140 Dr. M. W. Travers. The Origin of the Gases 



Helium and Argon. 



With regard to the state in which helium is present in the minerals 

 from which it is obtained by the action of heat, there is at present no 

 conclusive evidence. It must be present under one of three condi- 

 tions : 



I. In combination with some constituent of the mineral. 

 II. Occluded ; or in solution in the mineral. 

 III. Enclosed in cavities under pressure. 



Microscopic examination of the minerals, from which helium has 

 been derived, has failed to reveal the presence of cavities, and, indeed, 

 if we assume that cavities exist, and that they contain all the helium 

 present in the mineral, we have to make further assumptions to explain 

 why the helium escapes when the mineral is heated, although in most 

 cases no disintegration takes place. It seems improbable that the 

 helium is present in the state of solid solution or occlusion, a condition 

 of which we yet know little, for unless we assume that the rate of 

 diffusion of the helium through the mineral in which it is dissolved is 

 infinitely small, the gas should long ago have escaped into the atmo- 

 sphere. Further, it is probable that the supposed cases of solution of 

 gases, like hydrogen, in solids such as platinum, palladium, &c., are 

 really cases of chemical combination, as has recently been proved by 

 the researches of Ramsay, Mond, and Shields, and even in meteoric 

 iron it is improbable that the hydrogen is not present as such, but is 

 the product of secondary reactions. Indeed, in the state of our know- 

 ledge at present it is impossible to draw a distinction between occlusion 

 and chemical combination. 



We are, therefore, forced to the conclusion that helium is present in 

 the minerals in the state of combination with one of its constituents. 

 It may be well to review such positive evidence as exists in favour of 

 such a supposition. 



In the first place the gas is not found generally dispersed among 

 crystalline mineral substances, but seems only associated with certain 

 elements, uranium, yttrium, &c., in minerals which are invariably vein 

 products. 



It has been pointed out by Professor Kamsay and the author,* that 

 in certain cases the evolution of helium from the mineral is accom- 

 panied by a considerable evolution of heat, and in one case by a con- 

 siderable decrease in density. This matter has been dealt with fully 

 in the paper (loc. cit.), and is considered as evidence in favour of 

 chemical combination. Julius Thomsenf has confirmed this obser- 

 vation. 



* 'Roy. Soc. Proc.,' vol. 62, p. 325. 

 t ' Zeit. Pbys. Cliera.,' vol. 25. p. 112. 



