EXPERIMENTS
AND
OBSERVATIONS
ON
ANIMAL HEAT,
AND THE
INFLAMATION
OF
COMBUSTIBLE BODIES.
BEING AN ATTEMPT
TO RESOLVE THESE PHÆNOMENA
INTO A
GENERAL LAW OF NATURE.
BY ADAIR CRAWFORD, A. M.
PHILADELPHIA:
PRINTED FOR THOMAS DOBSON, IN SECOND-STREET, BETWEEN
MARKET AND CHESNUT-STREET.
M, DCC, LXXXVII. 



TO
JOHN WATKINSON, M. D.
DISTINGUISHED BY
HIS SKILL IN THE ART OF MEDICINE,
BY HIS KNOWLEDGE IN
PHILOSOPHY AND POLITE LITERATURE;
AND BY THOSE
PRINCIPLES OF PROBITY AND HONOUR,
WHICH SECURE THE ATTACHMENTS OF FRIENDSHIP,
AND THE
CONFIDENCE OF SOCIETY,
THESE EXPERIMENTS
AND 
OBSERVATIONS
ARE INSCRIBED,
BY HIS MOST SINCERE FRIEND,
AND OBLIGED HUMBLE SERVANT,
THE AUTHOR.
A2

ADVERTISEMENT.
IT is proper to apprize the reader who would wish to repeat
the experiments recited in the following pages, that such experi-
ments are liable to be affected by a variety of adventitious circum-
stances, so minute as to require the most attentive observation.
A change in the temperature of the air in the room, a variation
in the time that is employed in mixing together the substances
which are to have their comparative heats determined, a difference
in the shape of the vessel, or in the degree of agitation that is
given to the mixture, will often produce a considerable diversity
in the result of the same experiment. It is possible however, by a
series of repeated trials, with well constructed thermometers, in
the same circumstances, to make a very near approximation to the
truth, and, from a coincidence of facts, to obtain a degree of evi-
dence, on which the mind may rest with entire confidence.
Much time and labour have been employed in endeavouring
to render the following experiments accurate: But if, after all,
some mistakes of less moment should be discovered, it is hoped
that the candour of the public will excuse them, as the author
is persuaded, that the facts from which the general conclusions
have been drawn, are well founded. 

EXPERIMENTS
AND
OBSERVATIONS
UPON
ANIMAL HEAT, &c.
THE words heat and fire are ambiguous. Heat
in common language, has a double signification.
It is used indiscriminately to express a sensation of the
mind, and an unknown principle, whether we call it
a quality or a substance, which is the exciting cause
of that sensation.*
By many ingenious philosophers, in modern times,
the word Heat has been applied to the unknown
principle, and has been taken in a much greater ex-
tent than in common language. For, in common
language, it is used to express such a degree of the
unknown external cause, as will produce a given ef-
fect upon the senses: but in the philosophical accep-
tation,
* See Dr. Ried's Inquiry into the Human Mind.

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tation, it expresses the external cause in the abstract,
without regard to the effects which it may produce.
For the sake of greater accuracy, in the following
Dissertation, I shall, with the ingenious Dr. Irvine
of Glasgow, call the latter absolute heat; and what
is denominated heat in common language, I shall
call sensible heat.
From this view of the matter, it appears, that ab-
solute heat expresses that power or element, which,
when it is present to a certain degree, excites in all
animals the sensation of heat; and sensible heat ex-
presses the same power, considered as relative to the
effects which it produces.
Thus we say, that two bodies have equal quanti-
ties of sensible heat, when they produce equal ef-
fects upon the mercury in the thermometer; and
that the same body has a greater or less degree of
sensible heat, according as is produces a greater or
a less effect upon this fluid.
But it will hereafter appear, that bodies of differ-
ent kinds, have different capacities for containing
heat; and, therefore, in such bodies, the absolute
heat will be different, though the sensible heat be
the same. If a pound of water, for example, and
a pound of antimony, have the same temperature,
we say that their sensible heats are equal. But we
shall find, that the former contains a much greater
quantity of absolute heat than the latter.
Fire, in the vulgar acceptation of the word, ex-
presses a certain degree of heat, accompanied with
light; and is particularly applied to that heat and
light which are produced by the inflammation of
combustible bodies. But as heat, when accumulat-
ed in a sufficient quantity, is constantly accompanied
with light, or, in other words, as fire is always pro-
duced by the increase of heat, philosophers have ge-
nerally considered these phenomena as proceeding
from 

(7)
from the same cause;* and have, therefore, used
the word fire to express that unknown principle,
which, when it is present to a certain degree, excites
the sensation of heat alone, but when accumulated
to a greater degree, renders itself obvious both to
the sight and touch, or produces heat, accompanied
with light.
In this sense, the element of fire signifies the same
thing with absolute heat.
These definitions and remarks being premised, I
shall proceed to give the reader a concise view of
the general facts upon which the experiments, re-
cited in the following pages, are founded.
1. Heat is contained in great quantities in all bo-
dies, when at the common temperature of the at-
mosphere.
In the deserts of Siberia, as we learn from Baron
Demidoff, the mercury sometimes falls an hundred
and fifty degrees below the freezing point. This is
the most intense natural cold that has ever been
known. But a much greater degree of cold has
been produced by art. At Petersburg, in the year
1759, the heat was so much diminished by a mix-
ture of snow and spirit of nitre, in the time of a se-
vere frost, that the spirit of wine thermometer sunk
to 148, and the mercurial thermometer to 352 de-
grees below the beginning of Farenheit's scale. As
in this experiment the mercury was frozen, and as
before its congealation, it contracted suddenly and
irregularly, it has been concluded by Dr. Black and
Dr. Irvine, that the cold which was then produced,
was such as was indicated by the spirit of wine ther-
mometer; and, therefore, 148 degrees below 0 is
considered as the freezing point of mercury. This
is the greatest cold that has ever been observed in
nature;
* See Boerhaave, Martin M'Quer, &c. 

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nature; and yet we have no reason to believe that
the bodies which were exposed to it, were deprived
of their whole heat. From these facts, however,
we may conclude with certainty, that heat is con-
tained in great quantities in all bodies, when at the
common temperature of the atmosphere.
2. Heat has a constant tendency to diffuse itself
over all bodies, till they are brought to the same de-
gree of sensible heat.
Thus, it is found by the thermometer, that if two
bodies, of different temperatures, are mixed toge-
ther, or placed contiguous, the heat passes from the
one to the other, till they both come to the same
temperature; and that all inanimate bodies, when
heated, and placed in a cold medium, continually
lose heat, till, in process of time, they are brought
to the state of the surrounding medium.
3. If the parts of the same homogeneous body
have the same degree of sensible heat, the quantities
of absolute heat will be proportionable to the bulk
or quantity of matter. Thus the quantity of abso-
lute heat contained in two pounds of water, must
be conceived to be double of that which is contain-
ed in one pound, when at the same temperature.
This I think is evident from the similarity in the par-
ticles of the same homogeneous solid and fluid sub-
stances. For the particles being similar, their pow-
ers will be equal; their capacities for receiving heat
will be the same; and therefore the quantities of ab-
solute heat which they contain, will be in proportion
to the bulk or quantity of matter.
4. The mercurial thermometer is an accurate
measure of the comparative quantities of absolute
heat, which are communicated to the same homoge-
neous bodies, or separated from them, as long as
such bodies continue in the same form.
To 

(9)
To illustrate this, let us suppose two equal parts of
mercury, or of any other fluid. And let the part
A be raised to a higher temperature than the part
B, A will therefore contain a greater quantity of ab-
solute heat than B. Let them now be mixed; and
according to the second general fact, they will arrive
at the same common temperature. But they cannot
come to the same temperature, unless A communi-
cates to B, one half of the excess of its heat above
the original heat of B.
[illustration]
For, let the whole heat con- 
tained in A, previous to the mix- 
ture, be represented by the fi- 
gure, a b c. And let the heat 
contained in B be represented 
by f g h. When they are 
mixed together and brought to 
the same temperature, let the 
heat of A be diminished by the 
figure a l m c, and let that of 
B be increased by the figure 
i f h k. And since the heat which is taken from A
is the very same with that which is added to B, it is
manifest that a l m c must be equal to i f h k. Now
it is affirmed, that a l m c, or i f h k, is equal to l d e
m, or in other words, that A communicates to B
one half of the excess of its heat above the original
heat of B. For from the third general fact, it is e-
vident, that when A and B are brought to the same
temperature, they contain equal quantities of abso-
lute heat. Therefore the figure i g k, is equal to the
figure l m b. For the same reason the figure f g h,
is equal to the figure d b e. And therefore the re-
mainder i f h k, or a l m c, is equal to the remain-
der l d e m.
If, therefore, a mercurial thermometer, of the
same temperature with a quantity of water, the ab-
B solute

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solute heat of which is represented by d b e, be im-
mersed in the same water, when its absolute heat is
represented by l b m, and be again immersed in it,
when its absolute heat is represented by a b c; and
if the expansions of the mercury be in these in-
stances, in the ratio of one to two, it is evident
that the expansions, and the increments of absolute
heat, will be in exact proportion.
A variety of experiments were made upon this
principle by Mons. De Luc, from which it appears,
that the expansions of mercury between the freezing
and boiling points of water, correspond precisely to
the quantities of absolute heat applied.
The mercurial thermometer therefore is an accu-
rate measure of the comparative quantities of abso-
lute heat, which are communicated to the same ho-
mogeneous bodies, or separated from them, as long
as such bodies continue in the same form.
It has been already shown, that in the same ho-
mogeneous bodies, if the quantities of matter be
different, but the temperatures the same, the quan-
tities of absolute heat will be, in proportion to the
quantities of matter.
It now appears, that in the same homogeneous
bodies, if the temperatures be different, but the
quantities of matter the same, the quantities of ab-
solute heat will be in proportion to the tempera-
tures, or to the expansions in the mercurial ther-
mometer.
For let the whole of the absolute 
heat contained in a pound of ice, at
the temperature of o, the beginning 
of Farenheit's scale, be represented by 
the figure, a b c d, and let this heat 
be diminished till it becomes equal to
e b c f, which is the one half of a b c d. If a mer-
curial thermometer were deprived of its whole heat,
and

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and applied to the ice when its heat is represented by
e b c f, and were again applied to the same ice when
its heat is represented by a b c d; the expansion pro-
duced by the heat e b c f would be to that produ-
ced by a b c d, in the ratio of one to two*. If a
b c d were triple of e b c f, the expansions would
be in the ratio of one to three; and, universally,
the temperatures being different, but the quantity
of matter the same, the quantities of absolute heat
will be in proportion to the temperatures, as mea-
sured by the mercurial thermometer.
This conclusion is an inference from De Luc's ex-
periments, which prove, as was observed above, that
the expansions of mercury are in proportion to the
increase of the absolute heat, and its contractions to
the diminution of this element, in all the interme-
diate degrees, between the boiling and freezing
points of water: from whence it is inferred, that if
the mercury were to retain its fluid form, its con-
tractions would be proportionable to the decrements
of the absolute heat, though the diminution were
continued to the point of total privation.
COROLLARY. If therefore the sensible heat of a
body, as measured by the mercurial thermometer,
were to be diminished the one half, or the one third,
or in any given proportion, the absolute heat would
be diminished in the same proportion.
5. The comparative quantities of absolute heat
which are communicated to different bodies, or se-
parated from them, cannot be determined in a direct
manner by the thermometer. Thus if the tempera-
ture of a pound of mercury be raised one degree,
and that of a pound of water one degree, as indi-
cated by the thermometer, it does not by any means
B2 follow,
* It is here supposed that the mercury continues fluid when it is de-
prived of its whole heat. 

(12)
follow, that equal quantities of absolute heat have
been communicated to the water and the mercury.
It has been thought by some philosophers, that
the quantities of absolute heat in bodies, are in pro-
portion to their densities. Boerhaave was of opini-
on, that heat is equally diffused thro' all bodies, the
densest as well as the rarest, and therefore that the
quantities of heat in bodies are in proportion to their
bulk.
But it appears from experiment, that the law of
the distribution of heat throughout the various clas-
ses of natural bodies, is not according to the ratio
either of bulk or density.
The first attempt to determine by experiment the
comparative quantities of absolute heat in bodies,
was made by Farenheit, at the desire of Boerhrave.
The following is a short sketch of this attempt,
nearly in the words of the author:
If you take equal quantities of the same fluid,
and give them different degrees of heat, and mix
them intimately together, the temperature of the
mixture will be half the excess of the hotter above
the colder,
If, for example, a pint of boiling water at 212,
be mixed with a pint of the same fluid at 32, the
temperature of the mixture will be 122: the warm
water will be cooled 90 degrees, and the cold water
heated 90.
But if this experiment be made with water and
mercury, in the same circumstances, the effect will
be very different.
For if you take equal bulks of mercury and wa-
ter, and give the water a greater degree of heat than
the mercury, the heat of the mixture will always be
greater than half the excess of the heat of the water
above that of the mercury.
On 

(13)
On the other hand, if the mercury be hotter than
the water, the temperature of the mixture will con-
stantly be less than half the excess of the heat of the
mercury above that of the water. The changes which
are produced in the temperature of the water and
mercury, in the first of these instances, are found to
correspond to those which are produced, by mixing
three parts of hot water with two of cold; and in
the second instance, to those which take place, when
three parts of cold water are mixed with two of hot.
That is, the change produced in the heat of the mer-
cury, is to that produced in the heat of the water, as
three to two.
From the former of these experiments it was justly
concluded by Boerhaave, that in the same body, the
distribution of fire is in proportion to the bulk or
quantity of matter.
But from the experiments with water and mercury,
he concluded very unwarrantably, that heat is equal-
ly diffused thro' all bodies, the densest as well as the
rarest; and therefore that the quantities of heat in
different bodies, are in proportion to their bulks, or
to the spaces which they occupy.
These experiments have been repeated and varied
in the present age, and very different conclusions
have been drawn from them. 
I have already observed, that, if a pint of mercury
at 100, be mixed with an equal bulk of water at 50,
the change produced in the heat of the mercury, will
be to that produced in the heat of the water, as three
to two; from which it has been inferred, that the
absolute heat of a pint of mercury is to that of an
equal bulk of water, as two to three: or, in other
words, that the comparative quantities of their abso-
lute heats are reciprocally proportionable to the chan-
ges 

(14)
ges which are produced in their sensible heats, when
they are mixed together at different temperatures*.
The truth of this conclusion may be illustrated in
the following manner.
If four pounds of diaphoretick antimony at 20, be
mixed with one pound of ice at 32, the temperature
of the mixture will be very nearly 26. The ice will
be cooled six degrees, and the antimony heated six.
If we reverse the experiment, the effect will be the
same. That is, if we take six degrees of heat from
three pounds of antimony, and add it to a pound of
ice, the latter will be heated six degrees. The
same quantity of heat, therefore, which raises a
pound of ice six degrees, will raise three pounds of
antimony six degrees.
If this experiment be made at different tempera-
tures, we shall have a similar result. If, for exam-
ple, the antimony at 15, or at any given degree be-
low the freezing point, be mixed with the ice at 32,
the heat of the mixture will be half the excess of
the hotter above the colder. From hence we infer,
that the result would be the same, if the antimony
were deprived of its whole heat, and were mixed
with the ice at 32. But it is evident, that, in this
case, the ice would communicate to the antimony
the half of its absolute heat†. For, if 200 below
frost, be conceived to be the point of total priva-
tion, the antimony will be wholly deprived of its
heat, when its temperature is diminished 200 degrees
below 32; and the heat contained in the ice, when
at
* This fact has, for several years, been taught publicly by Dr. Black
and Dr. Irvine, in the Universities of Edinburgh and Glasgow: and it
has been applied by Dr. Irvine, to the solution of a variety of curious
and important phenomena.
It is hoped, that those learned and ingenious philosophers will soon
favour the world with their respective discoveries in this branch of sci-
ence.
† See the Corollary to the 4th general fact. 

