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1893. J Waterhouse — Electrical action of Light upon Silver. 11 



A great many observations have Ibeen made from time to time of 

 the electrical influence of light on metals immersed in water and various 

 saline solutions, and before going further, it seems desirable to give a 

 brief summary of these observations, and more particularly of those re- 

 lating to silver and its salts. 



More than half a century ago, in 1839, Edmond Becquerel was the 

 first to show that the electrical action accompanying the chemical 

 changes brought about by the influence of light upon various substances, 

 including several metals and the silver haloids, could be observed with 

 the aid of a very delicate galvanometer. He found that this action was 

 quite independent of any calorific radiation or heating of one electrode 

 more than another, but was powerfully affected by the different rays of 

 the spectrum, the greatest action being produced by the violet, indigo and 

 blue rays, while with the green, yellow and red rays there was little or 

 no action. Becquerel's observations are fully summarized in his work, 

 " La Lumiere, ses causes et ses effets," Vol. II. To observe these effects 

 he used a covered vessel divided into two parts by a thin membrane. In 

 each of the compartments he placed a plate of platinum or gold, previously 

 made red-hot to remove all impurities, the plates being connected with 

 the poles of a very sensitive galvanometer, and laid horizontally in the 

 apparatus. Each compartment had a moveable cover. He found that 

 when the two compartments contained an alkaline solution, the plate 

 exposed to the solar rays took negative electricity, while the reverse 

 occurred if the solution were acid. With alterable metals, such as silver 

 or brass, analogous effects were obtained and the electrical effect could 

 be largely increased by giving the plates a preliminary polarisation by 

 plunging them in water and then placing them in connection with the 

 positive pole of a battery. When two silver plates were immersed in 

 water acidulated with nitric acid exposure to light of one plate only 

 produced a very weak current and the exposed plate was always positive. 

 If the gold or platinum plates had been thoroughly cleaned, had 

 remained in strong nitric acid and had been made red hot, the differ- 

 ent parts of the spectrum were almost powerless to produce electric 

 currents. With well cleaned silver plates which had been heated 

 several times the effects were also almost nil, though not quite absent, 

 and from this fact it seemed possible that when the plates were not in 

 this state the effects produced might be due to the action of light upon 

 corpuscles of organic matter adhering to the plates which become 

 oxidised by the action of light, the water supplying the oxygen. If 

 this effect did not take place and there was no alteration in the plates 

 themselves the light must produce a disturbance of the particles, but 

 the former supposition seemed most probable. He found that when 



