Popular Science Monthly/Volume 56/April 1900/Latest Developments with the X Rays
| LATEST DEVELOPMENTS WITH THE X RAYS. |
By Prof. JOHN TROWBRIDGE,
DIRECTOR OF THE JEFFERSON PHYSICAL LABORATORY, HARVARD UNIVERSITY.
WE have become accustomed to seeing photographs of the bones of our hands, and we no longer stop at shop windows to look at X-ray photographs. Indeed, they are rarely displayed, and the lecturer who once gazed on a sea of faces as he endeavored to explain the most marvelous electrical sensation of this century now addresses a mere handful of listeners. Such patient hearers continuing to the end may still hear of marvelous performances of this strange light of which the great public are even now ignorant, and in this paper I shall take my readers into a physical laboratory and endeavor to make the generally unknown manifestation of the new rays plain and free from technical language. I am sure that we shall all leave the laboratory with our imagination full of thoughts of unknown movements in the air about us—thoughts of possible telepathic waves through space, conceptions of new ranges of nerve excitations, hopes of new lights, conceptions of the vastness of the electrical whirls in that elevated region where the molecules of the air, in their endeavor to fly into the abyss of space, are controlled by the earth's forces and are endowed with electrical energy by the sun.
In the first place, what-is the present state of our knowledge of the X rays? Have we more efficient methods of producing them, and can we see farther into the recesses of the human body? In regard to the first question, we can say that, although we may not be able to answer dogmatically that we know what these rays are, we have valuable hints in regard to their character, and our knowledge of their manifestations and their relation to light waves and magnetic waves has greatly increased during the four years which have elapsed since their discovery. They are now believed by the best authorities to be magnetic and electrical pulses, or waves of extremely short length. In the spectrum of sunlight formed by sending a beam through a prism of quartz the X-ray pulses or waves are to be found, according to this hypothesis, beyond the violet color of this spectrum—far into the dark region invisible to the eye, and only brought into view at present by the aid of photography. In this invisible region reside many singular manifestations of energy closely analogous to those of the X rays. The ultra-violet rays invisible to the eye have the property of refraction. They can be bent out of their course by prisms made of quartz. The X rays, however, can not be directed from a straight course. This is their greatest peculiarity, and many attempts, both mathematical and experimental, have been made to elucidate it. It is not a fatal objection to the X rays being classed with light waves, for, under certain conditions, even light waves can be made to lose the power of being bent aside.
Leaving for the present a further discussion of the question What are the X rays? let us examine what the actual condition of the art of using the rays is. Many attempts have been made to improve the Crooke's tube, in which the rays are produced, but, like the hand telephone, its form has remained substantially unaltered since the first flush of discovery. Its present form consists of a bulb of thin glass, exhausted of air, containing a little concave mirror of aluminum, and opposite to this, separated by a gap of several inches, is an inclined sheet of thin platinum, called the focus plane, or anticathode. The electrical discharge passes between this plane and the mirror, and the X rays are thrown off from the inclined sheet of platinum. They are not reflected in the ordinary sense of the term, but the electric rays converge from the mirror to a spot on the platinum which glows with a red heat, and the X rays emanate from the heated spot as if it were their source. Thousands of investigators have endeavored to improve the form of tube, but, with several important minor appendages, it still maintains the principal features of an aluminum concave mirror and an inclined plane of platinum. Aluminum is found to be the best metal for the mirror from which the rays are generated, largely because its metallic particles are not torn off by the discharge, as would be the case if it were made of platinum. It is also light, and can be easily fixed to a platinum wire. Among the important modifications of the tube are those which enable the operator to control the degree of vacuum in the tube. This is accomplished by sealing to the main tube an appendage containing certain chemicals which, on being heated, give off a small amount of vapor, and which take it up again on cooling. This modification is made necessary by the singular fact that after a Crooke's tube is submitted to an electrical discharge for some time the vacuum becomes more and more complete, and a higher and higher electro-motive force or pressure is needed to produce the discharge in the tube. It prefers in time to jump over the surface. Thus, at the very beginning of our use of the X rays we meet with a mystery. Where do
Fig. 1.—The evolution of the Crooke's tube.
the remaining particles of air go? It is surmised that they disappear in the platinum terminals.
The manufacture of the X-ray tubes tests technical skill and the 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.
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 arc called radio-active substances. It is possible
Fig. 3.—The burning of an iron wire by the most powerful electric discharge yet produced.
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.
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).[1]
The use of photographic films in the application of the X rays in surgery will doubtless extend; we can easily imagine cases
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.
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 go much definition. Here is a photograph of an elbow joint which shows the texture of the bones (Fig. 5).[2]
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.
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
Fig. 5.—Photograph of an elbow joint, showing the texture of the bones.
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 110000 of an inch and shorter than 1100000 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.
The electrical discharge can accomplish all this, but the story of its activity is not yet told. It can not be told, for each year adds information in regard to these activities, for there are thousands of investigators at work. Another far-reaching manifestation is this: the rays can separate the air or a gas into its constituent particles, much as a strong electrical current separates water into oxygen and hydrogen. They can communicate electrical charges to these particles—positive and negative charges. The charged air-particles, when forced through partitions of spun glass, does not give up their electricity as they do when they are charged by an electrical machine. This curious manifestation leads me to suspect that the electricity and magnetism of the earth may be caused by an X-ray effect on our atmosphere. The sun and the earth are separated like the terminals of a Crooke's tube—two conductors with a vacuum between. An electrical excitation from the sun may cause an electrical discharge between it and the earth. This discharge might consist of an X-ray effect which could separate the upper layers of the atmosphere into positive and negative charges. The velocity of the negatively charged particles is greater than that of the positively charged ones, and the revolution of the earth may cause such a movement of these electrified particles that electrical currents may be generated which in circulation around the earth could produce the observed magnetism of the north and south poles, together with the auroral lights characteristic of those regions. This, I am well aware, is an audacious theory. It is certainly a vast extension of the laboratory experiments I have described, but the electrical radiations developed in electrical discharges are as competent to produce powerful magnetic whirls as the heat radiations in our atmosphere to develop cyclones. In the lower regions of our atmosphere the air is an insulator like glass to the passage of an electrical current. A layer a foot thick can prevent the circulation of the most powerful current which is now used to generate horse power. When this air space is rarefied at a certain degree of rarefaction the electrical current passes, especially, as we have seen, if it is illuminated by the X rays. When, therefore, we ascend to a height of ten or twenty miles the rarefied air becomes an excellent conductor of electricity of high electro-motive force. To my mind the conditions exist for developing an electrical state in the earth's covering of air, which is competent to explain the electrical manifestations of the air, the auroral gleam, and the mysterious effect on the magnetic needle which keeps it directed to the magnetic north. Can not we conclude that the study of the X rays bids fair to greatly extend our conceptions of the constitution of matter and of the action and interaction of Nature's forces?