216 Prof. Minchin's Experiments in Photoelectricity. 



tin foil as it is obtained from the maker ; and, as must be 

 expected, not only were some pieces on exposure to light 

 observed to be at once negative while others (the great 

 majority) were positive, but there were various degrees of 

 sensitiveness. Occasionally, also, the curious jerks imme- 

 diately on exposure and immediately on screening, which 

 were observed in the case of silver and eosine, were observed 

 with these plates. 



Then succeeded a series of experiments in which a quad- 

 rant electrometer was used instead of the galvanometer, i, e., 

 the E.M.F.. and not the current, was measured. 



A curious result was very soon observed with the thin 

 tin foil which is wrapped inside small packages of tobacco. 

 The two sides of this foil are notably different in appearance, 

 one being somewhat dull, while the other has a bright silvery 

 aspect. Two strips of this were cemented on opposite sides 

 of a small glass plate, the dull surface of one and the bright 

 surface of the other being fixed to the glass. This plate was 

 immersed in a glass cell containing absolute alcohol, the two 

 pieces of tinfoil being connected with the poles of the electro- 

 meter. When the bright surface was exposed to daylight, a 

 small deflexion was obtained which showed the plate to be 

 negative ; when the dull surface was exposed, the deflexion 

 was more than doubled, the plate being, like the other, also 

 negative, and the magnitude of its E.M.F. being about 

 T V volt. 



A tin-foil plate, which at once on exposure to light is 

 negative, is produced by thoroughly cleaning a piece of foil 

 and coating it with sulphide of tin, either by exposing it to 

 SH 2 or by rubbing a little " mosaic gold " over the surface. 



When distilled water was used instead of alcohol, the 

 E.M.F. produced by light was in all cases diminished ; and 

 the same result always happened when any salt whatever was 

 dissolved in the alcohol with a view to diminishing its resist- 

 ance. 



This unfortunate result is characteristic of every photo- 

 electric cell that I have employed. It is unfortunate, because 

 it seems to preclude the possibility of obtaining really strong 

 and practically useful currents by the action of light, and it 

 appears to indicate some essential connexion of resistance and 

 electromotive force. 



The liquid is, of course, an essential element of the cell, 

 and a very large number of liquids was tried in these experi- 

 ments. Thus, for example, the two tobacco-foil plates when 

 immersed in peroxide of hydrogen were absolutely insen- 

 sitive. The alcohols are much the best liquids when tin 



