WHAT ARE THE X-RAYS? 
BY JOHN TROWBRIDGE, 
Rumford Professor of the Application of Science to the Useful Arts, Harvard University. 
H~T is now three years since Ront- 
gen, professor of physics at Wurz- 
burg, published an account of his 
discovery of the so-called X-rays. 
The scientific journals of the world were 
immediately flooded with articles describ- 
ing investigations of the remarkable phe- 
nomenon. In the year 1895-96 there were 
at least one thousand of these articles. 
During the past year this number had 
dwindled to less than one hundred; and the 
leading scientific periodical in Germany, 
Wiedemann's "Annalen der Physik und 
Chemie," has just published Rbntgen's origi- 
nal article, as if in irony of the futile at- 
tempts of the army of investigators to ex- 
tend the work of the original discoverer. 
Rbntgen seems, indeed, to have anticipated 
subsequent workers in many points. He 
found that the so-called rays could not be 
bent or refracted like ordinary light-rays in 
passing from air to a denser medium; and, 
apparently with the firm conviction that he 
had discovered a new manifestation of light- 
radiation, and since he could not discover re- 
flection and refraction of these rays, he asks: 
" Are these rays an evidence of longitudinal 
vibrations of the ether?" 
Now, we know that the light-waves move 
up and down in the ether of space with a 
motion which is transverse to the direction 
of their propagation. This transverse motion 
is like the rise and fall of the waves of the 
sea. A ship rises and falls with such trans- 
verse motion, and does not move to and fro 
in the direction of propagation of the waves. 
Such a to-and-fro or longitudinal movement 
has never been discovered in the case of 
light, and the suggestion of Rbntgen imme- 
diately awakened the utmost interest among 
scientific men. If the X-rays are due to a 
longitudinal movement in the ether, their 
absence of reflective and refractive power 
can be explained. No advance, however, has 
been made in connecting the mysterious 
phenomenon with longitudinal movements 
in the ether, and the general trend of 
scientific opinion is toward the belief that 
the X-rays are extremely short waves of, 
ultra-violet light, less than one hundred- 
thousandth of an inch in length; and no 
miscroscope now made could show such 
waves to the eye; for it is barely possible to 
separate lines which are one hundred-thou- 
sandth of an inch apart. I have said that 
the general belief is that the X-rays are due 
to a wave motion in the ether. The experi- 
ments, however, which I am about to describe 
lead me to believe that in the X-ray phenom- 
ena we have really two classes, so to speak, 
of phenomena-one an electrical polarization 
of matter in space, and another a manifesta- 
tion of light at surfaces where the electrical 
polarization is converted into ordinary fluores- 
cent and phosphorescent light. According to 
this electrical hypothesis, one should not ex- 
pect to observe reflection and refraction of 
the electrical rays in the ordinary sense, and 
one should expect to treat the light observed 
where the X-rays strike just as one treats 
ordinary fluorescent and phosphorescent 
light. 
Let us first consider what we mean by 
electrical polarization, electrical induction, 
phosphorescence, and fluorescence. In the 
magnetic needle we have a body which pos- 
sesses two poles-a south pole and a north 
pole. The needle is thus said to be polarized. 
If we had an infinite number of such needles, 
without sensible weight, we could stretch a 
chain from the earth to the sun, and we 
could call this a polarized chain of particles 
the subtle vibration of which, under certain 
conditions, could form a medium of physical 
communication between a distant body and 
the earth. Such a polarized chain can also 
be formed by electrical polarization and in- 
duction. If we suppose that the earth and 
the sun are both electrified, then an infinite 
number of pith-balls, without sensible weight, 
existing in space between the earth and the 
sun, would arrange themselves by electrical 
polarization and induction, also in invisible 
polarized chains, between these bodies, and 
the direction of these chains would consti- 
tute invisible lines of electric force. The 
slightest quiver in these chains or lines of 
force would constitute an interchange of 
energy through illimitable space. When 
these lines of force become sufficiently in- 
128 WHAT ARE THE X-RAYS? 
129 
tense, and when they undergo a rapid change 
at the surface of certain substances, these 
substances exhibit fluorescent or phosphores- 
cent light. We are familiar with phosphores- 
cence in the case of the brimstone match, and 
we can perceive fluorescence in kerosene oil 
by looking at the surface of this oil obliquely. 
