patience of the experimenter more highly, perhaps, than the preparation
of any apparatus used in science. Glass working is a difficult art,
and requires an absolute devotion to it. There is only one metal
known which will enable an electrical discharge to pass into and out
of a rarefied space inclosed by glass. This is platinum. A wire of
this metal can be sealed into glass so that no air can leak into an
exhausted space around the joints. All electric lamps, so commonly used
in electric lighting, have little wires of platinum at their bases, by
means of which the electric current enters and leaves the bulb. The
Crooke’s tube is in principle an Edison lamp with the filament broken.
The maker of Crooke’s tubes should complete the making of the tube
at one sitting, for reheating of the tube is very apt to lead to a
disastrous cracking of the glass. He must take the utmost precautions
against unequal heating and sudden cooling, and he must, above all,
have phenomenal patience.
Fig. 1 shows the evolution of the Crooke’s tube which is used to
produce the X rays. The first form of tube was barely larger than a
goose’s egg. The size has been gradually increased, and at present it
is three or four times larger than the original form. The interior
arrangement has not been materially changed, and consists, as we have
said, of a concave mirror, which constitutes the negative electrode,
and an inclined sheet of platinum, from which the X rays seem to
emanate.
The later forms of tube have accessory chambers, filled with certain
chemicals, which, on being slightly heated, reduce the vacuum to the
desired point. Certain forms of tubes have merely an additional chamber
which, on being heated, reduces the vacuum in the main vessel. The
latest form of tube, devised by Dr. William Rollins, of Boston, has a
hollow anode tube (_B C_, Fig. 1), through which a current of water
can circulate in order to save the tube from breaking. The end of this
anode tube is small, in order to form a sharp radiant point of light.
One of the platinum wires (_P_) inserted in the tube projects outside
some distance. When the vacuum becomes too high in the tube, this
platinum wire is slightly heated in a gas flame; then the flame is
blown out and the hydrogen is allowed to flow against the heated wire.
A sufficient amount of the gas is absorbed by the heated wire to reduce
the vacuum in the tube. This tube stands very powerful electrical
discharges, and is the most scientifically designed tube at the command
of the experimenter.
There are three methods of generating the electrical discharge which
produces the rays. The commonest method is that in which the Ruhmkorf
coil is used. This coil is what is now known as a transformer, and
consists of one coil of a few turns of coarse wire, which is connected
to a battery or other source of electricity, and of another coil
surrounding the first of a great number of turns of fine wire. Any
sudden change of the battery current produces an electric pressure or
electro-motive force at the ends of the fine coil of wire. By this
simple arrangement of two coils we can thus exalt a current of low
pressure to one of high electro-motive force. A battery current which
can barely produce an electric spark of one hundredth of an inch at the
ends of the coarse coil can cause a spark of eight inches or more at
the terminals of a fine coil.
In the second method one uses an ordinary electrical machine in which
the glass plates are supplanted by rubber ones, which are run at a
high rate of speed. Both of these methods have their advocates. The use
of the Ruhmkorf coil is the most universal.
The third method consists in charging a number of Leyden jars by a
storage battery and in discharging these one after another, so as to
obtain a high electro-motive force. This method is a very flexible one.
I can experiment with my apparatus over a range of electric pressure
extending from twenty thousand units to three million. The electrical
discharge produced by three million units or volts is over six feet in
length.
The apparatus for discharging the Leyden jars or condensers in series
is represented in Fig. 2.
[Illustration: FIG. 2.--Apparatus for producing electrical sparks six
feet in length.]
A fourth method, first used by Professors Norton and Goodwin, of the
Massachusetts Institute of Technology, consists in discharging a
quantity of electricity through the coarse coil of a Ruhmkorf coil.
This method obviates the necessity of a mechanical break to interrupt
the battery current which is employed to excite the current in the
coarse coil of this apparatus.
I have experimented with more powerful quantities of electricity than
have been hitherto used. The accompanying photograph gives an idea of
the magnitude of the quantity which I can use to excite the X rays.
