184 Mr. S. Bid well on the Sensitiveness 



reduced to less than one fortieth in the course of a year*". 

 During this period the selenium had been in contact with the 

 metallic electrodes; and it seems possible that a larger quan- 

 tity of selenide than was produced in the first instance by the 

 process of annealing was slowly formed. This would espe- 

 cially occur at the u marked end/' or anode, where there 

 would naturally be a quantity of free selenium. 



In the above argument it has been assumed that selenium 

 will combine directly with any metal with which it is brought 

 into contact, the combination being facilitated by the appli- 

 cation of heat. In the case of such metals as copper, brass, 

 and silver this is undoubtedly the fact. Indeed, an attempt 

 to make a selenium cell with silver wires was attended with 

 failure in consequence of the complete destruction of the 

 metal after contact with the melted selenium for only two or 

 three minutes. It is, however, questionable whether platinum 

 (which was the metal used by Adams and Day) is, in any 

 sensible degree, attacked by selenium either at the ordinary 

 temperature or at that reached in the process of annealing. 

 With sufficient heat the two substances will undoubtedly 

 unite; and I have found that the surface of platinum-foil upon 

 which melted selenium has been kept for an hour or two at a 

 temperature probably of about 250° C. acquires a bluish-grey 

 colour which may be due to selenide. But whether any ap- 

 preciable quantity of selenide is formed in the ordinary pre- 

 paration of crystalline selenium is a question only to be settled 

 by the aid of refined chemical operations which I am incom- 

 petent to undertake, and in the meantime the suggested theory 

 is left without direct confirmation. 



But certain indirect evidence in support of my views has 

 been forthcoming. Selenium is an element which, in its 

 properties, closely resembles sulphur, and attempts have from 

 time to time been made, hitherto without success, to develope 

 in sulphur that peculiar sensitiveness to light which is such a 

 remarkable characteristic of selenium. It occurred to me 

 that if this property of selenium were really due to the acci- 

 dental existence of metallic selenides, then the admixture 

 with sulphur of metallic sulphides might be expected to lead 

 to similar effects. It is not possible to " anneal" a stick of 

 sulphur or a sulphur " cell " previously furnished with metallic 

 electrodes, because sulphur does not, like selenium, solidify 

 and crystallize at a higher temperature than that of its first 

 melting-point. But if it is true that the virtue of annealing 

 really lies in the fact that a chemical union of the two elements 

 is promoted by the action of heat, it is clearly immaterial 



* Phil. Trans, he. cit. p. 348. 



