CHAPTER XIVTheoretical Considerations.

Before attempting to discuss the facts now known in regard to the Roentgen phenomena, it is well to review briefly the known ways in which radiant energy may be transmitted.

By radiant energy is, of course, meant energy proceeding outward from a source and producing effects at some distant point. There are two well understood ways in which energy may be transmitted,—first, by an actual transfer to the distant point of matter to which the energy has been imparted from the source, as in the flight of a common ball, a bullet, or a charge of shot. In this mode of transmission, it is evident that the flying particles, assuming that they are subject to no forces on the way, will move in straight lines from the source to the distant point. They constitute real rays, diverging from the source; an obstacle in their path, would, if the radiations proceeded from a point, cast a shadow with sharply defined edges.

Second,—by a transfer of the energy from part to part of an intervening medium, each part as it receives the energy, transmitting it at once to the parts around it, no part undergoing more than a slight displacement from its normal position. This mode of transmission constitutes wave motion. The source imparts its energy to the particles of the medium near it. Each of those particles transfers its energy to the particles all around it. Each of these particles in turn transfers its energy to the particles around it, and so on through the medium. It is plain that there are here no such things as genuine rays. As the energy is transferred from particle to particle, each in turn becomes a centre of disturbance transmitting its motion in all directions. It is only because the movements transmitted from different points annul one another except along certain lines, that we have apparent straight lines of transmission, and, therefore, fairly sharp shadows. But shadows produced by wave transmissions are never absolutely sharp. The wave movement is always propagated to some extent within the boundary of thegeometrical shadow, less as the wave lengths are shorter. With sound waves whose lengths are measured in inches or feet, the penetration into the shadow is considerable. With light waves 1/37000 to 1/70000 of an inch in length, the penetration into the shadow is very small and requires specially arranged apparatus to show that it exists.

This penetration into the geometrical shadow is characteristic of energy propagated by wave motion, and if the fact of such penetration can be demonstrated, it is conclusive proof of propagation by waves.

Another characteristic of wave motion is found in the phenomena ofinterference. This is the mutual effect of two wave systems, which, when meeting at a given point, may strengthen or annul each other according to the conditions under which they meet. Either of those characteristics should enable us to distinguish between propagation by wave motion and by projected particles. But when wave lengths are very short and radiations feeble, the tests are not easy to apply.

Again, a wave is in general propagated with different velocities in different media. This causes a deflection or deformation of the wave as it passes from one medium into another, and results inrefraction, as in the cases of light and sound. Absence of refraction would be strong though not conclusive evidence against a wave theory of propagation.

In wave propagation, each particle of the medium suffers a small displacement from its equilibrium position and performs a periodic motion about that position. This displacement may be in the line of propagation—longitudinal vibration—or it may be in a plane at right angles to that line—transverse vibration. All the phenomena mentioned above, diffraction, interference, refraction, and also reflection, belong equally to either mode of wave propagation. Other phenomena must be made use of to distinguish between these.

When the vibrations are transverse they may all be brought into one plane through the line of propagation. They may bepolarized, when the ray will present different phenomena upon different sides. When the vibrations are longitudinal, no such phenomena can be produced. Polarization, then, serves to distinguish between longitudinal and transverse vibrations.

Now let us consider briefly the Roentgen ray phenomena that bear upon the question of the nature of the propagation.

From Sciagraph of Normal Elbow-joint; Straight, in Position of Supination.By A. W. Goodspeed.Phot. Times, July, ’96.Copyright, 1896, by William Beverley Harrison, Publisher of “X-ray” Pictures, New York.

It seems to be settled beyond question that the origin of the Roentgen rays is the fluorescent spot in the discharge tube.§ § 107,108,111. The evidence seems overwhelming that withinthe tube, the phenomena are the result of streams of electrified particles of the residual matter, shot off from the cathode in straight lines, perpendicular to its surface.§ 57. This was Crookes’ original theory,§ 53,near centre, and it seems to have stood well the test of scientific criticism. These flying particles falling upon anything in their path, give rise to X-rays. It is preferable, but not essential, that the bombarded surface should be connected electrically with the anode.§ § 113, and116. The best results are obtained by using a concave cathode, and placing at its centre the surface which is to receive the bombardment, thereby concentrating the effect upon a small area.

Nearly all experimenters agree in locating the origin of the X-rays at this bombarded spot. The energy here undergoes a transformation, and the X-rays represent one of the forms of energy developed.

What are the characteristics of this particular form of radiant energy?

