182 -/. C. Sanderson — Probable Influence of the 



three hours, the rate of leak of the electroscope was 110 divi- 

 sions per minute. Taking 3-4X10 - ' gram as the amount of 

 radium in equilibrium with one grain of uranium, the amount 

 of radium in the sample was 2-34 XlO" 8 gram. Approximately 

 14 per cent of the radium emanation escapes from the specimen 

 of (powdered) uraninite taken as a standard,* and therefore the 

 emanation introduced into the electroscope was in equilibrium 

 with onlv S(i per cent of 2'34XlO _8 gram, or 2-OxlO" 8 gram of 

 radium. One scale division per minute therefore corresponded 

 to the ionization produced by the emanation, radium A, B, 

 and C, in equilibrium with r82xl0~'° gram of radium. 



The mean of several measurements of the activity of the 

 radium emanation, in equilibrium with radium A, B, and C, 

 contained in the underground air was five scale divisions per 

 minute. The total amount of radium emanation present in the 

 volume of underground air necessary to fill the electroscope 

 was therefore equal to that in equilibrium with 5xl*82XlO -ll) 

 or 9*1 X 10"'° gram of radium. The volume of the ionization 

 chamber of the electroscope was 3,800 cubic centimeters and 

 therefore each cubic centimeter of underground air contained 

 radium emanation equivalent to that in equilibrium with 

 2-4 XlO -13 gram of radium. The porosity of the earth was 

 36"6 per cent, that is to say each cubic centimeter of earth con- 

 tained - 366 cubic centimeter of air. Therefore the amount 

 of radium emanation produced by one cubic centimeter of earth 

 was equal to 2-4X10"' 3 times 0-366 or 8-9XlO" M gram of pure 

 radium. 



Discussion of Results. 



The results of experiments conducted by a number of inde- 

 pendent investigators have clearly demonstrated that the radio- 

 active properties of the normal atmosphere near the surface of 

 the earth are due to the presence of very small amounts of the 

 emanations of radium and thorium and the disintegration pro- 

 ducts of these substances. It is necessary to assume that these 

 emanations are formed by thorium and radium compounds pres- 

 ent in the solid material constituting the surface of the globe. 

 The radium emanation because of its relatively slow rate of dis- 

 integration might be expected to reach the surface by diffusion 

 from considerable depths and to escape from the interior of 

 masses of rock having a slight porosity. In contrast to this, 

 however, the thorium emanation disintegrates so rapidly that 

 the process of diffusion alone is insufficient to bring any appre- 

 ciable quantities of this to the surface, except from the upper- 

 most layers of a light and porous earthy covering. It is in the 

 * Boltwood, this Journal, xxv, 281, 1908. 



