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

cessful photograph of the hand shows the present limitations of the method. In order to obtain a fairly sharp shadow of a bone or of a shot, it should not be more than an inch away from the sensitive plate. The term shadow, however, is somewhat misleading. The photograph of the hand by the X rays is entirely different from one produced by resting the hand in a similar position to that above described against an uncovered sensitive plate in a dark room and then lighting a match. By the last method we should obtain a true shadow of the hand, the flesh would throw as dense a shadow as the bones, and the latter could not be detected in the general blackness. In the cathode photograph, on the other hand, a difference in absorptive power is shown: the flesh looks like a hazy film around the skeleton, and even the medulla cavities can be made out, and the varying thickness of the bones is more or less shown. This specific absorption is of great scientific interest as well as of practical importance.

Now, these X rays will penetrate several inches of wood, with varying amount of absorption, but they are almost entirely cut off by glass as thick as a window pane. They pass through thin layers of aluminium, even layers as thick as a silver ten-cent piece, while the silver coin almost entirely intercepts them.

It therefore immediately occurs to one, Why not return to Lenard's tube, provide a Crookes tube with an aluminium window, and thus save the great absorption of the glass walls of the tube? There are certain practical difficulties in the way. The aluminium must be very thin. Lenard used a window which we have seen was about one nine thousandth of an inch thick, and it was necessarily very small, in order to stand the atmospheric pressure. An aluminium window one eighth