OCCLUSION OF HYDROGEN AND OXYGEN BY PALLADIUM. 



121 



We have also included in the table the corresponding values which we have 

 calculated from three experiments by GRAHAM (' Researches/ p. 291) and from an 

 experiment on a larger scale by DEWAR ('Phil. Mag.,' (4) vol. 47, p. 334). The 

 numbers given in brackets are those calculated by GRAHAM on the basis that the 

 specific gravity of palladium wire is 12 '38. In the last four experiments the palladium 

 was charged electrolytically with hydrogen, and it follows directly from a considera- 

 tion of the table that, no matter whether the palladium exists as black, sponge, foil, 

 wire, or compact metal, or whether it is charged by exposure to hydrogen gas (the 

 proper conditions being observed), or charged electrolytically, the amount of hydrogen 

 occluded in each case is approximately the same. 



If we wish to express, by means of a formula, the composition of palladium 

 hydrogen, then, by choosing the nearest whole numbers, we arrive at the formula 

 Pd 3 H 2 , which was first proposed by DEWAR (loc. cit.), and which corresponds with 

 the ratio 1*5, instead of varying between 1'37 and 1'47, as above. 



It must be observed, however, that in all cases in which the palladium has received 

 the maximum charge of hydrogen the amount of hydrogen taken up is always in 

 excess of that required for the formation of a definite chemical compound Pd 3 H:,. 

 If we admit and this is not at all improbable that such a compound is still capable 

 of occluding or condensing hydrogen in the ordinary way, then the excess of 

 hydrogen which is absorbed does not exclude the possibility of the formation of a 

 definite compound of this composition. On the other hand, evidence in favour of its 

 existence is of a very meagre kind, and is, we think, chiefly confined to the approxi- 

 mation of the ratio Pd/H to the theoretical value 1*5. 



The views of chemists and physicists on the nature of palladium hydrogen have 

 from time to time undergone considerable change. In 1869 GRAHAM ('Researches,' 

 p. 290) wrote : " The idea forces itself upon the mind that palladium, with its 

 occluded hydrogen, is simply an alloy of this volatile metal (hydrogenium), in which 



* Details will be found on p. 125. 



VOL. CXCI. A. 



