INFLUENCE OF LIGHT ON ELECTRODES. 407 



the very long-wired instruments used by M. DuBois Reymond 

 and others, it has proved sufficiently delicate for most of the 

 effects I aimed at. 



The idea with which I started was to arrange two plates 

 of platinum in an electrolyte in such manner that a bright 

 beam of light should impinge on one while the other was in 

 darkness, and yet to allow free electrolytic communication. 

 After making a somewhat complex apparatus, which did not 

 answer the purpose, the following simple means of effecting 

 my object was adopted : In a cell similar to those used for 

 the nitric acid battery the outer cell being of thin glass and 

 the inner one of porous ware, two platinum plates were placed, 

 each of six inches by two four inches by two, or the immersed 

 portions of the plates, being platinised or coated with a de- 

 posit of black platinum. Both the outer and inner cell were 

 filled with distilled water slightly acidulated with sulphuric 

 acid ; and some tow steeped in the same solution was stuffed 

 into the upper part of the porous cell around the platinum, 

 so that this latter plate was perfectly excluded from light. 

 The extremities of the two plates were metallically connected. 

 A brass cylinder, covered at the top, was placed over the 

 whole, its lower circumference resting on a circular pad of 

 paper, so as to exclude light. 



The apparatus, thus disposed, was set aside for ten days, 

 so as to allow the local currents to subside. At the expiration 

 of this period the apparatus was taken into bright sunlight, the 

 position of the plates so arranged that the one in the outer or 

 glass cell should be opposite the sun, the terminals connected 

 with the galvanometer, and the temporary deflection occasioned 

 by polarisation allowed to subside, or rather to reach a fixed 

 point, for there was always a slight deflection. 



The brass cylinder which excluded light from the ap- 

 paratus was now removed ; and the galvanometer needle 

 instantly deviated to 10, the platinum exposed to sunlight 

 being positive to that in the dark, or as zinc to copper. The 

 platinum plates were now reversed, that which had been in 

 the outer cell placed in the porous cell, and vice versa, and the 

 apparatus again set aside for ten days ; at the end of this period 

 it was again taken out, the experiment repeated, and the same 