It represents the discharge, burning a fine iron wire, and it makes a
noise resembling the crack of a pistol. Now, this discharge can be used
in a variety of ways to excite various transformers in order to produce
the best conditions for exciting the X rays. The method of using this
powerful discharge to excite a transformer seems at present the most
promising one in seeking the best conditions for obtaining rays of high
penetrating power.
There is still another method of obtaining the rays yet in its
infancy--the simplest method of all, for no apparatus is required.
It has been discovered that certain substances, like the salts of
uranium, have the power of emitting rays which have all the properties
of the X rays. The list of such substances is constantly increasing,
and they are called radio-active substances. It is possible to take a
shadow picture of the hand through a board by placing the hand on a
covered sensitive plate, resting the board on the back of the hand,
and strewing the board with one of these radio-active substances in
the form of a powder. Can it be that all the skill and industry which
has been employed to perfect X-ray apparatus is to be supplanted by
a powder? The peculiar property shown by the radio-active substances
leads investigators to surmise that we have evidence of new substances,
and we have the waves radium and polonium.
[Illustration: FIG. 3.--The burning of an iron wire by the most
powerful electric discharge yet produced.]
The methods by which the X rays are detected in practical employment in
surgery have not been essentially changed. The ordinary photographic
plate, shielded in a plate holder, is still used to receive the
shadow cast by the bones, and salts of barium or of calcium strewn
on pasteboard serve as fluorescent screens to receive on their
luminous surfaces these shadows and to make them evident to the eye.
An interesting use of flexible sensitive films instead of glass plates
has been made in dentistry. The films are put in the mouth, and the
Crooke’s tube placed outside in such a position that the rays can pass
through the jaw. In this way the accompanying photographs were taken
(Fig. 4).[D]
[D] Kindness of Dr. Dwight M. Clapp, Boston.
[Illustration: FIG. 4.--1. Male patient, aged ten years. Temporary
incisors in position with the permanent incisors nearly ready to erupt.
The roots of the temporary teeth nearly absorbed. Right temporary
central cut incisor destroyed by a blow five years previous, showing
gutta-percha filling put in at the time which, with the dead root,
has been absorbed (same as the live root of the other central). It is
taught in the text-books that teeth with dead nerves are not absorbed.
2. Temporary molars in position with no signs of the permanent bicuspid
which should take its place. 3. Patient, aged ten years. Shows the open
ends of the incisor roots. 4. Shows one bicuspid and two molar teeth.
The roots of the teeth pass through the floor of and into the cavity
of the antrum. The spongy character of the bone is shown. 5 and 6.
Superior jaw, right and left sides, showing temporary cuspid teeth in
place, with the permanent cuspids imbedded in the jaw entirely covered.]
The use of photographic films in the application of the X rays
in surgery will doubtless extend; we can easily imagine cases
where the necessity of the use of the knife may be avoided by the
information which a carefully placed film might afford. In general,
X-ray photographs convey more information to the skilled eye of the
specialist than to the untrained inspector of them. They should be
studied from the negatives themselves, for the delicate details can not
be reproduced in a print. It is remarkable that shadow pictures can
show so much definition. Here is a photograph of an elbow joint which
shows the texture of the bones (Fig. 5).[E]
[E] Taken by Professor Goodspeed, University of Pennsylvania.
The use of the fluorescent screen, too, has been greatly extended. Dr.
Francis H. Williams, of Boston, has used it as a valuable instrument
in medical diagnosis, especially in studying lung diseases. It has
been used at the Harvard Medical School to follow the processes of
digestion. To accomplish this, in one instance a goose was fed with
food mixed with subnitrate of bismuth, a salt which absorbs X rays.
[Illustration: FIG. 5.--Photograph of an elbow joint, showing the
texture of the bones.]