(15)
at 32, will be 200 degrees. If we now suppose
them to be mixed together, the temperature of the
mixture will be half the excess of the hotter above
the colder; or the ice will be cooled 100 degrees,
and the antimony heated 100. The one half of the
heat, therefore, which was contained in the ice,
previous to the mixture, will be communicated to
the antimony: from which it is manifest, that, after
the mixture, the ice and antimony must contain e-
qual quantities of absolute heat.
To place this in another light, it has been proved,
that the same quantity of heat which raises a pound
of ice six degrees, will raise three pounds of antimo-
ny six degrees. From which it is inferred, that the
same quantity of heat which raises the ice 200 de-
grees, or any given number of degrees, will raise the
antimony an equal number of degrees *.
A pound of ice, therefore, and three pounds of
antimony, when at the same temperature, contain
equal quantities of absolute heat. But, it appears
from the third general fact, that three pounds of an-
timony contain three times as much absolute heat, as
one pound of antimony; and hence the absolute heat
of a pound of ice, is to that of a pound of antimony,
as three to one.
Again, if a pound of ice at 32, be mixed with a
pound of antimony at 12, the temperature of the
mixture will be 27; the ice will be cooled five de-
grees, and the antimony heated 15; or the change
produced in the sensible heat of the ice, will be to
that produced in the sensible heat of the antimony,
as one to three. But it was before proved, that the
absolute heat of a pound of ice, is to that of a pound
of antimony, as four to one. From which it is evi-
dent, that the comparative quantities of absolute heat,
in
* It is here supposed, that the ice continues in the same form. 

(16)
in equal weights of ice and antimony, are recipro-
cally proportionable to the changes which are produ-
ced in their sensible heats, when they are mixed to-
gether at different temperatures*.
Thus it appears, that the comparative quantities
of absolute heat in bodies may be determined, by
mixing them together as above, and observing the
changes which are produced in their sensible heats.
This rule, however, does not apply to those substan-
ces, which, in mixture, excite sensible heat or cold
by chymical action.
From the reasoning which was employed to esta-
blish the above proposition, it follows, that equal
weights of heterogeneous substances, as air and wa-
ter, having the same temperature, may contain un-
equal quantities of absolute heat. There must, there-
fore, be certain essential differences in the nature of
bodies, in consequence of which same have the pow-
er of collecting and retaining the element of fire,
in greater quantities than others. These different
powers are called in the following pages, the capaci-
ties of bodies for containing heat. Thus, if we find
by experiment, that a pound of water contains three
times as much absolute heat, as a pound of antimo-
ny, the capacity of water for containing heat, is said
to be to that of antimony, as three to one.
SECT.
* I have thus endeavoured briefly to establish the truth of the above
doctrine, as a necessary introduction to the experiments which follow,
but the more full and complete illustration of it I shall leave to Dr.
Black and Dr. Irvine.
This discovery opens a wide field for investigation, as by means of it
we are enabled to estimate the comparative quantities of absolute heat
in bodies, and to determine with certainly and accuracy, the various
proportions in which the element of fire is distributed throughout the
different kingdoms of Nature. 

(17)
SECT. II.
HAVING premised these general facts, I shall
now lay before the reader my Experiments on Ani-
mal Heat, and the Inflammation of Combustible
Bodies.*
It was observed above, that sensible heat has a
constant tendency to diffuse itself equally over all
bodies, till they are brought to the same tempera-
ture. From this property of heat, it is manifest,
that those animals which have a higher temperature
than the medium in which they live, must be con-
tinually communicating heat to the surrounding bo-
dies. Since therefore, in the animal kingdom, there
is a constant dissipation of heat, it follows, that there
must be a proportionable supply of this element, to
repair the waste. For, if the animal body had not
the power of exciting or collecting heat, it would
soon arrive at the temperature of the ambient me-
dium.
With a view to discover the nature of this power,
I made a variety of experiments, in the summer,
1777, on animal, vegetable, and mineral substances;
C some
* I think it proper to observe, that I was led to the consideration of
this subject, in consequence of having attended the chymical Lectures
of the learned and ingenious Dr. Irvine of Glasgow. And it is a tribute
of justice which I owe to this philosopher, to acknowledge, that the so-
lution which he has given of Dr. Black's celebrated discovery of latent
heat, or of the heat which is produced by the congelation of water,
spermaceti, bees-wax, metals, &c. suggested the views which gave rise
to my experiments.
I must also add, that Dr. Irvine, from the solution which he has
given of the above phenomena, and from the general fact, that hetero-
geneous bodies have different capacities for containing heat, concluded,
that there was a difference between the absolute heat of fixed and at-
mospherical air. It remained to ascertain this difference, and to deter-
mine by experiments, whether fixed or atmospherical air, contained the
greater quantity of absolute heat. 

(18)
some of which I shall now relate, as I think they
have led to the true source, from whence the heat
of animals, and the heat which is produced by the
inflammation of combustible bodies, is derived.
I must first observe, that experiments for deter-
mining the comparative quantities of absolute heat
in bodies, are liable to three causes of inaccuracy.
1. When the substances to be compared, are mixed
together, a certain time is required for the heat to
pass from the warmer to the colder body, till they
arrive at the same temperature. If they be intimately
mixed, and the vessel be a little agitated, a minute
is generally sufficient. During this interval, a part
of the heat is carried off by the surrounding atmos-
phere. It therefore becomes necessary, to calculate
the heat which is thus lost in the first minute.
A very simple and ingenious rule was laid down
for this purpose, by the illustrious Sir Isaac New-
ton.
Considering the heat in a body, as the excess
whereby it is warmer than the surrounding medium,
he supposed, that the quantities of heat lost in small
portions of time, would always be proportionable
to the heats remaining. Thus, if a body were 180
degrees hotter than the atmosphere, the quantity of
heat which it would lose in a given moment, would
be double of that which it would lose in an equal
portion of time, if it were only 90 degrees hotter
than the atmosphere. And, therefore, if the times
be taken in arithmetical progression, the decrements
of heat will be in geometrical progression. But, it
has been observed by Dr. Martine, that this rule is
not to be admitted without some restriction. He has
inferred from a variety of experiments, that the dif-
ferential decrements are in a proportion somewhat
greater than the inherent quantities of heat; and,
there- 

(19)
therefore, that the quantities of heat lost, are part-
ly equable, and partly in geometrical progression*.
This observation of Dr. Martine, it must be allow-
ed, is well founded. But when the body, which is
to be the subject of an experiment, transmits heat
very fast, and its temperature is much greater than
that of the ambient medium, the error produced,
by calculating according to Sir lsaac Newton's rule,
is so very inconsiderable, that it may be neglected.
On the contrary, when the experiment is made
in a vessel that transmits heat very slowly, and the
heat of the substance to be examined is not much
greater than that of the atmosphere, the quantities
of heat lost in any two small successive portions of
time, approach so nearly to an equality, that the
difference cannot be distinguished by the nicest ther-
mometer. That this observation is well founded,
appears from the following experiment:
A pound of water in an earthen vessel, being
raised to 120, the temperature at the end of
1 minute was 119
2 118
3 ll7
4 116
5 115
6 114
7 113
In the experiments which I shall hereafter relate,
the order of cooling was observed for several mi-
nutes; and, in general, the heat which was lost in
the first minute, was calculated according to Sir
Isaac Newton's rule, from the series of numbers
determined by observation.
I must here observe, that I believe Dr. Irvine was
the first who applied this rule of Sir Isaac Newton,
C2 to
* See Martine's Essays. 

(20)
to calculate the heat lost during the first minute, in
experiments for determining the absolute heat of
bodies.
2. If, in making these experiments, the substance
which has the greatest heat, be mixed with that
which has the least, in a cold vessel, a part of the
heat will be communicated to the vessel: But if the
experiment be made in a warm vessel, the cold sub-
stance will receive heat, not only from the warm
substance, but from the vessel in which it is con-
tained.
The most effectual method of avoiding this cause
of inaccuracy, is first to determine the capacity of
the vessel for receiving heat, compared with that of
one of the substances to be examined.
The relative capacity of the vessel, in which some
of the following experiments were made, compared
with that of water, were thus determined.
The air in the room being 61
A pound of water at 168
was poured into an earthen vessel at 68
The temperature of the water at the end of
1 minute was 155
2 150
3 145
To discover the heat communicated to the atmos-
phere in the first minute, say as 84 to 89 so is 94 to
a fourth proportional, which gives 99.5. From
which it appears that 5.5 were carried off by the
air in the first minute, adding 5.5 to 155 we have
160.5 for the true temperature of the water and of
the vessel.
Subtracting this from 168, we have 7.5 for the
quotient. The water was therefore cooled by the
vessel 7.5, and the vessel was heated by the water
92.5. And since the vessel received this heat from
the water, it is manifest, that the same quantity of
heat, 

(21)
heat, which changes the temperature of a pound of
water, 7.5 will change the temperature of the vessel
92.5. And by a parity of reasoning, the same heat
which raises the water one degree, will raise the ves-
sel 12⅟₃. If, therefore, in any experiment, we find
that the vessel has received 12⅟₃ of heat from a pound
of water, we may be sure that the separation of this
heat, has cooled the water one degree.
3. In experiments for determining the absolute
heat of bodies, water is generally the standard.
When the body which is to have its heat compared
with that of water, transmits heat very slowly, it
frequently happens, that the different parts of the
mixture, cannot be brought precisely to the same
temperature for several minutes. Thus I find that
in experiments upon vegetables and the calves of
metals, there is a considerable difference, at the end
of the first minute, between the heat of the mixture
at the surface, and that at the bottom of the vessel;
arising partly from the tendency of the warmest part
of the water to remain at the surface, and partly from
the difficulty with which the above-mentioned substan-
ces transmit heat. This may be remedied in some
degree, by mixing the bodies together intimately,
and agitating the mixture briskly. But by much a-
gitation, the heat is carried off suddenly and irregu-
larly, which makes it difficult to calculate the heat
that is lost in the first minute.
The method which I apprehend, least liable to
error, is to agitate the mixture moderately, and at the
end of the first minute, to take the arithmetical mean
between the heat at the surface, and that at the bot-
tom of the vessel; and if in a variety of experiments,
in different circumstances, we find that the result is
nearly the same, we may be sure that we have ap-
proached very near the truth.
With

(22)
With these precautions, the following experiments
were made, to determine the absolute heat of some
of the most common vegetable and animal sub-
stances, compared with that of water.
EXPERIMENT I.
Air in the room 69
A pound of wheat at 66
was mixed with a pound of water at 166
The mixture being agitated for a short time,
The temperature at the end of
surface bottom medium
1 minute was 138 128 133
2 134 125 129⅟₂
3 131 122 126⅟₂
4 127 120 123⅟₂
The mean temperature of the mixture at the end
of one minute was 133, at the end of two minutes
129⅟₂, at the end of three minutes 126⅟₂: And there-
fore the heat carried off by the air in the first minute,
being calculated according to Sir Isaac Newton's rule,
was very nearly 3⅟₂. If we add this to 133, we shall
have 136⅟₂ for the true temperature of the mixture.
It was proved that the capacity of the vessel for re-
ceiving, heat, was to that of the water as 1 to 12⅟₃.
In the experiment which we are now considering,
the vessel was raised from 66 to 133, or 67.5; di-
viding this by 12⅟₃, we have nearly 5.5 for the quo-
tient. From which it appears, that the water was
cooled by the vessel 5.5, or the vessel separated 5.5
degrees from the water. The true temperature of
the mixture was 136⅟₂, subtracting this from 166, we
have 29⅟₂ for the remainder. The water was there-
fore cooled 29⅟₂ by the wheat and the vessel together.
But we have shown that it was cooled 5.5 by the ves-
sel; it was therefore cooled 24 by the wheat. But
the wheat was raised from 66 to 136⅟₂ or 70.5. It
follows, 

(23)
follows, that the same quantity of heat which will
change the temperature of water 24, will change
that of wheat 70⅟₂. Therefore the absolute heat of
water is to that of wheat, as 70⅟₂ to 24, or very
nearly as 2.9 to 1.
EXPERIMENT II.
Air in the room 60.
One pound of oats, having the hulls
taken off, at 61,
was mixed with one pound of water, at 161;
The temperature of the mixture at the end of
surface bottom medium
1 minute was 127 123 125
2 125 122 123⅟₂
3 121 121 121
4 118 118 3/4
5 114 116
6 111 115
To 125, the mean temperature of the mixture,
adding 2⅟₂ for the heat carried off by the air in the
first minute, we have 127⅟₂ the true temperature of
the mixture.
By calculating, as in the preceding experiment, it
appears that the water was cooled by the vessel nearly
5.4. The oats were raised from 61 to 127⅟₂ or 66⅟₂;
subtracting 127⅟₂ from 161, we find that the water
was cooled by the oats and the vessel together 33⅟₂;
but it was cooled 5.4 by the vessel; it was therefore
cooled 28.1 by the water: and hence the absolute
heat of water is to that of oats, as 66.5 to 28., 0
as 2.36 to 1, that is, nearly, as 2⅟₃ to 1.
EXPERIMENT III.
Air in the room 59.
One pound of beans at 60,
was mixed with one pound of water at 160
The 

(24)
The temperature of the mixture at the end of
surface bottom medium
1 minute was 119 113 116
2 117 109 113
3 116 107 111⅟₂
4 113 106 109⅟₂
5 111 105 108
6 109⅟₂ 104⅟₂ 107
7 107⅟₂ 104 106₂/₄ 
11 101 101 101
To 116 adding 3 degrees for the heat carried off
by the air in the first minute, we have 119 for the
temperature of the mixture. It appears, from cal-
culation, that the water was cooled by the vessel 4.8.
The beans were raised from 60 to 119, or 59 de-
grees, subtracting 119 from 160, we find that the
water was cooled, by the beans and the vessel toge-
ther, 41 degrees. But it was cooled by the vessel
4.8. It was therefore cooled by the beans 36.2.
And hence the absolute heat of water is to that of
beans as 59 to 36.2, or nearly as 1.6 to 1.
EXPERIMENT IV.
Air 61.
A pound of barley at 60,
was mixed with a pound of water at 160;
The temperature of the mixture at the end of
surface bottom medium
1 minute was 126 120 123
2 122 115 118⅟₂
3 119 113 116
6 109 109 109
To 123, the mean temperature, adding 4.⅟₂ for
the heat carried off by the air in the first minute,
we have 127⅟₂ the true temperature of the mixture.
The vessel was raised by the water 66⅟₂, dividing this
by 12⅟₃, we have 5.3 for the quotient; by which it
appears

(25)
appears that the water was cooled by the vessel 5.3.
The barley was raised from 60 to 127⅟₂ or 67⅟₂. The
water was cooled by the vessel and the barley toge-
ther, from 160 to 127⅟₂, or 32⅟₂. But it was cooled
5.3 by the vessel. It was therefore cooled 27.2 by the
barley: and hence the absolute heat of water is to
that of barley as 67.5 to 27.2, or nearly as 2.4 to 1.
EXPERIMENT. V.
Air 60.
One pound of the lungs of a sheep at 59,
was mixed with a pound of water at 149;
The mixture at the end of
surface bottom medium
1 minute was 112 108 110
2 109 106 107⅟₂
3 106 105 105⅟₂
4 103 103 103
Adding 2.5 for the heat carried off by the air in
the first minute, we have 112.5 for the true temper-
ature of the mixture,
The vessel was raised from 59 to 112.5, or 53.5.
Dividing this by 12⅟₃, we have nearly 4.3 for the
quotient. From which it appears, that the water
was cooled by the vessel 4.3. The flesh was raised
from 59 to 112.5, or 53.5. The water was cooled
by the flesh and the vessel, from 149 to 112.5, or
36.5; but it was cooled 4.3 by the vessel; it was,
therefore, cooled 32.2 by the flesh. And hence the
absolute heat of water is to that of flesh, as 53.5 to
32.2; or, nearly as 1.3 to 1.
EXPERIMENT VI.
Air 60,
One pound of milk at 60.
was mixed with a pound of water at 160;
D The

