Lenard has described the phenomenon of cathode rays very fully, and has investigated it with an apparatus which, in its essentials, consisted of a Crookes tube (Fig. 28, page 193) provided with a window.

The window is hermetically closed by a thin aluminium sheet through which the rays pass into the outer room, and the whole apparatus is inclosed by a metallic vessel, which is connected to the ground to lead off disturbing electric charges. A quartz plate one half millimetre thick, placed between the window and a phosphorescing body, extinguishes the light. Thin plates of aluminium, gold, and silver allow the light to pass through. The light appears between such plates and the window, and also beyond the plates, while the plates themselves remain dark. The opacity of the quartz plates and the transparency of the metal plate to these rays form the marked difference between the cathode rays and the light rays. It is probable, however, that all substances in sufficiently thin sheets are transparent to the cathode rays. Soap films stretched on wire supports cut off the rays when they are thicker than 0.0012 millimetre. Aluminium plates, however, 0.027 millimetre thick allow the rays to pass.

In the case of light, one body in a layer one hundred millionth of a metre thick can be more opaque than another body a metre thick. Such enormous differences, however, do not exist in the case of the cathode rays. The ordinary atmosphere is an unfit medium for the passage of the cathode rays, for they speedily lose in it their movement in a straight line and become greatly diffused. The rays from the cathode window are diffused very much as the rays of sunlight are diffused in passing from a slit or opening into milk and water.

No heating effect of the cathode rays has been

detected. Lenard placed a delicate thermal junction in the rays which showed no heat effects, while a candle at the distance of fifty centimetres gave with the same thermal junction a marked effect.

It was noticed that electrified bodies lose their charges when the cathode rays fall upon them, or when they are placed in the neighbourhood of the window through which the rays pass. It is well known that when the exhaustion in the tubes in which the cathode rays are produced is pushed to an extreme, so that the vacuum is well-nigh perfect, the rays can no longer be produced, and, in fact, all electrical manifestations visible as light phenomena disappear. The vacuum seems to afford an infinite resistance to electricity. To test the question whether the cathode rays could travel in a vacuum, although they could not be excited in it, Lenard arranged a tube 1.50 metre long (4·92 feet), into which the cathode rays could pass from the rarefied tube in which they were generated. This long tube was then made as perfect a vacuum as modern methods permit. The pressure of the air remaining in the tube was only 0·000009 millimetre mercury, or 0.01×10- of an atmosphere.

vacuum.

The cathode rays streamed from the cathode through the window into the long cylinder which inclosed the They were not visible until they struck a little phosphorescent screen, which could be moved along the tube to different distances by means of a magnet, there being a bit of iron on the screen. The rays travelled in straight lines in the vacuum, and could be detected at the end of the tube, 1.50 metre from the window. Lenard remarks that the ether is therefore the medium by which the rays travel and in which they manifest their peculiar phenomena. The motions in the ether must be of such extremely minute order that the

size of the molecules is of relative importance. The molecules of gas muddle, so to speak, the ether, but it is noticeable that it is only the mass of the molecule which influence the phenomena.

If one could measure the velocity of the cathode rays in the ether, and observe their refraction by different media, one could connect the phenomena still more closely with ordinary light waves. It seems as if the study of such phenomena in the ether is destined to greatly increase our knowledge of the relations between the various forms of energy. Rays similar to the cathode rays could pass from the sun to the earth through the ether of space and exercise an effect in our atmosphere, although they would be invisible in the vacuum which exists in space between the sun and the earth.*

The electro-magnetic theory of light demands that a vacuum should be a nonconductor of electricity; for if it were a good conductor, it would be opaque to the electric waves, and, according to the electro-magnetic theory, no light would come to us from the sun. In experiments with the cathode rays we find that they can pass through certain conductors in thin layers. They are cut off, however, by layers of appreciable thickness.

* Ann. der Physik und Chemie, 51, 1894, p. 225.

CHAPTER XX.

THE X RAYS.

SINCE the writing of the previous chapter interest in the remarkable phenomena of the cathode rays has been reawakened to a marked degree by the discovery of Prof. Röntgen, who, by the use of ordinary dry plates and without the use of an aluminium window, has taken photographs through wood and through the human hand by means of what he terms the X rays, which he supposes are excited either in the glass walls of the Crookes tube or in the media outside the tube by means of the cathode rays.

We see, therefore, that the literature of the subject must be sought in the papers of Hittorf, Crookes, Hertz, Lenard, and Röntgen; and the interest in the mysterious manifestations of these invisible rays is twofold: first, in regard to the possible application of the phenomena to surgery, since the rays show a specific absorption, passing more easily through the flesh than through bones or glass or metallic particles; and, secondly, in relation to the questions whether we are dealing here with radiant matter shot forth from the negative pole or cathode or with longitudinal waves of electricity.

The term cathode, we have seen, is applied to the zinc pole or negative pole of an ordinary battery. It

is that terminal of an electrical machine which glows least in the dark when the machine is excited. It is the shortest carbon in the ordinary street electric lamp. The positive carbon or anode burns away twice as fast as the negative carbon or cathode. If the electric light is formed in a high vacuum by means of a great electro-motive force, we no longer have a voltaic arc or a spark; instead of this the exhausted vessel is filled with a feeble luminosity, and a beam of bluish rays is seen to stream from the negative terminal or cathode. When these rays strike the glass walls of the vessel they excite a strong fluorescence. If the glass contains an oxide. of uranium this fluorescence is yellow; if it contains an oxide of copper it is green. Röntgen supposes that this fluorescence excited by the cathode rays is connected in some way with the formation of what he terms the X rays. Now, a photograph of the bones in the hand, for instance, can be obtained by placing a sensitive plate in an ordinary photographic plate-holder, and by resting the hand on the undrawn slide in the daylight, with the palm of the hand outward and toward the cathode, and about six inches away from it; the bones of the hand are thus brought in the nearest possible position to the sensitive plate. At the time of the present writing, the breast and the abdomen of the human body present too great thickness for successful photographs, and the attempts to obtain representations of the cavity in which the brain is situated have been failures, since the rays do not show any marked difference in fleshy tissues. Nothing can be obtained in these attempts to photograph the brain but a contour of the cavity in which it is situated, and possibly a shadowy representation of a bullet which might be imbedded in the head. The method of obtaining a suc