122 DBS. L. MOND, W. RAMSA5T, AND J. SHIELDS, ON THE 



the volatility of the one element is restrained by its union with the other, and which 

 owes its metallic aspect equally to both constituents." 



In 1874, however, another view was suggested by TROOST and HAUTEFEUILLE 

 ('Ann. Chim. Phys,,' (5) vol. 2, p. 279). From experiments on the vapour pressure 

 of palladium hydrogen at different temperatures, they conclude that the first 

 addition of hydrogen to palladium forms a definite chemical compound Pd 2 H, and 

 that the excess of hydrogen over and above that required to form this compound is 

 then occluded or absorbed in the usual way. 



By subjecting the results of TROOST and HAUTEFEUILLE to a critical analysis, and 

 from more extended experiments by himself and ROOSEBOOM, HOITSEMA (' Zeitschr. 

 Physikal. Chem.,' vol. 17, p. 1, 1895) has arrived at an entirely different conclusion, 

 namely, that, as regards the vapour pressure experiments, there is no decisive 

 evidence for the formation of a definite chemical compound ; and he further suggests 

 that the results are in better agreement with the view that two immiscible solid 

 solutions are formed. This appears to resolve itself into the formation of two alloys 

 instead of one, as suggested by GRAHAM. 



In confirmation of the arguments of HOITSEMA against the supposed formation of 

 a compound having the formula Pd 2 H, we have also the fact, first observed by 

 FAVRE (' Comptes Rend.,' vol. 77, p.- 649, and vol. 78, p. 1257) and confirmed by 

 ourselves, that the heat of occlusion of hydrogen by palladium remains constant 

 throughout the whole range of absorption. 



If a chemical compound Pd 3 H were first formed, we should expect to get a certain 

 definite evolution of heat per gram of hydrogen combined for the hydrogen first 

 admitted, and then, after sufficient hydrogen had been added to form the compound 

 Pd 2 H (about 630 volumes), we should expect to find a different value for the heat 

 evolved per gram of hydrogen occluded or dissolved or absorbed. We have, 

 therefore, confidence in discrediting the supposed formation of Pd 2 H. 



It would seem that HOITSEMA'S conclusions apply equally well to the supposed 

 formation of Pd 3 H.,. But before finally dismissing the possibility of its existence we 

 consider that some additional evidence is desirable. It is proposed to attack the 

 problem from another and independent point of view, and it is hoped that all doubts, 

 which are liable to affect any process in which it is uncertain whether equilibrium 

 has really set in or not, will be overcome. 



IX. On the Heat of Occlusion and the Heat of Condensation. 



It has sometimes been suggested (cf. FAVRE, 'Ann. Chim. Phys.,' (5) vol. 1, 

 p. 209), that the heat of occlusion of a gas represents the heat of condensation or 

 liquefaction of the gas in the capillary pores of the absorbing substance. Now if 

 this is so, and we vary the absorbing substance, at the same time maintaining it 

 always at the same temperature, say C., so that its specific heat shall play no 



