body, however small, subject 
to the law of universal gravitation, is illimitable as space. It is impos- 
sible to imagine an infinite attribute belonging to a finite entity.! It is 
therefore in space that the energy that contributes the power of gravi- 
tation exists', and the element of matter merely gives to it a centripetal 
direction. This, as a consequent of the law of gravitation, seems note- 
worthy from it probably being applicable to molecular forces generally. 
It favors the idea that the function of the material element is to give 
direction to a living force that pervades space."! The first part of this 
proposition, (as an iteration of the previous one,) that gravitative force 
"exists in space," is derived as an inference from a metaphysical postu- 
late,-" It is impossible to imagine" it as belonging to a finite body. 
But our powers of " imagining" can hardly be accepted as the measure 
of scientific verity. "It is impossible to imagine" the nature of elec- 
trical action, chemical affinity, luminiferous vibration, aether, atom, 
force, or space ! Who is able to formulate in thought the co-existence 
of an equal repulsive and attractive energy in either pole of a bar 
magnet, simultaneously discriminating by opposite action between the 
reversed ends of two magnetic needles? But when it is said that the 
sole function of the material element is to give centripetal direction to 
the circumambient ocean of force, wonderful indeed is the conception of 
virtue in the " finite entity" thus drawing to itself the centripetal ten- 
dency in all directions throughout illimitable space, and instantaneously 
re adjusting these infinite lines of force with every momentary change 
of position I If difficulty of "imagining" were a criterion of error, 
surely it might be well applied to this hypothesis. 
" Section 3. The law of gravitation with respect to the element of 
radial space, is usually defined with reference to a constant element of 
time; the increment of velocity generated being proportional to the 
increment of time, whatever the direction or velocity of the motion, and 
* L. E. D. Phil. Mag., February, 1858, vol. xv, pp. 87, 88. 
t Is it impossible to imagine an atom having an eternal duration ? 
t Phil Mag., 1858, vol. xv, pp. 331,332. 42 
KINETIC THEORIES OF GRAVITATION. 
inversely as the square of the central distance. If we view it with ref- 
erence to a constant element of radial space, we find that the increment 
of square velocity generated by the force of gravitation is proportional 
th decrement of radial distance, and inversely as square of central dis- 
tance. This holds whatever the velocity or direction of the motion, 
whatever the orbit of the projectile . . . ." 
''Section 4. The mutual gravitation of two bodies develops mechan- 
ical force in each of them inversely proportional to its mass. Suppose 
two bodies to descend toward each other by their mutual gravitation, 
their common center of gravity being at rest. At any time before they 
meet, their acquired velocities being examined will be found inversely 
as their masses; which assume as one to ten. Suppose them removed 
to the earth's surface, and each projected up a vertical with their ac- 
quired velocities respectively: the smaller body rises 100 times the 
height ascended by the larger, and thus in again descending would be 
able to perform ten times t he work . . . ."* 
After some other propositions and discussions, not specially bearing 
on the present inquiry, Mr. Waterston gives a general statement of 
results as follows: " Even if we had not those proofs of the existence 
of a universal medium that the undulatory theory of light supplies, it 
would be well to admit the existence of media to which ordinary matter 
may be assumed to be entirely subordinate in all their potential rela- 
tions, so as to give order to our ideas in tracing out the dynamical 
sequence of nature. It would be taking too narrow a view if we limited 
the function of the luminiferous aether to the conveying of physical 
pulses only. The atmosphere also conveys physical pulses, but that is 
the least important of its functions in the economy of nature. There is 
nothing that should hinder us attributing to the media concerned in 
the radiation of heat and light the higher functions of electric polarity 
and gravitation. The special dynamic arrangements by which this is 
effected may ever elude research, but as there is no limit to the vis viva 
which such media may conserve in their minutest parts, so there is no 
physical impossibility in that vis viva being suddenly transferred to the 
molecules of ordinary matter in the proportions and sequence required 
to carry out the order and system of nature. The fundamental princi- 
ple of action in such media must be in accordance with elastic impact, 
for upon that the dynamic theory of heat and conservation of force rests 
as a foundation. The statical and dynamical characteristics of gravita- 
tion and transfusion of force have also been shown to conform to it, so 
that all the forces that hold the molecules of bodies together must also 
be in subjection to it." t 
From the above it would appear that the writer in previously affirming 
the integral of gravitation to be a function of space, intended rather the 
content of space, or the dynamic medium supposed to occupy it. The 
* Loco citat., p. 332. 
t Loco citat., p. 344. KINETIC THEORIES OF GRAVITATION. 
43 
whole suggestion is however, so indefinite that it must be accounted less 
a coherent hypothesis than a mere speculation,-a cast among the possi- 
bilities. To refer the great fact of gravitation to some unimagined and 
unimaginable aether-motion, the special arrangements of which for effect- 
ing the desired purpose " elude research," is not to proffer an explana- 
tion, but to indulge in an illusion; and although Mr. Waterston has in 
terms recognized all of the six propositions (excepting the last one) an- 
nounced as the necessary conditions of the problem,* he has failed to 
slow that one of these conditions can be satisfied by his speculations. 
Challis. 1859. 
Professor James Challis, of the University of Cambridge, England, 
in the prosecution of a " Mathematical Theory of Heat," published in 
the Philosophical Magazine for March, 1859, advanced in November of 
the same year, to a " Mathematical Theory of Attractive Forces," based 
on the assumption " that all substances consist of minute spherical atoms 
of different but constant magnitudes, and of the same intrinsic inertia; 
and that the dynamical relationsand movements of different substances, 
and of their constituent atoms, are determined by the pressures of the 
aether against the surfaces of the atoms, together with the reaction of 
the atoms against such pressure by reason of the constancy of their 
form and magnitudes. The aether is assumed to be a uniform elastic 
fluid medium pervading all space not occupied by atoms, and varying 
in pressure proportionally to variations of its density. The theory recog- 
nizes no other kinds of force than these two, the one an active force resi- 
dent in the aether, and the other a passive reaction of the atoms." 
After a formidable array of partial differential equations, the author 
concludes: " Having now shown that waves of large breadth attract a small 
spherical body toward their origin, and having previously shown that 
waves of small breadth may repel such a body in the contrary direction, 
the main difficulty in forming a theory of attractive and repul si ve forces 
seems to be overcome."! It is supposed by Professor Challis that by 
the disturbance of a material element, a series of undulations differing 
greatly in their order of magnitude and velocity maybe simultaneously 
propagated in the mtherial medium, giving rise to as many different 
manifestations of force; and that according to their relative wave- 
length, some of these will produce a permanent motion of translation 
on molecules of determinate mass subjected to their influence. This 
is partly in accord with the striking experiments of Guyot previously 
referred to. 
In a following paper the author undertook ''A Theory of the Force 
of Gravity;" remarking that, "As we have no conception from personal 
experience and sensation of any other species of force than pressure, the 
actio in distans does not admit of being explained by any previous or 
* Loco citat., p. 335. 
+ L. E. D. Phil. Mag., November, 1859, vol. xviii, p. 334. 44 
KINETIC THEORIES OF GRAVITATION. 
concomitant knowledge, but if it be a reality, must forever remain to 
us incomprehensible." How the school-boy's personal experience of the 
strain exerted in drawing by a cord his winter's sled can be resolved 
into a sensation of " pressure " does not seem easy of discovery. 
Assuming then an order of setherial waves having a much larger 
range than those of light, Professor Challis endeavors to deduce the 
several laws, of action proportional to the number of atoms acted upon, 
of the inverse square of the distance of action, and of simultaneous 
action in different directions. He infers that if such waves encountered 
a slight retardation of propagation in passing through the earth, they 
would be refracted, so to speak, by the form of the large inequalities of 
the terrestrial surface, producing the observed deflection of the plumb- 
line. 
' He also supposes that a small function involving r4 must be added 
to the usual formula increasing sensibly the action of the sun near 
its surface and diminishing its action notably through interstellar dis- 
tances. He thinks that this alone will explain why the sidereal system 
does not collapse toward its common center of gravity. "According 
to the theory of gravity I have proposed, although the ordinary law 
may be exact through the solar system and far beyond, there must be 
distances at which the condition that the excursions of the vibrating 
particles of the aether are large compared to the dimensions of the 
atoms ceases to be fulfilled. In that case the attraction changes to 
repulsion."* 
Renewing the discussion of "A Theory of Molecular Forces," the fol- 
lowing year, Professor Challis contends: " It is a matter of demonstra- 
tion that a theory of molecular forces cannot be constructed on the 
hypothesis that the forces vary according to some law of the distance 
from individual material particles, unless the law be such that the force 
changes sign with the distance so as to become attractive after being 
repulsive. But if force be a virtue resident in the particle, it must at its 
origin be either attractive or repulsive, and it seems impossible to con- 
ceive how by emanation to a distance it can change its quality. This 
difficulty, as will be shown, is not encountered in a theory of molecular 
forces which deduces their laws from the dynamical action of an elastic 
medium." While it is probably no more difficult to conceive an innate 
force or virtue which " at its origin " shall have a law of radial intensity 
whose value passes through zero, than to conceive any other mathemat- 
ical law of increment or decrement, there is certainly no necessity for 
assuming such a law. If we should suppose the attraction by inverse 
squares to be absolute, with the superposition of a repulsion of much 
higher inverse power, and of far greater intensity, it is evident that the 
two curves of force would cross each other, and that at the intersection 
the resultant would involve a " change of sign." However difficult it 
may be to realize such a conception, the actual superposition of oppos- 
* Phil. Mag., 1859, vol. xviii, p. 451. KINETIC THEORIES OF GRAVITATION. 
45 
ing forces is daily exhibited to us in the behavior of the magnet. 
Another possible conception is that repulsion is a positive material or 
cetherial atmosphere of definite radius. 
Indeed, the author's theory is really one of the superposition of two 
systems of waves, rather than one of a single system changing its sign. 
For be supposes that the attraction of gravity results from setlierial 
waves of great length and correspondingly large excursions or ampli- 
tudes, in which the diameter of the material atom is a vanishing quan- 
tity ("/2=0"), and there is no sensible difference between the velocities 
on its two hemispheres; while atomic repulsion results from such 
small waves (smaller even than those of light) that the atom is large in 
comparison, and the difference of the wave on its two hemispheres is 
very notable. " Thus the conditions assumed in the mathematical the- 
ory of heat are satisfied by supposing p to be very large and q [the ex- 
cursion of the wave] to be very small; and the fulfillment of these con- 
ditions accounts for the great energy of calorific repulsion. . . Hence 
atoms of very small size, acting upon each other by the intervention of 
waves of which the excursions are very small, mutually repel with a 
very great force ; and at the same time, as was shown in the Theory 
of Heat, the force varies very rapidly with the distance." 
Attributing to the spherical hard atom of matter only inertia, " it 
would be contrary to these principles to ascribe to an atom the property 
of elasticity, because from what we know of this property by experience 
it is quantitative, and being most probably dependent on an aggrega- 
tion of atoms, may admit of explanation by a complete theory of mole- 
cular forces." * Of this fundamental property however,-necessarily 
precedent to all theory of wave-action,-no explanation is given. 
That the author did not feel entirely satisfied with his vibratory theory 
of molecular forces, would appear from his returning to the subject two 
years later with the remark : " Such vibrations, when we calculate their 
effect only to the first power of the velocity, are found to produce simply 
oscillations of small spherical bodies submitted to their action, and not 
motion of translation. To account for the latter, it is necessary to pro- 
ceed to the consideration of effects due to the second power of the ve- 
locityLastly, there is yet another physical force, the 
relations of which to an setherial medium and to other modes of force 
are not readily made out: I mean the force of gravity. If however, all 
the other forces are modifications of aetherial pressure, it is reasonable to 
suppose that this one is of the same kind. I have ventured to reason 
on this supposition, and have attempted to deduce (I think with success) 
the known laws of gravity from the dynamical action of setherial waves 
of much larger magnitude than those which correspond to molecular 
forces." t 
While it is comparatively easy to explain the origin of heat-waves 
* L. E. D. Phil. Mag., February, I860, vol. xix, pp. 89-91. 
tL. E. D. Phil. Mag., April, 1862, vol. xxiii. pp. 319, 320. 46 
KINETIC THEORIES OF GRAVITATION. 
by the theory of the reactions of antecedent forces, the converse prob- 
lem, to explain forces by the hypothesis of antecedent waves, is by no 
means so simple. Accordingly, a very weak point in all undulatory 
theories of gravitation has been an account of the origin (to say nothing 
of the continuance) of the primum mobile. " On the source and main- 
tenance of the sun's heat," Professor Challis remarks as follows: " We 
are led to the conclusion that the undulations which emanate equally 
in all directions from each atom, and constitute by their dynamic action 
the repulsion of heat, are mainly produced by the reaction of the atoms 
due to their inertia and impenetrability. It is plain however, from 
this reasoning, that there must be an original and independent source 
of undulations. Now obviously such a supply may be conceived to be 
furnished to the sun by undulations emanating from the stars. We have 
ocular evidence that stars transmit light-undulations, and it is quite 
possible that they originate others not sensible to the sight."* As our 
sun is one of the stellar fraternity, surely this device of mutual borrow- 
ing is an extraordinary method of production. 
