28 Mr. A. H. Pfund: A Study 
Selenium prepared in this manner is deemed sufficiently 
pure for atomic and molecular weight determinations. 
Selenium Cell. 
A departure was made from the usual manner of making 
selenium cells in that carbon electrodes were used in place of 
metallic electrodes. The reason for this is, that when metallic 
electrodes are used a selenide is formed wherever metal and 
selenium come in contact, the amount formed being dependent 
upon the character of the metal and the length of time during 
which the cell is subjected to heat. The difficulty of knowing 
the exact amount of selenide in such a cell is obvious. As 
actual tests have proved that no conducting compound is 
formed when selenium is heated in contact with a carbon 
surface, it was thought that by using carbon electrodes the 
above difficulty would be ay oided, or, in other words, that it 
would be possible to know the exact amount of selenide con- 
tained in a completed cell. Furthermore, when electrodes 
of silver and mercury are used, the strong affinity of these 
metals for selenium brings about such a copious formation of 
selenide that, after a few minutes of heating, the cell becomes 
worthless. The use of carbon electrodes overcomes this 
difficulty entirely. 
Fiewel. 
selenium 
Carbon 
Mica 
Sitiuitv 
abieitinn Sat Mads 2) ec oe ea 
= w= ale r - - 
' SGI aaa a a 
i €---- 15 717727 -—-3 
on. eae wid ie 471772. 
«-5minry « mm» 
1 | ; 
J ' 
1-83 772772. 0:°8772772. 
The above sketch (fig. 1) represents a full-sized sele- 
nium cell as actually used in these experiments. Two small 
i ie 
