Disintegration of Metals at High Temperatures. 275 



random, the temperature required to produce nuclei of a 

 definite size, say, was found to depend very much upon the 

 recent treatment of the wire. When the conditions were 

 varied on more definite lines, as, for example, when the wire 

 was kept in air with no variation other than was incidental 

 to testing, it was found that the temperature required to 

 produce nuclei of a definite size depended upon the length 

 of time since the commencement of the experiments. Thus 

 there are two well-marked conditions governing the pro- 

 duction of nuclei : — 



1. The recent history of the wire previous to the 



commencement of the experiments. 



2, The length of time since the commencement of the 



experiments. 



If the wire had been standing in air, and had not been 

 heated for some hours or days, a comparatively low 

 temperature was all that was required to produce nuclei of 

 a definite size. If, however, the wire had just been heated, 

 then a second heating to the same temperature would 

 produce practically nothing, for this definite expansion ; a 

 little higher temperature would again produce nuclei, a 

 second heating at this new temperature would give practically 

 nothing, but a little higher temperature would again give 

 nuclei, and so on. Eventually a temperature would be 

 reached at which nuclei could be produced time after time. 



For simplicity it may be stated at once that there are two 

 sets of nuclei : the first set, which are got rid of after 

 continuous experimenting, depend upon the presence of 

 hydrogen and other gases in the wire ; the second set 

 depend upon the presence of oxygen round the wire: below 

 a certain temperature the second set are never obtained ; 

 above this temperature they are always obtained in the 

 presence, but never in the absence, of oxygen. 



If the wire were left until the following day similar 

 results were again obtained, beginning, however, at a 

 somewhat higher temperature than the previous initial 

 temperature, and ending at a slightly higher temperature 

 than the previous final temperature. This effect was very 

 marked in the earlier stages of the work, and decreased 

 gradually until, after about six weeks' experimenting, it 

 ■became inappreciable. At the same time that this variability 

 dies away, the upper temperature, that is, the minimum 

 temperature, for a given occasion, at which nuclei can be 

 produced repeatedly, attains a limit from which it never 

 •afterwards departs, provided that hydrogen is carefully 



