284 Mr. J. H. T. Roberts on the 



seems likely that in a vacuum the process of diffusion will 

 be hastened, and that the minimum temperature for nuclei 

 will be lower than in gas at atmospheric pressure ; and these, 

 as we have seen, are among the observed experimental facts. 

 There is still one point which requires explanation. Why 

 is it that when a wire has ceased to give nuclei in a vacuum, 

 the single process of admitting and removing hydrogen, 

 provided the wire has not been heated in the vacuum too 

 long, at once revives the power of the wire to emit nuclei, 

 while the admission and removal of oxygen, air, or nitrogen 

 does not have this effect ? We may imagine the mechanism 

 of occlusion and liberation to be pretty much the same in 

 the cases of the other occluded gases as in that of hydrogen, 

 except that in the case of hydrogen the processes, both of 

 occlusion and liberation, are more rapid. Thus it is probable 

 that when the hydrogen has been almost entirely removed, 

 and consequently traces of compounds with hydrogen cease 

 to be formed, there are nevertheless, remaining in the metal, 

 quantities of other occluded gases liberated with more 

 difficulty. On bringing hydrogen into contact with the 

 wire, the ability to form hydrogen compounds (possibly 

 H 2 0) would be at once restored *. 



This explains why the action of pure hydrogen fails if the 

 wire has been kept white hot in a vacuum for a long time, as 

 the other occluded gases would then be removed. The effect 

 of hydrogen in reviving the nucleating power in air is easily 

 understood, as, in the experiments in air, the wire, on account 

 of the frequent heatings, would have every opportunity of 

 absorbing oxygen, &c. It has been mentioned that the 

 nuclei may he produced by the action of the metal and merely 

 liberated by the heat. 



If a wire has been heated in a vacuum so long that the 

 admission of hydrogen fails to revive the power of emitting 

 the first set of nuclei, the subsequent admission of air will 

 restore the power. 



2. The second set of nuclei are in all probability particles 

 of an unstable oxide of the metal. The formation of this 

 oxide will be considered later. The formation of an oxide 

 agrees with the facts that : — 



{a) The second set of nuclei are not formed in nitrogen, 

 hydrogen, or a vacuum, but only in the presence of 

 oxygen. 



(6) The rate of loss of weight of the metal is zero in 

 nitrogen, hydrogen, or a vacuum. For platinum 



* H. A. Wilson, loc. cit. 



