W. K. Rontgen, “the x-‐rays” (1895) 1
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1 It seemed that nothing in the natural world could stop the West’s progress. Western scientists, engineers, and inventors appeared able to surmount every obstacle and to find solutions to every problem. Even the invisible realms revealed their secrets to them. On November 8, 1895, while experimenting with electric current flow, the German scientist Wilhelm Röntgen (1845–1923) produced and detected radiation on the electromagnetic spectrum in a wavelength range now known as X-‐rays or—in many countries—Röntgen rays. For this achievement he won the first Nobel Prize in Physics in 1901. In addition to medical uses, including radiography and radiation therapy, X-‐rays were also found to help determine the structure of crystals, to test the soundness of diverse materials, and of course to screen passengers in airports. In the passages below, Röntgen describes the systematic observations and experiments by which he confirmed the existence and behavior of X-‐rays and his hypotheses as to their nature. His careful use of the scientific method is clearly shown. For the full text online, click here .
THE X-‐RAYS By W. C. RÖNTGEN
I.—UPON A NEW KIND OF RAYS 1. If the discharge of a great Ruhmkorff 2 induction coil be passed through a Hittorf 3 vacuum-‐tube, or a Lenard 4 tube, Crookes 5 tube, or similar apparatus containing a sufficiently high vacuum, then, the tube being covered with a close layer of thin black pasteboard and the room darkened, a paper screen covered on one side with barium-‐platinum cyanide and brought near the apparatus will be seen to glow brightly and fluoresce at each discharge whichever side of the screen is toward the vacuum tube. The fluorescence is visible even when the screen is removed to a distance of 2 meters from the apparatus. The observer may easily satisfy himself that the cause of the fluorescence 6 is to be found at the vacuum tube and at no other part of the electrical circuit.
1 W. C. Röntgen, “The X-‐Rays,” in Annual Report of the Board of Regents of the Smithsonian Institution (Washington: Government Printing Office, 1898), 137–39, 141, 142, 143. 2 Heinrich Daniel Ruhmkorff (1803–77) devised an induction coil used to produce high-‐voltage current from a low-‐voltage supply. 3 Johann Wilhelm Hittorf (1824–1914) was a German physicist who successfully calculated the electric capacity of charged atoms and ions. 4 Philipp Eduard Anton von Lenard (1862–1947) was a German physicist who won a Nobel Prize for Physics in 1905 for his research on cathode rays. 5 William Crookes (1832–1919) was a British chemist and physicist who developed a device that controls electric current in a container. 6 The emission of light by a substance that has absorbed light or other electromagnetic radiation. 2
2. It is thus apparent that there is here an agency which is able to pass through the black pasteboard impenetrable to visible or ultra violet rays from the sun or the electric arc, and having passed through is capable of exciting a lively fluorescence, and it is natural to inquire whether other substances can be thus penetrated. It is found that all substances transmit this agency, but in very different degree. I will mention some examples. Paper is very transmissible. I observed fluorescence very distinctly behind a bound book of about 1,000 pages. The ink presented no appreciable obstacle. Similarly fluorescence was seen behind double whist 7 pack. A single card held between the fluorescent screen and the apparatus produced no visible effect. A single sheet of tin foil, too, produces hardly any obstacle, and it is only when several sheets are superposed that their shadow appears distinctly on the screen. Thick wooden blocks are transmissible. Slabs of pine 2 or 3 centimeters thick absorb only very little. A plate of aluminum about 15 millimeters thick diminished the effect very considerably, but did not cause the fluorescence to entirely disappear. Blocks of hard rubber several centimeters thick still transmitted the rays. Glass plates of equal thickness behave very differently according to whether they contain lead (flint glass) or not. The first class are much less transmissible than the second. If the hand is held between the vacuum tube and the screen, the dark shadow of the bones is seen upon the much lighter shadow outline of the hand. Water, carbon, bisulphide, and various other liquids investigated proved very transmissible. I could not find that hydrogen was more transmissible than air. The fluorescence was visible behind plates of copper, silver, lead, gold, and platinum, when the thickness of the plate was not too great. Platinum 0.2 millimeter thick is still transmissible, and silver and copper plates may be still thicker. Lead 1.5 millimeters thick is practically impenetrable, and advantage was frequently taken of this characteristic. A wooden stick of 20 millimeters square cross section, having one side covered with white lead, behaved differently when interposed between the vacuum tube and the screen according as the X-‐rays traversed the block parallel to the painted side or were compelled to pass through it. In the first case there was no effect appreciable, while in the second a dark shadow was thrown on the screen. Salts of the metals, whether solid or in solution, are to be ranged in almost the same order as the metals themselves for transmissibility. 3. These observations and others lead to the conclusion that the transmissibility of equal thicknesses of different substances depends on their density. At least no other characteristic exerts so marked an influence as this. The following experiment shows, however, that the density is not the sole factor. I compared the transmissibility of nearly equally thick plates of glass, aluminum, calcspar, 8 and quartz. The density of these substances is substantially the same, and yet it was quite evident that the calcspar was considerably less transmissible than the others, which are about alike in this respect.
