The Roentgen Rays

By EDWIN J. HOUSTON, Ph.D., and A. E. KENNELLY, Sc.D.

The announcement of a scientific discovery has seldom produced so intense an excitement throughout the civilised world as has that made in December last, by Prof. W. C. Röntgen, of Würzburg, Bavaria, concerning his ability to obtain a photograph of the bones of an animal through its living integument. His announcement, indeed, has created so intense an excitement and enthusiasm that, although the discovery is practically only a few weeks only, yet his experiments have been repeated in all parts of the world.

The authors have been requested, by the Franklin Institute, to present to its members a résumé of the facts that are known concerning Röntgen's discovery, as well as of some of the theoretical explanations which have been offered in connection therewith.

The paper in which Prof. Röntgen announced his discovery was addressed to the Würzburg Physico-Medical Society in December last. Prof. Röntgen describes in this paper the method he adopted for obtaining the peculiar form of radiant energy now generally known as the Röntgen rays, and for which he proposed the provisional name of X rays. On passing the discharge from a large Ruhmkorff coil through a suitably exhausted tube of the Hittorf, Lenard, or Crookes type, a peculiar effect, invisible to the eye, is produced in the region outside the tube. This effect, however, can be rendered visible by the fluorescence excited in a paper screen, painted with a solution of barium-platino-cyanide. In vacuum tubes of the type referred to, the passage of the proper discharge is attended by the production of rays, called cathode rays — that is, rays projected in straight lines from the cathode, or negative electrode, of the vacuum tube.

It was observed by Crookes that these cathode rays were not produced to an appreciable degree until a certain high vacuum was reached. Crookes showed by his experiments that, in so high a vacuum, matters could be regarded as existing in a fourth state or condition, to which he gave the name of the

Radiant or Ultra-Gaseous State.

The highly rarefied matter contained in these vacua is capable of producing characteristic effects when subjected to heat or electrification. For example, when an electric discharge is passed through the residual atmosphere in the tubes, the molecules are shot off in rectilinear paths from the cathode, and wherever they strike the walls of the glass tube, or suitable bodies placed in the tube, they produce a faint fluorescent light. Crookes showed that these cathode rays were capable of being deflected from their original rectilinear paths by the action of a magnet. They thus behave in conformity with the view that they consist of streams of negatively charged molecules, the deflecting action being merely a particular kind of electro-magnetic action, well seen in the deflection of a voltaic arc by a magnet, or in the electro-magnetic motion of the armature of the electric motor.

Hertz discovered that the cathode rays readily pass through thin films or screens of mica, or even of metal. He showed that, in fact, metallic films were even more transparent to the cathode rays than films of mica. For example, when such metallic screens were interposed in the vacuum tube between the cathode and the glass walls on the tube opposite the cathode, fluorescence was still developed in portions of the tube which would have corresponded to the shadow of the screen, had it been opaque.

Lenard, at one time an assistant of Hertz, took the matter up, and showed that fluorescence could be produced by the cathode rays outside the glass vacuum tube. For this purpose, he placed a small window of aluminium in the walls of his vacuum tube, so that the stream of cathode rays would impinge upon this window. A change of some sort appeared to occur at the window. While in the tube the rays proceeded along straight lines, unless deflected by a magnet; outside the window they preserved rectilinear paths, but radiated in all directions from the window as a source. These rays, produced outside the vacuum tube by the cathode rays within the vacuum tube, may be called

The Lenard Rays.

In common with the cathode rays, the Lenard rays possess the power of producing fluorescence in fluorescent substances upon which they impinge. They differ from the cathode rays in the fact that, in the air, they are visible to the eye as a faintly glowing stream or pencil, which rapidly diminishes in intensity, at comparatively short distances from the window. All the effects of the Lenard rays became lost at a distance of a few centimetres from the window, so rapid is their absorption in the air or other media. Hydrogen was found to be the least absorptive—that is, the most transparent medium; but hydrogen, under great pressure, becomes more opaque. Lenard showed that many of the metals are partially transparent to his rays, and that sensitive photographic plates, or films, are quickly blackened by their influence. In fact, Lenard took shadow photographs of opaque or semi-opaque bodies, by interposing them in the path of the rays before they impinged upon the sensitive photographic plate held in a light-tight box, provided with an aluminium cover or slide. Lenard found that the opacity of different substances to his rays varied, approximately, with their density.

