Disintegration of Metals at High Temperatures, 283 



(d) The ability of the wire to emit them is temporarily 



lost after an emission, but is slowly regained after 

 the lapse of time. 



(e) The wire immediately regains the ability to produce 



them on hydrogen being brought into contact with it; 

 air, oxygen, and nitrogen do not produce this effect. 

 ( f) If the wire has been kept at a white heat in a vacuum 

 for a very long time, the admission and withdrawal 

 of pure hydrogen does not revive the power to emit 

 nuclei in a vacuum. 



These nuclei are obviously not of the nature of solid 

 particles ; they are particles which are able to evaporate or 

 break up. They may be : 



(a) Minute water-drops. 



(b) Traces of compound gases produced by the combination 



with hydrogen of other occluded gases *. 



These nuclei are not necessarily produced by heat ; they 

 may be produced by the catalytic action of the metal, which 

 is well known, and may be merely liberated by the heat. 

 It is easy to see why they should last longer if condensation 

 of water-vapour takes place upon them, as extremely small 

 drops evaporate rapidly and the condensation, by increasing 

 the size of the drops, would diminish their rate of 

 evaporation. 



Recent investigations have shown that the occluded gases 

 in a metal wire are partly absorbed in the metal, and 

 partly adsorbed, that is, condensed or entangled upon the 

 surface ; and I have to thank Prof. Donnan for the following- 

 suggestion in explanation of the curves in fig. 2 : — On first 

 heating the wire to a fairly low temperature, some of the 

 adsorbed gas quickly goes off, but on again heating 

 immediately afterwards to the same temperature, practically 

 no more is liberated ; on heating to a higher temperature 

 more adsorbed gas goes off, but not again on a second heating 

 at that temperature, and so on. The amount of the adsorbed 

 gas which can be got rid of by this process has now been 

 removed. Now leave the wire for a day, and some of the 

 absorbed gas slowly diffuses out and becomes adsorbed, and 

 on a second set of heatings, results of the same nature as 

 before are obtained ; eventually the quantity of gas remaining 

 in the wire is too small to give nuclei. Thus it is the 

 adsorbed gas which goes off, and the absorbed gas slowly 

 diffuses out in the longer intervals of rest. On this view it 



* See ' Electrical Properties of Flames/ H. A, Wilson, p. 26. 



U2 



