Pa 
SS) 
I 
of the Selenium Cell. é 
light produces an increase in the conductivity of a cell by 
facilitating the molecular rearrangement in the surface-layer 
of a selenide through which an electric current is passing, 
the question has presented itself, does the resistance of a cell 
undergo changes if it be exposed to light while no current is 
flowing ? 
Selenium. 
It appears that, in spite of the most painstaking efforts, 
selenium cannot be obtained absolutely pure, although the 
process of purification may be carried to so high a degree as 
to make the amount of impurity still present very small 
indeed. Asa preliminary series of experiments, cells were 
made with metallic electrodes and of crude selenium to which 
definite selenides had been added ; these cells were subjected 
to light from various portions of the spectrum and the position 
of maximum sensibility was determined for each. Itappeared 
that the position of the maximum was the same for all cells. 
On account of the impurity of the selenium some doubt was 
east upon the trustworthiness of the results and it was thought 
advisable to repeat the experiments with purified selenium. 
As distillation does not effectively remove impurities, a 
chemical method* was resorted to. I wish to take this 
opportunity of acknowledging my indebtedness to Professor 
Victor Lehner, of the Department of Chemistry, for the details 
of this process, which yielded selenium of great excellence 
and purity. Briefly the method is as follows : 
Commercial selenium is dissolved in hot nitric acid and 
the resulting selenious acid is evaporated to hard dryness when 
the dioxide is formed. ‘This is dissolved in distilled water and 
barium hydrate is added until a permanent precipitate is no 
longer formed. After filtering, the solution is boiled to 
dryness in an evaporating-dish, and the residue is covered by 
an inverted funnel whose base fits snugly upon the inside of 
the evaporating-dish. Continued heating brings about a 
sublimation of the dioxide upon the inner walls of the funnel 
in the form of white needles. As the slightest amount of 
impurity imparts to the selenium dioxide a reddish colour, it 
is necessary to repeat the process of sublimation until pure 
white needles are obtained. These are dissolved in distilled 
water, the solution is acidified with hydrochloric acid and 
acid sodium sulphite is added, bringing about a liberation of 
sulphur dioxide, which in turn precipitates the selenium in 
the form of a red powder. By boiling the mass for a few 
minutes the selenium forms into a hard black lump which, 
when washed and dried, is ready for use. 
* Victor Lehner, Journal Am. Chem, Soc, vol. xx. no. 8. 

