PRESENT PROBLEMS IN RADIOACTIVITY. 33 



to slight traces of the known radioactive elements or to new kinds of 

 radioactive matter. It is not improbable that a close examination of 

 the radioactivity of the different soils may lead to the discovery of 

 radioactive substances which are not found in pitchblende or other 

 radioactive minerals. The extraordinary delicacy of the electroscopic 

 test of radioactivity renders it not only possible to detect the presence 

 in inactive matter of extremely minute traces of a radioactive sub- 

 stance, but also in many cases to settle rapidly whether the radio- 

 activity is due to one of the known radio-elements. 



The observations of Elster and Geitel render it probable that the 

 radioactivity observed in the atmosphere is due to the presence of 

 radioactive emanations or gases, which are carried to the surface by the 

 escape of underground water. Indeed it is difficult to avoid such a 

 conclusion, since there is no evidence that any of the known con- 

 stituents of the atmosphere are radioactive. Concurrently with ob- 

 servations 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 rela- 

 tive amount of active matter and of the ionization in the atmosphere 

 over land and sea will probably 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 pres- 

 ence 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 

 surface. 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 means 

 of calculating the age of the deposits of radioactive minerals. It seems 

 reasonable to suppose that the helium always found associated with 

 radioactive minerals is a product of the decomposition of the radioactive 

 matter present. In the mineral fergusonite, for example, about half of 

 the helium is removed by heating the mineral and the other half by solu- 

 tion. Thus it does not seem likely that much of the helium formed 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 



VOL. LXVII. — 3 



