OCCLUSION OP HYDROGEN AND OXYGEN BY PALLADIUM. 123 



part in the reaction, then it would seem justifiable to suppose that in the liquefaction 

 of one gram of one and the same gas, hydrogen for example, in different absorbing 

 substances, the same amount of heat, viz., the heat of condensation of one gram of 

 hydrogen, would always be evolved. 



We have found, for the heat of occlusion of one gram of hydrogen in platinum and 

 palladium, the following values : 



Platinum. . . . + 68'8 27 = 66'1 K. per gram. 

 Palladium . . . + 46'4 27 = 437 K. 



In order to obtain the difference between the initial and final values of the internal 

 energy of the system, we require to subtract the heat equivalent of the work done 

 by the atmosphere. For one gram molecule of a gas this amounts to 0'02 T.K. 

 (2 T. g. cal.), and hence for half a gram molecule of hydrogen at C. we require to 

 diminish the above values for the heat of occlusion by 27 K. Since the numbers 

 which we thus obtain are by no means approximately equal, we are of opinion that 

 the phenomenon of the occlusion of hydrogen by platinum and palladium black is 

 not simply the liquefaction or condensation of the gas in the capillary pores of the 

 metals. 



The same arguments would; doubtless, be valid in a comparison of the heat of 

 occlusion with the heat of solidification or fusion. 



X. The Influence of Increased Pressure on the Occlusion of Ili/drogcn by Palladium 



Black. 



We have already seen that the composition of palladium hydrogen, fully charged 

 with hydrogen at ordinary atmospheric pressure, corresponds approximately to the 

 formula Pd 3 H 2 . Although from the experiments of HOITSEMA and KOOSEBOOM it 

 would seem that increase of pressure should produce little or no effect on the 

 occlusion of hydrogen by palladium, an experimental demonstration was deemed 

 desirable. The details of the experiment were the same as in the corresponding 

 experiment with platinum black ('Phil. Trans.,' A, 1895, vol. 186, p. 675). A 

 quantity of palladium black was charged with hydrogen and shut up with an excess 

 of hydrogen in the shorter graduated limb of a glass U-tube. After equilibrium had 

 been established, the volume, temperature, and pressure of the enclosed gas were 

 noted. Successive quantities of mercury were then poured into the longer limb, and 

 the measurements repeated. 



The results are contained in the following table : 



R 2 



