Soil on Local Atmospheric Radioactivity. 



183 



free surface of such a covering that hy far the greater pro- 

 portion of the thorium emanation which ultimately enters the 

 atmosphere must originate, and its dissemination through the 

 lower strata of the air must be greatly facilitated by winds 

 and other air currents. 



It is, moreover, evident that the actual quantities of either 

 emanation which can obtain access to the free atmosphere 

 under the most favorable conditions of exposure, etc., will be 

 much less dependent on the actual amounts of thorium and 

 radium present in the surface materials than on the physical 

 and chemical conditions in which these elements exist and 

 which determine their emanating properties. If the thorium 

 and radium are in the form of compounds which permit the 

 escape of a large proportion of the emanation formed within 

 them, their contribution to the local atmospheric radioactivity 

 will be much more pronounced than that of much larger quan- 

 tities of the parent radioactive elements existing in a non- 

 emanating form. 



Sand and earth consist essentially of disintegrated rock 

 which has undergone more or less chemical decomposition. 

 For facilitating the escape of the emanations the advantage 

 derived from the mechanical subdivision alone is apparent. 

 The decrease in the thorium emanating power of a specimen of 

 earth treated with dilute hydrochloric acid, which was observed 

 in the course of the experiments described in this paper, also 

 justifies the assumption that a considerable proportion of the 

 thorium emanation which escapes originates in the superficial 

 coating covering the separate grains of the specimen of sand 

 examined. It is, therefore, highly probable that the thorium 

 in this surface coating exists in a chemical form which readily 

 permits the escape of emanation. 



Since by far the greater proportion of the land areas of the 

 globe are covered with soil of some description, it is apparent 

 that the part played by this in contributing to the radioactive 

 constituents of the atmosphere must be much greater and 

 much more important than the part played by the rocks which 

 underlie it. It is, therefore, to be expected that an examina- 

 tion of the radioactive properties of the soil and the under- 

 ground air in different localities will afford valuable data for 

 explaining the differences in the radioactivity of the atmos- 

 phere already observed in certain instances. 



As the average of results of measurements conducted during 

 a period of a year at Montreal, Eve* found the amount of 

 radium emanation present per cubic centimeter of atmospheric 

 air to be equivalent to that in equilibrium with 6xl0" u gram 



*Phil. Mag., xvi, 622, 1908. 