" That action at a distance is not a universal condition of force is 
proved by the modern discovery that light and heat, which are modes 
of force, are transmitted through space by the intervention of a me- 
dium. If one kind of force requires a medium of transmission, why not 
another"? Again, it is found by experience that the same portion of 
matter may attract or repel, according to circumstances. But inherent 
force cannot possibly be so changed by circumstances. In the same 
matter it must continue to be always the same."t 
" All physical force being pressure, there must be a medium by which 
the pressure is executed." And the origin of this pressure is therefore, 
the origin of " all physical force." The fundamental postulate of this 
medium is that it is a uniformly continuous elastic fluid, exerting a 
pressure always proportional to its density. As it is contrary to prin- 
ciple to ascribe elasticity to atomic matter, the question might arise, 
why is it more proper to ascribe this occult property to the cether ? In 
the case of air (of uniform temperature) the constant ratio of pressure 
to density results we are told, from a vis a tergo, the vibratory action 
of the interstitial cether. The author perceives the incongruity of deny- 
ing to the air a quality attributed to the tether, when the law is implic- 
itly the same in both; and he suggests in extenuation, " we can con- 
ceive of the existence of another order of tether having the same relation 
to the first as that has to air, and so on ad libitum." \ 
The very key-note of the hypothesis is dynamic tetherial " pressure?' 
"All the different kinds of physical force detected by observation and 
experiment are modifications of pressures of the sether."§ But when we 
* Phil. Mag., June, 1863, vol. xxv, p. 465. 
tPhil. Mag., October, 1863, vol. xxvi, p. 284. 
t Phil. Mag., June, 1866, vol. xxxi, pp. 468, 469. 
§ Ibidem, p. 470. KINETIC THEORIES OF GRAVITATION. 
47 
seek the cause of this pressure, it forever eludes us. Here then the 
system stands self-convicted of impotence to exercise its prime preroga- 
tive. At whatever point in the infinite series of descending orders of 
tether we stop, the secret of its power is ever one step backward. We 
must still " conceive of the existence of another order of aether having 
the same relation to it" that it had to the preceding. And that no pos- 
sible element of embarrassment may be wanting to our conception, the 
first aether is absolutely "continuous," without atoms and without in- 
terstices 1 
In 1869, Professor Challis published an elaborate extension of his 
mathematical discussion of kinetic theories of the physical forces, in a 
large octavo volume of some 750 pages; the first half of the work being 
devoted to a general mathematical treatise, of high merit and value, 
under the title of " Notes on the Principles of Pure and Applied Calcu- 
lation." In the latter portion of the work, (on theoretical physics,) the 
subjects treated of " are those of light, heat, and molecular attraction, 
force of gravity, electricity, galvanism, and magnetism, respecting which 
I make the general hypothesis that their phenomena all result from 
modes of action of an elastic fluid, the pressure of which is proportional 
to its density."* And in the "introduction " to the work, he has more 
explicitly stated: "The hypothesis respecting the tether is simply that 
it is a continuous elastic medium, perfectly fluid, and that it presses pro- 
portionally to its density." t The forces of elasticity, and of chemical 
affinity, are excluded as beyond the present reach of analysis. 
A distinction is made between the retherial radiations of light and of 
heat, not justified by any observed phenomenon. " Since in the theory 
I have proposed, the transverse vibrations of rays always accompany 
direct vibrations, and it was concluded that the sensation of light is en- 
tirely due to the former, we are at liberty to refer the action of heat, or 
other modes of force, to the direct vibrations." | This would leave the 
polarization of heat quite inexplicable; as obviously vibrations of the 
acoustic type are incapable of polarization. 
It is now familiar to opticians that fine rulings on glass, whose dis- 
tance apart is less than a half of the wave-length of light, are readily 
resolvable with optical distinctness by our modern microscopes, while 
the intimate texture of the glass is apparently as far removed from res- 
olution as with the unarmed eye. What part can mtherial vibrations 
play in giving cohesion to the ultimate molecules of the glass? Here 
then is apparently a new difficulty for the undulatory theory of force; 
for not only are the ultimate molecules of the silicate bound together 
with a powerful force, (giving seeming continuity of substance to our 
highest artificial vision,) but they are also held apart with a still more 
potent stress. Professor Challis does not shrink from the solution. 
* Principles of Mathematics and Physics, 8vo., Cambridge, 1069. p. 318. 
+ Introduction, p. xlv 
t Opere citat., p. 437. 48 
KINETIC THEORIES OF GRAVITATION. 
" Making the only hypothesis which is consistent with the theoretical 
principles advocated in this work, namely, that the ultimate atoms of 
the glass are kept asunder by the repulsive action of aetherial undula- 
tions which have their origin at individual atoms, it may be presumed 
that this atomic repulsion is attributable to undulations incomparably 
smaller than those which cause the sensation of lightThe only 
additional hypothesis that will now be made is that there are undula- 
tions of the aether for which the values of A are very much inferior in 
magnitude to those of the undulations which produce the phenomena of 
light. The origin of this class of undulations may, as well as that of 
all others, be ascribed to disturbances of the aether by the vibrations 
and motions of atoms. Although the periods of the aetherial vibrations 
may, under particular circumstances, be determined by the periods of 
the vibrations of the atoms, this is not necessarily the case. . . . "* 
"However small may be the condensation propagated from a single 
atom, the resulting condensation from an aggregation of atoms con- 
tained in a spherical space will be of sensible magnitude at distances 
from the center of the space very large compared to its radius, provided 
the space be not less than a certain finite magnitude, and the atoms 
contained in it be not fewer than a certain finite numberWe 
have hitherto had under consideration the waves of atomic repulsion 
and the waves of molecular attraction, and it was argued that the lat- 
ter might result from compositions of the former, and that in that case 
the values of A would be much larger for the composite waves than for 
the components."! 
In 1872, the author again writing " On the Hydrodynamical Theory 
of Attractive and Repulsive Forces," says in regard to the discussion of 
the first and second orders of small quantities, " Having in fact suc- 
ceeded in overcoming the mathematical difficulty of effecting a second 
approximation by this means, [starting from the first approximation,] 
I have ascertained that the solution contains terms of indefinite increase, 
whence it must be concluded that the logic of the process is somewhere 
at faultBoth in this Magazine and in my work on the Mathe- 
matical Principles of Physics, I have in various ways attempted to solve 
to the second approximation the problem of the motion of a small 
sphere acted upon by the vibrations of an elastic fluid. But I must 
confess that owing to the difficulty of including the effect of the spon- 
taneous vibrations, my efforts have been only partially successful." He 
remarks that as his equations involved two unknown constants and 
IZ2, (representing the amounts of wave condensation on the nearer and 
on the further hemispheres of the atom,) " on this account the theo- 
ries which attribute the forces of heat, molecular attraction, and gravity, 
to action on the atoms by pressure of the aether in vibration, are incom- 
plete." And he admits that for any purpose of quantitative determina- 
* Opere citat., pp. 456,459. 
t Opere cital., pp. 463, 489. KINETIC THEORIES OF GRAVITATION. 
49 
tion or verification, " The whole theory however of attractive and re- 
pulsive forces, regarded as due to vibrations of the aether, is incomplete 
for want of an a priori determination of the composition and value of 
the quantity H."* 
In 1876, the author returns to the discussion with a view "to correct 
in certain respects that investigation, [the one last cited,] and to carry it 
a step further." And in dealing with a residual effect of vibration, 
" which in fact is the attraction of gravity," and determining that the 
accelerative force is independent of the size of the atom or molecule, 
he says : " This result expresses the force of gravity as due to the at- 
tractive action of a molecule of a higher order as to magnitude than the 
molecule of molecular attraction. For distinction, a molecule of this 
superior order might be called a gravity-molecule. Its magnitude may 
still be considered to be so small that in comparison with the magni- 
tudes of verrestrial and cosmical masses, it may be treated as an infini- 
tesimal quantityLastly, it is to be noticed that on account of 
the large value of A for gravity-waves, they do not suffer sensible retarda- 
tion or refraction in passing through gross bodies. I have on several 
previous occasions, treated of the problem of gravitating force theoreti- 
cally, and by slow steps have approximated to its solution ; but before 
the present attempt, I had not succeeded in exhibiting satisfactorily the 
rationale of this kind of attraction by vibrations."! 
Such is an imperfect abstract of the most carefully-studied and the 
most diligently-elaborated exposition of the wave-theory of attraction 
which has yet been proffered to the scientific world. Discussed with 
the earnestness and candor of a fervent conviction, and with all the 
resources of a high order of mathematical culture and ability, the re- 
sult yet fails, sadly and fatally, to satisfy the conditions of the problem. 
Strong as is the author's assurance that he has successfully grasped by 
his formulas and equations the several functions embraced in the first 
three of our propositions,]: this is by no means mathematically estab- 
lished; and the last three propositions are hopelessly ignored and vio- 
lated. We have seen that Elasticity, that puzzling " occult quality," 
driven out from the sober presence of the purely rational atom, has in an 
inexplicable manner, slipped in by the back-door of cetherial pressure. 
Naturam expellee furca, tamen usque recurret. 
Et mala perrumpet furtim fastldia victrix. 
With the multitudinous duties imposed upon the much-suffering 
a?ther, in all the varying ranges and orders of undulations derived from 
atoms and from molecules with ever-changing motions, amid all the 
perturbations and transformations of the mechanical energy of matter, 
* L. E. D. Phil. Mag., September, 1872, vol. xliv, pp. 203, 204, 209. 
+ Phil. Mag., September, 1876, vol. ii, p. 191. 
+ Principles of Mathematics and Physics, p. 499. In the concluding chapter of the 
work, the author draws strength and encouragement from a quotation of the celebrated 
" Third Letter to Bentley." 
4 50 
KINETIC THEORIES OF GRAVITATION. 
there is one resultant alone which never by any accident incurs a com- 
position or experiences a commutation, the constant and unchangeable 
undulation of gravitation. 
Glennie. 1861. 
Mr. J. S. Stuart Glennie, in 1861, published in the Philosophical Mag- 
azine several papers on the subject of gravitation, in which he proposed 
to show that universal repulsion is the true explanation of this force; 
thus referring it rather to a static than a kinetic condition of pressure. 
In an essay " On the Principles of the Science of Motion," he sets out 
with the design " in this attempt to found a general theory, cleared of 
aethers and fluids, of properties and virtues." Commencing with the 
generalization that " a- mechanical force, or the cause of a mechanical 
motion, we know to be in general the condition of a difference of press- 
ure," he infers, "hence it appears that if a general mechanical theory is 
possible, the ultimate property of matter must be conceived to be a 
mutual repulsion of its parts, and the indubitable Newtonian law of 
universal attraction be deduced herefrom, under the actual conditions 
of the world. The general experimental condition of the fitness of the 
mechanical conception of pressure as the basis of a general physical 
and chemical theory evidently is that there be a plenum 
To give distinctness to this idea of the parts of matter as mutually re- 
pulsive, a molecule, or a body (an aggregate of molecules), is conceived 
as a center of lines of pressure; the lengths and curves of these lines 
are determined by the relative pressure of the lines they meet; and 
lines from greater are made up of lesser molecules and their lines, and 
so on ad infinitum. In speaking of a molecule or body as such a center 
of pressure, it will be convenient to have a technical name. Rather 
than coin a new term, it is proposed to use 'atom' in this sense 
Atoms, or mutually determining centers of lines of pressure, may also 
be defined and their relations analytically investigated, as mutually de- 
termining elastic systems with centers of resistance. This is the fun- 
damental conception (not hypothesis) of the theoryNow 
in a system of atoms as above defined, let the centers be of equal mass 
and at equal distances; there will be no difference of pressure on any 
one center, no moving force will be developed, and the conditions of 
equilibrium will be satisfied. But it is clear that forces will be devel- 
oped, or the general conditions of motion be fulfilled, either (1) by a 
difference in the masses of the centers, or (2) by a difference in the dis- 
tances of the centers, in consequence of a displacement of any one of 
them, or (3) supposing a state of dynamic equilibrium established in 
the system by its being brought in contact with another system in a 
different state of such equilibrium" 
" If all the masses of the system were equal, and all at the same dis- 
tance from each other, their mutual repulsions would be equal in all 
directions, and they would remain at rest. But if, though two masses KINETIC THEORIES OF GRAVITATION. 
