OCCLUSION OF HYDROGEN AND OXYGEN BY PALLADIUM. 115 



metal produced by the expansion during the first part of the charging. GRAHAM (loc. 

 cit., p. 267) remarks, "The foil was much crumpled and rather friable after repeated use." 



The fact that increase of temperature is necessary during the first part of the 

 absorption is to be ascribed not to an increase in the solubility of the hydrogen, but 

 to a more rapid diffusion, throughout the mass of the metal, of such hydrogen as can 

 be retained at that particular temperature. The meaning of this will be rendered 

 clearer by reference to DEVILLE and TROOST'S well-known experiment on the passage 

 of hydrogen through a red-hot platinum tube, or to the similar passage of hydrogen 

 through a palladium tube above 237 C. (cf. RAMSAY, 'Phil. Mag.,' August, 1894, 

 p. 206). Now platinum and palladium at this high temperature can only retain 

 under atmospheric pressure* a very minute quantity of hydrogen, but, minute as it is, 

 it can diffuse with great readiness through the hot metal, its place being immediately 

 taken by fresh hydrogen. At moderately high temperatures therefore, when palla- 

 dium can still absorb a considerable quantity of hydrogen, we should expect a more 

 rapid diffusion through, or permeation of, the foil, and consequently the production 

 of a spongy or fissured mass, caused by expansion on the occlusion of just as much 

 hydrogen as can be retained at this temperature, so that on cooling down to Iho 

 ordinary temperature the foil ought to behave more nearly like palladium sponge. 



At this stage an exact measurement of the total quantity of hydrogen absorbed 

 was made, and it was found that 94-50 cub. centims., or 851 volumes, had been taken 

 up by the palladium foil. 



That the absorption at 100 was complete was shown by the fact that on raising 

 the temperature to 100 C. about 100 volumes of gas were expelled, and of this about 

 three-quarters was reabsorbed on cooling down to the ordinary temperature, whilst 

 the remainder, along with seven additional volumes, was occluded on standing for 

 eighteen hours. The final measurement showed that 95'18 cub. centims., or 858 

 volumes, of hydrogen had been absorbed. 



Just as palladium foil which has been rendered more or less spongy in texture 

 requires a considerable time for the absorption of its quantum of hydrogen, so also 

 it gives off its occluded gas extremely slowly in vacuo at the ordinary temperature. 

 On applying the pump a practically complete vacuum was produced in a few minutes, 

 when it was found that only 0'57 cub. centim., or five volumes, had been removed. 

 Hydrogen was, however, given off very slowly, for in another hour 2' 1 4 cub. centims., 

 or nineteen volumes, were extracted. The results are contained, in the following 

 table, which shows that nearly the whole of the hydrogen is given off easily and 

 rapidly at 100 C. in vacuo. There is a difference of twelve volumes between" the 

 total hydrogen absorbed and the total hydrogen extracted or occluded. This may be 

 due to (1) experimental error, (2) all the hydrogen may not be extracted at a red 



* The most recent work on the subject by Professor DKVVAR (' Proc. Chem. Soc.,' 1897, No. 183, 

 192) shows that a rod of palladium is still capable of occluding about 300 volumes of hydrogen, under a 

 pressure of 100-120 atmospheres, at a temperature of 500 C. 



Q 2 