I mention this bold hypothesis to show how 
far-reaching the mysterious phenomena of 
these rays appear to many minds. These 
rays are produced by electrical currents, and 
the question also arises, Are they given off 
from our telegraph- and telephone-wires 
when messages fly over these wires-not 
sensibly, but even in a minute degree? The 
experiments which I am about to describe 
were instituted for the purpose of studying 
the effect of the X-rays on the electrical 
polarization of matter which I have en- 
deavored to describe. If we could stretch a 
telegraph-wire between the earth and the 
sun, and send powerful currents of electricity 
over it, could we fill the extremely rarefied 
space around the wire with X-rays? Would 
they be given off from every element of such 
a conductor when the electrical charge in 
the conductor was made to fluctuate? Since 
it is impossible to realize such an experiment, 
I resolved to imitate the conditions as nearly 
as possible in the laboratory. To do this 
required the expenditure of enormous elec- 
trical force. Instead of stretching a wire 
from the earth to the sun, I narrowed, so 
to speak, the distance between these bodies 
to six inches, and, inclosing a wire of this 
length in a glass vessel, I imitated the vast 
region of rarefied space by pumping out the 
air from this vessel. This was the form of 
vessel in which I studied the manifestation 
of the X-rays. The electrical apparatus 
used for the production of the intense elec- 
trical forces is probably the most powerful 
that has ever been used to study these rays. 
Electric discharges varying in length from 
one inch to eight feet can be studied by its 
means. The source of the electricity consists 
of ten thousand storage-batteries; and the 
effect of this battery is so heightened that 
an electric force of over two million volts 
can be obtained. The ordinary electric-arc 
street-lamp is generated by means of an elec- 
tric pressure of less than one thousand volts. 
The energy in the X-rays, however, does not 
manifest itself by a dazzling light. Its light- 
manifestation is a weird yellow glow which 
barely enables one to obtain a photograph 
of the tube in which it is generated. Its en- 
ergy is shown by the extraordinary activity 
which is given to small particles of matter. 
By means of the electric discharge of high 
pressure or electromotive force one can see 
through timbers a foot thick, and also see 
the beating of the human heart through the 
flesh. 
Immediately on sending such powerful dis- 
charges through the form of vessel I have 
FIG. 1. 
The principal difference between fluorescence 
and phosphorescence resides in this: phos- 
phorescent bodies glow in the dark after 
having been exposed to light (even a piece 
of ordinary paper is phosphorescent), while 
fluorescent substances in general cease to 
exhibit light in the dark. 
Now the X-rays excite both fluorescence 
and phosphorescence very powerfully, and 
when they were discovered many investiga- 
tors endeavored to discover them in ordinary 
sunlight, and in the electric light, which also 
excites these states. These attempts were 
failures. Nevertheless, many believe that 
ordinary sunlight is due to the conversion of 
the electrical energy of the sun, transmitted 
across the ninety millions of miles of space 
by electrical polarization or induction, into 
intense fluorescent and phosphorescent light, 
by an agency similar to that of the X-rays. 
Vol. LVI.-17-18. 130 
THE CENTURY MAGAZINE. 
described, I discovered that the rays were 
generated from every point of the six inches 
of wire. They made the walls of the tube 
gleam with a weird, fluorescent light, and, 
penetrating to the outer air, enabled me to 
detect their presence by photography. Our 
flight of imagination in picturing a telegraph- 
wire stretching from the earth to the sun, 
giving forth mysterious rays into space, has 
therefore a basis of fact. 