It causes certain salts to fluoresce,§ § 66,84, and132, and it affects the photographic plate.§ § 70and84. In these respects, it is like the short wave length radiations from a luminous source. It is, however, totally unlike these in its power of penetrating numerous substances entirely opaque to light, such as wood, paper, hard rubber, flesh, etc. In passing through hard rubber and some other opaque insulators, X-rays are like the long wave length radiations from heated bodies, but X-rays penetrate many substances that are opaque to these long wave length radiations, and they are especially distinguished from all forms of radiant energy previously recognized, in their relative penetrating power for flesh and bones which makes it possible to obtain the remarkable shadow pictures which have become within three or four months, so familiar to all the world.

But these phenomena, although they serve to distinguish the X-rays from all other forms of radiant energy, do not furnish any clew to the nature of the X-rays themselves.

In attempting to formulate a theory of X-rays, the idea that first naturally presents itself is that they are due to some form of wave motion.

From Sciagraph of Knee-joint, Straight, Side View, showing Patella, or Knee-cap.By Prof. Goodspeed.Phot. Times, July, ’96.

The characteristics of wave motion are diffraction and interference phenomena. So far, no positive evidence of diffraction,§ 110, nor interference,§ 89, have been recognized, although experiments, have been tried that would have shown plainly, diffraction phenomena, had light been used in place of the Roentgen radiations.§ 170. We must, therefore, conclude, either that the Roentgen radiations in the experiments weretoo feeble to produce a record of the diffraction effects, or, that they are not due to wave motion at all, unless of a wave length very small even when compared with waves of light. The absence of refraction is also opposed to any wave theory of the Roentgen radiations, for it is difficult to believe that waves of any kind could travel with the same velocity through all media, which they must do if they suffer no deviation.§ 86.

The next supposition naturally is, that the phenomena are due to streams of particles. It has been suggested that the rays may be streams ofmaterialparticles, but this theory cannot be maintained in view of the fact that the rays proceed, without hindrance, through the highest vacuum.§ § 72band133,near end. Neither is it consistent with the high velocity of propagation. Molecules of gas could not be propelledthrough airwith any such velocity or to any such distance as X-rays are propagated. Tesla has claimed§ 139, that the residual gases are driven out through the glass of the vacuum bulb by the high potential that he employs. This has not been confirmed by other experimenters. It has been observed that the vacuum may be greatly improved by working the bulb,§ 121, that is, sending the discharge through it, but experimenters generally have found that heating the bulb impairs the vacuum and restores the original condition. The gases, were, therefore, occluded during the electrical discharge, to be again set free by heating the bulb.§ 139b. The rays may be ether streams, perhaps in the form of moving vortices, but of such streams we have no independent knowledge, and can only determine by mathematical analysis, what their characteristics should be. They would not suffer refraction, and would not produce interference nor diffraction phenomena. Whether they woulddowhat the X-rays do, go through the flesh and not through bone, through wood and not through metal, excite fluorescence, or affect the photographic plate, cannot be said. There is evidence that there are at least two kinds of X-rays,§ 152, differing in penetrating power, though perhaps not differing in other respects.

From Sciagraph of Normal Knee-joint, Flexed.Phot. Times, July, 96.Copyright, 1896, by William Beverley Harison, Publisher of “X-ray” Pictures, New York.

X-rays have their origin only in electrical discharges in high vacua. They are absent from sun-light and from light of the electric arc, and other sources of artificial illumination,§ 136. Proceeding from the bombarded spot, they are not deflected by a magnet, except in an evacuated observing tube, as proved by Lenard,§ 72a, and show no evidence of carrying an electric charge like cathode rays,§ 61b, p.47. On the contrary, they will discharge either a negatively or positively charged body intheir path. The evidence seems conclusive (Chap.VIII.) that the ultra-violet rays from an illuminating source also discharge charged conductors. In this respect, therefore, there is a similarity between the X-rays and ultra-violet light.

The action of the waves of light upon a cell formed of selenium lowers the resistance of the latter and herein is circumstantial evidence at least, concerning the similarity of the properties of X-rays and light, because the former are also found to increase the conducting power of selenium.§ 171.

The experiments of Roentgen,§ 90, seem to show that the discharging effect of X-rays is due to the air through which the rays have passed.

It is certain that the discharge of electrified bodies by light occurs more generally for negatively than for positively charged bodies,§ § 99B,99I, and99S, that it depends upon the nature,§ 97b, and density,§ 97a, of the gas surrounding the body, and also upon the material of the charged body itself.§ 98. The discharge would, therefore, seem to be connected with a chemical action,§ 153,near end, which is promoted by the rays. This seems all the more probable, since it was found,§ 98, that the more electro-positive the metal, the longer the wave length that would influence the discharge. In this connection, it is well to note that Tesla found,§ 146a, that in their power of reflecting (or diffusing X-rays), the different metals stand in the same order as in the electric contact series in air, the most electro-positive being the best reflectors. It would be interesting to know whether connecting the reflecting plate to earth, would, in any way, vary its reflecting power.