51 
may be equal, either has on the other side of it a mass of greater size, 
or at a greater distance than the other, it is evident that the mutual 
pressures of these two equal masses will under such conditions, be un- 
equal, and hence as in the first case, they will approach. It is also 
evident that a body may thus cause the approach to itself of another 
body, whatever the number of interposed bodies. Thus if the concep- 
tion of atoms is applied to the unequal and unequally-placed bodies of 
such a world as that presented to us, the law of universal attraction 
follows, and gravity is mechanically explained, that is, is referred to a me- 
chanical conception. But it must be understood that the above propo- 
sition is given rather to show that as an actual law, universal attraction 
may be deduced from the theoretical conception of universal repulsion, 
than with any pretension to its being the best attainable form of an ex- 
planation of the law. It may however be remarked that such an expla- 
nation is in accordance with the chief characteristics of the force of 
gravity; it is not polar, and it seems to be so far different in kind from 
other physical forces that it is not interchangeable with them, as they 
are among each other; for the attraction of gravity is thus referred to 
difference of mass, either between the two attracted bodies or in the 
systems of which they are parts."* 
In a second article, " On the Principles of Energetics," Mr. Glennie 
proceeds: "As force is thus conceived, not as an absolute entity acting 
upon matter, but as a condition of the parts of matter itself, and as a 
condition determined by the relative masses and distances of these parts, 
any valid hypothesis of a force or of a motion to account for any set of 
phenomena is thus seen to imply an assertion as to relative masses and 
distances which can be more or less readily submitted to experiment or 
observation and analysis. . . . 
" The condition of the beginning of motion is a difference of pressure on 
the body that begins to move; the condition of a uniform continuous 
motion is a neutralization of the resisting pressure; the condition of an 
accelerated continuous motion is a uniform or varying resisting [effect- 
ive1?] pressure." t 
In a subsequent paper, in continuation of the last, the writer thus re- 
sumes his statement: "Here more clearly to express the idea in con- 
trast with the fundamental hypothesis of Professor Challis, anatom may 
be defined as a center of an emanating elastic tether, the pressure of which 
is directly as the mass of its center, and fjie form of which depends on 
the relative pressures of surrounding atoms. Thus if you will, matter 
may be said to be made up of particles in an elastic tether. But that 
tether is not a uniform circumambient fluid, but made up of the mutually 
determining tethers (if you wish to give the outer part of the atom a 
special name) emanating from the central particles. And these central 
particles are nothing but what (endeavoring to make my theory clear by 
* L. E. D. Phil. Mag., January, 1861, vol. xxi, pp. 41-45. 
f Loco citat., April number, p. 276. 52 
KINETIC THEORIES OF GRAVITATION. 
expressing it in tbe language of the theories it opposes) I may call sethe- 
rial nuclei. . . . In defining atoms as centers of pressure, they are 
thus no less distinguished on the one hand from centers of force than 
from the little hard bodies of the ordinary theories; for such centers of 
force are just as absolute and self-existent in the ordinary conception of 
them as those little bodies; and in a scientific theory there can (except as 
temporary conveniences) be no absolute existences, entities. Hence 
(mechanical) force, or the cause of motion, is conceived not as an entity 
but as a condition,-the condition namely of a difference of pressure; 
and the figure, size, and hardness of all bodies are conceived as relative, 
dependent, and therefore changeable. There are thus no absolutely ulti- 
mate bodies/'* In a later part of tbe same article (p. 356) the writer 
objects to the ordinary atomic conception that "an hypothesis of infin- 
itely hard atoms not merely requires in the consideration of the motion 
of such an atom abstraction to be made of the interior relative motions, 
also consequent on that difference of pressure which causes its external 
relative motion, but explicitly denies any internal motion." The force 
of the objection is not very obvious. There seems to be no more need 
of conceiving internal motions in the ultimate unit of matter, for the pur- 
poses of molecular physics, than there is for conceiving internal motions 
in the planets for the purpose of astronomical physics. 
In a brief summary of his views, in the succeeding volume, Mr. Glen- 
nie thus recapitulates: "Matter is conceived as made up, not of an 
elastic aether and inelastic atoms, but of elastic molecules of different 
orders as to size and density. If a rough physical conception of these 
molecules be required, they may be conceived as mtherial nuclei, the 
aether of the nuclei of a lower being made up of nuclei of a higher or- 
der, and so on ad infinitiim." t 
It is somewhat difficult to criticise a scheme of gravitative pressure 
so indeterminate in detail. The very function of a theory or hypothesis 
is to formulate the unknown in terms of the known; and in proportion 
as a writer fails to do this, he fails to present us with an intelligible 
theory or hypothesis. From the objection expressed, that "atoms " are 
ordinarily conceived as self-existent bodies, while " in a scientific theory 
there can be no absolute existences or entities," it might be supposed 
that the author held the constituents of matter to be merely mathemat- 
ical points (without dimension) forming the centers of repulsive 
spheres, " aetherial nuclei;" but when we learn that these elastic mole- 
cules are " of different orders as to size and density," built up of success- 
ive aggregates from the infinitely small, and that each exerts a repel- 
lant "pressure which is directly as the mass of its center," we are led 
to conclude that these " aetherial nuclei" do possess a determinate mag- 
nitude. If they are not to be considered as " absolute existences or enti- 
ties," they are supposed to have at least sufficient substance to be moved 
* Phil. Mag., May, 1861, vol. xxi, pp. 351, 352. 
tPhil. Mag., July, 1861, vol. xxii., p. 62. KINETIC THEORIES OF GRAVITATION. 
53 
about according to the differences of pressure to which they are ex- 
posed. A system composed of any number of these repellant centers 
having equal mass, and placed at an equal distance apart, will it is said 
remain in a condition of stable equilibrium, apparently whatever be 
the unit of distance. What fact of observation this deportment illus- 
trates is not stated. But if either masses or distances be unequal, mo- 
tion will result, and " forces will be developed." This certainly does 
not represent any ascertained fact of gravity or molecular physics. 
With a universe filled with such centers of repulsion energetic inversely 
as the square of their radii, it is not easy to see how strictly centripetal 
motions can result, or how such motions of approach (if possible) 
could exhibit an energy in directly the reverse ratio. 
In enthroning a universal repulsion to discharge the office of a uni- 
versal attraction, Mr. Glennie has not been successful in satisfying any 
of the conditions of the problem, and in investing his " atoms" with the 
pressure of elasticity he has hardly carried out his programme of a theory 
"cleared of properties and virtues." 
Keller. 1863. 
Messrs. F. A. E. and Em. Keller, in a joint "Memoir on the Cause of 
Weight, and of the Effects Attributed to Universal Attraction," (pre- 
sented to the French Academy of Sciences March 23, 1863,) announced 
as the motive force the agency of mtherial undulations. Referring the 
effect to the longitudinal vibrations of the tether, the writers think " the 
time has come to seek and to find a plausible explanation of weight- 
simple and natural-in the ceaseless action of these waves on resisting 
bodies, an action analogous to that of ocean-waves, which drive ships 
upon the coast by the vis viva of their flow over that of their ebb; for 
the longitudinal vibrations of the aetherial waves condensing and dilat- 
ing being simply impulsions followed by reaction, and the reactions 
being always more feeble than the impulsions, there follows definitively 
an excess of force in the direction of the propagation which should com- 
municate itself to dense bodies opposed to their propagation, and which 
should press them one toward the others. It is thus that inert bodies 
of slight density would transmit their impulsion to denser bodies, when 
thrown promiscuously into a long box, were we to strike repeatedly one 
of the ends of the box. Ultimately the denser particles will collect at 
the opposite end ; and if both ends are struck simultaneously these par- 
ticles will collect in the middle of the box, while the others will be ar- 
ranged in the order of decreasing density from the center." 
After considering the effect on a line of particles subjected to contin- 
uous shocks at each extremity, the writers proceed: "If instead of a 
single line of particles a certain volume betaken, and if instead of shocks 
in two opposite directions the shocks are supposed to be given in all 
directions, it is easy to see that the denser particles, mutually absorbing 
a part of the impulses directed from one to the other, would approach as if 54 
KINETIC THEORIES OF GRAVITATION. 
really attracted. As a resultant of all the impressions, the particles 
would act upon each other in the inverse ratio of the square of the dis- 
tance, and in the direct ratio of their number,-an action which at once 
presents a striking analogy with the law of universal attraction."* 
It is not believed that either of these ratios would be even approxi- 
mately attained. It will be observed that in this scheme the setherial 
vibrations are supposed to exert a precisely opposite action to the undu- 
lations exhibited in the system of Professor Challis, having their origin 
on the circumferences of enormous spheres of tether, and being accurately 
directed to a central point or points, whatever may be the variety of 
distribution or the changes of position in the material elements. 
Tait. 18G4. 
The professor of natural philosophy in the University of Edinburgh, 
P. G. Tait, has expressed himself with a cautious moderation on the 
probable origin of gravity, but with a sufficient distinctness to indicate 
his inclination to a kinetic hypothesis. In an able thbugh somewhat 
prejudiced and partisan review of "The Dynamical Theory of Heat," 
published in the North British Review for February, 18G4, after the 
very distinct affirmation of the great truth that " natural philosophy 
is an experimental anil not an intuitive science: no a priori reasoning 
can conduct us definitely to a single physical truth ;" the reviewer thus 
proceeds to suggest his inductive conclusions: 
"In the physical world we are congnizant of but four elementary or 
primordial ideas, beside the inevitable Time and Space. They are Mat- 
ter, Force, Position, and Motion. Of these, motion is simply change of 
position; and force is recognized as the agent in every change of 
motion. Till we know what the ultimate nature of matter is, it will be 
premature to speculate as to the ultimate nature of force; though we 
have reason to believe that it depends upon the diffusion of highly at- 
tenuated matter throughout space."! 
Indefinite as the statement is, the indication that "force" probably 
depends rather on "highly attenuated matter" than on ordinary gross 
or sensible matter would appear to be derived from a somewhat meta- 
physical reason to believe. If the conception of a material connecting- 
link throughout space may be supposed to rest on a perception of phys- 
ical fitness or necessity in such a transmitter of energy, this gives no 
physical reason to believe the origin of force resident in the one form of 
matter rather than in the other. For whether this "highly attenuated 
matter throughout space" is supposed to act statically or kinetically, 
and whether its function be to give or to receive impulses, we are cor- 
* Comptea Itendus, vol. Ivi, pp. 531,532. 
t North British Review, February, 1864, vol. xl, art. ii, p. 22 of Am. edition. This 
essay, with another one on "Energy" in the same periodical, has been republished as 
a separate treatise, under the title of " Sketch of Thermodynamics." 8vo., Edinburgh, 
1868. KINETIC THEORIES OF GRAVITATION. 
55 
rectly informed that "force is recognized as the fluent in every change 
of motion; " and therefore we are no nearer the source of this agency 
after the acceptance the hypothesis than we were before it. 
Still more recently, Professor Tait, in an evening lecture on " Force," 
delivered September 8, 187G, before the British Association at its Glas- 
gow' meeting, has recurred to his kinetic hypothesis. " Why two 
masses of matter possess potential energy when separated, in virtue of 
which they are conveniently said to attract one another, is still one of 
the most obscure problems in physics. I have not now time to enter 
on a discussion of the very ingenious idea of the ultramundane corpus- 
cles, the outcome of the life-work of Le Sage, and the only even appar- 
ently hopeful attempt which has yet been made to explain the mechanism 
of gravitation. The most singular thing about it is that if it be true, 
it will probably lead us to regard all kinds of energy as ultimately 
kinetic.* And a singular quasi-metaphysical argument may be raised 
on this point, of which I can give only the barest outline. The mutual 
convertibility of kinetic and potential energy shows that relations of 
equality (though not necessarily of identity) can exist between the two; 
and thus that their proper expressions involve the same fundamental 
units, and in the same way. Thus as we have already seen that kinetic 
energy involves the unit of mass and the square of the linear unit directly, 
together with the square of the time unit inversely, the same must be 
the case with potential energy; and it seems very singular that poten- 
tial energy should thus essentially involve the uuit of time, if it do not 
ultimately depend in some way on energy of motion."! 