The passage of the dark mass down the long neck of the bird could be
traced on the fluorescent screen, and the peculiarities of its motion
in the gullet could be studied. A cat was also fed with the same
substance, and the movements of its stomach noted. These movements were
analogous to those of the heart--in other words, were rhythmical when
the processes of digestion were going on normally and uninterruptedly.
When, however, the cat was irritated, it may be by the sight of a dog,
these pulsations instantly ceased. As soon as the source of vexation
was removed and the purring of the animal showed a contented frame of
mind, the stomach resumed its rhythmical movements. The dependence of
the digestive apparatus on the state of the nervous system was thus
clearly shown. The female cat was much more tractable under these
experiments than the male.
The use of the X rays is accompanied with some danger if the Crooke’s
tube is not properly used. A long exposure to the X rays is apt to
produce bad burns which are like sunburns, and lead in certain cases
to bad ulcerations. They are long in healing and are characterized by
a peculiar red glow, especially on exposure to a cold wind. To prevent
them one should place a sheet of thin aluminum between the Crooke’s
tube and the part of the body submitted to the rays. This sheet should
be connected to the earth. This fact should be borne in mind when we
come to speak of the electrical region outside a Crooke’s tube.
Many investigators, reflecting upon the singular fact that the rays
pass so freely through thin aluminum and that, on the contrary,
glass absorbs such a large percentage, concluded that Crooke’s tubes
provided with aluminum windows would be an improvement upon the thin
incandescent lamplike bulbs now used. The glass of these bulbs is very
thin, not more than one thousandth of an inch in thickness, where the
rays emerge, not thicker than a sheet of ordinary note paper, and
the absorption of such a sheet of glass is so small that it can not
be detected by photography. Thus a sliver of glass of this thickness
in the hand would not appear on the X-ray photograph of this member,
and would not cast a shadow in the fluoroscope. There does not
seem, therefore, any advantage in supplying a Crooke’s tube with an
aluminum window. The mechanical difficulties, too, in accomplishing
this are very great. There is no way of joining the thin aluminum
disk to the glass so that an air-tight joint can be made. In the
process of exhausting the Crooke’s tube, the tube must be heated to
a comparatively high temperature in order to drive off the air which
clings to the inside of the glass. The rise of temperature would soften
or melt any current which might be used to make the aluminum adhere to
the glass.
We can not expect, therefore, any improvement in the direction of
aluminum windows. At one time, I suppose that the rays were highly
absorbed in passing through atmospheric air, and that it would be an
improvement in the application of the rays to surgery to interpose, so
to speak, a vacuum chamber between the body and the source of the X
rays. The experiment led to some interesting results, but not in the
direction anticipated.
The vacuum chamber consisted of a glass cylinder three feet long and
about eight inches in diameter. The two ends were closed by sheets of
aluminum, and it could be exhausted through a side tube. The reader
will immediately ask, in view of what has been said, How could the
glass tube be hermetically closed with sheets of aluminum? This was
indeed a difficult matter, but less difficult than in the case of the
Crooke’s tube, for the ends of the glass cylinder were provided with
heavy brass flanges, which were perfectly flat, and the sheets of
aluminum lying smoothly could be confined by many bolts between the
flange and suitable brass heads. This cylinder, having been exhausted,
was placed between the Crooke’s tube and the arm, for instance, in
the hope that a greater depth of human flesh and tissue might be
penetrated by the rays. It was speedily seen that the absorption of
the layer of air three feet thick could not be detected either by
photographs or the fluorescent screen. The glass cylinder was then
filled with rarefied hydrogen, but no advantage was apparent. If the
photographs of the human hand were taken, one through the rarefied
cylinder and the other through an equivalent thickness of air, no
difference in clearness or depth of definition could be perceived. The
amount of absorption by a column of air three feet in length is less
than ten per cent. This result interested me greatly, for it shows the
remarkable difference between the X rays and the cathode rays, which
had been investigated by Crooke, Hittorf, and Lenard; for the cathode
rays are greatly absorbed by atmospheric air, being reduced in passing
through five or six inches of air to one four-hundredth part of their
value.