The X-rays seem to discharge some bodies, when positively charged, and other bodies when negatively charged. They will also give to some bodies a positive, and to others a negative charge (§ 90c). Is the order here also that of the electrical contact series in air? Are not all the phenomena of electrical charge and discharge, of reflection or diffusion, and of X-rays, connected with chemical action, as the apparent difference of potential, due to contact, undoubtedly is?§ 153.

An experiment by La Fay (§ 139a) seems to show that X-rays, in air, after passing through a charged silver leaf, acquire the property of being deflected by a magnet, as are the cathode rays inside the generating or exhausted observing tube,§ 72a. If this is confirmed, it would go far to support the theory that these rays are streams ofsomething.

From Sciagraph of Head by Prof. Goodspeed. Nasal Bones appear like Eyelashes.Inter. Med. Mag., June, ’96.The cervical vertebræ are distinguishable in the original, but barely so in the half-tone. Fillings are located.

The burden of proof, up to the present, seems to be against any wave theory of the X-rays, for, although they are like theultra-violet rays in producing fluorescence and in affecting the photographic plate, and have some points of similarity to these rays in their effect upon charged bodies, the X-rays are totally unlike the ultra-violet, in respect to diffraction and interference phenomena. In fact, the absence of such phenomena, if they are really absent, is conclusive proof that the X-rays cannot be wave motions, unless of a wave length extremely short even as compared to waves of light.

Since writing the above, I have seen an account of experiments in relation to diffraction of X-rays, presented to the French Academy by MM. L. Calmette and G. T. Huillier, in which the authors claim to have obtained evidence that diffraction occurs. The following translation of MM. Calmette and Huillier’s paper is taken from theElectrical Engineer, N.Y., for July 22, 1896.

“We have the honor of submitting to the Academy some photographic proofs obtained with the Röntgen rays by means of the following arrangement.”

“Very near the Crookes tube there is a screen “E” (diagram omitted), of brass, perforated by a slit, the width of which has rarely reached a half mm. A second metal screen,E´, is formed of a plate provided with two slits or pierced with a window in which is fixed a metal rod of 1 mm. in diameter. This screen is placed at the distance,a, behind the former. Lastly, a photographic plate, enfolded in two leaves of black paper, is placed at the distance,b, behind the second screen,E´.”

“The following table indicates, for each proof, what is the screenE´used, and the value ofaandb+a:

E´.

No.aCm.b+aCm.1.Rod of 1 mm. in diameter519.53.Rod of 1 mm. in diameter5.5205.Rod of 1 mm. in diameter8.9307.Two narrow slits, separated by a cylindrical rod of 1 mm. in diameter??

“On the proofs 1, 3, 5 the shadow thrown by the metallic rod is bordered on each side by a light band which shows a maximum of intensity. Within this shade we observe a zone less dark, which seems to indicate that the Röntgen rays penetrate into the geometrical shadow. Lastly, in proofs 3 and 5 we see, in like manner, a maximum of intensity along the margins of the window in which the rod is placed.”

“In the proof No. 7 we perceive, in the middle of the twowhite bands, a fine dark ray, while in the shadow of the rod which separates the two slits there is seen a light ray.”

“If we compare these results with those obtained with light in the same conditions, the slit being relatively wide and the intensity weak, it seems difficult not to ascribe them to the diffraction of the Röntgen rays.”

“The proofs obtained in these experiments—which we propose to continue—are not yet so distinct that we can measure the wave length with any precision. But we are still led to believe that this wave length is greater than that of the luminous rays.—Comptes Rendus.” Of course, if diffraction phenomena can be demonstrated, the question as to the radiations being wave propagations, is settled, though the question whether the vibrations are longitudinal or transverse, is still open.

Before accepting any stream or vortex motion theory, we need to know more about the X-ray phenomena, and more about stream and vortex motion.

Transcriber’s Notes:Since this is a collection of articles by different scientists, there may be differences in spelling and usage.There is no experiment 64. There last experiment on page51is number 63b, and the first experiment page52is 65. (A note attached to the 63btable of contents entry says that there is no experiment 64.)The missing entries in the Table of Contents for experiments 128aand 149awere added.There are two experiments labeled 61b, page46(Thomson’s Experiment) and47(Perrin’s Experiment). The instance on page47was relabeled 61c.There are two experiments numbered 159 in the Table of Contents. The first follows entry 110aand the other is in order, after experiment 158. The first instance was changed to 159a.Missing or obscured punctuation was silently corrected.Typographical errors were silently corrected.Inconsistent spelling and hyphenation were made consistent only when a predominant form was found in this book.

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