of Selenium and Sulphur to Light. 181 



trodes, and is perhaps diffused to some extent throughout the 

 mass of the selenium* ; and that the apparently improved 

 conductivity of the selenium, together with the electrolytic 

 phenomena which it exhibits, are to be accounted for by the 

 existence of this selenide. 



I have sometimes been tempted to think it possible that the 

 apparent conductivity of selenium may in fact be entirely due 

 to the impurities which it contains, and that perfectly pure 

 selenium would be as good an insulator when in the crystal- 

 line form as it is in the vitreous condition. Vitreous selenium 

 might contain a large percentage of conducting particles 

 without sensible increase of its conductivity, but that this 

 would not be the case with crystalline selenium, is rendered 

 more than probable by the results of some experiments which 

 I have described in a former communication!. If a conduct- 

 ing powder, such as graphite, is mixed with melted sulphur, 

 even in small proportions, the mixture when cold is found to 

 conduct electricity; while if a very large proportion of the 

 same powder is incorporated with melted shellac, the shellac 

 when cold remains sensibly as perfect a nonconductor as if 

 it w T ere pure. The explanation which I have given of these 

 facts, and in support of which a number of experiments are 

 quoted, is as follows: — The first mixture does not consist of a 

 uniform structureless mass of sulphur, having particles of 

 carbon imbedded in and completely surrounded by it : it is in 

 fact an aggregation of little crystals of sulphur with carbon 

 packed between them like mortar between bricks. The con- 

 duction thus takes place entirely through the carbon particles, 

 which may be considered as extending in a series of chains 

 from end to end of the mass. In the case of the shellac 

 mixture, though the proportion of carbon may be larger than 

 in the sulphur experiments, the resistance is still sensibly 

 infinite, because the structureless shellac penetrates between 

 and completely surrounds the carbon particles. Just in the 

 same manner, selenium, when in the vitreous condition, would 

 completely surround any particles of conducting selenides 

 which it might contain; while, when the selenium was 

 crystallized, the conducting particles would arrange them- 

 selves in the form of a network, capable of conveying a current 

 of electricity. 



Selenium which is free from impurities appears not to be 

 an article of commerce. An analysis of samples collected by 

 Professor Graham Bell from different parts of the world 



* The selenium is necessarily for some time in a liquid state, 

 t Phil. Mag. May 1882, p. 347. 



