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



temperature and so on, and eventually no more will be given, 

 •no matter how high the temperature may be raised. If the 

 wire be now left for a few hours in the vacuum, nuclei 

 will be again obtained on heating to a temperature very 

 little higher than the previous initial temperature, and after 

 successive heatings the wire will again cease to give nuclei at 

 any temperature. In other words, the behaviour of these nuclei 

 is exactly the same as the behaviour of what we have called 

 the first set of nuclei obtained in air. They are ultimately 

 .got rid of entirely, the only difference being that by the use 

 of the vacuum the emission of these nuclei can be prevented 

 after about three days' experimenting, whereas in gases 

 (excluding hydrogen) at atmospheric pressure it takes several 

 weeks to attain this result. 



Suppose a platinum wire has been heated repeatedly in a 

 vacuum until the first set of nuclei have been entirely got 

 rid of, then the wire can be left, heated or cold; in a vacuum 

 for an indefinite period, or it can be left for an indefinite 

 period, heated or cold, in air, nitrogen, or oxygen at any 

 pressure, and on again testing in a vacuum no nuclei will be 

 obtained at any temperature. Thus we see that in the pro- 

 duction of the first set of nuclei the action of the hydrogen is 

 unique. 



If a wire which has ceased to give nuclei in a vacuum is 

 now tested in air or oxygen, it is found that the first set of 

 ^nuclei are entirely absent, that is, the behaviour indicated by 

 fig. 2 is not observed ; the second set of nuclei begin at the 

 usual temperature, about 500° C, and increase in size as the 

 temperature is raised. 



We see, then, that the wire is capable of emitting two 

 kinds of nuclei, in addition to the positive and negative ions. 

 One set of nuclei depends upon the presence of hydrogen in 

 the wire, and the other upon the presence of oxygen round 

 the wire. In working with the expansion apparatus it is 

 quite easy to tell which nuclei are being dealt with ; the first 

 set of nuclei rapidly diminish in size, and disappear in two 

 or three minutes after their production, but last longer if 

 condensation of water- vapour has taken place upon them 

 than otherwise ; in general, they are not so great in number, 

 at any rate by the time the expansion takes place, as are the 

 oxygen nuclei ; the latter nuclei are very great in number, 

 except at the temperature at which they just begin to be 

 produced; they are of a fairly definite maximum size for 

 each temperature, and of smaller sizes in addition, as can be 

 shown by following up with higher expansions ; and they are 

 extremely persistent. 