The small amount of absorption of the X rays lifts them into the realm
of very short wave lengths of light, for their behavior in regard to
the absorption by air is very analogous to that of ultra-violet rays.
Although the vacuum chamber, by which I looked, showed no absorption
of the X rays, it disclosed a beautiful phenomenon. In a dark room
this large tube, three feet long and eight inches wide, was filled
with a roseate light, which wavered like the northern lights when the
Crooke’s tube was emitting the X rays. If the finger was brought near
the glass walls of the cylinder a stream of light apparently emanated
from a point on the inside wall of the cylinder. The hand thus had
ghostly streamers giving an image of it, although the hand itself was
invisible. These banners of light could be diverted in any direction
by the hand or by any conducting body brought near, and gave a vivid
conception of how the streaming of the aurora can be brought about by
this flitting of conducting clouds or the drifting of moisture-laden
strata of air below the rarefied space in which the beams of the
northern light dart back and forth. Both in the case of the Crooke’s
tube and the aurora these streamers are produced by electrical
discharges through rarefied air. The experiments show that outside the
Crooke’s tube there is a strong electrical attraction and repulsion,
which is only revealed in darkness and in a cold, lifeless, airless
space, such as exists between us and the sun. Can we not extend our
thoughts from the contemplation of this laboratory experiment to that
of the immensely greater play of electrical forces between the earth
and the sun across the immense vacant space ninety millions of miles in
distance?
The mysterious effects of the X rays on the molecules in the air form
a great subject of inquiry, and the investigation of it promises to
extend our knowledge of electricity and light and heat. When the
Crooke’s tube is excited we are conscious of a mysterious activity
within it, for its glass walls glow with a phosphorescent light, and
if certain crystals, like the diamond or the ruby, are placed in the
tube, this phosphorescent light is vivid. Outside the tube, in free
air, these luminescent effects are also present. The air is under an
electrical strain, which is shown by the auroral streamers when this
air is rarefied, and an electrical charge can not be maintained on a
pith ball--it is dissipated in some strange manner. Still stranger,
an electrical current is greatly aided by the X rays in its endeavor
to pass through air--they make for the time being air a conductor.
Furthermore, these rays separate the air into positively laden and
negatively laden particles.
The electrical discharge in the Crooke’s tube is many-sided in its
manifestations. Its energy seems all-pervading in the room where it
occurs. Before the discharge passes through the rarefied space in the
tube its energy manifests itself by a crackling spark, a miniature
lightning discharge. This spark, five or six inches in length, can
send out magnetic waves which extend far beyond the narrow limits of
the room. They can be detected, by the methods of wireless telegraphy,
fifty miles. When the same amount of energy is developed in a Crooke’s
tube the magnetic waves hardly pass beyond the walls of the room, and
the phenomenon of phosphorescence and fluorescence and the strange
molecular effects outside the Crooke’s tube spring into prominence.
The crackling spark outside the tube is far-reaching in its effect,
yet it shows no signs of the X rays, its light can not penetrate the
human body, it excites only a feeble phosphorescence at a distance
of even two or three feet, while the same energy excited in the
Crooke’s tube can cause luminescence at a distance of twenty feet.
The crackling spark, however, can be seen much farther than the light
of the Crooke’s tube, and it can also impress a photographic plate at
much greater distance. The following experiments will illustrate the
different manifestations of energy of which an electrical discharge
is capable. I produced an electrical spark about six inches in length
and exposed a photographic plate for six seconds, at a distance of
two, ten, and twenty feet, to its light. A thin strip of tin, with a
circular hole cut in it, served as a shutter. The sensitive plate was
thus protected, except in front of this aperture. The images exhibit
the decrease in light with the increase of distance. Another portion
of the sensitive plate was exposed in the same manner during the same
length of time to the light of a Crooke’s tube which was excited by
this same spark. No image was obtained at a distance of ten feet,
and barely one at three feet. The spark in air, therefore, was far
more energetic photographically than the X rays, but it could not
penetrate solid materials. This property was given to it by its passage
through rarefied space. I then covered a screen with a phosphorescent
substance, and exposed it to the spark in air. The phosphorescent light
could barely be detected at a distance of three feet, while with a
spark in rarefied air it could be seen at a distance of twenty feet.
