372 Prof. Thomson & Mr Skinner, On chemical effect, etc. [Nov. 22, 



In potassium vapour the aluminium kathode is not evaporated 

 to any marked degree. In this case the action on the glass may 

 be attributed to the potassium which under the action of the 

 discharge attacks the glass forming soluble potassium silicate ; 

 so when the bulb is washed out with water and dried a frosted 

 surface is left. 



Opposite the kathode both in the mercury vapour and po- 

 tassium vapour bulbs a dark annular stain of the shape of the 

 kathode is formed. This stain resists the action of strong hot 

 hydrochloric acid, nitric acid, aqua regia and potash solution. 

 The action of hydrofluoric acid removes it apparently by dissolving 

 the glass. These tests indicate carbon rather than silicon, but 

 the quantity of the stain is too small to make certain. The stain 

 is also formed on screens of mica, quartz and calcite ; and this 

 supports the view that it is carbon fractionally evaporated from 

 the aluminium and iron cathodes. The sheet aluminium and iron 

 used for kathodes was not pure. 



For comparison some pieces of an old incandescent lamp which 

 was blackened inside with carbon, evaporated from the filament 

 by long use, were treated with hydrofluoric acid and examined 

 microscopically. The visible effects in this case were very like 

 those observed when the stain was treated with the acid. The 

 stain in each case was apparently in a state of tension and portions 

 of it curled up when broken. 



Monatomic gases appear to permit the evaporation of alu- 

 minium, as Professor Callendar has observed its evaporation in an 

 argon vacuum tube. 



(4) On the effect of zinc and other metals on a photographic 

 plate. By Professor J. J. Thomson. 



In the course of a discussion at the Cavendish Physical Society 

 on Dr Russell's paper on the photographic effect produced by 

 certain metals, Sir George Stokes suggested that possibly light 

 might be thrown on the question as to whether these effects were 

 due to radiation or to the vapour of the metals, if photographs 

 were taken with a stream of air flowing between the metal and 

 the photographic plate. In consequence of this suggestion a series 

 of the photographs made by zinc and amalgamated zinc (1) with 

 nothing but air between the zinc and the photographic plate, 

 (2) when the zinc was covered with a film of celluloid, were taken 

 both with and without an air blast. The photographs with the 

 air blast were found in both cases to be distorted, which is in 

 favour of the view that the effects on the photographic plates are 

 due to the vapour of the metals, 