In the paper before referred to, Röntgen announced that, when a suitably exhausted vacuum tube is placed under a cover of blackened pasteboard, fluorescent effects can be obtained, not merely at a distance of a few centimeters, but even at a distance of two meters, in air. He employed for this purpose a paper screen painted with a solution of barium-platino-cyanide. The Röntgen rays, unlike the Lenard rays, are invisible to the eye, even close to the vacuum tube. No window is necessary in the vacuum tube to produce the Röntgen effect, although Röntgen has produced the effect through an aluminium window. As in the case of the Lenard rays, the Röntgen rays are radiated in rectilinear paths in all directions from the portions of the walls of the tube subjected to the bombardment of the cathode rays.

It will thus be seen that a similarity exists between the Lenard and the Röntgen rays, so far as their method of production is concerned. In each, the source of the rays is to be traced to the impact of the cathode rays upon the wall of the vacuum tube in which they are produced. In each, the rays possess the power of exciting fluorescence in suitable bodies on which they impinge; in each, they possess the power of passing through substances opaque to ordinary light, the opacity in each case increasing apparently with the density, although not in direct proportion.

The Röntgen rays appear to differ from the Lenard rays in the fact that they are not so readily absorbed by air or other media. If no absorption existed, the intensity of the rays would, of course, diminish as the square of the distance from their source. Röntgen claims that he has observed that this rate of diminution in the intensity of his rays does exist within the limits of observational error. This peculiarity of the Röntgen rays has greatly enhanced their practical value.

Similarity of the Lenard and Roentgen Rays.

It will be seen, therefore, that the difference between the Lenard and the Röntgen rays is not marked. The difference appears to be one of degree rather than one of kind. There is, however, at least one difference in the rays under discussion. Lenard found that his rays are not appreciably deflected by a magnet when passing through air, but that deflection does occur when his rays were allowed to traverse a highly-exhausted chamber. Röntgen mentions that, up to the present time, he has not been able to produce any deflection of his rays by a magnetic field. This has been confirmed by Dr. Oliver Lodge. Röntgen is also experimenting with the action of an electrostatic field, but we believe he has not as yet announced any result.

The curious fact pointed out by Röntgen in the paper before referred to of the ability of his rays to produce upon a sensitive photographic plate photographs of the shadows of, say, the bones of the hand in the living subject, has naturally caused great popular excitement and interest. There can be no doubt that this fact will prove of great value to the surgeon, especially if, as would seem probable, means are perfected for the production of these rays with increased power.

Röntgen states, as a result of observation, that his rays are apparently incapable of reflection, refraction, or interference. If refraction does exist, it must be of very limited amount. These facts have led Röntgen to suggest that his rays are not light in the ordinary physical sense of the term; that is, that they do not consist of a transverse vibration propagated in the ether, according to electro-magnetic laws, but that possibly they may be a longitudinal motion propagated in the ether, a condition which Prof. S. P. Thompson has happily named "ultra-violet sound," and which others have called "ethereal sound" and "longitudinal light."

The details of the method for producing Röntgen rays do not appear yet to have been published by Prof. Röntgen. Physicists, however, have very generally reproduced them, although thus far the shadowgraphs obtained in this country do not present the sharpness and depth which apparently characterise Prof. Röntgen's shadowgraphs.

Modus Operandi.

The authors have successfully obtained Röntgen rays by the use of a large induction coil excited directly by storage cells through an interrupter, the secondary terminals of the induction coil being led directly to the terminals of the Crookes tube. In this case the cathode is the source of cathode rays in the tube, and the surface of the glass opposite to the cathode, where fluorescence is produced, is the source of the Röntgen rays. We have found, however, as others have done, that by far the best results are obtained by using a high-frequency discharge through the Crookes tube. This is conveniently done by exciting the primary of the induction coil from a 50-volt alternating-current circuit, such as is employed for supplying incandescent lamps on commercial alternating-current circuits at, say, 15,000 alternations per minute. The secondary terminals of the induction coil are led to a battery of Leyden jars through the primary coil of a Tesla induction coil immersed in oil. The secondary terminals of the Tesla coil are then connected directly to the Crookes tube. Under these conditions torrents of high-frequency discharges pass between the discharging knobs of the induction coil, which are separated to a distance of perhaps five millimeters, the frequency being determined by the capacity and inductance of the Leyden-jar circuit, including the Tesla primary. These high-frequency discharges induce in their turn high-frequency and high-tension discharges in the Crookes tube. In such cases both electrodes of the Crookes tube are alternately cathodes, and the glass wall opposite to each electrode becomes fluorescent, and, therefore, the source of Röntgen rays.