(26)
The temperature at the end of
surface bottom medium
1 minute was 109 107 108
2 106 104 105
3 103 102 102⅟₂
To 108, the mean temperature, adding 2.8, we
have 110.8, the true temperature of the mixture.
The vessel was raised by the water 50.8. The
water was therefore cooled by the vessel 4.1. But
it was cooled by the milk and the vessel together
49.2. It was consequently cooled by the milk 45.1.
The milk was raised from 60 to 110.8, or 50.8.
And hence the absolute heat of water is to that of
milk, as 50.8 to 45.1; or, nearly as 1.1 to 1.
EXPERIMENT VII.
Half a pound of water at 47,
was mixed with a half pound of blood at 98;
Having agitated the vessel, the heat was in a short
time nearly equally diffused over the whole, and the
mixture continuing fluid for the space of two mi-
nutes, its temperature at the end of
1 minute was 71
2 70 coagulated.
3 70
4 70
5 70
6 69
The blood, which was the subject of this experi-
ment, was procured from a sheep, by dividing the
veins and arteries of the neck; and appeared by its
colour to be a mixture of venous and arterial blood,
though consisting chiefly of the latter. The capaci-
ty of the vessel in which this experiment was made,
for containing heat, was to that of the water, as 6
to 94; or, nearly as 1 to 15.6.
The 

(27)
The mixture cooled (if we except the time of its
coagulation) at the rate of one degree in a minute.
Adding one degree for the heat lost in the first mi-
nute, we have 72 for the heat of the mixture. The
vessel was raised by the blood, from 47 to 72, or
25 degrees. The blood was therefore cooled by the
vessel nearly 1.6: it was cooled by the water and
the vessel together 26 degrees. It was therefore
cooled by the water 24.4. The water was raised 25
degrees. And hence the absolute heat of a mixture
of venous and arterial blood is to that of water, as
25 to 24.4.
These experiments prove, in general, that flesh,
milk, and vegetables, contain less absolute heat than
water, and water less than blood. Blood, therefore,
contains a greater quantity of absolute heat, than
the principles of which it is composed.
The remarkable accumulation of heat in this fluid,
led me to suspect, that it absorbs heat from the air,
in the process of respiration. And in this suspicion,
I was much confirmed by the following considera-
tions:
1. Those animals which are furnished with lungs,
and which continually inspire the fresh air in great
quantities, have the power of keeping themselves at
a temperature considerably higher than the surround-
ing atmosphere. But animals that are not furnished
with respiratory organs, are very nearly of the same
temperature with the medium in which they live.
2. Among the hot animals, those are the warmest,
which have the largest respiratory organs, and which
consequently breathe the greatest quantity of air in
proportion to their bulk. Thus, the respiratory or-
gans of birds, compared with their size, are more
extensive than those of any other animal; and birds
have the greatest degree of animal heat.
D2 3. In

(28)
3. In the same animal, the degree of heat is in
some measure proportionable to the quantity of air
inspired in a given time. Thus, we find that animal
heat is increased by exercise, and by whatever acce-
lerates respiration.
From these considerations, I was naturally led to a
more particular examination of this subject: the re-
sult of which, is comprehended in the following pro-
positions:
PROPOSITION I.
Atmospherical air contains a greater quantity of
absolute heat, than the air which is expired from the
lungs of animals; and the quantity of absolute heat
contained in any kind of air that is fit for respiration,
is very nearly in proportion to its purity, or to its
power in supporting animal life.
Before I proceed to the direct proof of this propo-
sition, it is necessary to consider the nature of the
change which the air undergoes in the lungs. It is
well known, that the air expired from the lungs,
occasions a precipitation in lime water. A part of it,
therefore, consists of fixed air. It has been found,
by Dr. Priestley, that the residuum of this air, is a
mixture of atmospherical, and what he has called phlo-
gisticated air, a species of air which occasions no pre-
cipitation in lime water, but which extinguishes flame,
and is noxious to animal life.
That the fixed air produced in respiration, depends
upon a change, which the atmospherical air under-
goes in the lungs, is, I think, evident, from the fol-
lowing facts.
Air is altered in its properties by phlogistic pro-
cesses, and though many of these processes are total-
ly different from each other, yet the change produced
in the air, is in all cases very nearly the same. It
is diminished in its bulk. It is rendered incapable of
main- 

(29)
maintaining flame, and of supporting animal life.
And, if we except a very few instances, where the
fixed air is absorbed, it universally occasions a preci-
pitation in lime water. We have therefore reason to
believe, that there is no instance of a phlogistic pro-
cess in nature, which is not accompanied with the
production of fixed air.
It may be supposed, indeed, that this air is dischar-
ged from the substance which furnishes the phlogi-
ston.
In some cases, a part of the fixed air may proceed
from this source, as in various instances of combus-
tion and putrefaction. But there are other cases, in
which we are certain, that the fixed air is derived
from a change produced in the atmospherical air.
Thus, air diminished by the burning of alcohol
and sulphur, occasions a precipitation in lime water.
The same effect is produced, when atmospherical air
is diminished by nitrous air, and when it is exploded
with inflammable air; and yet it is not found that
brimstone, alcohol, nitrous or inflammable air,
contain fixed air.
But Dr. Priestley has given us the most decisive
proof of this fact, in the following experiment.
Having caused the electric spark to pass through a
glass tube, the lower part of which contained some
water tinged with turnsol, he observed, that the blue
colour of the liquor was, in a few minutes, changed
to red, and that the included air was diminished in
its bulk, and rendered highly noxious. He likewise
observed, that when the spark was taken in air over
lime water, the lime was precipitated*.
Since, therefore, a quantity of fixed air, is, in
these processes, produced by a change in the atmos-
pherical air, we may conclude by induction, that
the
* See Dr. Priestley's experiments and observations upon air, Vol. I. 

(30)
the same effect is produced in every other phlogistic
process.
Thus it appears, that in respiration, atmospherical
air is converted into fixed and phlogisticated air.
It is therefore necessary, in order to determine the
truth of the proposition, that we should compare the
absolute heat of fixed and phlogisticated air, with
that of atmospherical air.
The following experiments were made to deter-
mine the heat of these different species of air.
EXPERIMENT I.
Air in the room 52.
A bladder containing a pint of atmospherical air
at 102, was immersed in a pint of water at 52.
The heat of the water at the end of
surface bottom
1 minute was 53 52⅟₄
2 53 52⅟₂
3 53⅟₄ 53
4 53⅟₄ 53⅟₄
In the above experiment, the air and the water
do not seem to have been brought to the same tem-
perature, till the end of 4 minutes. That this was
really the case, is proved by the following experi-
ment.
EXPERIMENT II.
Air in the room 64. A pint of water was taken,
the temperature of which, was 63 at the surface, and
62⅟₃ at the bottom. A pint of atmospherical air con-
fined in a bladder, was raised to 163. The bladder
containing the air being immersed in the water the
heat was determined by two thermometers, one of
which was placed near the surface of the water, in
contact with the bladder and the other near the bot-
tom.
At 

(31)
At the end of 1 minute the thermometer at the
surface was 67⅟₂, thermometer at the bottom 62⅟₃
2 minutes 65 63
3 65 63⅟₃
4 64⅟₂ 63⅔
5 64⅟₃ 63⅔
In this experiment, which was made with the same
bladder that was used in the former, the heat of the
air in the bladder near the surface, exceeded the heat
of the water for several minutes. And hence we
may perceive the reason why, in the first expe-
riment, the temperature of the water rose gradually
at the surface and the bottom, till the end of 4 mi-
nutes; for during that time, it continued to receive
heat from the air which had been immersed in it.
In that experiment, the heat communicated to the
water by the air, and the bladder which contained it,
was 1⅟₄. Of this quantity of heat, the portion which
was yielded by the bladder, will be seen by the ex-
periment which follows:
EXPERIMENT III.
A pint of water was taken at 52. The bladder
having been dried and freed from air, was raised to
102, and being immersed in the water, the heat of
the water at the end of
surface bottom
1 minute was 52⅟₄ 52
2 52⅟₄ 52⅟₄
We must allow, therefore, one quarter of a degree,
in the experiment in question, for the heat imparted
by the bladder. And hence it follows, that one de-
gree was communicated by the air.
To discover, from the above experiment, the ab-
solute heat of atmospherical air compared with that
of water, we may consider what would have been the
effect produced, if atmospherical air contained the
same 

(32)
same absolute heat with water. In that case, if the
air were only the one hundredth part as dense as wa-
ter, and were raised 100 degrees above the tempera-
ture of the water, it would communicate to it nearly
one degree of heat. If it were only the one eighth
hundredth part as dense, and were raised 100 degrees
above the heat of the water, it would communicate
to it nearly the one eighth part of a degree. If it
were raised only 50 degrees above the heat of the
water, it would communicate the one sixteenth part
of a degree.
Now, the density of atmospherical air is to that
of water, in a proportion somewhat less than that of
1 to 800. If, therefore, in the experiment in que-
stion, the atmospherical air had contained the same
absolute heat with water, it would have communi-
cated to the water, nearly the one sixteenth part of
a degree of heat. But it communicated to it one
entire degree of heat. Atmospherical air must there-
fore contain at least 16 times as much absolute heat
as water.
Dr. Irvine has pointed out a general rule, by
which the comparative quantities of absolute heat in
bodies may be estimated, when the quantities of
matter, and the changes produced in the sensible
heats, are unequal. In that case, the quantities of
absolute heat, are reciprocally as the changes in the
sensible heats, multiplied into the quantities of mat-
ter.
By the help of this rule, the ratio of the heat of
atmospherical air to that of water, may be more
accurately calculated in the following manner.
It is evident, that, as equal bulks of air and wa-
ter were taken, if the densities of the water and air
had been equal, the absolute heat of the air would
have been to that of the water, as 1 to 49, which
is 

(33)
is the reciprocal proportion of the changes produ-
ced in the sensible heats.
Again, if the changes produced in the sensible
heats had been equal, the absolute heats would have
been reciprocally as the quantities of matter. It fol-
lows, that neither being equal, the absolute heats
are in the compound ratio, of the sensible heat gain-
ed by the water, to that separated from the air, and
of the quantity of water to that of the air.
The experiment was made in a quart pewter vessel,
the capacity of which for receiving heat was to that
of the water very nearly as 1 to 16. The quantity
of water was 16 ounces; and therefore, the heat re-
ceived by the vessel was equal to that which would
have been received by the one sixteenth part of 16
ounces, or by one ounce of water. The water and
vessel together were consequently equal to 17 ounces
of water; and the specific gravity of air being to
that of water in the exact proportion of 1 to 862;
it follows that the quantity of matter contained in
17 ounces of water is to that contained in a pint of
air, nearly as 915 to 1. Hence the absolute heat
of air is to that of water in the compound ratio
of 915 to 1, and
of 1 to 49
From which it appears, that the quantity of heat
contained in the former of these elements, is to that
contained in the latter, as 915 to 49, or nearly, as
18.6 to 1.
In calculating this experiment, no allowance was
made for the heat carried off by the external air;
for as the temperature of the water, after the air
was immersed in it, exceeded that of the atmos-
phere, only 1⅟₄, the portion of heat which was thus
carried off was so very small, that it may be ne- 
glected.
E I must 

(34)
I must farther observe, that, from the imperfec-
tion of thermometers, and from the difficulty of
judging by the eye of one fourth or one third of a
degree, perfect accuracy in experiments of this kind
is not to be expected.
The experiment, however, for determining the
heat of atmospherical air, has been frequently repeat-
ed——every repetition has tended to confirm the gene-
ral conclusion, and from the result of a variety of
trials, I have the greatest reason to believe that the
ratio of the heat of atmospherical air to that of water,
as deduced from the above experiment, does not ex-
ceed the truth.
I propose, in future, to endeavour to ascertain,
with as much accuracy as possible, the heat of the
different species's of air, by a greater variety of trials,
and by thermometers constructed for the purpose
I next proceed to determine the heat of fixed and
phlogisticated air.
EXPERIMENT IV.
Air in the room 52. A pint of fixed air, extri-
cated from chalk by the vitriolic acid, was confined
in a bladder, and raised to 104. A pint of water
was taken at 54. The bladder containing the air
being immersed in the water, the temperature of the
water at the end of
surface bottom
1 minute was 54 54
2 54⅟₄ nearly, 54⅟₄ nearly
3 54⅟₄ 54⅟₄ 
The bladder in which this air was contained being
freed from air, and raised 50 degrees above the tem-
perature of a pint of water, communicated to the
water, one fourth of a degree.
This experiment, was frequently repeated with
the same result.
EXPE- 

(35)
EXPERIMENT V.
Air in the room 67. A pint of air, elevated from
rosin by heat, was confined in a bladder, and was
raised to 104.
A pint of water was taken at 64. The bladder
containing the air, being immersed in the water, the
temperature at the end of
surface bottom
1 minute was 64⅟₂ 64
2 64⅟₂ 64
3 64⅟₂ 64
4 64 3/4 64 3/4
The bladder in which the air was contained, being
freed from air, communicated to a pint of water in
the same circumstances, half a degree of heat.
To obtain this air, a small quantity of rosin, was
put into a gun barrel, the end of which was heated
red hot. When the rosin was violently inflamed,
fresh air was blown into the touch hole, and the
fumes immediately beginning to issue copiously, a
flaccid bladder was fixed upon the end of the bar-
rel.
The air which was thus obtained, was partly in-
flammable; when it was forced upon a candle, it
burned with a pale bluish flame. It consisted chiefly
of fixed and phlogisticated air.
EXPERIMENT VI.
A pint of air elevated from tallow, as in the for-
mer experiment, and confined in a bladder, was
railed to 113.
A pint of water was taken at 63. The bladder
containing the air being immersed in the water, the
temperature, at the end of
E2 1 mi-

(36)
surface bottom
1 minute was 63 3/4 63
2 63⅟₂ 63⅟₄
3 63⅟₂ 63⅟₂
4 63⅟₂ 63⅟₂ 
The bladder being then freed from air, and im-
mersed in a pint of water, in the same circumstan-
ces, raised the water half a degree.
This air was also partly inflammable, but consisted
chiefly of fixed and phlogisticated air.
From these experiments it appears, that the quan-
tity of sensible heat communicated by fixed and phlo-
gisticated air, to an equal bulk of water, (the differ-
ence of temperature being 50) is so small, that it
cannot be measured by the thermometer. But we
have seen, that in the same circumstances, a pint of
atmospherical air communicates one degree of heat
to a pint of water. From hence we may conclude,
with certainty, that the absolute heat of atmosphe-
rical air is greater than that of fixed or phlogisticated
air.
The specific gravity of fixed air was found by the
honourable Mr. Cavendish, to be to that of water, as
1 to 511. If, therefore, it had contained the same
absolute heat with water, it would have communi-
cated to the water nearly the eleventh part of a de-
gree. If it had contained less absolute heat than
water, it would have communicated less than the
eleventh part of a degree. But such minute varia-
tions of heat cannot be distinguished by the thermo-
meter; and, therefore, from this experiment, we
can draw no precise conclusion, with regard to the
comparative heat of water and fixed air.
It is well known, that a great quantity of fixed
air is contained in the crude calcareous earth. Chalk
and limestone contain more than a third of their
weight of this species of air. It has been proved 
by 

(37)
by Dr. Black, that, when these substances are de-
prived of the fixed air, with which they are com-
bined in their natural state, they are converted into
quicklime; and as no sensible heat or cold is pro-
duced, by the separation of fixed from the calcari-
ous earth (as I shall afterwards endeavour to show,)
it is possible, by comparing the heat of the crude
calcarious earth with that of quicklime, to ascertain,
with accuracy, the absolute heat of fixed air.
The following experiments were made to deter-
mine the heat of chalk and quicklime.
EXPERIMENT VII.
A pound of chalk at 58,
was mixed with a pound of water at 158;
The temperature of the mixture at the end of
1 minute was 135,
2 134.
Adding one degree for the heat carried off by
the air in the first minute, we have 136 for the
temperature of the mixture.
The chalk was raised from 58 to 136, or 78 de-
grees.
The water was cooled by the chalk and the ves-
sel together 22 degrees. It was cooled nearly 2 de-
grees by the vessel——it was, therefore, cooled 20 de-
grees by the chalk. And hence the heat of water
is to that of chalk, as 78 to 20, or as 3.9 to 1.
It is to be observed, that no sensible heat was
produced by mixing equal parts of chalk and water
together, when they were both at 57, which was
the temperature of the air in the room.
The absolute heat of quicklime cannot be ascer-
tained with accuracy, by making water the standard,
as these substances, when mixed together, produce
much sensible heat.
For 

