Action of Selenium Cells. 215 



point of view and not merely as an isolated phenomenon in. 

 selenium, but, by finding the general law, to remove selenium 

 from its isolated position. This, however, may be done either 

 by comparing other bodies with it (which is the method of 

 previous investigations on the subject, and was my own also 

 till now), or this may be effected by inserting selenium in 

 the series of other bodies. The latter method I shall now 

 describe. 



I intended to make such cells of copper, selenium, copper, or 

 of zinc, selenium, copper plates, in the same way as the above- 

 described photophonic cells. On a copper plate I melted 

 amorphous selenium, and put on it the second metal plate of 

 zinc or copper. Then I heated gradually, so that the selenium 

 became crystalline; and I then annealed it. Whilst the amor- 

 phous selenium adheres very well to the metal plates, these 

 cells proved very brittle when the selenium was crystalline ; 

 the selenium always split off from the copper plate. In order 

 to find how to avoid this splitting-off, I examined the matter 

 more closely; thus, I left out the second metal plate, and ex- 

 perimented with copper and selenium only. 



The preparation was, in accordance with that of the photo- 

 phonic cells, the following : — One of the copper plates to be 

 used for the cells was heated on a large brass plate of about 

 3 millim. thickness ; this was covered by a thin sheet of 

 mica, on which the selenium which eventually flowed off was 

 collected. The copper plate having reached the melting- 

 point of amorphous selenium, was covered with this substance 

 and thereupon removed from the brass plate. On being re- 

 moved it cooled quickly; and on it now a black- brilliant layer 

 of amorphous selenium remained. The larger brass plate having 

 also cooled, the copper plate covered with selenium was again 

 laid on it and again slowly warmed, so that the selenium crys- 

 tallized. The temperature was still further raised, near to the 

 point at which the crystalline selenium begins to melt. The 

 selenium was then annealed. 



But, notwithstanding this caution, it was not possible to fix 

 the selenium on the copper plate. It exfoliated and split off. 

 Every lamina which exfoliated was on the upper side light 

 grey, and on the lower side blue-black, not brilliant but dull. 

 In the same way the copper plate had now a similar dull 

 blue-black covering. This blue-black body is cuprous sele- 

 nide, Cu 2 Se. There were thus three layers — copper, cuprous 

 selenide, and selenium; and in the copper-selenium-zinc cell 

 two other layers — selenide of zinc and zinc; so that all together 

 there were five layers. 



I took thinner copper plates ; but the splitting-off still 

 occurred. 



