246 Prof. De la Rive on the Propagation of Electricity 



a very decided blue colour, but darker with zinc than with silver. 

 With points of copper, cadmium, aluminium, and magnesium 

 the tint is green, very dark with copper, apple-green with cad- 

 mium, very light green with magnesium, and a whitish gvctn 

 with aluminium. With points of gas-carbon, the colour of the 

 discharge is light blue, and changes to bluish when the arc 

 ceases ; this is owing to the production of a small quantity of 

 carburetted hydrogen gas. The effects are most distinct at the 

 upper part of the globe, where the vapours resulting from the 

 voltaic arc collect. The striae or st ratifications of the electric 

 light are even more marked in these vapours than they are in 

 rarefied gases. 



The intensity of the electric discharge is increased in conse- 

 quence of the presence of metallic vapours within the globe ; this 

 increase depends on the nature of the vapour : with silver and 

 copper it is very considerable, the galvanometer going suddenly 

 from 30° to 60° at the moment when the change of colour of the 

 discharge shows that it has begun to be transmitted by the me- 

 tallic vapour. The increase, though smaller, is still distinct with 

 aluminium-vapour ; with the vapours otzinc, cadmium, and mag- 

 nesiuni it is much smaller, namely from 10° to 20° only. It. is 

 very large in the case of the arc produced by means of points of 

 gas-carbon, perhaps in consequence of the presence of small 

 quantities of carburetted hydrogen gas. 



I have also tried points of iron and platinum : with the former 

 I observed certainly a change of colour in the electric discharge 

 and a slight increase of its intensity; but with the latter I ob- 

 tained nothing more than a very small increase in the intensity 

 of the discharge, which might be due to the influence of the 

 enormous elevation of temperature on the conductivity of the 

 rarefied nitrogen — an effect, however, which is too slight to exert 

 any sensible influence upon the foregoing experiments. 



These experiments appear to me to prove, not only that ex- 

 cessively rarefied metallic vapours have a conducting-power for 

 electricity which greatly exceeds that of other elastic fluids, but 

 also that this power varies with their nature, and is possessed in 

 the highest degree by the vapours of those metals which are 

 themselves the best conductors. As to the appearance of the 

 discharge, its colour depends upon the nature of the vapour, and 

 is the same as that which the metal yielding the vapour produces 

 on combustion; and, what is of some importance to notice, the 

 stratifications are more distinct than they are in ordinary rarefied 

 gases *. 



* I was not aware* when I made the experiments upon metallic vapours 

 which I have just described, that M. Faye had made analogous experiments 

 (Comptes Renews de VAcad. dss Sciences, vol. liii. pp. 493 el seq.). 



