beyond the ken of science and which are involved in metabolism cannot fail to be affected. Any, it is not surprising that as the result of eaperinient it is found that the radiations are agents which may be used either for the stimulation of the natural events of growth or used for the actual destruction of the cell. It is easy to see that the feeble radiation should produce the one effect, the strong the other. In a similar way by a moderate light stimulus we create the latent image in the photographic plate; by an intense light we again destroy this image. The inner mechanism in this last case can be logically stated.[1] _There is plainly a true physical basis here for the efficacy of radioactive treatment and, what is more, we find when we examine it, that it is in kind not different from that underlying treatment by spectral radiations. But in degree it is very different and here is the reason for the special importance of radioactivity as a therapeutic agent._ The Finsen light is capable of influencing the soft tissues to a short depth only. The reason is that the wave length of the light used is too great to pass without rapid absorption through the tissues; and, further, the electrons it gives rise to--_i.e._ the ss-rays it liberates--are too slow-moving to be very efficient ionisers. X-rays penetrate in some cases quite freely and give rise to much faster and more powerful ss-rays [1] See _The Latent Image_, p. 202. 251 than can the Finsen light. But far more penetrating than x-rays are the y-rays emitted in certain of the radioactive changes. These give rise to ss-rays having a velocity approximate to that of light. The y-rays are, therefore, very penetrating and powerfully ionising light waves; light waves which are quite invisible to the eye and can beam right through the tissues of the body. To the mind's eye only are they visible. And a very wonderful picture they make. We see the transmuting atom flashing out this light for an inconceivably short instant as it throws off the ss-ray. And "so far this little candle throws his beams" in the complex system of the cells, so far atoms shaken by the rays send out ss-rays; these in turn are hurled against other atomic systems; fresh separations of electrons arise and new attractions and repulsions spring up and the most important chemical changes are brought about. Our mental picture can claim to be no more than diagrammatic of the reality. Still we are here dealing with recognised physical and chemical phenomena, and their description as "occult" in the derogatory sense is certainly not justifiable. Having now briefly reviewed the nature of the rays arising in radioactive substances and the rationale of their influence, we must turn to more especially practical considerations. The Table given opposite shows that radium itself is responsible for a- and ss-rays only. It happens that 252 Period in whioh 1/2 element is transformed. URANIUM 1 & 2 { a 6 } x 109 years.