INFLUENCE OF LIGHT ON ELECTRODES. 409 



at the actual point of contact of the platinum and liquid than 

 any non-luminous heat would produce ; and I was therefore 

 anxious to ascertain whether the different coloured rays of 

 light showed any difference in their effects. To this end I 

 procured three plates of coloured glass, one blue, the second 

 yellow, and the third red ; a strip of thick brown paper was 

 pasted to the opposite sides of each of these plates of glass, so 

 as to form a nearly cylindrical chamber cut by the plane of the 

 glass. A cover was placed over each of them ; and the cham- 

 bers so formed could be placed over the cell containing the 

 platinum plates, the coloured glass plates intervening between 

 the sun and the platinum in the outer cell. A great number 

 of experiments were made with these apparatus ; and in all 

 the deviations of the galvanometer were notably greater with 

 the blue glass than with the yellow or red, and, of the latter 

 two, the yellow gave slightly greater deflections than the red 

 glass. 



This result is, I think, conclusive in favour of the effect 

 being due to the chemical, not to the calorific rays of the sun, 

 the more so when we consider that the yellow allowed a far 

 larger quantity of light to pass than the blue glass. I may 

 also add that I have obtained a slight galvanometric deflec- 

 tion when diffused daylight was allowed to impinge on the 

 platinum plate, and when there was no perceptible difference 

 of temperature between the illuminated and the non-illumi- 

 nated plates. 



The superiority of the yellow over the red was not so 

 strongly marked; and, considering that the yellow glass 

 allowed much more light to pass than the red, I am not dis- 

 posed to think that there was any actual superiority in the 

 former ; the effects observed with these two colours are, how- 

 ever, corroborative of the effects not being due to the red or 

 heating rays of the sun. 



I substituted for the water acidulated with sulphuric acid 

 (which may be regarded electrically as pure water with its con- 

 ducting power improved), muriatic and nitric acids; the effects 

 were the same, but less marked with the nitric acid, probably 

 from its more completely depolarising the plates. 



In a small number of experiments the following effect took 



