Another and quite different kind of radiation is given out by many of the transmuting elements:--the y-ray. This is not material, it is ethereal. It is known now with certainty that the y-ray is in kind identical with light, but of very much shorter wave length than even the extreme ultraviolet light of the solar spectrum. The y-ray is flashed from the transmuting atom along with the ss-ray. It is identical in character with the x-ray but of even shorter wave length. There is a very interesting connection between the y-ray and the ss-ray which it is important for the medical man to understand--as far as it is practicable on our present knowledge. When y-rays or x-rays fall on matter they give rise to ss-rays. The mechanism involved is not known but it is possibly a result of the resonance of the atom, or of parts of it, to the short light waves. And it is remarkable that the y-rays which, as we have seen, are shorter and more penetrating waves than the x-rays, give rise to ss-rays possessed of greater velocity and penetration than ss-rays excited by the x-rays. Indeed the ss-rays originated by y-rays may attain a velocity nearly approaching that of light and as great as that of any ss-rays emitted by transmuting atoms. Again there is demonstrable evidence that ss-rays impinging on matter may give rise to y-rays. The most remarkable demonstration of this is seen in the x-ray tube. Here the x-rays originate where the stream of ss- or cathode-rays 247 are arrested on the anode. But the first relation is at present of most importance to us--_i.e._ that the y-or x-rays give rise to ss-rays. This relation gives us additional evidence of the identity of the physical effects of y-, x-, and light-rays --using the term light rays in the usual sense of spectral rays. For it has long been known that light waves liberate electrons from atoms. It has been found that these electrons possess a certain initial velocity which is the greater the shorter the wave length of the light concerned in their liberation. The whole science of "photo-electricity" centres round this phenomenon. The action of light on the photographic plate, as well as many other physical and chemical phenomena, find an explanation in this liberation of the electron by the light wave. Here, then, we have spectral light waves liberating electrons--_i.e._ very minute negatively-charged particles, and we find that, as we use shorter light waves, the initial velocity of these particles increases. Again, we have x-rays which are far smaller in wave length than spectral light, liberating much faster negatively electrified particles. Finally, we have y-rays--the shortest nether waves of all-liberating negative particles of the highest velocity known. Plainly the whole series of phenomena is continuous. We can now look closer at the actions involved in the therapeutic influence of the several rays and in 248