(38)
For this reason, I endeavoured to determine the
absolute heat of quicklime, by mixing together e-
qual quantities of chalk and quicklime at different
temperatures. But, to determine, whether the ex-
periment could be made with accuracy in this way,
I first made the following experiment on chalk.
EXPERIMENT VIII.
Air 64.
Half a pound of powdered chalk at 64
was mixed with an equal weight powdered
chalk at 164;
The mixture at the end of
1 minute was 116
2 114
3 112
4 111⅟₂
5 111
6 110⅟₄
7 110⅟₄
8 110
The vessel in which this experiment was made, was
heated to 114, previous to the mixture. Hence it
appears, that equal quantities of chalk, at different
temperatures, being mixed together; the tempera-
ture of the mixture, at the end of two minutes, was
half the excess of the hotter above the colder.
From a variety of trials, I have found that chalk
heats and cools very slowly. And this seems to have
been the reason, why the hot and cold chalk were
not brought to a common temperature, till the end
of the second minute.
EXPERIMENT IX.
A pound of quicklime at 61
was mixed with a pound of chalk at 161.
The 

(39)
The mixture at the end of
1 minute was 110
5 110
6 111
7 111⅟₂
8 112
9 111⅟₂
10 111
11 110⅟₂
12 110
The vessel was heated to 111, previous to the
mixture.
In this experiment we find, that when the chalk
and quicklime were mixed together, the thermome-
ter at first fell to 110; afterwards it gradually rose
to 112, which it never exceeded; the mixture then
cooled at the rate of half a degree in a minute.
If, therefore, we take 112 for the temperature of
the mixture, we shall probably be very near the
truth. At least we shall not make the heat of quick-
lime greater than it really is. For tho' some sensi-
ble heat seems to have been produced by the mix-
ture, yet the whole effect, which the production of
sensible heat can have in this calculation, is to di-
minish the heat of quicklime, compared with that
of chalk.
Taking 112 for the common temperature, we
have 49 for the heat separated from the chalk, and
51 for that gained by the quicklime. And hence
the absolute heat of chalk is to that of quicklime
as 51 to 49.
That cold is not produced by the separation of
fixed air from the calcarious earth, I endeavoured
to satisfy myself in the following manner.
If cold were produced by the separation of these
substances, heat would be produced by their union.
This conclusion is not conjectural. It is founded up-
on 

(40)
on an induction of facts. It is an inference drawn
from what in similar cases actually takes place in
nature. Thus cold is produced by the evaporation
of water, and heat by the condensation of vapour.
Like effects have been observed by the ingenious Dr.
Black, in a very great variety of natural phenomena.
And as no instance can be shown to the contrary, we
may safely conclude, in general, that when a body
produces cold in consequence of a change of form,
it will produce heat when it returns to its former
state.
To discover whether sensible heat is produced by
the union of quicklime with fixed air, I exposed an
ounce and a half of quicklime at 66, to the vapour
arising from a mixture of chalk and the vitriolic acid.
In a few minutes, the thermometer in the mixture
rose to 88; the heat of the vapour was 82; and the
thermometer in the quicklime stood at 78.
If sensible heat had been produced, in this expe-
riment, by the union of the quicklime with the fixed
air, the heat of the quicklime would have been great-
er than that of the vapour.
I have also found, that no sensible heat is produ-
ced, when quicklime is precipitated from lime water,
by fixed air.
We may therefore conclude, with great probabili-
ty, that sensible heat is not produced by the union
of fixed air with the calcarious earth. At least it is
certain, from these experiments, and from a variety
of phenomena, that if heat is at all produced by the
combination of these substances, the quantity is so
very inconsiderable, that it cannot affect the conclu-
sions, which are contained in the following pages.
As heat therefore, is not produced by the union
of these substances, we may conclude that cold is not
produced by their separation, and consequently dur-
ing this process, they will not absorb heat from the
surround- 

(41)
surrounding bodies. Hence it may be inferred, that
the quantity of heat contained in the earth and air
when separated, is not greater than the heat which
they contained previous to their separation.
The following is a brief illustration of the truth of
this conclusion.
It is found by experiment, that the same heat
which raises the regulus of antimony one degree, will
raise the calx of antimony only the one third of a de-
gree.
If, therefore, we suppose that the regulus, when
at the common temperature of the atmosphere, con-
tains 200 degrees of heat, and if we conceive it to
be suddenly calcined, the heat contained in the re-
gulus, will raise the calx, only the one third of
200 degrees, or 66 degrees and ⅔; the latter will
therefore during the calcination absorb 133 degrees
and ⅟₃ of heat. And hence the calyx is found to con-
tain three times as much absolute heat as the regu-
lus.
In like manner, if the fixed air and calcarious
earth when disunited, contained a greater quantity
of absolute heat than when combined, the separation
of these substances would necessarily be attended
with the absorption of heat. But we have proved
that no heat is absorbed during their separation. We
may therefore conclude, that, when, a given quantity
of chalk, is resolved into its principles, by convert-
ing it into quicklime and fixed air, the absolute
heat of the quicklime and fixed air taken together,
is not greater than the heat which was originally
contained in the chalk*.
F From
* This conclusion is farther confirmed by the ingenious Dr. Irvine's
discoveries with regard to the cause of the phenomena of latent heat.
As these discoveries, however, have not been communicated to the
world, I have not taken the liberty to point out their connection with
this part of my subject.

(42)
From these data, the absolute heat of fixed air
compared with that of chalk, may be calculated in
the following manner.
It has been proved that the absolute heat of chalk
is to that of quicklime, as 51 to 49, or nearly as
25 is to 24. We shall suppose that chalk contains
one third of its weight of fixed air; and that the
absolute heat, contained in the quicklime and fixed
air which are produced by the calcination of a given
quantity of chalk, is equal to that which was con-
tained in the chalk, previous to its calcination. If
we conceive the whole heat in the chalk divided
into 25-equal parts, the heat contained in the quick-
lime, after the separation of the fixed air, will be
to the original heat of the chalk, as 16 to 25. For
if the quicklime were equal in quantity to the chalk,
its heat would be to that of the chalk as 24 to
25. But as it is only equal to two thirds of the
chalk, it will be as 16 to 25. And since the heat
of the quicklime and fixed air taken together, is
equal to the original heat of the chalk, the heat of the
fixed air will be equal to the difference between the
heat of the chalk and quicklime, or it will be repre-
sented by the difference between 16 and 25. That
is, the heat contained in the fixed air, after the se-
paration, will be the original heat of the chalk, as
9 to 25, the quantity of matter in the fixed air being
one third of that in the chalk. And therefore tak-
ing equal quantities of chalk and fixed air, the heat
of the fixed air will be to that of the chalk, as 9
multiplied by three, or as 27 to 25, or as 1 2/27 to one.
It has been already shown that the heat of chalk is
to that of water, as 20 to 78, or as one to 3.9. But
the heat of fixed air is to that of chalk as 1 2/27 to one;
therefore the heat of fixed air is to that of water,
nearly as 1 to 3.6.
From 

(43)
From these principles, we may determine the
comparative heat of fixed and atmospherical air.
The absolute heat of atmospherical air is to that of
water, as 18.6 to one. The absolute heat of water
is to that of fixed air as 3.6 to 1. The heat of at-
mospherical air is therefore to that of fixed air, as
18.6 multiplied by 3.6 to 1; or very nearly as 67
to 1.
EXPERIMENT X.
Air in the room 52.
Fifteen ounces of water were taken at 51;
A quantity of dephlogisticated air, equal in
bulk to 10 ounces of water, was raised to 101;
The bladder containing the air being immersed
in the water, and the ball of the thermometer be-
ing kept in contact with the bladder for the first two
minutes, the temperature at the end of
l minute was 57,
2 55.
The thermometer being then removed from the
bladder, the water at the end of
3 minutes was 54,
4 54,
5 54,
6 54:
And this was found to be the heat of the water,
at the centre, as well as at the surface. The blad-
der in which this air was contained, communicated
to the water, in the same circumstances, the one
fourth of a degree, as nearly as could be judged by
the eye.
Fifteen ounces of water, being heated in the same
vessel, 2 degrees above the temperature of the at-
mosphere, cooled, in 20 minutes, 1 degree, or 1
fourth of a degree in 5 minutes. If, therefore, we
allow the heat imparted by the bladder, for that
F2 which 

(44)
which was carried off by the atmosphere in the first
5 minutes, we have 3 degrees for the heat commu-
nicated to the water, by the dephlogisticated air.
The specific gravity of dephlogisticated air, was
found, by Dr. Priestley, to be to that of atmosphe-
rical air as 187 to 185. Its specific gravity is con-
sequently to that of water, nearly as 1 to 852.
But the bulk of the water, in the above experiment,
was one third greater than that of the air. Since,
therefore, if equal bulks had been taken, the wa-
ter would have been to the air as 852 to 1; it fol-
lows, that as the water was one third greater in
bulk than the air, the quantities of matter were as
1278 to 1. The heat received by the vessel was e-
qual to that which would have been received by one
ounce of water. The water and vessel together
were therefore equal to 16 ounces of water; and
the quantity of matter in 16 ounces of water being
to that contained in 10 ounce measures of dephlo-
gistiecated air, as 1363 to 1, it follows, that the ab-
solute heat of dephlogisticated air, is to that of wa-
ter in the compound ratio of 1363 to 1, and of 3
to 47, or, as 87 to 1.
To obtain this air, a quantity of red lead was;
moistened with yellow spirit of nitre, and the salt
being dried and put into a glass vessel, the air was
separated by an intense heat, and caught in bladders.
It appeared to be of a very pure kind, as a candle
burned in it with a crackling noise, and with a bright
and vivid flame.
From this experiment, compared with experiment
the 1st, it appears, that the absolute heat of dephlo-
gisticated air, is to that of atmospherical air, as 87
to 18.6, or nearly as 4.6 to 1. And Dr. Priestley,
whose discoveries on this subject are deservedly
much admired, has proved that its power in support-
ing 

(45)
ing animal life, is 5 times as great as that of atmos-
pherical air.
We have, therefore, upon the whole, sufficient
evidence for concluding, that atmospherical air con-
tains a greater quantity of absolute heat, than the
air which is expired from the lungs of animals; and
that the quantity of absolute heat contained in any
kind of air that is fit for respiration, is very nearly
in proportion to its purity, or to its power in sup-
porting animal life.
PROPOSITION II.
THE blood which passes from the lungs to the
heart, by the pulmonary vein, contains more abso-
lute heat, than that which passes from the heart to
the lungs, by the pulmonary artery.
As the former is the blood which is returned by
the veins in the aortic system, and the latter is that,
which, in the same system, is propelled into the ar-
teries, I shall call the first venous, and the last ar-
terial blood.
The following experiments were made to deter-
mine the heat of venous and arterial blood.
EXPERIMENT I.
Air in the room 68
Half a pound of water, averdupoise weight,
at 53,
was mixed with half a pound and 400 grains
of arterial blood at 102.
The mixture at the end of
1 minute was 78,
2 77 3/4 nearly,
3 77⅟₂ when it coagulated,
4 77⅟₂ 
EXPE- 

(46)
EXPERIMENT II.
Half a pound of water, averdupoise weight,
at 53⅟₂
was mixed with nine ounces and a half, and
14 grains of venous blood, at 99⅟₃
The mixture at the end of
1 minute was 76,
3⅟₂ 76 when it coagulated,
8 76,
9 75 3/4.
In making these experiments, it was necessary to
use as much expedition as possible, that the heat of
the mixture might be determined previous to the
coagulation; and, therefore, the water was first ac-
curately weighed.——Half a pint of blood was taken
from the carotid artery of a sheep, for the first expe-
riment, and from the jugular vein for the second:
the heat of the mixture was then ascertained by the
thermometer, and the weight of the blood was de-
termined at the conclusion of the experiment.
We learn from these experiments, that the specific
gravity of venous blood is greater than that of the
arterial. For the measures were nearly equal, and
the weight of the former was found considerably to
exceed that of the latter. The arterial blood appear-
ed also to be much more fluid than the venous; and
we have seen, that when they were mixed with equal
quantities of water, the venous blood was somewhat
later in coagulating, than the arterial.
To determine the heat of arterial blood, from the
former of the above experiments, we may observe,
that as, in this experiment, the blood was poured
upon the water, a small portion of heat was lost in
its passage through the air. I have found by a sub-
sequent trial, that the quantity of heat which was
thus lost, was very nearly one degree. If this heat
had been added to the mixture, it would have raised
it 

(47)
it nearly half a degree; and as the mixture, previous
to its coagulation, cooled at the rate of one fourth
of a degree in a minute, we may add, at least, half
a degree for the heat lost in the first minute; which
gives 78⅟₂ for the temperature of the mixture.
This experiment was made in a pint pewter vessel,
the capacity of which, for receiving heat, was to that
of the water, nearly as 16 to 1. The quantity of
water was eight ounces; the heat received by the
vessel, was consequently equal to that which would
have been received by the one-sixteenth part of eight
ounces, or by half an ounce of water. It follows,
that the effect of the vessel and the water together,
was equal to that which would have been produced
by eight ounces and a half of water.
The temperature of the mixture was 78⅟₂: sub-
tracting this from 102, we have 23⅟₂ for the heat se-
parated from the blood. The water and the vessel
were raised from 53 to 78⅟₂, or 25⅟₂. The quantity
of blood was eight ounces and 400 grains averdu-
poise, or 3899 grains. The water and the vessel to-
gether were equal to eight ounces and a half of wa-
ter, or to 3717 grains. And, therefore, the heat
of arterial blood is to that of water, in the compound
ratio of eight ounces and a half to eight ounces and
400 grains, and of twenty-five and a half to twenty-
three and a half, or as 103 to 100; consequently
the heat of water is to that of arterial blood, as too
to 103, or nearly as 97.08 to 100.
In the second experiment, adding half a degree
for the heat lost in the first minute, we have 76⅟₂
for the temperature of the mixture.
The blood was cooled from 99⅟₃ to 76⅟₂ or nearly
22.83. The water and vessel were raised from
53⅟₂ to 76⅟₂, or 23 degrees. The quantity of venous
blood was 9⅟₂ ounces and 14 grains averdupoise, or
4168 grains. The water and vessel were together
equal 

(48)
equal to 8⅟₂ ounces of water. Therefore the heat
of venous blood is to that of water, in the com-
pound ratio of 8⅟₂ ounces to 9⅟₂ ounces and 14 grains,
and of 22.83 to 23, or as 100 to ll2.
Putting A for arterial blood, V for venous, and
W for water, the ratio of the heat of venous to
that of arterial blood, is determined in the follow-
ing manner: V. W. A.
97.08 100 112.
Therefore V: A:: 97.08: 112, or nearly as 10 to
11⅟₂. Thus it appears, that the blood which passes
from the heart to the lungs, by the pulmonary ar-
tery, contains less absolute heat than that which pas-
ses from the lungs to the heart by the pulmonary
vein.
PROPOSITION III.
THE capacities of bodies for containing heat, are
diminished by the addition of phlogiston, and in-
creased by the separation of this principle.
As bodies, when inflamed, appear to emit light,
and give out heat, from an internal source, and as
those bodies only are combustible, which contain the
phlogiston in a considerable quantity, it has been an
opinion generally received among philosophers, that
this principle is either fire itself, or intimately con-
nected with the production of fire. If this were
true, bodies, when united with phlogiston, would
contain a greater quantity of fire, or of absolute
heat, than when separated from it: metals would
contain more absolute heat than their calces; and
sulphur more than the vitriolic acid. But that the
contrary is the fact, as stated in the above propo-
sition, appears from the following experiments.
EXPE- 

(49)
EXPERIMENT I.
Air in the room 64.
Half a pound of water at 62,
was mixed with half a pound of tin at 162;
The mixture at the end of
surface bottom
1 minute was 68 68,
2 68 68.
EXPERIMENT II.
Half a pound of water at 62,
was mixed with half a pound of the grey calx
of tin at 162;
The mixture at the end of
surface bottom medium
1 minute was 68 72 70,
2 68⅟₄ 70 69⅜,
3 68⅟₂ 69⅟₂ 69,
4 69 69 69.
These experiments were made in a pewter vessel,
the capacity of which, for receiving heat, was de-
termined thus:
Half a pound of water at 160,
was poured into the vessel at 60;
The temperature of the water at the end of
1 minute was 150,
2 146,
3 142.
Adding four degrees for the heat carried off by
the air in the first minute, we have 154 for the
common temperature of the water and the vessel.
The absolute heat of the vessel, therefore, is to that
of the water, as 6 to 94, or as 1 to 15⅔. That is,
the heat contained in the vessel, was equal to the
heat contained in the 1/15⅔ part of eight ounces of wa-
ter. Or nearly equal to the heat contained in half
an ounce of water.
G In 

