88. The Propagation of X-Rays Rectilinear.—There would be no difficulty in producing photographs of the bones of the hand with the rays of light, if it were not for the tremendous amount of reflection and refraction causing so much diffusion that no sharply defined shadow of the bones would be produced. By means of a powerful lens and a funnel pointed into a dark room, the author noticed that the condensed light thereby obtained when passed through the hand, and when the incident rays were parallel, came out so diffused that one would think that the light went through bones as easily as any part of the hand. An experiment of this kind serves to emphasize that the success of sciagraphy by X-rays is due not only to the great penetrating power, but to practically no refraction nor reflection. In view of the sharp shadows cast of objects even when located in vegetable or animal media, Roentgen was justified in giving the name of ray to the energy. He tested the sharpness of the shadow by making sciagraphs and fluorescent pictures not only of the bones of the hand, but of a wire wound upon a bobbin, of a set of weights in a box, of a compass, card and needle, conveniently closed in a metal case, and of the elements of a non-homogeneous metal. To prove the rectilinear propagation further, he received the image of the discharge tube upon a photographic plate by means of a pinhole camera. The picture was faint but unmistakable.
89. Interference. The rays of light may be caused to interfere with each other. See Newton’s Principia, Vol. III.; Young’s Works, Vol. I.—Theory points out that waves of ether of two pencils of light, when caused to be propagated at certain relative phases partially or wholly neutralize or strengthen each other. Roentgen could obtain no interference effects of the X-rays, but did not conclude that the interference property was absent. He was not satisfied with the intensity of the rays and therefore could not test the matter severely.
Fig. L.
90. Electrified Bodies Discharged by X-Rays. p. [47].—After Roentgen’s first announcement, others, and probably J. J. Thomson as the first, found that the X-rays would discharge both negatively and positively electrified bodies. Roentgen, in his second announcement, stated that he had already made such a discovery, but had not carried the investigation far enough to report satisfactorily on the details. At last he put forth an account of the whole phenomena and stated that the discharge varied somewhat with the intensity of the rays, which was tested in each instance by the relative luminosity of the fluorescent screen, and by the relative darkness produced upon the photographic plate in several instances. Electrified bodies, whether conductors or insulators, were discharged when placed in the path of the rays. All bodies whatsoever behaved in the same manner when charged. They were all discharged equally by the X-rays. He noticed that “If an electrical conductor is surrounded by a solid insulator such as paraffin instead of by air, the radiation acts as if the insulating envelope were swept by a flame connected to earth.” Upon surrounding said paraffin by a conductor connected to earth, the radiation no longer acted on the inner electrified conductor. The above observations led him to believe that the action was indirect and had something to do with the air through which the X-rays passed. In order to prove this, it was necessary for him to show that air ought to be able to discharge the bodies if first subjected to the rays, and then passed over the bodies. The apparatus for performing an experiment to test this prediction is shown in Fig. [L], which serves to illustrate also the manner in which he prevented electro-static influences of the discharge tube, leading in wires and induction coil. [§ 71], near centre. For this purpose he built a large room in which the walls were of zinc covered with lead. The door for his entrance and exit was arranged to be closed in an air-tight manner. In the side wall opposite the door there was a slit 4 cm. wide, covered hermetically with a thin sheet of aluminum for the entrance of X-rays from the vacuum tube outside of the room. All the electrical apparatus connected with the generation of the X-rays was outside of the room. No force whatever came into the room, therefore, except the X-rays through the aluminum. [§ 71]. In order to show that air which had been subjected to the X-rays would discharge a body immediately afterwards upon coming in contact therewith, he arranged matters so that the air was propelled by an aspirator. He passed air along a tube made of thick metal so that the rays could enter only through a small aluminum window near the open end. At over a distance of 20 cm. from the window was an insulated ball charged with electricity, and connected to any electroscope or electrometer. The professor used a Hankel electroscope. No published sketch was made by Roentgen; therefore, that shown in the figure was produced by inference from the description. The operation was as follows: The X-rays passed into the room through the aluminum window, and then into the metal tube through its aluminum window. When the air was at rest, the ball was not discharged. When the aspirator was at work, however, so that the air moved past the aluminum window and past the ball, the latter was discharged whether electrified positively or negatively. He modified the operation by maintaining the ball at a constant potential by means of accumulators, while the air which had been treated by X-rays was passed by the ball. “An electric current was started as if