The practical difficulties in connection with the production of the Röntgen rays arise from the fact that the Crookes tubes, as ordinarily constructed, are not designed to stand the molecular bombardment of the cathode rays, when of fairly great power, for any considerable length of time. Since the ordinary Röntgen shadowgraphs require exposures lasting usually half an hour, and sometimes several hours, a great liability exists to excessive heating of the tubes and the destruction of their vacua. Indeed, at times, the heat produced by the molecular bombardment of the cathode rays is sufficient to melt the glass walls of the tubes, as the authors, most probably in common with others, have ascertained to their loss.

Since the high-frequency discharges obtained by the use of a Tesla coil are not characterised by such marked heating effects, either at the walls of the tubes or in the electrodes, they are greatly to be preferred for producing shadowgraphs. Independently of this, however, they appear to produce a more powerful image. Possibly the velocity with which the molecules are projected from the cathode, or their number, or both, are increased.

Tesla Coil.

A simple form of Tesla coil may be made by winding a primary of about eighty turns of No. 19 A.W.G. cotton-covered copper wire over a glass tube about ¾in. in diameter, and inserting this inside a slightly larger glass tube, over the surface of which has been wound a secondary of about 400 turns of about No. 31 A.W.G. fine silk-covered copper wire. The coil is then immersed in some high insulating oil, such as rosin oil.

In the shadowgraph of such portions of the human body as, for example, the hand, the bones cast the most marked shadows, and the surrounding tissues produce shadows intermediate in intensity. The density of the shadow which is cast upon the plate depends apparently upon the total quantity of matter which the rays have to traverse. Thus, a short passage through a dense substance produces the same depth of shadow as a long passage through a rare medium. In order to obtain sharp definition, the use of a diaphragm is recommended. An iron or lead plate provided with a suitable aperture, and placed between the tube and the photographic plate, is suitable for the purpose.

It is stated by Prof. Silvanus P. Thompson that distinct shadow photographic effects have been produced by the ordinary arc light, so that it would appear that some Röntgen effect is produced by the voltaic arc.

It has recently been discovered by Prof. Pupin and others that, in order to obtain the cathode rays in a vacuum tube, it is not necessary to employ electrodes within the tube itself; but if pieces of tinfoil are placed outside the vacuum tube, and the terminals of the high-tension coil are connected therewith, discharges are produced in the vacuum tube by electrostatic induction. These discharges produce fluorescence in the tube, and the fluorescent glass is again the region of Röntgen rays.

Molecular Bombardment.

This would appear to show that the Röntgen rays are produced by molecular bombardment by the molecules of the residual atmosphere of the high vacuum, and that the metallic cathode is only to be regarded as an auxiliary means for producing this bombardment.

Prof. J. J. Thomson has recently made the extremely important discovery that the Röntgen rays tend to discharge an electrified body on which they impinge, and this whether the charge of the body be positive or negative. Prof. Thomson finds that a charged body constitutes a more sensitive test for the presence of the Röntgen rays than is afforded by either a sensitive photographic plate or a fluorescent screen. This effect takes place whether the charged body be covered by a layer of paraffin, sulphur, or vulcanite. In other words, as Prof. Thomson points out, the remarkable fact exists that an insulating layer becomes electrically leaky while being traversed by the Röntgen rays.

Hitherto no means have been known, beyond mere thermal influence, for altering the electrical insulating power of solid materials by radiant energy of any character. The discovery of this property of effecting electric discharge makes the ray far more wonderful and important to physicists than its property of penetrating opaque solids as displayed in shadowgraphs.