This is the unavoidable inference of the kinetic system of force, if 
consistently maintained. But if there be any induction impregnable, as 
the generalization of a life-long, a continuous, and an unvarying exper- 
ience, it is that potential energy does not "involve the unit of time." 
The carbon that has lain protected in the bowels of the earth for untold 
ages (certainly for many millions of years) has precisely the same rela- 
tion to oxygen as the carbon prepared from last year's wood, and 
holds stored in the same mass the same exactly measurable potential 
energy. The stone ball that may have lain a thousand years undis- 
turbed on the brink of a precipice has during that time lost no fraction 
of its static tension, but will fall with absolutely the same dynamic effect 
as if thrown up to its seat by a cannon but a moment before. The 
familiar case of a wound-up clock or watch, with the pendulum or the 
balance-wheel at rest, is equally irreconcilable with any scheme of kinetic 
action or of a force involving as a function any " unit of time." As 
Professor Maxwell points out: " In both cases, until the clock or watch 
is set agoing, the existence of potential energy, whether in the clock- 
weight or in the watch-spring, is not accompanied with any visible 
* This was, of course, its very purpose. 
t Nature, September 21, 1876, vol. xiv, p. 463. 56 
KINETIC THEORIES OF GRAVITATION. 
motion. We must therefore admit that potential energy can exist in a 
body or system all whose parts are at rest."* 
The degree of accordance between the logical conclusion and the un- 
reasoned fact, is a measure of the value of the " singular quasi-meta- 
physical argument" by which it is deduced. "In dealing with physical 
science, it is absolutely necessary to keep well in view the all-important 
principle that nothing can be learned as to the physical world save by 
observation and experiment, or by mathematical deductions from data so 
obtained."! 
Saigey. 1866. 
The following presentation of the theme, though from a litterateur 
rather than a physicist, is interesting as showing what may be called 
the percolation of ideas. In a series of essays contributed by Emile 
Saigey (under the nom de plume of "Edgar Saveney") to the Revue de 
Deux Mondes for November 1, November 15, and December 15, 1866, 
the writer proposed to give a popular exposition of " Modern Physics 
and Recent Views on the Unity of Natural Phenomena." After a brief 
sketch of the received correlation of forces, he proceeds: " What then 
is gravity? What is that mysterious force which causes two bodies to 
attract each other in the direct ratio of their masses and an inverse 
ratio of their distance ? Two bodies attract each other ! Then matter 
is not inert I Is there not then an apparent contradiction between the 
two terms, matter and inertia ? . . . Now what light is this new 
theory going to throw upon the principle of gravity ? Here is the 
answer. A substance to which the name of aether has been given is 
diffused throughout the entire universe. It envelops bodies and pen- 
etrates into their interstices. The existence of this substance is deduced 
from a series of proofs, among which luminous phenomena hold the 
first rank. 2Ether is composed of atoms which impinge upon each other 
and upon neighboring bodies. It forms in this way a universal medium 
which exerts a constant pressure upon the molecules of ordinary 
matter. ... It becomes evident that bodies do not owe their 
gravity to an intrinsic force, but to the pressure of the medium in 
which they are immersed. The motion of heavy bodies no longer ap- 
pears to us other than as a transformation of the aetherial motions; 
and gravity henceforth enters into that majestic unity to which we 
have conducted all physical forces. . . . Are there then strictljT 
speaking, two hinds of matter? We can hardly conceive it, now that 
we have resolved everything into motion. In what respect would these 
two kinds of matter differ ? Would not the one be subject to the same 
laws of motion as the other ? Can there be two systems of mechanics? 
Certainly not; since there is but one law for motion, there can be but 
* Theory of Heat: by J. Clerk Maxwell, London, 1872, chap, xxii, p.282. 
t Nature, 1876, vol. xiv, p. 460. KINETIC THEORIES OF GRAVITATION. 
57 
a single essence for matter, and the molecules of ordinary matter must 
appear to us as aggregates of setberial atoms."* 
In his concluding essay, M. Saigey thus expounds the mtherial action: 
" Let us imagine the aether uniformly diffused throughout space. Its 
atoms endowed with motions of progression and rotation, strike each 
other in the manner mentioned. Let us now suppose that at some point 
within the medium there is a special and disturbing cause, as for ex- 
ample, a molecule having weight [inertia ?] and itself endowed with a 
vibratory motion. The disturbance goes on extending throughout the 
aetherial mass, and by reason of the nature of this medium is propa- 
gated in all directions. The atoms nearest to the heavy molecule will 
receive violent shocks; they will be powerfully urged and their ranks 
will grow thin in the neighborhood of the center of disturbance, and the 
layer contiguous to the molecule will become less dense than the rest 
of the medium. The motor action continuing, this same effect becomes 
propagated from layer to layer throughout space. As a final result, 
the aether becomes arranged around the center of disturbance in concen- 
tric layers, the first of which and nearest to the molecule will be least 
dense, and they will go on indefinitely increasing in density. This con- 
dition of things might be easily represented and the figure traced : the 
molecule at the center, around it spheres of atoms, wide apart at first, 
then nearer and nearer to each other. Let us remark, in passing, that 
the difference in density of contiguous layers, like all effects which are 
propagated by concentric spheres, is inversely proportional to the sur- 
face of these spheres, that is to the square of their radii. [?] This es- 
tablished, suppose a second molecule to be situated at any point of this 
system. It will encounter on the side toward the first molecule, layers 
of aether less dense than upon the opposite side: pressed upon by the 
aether in all directions, it will however receive fewer shocks on the side 
toward the first molecule, and it will consequently tend to approach it. 
Such would seem to be the cause of gravity."! The law of inverse 
squares does not appear to be well made out, although this is a point 
to which all kinetic theorists make ostentatious reference. The density 
of the aether, instead of following this ratio, should ex hypothesi, 
follow directly the opposite ratio; or rather the ratio of increase directly 
as the distance;-which is perhaps what was intended. Of the other 
five conditions it is quite unnecessary to speak. 
Croll. 1867. 
In a communication to the Philosophical Magazine, in 1867, by James 
Croll, of Edinburgh, " On certain Hypothetical Elements in the Theory 
of Gravitation," the author revived the difficulties which had been felt 
and proclaimed by Faraday. He says: " It was demonstrated by New- 
ton, and has been proved by general observation and experience, that 
* llevue de Deux Mondes, 1st November, 1866, vol. Ixvi, pp. 148, 149. 
t It true de Deux Mondes, 15th December, 1866. vol. Ixvi, pp. 922, 923. 58 
KINETIC THEORIES OF GRAVITATION. 
bodies tend toward each other with a force varying inversely as the 
square of the distance, and directly as the mass of the bodies. But it 
never was demonstrated or proved by any one that the bodies attract 
each other. The thing which has been demonstrated is that B tends 
toward A; but the theory does not rest here; it goes on to account for 
this tendency by referring it to a hypothetical cause, viz, to the ' attrac- 
tion ' of A. This however is a mere hypothesis, and no way essential 
to the theory. All that the theory requires is that it be demonstrated 
that A tends to move toward B. It is not necessary that we should go 
beyond this, and attempt to explain the cause of this tendency. Trifling 
as this assumption included in the theory may at first sight appear, it 
will be found that almost all the difficulties and objections which have 
been urged against the theory of gravitation are due in some form or 
other to that assumption. At the very outset we have the objection 
urged against the theory that it implies the absurdity of action at a 
distance. Now the mere facts of gravitation imply no such thing. 
That A and B placed at a distance should tend toward each other does 
not imply action at a distance. A moves by virtue of a force, but it 
does not follow that this force is at a distance from A. But if we assert 
that A and B 'attract' each other, then we imply action at a distance; 
for A is then affirmed to move in consequence of the force of B, and B 
in consequence of the force of A. ' The very idea of attractive force,' 
as Professor Briicke remarks, 'includes that of an'action at a distance.' 
No principle will ever be generally received that stands in opposition 
to the old adage, 'A thing cannot act where it is not,' any more than it 
would were it to stand in opposition to that other adage, 'A thing can- 
not act before it is, or when it is not.' " * 
These venerable " adages " are about as valuable in directing us to 
the actual facts of nature, as that other celebrated adage of Zeno, " a 
body cannot move where it is not;" and conversely, "it cannot move 
where it is." An equally profound dictum is, that a " cause " cannot 
properly be said to precede its " effect," since succession implies discon- 
tinuity. • 
It may be a fact of natural law that everything "acts where it is not," 
including even an setherial vibration ; and certainly there is no difficulty 
in believing it; and the other metaphysical axiom maybe easily dis 
credited by the simple reflection, that were our sun suddenly blotted 
from existence by supreme power, though all " attraction" of the planets 
would instantly cease, its full dynamic action on the earth would con- 
tinue unimpaired for eight minutes. Were Sirius annihilated this year, 
it would still continue to pour upon us its full measure of dynamic ac- 
tion for twenty years " when it was not." The difficulty is not in the 
possibility of posthumous action, but in the possibility of annihilation. 
As Mill has very properly stated in answer to Sir William Hamilton, 
"Action at a distance is intrinsically quite as credible as action in con- 
* L. E. D. Phil. Mag., December, 1867, vol. xxxiv, p. 450. KINETIC THEORIES OF GRAVITATION. 
59 
tact, and there is no reason, apart from specific experience, to regard the 
one as in any respect less probable than the other."* 
In our profound ignorance of all beyond the ascertained facts of grav- 
itation, there could be no objection whatever to the substitution of the 
word " tend" for the word " attract," did it adequately express the 
observed fact that A induces in B a " tendency" to approach ; the quan- 
tity of tendency in B being found to be proportional not merely to its 
own mass, but notably to the mass of the distant body A. 
Mr. Croll proceeds: " The attraction theory is also in opposition to 
the principle of the conservation of force, as has been shown, I think 
clearly, by Faraday. When a stone for example is thrown upward 
from the earth, it not only loses all its motion, but it loses its attraction 
in proportion to the square of its distance from the center of the earth. 
What becomes of the motion imparted to the stone"? It is not trans- 
formed into attraction, for the attraction diminishes as well as the motion. 
When the stone again falls to the earth it gains both motion and attrac- 
tion. In the former case, the attraction is said to consume the motion, 
and instead of becoming stronger becomes weaker in consequence; and 
in the latter case it imparts this same motion, and yet after imparting 
the motion, it is actually found not only not to have lost but to have 
gained force thereby. Faraday justly asks what becomes of the force or 
motion imparted to the stone? It is not converted into attraction, for 
the attraction becomes less instead of greater in consequence. And in 
the case of the falling stone, where does the motion come from ? If the 
motion arises from the attraction of the earth, then there must be a 
certain amount of this attractive force converted into motion; and if so, 
the attractive force should be so far reduced; but instead of this it is 
actually increased. There is therefore no account given of what be- 
comes of the motion externally imparted to the stone when thrown 
upward, or whence the increase both of attraction and motion is derived 
as it descends. If the attraction theory be correct, then there is a des- 
truction of force in the one case and a creation of force in the other ; and 
if so, then the conservation of force is violated." f 
Although this is merely a more prolix statement of the objection 
urged by Faraday, it may be again specifically answered. It will be 
found on careful examination, that the whole difficulty really proceeds 
from an hypothesis tacitly adopted by the writer, while ostensibly 
opposing " certain hypothetical elements in the theory." From the nec- 
essary limitations of language, we have constantly to make words do 
double or multiple duty in carrying different ideas; and to many this is 
a source of constant confusion and misconception. The tacit assump- 
tion underlying this supposed violation.of the conservation of "force" 
is that the force of gravity is the same in kind as the force of the fall- 
'Examination of Sir William Hamilton's Philosophy, by John Stnart Mill, chap, 
xxiv, vol. ii, p. 245, of A.m. edition. 
f L. E. D. Phil. Mag., 1867, vol. xxxiv, p. 451. 60 
KINETIC THEORIES OF GRAVITATION. 
ing stone; and that as the latter is an example of kinetic energy, capa- 
ble of transmuting itself into heat, so gravity must be in essence also 
a kinetic energy capable of similar transformation. On this " hypo- 
thesis" all the consequences so incongruous with experience, as above 
indicated, would result. The ascending stone would have its energy 
"transformed into attraction," and the latter would be correspondingly 
increased. It would then be truly " said to consume the motion " of the 
stone. And when the stone was falling there would be "a certain 
amount of the attractive force converted into motion, and so the 
attractive force should be so far reduced." The reasoning is undoubt- 
edly correct. " But instead of this, it is actually increased. There is 
therefore, [by the kinetic hypothesis,] no account given of what becomes 
of the motion externally imparted to the stone when thrown upward." 
