30 SODDY, Evolution of Matter by Radio-active Elements. 



above number, or 6x lo^" gives the number of atoms in a 

 gram of radium that disintegrate per second. This is the 

 maximum estimate, and the minimum is about 40 times 

 smaller. Professor J. J. Thomson* has deduced from his 

 <5wn results and those of Townsend that there are 

 87 X 10^ atoms in a gram of hydrogen. The number in 

 a gram of radium is therefore 3'4X io^\ On the maximum 

 . estimate, the proportionate fraction of the radium changing 

 per second is therefore 17X io~", and the average life is 

 about two thousand years. This is almost exactly the 

 experimental estimate obtained directly from the volume 

 of the emanation, and the result indicates that probably 

 ■only one a particle is expelled at each disintegration. 



The emanation of radium is a very interesting type 

 of matter, for one half of it undergoes change in four 

 ■days. Hence the behaviour of the tiny bubble of gas was 

 investigated from day to day. In one experiment the 

 .rvolume steadily diminished with time roughly at the same 

 rate and according to the same law as the activity decays. 

 In about three weeks the volume had shrunk practically 

 to zero, being less than one per cent, of that initially 

 occupied. The question will be asked " What becomes of 

 the helium produced?" This point is under examination. 

 Rutherford has suggested that the a particle is actually an 

 atom of helium, and since it is expelled with the velocity 

 ■of 20,000 miles a second it is to be expected that it will 

 penetrate the surface of the glass and remain imprisoned 

 therein. We have actually found that glass which is 

 subjected to this bombardment, when broken up and 

 heated, gives off a quantity of helium, recognisable by its 

 spectrum. A similar experiment, which however gave a 

 negative result, is described by Curie and Dewar in the 

 Chemical News for Feb. 19th, 1904 (p. 85). The decrease 



* Phil. Mag., 1903, S.6, v. 5, p. 355. , , 



