OCCLUSION OF HYDROGEN AND OXYGEN BY PALLADIUM. 107 



palladium black in the air or in vacua, unless conducted at an extremely high 

 temperature, is insufficient to remove the oxygen initially contained in it. 



Palladium foil. A portion of thin foil was procured from Messrs. JOHNSON and 

 MATTHEY, and was guaranteed to contain over 99 per cent, of palladium. The 

 special treatment to which this was subjected will be described in Section VI. 



III. Density of Palladium. 



It has always been customary, in stating the quantity of hydrogen occluded by 

 palladium, to express the result as so many volumes of hydrogen occluded by unit 

 volume of palladium. 



This makes a knowledge of the density of palladium necessary. 



The density of the foil employed was found to be 12 - 1, whilst a direct determina- 

 tion of the density of palladium black, dried at 100 0. in a specially constructed 

 pyknometer, gave the value 10 '6. This refers to palladium black prepared as 

 described. An analysis of the black soon after it was dried showed that it contained 

 0'72 per cent, of water and l'G5 per cent, of oxygen, existing probably as oxide. If 

 we make an allowance for this amount of water contained in the black the density 

 rises to 11 '4. 



The palladium black used for the density determination was the remainder (over 

 5 grams) contained in a bottle which had been frequently opened during about four 

 months, and which in all probability had absorbed more water. If we assume that 

 the percentage of water had gone up from 072 per cent, to 1 per cent, during the 

 four months, then the density of dry palladium black would become ITS. No 

 allowance, however, has been made for the fact that the palladium black contained 

 oxygen, which would also tend to lower the density. 



Since the knowledge of the density was only essential for the purpose of translating 

 the actual quantity of hydrogen occluded into the number of volumes of hydrogen 

 occluded by unit volume of palladium, we have thought it better, in view of the 

 above circumstances and of the fact that the determination of the density of a fine 

 powder like palladium black is not at all easy, to adopt, for comparative purposes, 

 the value 12 - as the density of palladium in its three states of aggregation, viz., 

 black, sponge, and foil. As the actual measurements are always given in addition, 

 this assumption ought not to give rise to any error, and is to a certain extent justified 

 by the fact that, under proper conditions, the number of volumes of hydrogen 

 occluded by unit volume of the different varieties of palladium remains practically 

 the same. 



IV. Analysis of Palladium Black. 



Before beginning any extended experiments with palladium black a preliminary 

 examination was made. From the fact that platinum black prepared in the same 



r 2 