(50)
In the first experiment, the tin was cooled 94 de-
grees, and the water heated 6. Since, therefore,
the tin heated eight ounces of water, and the vessel
which was equal to half an ounce of water, six de-
grees, it follows, that it would have heated eight
ounces and a half of water six degrees. And hence
the absolute heat of tin is to that of water, in the
compound ratio of 6 to 94, and of 8.5 to 8, or as 1
to 14.7.
In this calculation, no allowance has been made for
the heat which was lost in the first minute. For,
though a portion of heat must have been separated
from the tin by the air, when it was mixed with the
water, yet the quantity was so very small, that it
may be neglected. If one degree of heat were thus
separated, it would not have raised the mixture more
than one-fourteenth part of a degree. And it ap-
pears, that, after the tin and water were brought to
a common temperature, the mixture cooled so very
slowly, that the heat which was lost, could not be
measured by the thermometer. If, however, we
were to add one-fourth of a degree for the heat im-
parted to the air in the first minute, the absolute
heat of tin would be to that of water, nearly as 1
to 14.1.
In the second experiment, the mean temperature
of the mixture, at the end of one minute, was 70.
It cooled nearly at the rate of one-fourth of a degree
in a minute. Adding therefore one-fourth of a de-
gree for the heat lost in the first minute, we have
8⅟₄ for the heat communicated to the water and the
vessel, and 91 3/4 for that separated from the calx.
But the quantity of the calx was eight ounces. The
water and the vessel were together equal to eight
ounces and a half of water; and, therefore, the
heat of the calx, is to that of water, in the com-
pound ratio of 8.25 to 91.75, and of 8.5 to 8, or
as 

(51)
as 1 to 10.4. Hence the absolute heat of the calx
of tin is to that of tin, as 14.7 to 10.4.
EXPERIMENT III.
A quarter of a pound of water at 55,
was mixed with a quarter of a pound of calx of
iron at 155:
The mixture at the end of
surface	bottom medium
1 minute was 75	77 76,
2 72 76 74,
3 71 74 73⅟₂
4 71⅟₂ 72⅟₂ 72,
5 71⅟₂ 71⅟₂ 71⅟₂
During five minutes, the mixture cooled nearly
at the rate of a degree in a minute.
Adding one degree for the heat lost in the first
minute, we have 77 for the mean temperature of the
mixture. The water, therefore, was heated 22 de-
grees, and the calx cooled 78.
The quantity of the calx was four ounces. The
water and the vessel together were equal to four
ounces and a half of water. And, hence, the heat
of the calx is to that of water, in the compound
ratio of 22 to 78, and of 4.5 to 4; or, nearly as
1 to 3.1.
The heat of iron was found, by Dr. Black, and
Dr. Irvine, to be to that of water, as one to eight.
I have since repeated this experiment with nearly
the same result*.
It appears, therefore, that the absolute heat of the
calx of iron is to that of the metal, as 8 to 3.1.
G2 EXPE-
* I must farther observe, that, before I made the experiments which
are recited in this section, the heat of lead and tin, in their metallic
state, had also been determined by the above mentioned philosophers. 

(52)
EXPERIMENT IV.
Air in the room 61,
Half a pound of water 58,
was mixed with half a pound of lead 158;
The mixture at the end of
 surface bottom
1 minute was 62⅟₂ 62⅟₂,
2 62⅟₂ 62⅟₂. 
EXPERIMENT V.
Air in the room 61,
One half pound of water 60,
was mixed with half a pound of red lead 160;
The mixture at the end of
 surface bottom medium
1 minute, was 64⅟₂67 65 3/4,
2 64⅟₂ 66⅟₂ 65⅟₂,
3 65 66 65⅟₂,
4 65 65⅟₄ 65⅛,
5 64⅟₂ 64⅟₂ 64⅟₂.
The heat received by the vessel, in the fourth ex-
periment, was equal to that which would have been
received by one half ounce of water. The quantity
of water was eight ounces. The water and the ves-
sel together were equal to eight and a half ounces
of water: and, therefore, the quantity of lead be-
ing eight ounces, the absolute heat of lead is to that
of water, in the compound ratio of 4.5 to 95.5, and
of 8.5 to 8, or nearly as 1 to 19.9.
In experiment fifth, the mean temperature of the
mixture, at the end of one minute, was 65 3/4. Du-
ring five minutes, it cooled nearly at the rate of one
fourth of a degree in a minute. Adding, therefore,
one fourth of a degree for the heat lost in the first
minute, we have 66 for the temperature of the
mixture.
The

(53)
The calx was cooled 94; the water and the ves-
sel were heated 6; and hence the absolute heat of
the calx is to that of water, in the compound ratio
of 6 to 94, and of 8.5 to 8, or as 1 to 14.7.
It follows, that the absolute heat of the calx is to
that of lead, as 19.9, to 14.7.
EXPERIMENT VI.
Air in the room 58.
One fourth of a pound of water at 59, was mix-
ed with one fourth of a pound of a compound,
consisting of equal parts of the calves of lead and
tin, at 159.
The mixture at the end of
surface bottom medium
1 minute was 65 69 67, 
2 63 67 65⅟₄
3 64 66 65,
4 66 65⅟₂ 65⅟₄,
5 65 65⅟₄ 65⅟₈,
6 65 65 65.
The mean temperature of the mixture, at the end
of one minute, was 67. It cooled two degrees in
six minutes; adding therefore, one third of a degree
for the heat lost in the first minute, we have 67⅟₃, or
67.3, nearly for the true temperature of the mixture.
The calx was cooled 91.7. The water heated 8.3.
The absolute heat of the calx therefore is to that of
water, in the compound ratio, of 8.3 to 91.7, and
of 4.5 to 4, or as 1 to 9.8.
It is to be observed that these metals calcine more
perfectly when mixed, than when separate.
EXPERIMENT VII.
Air 64,
Half a pound of water at 63,
was mixed with half a pound of the regulus
of antimony at 163,
The 

(54)
The mixture at the end of
1 minute was at the surface and bottom 70,
2 ———— 68⅟₂,
3 ———— 68.
EXPERIMENT VIII.
Air 61,
One fourth of a pound of water at 58⅟₂.
was mixed with one fourth of a pound of
calx of antimony at 158⅟₂,
The mixture at the end of
surface bottom medium
1 minute was 72⅟₂ 77 74
2 71⅟₂ 74⅟₂ 73
3 71 72 71⅟₂
4 70 3/4 71 70⅞
In Experiment VII. the mixture cooled at the rate
of half a degree in a minute. The water and vessel
were heated 7.5. The regulus was cooled 92.5. The
heat of the regulus, therefore, is to that of water in
the compound ratio, of 7.5 to 92.5, and of 8.5 to 8,
or nearly as 1 to 11.6. In Experiment VIII. the
mean temperature of the mixture was 74 3/4. It cooled
in 4 minutes, nearly at the rate of one degree in a
minute. Adding therefore one degree for the heat
lost in the first minute, we have 75 3/4 for the true tem-
perature; consequently the absolute heat of the calx
of antimony, is to that of water in the compound
ratio of 17⅟₄ to 82 3/4, and of 8⅟₂ to 8, or nearly as 1 to
4.5, and hence the absolute heat of the calx of anti-
mony, is to that of the regulus, as 11.6 to 4.5. By
similar experiments, it may be demonstrated, that the
vitriolic acid contains more absolute heat than sul-
phur. We may therefore conclude, in general, that
bodies, when joined to phlogiston, contain less abso-
lute heat than when separated from it; and conse-
quently, that, in the former case their capacities for
contain- 

(55)
containing heat are diminished, and in the latter, in-
creased.
It follows, that if phlogiston be added to a body,
a quantity of the absolute heat of that body will be
extricated; and if the phlogiston be separated again,
an equal quantity of heat will be absorbed.
The calx of antimony, for example, contains nearly
three times as much absolute heat as the regulus:
when, therefore, by the addition of phlogiston, the
calx is revived, it will lose two thirds of its absolute
heat, and, on the contrary, when the regulus is by
calcination deprived of its phlogiston, the calx will
recover the heat which it had formerly lost.
In this point of view, the separation of heat from
a body, by means of phlogiston, and the reabsorption
of it, when the phlogiston is again disengaged,
seems to be analogous to the separation of air from
earths and alkali's, by means of an acid, and the reuni-
on of these substances with this element, when the acid
is separated. If the vitriolic acid, for instance, be
added to a mild alkali, the fixed air will be extrica-
ted, and will fly off in the form of an elastic vapour;
if phlogiston be added to a metallic earth, a portion
of the absolute heat will be separated, and will fly
off in the form of sensible heat; when the acid is
again separated from the alkali, the latter recovers
the air which it had lost, and when the phlogiston is
again disengaged from the metallic earth, the earth
reabsorbs the heat which had formerly escaped from
it.
Heat, therefore, and phlogiston, appear to be two
opposite principles in nature. By the action of heat
upon bodies, the force of their attraction to phlogi-
ston is diminished; and by the action of phlogi-
ston, a part of the absolute heat, which exists in all
bodies as an elementary principle, is expelled.
SECT. 

(56)
SECT. III.
FROM the facts which have been established by
the above experiments, the following explanation
may be given of animal heat, and of the heat which
is produced by the inflammation of combustible bo-
dies.
I. Of Animal Heat.
It has been proved, that the air which is expired
from the lungs of animals, contains less absolute
heat than that which is inhaled in inspiration. It
has been shown, particularly, that, in the process
of respiration, atmospherical air is converted into
fixed air; and that the absolute heat of the former
is to that of the latter, as 67 to 1.
Since, therefore, the fixed air which is exhaled
by expiration, is found to contain only the one six-
ty-seventh part of the heat which was contained in
the atmospherical air, previous to inspiration, it fol-
lows, that the latter must necessarily deposit a very
great proportion of its absolute heat in the lungs.
It has moreover been shown, that the absolute heat
of florid arterial blood, is to that of venous, as 11⅟₂
to 10. And hence, as the blood which is returned
by the pulmonary vein to the heart, has the quanti-
ty of its absolute heat increased, it is evident that it
must have acquired this heat in its passage thro' the
lungs. We may conclude, therefore, that, in the
process of respiration, a quantity of absolute heat is
separated from the air and absorbed by the blood.
That heat is separated from the air in respiration,
is farther confirmed by Experiment X. Prop. I.
from which experiment, compared with Dr. Priest-
ley's discoveries, it is manifest, that the power of
any species of air in supporting animal life, is nearly
in 

(57)
in proportion to the quantity of absolute heat which
it contains, and is consequently proportionable to
the quantity which it is capable of depositing in the
lungs.
The truth of this conclusion, will perhaps appear
in a clearer light, from the following calculation,
by which we may form some idea of the quantity of
heat yielded by atmospherical air, when it is con-
verted into fixed air, and also of that which is ab-
sorbed, during the conversion of venous into arteri-
al blood.
We have seen that the same heat, which raises
atmospherical air one degree, will raise fixed air
nearly 67 degrees. And, consequently, that the
same heat, which raises atmospherical air any given
number of degrees, will raise fixed air the same
number of degrees, multiplied by 67. In the Pe-
tersburgh experiment, the heat was diminished 200
degrees below the common temperature of the at-
mosphere. We are, therefore, certain that atmos-
pherical air, when at the common temperature of
the atmosphere, contains at least 200 degrees of
heat. Hence, if a certain quantity of atmospherical
air, not in contact with any body that would imme-
diately carry off the heat, should suddenly be con-
verted into fixed air, the heat which was contained
in the former, would raise the latter 200 degrees
multiplied by 67, or 13400 degrees. And the heat
of red hot iron being 1050, it follows, that the
quantity of heat, which is yielded by atmospherical
air, when it is converted into fixed air, is such, (if
it were not dissipated) as would raise the air so
changed to more than 12 times the heat of red hot
iron. 
If, therefore, the absolute heat which is disenga-
ged from the air in respiration, were not absorbed by
H the 

(58)
the blood, a very great degree of sensible heat would
be produced in the lungs.
Again, it has been proved, that the same heat
which raises venous blood 115 degrees, will raise
arterial only 100 degrees; and, consequently, that
the same heat, which raises venous blood any given
number of degrees, will raise arterial, a less num-
ber, in the proportion of 100 to 115, or 20 to 23.
But we know that venous blood contains at least
230 degrees of heat. Hence, if a certain quantity
of venous blood, not in contact with any body
that would immediately supply it with heat, should
suddenly be converted into arterial, the heat which
was contained in the former would raise the latter
only 20/23 of 230 degrees, or 200 degrees; and con-
sequently the sensible heat would suffer a diminution,
equal to the difference between 230 and 200, or 30
degrees. But the common temperature of blood is
96; when, therefore, venous blood is converted in-
to arterial in the lungs, if it were not supplied by
the air, with a quantity of heat proportionable to
the change which it undergoes, its sensible heat
would be diminished 30 degrees, or it would fall
from 96 to 66.
That a quantity of heat is detached from the air
and communicated to the blood in respiration, is
moreover supported by the experiments which have
been brought in proof of the third Proposition;
from which it appears, that, when bodies are joined
to phlogiston, they lose a portion of their absolute
heat, and that when the phlogiston is again disen-
gaged, they reabsorb an equal portion of heat, from
the surrounding bodies.
Now it has been demonstrated, by Dr. Priestley,
that, in respiration, phlogiston is separated from the
blood and combined with the air. During this pro-
cess, therefore, a quantity of absolute heat must
neces- 

(59)
necessarily be disengaged from the air, by the action
of the phlogiston; the blood, at the same moment,
being left at liberty to unite with that portion of
heat, which the air had deposited,
And hence animal heat seems to depend upon a
process, similar to a chemical elective attraction.
The air is received into the lungs, containing a great
quantity of absolute heat. The blood is returned
from the extremities, highly impregnated with phlo-
giston. The attraction of the air to phlogiston, is
greater than that of the blood. This principle will,
therefore, leave the blood to combine with the air.
By the addition of the phlogiston, the air is obliged
to deposit a part of its absolute heat; and as the
capacity of the blood is at the same moment increas-
ed by the separation of the phlogiston, it will in-
stantly unite with that portion of heat which had
been detached from the air.
We learn from Dr. Priestley's experiments, with
respect to respiration, that arterial blood has a strong
attraction to phlogiston: It will consequently, during
the circulation, imbibe this principle from those
parts which retain it with least force, or from the
putrescent parts of the system: And hence the ve-
nous blood, when it returns to the lungs, is found
to be highly impregnated with phlogiston. By this
impregnation, its capacity for containing heat is di-
minished. In proportion, therefore, as the blood
which had been dephlogisticated by the process of
respiration, becomes again combined with phlogi-
ston, in the course of the circulation, it will gradu-
ally give out that heat which it had received in the
lungs, and diffuse it over the whole system.
Thus it appears, that, in respiration, the blood
is continually discharging phlogiston and absorbing
heat; and that in the course of the circulation, it
H2 is 

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is continually imbibing phlogiston and emitting
heat.
It may be proper to add, that, as the blood by its
impregnation with phlogiston, has its capacity for
containing heat diminished; so, on the contrary,
those parts of the system from which it receives this
principle, will have their capacity for containing
heat increased, and will consequently absorb heat.
Now, if the changes in the capacities, and the
quantities of matter changed in a given time were
such, that the whole of the absolute heat separated
from the blood were absorbed, it is manifest, that
no part of the heat, which is received in the lungs,
would become sensible in the course of the circu-
lation.
That this, however, is not the case, will, I think,
be evident, from the following considerations.
We know that sensible heat is produced by the
circulation of the blood; and we have proved by
experiment, that a quantity of absolute heat is com-
municated to that fluid in the lungs, and is again
disengaged from it, in its progress thro' the system.
If, therefore, the whole of the absolute heat, which
is separated from the blood, were absorbed by those
parts of the system from which it receives the phlo-
giston, it would be necessary to have recourse to
some other cause, to account for the sensible heat
which is produced in the circulation. But, by the
rules of philosophising, we are to admit no more
causes of natural things, than such as are both true,
and sufficient to explain the appearances; for nature
delights in simplicity, and affects not the pomp of
superfluous causes*.
We may, therefore, safely conclude, that the ab-
solute heat which is separated from the air in respira-
tion.
* See Newton's Principia b. iii. p. 202. 

