Disintegration of ]\ fetal s at High Temperatures. 291 



with platinum and rhodium takes many hours. No deposits 

 are obtained from platinum, rhodium, and iridium if these 

 are heated in a vacuum. 



The behaviour of palladium at low pressures is different 

 from that of the other three metals named. It has been 

 found by Stewart * and by Holborn and Austin * that : — 



1. The rate of loss of weight of platinum, rhodium, and 



iridium is practically zero when oxygen is absent, 

 and when oxygen is present the rate of loss of weight 

 diminishes with diminution of pressure of the oxygen. 



2. The rate of loss of weight of palladium increases with 



diminution of pressure, and is practically independent 

 of the nature, of the surrounding gas. 



3. With palladium the deposit is always black when 



oxygen is present, but is bright and metallic in 



hydrogen at a pressure of 1*25 mm. 

 It is evident that the mechanism of the disintegration 

 in the case of palladium is different from that in the cases 

 of the other metals. Several explanations of this pheno- 

 menon in the case of palladium suggest themselves : — 



1. Since palladium absorbs gases to a greater degree than 



the other platinum metals, the greater rate of loss of 

 weight at low pressures might be due to the greater 

 rate of escape of these gases, the loss of weight being 

 equal to the weight of escaped gas. 



2. The evolution of the gas might cause particles of the 



metal to be loosened and detached. 



3. The bright deposits mentioned by Holborn and Austin 



in hydrogen at 1*25 mm. might be due to the 

 reduction of an oxide. 



4. The effects might be due to simple volatilization of the 



metal. 



In order to decide between these possible explanations, I 

 have repeated Holborn and Austin's experiment and found 

 that 1 and 3 are disposed of by the fact that the greater 

 rate of loss of weight at low pressures continues indefinitely, 

 and the bright deposits are obtained in a maintained vacuum, 

 where they cannot be due to the reducing action of hydrogen. 

 2 and 3 are disproved by the fact that no nuclei whatever 

 are detected from palladium in a vacuum. I think that 4 

 is the true explanation. Incidentally it may be noticed 

 that volatilizing palladium does not give nuclei. This 

 agrees with Aitken's observations on camphor, and suggests 

 that a pure solid substance does not give nuclei when 



* Loc. cit. 