7 A card game. 8 Calcspar is a mineral and polymorph of calcium carbonate. 3
4. All bodies became less transmissible with increasing thickness. For the purpose of finding a relation between transmissibility and thickness I have made photographic exposures, in which the photographic plate was partly covered with a layer of tin foil consisting of a progressively increasing number of sheets. I shall make a photometric measurement when I am in possession of a suitable photometer. 5. Sheets were rolled from platinum, lead, zinc, and aluminum of such thickness that all appeared to be equally transmissible. The following table gives the measured thickness in millimeters, the relative thickness compared with platinum, and the specific gravity:
Thickness Relative Specific Thickness Gravity
Platinum . . . . . . . . . . . . . . . . 0.018 1 21.5 Lead . . . . . . . . . . . . . . . . . . . . 0.05 3 11.3 Zinc . . . . . . . . . . . . . . . . . . . . . 0.20 6 7.1 Aluminum . . . . . . . . . . . . . . . 3.5 200 2.6
From these values it may be seen that the transmissibility of plates of different metals so chosen that the product of the thickness and density is constant would not be equal. The transmissibility increases much faster than this product falls off. 6. The fluorescence of barium-‐platinum-‐cyanide is not the only action by which X-‐rays may be recognized. It should be remarked that they cause other substances to fluoresce, as for example the photophorescent calcium compounds, uranium glass, common glass, calcspar, rock salt, etc. It is of particular importance from many points of view that photographic dry plates are sensitive to X-‐rays. It thus becomes possible to fix many phenomena so that deceptions are more easily avoided; and I have where practicable checked all important observations made with a fluorescent screen by photographic exposures. It appears questionable whether the chemical action upon the silver salts of the photographic plate is produced directly by the X-‐rays. It is possible that this action depends upon the fluorescent light which, as is mentioned above, may be excited in the glass plate, or perhaps in the gelatine film. “Films” may indeed be made use of as well as glass plates. I have not as yet obtained experimental evidence that the X-‐rays are capable of giving heat. This characteristic might, however, be assumed as present, since in the excitation of fluorescent phenomena the capacity of the energy of the X-‐rays for transformation is proved, and since it is certain that of the X-‐rays falling upon a body not all are given up. The retina of the eye is not sensitive to these rays. Nothing is to be noticed by bringing the eye near the vacuum tube, although according to the preceding observations the media of the eye must be sufficiently transmissible to the rays in question. . . . 4
Taking this result together with the observation that powder is as transmissible as coherent substance, and further, that bodies with rough surfaces behave in the transmission of X-‐rays and also in the experiments just described exactly like polished bodies, the conclusion is reached that there is, as before remarked, no regular reflection, but that the bodies behave toward X-‐rays in the same manner as a turbid 9 medium with reference to light. As I have not been able to discover any refraction in the passage from one medium to another, it appears as if the X-‐rays travel with equal velocity in all bodies, and hence in a medium which is everywhere present and in which the particles of the bodies are embedded. These latter act as a hindrance to the propagation of the X-‐rays, which is in general greater the greater the density of the body in question. 9. In accordance with this supposition it might be possible that the arrangement of the molecules of the body would exert an influence on its transmissibility, and that, for example, a piece of calcspar would be unequally transmissible for equal thicknesses when the rays passed along or at right angles to the axis. Experiments with calcspar and quartz gave, however, a negative result. 10. It will be recalled that Lenard, in his beautiful experiments on the transmission of the Hittorf cathode rays through thin aluminum foil, obtained the result that these rays are disturbances in the ether, and that they diffuse themselves in all bodies. We may make a similar statement with regard to our rays. . . . Most other substances are, like the air, more transmissible for X-‐rays than for the cathode rays. . . . 12. According to the results of experiments particularly directed to discover the source of the X-‐rays, it is certain that the part of the wall of the discharge tube which most strongly fluoresces is the principal starting point. The X-‐rays therefore radiate from the place where, according to various observers, the cathode rays meet the glass wall. If one diverts the cathode rays within the tube by a magnet, the source of the X-‐ray is also seen to change its position so that these radiations still proceed from the end points of the cathode rays. The X-‐rays being undeviated by magnets cannot, however, be simply cathode rays passing unchanged through the glass wall. The greater density of the gas outside of the discharge tube cannot, according to Lenard, be made answerable for the great difference of the deviation. I come therefore to the results that the X-‐rays are not identical with the cathode rays, but that they are excited by the cathode rays in the glass wall of the vacuum tube. . . . 17. If the question is asked what the X-‐rays—which certainly are not cathode rays—really are, one might at first, on account of their lively fluorescent and chemical action, compare them to ultra-‐violet light. But here one falls upon serious difficulties. Thus, if the X-‐rays were ultra-‐violet light, then this light must possess the following characteristics: (a) That in passing from air into water, carbon bisulphide, aluminum, rock
9 Cloudy, opaque. 5
salt, glass, zinc, etc., it experiences no notable refraction. (b) That it is not regularly reflected by these substances. (c)That it cannot be polarized by the usual materials. (d) That its absorption by substances is influenced by nothing so much as by their density. In other words, one must assume that these ultra-‐violet radiations comport themselves quite differently from all previously known infra-‐red, visible, and ultra-‐ violet rays. I have not been able to admit this, and have sought some other explanations. A kind of relation seems to subsist between the new radiation and light radiation, or at least the shadow formation, the fluorescence, and the chemical action, which are common phenomena of these two kinds of radiation, point in this direction. It has been long known that longitudinal as well as transverse vibrations are possible in the ether, and according to various physicists must exist. To be sure, their existence has not, up to the present time, been proved, and hence their characteristics have not thus far been experimentally investigated. Should not the new radiations be ascribed to longitudinal vibrations in the ether? I may say that in the course of the investigation this hypothesis has impressed itself more and more favorably with me, and I venture to propose it, although well aware that it requires much further examination. WÜRZBURG, PHYSIK. INSTITUT D. UNIV., December, 1895. Download 51.98 Kb. Do'stlaringiz bilan baham: 
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