The continuous wire tube may have va- 
rious forms. One of the most interesting, 
from a scientific point of view, is a spheri- 
cal bulb through the center of which runs 
a straight, continuous wire at the center of 
which is a little 
mirror of alumi- 
num. When a 
powerful electric 
discharge is sent 
along this wire at 
a certain stage of 
the vacuum in the 
tube, the mirror 
reflects a beam 
like a search-light 
to the walls of 
the tube, and 
the point where 
this beam strikes 
glows with a phos- 
phorescent light 
and emits the X- 
rays. Moreover, 
if one should stand 
on an insulated 
stool (Fig. l),and 
touch with the 
finger this spot 
on the outside of the bulb, one could reflect 
back another search-light of X-rays to 
the opposite side of the bulb, and throw a 
shadow of the mirror and the wire on the 
inside of the bulb. This shadow can be 
thrown to one side or the other, according 
to the position of the touching finger. These 
so-called search-lights contain the X-rays, 
for they show all the manifestations of the 
latter, such as their power to pass through 
thin sheets of aluminum, to produce light in 
fluorescent substances, and to exhibit the 
skeleton of the hands. Now these effects can 
be produced by making the continuous wire 
either positive or negative-that is, by mak- 
ing it either an anode, the way in, or a 
cathode, the way out. We have hitherto 
thought of the cathode rays as a phenome- 
non of the cathode-that is, of the terminal 
in a Crookes tube by means of which the 
discharge is conducted out of the tube; and 
we have never spoken of anode rays. My 
experiments show conclusively that the term 
" cathode rays," which are accompaniedby the 
X-rays,-the latter probably being a height- 
ened manifestation of the former,-is only a 
limited name for a more general phenomenon 
which I am tempted to call electrostatic rays. 
The anode rays have all the qualities of 
the cathode rays; they are not, however, so 
powerful. 
It is highly important that the investi- 
gator of the phenomena of the X-rays should 
himself exhaust the Crookes tubes, and 
should study their manifestations at differ- 
ent stages of the 
rarefied medium 
in which they are 
produced. The 
effects produced 
by electricity in 
such tubes as the 
air is gradually 
withdrawn are 
very beautiful. 
At first there is a 
bright pink glow 
which fills the en- 
tire tube; then 
there are cloud- 
like masses of 
white light, which 
float like feathers 
through the tube; 
then comes a yel- 
low fluorescent 
light which makes 
the whole interior 
of the tube lu- 
minous. This last effect is produced by the 
cathode and anode and X-rays, or by what 
I prefer to call the electrostatic rays. For 
a time there was a long discussion in re- 
gard to the source of the X-rays. Some 
maintained that they came only from the 
cathode, others that they proceeded from 
the anode, and others that they emanated 
from any surface where the cathode rays 
struck. My experiments show that the con- 
testants were like those who are said to 
have disputed whether a shield was gold 
or silver. Each contestant saw only one 
side of the shield. In truth, one side of it 
was gold and the other silver. There are an- 
ode rays as well as cathode rays, and either 
produce by electrical induction a manifesta- 
tion in any desired direction. This inductive 
effect is shown by touching the Crookes bulb 
containing the continuous conductor with 
FIG. 2. WHAT ARE THE X-RAYS? 
131 
the insulated finger or with an insulated 
piece of metal. 
The continuous wire tube has shown that 
lines of electric force radiate from the sur- 
face of a conductor in a rarefied medium, 
and produce the X-rays at every point of 
this conductor. This is true whichever 
way the electric current flows in the con- 
ductor; with such a tube the distinction 
of cathode rays disappears, and we have a 
more general manifestation of X-rays. 