When we consider these experiments we see that the X rays act toward
phosphorescent matter much as the spark in air behaves toward the
photographic plate. Now, these results, taken in connection with the
strong electrical effects in the neighborhood of an excited Crooke’s
tube, points to a certain connection between phosphorescence and
electricity. Can it be that the strange light is excited by very short
electrical waves sent out from the tube, which can not travel far
but are very active in producing molecular effects? This activity,
indeed, may prevent their extending to great distances. Wireless
telegraphy evidently depends upon one set of waves sent out by a spark,
and X-ray photographs upon another set developed only in rarefied
air. Phosphorescence can not be produced with ease by the spark in
air. On the contrary, it is developed to a remarkable degree and
at comparatively great distances by the discharge in rarefied air.
It has been shown by Mr. Burbank and myself that electrical force
can develop phosphorescent light in certain crystals. The sunlight
can do the same. Is sunlight an electrical phenomenon? That it is
constitutes the greatest hypothesis in physics of this century. When
we reflect, too, that the phosphorescence of the firefly is excited by
some manifestation of a living organism--nerve force or some related
force--shall we not include nerve force in the electrical category?
The X rays, therefore, bring into prominence strange lights which had
heretofore been noticed chiefly by keen-eyed investigators, and which,
with their names, phosphorescence and fluorescence, were unknown to the
bulk of mankind. The fluorescent screen, by means of which surgeons
observe the skeleton of the body, has now taken its place in medical
practice with the stethoscope, by which the mechanism of the lungs is
studied, and hopes have been excited that the blind may yet use the
X rays in detecting objects and in regaining a sense of vision, even
though this sense may be only partial. It is a curious fact that the
retina of the eye is phosphorescent and fluorescent, and that one can
see the shadow of certain objects in the dark when one stands so that
the feeble X rays fall upon the eye. In other words, the retina acts
as a fluorescent screen. The eye at present recognizes only a limited
number of the waves that are surging about us. We can see the colors
from red to violet, but the dark colors, so to speak, formed by waves
longer than 1/40000 of an inch and shorter than 1/100000 of an inch
make no recognizable impress upon our retina, unless, indeed, they
constitute telepathic signals which apparently stir our consciousness
and make us believe that friends are communicating from a distance. The
electrical discharge has lifted, so to speak, a realm of short waves of
energy out of the darkness and made them visible. Can the human brain
be made conscious of other waves which fill space?
But we have not by any means exhausted the protean manifestations
of the X rays. Besides the photographic, the phosphorescent, and
fluorescent effects, there are still more singular properties of these
rays. One of the most striking consists in their opening a path for a
current of electricity. The electrical discharge, feeble in itself,
not capable of lifting by means of a motor a pound weight a foot from
the floor, is yet competent to open a path for a current which can
set all the trolley cars of a great city in motion. To exhibit this
mysterious effect we bring the ends of the electrical current which
we wish to excite near each other, but not touching, in a glass tube
with thin walls, from which the air has been exhausted. When the X rays
fall on the gap between the wires the electrical current immediately
jumps across the gap with a vivid light. We have here the mechanism
of an electrical relay--the feeble energy of the electric discharge
can call into play a giant energy. By what energy does it accomplish
this? Is it by compelling molecules to put themselves in line, so that
the electrical current can bridge the gap? Is it by breaking down
this mysterious ether of space, as if we threw a stone at a turbid
bull’s eye in a prison chamber and let in a flood of sunlight? How the
imagination is stirred by this process, what seems dead and lifeless
can, by a physical agency, be stirred to endless activity! The rays are
like the touch of Ithuriel’s spear.