the ball had been connected with the wall of the tube by a bad conductor.” He was not sure whether the air would retain its power to discharge bodies as long as it remained out of contact with any bodies. He determined, however, that any slight “disturbance” of the air by a body having a large surface and not electrified, rendered the air inoperative. He illustrated this by saying that “If one pushes, for example, a sufficiently thick plug of cotton-wool so far into the tube that the air which has been traversed by the rays must stream through the cotton-wool before it reaches the ball, the charge of the ball remains unchanged when suction is commenced.” With the cotton-wool immediately in front of the window, it had no effect, showing, therefore, that dust particles in the air are not the cause of the communication of the force of the discharge from the X-rays to the electrified body. Very fine wire gauze in several thicknesses also prevented the air from discharging the body when placed between the aluminum window and the ball within the thick metal tube, as in the case of the cotton plug. Similar experiments were instituted with dry hydrogen instead of air, and, as far as he could discern, the bodies were equally well discharged, except possibly a little slower in hydrogen. He experienced difficulty in obtaining equally powerful X-rays at different times. All experimenters are acquainted with this difficulty. Further, he called attention also to the thin layer of air which clings to the surface of the bodies, and which, therefore, plays an appreciable part in connection with the discharge. [§ 16], near end. In order to test the matter further as to discharge of electrified bodies, he placed the same in a highly exhausted bulb and found that the discharge was in one case, for example, only 1/70 as rapid as in air and hydrogen at ordinary pressure, thereby serving as another proof that gas was the intermediate agency. Allowance should be made in all experiments in connection with the discharging quality of X-rays. The surrounding gas should be taken into account.
90a. Application of Principle of Discharge by X-Rays.—Professor Robb, of Trinity College, (Science, Apr. 10, ’96), proposed and explained and practically tested the principle of the discharge of X-rays to determine the relative transparencies of substances to X-rays. He plotted a curve in which the co-ordinate represented the charge of the condenser in micro-coulombs, and the abscissæ the time between charging and discharging the condenser. The same plan could be adopted, he suggested, for making quantitative measurements of the intensity of X-rays from different tubes or the same discharge tube at different times. J. J. Borgmann, of St. Petersburg, probably was the first to show that X-rays charged as well as discharged bodies. See The Elect., Lon., Feb. 14, ’96, p. 501. Soon, a similar announcement was made by Prof. Righi, of Bologna. [§ 90].
90A. Borgmann and Gerchun’s Experiments. Action of the X-Rays on Electro-static Charges and (La Distance Explosive.) Comptes Rendus, Feb. 17, ’96; from Trans., by Louis M. Pignolet.—A positively charged zinc disk connected to an electroscope lost its charge almost instantly and acquired a negative charge. When the charge on the zinc disk was negative, the loss was much slower and was not complete, a certain charge remaining. When the rays fell upon two small platinum balls connected to the terminals of an induction coil but separated beyond its sparking distance, sparking took place between them, showing that X-rays, like ultra-violet rays, increase the sparking distance of static charges.
90b. Righi’s Experiments. Bodies In The Neutral or Negative State, Positively Electrified By X-Rays. Comptes Rendus, Feb. 17, 1896. From Trans. by Louis M. Pignolet.—The measurements were made by this eminent Italian physicist, with a Mascart electrometer connected with the bodies upon which the X-rays impinged and enclosed in a grounded metallic case (Faraday cylinder) provided with an aluminum window for the entrance of the rays. A metallic disk connected with the electrometer lost its charge rapidly whether positive or negative.
[§ 99S]. Initial positive charges were not completely dissipated; negative charges were not only completely dissipated but the bodies acquired positive charges. Disks in the neutral state were charged positively by the X-rays the same as takes place with ultra-violet rays. The final positive potential was greater for copper than for zinc and still greater for retort carbon (“le carbon de cornue”) [90c]. at end. The various results are not conflicting if the particular materials are taken into accounts. [90c] at end.
90c. The experiments of Prof. Minchin, an expert in such measurements, are properly described here, in that they seem to clear up the superficial ambiguity. He formulated the conclusion (The Elect., Lon., Mar. 27, ’96, p. 736) thus:—“The X-rays charge some bodies positively and some negatively, and whatever charge a body may receive by other means, the X-rays change it, both in magnitude and sign, to the charge which they independently give to the body.” Thus, in the case of magnesium, if the same is first positively charged by any suitable means, then will the X-rays not only discharge it, but electrify it negatively, while if this metal is first negatively charged, the X-rays either diminish or increase the discharge. It must be remembered, however, that this is not true with all metals, for he found that gold, silver, copper, platinum, iron, aluminum, bismuth, steel and antimony, are all positively electrified.