H. xvni.] RADIO-ACTIVITY 167 



show it to an extraordinary degree : Dr. Russell, 

 however, appears to have traced his at first interesting 

 effects to the merely chemical action of hydrogen 

 peroxide. He has quite recently shown that leaves 

 of growing plants have a spontaneous photographic 

 power of the same kind. 



The whole subject, together with the allied one of 

 the loss of charge from hot bodies,* first discovered by 

 Dr. Guthrie long ago (see Phil. Mag., [4], xlvi. p. 273), 

 is one that demands special attention and treatment. 



Crookes discovered that the alpha rays, or particles 

 projected by radium, polonium, and other strongly 

 radio-active substances, were able by their concussion 

 with a target of zinc-sulphide to produce luminous 

 flashes, visible under slight magnification : and the 

 evidence on the whole is in favour of the view that 

 many of the impinging atoms may have speed enough 

 to be able to cause a separate flash, by its own 

 individual action on the crystalline compound : a 

 phenomenon popularised in the little pocket-instru- 

 ment called a * spinthariscope.' When we consider 

 the speed with which these particles are ejected, such 

 an idea is not surprising, although it is novel to have 

 to contemplate any perceptible effect produced by a 

 single atom. But taking these projectiles to be 

 atoms of hydrogen or helium, of mass 10~ 24 grammes, 

 flying with say one-tenth the speed of light, the 

 stoppage of one of them within molecular dimension, 

 that is within its own thickness as ordinarily estimated, 

 10~ 8 centimetre, would require, for an exceedingly v 

 minute fraction of time, the expenditure of 80 horse- 

 power. 



* See, for instance, Strutt on leakage from hot bodies, Phil. 

 July, 1902 ; J. J. Thomson, ditto, Phil. Mag., August, 1902 ; further 

 developed by O. W. Richardson, Phil. Trans., vol. 201, p. 516 (1903). 



