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



way invariably contains oxygen, it was thought that palladium black would also 

 contain oxygen. On igniting T5650 gram palladium black in a hard glass tube in 

 vacuo, however, only 0'33 cub. centim. oxygen and 0'56 cub. centim. carbon dioxide 

 were extracted. This proves that the oxygen, if any be present, is more firmly 

 retained by palladium than by platinum. 



Another portion of palladium black, weighing 1'9795 gram, was placed in a hard 

 glass bulb tube, one end of which was attached to a weighed U-tube containing 

 P 2 r) , and thence to the pump, whilst the other end was connected with an apparatus 

 supplying pure dry hydrogen. On ignition in vacuo 0'0142 gram H 2 was collected 

 in the U-tube. This corresponds to the presence of 072 per cent, of water. No 

 oxygen was given off, but on passing hydrogen over the sponge remaining in the 

 hard glass tube 0'032G gram H 3 O was obtained. The palladium black therefore 

 contained l'G5 per cent, of oxygen, corresponding to 23'G9 per cent. Pd 2 O, or 12'G9 

 per cent. PdO. If we assume that the oxygen exists as PdO, and if we estimate the 

 pure palladium by difference, we get for the analysis of palladium black : 



Palladium 8G'59 per cent. ; 



.Palladium oxide (PdO) . . 12'G9 = TG5 per cent. Oo ; 

 Water 072 



From the weight of water formed, it follows that the palladium black contained 

 22'82 cub. centims., or 138 volumes of oxygen. The oxygen initially present in 

 palladium black was also determined in another way. 



Palladium black was fully chai'ged with hydrogen, a portion of which was really 

 occluded, whilst the remainder formed water with the oxygen pre-existing in the 

 black. The occluded hydrogen is easily determined by extracting at a red heat in 

 vacuo, and half the difference between the volume of hydrogen used and that 

 pumped off represents the volume of oxygen initially present. In this way it was 

 found (see table, p. 125) that the palladium black contained 140, 141, and 143 

 volumes of oxygen, whilst the gravimetric determination indicated the presence of 

 138 volumes. 



Pure palladium sponge is almost white, whilst that obtained by the direct ignition 

 of palladium black in vacuo is much darker in colour owing to the presence of oxygen 

 or oxide in it. 



V. Absorption of Oxygen by Palladium. 



We have already shown ('Phil. Trans.,' A, 1895, vol. 18G, p. 682) that when 

 platinum black is heated in an atmosphere of oxygen, kept approximately at 

 ordinary pressure, absorption of oxygen takes place until the temperature rises to 

 about 360 C., when the gas is again expelled. If this is really a process of 

 oxidation, then, since palladium is usually regarded as a more easily oxidisable 