While trying 
a great variety 
of forms of 
tube, I came 
across many in- 
teresting mani- 
festations of 
electrification 
outside the 
tubes. One of 
these is of prac- 
tical interest to 
surgeons and 
physicians; for 
I obtained what 
is called the 
X-ray burn by 
electrification 
when there 
were no X-rays 
which could be 
detected. I say 
by electrifica- 
tion, for the 
burn was evi- 
dentlyproduced 
by the imping- 
ing of the elec- 
trostatic lines 
of force on the 
skinof thehand. 
The latter was exposed to the neighbor- 
hood of a tube containing a continuous con- 
ductor such as is shown in Fig. 2. At a 
certain stage in the vacuum, before the X- 
rays could be distinguished, peculiar forked 
brush discharges proceeded from the tube; 
and these discharges, impinging on the skin, 
produced the peculiar so-called X-ray burn, 
which often results from exposure to the X- 
rays. The skin shows a peculiar red tint, 
especially after exposure to the cold; it 
is extremely irritable, and after about three 
weeks the surface peels. The effect is like 
that of a severe sunburn. These forked 
brush discharges can pass through sheets of 
glass half an inch thick, and leave their im- 
pression on photographic plates which are 
carefully insulated from the ground, and 
which are shielded from all light in plate- 
holders. When these photographic impres- 
sions (Fig. 3) are examined they resemble in 
a striking manner the centers of disturbance 
on the burnt hand when the latter are ex- 
amined with a microscope. The photographs 
resulted from minute electrical discharges 
on the surface of the plate, and the burn 
was also in this case, and probably in all 
cases, due to similar discharges on the skin. 
There is an- 
other remarka- 
ble phenomenon 
shown by the 
X-rays, which 
further sup- 
ports my belief 
that these rays 
are a manifes- 
tation of anelec- 
trical disturb- 
ance in space. 
It is well known 
that an ordi- 
nary electrical 
current cannot 
pass through a 
vacuum. At a 
certain degree 
of extreme te- 
nuity of the air 
or any gas the 
so-called vac- 
uum stops elec- 
trical dis- 
charges, just as 
if a piece of 
glass should be 
interposed in an 
electrical cir- 
cuit. If the X- 
rays illuminate such a vacuum, however, 
an electrical current can be made to 
pass with extreme ease over spaces which 
had completely stopped its flow. No ef- 
fect of ultra-violet light with which I have 
been able to experiment can produce a 
similar effect. The phenomenon is an elec- 
trical one. 
The phosphorescent effects produced by 
the X-rays also support the electrical theory. 
Whenever such rays strike certain crystals, 
the latter shine vividly in the dark. The X- 
rays can lead one to the spot where there is 
a Crookes tube entirely concealed from view 
behind a thick door, or behind timbers a foot 
thick. All that is necessary to discover such 
rays is a diamond ring and a darkened room. 
FIG. 3. 132 
THE CENTURY MAGAZINE. 
As one approaches the hidden tube the dia- 
mond emits a lambent flame. 
The phosphorescent effects produced by 
the X-rays can also, in certain notable in- 
stances, be produced by directly electrifying 
the phosphorescent bodies, even when the 
most intense ultra-violet light fails to pro- 
duce any trace of phosphorescence. 
Have we, then, answered the question, 
What are the X-rays? I believe that the 
experiments which I have described support 
the theory that there are really two classes 
of phenomena-one an electrical disturbance 
in a medium, another the conversion of this 
electrical disturbance into fluorescent and 
phosphorescent light at the surfaces of suita- 
ble screens or in the body of suitable crys- 
tals. My experiments certainly show that 
there are anode rays as well as cathode rays, 
and that both are subject to the well-known 
laws of electrical induction. One should not 
expect, therefore, that the electrical rays or 
lines of force should be reflected and re- 
fracted like waves of light. I believe that 
when we have answered the question, What 
are the X-rays? we shall be able to state 
more exactly than at present the relations 
between light and electricity. The question, 
therefore, has become one of the most im- 
portant in physical science.