(61)
tion, and absorbed by the blood, is the true cause of
animal heat.
It must nevertheless be granted, that those parts of
the system which communicate phlogiston to the
blood, will have their capacity for containing heat
increased; and therefore, that a part of the absolute
heat which is separated from the blood will be ab-
sorbed.
But from the quantity of heat, which becomes
sensible in the course of the circulation, it is mani-
fest that the portion of heat which is thus absorbed,
is very inconsiderable.
It appears, therefore, that the blood, in its pro-
gress thro' the system, gives out the heat which it
had received from the air in the lungs; a small por-
tion of this heat is absorbed by those particles which
impart the phlogiston to the blood; the rest becomes
redundant, or is converted into moving and sensible
heat.
I shall hereafter show, that the heat which is pro-
duced by this process, is similar to that which is pro-
duced by the inflammation of combustible bodies,
with this difference, that, in the latter instance, the
fire is separated from the air, in the former, from
the blood.
Of the Inflammation of combustible Bodies.
From the above experiments we learn, that at-
mospherical air contains much absolute heat; that
when it is converted into fixed and phlogisticated air,
the greater part of this heat is detached; and that
the capacities of bodies for containing heat, are di-
minished by the addition of phlogiston, and increas-
ed by the separation of it. From hence we infer,
that the heat which is produced by combustion, is
derived from the air, and not from the inflammable
body.
For

(62)
For inflammable bodies abound with phlogiston,
and contain little absolute heat; atmospherical air,
on the contrary, abounds with absolute heat, and
contains little phlogiston. In the process of inflam-
mation, the phlogiston is separated from the inflam-
mable body, and combined with the air; the air is
converted into fixed and phlogisticated air, and at
the same time gives off a very great proportion of
its absolute heat, which, when extricated suddenly,
bursts forth into flame, and produces an intense de-
gree of sensible heat. We have found by calcula-
tion, that the heat which is produced by the conver-
sion of atmospherical into fixed air, is such, if it
were not dissipated, as would be sufficient to raise
the air so changed, to more than twelve times the
heat of red hot iron. It appears, therefore, that in
the process of inflammation, a very great quantity
of heat is derived from the air.
It is manifest on the contrary, that no part of the
heat, can be derived from the combustible body. For
the combustible body during the inflammation, being
deprived of its phlogiston, undergoes a change simi-
lar to that which is produced in the blood, by the
process of respiration; in consequence of which, its
capacity for containing heat is increased. It, there-
fore, will not give off any part of its absolute heat,
but, like the blood in its passage thro' the lungs, it
will absorb heat.
The calx of iron, for example, is found to con-
tain more than twice as much absolute heat, as the
iron in its metallic form; from which it follows,
that, in the process of inflammation, the former must
necessarily absorb a quantity of heat, equal to the
excess of its heat above that of the latter. Now,
from whence does it receive this heat? It cannot re-
ceive it from the iron. For the quantity of heat in 
the

(63)
the calx, is more than double of that which was con-
tained in the iron, previous to the calcination.
But in the burning of iron, the phlogiston is se-
parated from the metal, and combined with the air;
and it has been proved, that, by the combination of
phlogiston with air, a very intense heat is produced.
From hence it is manifest, that, in the inflammation
of iron, the atmospherical air is decomposed, a very
great proportion of its absolute heat is separated, part
of which is absorbed by the calx, and the rest ap-
pears in the form of flame, or becomes moving and
sensible heat.
We may conclude, therefore, that the sensible heat
which is excited in combustion, depends upon the
separation of absolute heat from the air by the action
of phlogiston.
In confirmation of this conclusion, it may be pro-
per to add, that, (if we except the change that the
air undergoes in the process of respiration, in which
the heat is absorbed) the sudden conversion of at-
mospherical, into fixed and phlogisticated air, is in-
variably accompanied with the production of sensible
heat. Thus sensible heat is produced when common
air is mixed with nitrous air, when it is exploded
with inflammable air, when it is diminished and ren-
dered noxious, by putrefaction, by combustion, and
by the electric spark. If the quantity of air which
is changed, by these processes, in a given time, be
very great, the change is attended with much light,
with a vivid flame, and with intense heat; but if
the alteration in the air be slow and gradual, the
heat passes off imperceptibly to the surrounding bo-
dies.
It appears, upon the whole, that atmospherical
air contains, in its composition, a great quantity of
fire or of absolute heat. By the separation of a
portion of this fire in the lungs, it supports the tem-
perature 

(64)
perature of the arterial blood, and this communi-
cates that pabulum vitæ, which is so essential to the
preservation of the animal kingdom. And, finally,
by a similar process, it maintains those natural and
artificial fires, which are excited by the inflammation
of combustible bodies.
Assuming this doctrine as true, I shall next endea-
vour to shew, that it affords an easy solution of the
most remarkable facts, relating to animal heat and
combustion.
SECT. IV.
Of the principal Facts relating to Animal Heat.
1. THE above doctrine explains the reason why
the breathing animals have a higher temperature than
those which are not furnished with respiratory organs;
for it has been proved that the former are continu-
ally absorbing heat from the air: And, it is proba-
ble, that, to provide an apparatus for the absorption
of heat, was the chief purpose of nature, in giving
to so great a part of the animal creation, a pulmo-
nary system, and a double circulation.
We have shown that animal heat, and the inflam-
mation of combustible bodies, depend upon the
same cause, that is, upon the separation of absolute
heat from the air, by the action of phlogiston.
The quantity of air phlogisticated by a man in a
minute, is found, by experiment, to be equal to that
which is phlogisticated by a candle, in the same space
of time. And hence a man is continually deriving
as much heat from the air, as is produced by the
burning of a candle.
It is remarked by naturalists, that the cold animals
have also the power of keeping themselves at a tem-
perature, 

(65)
perature, somewhat higher than the surrounding
medium. To account for this, we may observe,
that animal heat depends, indirectly, upon a change
produced in the air by respiration, and directly upon
a change which the blood undergoes in the course of
the circulation.
In consequence of the tendency of the system to
putrefaction, the blood is impregnated with phlogi-
ston, and by this impregnation is obliged to give
out a part of its absolute heat. The source from
which it is again supplied with heat, in such of the
cold animals as are not furnished with lungs, can
only be determined by experiment. It is probable,
that, in those animals, the aliment contains more
absolute heat than the blood. If this be the case,
the blood will be supplied with heat from the ali-
ment.
2. From the experiments in heated rooms, it ap-
pears, that the animal body has, in certain situa-
tions, the power of producing cold, or of keeping
itself at a lower temperature than the surrounding
medium.
This power has been attributed by some philoso-
phers to the evaporation from the surface of the bo-
dy; and indeed it must be allowed, that the increas-
ed evaporation, will have a very considerable influ-
ence in diminishing the heat. But the experiments,
which have been related above, point out another
cause, which, I apprehend, conspires in producing
the same effect.
By the heat of the surrounding medium, the eva-
poration from the lungs is increased. Now it may
be shown, that if the evaporation from the lungs be
increased to a certain degree, the whole heat which
is separated from the air, will be absorbed by the
aqueous vapour.
I From 

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From the calculation in Sect. III. page 58, it
appears, that the capacity of the blood for contain-
ing heat, is so much increased in the lungs, that if
its temperature were not supported by the heat
which is separated from the air, in the process of re-
spiration, it would fall from 96 to 66. Hence if the
evaporation from the lungs be increased to such a
degree, as to carry off the whole of the heat that
is detached from the air, the arterial blood, when it
returns by the pulmonary vein, will have its sensible
heat greatly diminished, and will, consequently, ab-
sorb heat from the vessels which are in contact with
it, and from the parts adjacent. And thus the very
same process which formerly supplied the animal with
heat, will now become the instrument of producing
cold, and the quantity of cold produced, will be in
proportion to the velocity of the blood through the
lungs and the fulness and frequency of the respi-
ration.
Hence also we may perceive the reason, why the
heat of animals is nearly the same, in all parts of
the earth, notwithstanding the very great variations
in the heat of the atmosphere, arising from the vi-
cissitudes of the weather, and the difference of sea-
son and climate.
The quantities of heat lost by bodies, when heat-
ed and placed in the cold air, are in proportion to
the excess of their sensible heat, above that of the
surrounding atmosphere. The heat of the human
body is very nearly, at all seasons of the year, 96;
and, consequently, other circumstances continuing
the same, the quantity of heat lost in a given time,
when the air is at 36, will greatly exceed that
which is lost in an equal portion of time, when it
is at 66. It is therefore, necessary, that, in the
former case, a greater quantity of heat should be
absorbed from the air to supply the waste.
To

(67)
To account for this, we may observe, that, by
the tonic and stimulant powers of cold, the vigour
of the animal body is increased. The vessels on
the surface are constricted. The blood is determin-
ed to the lungs. The pulse and the respiration are
rendered full and frequent. And hence, as the
cold advances in winter seasons, and in northern
climates, the quantity of heat absorbed from the
air is proportionably increased.
In summer, the blood is determined to the sur-
face, the velocity of the circulation through the
lungs, is diminished, and hence, a proportionable
diminution in the quantity of heat absorbed. We
may add to this, that, if the quantity of heat ab-
sorbed by the vapour, which is exhaled in respira-
tion, be so great, as that the remaining portion of
the heat deposited by the air, is not sufficient to
support the temperature of the arterial blood, some
degree of cold will be produced in consequence
of the change which the blood undergoes in the
lungs.
As animals are continually absorbing heat from
the air, if there were not a quantity of heat carried
off, equal to that which is absorbed, there would
be an accumulation of it in the animal body. The
evaporation from the surface, and the cooling pow-
er of the air, are the great causes which prevent
this accumulation. And these are alternately in-
creased and diminished, in such a manner, as to pro-
duce an equal effect. When the cooling power of
the air is diminished by the summer heats, the eva-
poration from the surface is increased; and when,
on the contrary, the cooling power of the air is in-
creased by the winter colds, the evaporation from
the surface is proportionably diminished. The in-
fluence of this cause in preserving the equality of
I2 heat 

(68)
heat in animals, was first suggested to me by my
ingenious friend, Mr. Cleghorn.
3. Among different animals, those are the hot-
test, which breathe the greatest quantity of air in
proportion to their bulk; and in the same animal,
the degree of heat is in some measure proportiona-
ble to the quantity of air inhaled in a given time.
These varieties appear to be the necessary conse-
quence of the general fact, that the heat of the
breathing animals is derived from the air. For if
animal heat depends upon a change which the air
undergoes in the lungs, it is evident, that, all other
circumstances being equal, the greater the quantity
of air which is changed in a given time, the greater
must be the heat produced.
In exercise, by the action of the muscles, the ve-
nous blood is returned in greater quantities than u-
sual, from the extremities, to the right auricle of
the heart. By the action of the heart it is determin-
ed to the lungs. The respiration is accelerated; the
velocity of the circulation is increased; and hence,
a proportionable increase in the quantity of phlogi-
ston discharged, and the quantity of heat absorbed.
The cold stage of fevers is preceded by langour,
a sense of debility, and a diminution in the action
of the heart and arteries. The respiration is small,
the pulse is weaker than natural——the quantity of
blood which passes thro' the lungs, in a given time,
is diminished——and hence, less phlogiston will be
discharged from the blood, and, consequently, less
heat will be separated from the air.
In the progress of the cold stage, a spasm is form-
ed upon the surface*. By the constriction of the
vessels on the surface, the blood is determined to
the heart. The heart is stimulated to more frequent
and
* See Cullen's first Lines of the Practice of Physick. 

(69)
and violent contractions. The velocity of the blood
through the lungs is increased————the respiration is
accelerated; and hence a greater quantity of heat
will be absorbed.
We may observe, that the absorption of heat, and
the accelerated velocity of the blood through the
lungs, will act and react upon each other, in such
a manner, as that the heat will have a constant ten-
dency to increase. For the accelerated velocity of
the blood occasions a greater absorption of heat;
and the increased absorption of heat, by stimulating
the heart and arteries to more frequent and powerful
contractions, will again accelerate the velocity of the
blood, which will still farther increase the absorp-
tion. And therefore, the heat will continue to be
accumulated, till counteracted by the operation of
some other cause. From the sudden diminution in
the weight of the body, notwithstanding the quan-
tity of watery fluids that are taken in, and the ob-
struction of the urinary secretion, it appears, that,
in the hot stage of fevers, there is a very great eva-
poration from the surface; we may, therefore, con-
clude, that this is one of the means which Nature
employs, for moderating the heat, and restraining
the violence of the disease.
Another cause which prevents the accumulation of
heat, is the cooling power of the external air. We
have already observed, that the quantities of heat
lost by a body in a given time, are in proportion to
the excess of its heat, above that of the surrounding
medium.
If, therefore, the sensible heat of the body increase,
while the temperature of the air continues the same,
the quantities of heat carried off by the latter, in a
given time, will be proportionably increased.
In putrid fevers, to the accelerated velocity of the
blood through the lungs, there is added a putrescent
state 

(70)
state of the system; in consequence of which, the air
inhaled in inspiration, will be more copiously supplied
with phlogiston, than when the body is in a found
and healthy state. If, in the latter instance, the air
which is received into the lungs, were completely
saturated with phlogiston, the quantity of heat sepa-
rated from it, would always be proportionable to the
quantity of air inhaled in a given time. But it ap-
pears from experiment, that the air which is expired
by a healthy animal, is not completely saturated with
phlogiston. It is capable of being farther diminished
by nitrous air, and not more than the eighth part of
it consists of fixed air. The quantity of heat, there-
fore, which is separated from the air in respiration,
will be partly in proportion to the quantity of air in-
spired, and partly to the quantity of phlogiston dis-
charged from the blood, in a given time.
In fevers of the putrid kind, as the solid and fluid
parts of the system are in a putrescent state, and, con-
sequently, retain their phlogiston with less force, a
greater quantity of this principle will be discharged
from the lungs, the air will be more copiously sup-
plied with it in the process of respiration, and will,
therefore, impart to the blood a greater proportion
of its absolute heat. To these causes it is probably
owing, that the heat of the human body never rises
so high as in putrid fevers.
4. Topical inflammation is accompanied with red-
ness, with tumour, and with unusual heat*. From
the throbbing of the vessels, and from microscopi-
cal observations, it appears, that the velocity of the
blood through the part inflamed, is accelerated;
and it is manifest, that a tendency to putrefaction must
be produced by the violent reaction, and by the stag-
nation of the serous matter which is sometimes effus-
ed into the adjoining cellular texture. It has been
already
* See Cullen's first Lines of the Practice of Physick. 

