Negative Corpuscles by the Alkali Metals. 585 



with these metals there is a small emission of corpuscles, 

 even when all external light is excluded. The following- 

 experiments show, however, that rubidium and the liquid 

 alloy of sodium and potassium give out corpuscles in the 

 dark. 



The experiments showing this were made as follows : — A 

 gold-leaf electroscope with quartz insulation was enclosed 

 in a glass vessel, which was exhausted to an exceedingly low 



Fiff. 1. 



vacuum by means of charcoal cooled to a very low tem- 

 perature by liquid air in the way discovered by Sir James 

 Dewar. The rubidium or Na-K alloy was placed below the 

 gold leaves of the electroscope, care being taken to have 

 the surface of the metal as clean as possible ; the metal was 

 earthed by means of a wire fused through the glass. The 

 vessel was placed inside a box made light-tight by means of 

 felt, the tube containing the charcoal which protruded from 

 the box was painted over with lamp-black. To measure the 

 divergence of the leaves of the electroscope, these were 

 momentarily illuminated by a faint light transmitted through 

 a red glass window, the position of the leaves was deter- 



