298 Sir James Maclceyizie Davidson [Feb. 2, 



immediate visil)le effect is produced. But if the duration of exposure 

 has been sufficiently prolonged to bring about what is called a reaction, 

 the period intervening between the time of application and the 

 appearance of the reaction in the skin will be found generally to vary 

 between a couple of days and about three weeks. The duration of 

 this " latent period " depends upon the amount of the dose. The 

 stronger the dose, the shorter the latent period. There is no doubt 

 that the a?-rays produce certain changes or injuries to living cells, 

 which remain hidden until the absorption of the abnormal products 

 due to these changes causes redness and inflammation to become 

 evident. At first it was thought that these effects were confined to 

 the skin, but it soon became apparent, on researches by Haenisch 

 and others, that some of the deep organs were more susceptible to 

 the action of ;c-rays than the skin itself. The most sensitive of all 

 the tissues are the lymphoid tissues generally, especially the spleen, 

 which shrinks and becomes strongly pigmented under their attack. 



Plant Growth. — In considering the effect of 2--rays upon the lower 

 forms of life, we have first to note their influence upon plant growth. 

 Wetterer, the German radiologist, relates an interesting experiment in 

 this connexion. He has taken collections of sunflower seeds and ap- 

 phed ^--ray doses of varying strengths. The seeds which had received 

 very small doses, equal to or slightly above a dose which would pro- 

 duce an inflamation of the skin, sprouted in about the same time as 

 seeds which had been untouched by the rays. In seeds which had 

 received heavier doses the sprouting took place much later, and the 

 plant showed clear symptoms of degeneration. Seeds which had re- 

 ceived still stronger doses gave no sign of growth whatever. The 

 effect of x-rays upon bacteria, however, seems to be very feeble, if, 

 indeed, it exist definitely at all. And this is in marked contrast to 

 the bactericidal action of ultra-violet light previously mentioned. 



Skin Diseases. — It is evident that an ag'ent having such vital 

 effects as those described — and there are many other effects which 

 could be demonstrated if time permitted — will find a wide field of 

 application in the treatment of disease. In many skin diseases it is 

 of signal service. In certain diseased conditions of the spleen also its 

 action is very marked. In malignant tumours, while it may be of 

 service in arresting the rapidity of the growth, it cannot be looked 

 upon as a method of cure, although, after the removal of such growths 

 by operation, the application of the rays to the involved area may 

 assist in destroying any of the malignant cells which the surgeon's 

 knife has missed, and preventing re-development. And as the tech- 

 nique of the aj-rays is being improved, the field of their utility in 

 therapeutics is being gradually extended. 



Radium. — We may now proceed to consider the action of radium 

 and other radio-active substances. It is necessary before approaching 

 the question of the vital effect of these rays to review briefly their 

 physical properties. Immediately after the discovery of the x-rays, in 



