OCCLUSION OF HYDROGEN AND OXYGEN BY PALLADIUM. 113 



When palladium foil is charged electrolytically with hydrogen it undergoes a 

 considerable increase in volume, as has been pointed out by GRAHAM, DEWAB, 

 THOMA, and others, and when discharged it again contracts, and it is said that its 

 volume becomes less than the original volume. In order to see whether the occlusive 

 power of the foil was altered by successive expansion and contraction, it was charged 

 successively and alternately with hydrogen and oxygen in a voltaic cell. After 

 exhaustion at a red heat in vacua, hydrogen was admitted at the ordinary tempera- 

 ture. The following results were obtained : 



On first admission of hydrogen . 0'3 cub. centim. = 27 volumes were occluded; 

 After gently heating for two hours 3'6 = 32'2 ,, 



After eighteen hours at ordinary"]^ 

 temperature 



1 0-3 = 27 



Altogether, therefore, 4'2 cub. centims., or 37'6 volumes, of hydrogen were occluded. 

 The occlusive power is thus not much greater than before. If the compactness of the 

 metal has anything to do with the rate of occlusion, this result is what might have 

 been anticipated, since on charging electrolytically with hydrogen and then dis- 

 charging the volume of the metal becomes less, and presumably the degree of 

 compactness greater. 



There can be little doubt that the phenomenon of occlusion consists first of the 

 solution or combination of the gas in the outer skin of the metal, and then of diffu- 

 sion inwards towards the centre. 



In the case of compact foil, produced by the welding together of particles of 

 sponge, the hydrogen is possibly condensed, dissolved, or combined in the two chief 

 outer surfaces of the foil, and then diffuses inwards in both directions through half 

 its thickness. In the case of palladium sponge or black, however, the surface itself 

 is very much greater, and the diameter or radius of the individual particles probably 

 very much less than the thickness of the foil ; and consequently it would seem that 

 with palladium sponge we have to deal chiefly with the rate of solution, and that wo 

 are less concerned with the factor of diffusion or rate of diffusion. 



Hence, if we could by any possible treatment so change new compact palladium foil 

 that it becomes spongy in texture, it is to be expected that it would behave more 

 nearly like palladium sponge as regards the amount of hydrogen occluded and the 

 rate at which occlusion takes place. 



It is well known that when copper is oxidised and then reduced it becomes 

 spongy. Now, a favourite device for cleaning platinum and palladium goods is by 

 ignition in the blowpipe flame. Palladium, however, differs from platinum inasmuch 

 as it is more easily oxidised. At a still higher temperature (higher than can con- 

 veniently be maintained in an evacuated hard glass tube), it is again reduced to 

 metal. By repeatedly igniting palladium foil in this way it probably acquires a 

 VOL. cxci. A. Q 



