58 V. A. SOKOLOV 



metric tons) constitutes only a very small fraction of the quantity which has been 

 formed in the superficial parts of the lithosphère. As to argon there is about 

 6-5 X lo'-^ metric tons of it in the atmosphere and hydrosphere at present, 

 i.e. about 80*^0 of the amount mentioned above. 



The opinion has been expressed, however, that the mean amount of K in 

 the crust of the Earth is considerably less than that suggested above. Urey has 

 therefore put forward the suggestion that part of the argon was left over from 

 the earlier p)eriods of the history of the Earth. 



The proportion of He retained by the different rocky formations varies very 

 widely, from 2';,, (when almost all has been lost) up to 95 ^^o (when almost all 

 has been retained). The mean value of the retention of He would seem to be 

 about 33"o [18]. Owing to escape through crevices in the rocks, emission by 

 volcanoes, by diffusion, by being carried away in water and by other processes 

 of migration, some part of the helium which has been formed must have been 

 given off into the atmosphere. The fraction of helitmi given off into the atmos- 

 phere must have been greater than that of argon, owing to the higher molecular 

 weight of argon and its slower rate of migration. 



The insignificant concentration of hehum present suggests that it has been 

 lost from the atmosphere. There may also have been some loss of argon, though 

 this will have been small in comparison with that of helium. 



Thus, the radioactive breakdown of U and Th, and the transformation of K 

 would provide for the appearance of helium and argon in the atmosphere, although 

 the overwhelming bulk of the helium was lost again. 



The radiochemical processes of gas formation were mostly effected by the 

 action of a-radiations in that these account for about 90% of the radiant energy 

 given off by the radioactive elements. Each a-particle disrupts a large number 

 of the molecules of the compounds which it encotmters in its path. For example, 

 during the action of one curie of radium emanation (and its breakdown products) 

 on water, tens of cm^ of oxyhydrogen gas (2H2 + O?) are formed every 24 hours. 

 One a-particle can, on the average, disrupt tens of thousands of molecules of 

 water. Similarly, a-particles can also act on other compounds. 



The a-particles (in the form of hehum) mentioned above, therefore, gave rise 

 to chemical actions, disrupting considerable amounts of various compounds in 

 the crust of the Earth and thus providing the necessary conditions for the occur- 

 rence of different synthetic processes. 



If we allow that the greater part of the crust of the Earth affected by a-radiations 

 consists of compounds and not of elements, we may calculate roughly that the 

 amount of material subjected to chemical changes of this kind is about lo^^ 

 — 10' ^ metric tons, reckoning the thickness of the crust as 40 km. To put it 

 otherwise, no less than io-2o'/o of the molecules of the material of the litho- 

 sphère, and maybe even more, underwent changes as a result of radiochemical 

 processes. No doubt the fragments of the disrupted molecules (individual atoms 

 and ions) entered into combination with one another so that the original com- 

 pounds were, in part, formed again. Nevertheless, some fraction of the gases 

 formed left the hthosphere and hydrosphere and entered the atmosphere of 

 the Earth, 