And the undisputed facts of observation therefore, show us that if the 
kinetic hypothesis " be correct, then there is a destruction of force in 
the one case and a creation of force in the other." The conclusion is 
incontrovertible. 
Seeing then the incongruity and inadmissibility of the assumed 
hypothesis, let us try a new departure. Let us, recurring to that only 
safe guide experience, recall as the necessary outcome of the fourth, 
fifth, and sixth propositions, that "gravitation is a property im- 
mutable and inconvertible." Let us, to avoid confusion of idea by 
the unconscious double entendre of the word " force," limit the term for 
the present to that innate and primitive tendency or tension which 
appears as the last result of dynamic analysis, and which obviously 
differs as much from the action of the falling stone, as the flying arrow 
differs in function from the elasticity of the bow which has impelled it. 
We shall thus have a term comparable in derivation and use to the 
"element" or the "atom" of the chemist, designating simply that 
which as a matter of fact, has not yet been further resolved. If now we 
deny (for the present purpose) the application of this term "force," to 
the dynamic action of the falling stone, and call the latter " energy," a 
term which conversely we deny to the primitive vis motrix, all confu- 
sion and inconsistency will disappear. Obviously, "conservation" can 
be intelligently applicable only to that which is capable of expenditure, 
transformation, or dissipation; as to matter, or to energy. To speak of 
the conservation of immutable gravitation is as unmeaning as to speak 
of the conservation of the equally immutable molecular cohesion or 
atomic elasticity. As a fact of daily observation, motion is a variable 
function, and like heat, color, form, or density, is not conserved. 
When a stone is thrown upward therefore, it loses tension, because 
this has been found empirically to be the inflexible law of distance-ratio 
for the gravitative force, and for no other human reason. It gains in 
potential energy by the ascent, because there has been a corresponding 
expenditure of kinetic energy in effecting the ascent and all experi- 
mental research proves the absolute constancy of the sum of these two KINETIC THEORIES OF GRAVITATION. 
61 
forms of energy. And this constancy is all that is signified by the oft- 
quoted but not always justly apprehended " conservation of force." So 
far from there being any fixed relation between gravitative force and the 
conversion of motion, the ratio varies in every planet; and while the 
height to which a pound of gunpowder would project a ball upward 
would differ widely in different planets, the velocity of projection and 
the returning energy of the fall would be precisely the same in all. 
Were we to rigorously deny that gravitation is energy, or that energy 
is force, we could not correctly affirm the conservation of "force." The 
thing truly conserved would be energy, and this is undoubtedly the 
more accurate and less misleading form of expression. 
Mr. Croll says in concluding his essay: "In the case of the loaded 
piston rising under the pressure of the steam, we have the pressure of the 
steam and length of space both diminishing as the vis viva or mechan- 
ical work increases. This is in harmony with the principle of conserva- 
tion, for pressure or force diminishes as energy or work increases. But 
in the case of gravitation matters are reversed, for the force increases 
along with the work. As the weight descends and performs work, the 
pressure of the weight, the thing which performs the work, increases 
also; and when the weight is rising and energy diminishing, the force 
or pressure of the weight is not increasing but actually diminishing 
also." A very sufficient demonstration that steam and gravity are not 
correlated; that they are not both "forces" in the same sense of the 
term. Similarly a bar-magnet employed in educing magnetism in 
another steel bar, so far from losing what the other gains, has its own 
magnetism re-enforced by the operation. Vires acquirit eundo. 
" This difficulty," says Mr. Croll, " along with all the others which we 
have been considering, will entirely vanish if we adopt the view of gravity 
which has been ably advocated by Faraday, Waterston, and other physi- 
cists, viz, that it is a force pervading space external to bodies, and that on 
their mutual approach this force is not increased, as is generally supposed; 
the bodies merely pass into a place where the force exists with greater 
intensity; for in such a case, the intensity of the force in the space 
external to any body is inversely as the square of the distance from 
the center of convergence of these lines of force. As the stone recedes 
from the earth its vis viva is transferred to space, and exists there as 
gravity. When the stone approaches to the earth, the force existing in 
space is transferred back to the body, and appears again as vis viva."* 
Here then is an hypothesis which based on an a priori sentiment of 
fitness rather than on any direct induction, can be submitted to the 
test of observation. As a fact of observation, gravitation is always 
found to be a property of gross tangible matter, with its tensions math- 
ematically directed to the mass-centers, and in quantity always directly 
proportioned to the number of material molecules, agreeably to the first 
and second propositions. As a fact of observation, the moon may pass 
* Phil. Mag., loco citat., pp. 456,457. 62 
KINETIC THEORIES OF GRAVITATION. 
through the very center of the space occupied by our earth only two 
weeks previously without any perturbation or consciousness of increased 
tension in this mystic " space." To affirm that the projected stone 
transfers its vis viva to space without suggesting any conceivable method 
by which such transfer could physically be effected, is not to proffer an 
hypothesis. To affirm that the falling stone receives from the illimitable 
ocean of space its just supply of vis viva-whether required at the in- 
stant or ten years afterward-is to bid us hope that the steam exhausted 
from the cylinder into the atmosphere may be induced to return to its 
duty w hen needed, in order to justify our faith in its conservation. 
Nine years later, in a communication read before the British Asso- 
ciation at Glasgow, September, 1876, "On the Transformation of 
Gravity," Mr. Croll repeats very much the line of argument just re- 
ferred to, showing that the interval had not served to remove his diffi- 
culties. He commences his memoir with the query: " Js gravity con- 
vertible into other forms of energy? Can gravity be converted into 
heat, electricity, magnetism, etc.? or can those forms of energy be con- 
verted into gravity?" It might be supposed that the question would as 
soon occur to the physicist, Can the flight of the arrow be converted 
into elastic tension? The answer to his inquiry is directly involved in 
the fifth and sixth propositions. Mr. Croll, however, gives a different 
answer. "It may be true that gravity cannot be directly transformed 
into heat, electricity, magnetism, chemical affinity, etc., nor these forms 
directly transformed into gravity; but nevertheless, the thing may be 
done indirectly. . . . If the electricity produced by the descent of the 
water be gravity transformed into electricity, then the ascjnt of the 
water produced by electricity must be electricity transformed into grav- 
ity ; for it is the same process merely reversed."* The alternatives are 
doubtless equally correct. 
" If gravity be correlated to other forms of energy, it must like them 
come under the great principle of conservation. But here we enter 
upon debatable ground. It is admitted that gravity can perform me- 
chanical work, and the mechanical work can be converted into other 
forms of energy. Here we have correlations; but it is generally denied 
that there is a decrease or loss of gravity resulting from such trans- 
formations. But this appears to me to be a virtual denial of the prin- 
ciple of conservation. . . . The reasons which appear to have led to 
this opinion are I think, mainly the two following: 1. It has been as- 
sumed [I] that the weight of a body is not affected by the work which 
it performs. 2. The force by which bodies are drawn toward each other 
does not diminish as they approach, but on the contrary increases. 
. . May not a stone when in the act of falling be acted upon by 
gravity with less force at any given moment than it would be were the 
stone at rest at that instant ? The point has never yet been determined 
* Phil. Mag., October, 187G, vol. ii, pp. 241, 242. KINETIC THEORIES OF GRAVITATION. 
63 
either by experiment or by observation."* There is no conceivable 
reason why the falling stone should not" be acted upon by gravity with 
less force" than if it were at rest, (or why our fifth and sixth propo- 
sitions should be true,) excepting the cogency of ascertained fact. 
Natural philosophy is an experimental science; and this point has been 
determined both " by experiment and by observation." The sensitive gal- 
vanometer needle of Faraday would have betrayed a sigh of commuting 
energy in the falling weight employed, while the actual increments of 
vis viva were really so large that they would have been a notable duty 
for the coarsest scales. And the ceaseless fall of planets from the tan- 
gents of their orbits, without any reduction of their own centripetal 
tensions, or of their satellitic control, is a constant and conclusive 
observation to show that this law or condition of gravitation-embraced 
in our sixth proposition-has not (as above suggested) been " assumed." 
Mr. Groll continues: " But if the force of gravity does not sustain 
any loss as work is performed by it, what then is it that is supposed to 
sustain the loss? Some form of energy must diminish as work is per- 
formed; and if it be not gravity it must be something else. The 
generally received explanation is this: when a body is projected upward, 
the potential form of energy into which the upward motion of the body 
is transformed does not consist in the simple force of gravity or tend- 
ency of the body to descend, but consists in this force or tendency 
multiplied by the distance through which it is capable of descending 
. . . This mode of viewing the matter, it is perfectly true, com 
pletely meets the mathematical and mechanical conditions of the prob- 
lem ; but for this very reason it seems to me to hide somewhat the 
real physical nature of the process. [I] . . . Space and time are 
conditions, but conditions absolutely necessary to the transformation of 
potential energy into kinetic, and of kinetic energy into potential; but 
they themselves cannot be forms of energy. But if it be true that the 
mere force of gravity or tendency of the stone to fall to the ground is 
not the potential energy, but that this potential energy is the force 
multiplied by the space through which it can act, then space must 
become a form of potential energy. This is evident; for the potential 
energy in this case consists of two factors, one of which is the space 
through which the force acts. It thus becomes just as much a form of 
energy as the other factor, viz, the force."! 
The conclusion that space is " a form of" action, because all action is 
necessarily copditioned by space, does not appear so " evident." Mr. 
Croll correctly states: 11 But it is not in reference to gravity alone that 
this space-condition is essential to the transformation of potential into 
kinetic energy. It is as we shall shortly see, a condition absolutely 
necessary to the transformation of energy under every possible form. 
In the unbending of a spring the amount of work which can be per- 
* Loco citat., pp. 242-244. 
t Loco citat., pp. 244, 245. 64 
KINETIC THEORIES OF GRAVITATION. 
formed is proportionate to the pressure of the spring multiplied by the 
space through which the pressure can act. The potential element, 
which in this case decreases as work is performed, is the tension or 
elastic force of the spring. Space is necessary simply to allow this ten- 
sion to expend itself in work. It is this tension, not space, which re- 
appears as work or kinetic energy."* In. the case of a sling with a per- 
fectly elastic cord whose elongation is proportional to the strain, the 
tension or potential element does not " decrease as work is performed ; " 
on the contrary, it increases directly with the work; and whatever the 
centrifugal velocity of the ball, the period of revolution remains con- 
stant.! The elasticity does not ''expend itself in work." Vires ac- 
quirit eundo. As this is simply the " law of force " for this particular 
tension, it has no relation whatever to " conservation," which pertains 
entirely to " work." 
Again comparing gravitation to steam, whose pressure is weakened 
by the absorption of molecular motion in the rising piston, the writer 
repeats his questionings, " May not the same be equally true of gravity * 
In fact, gravity has a dynamical origin, it must hold equally true of 
gravity and of heat." Again recurring to Mr. Waterston's conception 
that " gravitation is a function of space," he would almost lead us to for- 
get that " space cannot be a form of energy." Again endeavoring to fortify 
his assumptions by scholastic dogmas, he reiterates, " the conception of 
attraction does not represent the modus operandi of gravitation, because 
attraction implies'action at a distance, or in other words, that a thing 
acts where it is not, which is just as impossible as that a thing can act 
when it is not." Again affirming that " there is no necessity for forming 
any conception of the cause of which it is the effect," his whole disserta- 
tion appears impelled by the necessity to " represent the modus operandi 
of gravitation." 
The general conclusion arrived at by Mr. Croll appears to be: "Grav- 
ity in all probability is of the nature of an impact or a pressure. Some of 
our most eminent physicists state that the force of gravity must either re- 
sult from impact of ultramundane corpuscles, in some respects analogous 
to that of the particles of a gas, (which has been found to be capable of 
accounting for gaseous pressure,) or it must result from difference of 
pressure in a substance continuously filling space, except where matter 
displaces it. That gravity is a force of the nature of pressure is I think 
beyond all doubt; but that this pressure results from the impact of cor- 
puscles, or from difference of pressure in a substance filling space, is 
purely hypothetical. Why not assume it to be a force, without calling 
in the aid of corpuscles or a medium filling spaceF'J 
If gravity be an abstract force, not requiring " the aid of corpuscles or 
* Phil. Mag., 1876, vol. ii, p. 247. 
tThe same isochronism exists with the revolving or conical pendulum whose vertical 
height is constant, or with one whose ball revolves in the surface of a paraboloid. 
t Loco citat., p. 252. KINETIC THEORIES OF GRAVITATION. 
