PRESENT PROBLEMS OF RADIOACTIVITY 185 



radioactive emanations or gases, which are carried to the surface 

 by the escape of underground water and by diffusion through the 

 soil. Indeed, it is difficult to avoid such a conclusion, since there is 

 no evidence that any of the known constituents of the atmosphere 

 are radioactive. Concurrently with observations of the radioactivity 

 of the atmosphere, experiments have been made on the amount of 

 ionization in the atmosphere itself. It is important to settle what 

 part of this ionization is due to the presence of radioactive matter in 

 the atmosphere. Comparisons of the relative amount of active matter 

 and of the ionization in the atmosphere over land and sea will prob- 

 ably throw light on this important problem. 



The wide distribution of radioactive matter in the soils which have 

 so far been examined has raised the question whether the presence 

 of radium and other radioactive matter in the earth may not, in 

 part at least, be responsible for the internal heat of the earth. It 

 can readily be calculated that the presence of radium (or equivalent 

 amounts of other kinds of radioactive matter) to the extent of about 

 five parts in one hundred million million by mass would supply as 

 much heat to the earth as is lost at present by conduction to its sur- 

 face. It is certainly significant that, as far as observation has gone, 

 the amount of radioactive matter present in the soil is of this order 

 of magnitude. 



The production of helium from radium indirectly suggests a method 

 of calculation of the age of the deposits of radioactive minerals. 

 It seems reasonable to suppose that the helium always found asso- 

 ciated with radioactive minerals is a product of the decomposition 

 of the radioactive matter present. About half of the helium is re- 

 moved by heating the mineral and the other half by solution. It 

 thus does not seem likely that much of the helium found in the 

 mineral escapes from it, so that the amount present represents the 

 quantity produced since its formation. If the rate of the production 

 of helium by radium (or other radioactive substance) is known, the 

 age of the mineral can at once be estimated from the observed 

 volume of helium stored in the mineral and the amount of radium 

 present. All these factors have, however, not yet been determined 

 with sufficient accuracy to make at present more than a rough esti- 

 mate of the age of any particular mineral. An estimate of the rate of 

 production of helium by radium has been made by Ramsay and 

 Soddy by an indirect method. It can be deduced from their result 

 that 1 gram of radium produces per year a volume of helium of about 

 25 cubic mms. at standard pressure and temperature. They, how- 

 ever, consider this to be an underestimate. On the other hand, if the 

 a particle is a helium atom, it can readily be calculated that 1 gram 

 of radium produces per year about 200 cubic mms. of helium. 



Let us consider for example the mineral fergusonite. Ramsay and 



