21(5 



K. i;rniKi;FOKi> ON Till. 



gramme. The value of N for each of the products is shown in the following tahle, 

 along with the corresponding weights: 



With the small quantities of radium available, the amounts of the products 

 radium A, B, and C are too small to weigh. It may be possible, however, to detect 

 their presence by means of the spectroscope. 



In the case of thorium, the weight of product ThX, which is present in greatest 

 quantity, is far too small to detect. Since the value of X for ThX is about the same 

 as for the radium emanation, the maximum weight present per gramme is about 

 1/2 X 10~ c of the weight of the emanation, i.e., about 10~ n gramme. Even from a 

 kilogramme of thorium, the amount of ThX is too small to detect by its weight. 



The evidence at present obtained points strongly to the conclusion that the 

 expelled a particle becomes helium after its charge is neutralized. If this is the 

 case, the volume of helium produced from radium per year can at once be estimated. 

 Since the emanation, radium A, and radium C, expel a particles, the volume of helium 

 produced during the life of the emanation should be three times the initial volume of 

 the emanation. It is difficult to estimate experimentally the volume of helium pro- 

 duced, on account of its absorption by the walls ol the tube, but the experiments of 

 RAMSAY and SODDY show it is about this order of magnitude. Since 2 X 10 11 atoms 

 of helium are expelled per second, the number expelled per year is 6'3 X 10 18 . The 



6'3 X 10 18 

 volume of helium produced per gramme of radium per year is ' ' ^ , or '18 cub. 



centim. 



32. Origin of the. Radio-elements. -We have seen that 1/1000 of a given quantity 

 of radium is transformed per year and half after a period of 800 years. At the end 

 of 8000 years only about 1/1000, and at the end of 16,000 years only 1/10 of a 

 given quantity of radium would remain unchanged. Thus if the whole earth had 

 been initially composed of pure radium, its activity 16,000 years later would not be 

 greater than a good specimen of pitchblende to-day. Since there is little doubt that 

 the radioactive minerals are very much older than this, we are forced to the conclusion 



