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



The more rapid disintegration in the early stages is due to 

 the small quantities o£ iridium which are usually present in 

 commercial platinum. Iridium disintegrates much more 

 rapidly than platinum. 



Expansion Method of Detecting Disintegration. 



The disintegrated particles serve as centres for the con- 

 densation of water vapour, as was found by Aitken *. The 

 emission of particles is certainly complicated with the 

 emission of positive and negative ions. That the electric 

 charges are not carried entirely by the particles is now well 

 known |. It has been suggested by Child % that so far from 

 carrying the charges the particles load them up, and so 

 hinder their motion. Though this effect may exist, it cannot 

 be to any important degree, for I have found that the 

 passing of hot-wire nuclei through a strong electric field 

 on their way to the expansion chamber is entirely without 

 detectable effect. 



As the super-saturation of water-vapour is an extremely 

 delicate test for the presence of particles, I have used this 

 method to investigate the minimum temperature at which 

 disintegration begins. As a proof of its superiority over 

 weighing methods, it may be mentioned that Sir W. Crookes 

 was unable to detect the slightest loss of weight in a platinum 

 crucible kept for 20 hours at 900° 0. ; on repeating the 

 experiment with a platinum wire, I was unable to detect 

 any alteration of weight after 13 hours at 650° C. On 

 inserting the wire in the nuclei-chamber, and heating by 

 means of the same current for two seconds, a dense cloud 

 was obtained at expansion of 1*15, showing that the wire 

 was disintegrating. This result is confirmed by tests under 

 various circumstances. 



Condensation ^Nuclei from Dot Wires. 



Apparatus. — The general form of: the apparatus is shown 

 in fig. 1. The experimental wire W was not placed in the 

 expansion chamber Gr, but in a separate nuclei-chamber C, 

 so that no water-drops could fall on it ; experiments could 

 be performed with the wire in different gases and in a 

 vacuum; the wire was supported in two platinum electrodes, 

 P P, 2 mm. diameter and 10 cm. long. The temperature 

 of the hottest part of the wire was determined by means of 

 a platinum platinum-rhodium thermocouple, the wires beings 



* Aitken, Trans. E. S. E. 1883. 



t J. J. Thomson, ' Conduction through Gases,' 1903, p. 184. 



t Child, Phys. Rev. 1902. 



