212 Prof. Minchin's Experiments in Photoelectricity. 



and then exposing this plate to light, it was found that it 



took positive electricity from the liquid With a thick 



layer of iodide on the surface of the silver, there is, on the 

 contrary, a current the inverse of the preceding ; that is to 

 say, the plate exposed to light took negative electricity. This 

 result shows that, in this case, iodine acts on silver under the 

 influence of light."" He then gives a table of deflexions 

 obtained when various coloured glasses were placed in front 

 of a silver plate coated with a thick layer of iodide, the 

 greatest effect being produced by violet rays, and the least by 

 red, the former being 22 times the latter. 



M. Becquerel continues : — " Thus, whilst with chloride and 

 bromide of silver precipitated, placed on plates of platinum, 

 there is always produced a current of the same sense, the 

 exposed plate being positive, with a layer deposited on silver, 

 there is an effect depending on the thickness of this layer. 

 These two inverse effects indicate that there should be neces- 

 sarily a thickness for which the electric effect is almost 

 nothing/' 



These observations of M. Becquerel may possibly explain 

 the above inverse effects in the eosine-silver cell ; but his last 

 conjecture would not be practically verifiable, unless, when 

 the critical thickness was reached, the state of the sensitive 

 surface was one of stability. One result, however, would pretty 

 evidently follow, viz., that if on the same plate a portion of 

 the layer was below the critical thickness, while the remain- 

 der was above it, the plate, on exposure, might be, on the 

 whole, positive, negative, or inert. In the case of photo- 

 electric cells to be subsequently described, it is possible that 

 the observed results are due to this cause. 



Certain mordants were tried for the purpose of preventing 

 the eosine from leaving the film. The sensitized plate was 

 washed with a solution of borax, which had the effect of 

 keeping the film on the plate, but almost completely de- 

 stroyed its sensitiveness to light. A similar effect followed 

 from the employment of a solution of chloride of aluminium. 

 The best effect was produced when a silver plate coated with 

 an emulsion of eosine and photographic gelatine was immersed 

 for a few minutes in a strong solution of alum. 



Naphthalene red, a substance soluble in alcohol and only 

 slightly soluble in water, was also used in place of eosine. A 

 little of this was dissolved in alcohol and then emulsified with 

 gelatine. It was then poured on a silver plate, and the film 

 was allowed to set thoroughly. The photoelectric currents 

 produced were not quite so strong as those obtained with 

 eosine. The effect is much greater in the blue than in any 



