1893.] Waterhouse — Electrical action of Light upon Silver. 17 



increase the existing current, the effect being merely impulsive, after 

 which the current generally disappeared. This cell having been kept 

 in the dark for a fortnight, it was found that while the inverse currents 

 were produced as before, the initial current on exposure was enor- 

 mously increased in magnitude and duration. It then disappeared 

 gradually and was succeeded by a current in the reverse direction. 

 When one of these plates was removed from the cell and immersed in 

 water in presence of a clean silver plate, it was at once on exposure to 

 light negative, like a silver plate coated in the ordinary way with an 

 emulsion of eosine. In preparing these eosine-gelatine films, it was found 

 to be an advantage to immerse them for a few minutes in a strong 

 solution of alum in order to prevent the dye from washing out of the 

 film too readily. 



With silver plates coated with napthalene red and gelatine the 

 effects were not so strong as with eosine ; the exposed plate was positive 

 and with strong red rays there appeared to be a reversal of the sign 

 of the E. M. F. 



Plates coated with iodine green and exposed to sunshine gave 

 currents with an E. M. E. amounting to about -~ volt. 



M. F. Griveaux, experimenting on silver plates coated with a film 

 of silver iodide, plunged into solutions of iodine of different strengths, 

 circulating through the cell, found that the maximum value of the 

 E. M. F. developed by light acting on one of the plates decreased 

 as the strength of the iodine solution increased, till a certain point 

 was reached at and above which the E. M. F. was nil. Also that this 

 point was regulated by the distance of the plates from the source of 

 light ; the nearer the plates the higher the concentration point of the 

 solution and vice versa. ' The same effects were observed with silver 

 chloride and bromide. (Comptes Bendus Acad. Franc, CVII, 1888, 

 P . 837.) 



I have entered somewhat fully into these previous experiments 

 because very little appears to be generally known about the subject 

 and it seemed desirable to bring together the scattered observations. 



In carrying out my experiments I have used two kinds of cells, 

 one horizontal and one vertical, more usually the latter. It consists 

 of a glass cell in which the plates can be coupled face to face or back 

 to back, one being screened from light by the other and by one or two 

 interposed screens of ruby or yellow glass, the cell being covered all 

 round except at an opening on one side. This glass cell is enclosed in 

 a wooden box with a shutter on one side sliding in front of an open- 

 ing about 1"5" X "5", corresponding to the one in the glass cell. In 

 front of this shutter tlaere are grooves in which coloured glasses can 

 J. ii. 3. 



