612 Dr. B. B. Boltwood on 



probability both polonium and radio-lead are slow trans- 

 formation-prodcuts of the radium emanation, and it would 

 therefore be expected that minerals with a high emanating 

 power would contain a smaller proportion of these trans- 

 formation-products than directly corresponded to the quantities 

 of radium contained in them. Some rough preliminary 

 experiments by the writer have given indications that the 

 highly emanating minerals do actually contain a smaller 

 proportion of both polonium and radio-lead than is present 

 in the minerals having a low emanating power, but the 

 experimental difficulties encountered do not permit of a 

 positive statement to that effect at present. That the loss of 

 emanation is not in any sense proportional to the content of 

 uranium is evident, since one of the most strongly emanating 

 minerals tested was xenotime (No. 17) which contains only 

 0*7 per cent, of uranium. The loss of emanation is probably 

 dependent on the porosity of the mineral, but it is a curious fact 

 that the two samples which to all appearances were the most 

 porous (Nos. 5 and 9), were not the most highly emanating. 

 Those minerals with a highly compact and vitreous structure 

 (Nos. 11 to 16 and 18 to 21), with the exception of thorite 

 (No. 14), had the lowest emanating powers of any of the 

 samples examined* . 



The possibility of percolating water removing radium from 

 uranium minerals, which has been suggested by Rutherford, 

 is best illustrated in the case of the North Carolina minerals 

 uraninite, gum mite, thorogummite, and uranophane. The 

 three latter minerals owe their formation to the action of 

 percolating water on uraninite ; and it is a very common 

 occurrence to find specimens containing a nucleus of un- 

 decomposed uraninite, surrounded successively by a layer 

 of gummite and a layer of uranophane. The samples of 



* An intimate relation would seem to exist between the emanating 

 power of a mineral and the loss of helium from the same substance, and 

 those minerals with high emanating powers might be expected to contain 

 relatively little helium. Moss (Trans. Roy. Dublin Soc. 1904) has 

 observed that a considerable quantity of helium is set free when pitch- 

 blende is finery pulverized in vacuo, a phenomenon which Travers (Nature, 

 lxxi. p. 248, 1905) attributes solely to the heating of the substance by 

 attrition. Travers expresses the belief that helium exists in minerals in 

 a state of supersaturated solid solution, and that grinding to an impalpable 

 powder under conditions which precluded a rise in temperature, should 

 result in the evolution of only minute traces of helium. It would seem 

 more probable, however, that the state of the radium emanation during 

 its temporary existence in the mineral is similar to that of the helium, 

 and that the highly emanating minerals in the form of impalpable 

 powders would lose at ordinary temperatures a very considerable pro- 

 portion of the helium formed within them. It would, of course, be 

 quite impossible to detect the escape of the helium by ordinary methods 

 because of its minute quantity. 



