110 DRS. L. MOND, W. RAMSAY, AND .T. SHIELDS, ON THE 



excess of oxygen over and above that required for the formation of Pd 2 could he 

 removed in vacuo at any intermediate temperature. 



After being placed in communication with the pump the experimental tube was 

 then exposed to gradually increasing temperatures, and finally to as high a tem- 

 perature as the hard glass tube would stand without collapsing. 



Only a few bubbles of gas (about 1 cub. centim.) could be extracted, and hence 

 it appears that if either or both of the oxides Pd 2 and PdO are formed, they must 

 be stable in vacuo at a dull red heat. The substance formed during the absorption 

 of oxygen has a dark brown colour, has lost all the appearance of palladium sponge, 

 and is, without doubt, an oxide or mixture of oxides. 



According to WILM (' Berichte,' 1882, p. 2225), the sub-oxide of palladium, Pd 2 0, 

 may be prepared by heating the sponge in a current of air until it attains a constant 

 weight. If this is so, it is curious that we have not been able to observe any 

 discontinuity in the absorption of oxygen, and we are inclined to think that palladium 

 sponge, if heated for a sufficiently long time in a current of air, should yield not the 

 sub-oxide, but palladium oxide, PdO. 



We have already seen that palladium black, prepared in the way described, 

 contains .about 140 volumes, or l'G5 per cent, of oxygen, and the fact that the 

 substance was dried at 100 C. probably accounts for the presence of a certain 

 quantity of this oxygen. In connection with the calorimetric experiments to be 

 described later on, it was of interest to us to know how much oxygen palladium 

 black, rendered free from oxygen, would absorb directly at or at the ordinary 

 temperature. We found it impossible to remove the oxygen without converting the 

 black into sponge, but the required information was obtained in the following way : 



The sample of palladium black was fully charged up with hydrogen, whereby all 

 the oxygen was removed as water. As will be seen presently, the bulk (over 

 90 per cent.) of the occluded hydrogen was next extracted by exhausting at 100 C. 

 To the palladium black containing only a comparatively small quantity of hydrogen, 

 oxygen was then admitted, in the first instance very slowly, whilst the experimental 

 tube was kept cold by immersion in melting ice, and in the second rapidly, whilst 

 the tube was simply exposed to the atmosphere. Of course a portion of the oxygen 

 formed water with the residual hydrogen, the rest being absorbed, in the first case 

 without appreciable rise of temperature, and in the second with considerable rise of 

 temperature. The absorbed oxygen was then determined in both specimens by 

 charging up fully with hydrogen and exhausting at a red heat. It was found that 

 about 40 volumes and 120 volumes respectively of oxygen were absorbed. 



At C., therefore, the absorptive power of palladium black for oxygen is con- 

 siderably lessened. 



It will readily be admitted that the absorption of oxygen by palladium black is 

 really a process of oxidation, and although palladium and platinum belong to different 

 sub-divisions of the platinum group of metals, we think the absorption of oxygen by 



