106 DRS. L. MOND, W. RAMSAY, AND J. SHIELDS, ON THE 



('Am. Chem. Journ.,' vol. 10, p. 249) to this method of weighing hydrogen in one of 

 the most delicate of chemical operations, viz., that of atomic weight determination. 



Any addition to our knowledge of this substance may therefore be expected to be 

 of interest. It will readily be admitted that the phenomenon of occlusion is one 

 which requires further elucidation, and in submitting the present communication to 

 the Society we hope that the additional work we now bring forward will not only be 

 of interest for its own sake, but that it will finally assist us in arriving at a definite 

 decision with regard to the nature of occlusion. 



In previous communications ('Phil. Trans.,' A, vol. 186, p. 657, and A, vol. 190, 

 ]>. 129) we discussed the relations of platinum black to oxygen, hydrogen, and other 

 gases, and on p. 152 of the latter paper we pointed out that until the relation of the 

 palladium to the hydrogen in the comparatively well defined substance palladium 

 hydrogen was ascertained, there was little hope cf solving the corresponding problem 

 for the less well defined substance platinum hydrogen. 



We therefore turned our attention to the general behaviour of palladium to oxygen 

 and hydrogen, and now give an account of our work on the subject, which includes 

 measurements of the heat evolved on the absorption of these gases, and a compara- 

 tive study of the occlusion of hydrogen by palladium in different states of 

 aggregation. 



II. P reparation oftJie Palladium. 



The palladium employed in these experiments was in the form of () black, 

 (6) sponge, and (c) foil. 



Palladium black. Some strips of metallic palladium, which were said to have 

 been prepared by WOLLASTON, were purified in the following manner : After dis- 

 solving in aqua rc.gia and evaporating several times successively with water and 

 hydrochloric acid, the solution was treated with excess of ammonia, and filtered. 

 From the filtrate pure palladosamine chloride was precipitated by leading a stream 

 of hydrochloric acid gas through it. This was filtered off, washed with strong 

 hydrochloric acid, dried, and ignited. The residue, consisting of palladium sponge, 

 was then again brought into solution by digesting with aqueous hydrochloric acid 

 through which a current of chlorine gas was led. After evaporating, to get rid of 

 the excess of hydrochloric acid and chlorine, the solution of palladium was neutralized 

 with sodium carbonate, and slowly poured into a boiling, very dilute solution con- 

 taining excess of sodium formate. The palladium black, precipitated in this way, 

 was then repeatedly boiled out with distilled water until free from alkalis and 

 alkaline chlorides, and dried at 100 C. 



Palladium sponge. The palladium sponge employed in the course of these experi- 

 ments was prepared by fully charging the necessary quantity of palladium black 

 with hydrogen, and then igniting at a red heat in vacuo, since simple ignition of 