65 
a medium," it is not easy to perceive why it should so indubitably be 
"of the nature pressure" rather than of tension. Is the phenomenon 
of the terrestrial tidal wave rendered any more intelligible by referring 
it to the action of a "pressure" from beneath, than by referring it, as 
the " most eminent physicists" do, to the action of a lunar tension from 
above ? 
Rejecting a pressure-medium as " purely hypothetical," the author yet 
thinks a pressure-force in gravity " is beyond all doubt"! not perceiving 
that the latter conception is as " purely hypothetical" as the former. A 
speculation born of metaphysical imaginings as to the " possible," framed 
in no physical relations to associate it with any known action, supported 
by no fact of observation, result of experiment, or sound induction, 
based on assumptions directly at variance with all ascertained laws or 
conditions of gravitative action,-if such a speculation be not "purely 
hypothetical," in what propulsive undulations, corpuscular chaos, or 
setherial vortex, shall we seek to find a fitting subject for the appellation? 
And yet hypothetical as the speculation pre-eminently and undoubtedly 
is,-baseless, formless, insubstantial,-it comes to us with the prestige of ' 
a distinguished physicist as its propounder, and of a learned association 
as its audience and recipient. 
Leray. 1869. 
Within the last twenty years, probably more than a dozen "original" 
discoveries of the cause of gravitation have been announced to the 
French Academy of Sciences. Two brief essays of the same year may 
here be noticed. A Note by P. Leray, entitled a " New Theory of Grav- 
itation," was presented to the Academy through M. Faye September 6, 
1869. 
" This new theory rests on the assumption of an aether-a fluid ex- 
ceedingly rare and perfectly elastic-and on the two following principles: 
First, that in the free aether (that is, undisturbed by the presence of 
other bodies) there exist at every point equal currents crossing each 
other in all directions; second, that in passing through bodies, the 
currents of the aether are retarded proportionally to the thickness trav. 
ersed, and to the mean density of the path. It may be added, that the 
currents thus enfeebled, on passing again into the aether, recover but 
slowly their former force, and may be considered approximately-within 
the limits of our solar system-as preserving a constant value." This 
gravific fluid evidently does not differ essentially from that of Lesage. 
These simple hypotheses, says the writer, " conduct in effect to the same 
results as the law of universal attraction, without requiring any action 
at a distance; and give moreover the key to many phenomena which 
this law Qoes not explain." 
Considering first, the case of an isolated body, it is evident that the 
currents, being equal in all directions, neutralize each other, effecting 
therefore no change of position in the isolated body; although by the 
• 5 66 
KINETIC THEORIES OF GRAVITATION. 
vis viva absorbed from the aether, heat light and magnetism are produced 
in the body as shown in the stars. " This cause being permanent, explains 
without difficulty why the light and heat of the sun are constant "! A 
body in motion is of course retarded by the relative differential of the 
currents, although this retardation would be sensible only with very 
light bodies. 
Considering secondly, the case of two bodies, (exterior to each other,) 
the currents enfeebled by traversing the bodies would impinge less 
strongly on the mutually-facing sides of the two bodies than on their 
exterior sides, thus producing an approach by the difference of impulse. 
And lastly, in regard to weight, this is the result of the quantity of 
motion communicated to a body by the shocks of the aetherial atoms. 
The two facts of experience, that the weight of a body remains invaria 
ble whatever its position, and that all bodies fall with the same velocity 
in a vacuum, prove that ponderable atoms are spherical, having an equal 
superficial density in all directions, and that all ponderable atoms are 
alike, or that there is ultimately but a single elementary substance.* 
Boisbaudran. 1869. 
Another short paper, entitled a "Noteon the Theory of Weight," was 
presented to the Academy September 20, 1869, by M. Lecoq de Bois- 
baudran, called forth by the previous communication of M. Leray. He 
remarks: "Having been myself occupied on this question, I have the 
honor to communicate to the Academy the actual state of my researches. 
On certain points, an£ particularly on the explanation of the central 
heat of the stars, I am happy to agree with M. Lerayj on other points 
my conclusions differ from his. I attribute to matter, whatever may be 
its state of division, no other essential properties than those established 
by experimental physics and mechanics. I designate as atoms the last 
stage of division of matter. I admit that two bodies separated by an 
absolute void cannot act on each other; that action takes place only by 
contact, the play of forces following the laws of ordinary mechanics. 
If there existed but a single kind of atoms, the interchange of forces 
occurring between equal masses, two atoms could not unite. Force and 
matter would exist, but not attraction. There are then at least two 
kinds of primordial atoms of different masses. The smaller may be 
called aether; the others, ponderable atoms." The writer then goes on 
to say that a ponderable atom in the midst of a vibrating aether would 
itself receive a vibration (though of less activity than that of the aether) 
without being displaced in space, the portion of energy lost by the aether 
being transformed into heat, etc. A second atom being placed near the 
first, the aether vibrations would be feebler between the two than in ex- 
terior space, which would result in approach. 
" I prefer the notion of vibrations of the aether to that of ' equal cur- 
rents crossing each other in all directions.' The attraction exerted on 
* Comptes Rendus, 6th September, 1869, vol. Ixix, pp. 616-620. KINETIC THEORIES OF GRAVITATION. 
67 
a ponderable atom is not in the simple ratio of its mass; for if this mass 
equalled that of an aether atom, the attraction would be nil. If there 
exist ponderable atoms of different masses their rate of fail would be 
uneqal. If there were but one kind of ponderable atom, all bodies would 
fall with equal velocity. Experience seems to justify the last hypothesis; 
but very slight differences of mass in the ponderable atoms would suffice 
to determine the formation of chemical elements of very different 
atomic weights, and an inequality of their fall would escape unadvised 
observers." 
After affirming the law of distance, M. Boisbaudran says: " In conse- 
quence of the inertia of the sether, attraction is not proportional to the 
real masses, but no more is it to the number of ponderable atoms con- 
tained in a body. The vis viva of the atoms of aether, however great, 
has a finite value." Lastly, he remarks: "It is to the longitudinal 
vibrations of the aether that I attribute the cause of weight."* 
Neither the hypothesis of M. Leray, nor that of M. Boisbaudran, pre- 
sents any feature of special novelty, requiring comment. 
GUTHRIE. 1870. 
In 1870, Prof. Frederick Guthrie published an account of some inter- 
esting experiments "On Approach caused by Vibration," unaware at 
the time of the earlier labors of Dr. Guyot in the same direction. He 
found that a card suspended near a vibrating tuning-fork was urged 
toward the fork, and by varying the experiment with smoke, with cork, 
with calcined magnesia, with floss cotton, with a second suspended 
tuning-fork, with brass disks, etc., he obtained similar results. At the 
termination of his experiments he thus sumsup: "The experimental 
results appear to me to point to the following conclusions: whenever 
art elastic medium is between two vibrating bodies, or between a vibrat- 
ing body and one at rest, and when the vibrations are dispersed in con- 
sequence of their impact on one or both of the bodies,, the bodies will 
be urged together. The dispersion of a vibration produces a similar 
effect to that produced by the dispersion of the air-current in Clement's 
experiment; and, like the latter, the effect is due to the pressure ex- 
erted by the medium, which is in a state of higher tension [or pressure] 
on the side of the body furthest from the origin of vibration than on 
the side toward it. In mechanics-in nature-there is no such thing 
as a pulling force. Though the term attraction may have been occa- 
sionally used in the above to denote the tendency of bodies to approach, 
the line of conclusions here indicated tends to argue that there is no 
such thing as attraction in the sense of a pulling force, and that two 
utterly isolated bodies cannot influence one another. If the setherial 
vibrations which are supposed to constitute radiant heat resemble the 
serial vibrations which constitute sound, the heat which all bodies pos- 
* Comptes Rendus, 20th September, 1869, vol. Ixix, pp. 703,704. 68 
KINETIC THEORIES OF GRAVITATION. 
sess, and which they are all supposed to radiate in exchange, will cause 
all bodies to be urged toward one another.7'* 
This hypothesis would make gravitation, or molecular attraction, a 
function of temperature, contrary to all observation; and is indeed en- 
tirely incompatible with the fourth, fifth, and sixth conditions of gravi- 
tative action. In short these experiments, striking and instructive as 
they unquestionably are, will be found on careful scrutiny to really sim- 
ulate the effects of gravity in no one particular. 
Crookes. 1873. 
A 
From some very ingenious and interesting experiments made by 
William Crookes, in 1873, with light disks attached to a delicate torsion- 
balance and hermetically sealed within a nearly perfect vacuum, in 
which experiments the radiation of heat was observed to exert a repul- 
sive effect upon the blackened side of the disks, he inferred that a clue 
was thereby probably furnished to the mystery of gravitation. He thus 
concludes his first memoir : "It is not unlikely that in the experiments 
here recorded may be found the key of some as yet unsolved problems 
in celestial mechanics. In the sun's radiation passing through the quasi- 
vacuum of space we have the radial repulsive force, possessing success- 
ive propagation, required to account for the changes of form in the 
lighter matter of comets and nebulae; .... but until we meas- 
ure the force more exactly, we shall be unable to say how much influ- 
ence it may have in keeping the heavenly bodies at their respective dis- 
tances. So far as repulsion is concerned, we may argue from small 
things to great, from pieces of pith up to heavenly bodies 
Although the force of which I have spoken is clearly not gravity solely, 
as we know it, it is attraction developed from chemical activity, and 
connecting that greatest and most mysterious of all natural forces, 
action at a distance, with the more intelligible acts of matter. In the 
radiant molecular energy of solar masses may at last be found that 
' agent acting constantly according to certain laws,'! which Newton held 
to be the cause of gravity." J 
Similar expositions are announced in Mr. Crookes' Journal for 1875, § 
and various modifications of the apparatus employed in the experiments 
are detailed. These little instruments, inclosed within an exhausted 
bulbous tube of glass, and with the disk-arms mounted on a pivot to 
permit continuous rotation, have since become quite familiar under the 
name of " radiometers," and have received a careful investigation from 
a number of observers. It is now known that they do not act by any im- 
pulsion of radiation, but solely by the differences of heat-absorption by 
* L. E. D. Phil. Mag., November, 1870, vol. xl, p. 354. 
t Referring, of course, to the 11 Third Bentley Letter." 
t Philosoph. Trans. Roy. Soc., 1874, vol. clxiv, p. 527. 
§ Quarterly Journal of Science, July, 1875, vol. v, p. 351. KINETIC THEORIES OF GRAVITATION. 
69 
the two sides of the disks and the reactions of the residual air of the 
exhausted chamber.* 
In a later essay on " The Mechanical Action of Light," Mr. Crookes 
estimates that "the pressure of sunshine" amounts "to 2 cwts. per 
acre, 57 tons per square mile, or nearly three thousand million tons on 
the exposed surface of the globe;-sufficient to knock the earth out of 
its orbit if it came upon it suddenly.! It may be said that a force like 
this must alter our ordinary ideas of gravitation ; but it must be remem- 
bered that we only know the force of gravity between bodies such as 
they actually exist, and we do not know what this force would be if the 
temperatures of the gravitating masses were to undergo a change. If 
the sun is gradually cooling, possibly its attractive force is increasing, 
but the rate will be so slow that it will probably not be detected by our 
present means of research."! 
This possibility is denied by our sixth proposition. The generaliza- 
tions embraced in our first, second, and third propositions are assumed 
to be equally true at all possible temperatures; and the ground for 
this assumption is, that they have been actually ascertained to be 
true for all observed variations of temperature. The induction can 
therefore be arraigned only by a conflicting fact of observation or 
experiment. Varied and delicate experiments have actually been re- 
peatedly made from the time of Fresnel to the present, to detect if pos- 
sible an influence of heat on weight, but without result. Indications 
sometimes observed have always been found to be due either to cur- 
rents in the best approximate vacuums or to the molecular reactions 
from unequal absorption. It is evident that if heat radiation could 
exercise the slightest influence upon gravitative attraction, it would be 
possible in many cases to interpose a screen to its action, contrary to the 
observed fact generalized in our fifth proposition. 
We are correctly reminded that " we only know the force of gravity 
as between bodies such as they actually exist;" but this knowledge 
teaches us that the ratio ™ is invariable; that therefore the exponent 2 
of the radius or distance is integral, and is not a residual of an exponent 
2-p n. Finally, we know that heat radiations require eight minutes to 
pass from the sun to the earth. The inferences then that the conduct of 
the " radiometer" affords anykey to the problem of celestial mechanics, 
or that it illustrates or suggests any analogous " agent acting con- 
stantly according to certain laws" as " the cause of gravity," are evi- 
dently unfounded and erroneous. "Nor must we forget that the more 
rigidly we scrutinize our received theories, our routine explanations and 
interpretation of nature, and the more frankly we admit their short- 
comings, the greater will be our ultimate reward."§ 
* This view has since been accepted by Mr. Crookes himself. 
tlf any such action really existed, its effects would be just as certain and as disas- 
trous if applied gradually as if applied suddenly. 
t Quarterly Journal of Science, April, 1876, vol. vi, p. 254. § Loco citat., p. 256. 70 
KINETIC THEORIES OF GRAVITATION. 