The degree of exhaustion in the vacuum tube, which is the source of the Röntgen rays, appears to be that required for rectilinear molecular bombardment. Edison has shown, experimentally, that below a certain exhaustion the Röntgen effect is negligibly small, while above a certain exhaustion the intensity of the effect again decreases. In general, it may be said that a vacuum which produces a bluish violet light on the passage of the discharge is too low to produce the Röntgen effect. The best results are obtained when little or no discharge is visible within the tube; but the walls are rendered fluorescent with a peculiar greenish tinge. This is usually obtained at about one-millionth of an atmosphere.

Since the Röntgen effect is evidently due to molecular bombardment, and varies with the degree of the vacuum, it would appear that good experimental work can be conducted, both by ascertaining the best degree of vacuum in tubes containing rarefied air, and by ascertaining whether gases other than air will produce, in such tubes, better results. It would appear that the Röntgen rays must be either a material propagation or an ethereal propagation. A material propagation would require either that ordinary matter be projected rectilinearly as a whole along the rays, or that some disturbance in matter is projected along them.

The burden of probability seems to be against the assumption that the Röntgen rays consist of a material propagation, since they not only traverse opaque, dense substances, but also appear to be unaffected by a magnet. Assuming, therefore, that the Röntgen rays are an ethereal propagation, they must either consist of a translatory movement of the ether, or of some disturbance propagated along it. Such a disturbance would, probably, have to be periodic or vibratory. The vibrations might be transverse, or they might be longitudinal to the path of the ray. Physical science is acquainted with transverse ethereal vibrations, such being, in fact, electro-magnetic vibrations, or ordinary light. These vibrations differ in their frequency, and hence in their wave-lengths, but are not known to differ in their speed through free ether. In the ether within material substances their velocity of propagation is reduced, as is evidenced by their refrangibility. This is true of all known transverse vibrations from the lowest frequency, far below the red, produced by the discharge of a Leyden jar, and detected only by electrical methods; through the higher-frequency waves, still in the infra-red and constituting direct heat; throughout the limits of the visible spectrum, and beyond the violet in that frequency which is detectable only by chemical or by electric eyes. The marked characteristics of these transverse vibrations are their refrangibility, their reflectibility, and their possibility of interference — characteristics which are all apparently wanting in the Röntgen rays.

It would, therefore, appear that the Röntgen rays are not transverse vibrations of the ether. If they are, they do not appear to lie within the range of frequency hitherto explored, a range extending from about 100,000 to about 1,000,000,000,000,000 (a quadrillion) waves per second. There is reason to believe that, if they have a wave-length, this wavelength is long, compared with that of visible light, since they pass through appreciable distances of gross matter, distances measured in centimètres or hundredths of a mètre, rather than in microns or millionths of a mètre. Such waves would correspond to dark heat-waves, in the infra-red.

It is known that ultra-violet light possesses the power of discharging a negatively charged body upon which it falls directly, while, as Prof. J. J. Thomson has discovered, the Röntgen rays can discharge bodies either positively or negatively charged, even while imbedded in a layer of highly insulating material.

Longitudinal Ethereal Vibrations.

Dr. Lodge has shown that, when polarised ultra-violet light strikes such a negatively charged polished plate, it is most effective in producing discharge when striking at an angle, instead of perpendicularly, and, consequently, the discharge may, perhaps, be effected by a longitudinal component of the impact. This seems to favour the supposition that the Röntgen rays are longitudinal ethereal vibrations.

The material substances with which we are familiar, and which are capable of transmitting vibrations transversely to the direction of propagation, are also capable of transmitting longitudinal vibrations. The fact that longitudinal vibrations in the ether have hitherto remained undiscovered has caused no little inconvenience in the framing of hypotheses concerning the action and properties of the ether. If ether resembles material substances, its velocity of propagation for longitudinal vibrations would be much greater than that of its velocity of propagation for transverse vibrations; consequently, for any given frequency of vibration, the wavelength would be greater than that of light waves.

Summing up, then, while it is far too early to form a reliable conclusion concerning the nature of the Röntgen rays, such evidence as does exist would appear to favour the theory of longitudinal vibrations in the ether.

Whatever may be the future of the Röntgen rays, so far as their practicable applicability or scientific value is concerned, great praise must be accorded to Prof. Röntgen for the exceedingly interesting and important contribution he has made to this branch of molecular physics,

* From the Journal of the Franklin Institute.