Prof. Mincum's Experiments in Photoelectricity. 217 



plates are used. Large electromotive forces have been 

 obtained with all of them i but they are not all of quite 

 equal value. As has been already stated, if any substance 

 containing chlorine is put into the liquid, the E.M.F. of light 

 completely ceases. 



If a small quantity of nitrate of potassium is dissolved in 

 the alcohol, a Thomson high-resistance galvanometer will be 

 very easily and strongly affected by the photoelectric currents 

 of these cells. 



Among liquids an aqueous solution of pyrogallic acid is 

 one which presents itself as worthy of trial, since pyrogallic 

 acid is a greedy absorber of oxygen ; and I have on record a 

 striking result of its employment. A small plate of tin foil, 

 the surface of which had been cleaned, was covered over with 

 a thin layer of water, with which a drop of nitric acid had 

 been mixed ; the foil was placed on a glass plate, under 

 which was applied a gas-flame until the liquid had quite 

 evaporated, and the tin foil remained coated with some salt of 

 tin. Whatever this salt was, when this plate was immersed 

 in a cell containing absolute alcohol in presence of a cleaned 

 tin-foil plate, the first was, on exposure to light, strongly 

 negative to the second. The sensitive plate was then taken 

 out of this cell and immersed in another containing a clear 

 aqueous solution of pyrogallic acid. It was now absolutely 

 insensitive to light. Taken out of this cell, and dried with 

 blotting-paper, it was again put into the alcohol ; and now, 

 on exposure to light, it was found to be strongly positive. 

 Here, then, is a case of complete reversal. This change was 

 accompanied by a slight jerk or kick in the E.M.F., such as 

 has been already described in connexion with silver and 

 eosine ; that is, on exposing the plate for the second time in 

 alcohol, the initial indication of the electrometer was a very 

 slight kick indicating a negative E.M.F. , but this was 

 immediately followed by a large deflexion in the opposite 

 direction. This result was repeated many times ; the plate 

 was taken out of the alcohol and dried in the dark for about 

 two hours, and when replaced in the alcohol cell the jerk and 

 large contrary deflexion were obtained. 



A similar experiment has been recently tried with a tin-foil 

 plate sensitized by the process which I now adopt, and which 

 will be presently described, and the result obtained was not 

 the same as the above. In the new process the plate appears 

 to be coated with the white oxide of tin, Sn0 2 ; but whether 

 it really is so or not, I cannot say. However, such a plate 

 which had been in a cell with methyl alcohol for two years 

 was taken out, together with the back or unexposed plate, 