(71)
already observed, that the arterial blood has a strong
attraction to phlogiston, and that by its union with
this principle, in the course of the circulation, it is o-
bliged to give out that heat which it had received in
the lungs. In the state of health, the velocity of
the blood through the different parts of the system,
and the quantities of phlogiston with which it is sup-
plied in those parts, are adjusted to each other in
such a manner, that the heat is equally diffused
over the whole. But, if by any irregularity, the
balance be destroyed; if, by the increased action
of the vessels, the blood be urged with greater vio-
lence than usual through any particular part, or, in
consequence of a greater tendency to putrefaction, be
more copiously supplied with phlogiston, it is mani-
fest, that a greater quantity of heat will be extricated
in that part, in a given time. This heat will stimu-
late the vessels into more frequent and forcible con-
tractions, by which the velocity of the blood, and
the consequent extrication of heat will be still far-
ther increased. On this principle we may probably
account for the partial heats which are produced by
topical inflammations, and for those which arise in
hectic and nervous diseases.
It will hereafter appear, that the heat is accumu-
lated in topical inflammation, by the increased veloci-
ty of the blood through the part inflamed, in the
same manner as it is accumulated upon the fuel, in
combustion, by directing a stream of fresh air into
the fire.
Of the principal Facts relating to the Inflammation of
Combustible Bodies.
1. We have proved, that the sensible heat in
combustion is derived from the air, and depends
upon the separation of absolute heat from this ele-
ment, 

(72)
pent, by the action of phlogiston. From hence it
is evident, that when the air, in which an inflamed
body is confined, is saturated with phlogiston, and
deprived of the greater part of its absolute heat, the
source of inflammation will be exhausted, and the
flame will necessarily be extinguished. And this
explains the reason, why a constant succession of
fresh air is necessary to inflammation, as well as to
the support of animal life.
2. It is highly probable, from Dr. Priestley's
Experiments, that dephlogisticated air contains less
phlogiston, than any other species of air. And this
conclusion will, I think, be farther confirmed, if
we compare his Discoveries with the Experiments
which have been related above.
The substances from which dephlogisticated air is
obtained, either contain in their natural state very
little phlogiston, or they are such as have the greater
part of their phlogiston separated, by the force of
fire, and by the action of the nitrous acid.
We have seen that phlogiston diminishes the abso-
lute heat of bodies; that dephlogisticated air abounds
with absolute heat; that when a certain quantity of
phlogiston is added to this species of air, so as to re-
duce it to the state of common air, its absolute heat
is proportionably diminished; and that when a still
greater quantity is added, so as to convert it into
fixed and phlogisticated air, it is, in a great measure,
deprived of its absolute heat. From all which it ap-
pears, that dephlogisticated air contains much abso-
lute heat, and little phlogiston. As the sensible heat
which is produced by combustion, depends upon the
separation of absolute heat from the air, by the action
of phlogiston, it is manifest that the less phlogiston,
and consequently the more absolute heat any species
of air contains, the longer it must contribute to the
support of flame, as well as to the preservation of
animal 

(73)
animal life. Agreeably to this conclusion, we find,
from Experiments I. and X. Prop. i. Sect. ii. that
dephlogisticated air contains nearly five times as
much absolute heat as common air. And Dr. Priest-
ley has shown, that five times as much sensible heat
is produced by the conversion of it into fixed and
phlogisticated air, as by that of common air; for a
candle will continue to burn five times as long in the
former, as in the latter species of air: Add to this,
that it burns with a much more bright and vivid
flame; and as the quantity of phlogiston is increas-
ed, the brightness and vivacity of the flame dimi-
nish, till at length the air becomes saturated with this
principle, and the flame is extinguished.
3. As the sensible heat, in combustion, depends
upon a change produced in the air, by the phlogi-
ston, which is separated from the inflammable body;
it is manifest, that (all other circumstances being
equal) the intensity of the heat, must be in propor-
tion to the quantity of air which is changed in a
given time. And hence the heat may be increased
to a very great degree, if a stream of fresh air be
directed upon the fuel, by bellows or by the blow
pipe.
We have found, that by the conversion of atmos-
pherical into fixed air, a quantity of heat is disenga-
ged, which would be sufficient to raise the air so
converted to more than twelve times the heat of red
hot iron. And indeed the degree of heat, excited
by the inflammation of combustible bodies, would
not be less than this, if the fire that is thus extricat-
ed, were applied to the fixed air alone, and were
to remain in the same concentrated state, in which it
is at first separated from the atmospherical air.
But as sensible heat has a constant tendency to an
equal diffusion, it will instantly flow from the point
inflamed, and spread itself over the surrounding bo-
K dies. 

(74)
dies. It will be accumulated upon the fuel, absorb-
ed by the vapour, and communicated to the atmos-
phere: And from the principles which have been
established above, it is manifest, that the same heat
which raises fixed air 13400 degrees, would raise an
equal quantity of atmospherial air, only the 1/67 part
of 13400, or 200 degrees. This explains the rea-
son, why the heat is so intense in the flame of a can-
dle, and is so greatly diminished at the smallest di-
stance from the flame.
4. Tho' it is highly probable that all bodies have
their capacities for containing heat changed in con-
sequence of the addition or separation of phlogiston,
yet from the experiments which have been recited
above, it is manifest, that the degree of this change
is very different in different bodies. We have seen
that the capacity of the calx of iron is to that of
iron, as 3.1 to 1; that the capacity of minium is
to that of lead, as 19.9 to 14.7; that the capacity
of the calx of antimony is to that of the regulus,
as 11.6 to 3.9; and that the capacity of arterial
blood is to that of venous, as 11⅟₂ to 10.
It appears, moreover, that different quantities of
phlogiston, are required to saturate different bodies.
Some of the metals abound more with this principle
than others. The quantity of phlogiston which is
required to saturate dephlogisticated air, is much
greater than that which will saturate common air;
and a greater quantity of phlogiston is required to
saturate common air, than an equal weight of arte-
rial blood.
From these facts it follows, that when phlogiston
passes from one body to another, the changes in the
capacities of the bodies for containing heat will be
different, and unequal quantities of matter will be
changed in a given time. Thus, in respiration, the
phlogiston is separated from the blood and combin-
ed 

(75)
ed with the air. The diminution which is produ-
ced, by this process, in the capacity of the air for
containing heat, is greater than the increase in that
of the blood; and as more phlogiston is required
to saturate atmospherical air, than an equal weight
of arterial blood, the quantity of the latter which
is changed in a given time, will be greater than that
of the former.
When two contiguous bodies, at the same mo-
ment, have their capacities for containing heat re-
spectively increased and diminished, it the changes
be such, that the whole heat separated from the
one is absorbed by the other, no sensible heat or
cold will be produced.
I shall proceed to determine the cases in which
this will happen, having first pointed out some of
the chief circumstances, by which, the sensible heat
of bodies, the capacities for containing heat, and
the absolute heat contained, may be distinguished
from each other.
The capacity for containing heat, and the abso-
lute heat contained, are distinguished as a force from
the subject upon which it operates. When we
speak of the capacity, we mean a power inherent
in the heated body; when we speak of the absolute
heat, we mean an unknown principle which is re-
tained in the body, by the operation of this power;
and when we speak of the sensible heat, we consider
the unknown principle as producing certain effects
upon the senses and the thermometer.
The capacity for containing heat may continue
unchanged, while the absolute heat is varied without
end. If a pound of ice, for example, be supposed
to retain its solid form, the quantity of its absolute
heat will be altered, by every increase or diminution
of its sensible heat: but as long as its form continues
the same, its capacity for receiving heat will not be
K2 affected 

(76)
affected by an alteration of temperature, and would
remain unchanged, though the body were wholly
deprived of its heat.
The alterations which are produced in the sensi-
ble heats of different bodies, by given quantities of
absolute heat, are greater or less, according as the
body, to which the heat is applied, has a less or
greater capacity for containing heat. Thus it has
been proved, that if a quantity of heat be added to
a pound of water, which is sufficient to produce an
increase in its temperature as 1, the same quantity
of heat being added to a pound of antimony, will
produce an increase as 4. The body, therefore,
which has the less capacity for containing heat, has
its temperature more augmented by the addition of
a given quantity of absolute heat, than that which
has the greater. Hence the sensible heat of a body
depends partly upon the quantity of its absolute heat,
and partly upon the nature of the body in which this
heat is contained, and consequently the sensible heat
may be varied, either by a change in the nature of
the body itself, or by a change in the quantity of
its absolute heat. If the variation of sensible heat
arises from the first of these circumstances, it fol-
lows, that, in the same body, the sensible heat may
vary, though the absolute heat continues the same*.
The following proportions obtain, with respect to
the capacities, the sensible heats, and the quantities
of absolute heat.
The capacities of bodies for receiving heat, are
considered as proportionable to the quantities of ab-
solute heat which they contain, when the quantities
of matter are equal, and the temperature are the
same. Calling, therefore, the sensible heat S, the
capa-
* See the observations on fixed and atmospherical air, venous and
arterial blood, page 57.

(77)
capacity C, and the absolute heat A, if the quantities
of matter, and the sensible heats be given, the capa-
cities will be as the absolute heats; or if S be given,
A will be as C. Again, it has been shown, that, in
the same body, if the form remain unchanged, or,
in other words, if the capacity be given, the quantity
of absolute heat will be in proportion to the sensible
heat. That is, if C be given, A will be as S. Since,
therefore, if S be given, A will be as C, and if C
be given, A will be as S, it follows that if neither
be given, A will be as SxC. Therefore, C will be
as A/S and if A be given, C will be reciprocally as
S. Consequently, if the capacities be reciprocally as
the sensible heats, the quantities of absolute heat will
be equal. Thus, if the capacity of the calx of anti-
mony be to that of the regulus as 3 to 1, and if the
sensible heat of the former be to that of the latter 1
to 3, the calx and regulus will contain equal quanti-
ties of absolute heat*.
It appears, therefore, that in heterogeneous bo-
dies, if the sensible heats be different, the capacities
may vary, though the absolute heat be the same. And
it is manifest from the foregoing experiments, that,
in such bodies, if the capacities be different, the ab-
solute heats may vary, though the sensible heat be the
same.
These observations being premised, the cases in
which no sensible heat or cold will be produced, by
the passage of phlogiston from one body to another,
may be determined in the following manner.
PROPO-
* The sensible heat is here supposed to be computed from the point of
total privation 

(78)
PROPOSITION I.
[illustration]
LET there be two
bodies, A and B, which  
have their capacities for 
containing heat changed 
at the same moment, the 
capacity of A being di-
minished, and that of B increased; let the capacity
of A before the change, be denoted by C, and after
the change, by c; the capacity of B before the
change by k, and after the change by K. Then C——c
will be the difference of the capacities of A before
and after the change; and K——k the difference of
the capacities of B. It is affirmed, that, if the tem-
peratures and the quantities of matter changed in a
given time, be equal, the differences of the capacities
will be as the differences of the absolute heats.
For the capacities of bodies for receiving heat,
are estimated (as was observed above) by the com-
parative quantities of absolute heat which they are
found to contain, when the quantities of matter are
equal, and the temperatures are the same. The
temperature and the quantities of matter, therefore,
in A and B being equal, the capacities will be di-
rectly as the absolute heats. If the absolute heat
of A be double that of B, the capacity will be
double, if triple, triple, &c. And therefore, call-
ing the absolute heats of A and B before the change,
H and p, and after the change h and P, it will be
C: c: :H: h. And by conversion C——c: c::H——h: h.
For the same reason K——k: k::P——p: p. But be-
cause the quantities of matter in A and B are equal,
and the bodies before and after the change are con-
ceived to be brought to the same temperature, it will
be, c: k::h: p.
Since 

(79)
Since therefore C——c: c: k: K——k
H——h: h: p: P——p, by equality
C——c: K——k:: H——h: P——p.
That is, the differences of the capacities are as
the differences of the absolute heats, the tempera-
tures and the quantities of matter being equal.
PROPOSITION II.
IF the differences of the capacities be equal,
the differences of the absolute heats will be as the
quantities of matter*.
[illustration]
The same things re- 
maining as above, if the 
quantities of matter in A 
and B be equal, C——c: 
K——k:: H——h: P——p.
[illustration]
Let the quantity of matter in A 
be increased, in any proportion, as 
in Fig. 3, and, after the increase, let 
it be called T. Let the absolute 
heats be denoted by R and r, and
the quantities of matter in A (or B) and T, by
q and Q respectively.
Since the capacities of A and T are equal, the
absolute heats before the change, will be as the
quantities of matter. Therefore H: R:: q: Q.
For the same reason the absolute heats, after the
change, will be as the quantities of matter. That
is, h: r :: q: Q. Therefore, H: R:: h: r, and
H: h:: R: r, and H——h: h:: R——r: r, and H——h:
R——r:: h: r. But h: r:: q: Q. Therefore H——h:
R——r:: q: Q. But H——h is equal to P——p. There-
fore P——p: R——r:: q: Q. And hence the differences
of
* In this and the following proportion, the bodies are supposed to
be brought to the same common temperature before and after the
change. 

(80)
of the capacities being equal, the differences of
the absolute heats will be as the quantities of mat-
ter.
PROPOSITION III.
IF the differences of the absolute heats be equal,
the differences of the capacities will be reciprocally
as the quantities of matter.
For, by the first proposition, the 
quantities of matter being given, the 
differences of the absolute heats are directly as the
differences of the capacities; and, by the second
proposition, the differences of the capacities being
given, the differences of the absolute heats are as
the quantities of matter; it follows, that neither be-
ing given, the differences of the absolute heats, are as
the differences of the capacities, multiplied into the
quantities of matter. That is, H——h is as C——c x Q.
Therefore C——c will be as H——h/Q; and if H——h be
given, C——c will be reciprocally as Q. Consequent-
ly if the differences of the absolute heats be equal,
the differences of the capacities will be reciprocally
as the quantities of matter.
COR. I. It is, therefore, required, in order that
neither heat nor cold should be produced, that the
differences of the capacities should be reciprocally as
the quantities of matter changed in a given time.
For in that case, by the converse of this proposition,
the differences of the absolute heats will be equal.
COR. II. See the Fig. Prop. I. If, therefore, the
diminution in the capacity of A, be to the increase
in that of B, in a greater proportion, than the
quantity of matter in B to that in A, the whole of
the heat which is separated from A will not be ab-
sorbed 

(81)
sorbed by B, a part of it will become redundant,
or will be converted into moving and sensible heat.
COR. III. The differences of the capacities being
reciprocally as the quantities of matter, if the quan-
tities of matter changed in a given time be equal,
the differences of the capacities will be equal.
COR. IV. See the fig. Prop. I. If the quantities
of matter changed be equal, and if the heat of A,
after the change, be equal to that of B previous to
the change, that is, if h be equal to p, in order that
neither heat nor cold should be produced, it is re-
quired that P should be equal to H. For in that
case H——h = P——p.
COR. V. The same things being supposed, if h
be less than p, then P must be greater than H, and
P will be equal to H + p——h. For in that case
H——h will be equal to P——p.
COR. VI. If h be greater than p, P must be less
than H, and P will be equal to H——h——p. For sub-
tracting h——p from H, we have H——h + p = P. And,
therefore, P——p = H——h.
The rule which is expressed in the first corollary,
applies to the separation of heat from the air, and
the absorption of it by the blood, in the process of
respiration. For as the sensible heat in the lungs, is
not greater than in the other parts of the body, it
is manifest, that the whole of the heat which is se-
parated from the air must be absorbed. And there-
fore the changes produced by the passage of the
phlogiston, from the blood to the air, are such, that
the difference of the capacities of venous and arte-
rial blood, is to the difference of the capacities of
fixed and atmospherical air, as the quantity of air
changed in a given time, is to that of blood: in
which case, by the above corollary, no part of the
heat will become redundant. The opposite changes,
therefore, which the air and the blood undergo in
L the

(82)
the lungs, precisely balance each other. For as the
quantity of blood, which is altered by respiration,
in a given time, is much greater than that of air, so
the change which is produced in the air, during this
process, is proportionably greater than that which
it produced in the blood.
The rule which is expressed in the second corrol-
lary, applies to the sensible heat produced in the
course of the circulation, and to that which is pro-
duced by the inflammation of combustible bodies.
As we find, for example, that a part of the heat
which is disengaged from the blood, in its progress
through the system, becomes redundant, we may
conclude, that the diminution in the capacity of the
blood, is to the increase in the capacity of those
parts of the body from which it receives the phlo-
giston, in a greater proportion, than the quantity of
matter in the latter to that in the former: in which
case, the whole of the heat separated from the blood
will not be absorbed; a part of it will be converted
into moving and sensible heat.
That this rule is also applicable to the inflamma-
tion of combustible bodies, appears from the fol-
lowing experiments and observations.
In the burning of oil, the phlogiston is separated
from its former basis, and combined with the air.
The air is converted into fixed and phlogisticated
air——the oil into vapour. By this process, the capa-
city of the air for containing heat is diminished, and
that of the oil increased. And, therefore, from the
first and third Corollaries, it follows, that if, in the
inflammation of oil, equal quantities of air and oil
were changed in a given time, and if the differ-
ence of the capacities of oil and the vapour of oil,
were equal to the difference of the capacities of fixed
and atmospherical air, the whole heat separated from
the air, would be absorbed by the vapour.
The 

