354: Disintegration of the Aluminium Cathode, 



as soon as the discharge was sent through the tube. Hence 

 it would seem that a very small amount of water vapour 

 present hinders or prevents the disintegration of the cathode. 

 The delay in other instances may be due to the fact, that the 

 thin and invisible layer of oxide or nitride on the surface of 

 the cathode must first be removed hj bombardment by the 

 positively charged particles. 



In support of the necessity of this initial " cleaning " of 

 the cathode surface, may be mentioned the fact, that in the 

 cases where the disintegration was pronounced, the metal of 

 the cathode appeared bright after the discharge ceased, and 

 when the tube was opened to the air, the peculiar " fibrous 

 growth " took place on the surface of the cathode. This 

 growth of aluminium hydroxide is supposed to take place 

 only when a perfectly clean surface of the metal is exposed 

 to the air. 



This ability of aluminium to long delay the process of 

 disintegration may be the reason it is so much used and yet 

 disintegrates so little. 



General Considerations, 



1. The aluminium cathode, which had been supposed not 

 to disintegrate to any marked extent, except in the monatomic 

 gases, does " splutter " violently in the presence of the 

 halogens, certain metallic vapours and other gases. 



2. The disintegration of the cathode in question seems to 

 take place best when the pressure in the tube has reached the 

 ordinary X-ray stage, and when the cathode dark-space has 

 reached the walls of the tube next the cathode. 



3. The rate of disintegration seems to increase in general 

 with the molecular weight of the discharge gas. 



4. It seems probable, that under proper conditions, the 

 aluminium cathode will disintegrate in all gases, more or 

 less rapidly. It is intended to investigate the behaviour of 

 this cathode in other gases, and to make quantitative deter- 

 minations of the rate of the disintegration, the cathode-fall, 

 and the current strength necessary to bring about this 

 phenomenon. 



The writer wishes to express his gratitude to Prof. 

 Sir J. J. Thomson for suggesting and directing this pre- 

 liminary investigation. 



Cavendish Laboratory. 

 June, 1914. 



