440 Professor Sir James Deivar [June 8, 



charcoal be expelled by heating it to the ordinary temperature, it will 

 contain on the average some 60 per cent, of oxygen. Fig. 5 represents 

 roughly the molecular composition of the air initially present in the 

 pores of the charcoal, namely, about four molecules of nitrogen to one 

 of oxygen. The final average molecular composition of the absorbed 

 gas after the passing of the slow current of air, cooled to — 185°C., 

 over the charcoal is similarly represented in Fig. 6, where it can be 

 seen that diffusion, or fractional distillation at constant temperature, 

 has gone on until about two of the molecules of nitrogen have been 

 replaced by two of oxygen. If the charcoal, in equilibrium with air 

 gases in the condition specified in Fig. 6, has a current of hydrogen 

 at the temperature of liquid air passed over it, the hydrogen will dif- 

 fuse in, until about one molecule in five is present, as shown in Fig. 7. 

 Again, if charcoal be saturated with oxygen to begin with, and 

 hydrogen be passed over it when cooled to — 185° C, the hydrogen will 

 diffuse in, until it constitutes one-third of the gas present in the 

 pores (Figs. 8 and 9). Similarly, if we employ nitrogen in place of 

 oxygen, the hydrogen will displace about two-thirds of the nitrogen 

 (Figs. 10 and 11). On the other hand, if the charcoal be initially 

 saturated with hydrogen at — 185°C. (Fig. 12), and air passed over it 

 at that temperature, the whole of the hydrogen is practically dis- 

 placed, and the gas remaining in the charcoal is represented by the 

 composition shown in Fig. 13, which is the same as in Fig. 6. 



If, instead of analysing the whole of the occluded gas that is given 

 off, samples are taken from the charcoal between definite temperatures, 

 from the boiling point of air up to the ordinary temperature, then a 

 regular fractionation of the occluded gases takes place, resembling that 

 of an ordinary mixed liquid, and the problem is an entirely different one. 

 Further, even in the simpler mode of treatment as described above, 

 the relative proportions of the absorbed gases depends at any time 

 after the experiment has started upon the relation between the current 

 of gas used at the constant temperature of — 185°C., and the nature 

 and condition of the charcoal. The simplification of the problem is 

 due to making the time very long. 



Chemical Interactions at High Vacua. 



If a piece of pure sulphur (S) is put in one end and some mercury 

 (Hg) in the other end of a U-tube (Fig. 14), both substances being kept 

 at the temperature of liquid air during the time required to reach a high 

 vacuum by the mercurial pump or by charcoal exhaustion, and the 

 whole sealed off and left to stand at the ordinary temperature ; then 

 in a few hours, it will be noted that the surface of the mercury 

 which was at first a brilliant reflecting one gets tarnished from 

 the formation of a film of sulphide of mercury. The mercury 

 vapour pressure being much greater than that of sulphur, it would be 

 expected that any formation of sulphide of mercury taking place 