In this uhume of kinetic theories, doubtless other names, well de- 
serving notice, have been overlooked from want of more extended re- 
search ; but it is believed that the foregoing survey comprises essentially 
all the forms of the hypothesis of primeval motion which have been 
propounded. The general tenor of this line of speculation has been 
well set forth in an able review of " The Atomic Theory of Lucretius," 
which appeared in the North British Review for 1868. 
"The most plausible suggestion yet made by this school is that a 
single omnipresent fluid tether fills the universe; that by various 
motions of the nature of eddies, the qualities of cohesion, elasticity, 
hardness, weight, mass, or other universal properties of matter are 
given to small portions of the fluid which constitute the chemical 
atoms; that these, by modifications in their combination, form, and 
motion, produce all the accidental phenomena of gross matter; that 
the primary fluid by other motions, transmits light, radiant heat, mag- 
netism, and gravitation; that in certain ways, the portions of the fluid 
transmuted into gross matter can be acted upon by the primary fluid 
which remains imponderable or very light, but that these ways differ 
very much from those in which one part of gross matter acts upon 
another ; that the transmutation of the primary fluid into gross matter, 
or of gross matter into primary fluid, is a creative action wholly denied 
to us; the sum of each remaining constant?'* The hypothesis of the 
transmutation of tether, or of the evolution of " matter," is not how- 
ever necessarily involved in that of referring all conditions, properties, 
and powers of matter to "a mode of motion." 
General Conclusions. 
In the conception of a statical pressure of tether constantly acting 
inwardly on concentric spherical surfaces, there is the obvious irration- 
ality of a stable non-equilibrium. Nor is there any real difference in 
the conception of an tether having concentric increments of density 
outward, in which ordinary matter is buoyed, as it were, toward the 
center: a scheme in which every particle of matter finds itself in the 
midst of its own little spheres of rarefaction, and in which such center 
of tetherial rarity is supposed to blindly follow the flying stone in what- 
ever direction hurled at the caprice of an idle boy. Entirely too much 
importance has been attached to this conception as a speculation of 
Newton, when it was evidently an unconsidered and merely obiter dictum 
suggestion, utterly repudiated by him in later years. 
Omitting then any further consideration of the statical method of 
explaining gravitation by pressure, we find that kinetic systems are 
essentially of two classes, the hypotheses of emissions or corpuscles, 
and the hypotheses of fluid undulations. It is proposed to show that 
no form of either hypothesis can satisfy the two Newtonian conditions 
of a scientific theory-verity and sufficiency. 
* North British Review, March, 18G8, vol. xlviii, p. 127 of Am. edition. KINETIC THEORIES OF GRAVITATION. 
71 
The corpuscular hypothesis is clearly invented pro hac 'vice. Nor can 
even an analogue of the invention be found in the corpuscular theory 
of gases; for in the latter case, the free path of the particle is assumed 
to extend only for the short distance between neighboring molecules, 
and by reason of incessant collision and deflection among the elements 
the hydro dynamic pressure of the medium is equal in all directions, 
quite irrespective of the number or character of intervening obstruc- 
tions : while the gravific or ultramundane corpuscle must ex hypothesi 
travel from infinity in a perfectly straight and undisturbed path to fulfill 
its appointed mission. It is therefore a special creation, neither ascer- 
tained nor suggested by any other consideration. 
Secondly, even if a rera causa, it is wholly insufficient. The first 
great triumph of the corpuscular assumption was its supposed happy 
expression of the distance-ratio of gravitative intensity: and this 
appears to have been admitted by all the mathematicians who have 
referred to the problem. But a very brief consideration will show that 
while this may be approximately true for small masses, there is a limit 
of magnitude as well as of number in the masses of gross matter, beyond 
which no assignable number of corpuscles is capable of acting. Let us 
suppose for definiteness (not as even suggesting a higher limit) that 
normal to a plane whose area is d2, (d representing the distance between 
two adjacent molecules of ice or of water,) a million million corpuscles 
are passing simultaneously with any desired frequency. This neglects 
for the moment the far greater number passing through the same area 
at all other angles. The molecule of ice or water being a very small 
though appreciable fraction of the value of d, (a not improbable estimate 
of which value may be the five hundred millionth of an inch,) it is ob- 
vious that with an equable distribution of the material molecules in a 
continued right prism having d2 for its base, no very great depth is 
required for a million million of such molecules to lie each in the line of 
motion of one of the parallel moving corpuscles. Beyond this depth or 
distance, of any number of molecules continued, no one will receive an 
impact, or in other words, will have any gravitation.* This reasoning 
remains true, whatever be the number and direction of the prisms chosen, 
and whatever be the number of corpuscles passing through its base 
d2. If it be said that this number is infinite, this is simply an uncondi- 
tional surrender of the hypothesis. It is proved then that the number 
of corpuscles passing through a large mass must be very much less than 
the number impinging on its exposed strata or passing through a small 
mass; and that if the mass be large enough, the side presented toward 
another mass will receive no gravitative impulse thereto; and a tidal 
* This idea may be more popularly illustrated by supposing on a plane of indefinite 
extent a grove of small, vertical trees, planted at irregular but equable intervals, at 
any assignable distance apart. It is evident that it would require no indefinite extension 
of the woods to absolutely exclude a horizontal ray of light in any direction, even 
though suns or electric lights were blazing with unimaginable splendor. 72 
KINETIC THEORIES OF GRAVITATION. 
attraction between the bodies would be impossible I The scheme there- 
fore fails not only to satisfy the second condition, (as formerly stated,) 
but it equally fails to satisfy "indefinitely" the third condition. And 
this objection lies equally against every variety of the corpuscular hypo- 
thesis, from that of Bernouilli to that of Leray. 
In the next place, the hypothesis of undulations similarly fails. First, 
because the motion assumed is not a vcra causa. If it be answered 
that an all-pervading aether and its system of undulations are otherwise 
established, by the large array of well-observed phenomena shown to 
be in exact accordance both in quantity and in quality with the mathe- 
matical deductions from such postulated undulations, the rejoinder is, 
that such undulations as have thus been established are demonstrably 
incapable of inducing anything in the slightest degree analogous to 
gravitation; and therefore the new undulations required have not been 
otherwise ascertained. And this is a radical defect in all the wave sup- 
positions. We are not at liberty to assume unknown and undemon- 
strated actions, entirely different in character from anything which has 
been actually7 observed or necessarily inferred. Great as is the credit 
due to Professor Challis, for instance, for his dexterous, persevering, and 
laborious efforts to develop his theory by the stern logic of mathematics, 
the whole demonstration is vitiated by the unwarranted prelimnary 
assumption of qualities and modes of action in the eether analogous 
to those of gases; as for example, that the pressure and the density are 
proportional. Mow as a matter of fact, the eether is a medium so wholly 
and so radically dissimilar to any known gas in structure and in proper- 
ties, that the most characteristic features of its luminiferous undula- 
tions,-rectilinear propagation,with its special incident inflection; select- 
ive refraction or chromatic aberration ; birefraction ; and polarization in 
its various forms,-find no representative therein ; leading a Newton to 
the reluctant conviction that a gaseous aether could not be the vehicle of 
light.* The denial therefore, of the verity of fluent undulations in the 
tether is based upon the skeptical affirmation that beyond experience 
and safe induction therefrom, we can tell nothing as to the behavior of 
an unknown agent. 
But if the character of undulation required were established, sec- 
ondly, it would as in the former case, be entirely insufficient for the 
purpose. The aether must have the elasticity and the mass or inertia 
* Dr. Young, in referring to Fresnel's discussion of " polarization," says, " it might be 
inferred that the luminiferous tether pervading all space, and penetrating almost all 
substances, is not only highly elastic but absolutely solid." (Miscellaneous Works of 
Thomas Young, London, 3 vols. 8vo, vol. i, No. 18, p. 415.) And Sir John Herschel re- 
marks : " Every phenomenon of light points strongly to the conception of a solid rather 
than a fluid constitution of the luminiferous aether, in this sense,-that none of its 
elementary molecules are to be supposed capable of interchanging places, or of bodily 
transfer to any measurable distance from their own special and assigned localities in 
the universe." (Familiar Lectures on Scientific Subjects, London, 1867, Icct. vii, p. 
285.) On the other hand, recent mathematical discussions have found some difficulties 
even in this hypothesis. KINETIC THEORIES OF GRAVITATION. 
73 
necessary to execute precisely its ascertained dynamic function.* This 
specific and very limited inertia is manifestly inadequate to the indefi- 
nite accumulation of energy required to give the same proportional 
tendency between large masses as between small ones. The experiments 
of Guyot and of Guthrie quite conclusively show that neither in the 
law of quantity nor in the law of intensity can gaseous vibrations rep- 
resent even approximately the ascertained facts of gravitation embodied 
in the second and third propositions. 
But if in dynamic action a gaseous undulation of (ether, with its 
existing co-efficient of inertia, is found to be so palpably inadequate to 
the known effect, on the other hand the elasticity must be assumed to 
be incomparably more active than the limit expressed by the actual 
velocity of a luminous undulation. If its rate of propagation be assumed 
to be very many millions of times more rapid than that of light, the 
inertia must be correspondingly reduced; in short, must be practically 
nil; and the action must be really kinematic rather than dynamic. 
Failing thus at every point, the hypothesis leaves still more inscruta- 
ble the origin of the undulation. The center of disturbance is supposed 
to be the vibrating m aterial element; but the cause of the vibration 
is never stated. If innate tendency be the answer, never surely was a 
more mysterious " occult quality " attributed to matter in the history 
of human excogitations. Either misapprehending altogether, or quite 
ignoring the origin of thermal vibrations, the kinetic theorist not un- 
frequently appeals to the dynamical doctrine of heat as the type and 
the warrant of his assumptions. But in the case of heat, as in every 
other observed case, motion is a phenomenon to be accounted for; and 
no physical theory is complete, until in origin, in quantity, and in di- 
rection, it is accurately explained by a true and sufficient antecedent 
cause.j 
But granting a prime mover-the immediate operation of a Demi- 
urgus, if necessary,-how is the initial impulse converted into vibration ? 
What is the resisting power deflecting the element in motion from that 
rectilinear direction, which is its first law of action And by what 
opposing battledoors (for two are absolutely necessary) does the deflected 
particle become a shuttlecock ? Upon these important questions there 
is a very remarkable reticence. We know that when a bell is struck 
with a hammer the moving impulse of a segment of the free edge is 
resisted by a molecular pressure which (for want of a better name) has 
been called repulsion; and that this reaction is in turn resisted by a 
molecular tension which (for want of a better name) has been called 
attraction; and that this reciprocating play very rapidly declines as 
* From the actual dynamic; energy transmitted by the aether, Sir William Thomson, 
in 1855, estimated that a cubic foot should weigh not less than the quadrillionth 
(1 - 1024) of a grain ; or that the inertia of aether had for its higher limit about one 
two-thousand-trillionth (15- 1021) of that of ordinary air. (L. E. D. Phil. Mag., Jan- 
uary, 1855, vol. ix, p. 39.) Its lower limit may be one hundred times this amount, or 
one twenty-trillionth -? 1019) of the inertia of air. 74 
KINETIC THEORIES OF GRAVITATION. 
the momentum of vibration is transferred to the elastic air. And these 
two qualities of the vibrating bell-cohesion and elasticity-as Sir 
John Herschel has well remarked, "we have no means of analyzing 
further, and must therefore regard them, till we see reasons to the con- 
trary, as ultimate phenomena, and referable to the direct action of an 
attractive and a repulsive force."* But in a kinetic system there is no 
room for such abstractions. How then are they to be displaced and su- 
perseded ? We wait in vain for an intelligent or an intelligible answer. 
But after this long list of difficulties, supposing the vibrating parti- 
cles set in motion, still by the Demiurgus, with his right and left pro- 
pulsions, by what power (short of his incessant repercussion) is the 
vibrating particle enabled to transmit without decline or interruption, 
these tetherial waves of gravitation-let us say for a single year, we 
might as well have said for a single minute ? Here again we listen 
vainly for an answer! 