(83)
The difference of the capacities of fixed and at-
mospherical air is 66. The capacity of oil is to that
of fixed air, nearly as three to one; and, there-
fore, by the fifth corollary, it the whole heat se-
parated from the air were absorbed, and if the quan-
tities of air and oil changed in a given time, were
equal, the capacity of the vapour of oil would be
equal to 67 + 3——1. It would be to that of atmos-
pherical air, as 69 to 67. But we have seen, in the
above Experiments, that a pint of atmospherical air
communicates one degree of heat to a pint of water,
the difference of temperature being fifty; and that,
in the same circumstances, the quantity of sensible
heat communicated by a pint of the vapour, produ-
ced by the inflammation of oil, is so small, that it
cannot be measured by the thermometer.
In the second place, supposing that atmospherical
air has a greater capacity for containing heat, than
the vapour of oil, if the quantity of oil changed in
a given time, were proportionably greater than that
of air, in this case, as appears from the third Pro-
position, the whole heat separated from the air,
would be absorbed.
The comparative quantities of air and oil, which
were changed by combustion, in a given time, were
determined in the following manner:
A candle, weighing nearly three ounces, and
burning with a large wick, was found to lose a fourth
of an ounce in twenty four minutes, or five grains
in a minute. Now, if a candle consumes a gallon
of air in a minute, and if the one-eighth part of this
consists of fixed air; it follows, that about eight
grains of fixed air, will be produced by the burning
of a candle, in a minute. From which it appears,
that the quantity of air changed in a given time, is
much greater than that of oil; and it has been al-
ready proved, that the diminution in the capacity
of the air for containing heat, is also greater than
L2 the 

(84)
the increase in that of the oil; we may, therefore,
conclude, that only a small part of the heat separa-
ted from the air will be absorbed; the rest will be
converted into sensible heat.
To place this in another light: if, during the in-
flammation of oil, the whole heat separated from
the air, were absorbed by the vapour, it would
follow, that the oily vapour mixed with the fixed
and phlogisticated air, which are produced by the
inflammation of a grain of oil, would contain as
much absolute heat, as an equal quantity of atmos-
pherical air, mixed with a grain of oil. Now it is,
well known, that when a candle is suffered to burn
out in air, the whole mass of fixed and phlogistica-
ted air and oily vapour, is contained in less space,
than the atmospherical air, previous to the inflam-
mation. And yet it appears, from Experiment I.
and VI. Prop. I. Sect. II that a pint of this com-
pound, contains much less absolute heat than a pint
of atmospherical air.
I have made several experiments to determine the
quantity of air which is phlogisticated by the calcina-
tion of iron; and have reason to believe, that it is
at least equal to the quantity of metal calcined; from
which we may calculate the heat produced by that
process.
The capacity of atmospherical air is to that of fixed
air, as 67 to 1; or as 147.4 to 2.2: The capacity
of fixed air is to that of iron, nearly as 2.2 to 1;
and the capacity of the calx of iron is to that of iron,
nearly as 2.5 to 1.
Calling, therefore, the capacity of atmospherical
air, 147.4, and that of fixed air, 2.2, it appears
from Cor. 6. Prop. III. that if the whole heat sepa-
rated from the air, were absorbed by the calx, the
capacity of the calx would be equal to 147.4——2.2——1
= 146.2. The heat of the calx would consequently
be 

(85)
be to that of the metal as 146.2 to 1. But it is as
2.5 to 1; and hence, the heat which becomes redun-
dant during the calcination of iron, is to the original
heat of the iron, as 143.7 to 1; and it is to that of
the calx as 143.7 to 2.5, or as 57.4 to 1.
It has been before shown, that bodies, when at the
common temperature of the atmosphere, contain at
least 200 degrees of heat. During the inflammation
of iron, therefore, a quantity of heat becomes re-
dundant, which would be sufficient to raise the calx
200 degrees, multiplied by 57.4, or 11480 degrees.
And hence we may account for the heat which is
produced by the percussion of flint and steel. A par-
ticle of the metal is struck off by the force of the
flint. The phlogiston is separated from this particle,
and is left at liberty to combine with the air, in con-
sequence of which a quantity of fire is disengaged
from the latter; and the heat which is produced
during this process, is so intense, that the particle of
the metal, which is struck off, is converted into glass.
Upon the whole, there is sufficient evidence for
concluding, in general, that the heat in combustion,
depends upon the separation of fire from the air, by
the action of phlogiston; that a part of this fire is
absorbed by the body, which supports the inflam-
mation; and that the rest becomes redundant, or is
converted into sensible heat. It follows, therefore,
that the quantity of fire which is separated from the
air, will be in proportion to the quantity of phlogi-
ston which is joined to it in a given time; and the
degree of sensible heat which is produced, will be
greater or less, according as a less or a greater
quantity of this fire is absorbed by the body, which
discharges the phlogiston.
In the inflammation of alcohol and sulphur, a ve-
ry great proportion of the fire which is detached
from the air, is imbibed by the aqueous and sulphu-
reous

(86)
reous vapour; and, therefore, alcohol and sulphur
burn with a pale and weak flame. On the other
hand, those inflammable bodies which produce little
vapour, or which produce a vapour that is capable
of absorbing but little heat, as pit-coal, oil, wax,
phosphorus, burn with a strong and vivid flame;
for, in these cases, a great part of the fire which is
yielded by the air, is converted into sensible heat.
I have thus endeavoured to account for the pheno-
mena of combustion and of animal heat, from the
general principle, that the capacities of bodies for
containing heat, are diminished by the addition of
phlogiston, and increased by its separation.
On this principle, a variety of phenomena may be
explained, besides those which have been already
mentioned.
When the nitrous acid is mixed with oil of tur-
pentine, the phlogiston is separated from the oil,
and combined with the acid; the latter is forced to
give out a portion of its absolute heat; part of which
is absorbed by the basis of the oil, and the rest be-
comes redundant, or is converted into sensible heat.
If the sensible heat be increased to a certain degree,
the phlogiston will suddenly combine with the air,
in consequence of which a great quantity of fire
will be extricated, and the whole will explode, with
a vivid flame, and with intense heat.
It is probable, that the vapour of the pure nitrous
acid contains as much absolute heat as atmospheri-
cal air; for the power of the former in maintaining
flame, is nearly as great as that of the latter. In
the deflagration of nitre, the acid is converted into
vapour; which being at the same moment combined
with the phlogiston of the coal, the fire is instantly
disengaged, an elastic fluid is suddenly expanded,
and a loud explosion produced.
The 

(87)
The ingenious Mr. Bewley has given the following
explanation of the spontaneous ascension of phospho-
rus. He supposes, with Dr. Priestley, that atmos-
pherical air contains the nitrous acid, as a constituent
principle; and observing that much heat arises from
the sudden combination of phlogiston, with this acid;
he concludes that the phlogiston of the phosphorus
is capable of decomposing the air; and that by the
union of the phlogiston with the aerial acid, a de-
gree of heat is produced sufficient to inflame the
phosphorus.
The experiments which have been recited above,
seem to prove, as Mr. Bewley has supposed, that
the production of heat is the necessary consequence
of the combination of phlogiston with air: But
these experiments appear moreover to shew, that
this heat arises from the separation of a quantity of
fire, which was contained in the air as a constitu-
ent principle, and which, in the process of combu-
stion, is detached from it, by the action of phlogi-
ston.
Dr. Priestley, has proved that the electric fluid is
capable of communicating phlogiston, to atmosphe-
rical air, and of converting it into fixed and phlo- 
gisticated air. This fact explains the cause of the
heat which is produced by the electric spark.
If a great quantity of electric matter be suddenly
disengaged from a cloud or from the earth, it will
extricate a proportionable quantity of fire in its pas-
sage through the air; and thus we may account for
the sudden rising of the thermometer in the time
of thunder and lightning.
It is found, by experiment, that the phenomena
of an earthquake may be imitated by a mixture of
iron filings and brimstone, made into a paste with
water, and buried in the earth. May not the heat
which 

(88)
which is thus produced be explained in the follow-
ing manner?
The attraction of the phlogiston to the acid of the
sulphur, will be diminished both by the attraction of
the iron to this acid, and by that of the water. In
the degree of heat which is necessary to the inflam-
mation of sulphur, atmospherical air is capable of
separating the phlogiston from the vitriolic acid. Is
it not probable that by the assistance of the iron and
the water, it may be capable of producing this effect
in the common temperature of the atmosphere? If
this be the case, it follows, that by the action of the
air, which is diffused through the substance of the
earth, upon the phlogiston of the sulphur, and by
that of the iron and water upon the acid, the sulphur
will be decomposed; the air will unite with the phlo-
giston; the iron with the acid; a quantity of fire
will be disengaged from the former, and an inflam-
mable elastic fluid from the latter; and hence a com-
motion will be excited, accompanied with noise and
the eruption of flame, resembling the phenomena of
an earthquake.
May not a similar mixture of sulphureous and me-
tallic bodies be produced in consequence of the
changes which take place in the bowels of the earth?
May not these bodies be brought into contact with
the water and the atmospherical air which are diffu-
sed through the earth's substance, or lodged in ca-
vities beneath its surface? By the action of the air
upon the phlogiston, and of the water and the ore
upon the acid, may not the sulphur be decomposed,
as in the mixture of iron filings and brimstone? In
which case a quantity of fire will be disengaged,
and an elastic vapour produced, the latter of which,
by its sudden expansion, will excite a commotion in
the bowels of the earth, and will at length force its
way through the superincumbent strata.
If

(89)
If much combustible matter be lodged in the
regions where the subterraneous fires have been
kindled, and if this matter be mixed with atmosphe-
rical air, or with substances, which, by the applica-
tion of heat, produce a fluid that is capable of main-
taining fire, the inflammation may be augmented to
a prodigious degree, and the rarified vapours may
carry along with them in their ascent, a great quan-
tity of ignited materials abounding with phlogiston,
by the exposure of which, the phlogiston will be
discharged, and the flame extended through a large
tract of air.
In this manner we may probably account for vol-
canos, those awful instances of combustion which
are exhibited by nature in the fossil kingdom.*
It appears, upon the whole, that a variety of im-
portant effects are produced in the universe in con-
sequence of the mutual opposition of phlogiston and
fire.
Vegetables are elaborated by the assistance of heat
and moisture, from the elements of earth, air, and
water, and by the action of the solar light, the
principles of which the vegetable tribe is composed,
are intimately combined with phlogiston, and are
obliged to resign a portion of their absolute heat.
In combustion, the phlogiston is disjoined from its
vegetable basis, and is combined with the air; and
thus those artificial fires are maintained which are so
necessary in the economy of human affairs. In like
manner, by the powers of animal life, the phlogist-
on is separated from the blood and discharged by
respiration, in consequence of which a quantity of
fire is absorbed from the air, and is communicated
to the animal kingdom.
M The
* The same doctrine seems to afford an easy solution of the heat
which is produced by fermentation and putrefaction. 

(90)
The air which was tainted by combustion and res-
piration, is again purified by the growth of vegeta-
bles†; and if this effect be produced by the separa-
tion of phlogiston, it follows that vegetation will
restore to the air that heat which had been detached
from it in the processes of respiration and combus-
tion; and thus the principles of phlogiston and fire,
by the medium of atmospherical air, will be conti-
nually circulating through the animal and vegetable
kingdom.
It may be proper to observe, that the doctrine
which is advanced in the preceding pages, with re-
spect to cause of animal heat and of combustion,
is the result of the general fact, that the changes
which are produced in the temperatures of different
bodies, by the application of given quantities of
heat, are different; or, that, the quantities of mat-
ter being equal, the same quantity of heat which
raises one body a certain number of degrees, will
raise another a greater or a less number, according
to the nature of the body to which it is applied.
This fact appears to have been sufficiently verified by
experiment; and, therefore, the consequences which
have been deduced from it, must, I apprehend, be
considered as well founded, whatever be the hypo-
thesis which we adopt concerning the nature of heat.
For this reason, I have not entered into the en-
quiry, which has been so much agitated among the
English, the French, and the German philosophers,
whether heat be a substance or a quality. It is true,
I have, in some places, made use of expressions,
which seem to favour the former of these opinions.
But my sole motive for adopting this language, was
because it appeared to be more simple and natural,
and more consonant to the facts which had been e-
stablished by experiment. At the same time, I am
per-
† See Dr. Priestley's Experiments on Air. 

(91)
persuaded, it will be found to be a very difficult mat-
ter to reconcile many of the phenomena with the
supposition, that heat is a quality. It is not easy to
conceive, upon this hypothesis, how heat can be se-
parated from bodies, by the addition of phlogiston,
or how the absolute heat can be augmented by the
separation of this principle; how the quantity of
heat in the air can be diminished, and that in the
blood, increased, by respiration, though no sensible
heat or cold be produced.
Whereas, if we adopt the opinion, that heat is a
distinct substance, or an element sui generis, the
phenomena will be found to admit of a simple and
obvious interpretation, and to be perfectly agreeable
to the analogy of nature.
Fire will be considered as an elementary principle
which enters into the composition of all known bo-
dies. In consequence of the addition of phlogiston,
a portion of the fire will be detached, in the same
manner as the nitrous acid, is detached, by the vitri-
olic, from an earth or alkali, and therefore respira-
tion and combustion will be truly chymical processes,
in which, by the exchange of fire and phlogiston, a
double decomposition will take place, and two new
compounds will be formed; the blood, or the in-
flammable body, parting with phlogiston and receiv-
ing fire, and the atmospherical air parting with fire
and receiving phlogiston.
I may add, in the last place, that, if fire be con-
sidered as an element, which is capable of uniting
chemically with bodies, a table may be formed, exhi-
biting the respective attractions of phlogiston and fire.
As phlogiston separates from bodies, a part of their
absolute heat, it should, be placed at the head of the
first column: And as we do not know of any sub-
stances, that attract this principle with greater force
than the earths of the perfect metals, these, perhaps,
M2 should 

(92)
should stand immediately under phlogiston. In great
degrees of heat, atmospherical air separates phlogiston
from all inflammable bodies, and in the common
temperature of the atmosphere, from nitrous air and
phosphorus.
If, therefore, the attractions be arranged as they
take place in consequence of the application of heat,
under the earths of the perfect metals should stand
dephlogisticated and atmospherical air; after which
should be placed, the bates of all the inflammable
bodies, disposing them according to the degrees of
heat which are necessary to their inflammation, when
combined with phlogiston; and at the foot of the
column, should stand nitrous air.
Fire should be placed at the head of the second
column; and if the attractions of bodies to this prin-
ciple, be proportionable to the quantities of it, which
they are found to contain, when the quantities of
matter are equal; under fire should stand dephlo-
gisticated and atmospherical air——the vapour of the
nitrous acid, and probably of some other fluids——
arterial blood, water, &c.
It is manifest, however, that much time, and a
series of accurate experiments, would be required in
order to the complete investigation of this subject.
APPEN- 

APPENDIX.
IT may not be amiss to give the Reader a brief
account of the Experiments referred to in page 10,
which were instituted by Mr. De Luc, with a view
to determine the question, whether the thermometer
be an accurate measure of heat; or, in other words,
whether the expansions of the fluid contained in the
thermometer, be in proportion to the quantities of
heat applied?
It was laid down as a principle, by this philoso-
pher, that if equal quantities of the same fluid be
mixed together at different temperatures, the heat
will be equally divided between them, from which it
was concluded, that a thermometer being immersed
in the warmer substance, and also in the colder,
previous to the mixture, if its expansions were in
proportion to the quantities of heat applied, it would
point, after the mixture, to the arithmetical mean,
or to half the difference of the separate heats.
Proceeding upon this principle, he found that
when a given quantity of water at 32, was mixed
with an equal quantity of the same fluid at 212, the
mercurial thermometer, being immersed in the mix-
ture, pointed to 180, or to the arithmetical mean
between 32 and 212: but the spirit of wine ther-
mometer, 

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mometer, in the same circumstances, did not point
to 180. These Experiments were repeated at dif-
ferent temperatures with a similar result; from which
it was inferred, that alcohol expands irregularly,
but that the expansions of mercury correspond, pre-
cisely, to the quantities of heat applied.
FINIS.