80 



Scientific Proceedings^ Royal Dublin Society. 



absorption and conversion to heat of the kinetic energy of all these countless 

 atoms of radiant matter ; and I may here remind you that radiant matter 

 possesses yet another geological application of interest. It enables estimates of 

 geological time to be arrived at by the careful measurement of the amount of 

 stored helium, and a comparison of this with tlie quantity of the primary 

 substances, uranium and thorium, present. Strutt, by a series of laborious 

 and brilliant researches, has carried this method to its consummation. 



We must now study some circumstances attending the flight of radiant 

 matter. We first notice the interesting fact that the velocity with which 

 helium is cast out by radioactive bodies at the moment of eliange varies 

 considerably from one element to another. Thus the radiant atoms of 

 radium possess a far liigher velocity than those of uranium or ionium. 

 This fact is apparent in the greater distance to whicli tlie a rays of the former 

 will penetrate in air or in any other substance. The distance traversed in 

 air is known as tlie " range." Tlie following table shows the ranges of a rays 

 from the various known radioactive elements. Thus we see that whereas tlie 

 helium from Ra is projected nearly 7 centimetres, that from uranium only 

 reaches 2'7 centimetres. In the thorium series, one of the elements, Th C, 

 attains a range of 8" 6 centimetres. This is tlie longest known. 



By a most ingenious series of observations, Bragg has revealed some 

 unexpected and interesting features attending the ionization effects of the 

 a rays upon gases through which they are projected. By measuring the 

 amount of ionization effected at different points along the path of the ray, 

 Bragg and Kleeman have shown that at first, when the velocity is greatest, 

 the ionization eft'ected is least, and that the amount of ionization — that is, the 

 number of ions created — greatly increases juot before the atom comes to rest. 