And thus, step by step, have we been led to the culminating vice of 
every kinetic sytem ; its utterly reckless violation of any rational concep- 
tion of the conservation of energy. And yet remarkably enough, the 
ostensible impulse and occasion of such creeds have usually been a 
strong veneration for this much-abused principle, and the consciousness 
of a special mission to restore and to vindicate its neglected authority ! 
Not unfrequently the vibrations communicated to the telegraphic aether 
by a trembling atom have been supposed to be transmitted unimpaired 
to that or to other atoms, and back again, in endless and magnificent 
cycles of "perpetual motion." And as "there is no limit to the vis viva 
which such a medium may conserve" within its boundless bosom, such 
projectors have the Bank of the Infinite on which to draw7 in every 
dynamic emergency, without the fear of a depleted treasury, and with- 
out any necessity being felt for inquiring too nicely into the balance of 
the depositor's account. And thus, as Leray has intimated, suns and 
stars are maintained for everblazing on a borrowed capital of motion. 
In opposition to all this ideal programme of an illimitable ocean ot 
dynamic, with its treasures lying loose in space, to be absorbed by every 
projected ball or stone, it may be simply declared that from observation 
we have no reason whatever to believe the tether to be in any case a 
source of energy. We have absolutely no experience of any undulations 
originating in its broad expanse. It is never self-luminous; and even 
in the case of electricity there is always required the disturbance of a 
material element. Nor is there any ascertained fact to warrant the sup- 
position that the tether is a reservoir of force,-in any other sense than 
that without the possession of intrinsic tension it would be incapable of 
transmitting energy. So far therefore as sober experience is accepted 
as our guide, we only know that a mechanical impulse of suitable char- 
acter being committed to the tether, it is a faithful vehicle of energy, 
never adding to, and never abstracting from its charge. 
* Prelim. Discourse on the Study of Natural Philosophy, part ii, chap, ii, sec. 80. KINETIC THEORIES OF GRAVITATION. 
75 
Among the names presented in the foregoing selection from kinetic 
theorists are several eminent in the domains of physics and mathemat- 
ics. Their labors, animated with the zeal, the diligence, and the persist- 
ency of purpose and conviction here displayed, might be expected to 
enlarge notably the boundaries of our knowledge to whatever depart- 
ment of natural law they were devoted. But when we look for any 
actual results from these elaborated speculations, how barren is the 
prospect. How wonderfully contrasted in character is the prime as- 
sumption with the fruitfulness of that generalization which it aims to 
subordinate and to comprehend. How striking its failure, not in the 
higher role of prophecy, but in the humbler one of mere interpretation. 
How clumsy the mechanism by which it vainly strives to accomplish its 
results. 
But it is not simply in the negative aspects of an unsuccessful effort 
that these varied speculations, prompted by a common sentiment and 
motive, teach us their most suggestive lesson. Beyond the facts of con- 
stant and of signal failure, these restless, resolute probings by the human 
mind in all directions, serve as the cumulative bases of a new induc- 
tion ; and by the very sharpness of the contrasts brought to view at 
every point between the tentative conjecture and the determinate expe- 
rience, they enforce the assurance, that whatever else the principle of 
gravitation may be, it is not in its essence any form of motion. And 
that gravitation is not a resultant of pressure, appears to be very clearly 
made out by the inability of every scheme of such hypothesis, in the 
hands of its most skilful adherents, to give any rational account of the 
semi-diurnal ocean-tides. 
Another lesson no less striking, is the utter worthlessness of metaphys- 
ical axioms as a criterion of physical truth, or as a foundation for ra- 
tional physical theory. Such propositions as " It is impossible to im- 
agine an infinite attribute belonging to a finite entity," or " It is im- 
possible to conceive anything to act where it is not," are not merely quite 
irrelevant to scientific verity, but if gravely accepted as physical postu- 
lates, prove only to be positive obstructions to scientific investigation. 
Professor Challis has well said, " I do not admit that any metaphysical 
argument can be adduced either in support of or against a physical hy- 
pothesis. Jfeta-physics come after physics." * And Sir John Herschel, 
some thirty years ago, laid down as a rule of sound philosophy, " Expe- 
pience once recognized as the fountain of all our knowledge of nature, 
it follows that in the study of nature and its laws we ought at once to 
make up our minds to dismiss as idle prejudices, or at least suspend as 
premature, any preconceived notion of what might or what ought to be 
the order of nature in any proposed case, and content ourselves with 
observing, as a plain matter of fact, what is."t And again, long before 
him, Newton condemned the "feigning hypotheses for explaining all 
things mechanically, and referring other causes to metaphysics. Where- 
* L. E. D. Phil. Mag., 1861, vol. xxi, p. 505. 
+ Discourse on Study of Natural Philosophy, part ii, chap, i, sec. 68. 76 
KINETIC THEORIES OF GRAVITATION. 
as the main business of natural philosophy is to argue from phenomena 
without feigning hypotheses, and to deduce causes from effects, till we 
come to the very first cause, which certainly is not mechanical." * 
It has already been noticed that elasticity has proved a stumbling- 
block to every kinetic hypothesis. A system of molecular physics 
without this property would represent a chaos; but a chaos destitute 
of energy. As Lam6 has well observed in his admirable treatise on the 
subject, " The role of Elasticity in nature is as important as that of 
Gravitation! " How then are we to formulate this prime and potent 
force? Evidently no phase or form of motion can simulate its action. 
Here all kinetic theory fails: and there is then something in the world 
beside "inertia and motion." Until some rational analysis be discov- 
ered, we must accept the fact (in the language of Sir John Herschel) 
as an " ultimate phenomenon." And as we find that the molecules of 
all material substance resist compression with a force proportional in 
some high ratio thereto, we can only conceive that they are separated 
by an interval through which this repellant resistance is exercised. 
Here then it seems, we are at once confronted with an "occult quality" 
or virtue, a force of repulsion, quite beyond the reach of any explana- 
tion or relegation. And worse than all, with a quality or virtue which 
proves to be that ever-dreaded and in fact "impossible" actio in dis- 
tans! Are we then driven in the last resort to admit that devoid of all 
perception of propriety, the insignificant physical molecule does pos- 
itively "act where it is not," and where it ought not ? 
When we endeavor to penetrate into the secrets of molecular physics, 
again are we borne on the waves of a large induction that the atoms of 
matter never are and never can be in contact: but that in Seguin's 
fine conception, they are really circulating in perpetual orbits of vary- 
ing magnitude, according to the resultants of impulsions received by 
mutual impacts on their dyuaspheres, or on the mystic boundaries of 
their elastic repulsions: in orbits whose magnitude determines the 
character of their aggregation, and which notwithstanding constant 
perturbation, could be suppressed only by the total absorption of mo- 
tion, in an absolute zero of temperature. And thus, whether we con- 
template the infinitely small or the infinitely grand, in every case comes 
back upon us the wide induction, that the action of matter in atom, in 
molecule, or in mass, is ever at a distance! Of actual contact there is 
probably no instance afforded in nature, excepting in the intimate sub- 
stance of the ultimate atom. And notwithstanding the petulant sneer 
of some distinguished physicists, there is nothing to forbid (on the con- 
trary much to favor) the Newtonian conception of infinitely hard spher- 
ules of determinate magnitude, as the real substratum of the physical 
universe. 
The assumption therefore of a material band or bar between bodies, to 
transmit energy from one to another, whether by a pull or by a push, is 
* Optics, Book iii, Query 28. KINETIC THEORIES OF GRAVITATION. 
77 
still very far from establishing an action by contact, or from sustaining 
the fond hypothesis that an atom " acts where it is !v Admitting a con- 
necting rod for drawing the railway train of dynamic, as Professor Max- 
well has remarked, " the action of the rod is explained by the existence 
of internal forces in its substance; and the internal forces are explained 
by means of forces assumed to act between the particles of which the 
rod is composed, that is between bodies at distances which, though 
small, must be finite. The observed action at a considerable distance is 
therefore explained by means of a great number of forces acting between 
bodies at very small distances, for which we are as little able to account 
as for the action at any distance, however great."* 
To the wondering inquiry what possible explanation can be given of 
* A Treatise on Electricity and Magnetism: by James Clerk Maxwell. Oxford, 2 vols. 
8vo, 1873. Parti, chap, v, sec. 105, vol. i, p. 123. 
If the attempts hitherto made by kinetic theorists to explain the tensile strength of 
a rope or of a chain by the pressure of a vis a tergo have been exceedingly lame and 
unsatisfactory, even the more direct examples of actual impact and propulsion are 
really as little serviceable to the hypothesis of contact action. If we have good reason 
to believe that the constituent molecules of a steel bar are actually separated by rela- 
tively large spaces of intense repulsion,-a fortiori must the physical impact of the 
most violent percussion be resolved into an action through a vacant space. Taking 
the case of a steel ball struck suddenly by a steel bat, the interval of distance between 
the first impression of the moving mall and its nearest approach to the ball, is suffi- 
cient to permit the acceleration of motion in the missile through every gradation, from 
zero to its full velocity. 
Perhaps no better " prerogative wf instance " of a physical contact could be sug- 
gested than that of a thick glass plate resting on the convex surface of a large glass 
lens, since the perfectly-ground plane surface of the upper glass and the perfectly- 
ground spherical surface of the lower one are best adapted to exclude a possible film 
of air. Such an arrangement represents the well-known experiment by which Newton 
determined from the measurable variation of distance between the glasses (when 
closely pressed together) the wave-length of light for different colors. On this very 
beautiful experiment Dr. Robison forcibly remarks in his excellent work on Natural 
Philosophy: 
" The conclusion seems unquestionable that we have no proof from the black spot 
between the glasses, that they are in mathematical contact in that place. We know 
by the first experiment that a very considerable force is necessary for producing the 
black spot. A greater pressure makes it broader, and in all probability this is partly 
by the mutual yielding of the glasses. I found that before a spot, whose surface is a 
square inch can be produced, a force exceeding one thousand pounds must be em- 
ployed. When the experiment is made with thin glasses, they are often broken before 
any black spot is producedThere is therefore an essential difference be- 
tween mathematical and physical contact; between the absolute annihilation of dis- 
tance, and the actual pressure of adjoining bodies. We must grant that two pieces of 
glass are not in mathematical contact till they are exerting a mutual pressure not less 
than one thousand pounds per square inch. For we must not conclude that they are 
in contact till the black spot appears ; and even then we dare not positively affirm it. 
My own decided opinion is, that the glasses not only are not in mathematical contact 
in the black spot, but that the distance between them is vastly greater than the 
eighty-nine-thousandth part of an inch, the difference of the distances at two suc- 
cessive rings."-A System of Mechanical Philosophy : by John Robison. 4 vols. 8 vo., 
Edinburgh, 1822, vol. i, sec. 241, 242, pp. 250,251. 78 
KINETIC THEORIES OF GRAVITATION. 
such a theory or hypothesis of attraction, the obvious answer is, there 
is neither "theory" nor "hypothesis" in the case. The observed fact 
that one body does actually induce approach in another body at a dis- 
tance from it, if accepted as an "ultimate" one, is of course thereby ex- 
cluded from all idea of "explanation." And as this observed fact, by 
the multiplicity of instances in which no intermediate agent has yet 
been detected, or rendered rationally plausible, is generalized into an 
" induction," the burden of proof lies entirely on the shoulders of those 
who with a keener vision through nature's veil, are ready to proffer 
hypotheses " without assumptions," and endless motions without the 
aid of " occult qualities." 
But this stern requirement of a demonstrated instance of material 
link, or mechanical connection, to impugn the large induction of an 
actio in distans, cannot be evaded by the logical artifice of designating 
such "induction" as a competing "hypothesis!" If by the scientific 
method of a carefully registered experience such theorists should ever 
be successful in physically justifying their ill-advised scruples, their 
mala fastidia, (to use the phrase of Horace,) none will be more heartily 
rejoiced at this new conquest over ignorance than the astronomers and 
physicists who hitherto have given themselves but small concern re- 
specting the metaphysical paradoxes supposed to be involved in the 
more usual statement of the law of gravitative action. 
But as yet this generalization, after two centuries of busy thought 
and daring speculation, still remains the largest, clearest, surest, yet 
attained by man ; and with each revolving year new demoustrations of 
its absolute precision and of its universal domination serve only to fill 
the mind with added wonder and with added confidence in the stability 
and the supremacy of the power in which has been found no variable- 
ness, neither shadow of turning, but which, the same yesterday, to-day, 
and forever, 
"Lives through all life, extends through all extent, 
Spreads undivided, operates-unspent!"