1893.] Waterhouse — Electrical action of Light upon Silver. 29 



some months, the deflection in sunshine was fairly large, amounting to 

 about 10 divisions, or rather more than the deflection caused by the con- 

 tact of dry zinc and copper. When, however, the same plate had been 

 carefully cleaned with a solution of cyanide of potassium followed by the 

 usual rubbing with emery cloth, the deflection was found to be still posi- 

 tive, but much smaller, being only about 1*5 divisions on first exposure, 

 and by repeated exposure it was reduced to about - 25 division. 



With subsequent exposures the deflection was generally in the 

 same direction, but once, after fresh cleaning, it was negative. With 

 a plate of pure silver deposited on glass, freshly polished, the first 

 exposure gave after a short interval, a fairly strong negative deflection, 

 but with subsequent exposures at intervals the deflections have been 

 sometimes negative and sometimes positive, but always very small, so 

 that the observations are somewhat uncertain. Plates of almost perfect- 

 ly pure silver, 999*5 touch, obtained through the kindness of the Mint 

 Master, Lt. Col. Baird, R. E., F. R. S., gave also rather indefinite results, 

 owing to the smallness of the currents, and though the deflections were 

 generally positive on first exposure of the plates, they were sometimes 

 negative, or became so by prolonged exposure. The general tendency, 

 however, appeared for the plates to be positive under the influence of 

 light, and, if this is tbe case it would seem to point to some slight 

 oxidising action on the surface. At the same time, the results obtained 

 with pure silver and the fact that in so many cases the deflections have 

 been first positive and then negative, appear to favour the conclusion 

 that such plates are really negative. It was clearly ascertained that the 

 currents produced were not due to the action of heat, because with the 

 plate first observed and with the purest silver plates, the action of heat 

 applied at the exposed end of the plate was to give a positive deflection, 

 but with the less pure silver plates used in the cells and others largely 

 alloyed with copper, the heated end of the plate was always negative 

 to the cool end. The deflection invariably increased with the continuance 

 of the heating, and was always in the same direction on repetition of it. 



The light currents, on the other hand, showed a decrease of deflec- 

 tion from repeated exposures and sometimes a change of sign in a 

 direction contrary to the heat currents shown by the same plates. The 

 observation is rather a difficult one and requires further repetition under 

 more favourable conditions of light, in order to obtain definite results. 



I have also tried the effect of solutions of alkaline haloid salts 

 upon silver plates, but as this paper is already beyond the usual 

 limits, it may be well to defer the account of these and other 

 experiments on photographic plates containing the haloid salts of silver 

 to a futux'e paper. 



