176 POPULAR LECTURES AND ADDRESSES. 



centimetre, when in fact it is so very thin as to 

 produce absolutely no perceptible effect on the 

 reflected light, that is to say, so thin as to be 

 absolutely invisible. If, in the apparatus for 

 measuring contact electricity, of which the drawing 

 is before you (Nature ^ vol. xxiii. p. 567), two 

 plates of freshly polished copper be placed in the 

 Volta condenser, a very perfect zero of effect is 

 obtained. If, then, one of the plates be taken 

 out, heated slightly by laying it on a piece of hot 

 iron, and then allowed to cool again and replaced 

 in the Volta condenser, it is found that negative 

 electricity becomes condensed on the surface thus 

 treated, and positive electricity on the bright 

 copper surface facing it, when the two are in 

 metallic connection. If the same process be re- 

 peated with somewhat higher temperatures, or 

 somewhat longer times of exposure to it, the 

 electrical difference is augmented. These effects 

 are very sensible before any perceptible tint 

 appears on the copper surface as modified by heat. 

 The effect goes on increasing with higher and 

 higher temperatures of the heating influence, until 



