322 Adams—^ Absence of Helium from Carnotite. 



this absence of helium may be explained by the physical pro- 

 perties of the mineral. Carnotite is a very fine powder which 

 is usually found disseminated through sandstone. Now even 

 the most compact specimens of this sandstone containing car- 

 notite are exceedingly permeable to gases. This was shown 

 by closing one end of a glass tube with a piece of the mineral 

 about 2 cm in thickness, and filling it with illuminating gas over 

 water. In a few minutes the water rose a distance of 6-7 cm in 

 the tube. If we then assume helium to be formed in this 

 mineral by the disintegration of the radium, it appears reason- 

 able to suppose that it rapidly diffuses away. The minerals 

 that contain helium are known to be massive, impervious sub- 

 stances, which are therefore able to retain the helium formed 

 in them. 



This explanation of the absence of helium from carnotite 

 seems to be supported by the views of Travers* on the state 

 in which helium exists in minerals. According to him, the 

 helium is present in the minerals in a state of supersaturated 

 solid solution ; the minerals being impermeable to the gas at 

 ordinary temperatures, the velocity with which equilibrium is 

 established between the helium in solution and the helium in 

 the gaseous phase is very small, but increases rapidly with rise 

 of temperature. In the case of carnotite, however, the min- 

 eral is permeable to the gas even at ordinary temperatures, and 

 therefore we could not expect to find any appreciable amount 

 of helium in this mineral. 



Princeton University, Physical Laboratory. 



* Nature, Jan. 12, 1905. 



