W. K. RÖNTGEN

WILHELM KONRAD VON ROENTGEN was born at Lennep, Prussia, in 1845. He was educated at Zurich, and became professor of physics at Strasburg, and in 1885 at Würzburg. His discovery of the so-called X-rays was made in 1895.

THE X-RAYS

1.-UPON A NEW KIND OF RAYS

If the discharge of a great Ruhmkorff induction coil be passed through a Hittorf vacuum tube, or a Lenard's, Crookes', 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 is to be found at the vacuum tube and at no other part of the electrical circuit.

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 a double whist 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, 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.

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:

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.

7. After I had discovered the transmissibility of various bodies of relatively great thickness I hastened to investigate whether or not the X-rays were refracted in passing through a prism. Experiments with water and carbon bisulphide in mica prisms of 30 degrees refracting angle showed no deviation either when observations were made with the fluorescent screen or with the photographic plate. For comparison, the deviation of light rays was observed under the same conditions. The refracted portion lay from 10 to 20 centimeters distant from that not refracted. With prisms of hard rubber and aluminum of about 30 degrees refracting angle I obtained exposures on a photographic plate which perhaps indicated a slight refraction. This is, however, very doubtful and the deviation is, if present, so small that the index of refraction for X-rays in these substances can not exceed 1.05. I could not observe with the fluorescent screen any deviation in these cases. Experiments with prisms of the denser metals have so far yielded no certain results on account of the slight transmissibility and the consequent decrease of the intensity of the transmitted ray.

In consideration of these results on the one hand, and on the other of the importance of the question whether or not the X-rays in passing from one medium to another undergo refraction, it is very gratifying that this question may be investigated by other means than by the help of prisms. Finely pulverized bodies in suitable layers allow but little. light to pass, in consequence of refraction and reflection. If now the X-rays are transmitted equally well through powder as through the coherent substance, equal masses being presupposed, it is proved that neither refraction nor regular reflection is present in any marked degree. This experiment was performed using finely pulverized rock-salt, finely divided silver, obtained by electrolysis, and the zinc dust so frequently utilized in chemical processes. In no case was any difference in transmissibility between the powder and the coherent substance detected either by the use of the fluorescent screen or the photographic plate.

It follows of course from the results thus obtained that the X-rays can not be concentrated by the use of lenses; and, indeed, a great hard