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



Effect of Light on Nuclei. 



The nuclei obtained in a vacuum from various platinum 

 wires containing hydrogen were allowed to remain, in air 

 at atmospheric pressure, in the nuclei-chamber, for various- 

 intervals of time before the expansion was made. Experi- 

 ments were tried alternately with the chamber strongly 

 illuminated by means of a Nernst lamp and other sources of 

 light, and with the chamber in darkness. It was found that 

 there was no difference in the behaviour o£ these nuclei in 

 the dark and in the light. 



Effect of Electric Field on Nuclei. 



The nuclei obtained under various conditions were tested 

 for electrification by means of the tubes E, fig. 1. No 

 difference was ever detected between the behaviour with the 

 field on and that with the field off. The nuclei, however 

 (particularly the second set, that is, the oxygen nuclei), are 

 usually so numerous that if only a few of them were removed' 

 there would not be any appreciable alteration in the nature 

 of the cloud formed upon the remainder. Thus, if any 

 nuclei are attached to ions, the number of such nuclei must 

 be a very small percentage of the total number of nuclei. 



The behaviour of iridium, rhodium, and ruthenium was in 

 general similar to that of platinum — that is, nuclei were 

 produced in air or oxygen, but not in a vacuum, after all the 

 hydrogen had been got rid of. There is one important point 

 of difference : platinum gives no nuclei in a vacuum after 

 being heated in oxygen : iridium and ruthenium, however, 

 will give nuclei in a vacuum after being heated in oxygen, 

 these nuclei being due to the removal of the oxide, which, in 

 the case of iridium, forms a film upon the cooler ends oE the 

 wire. After a few heatings, with the Gaede pump working,, 

 these nuclei cease to be emitted. 



Nature of the Nuclei. 



1. We see that the properties of the first set of nuclei are 

 as follows : — 



(a) They are emitted by the wire at comparatively low 



temperatures. 



(b) The minimum temperature required to produce them 



is lower the less the pressure of the surrounding 

 gas. 



(c) They alter in size and disappear very soon after their 



liberation, but last longer if water-vapour has 

 been caused to condense upon them. 



