170 Proceedings of Royal Society of Edinburgh. [sess. 
electro-chemical attempts which were made to solve this interest- 
ing problem, a short summary of similar attempts, and of the 
views of other observers, may not be out of place. 
What is really required is some criterion which is conclusive 
either for or against the idea of chemical combination or solid 
solution. Of course, we may possibly have to deal with both of 
these phenomena; but a third alternative, namely, that the 
occluded hydrogen is simply condensed or liquefied in the capil- 
lary pores, seems to be altogether inadmissible. In support of this 
conclusion, we have the fact (Mond, Ramsay, and Shields, Phil. 
Trans., cxci. 105) that palladium in all its different states of aggre- 
gation, which presumably differ with respect to the degree of 
porosity, always occludes, under proper conditions, approximately 
the same quantity of hydrogen. Again, if the occlusion of hydro- 
gen was merely the liquefaction of the gas in the capillary pores, 
we should expect the heat of occlusion to be identical with the 
heat of condensation of hydrogen. Although we do not know the 
magnitude of the heat of condensation of hydrogen, w r e can arrive 
at the required result in an indirect manner. 
Assuming that the occlusion of hydrogen by finely-divided pal- 
ladium is precisely the same sort of phenomenon as the occlusion 
of hydrogen by finely-divided platinum, then, if in both instances 
we were dealing simply with the liquefaction of hydrogen, we 
would expect the heat of occlusion of one gram of hydrogen in 
palladium to be identical with the corresponding heat of occlusion 
of one gram of hydrogen in platinum. The numbers actually found, 
however, under comparable conditions (Phil. Trans., loc. cit.) were 
+ 46*4K (4640 g - cal) and + 68.8K (6880 g - cal) respectively. 
We may therefore reject the hypothesis that the phenomenon of 
occlusion represents the condensation of the gas in the capillary 
pores of the absorbing substance. 
A knowledge of the dissociation pressures of a substance pro- 
vides us with a valuable criterion for determining whether we are 
dealing with a compound or simply with a solid solution. Accord- 
ing to the Phase Rule it can be shown that when a solid substance 
dissolves a gas the vapour pressure varies continuously, at one and 
the same temperature, with the increase of concentration of the 
dissolved gas. The pressure-concentration diagram will in general 
