1878.] 



Experiments in Electro Photometry. 



355 



Becquerel's form, acting as a differential combination, the plates of 

 silver being coated with iodide instead of chloride. 



The modifications of the halogen salts of silver when subjected to 

 the action of light have up to the present time been nsed most success- 

 fully in the production of electric currents, and although mixtures of 

 photographically sensitive salts have been shown by Smee to produce 

 currents of a similar kind, yet no attempt has been made to examine 

 the proper form of instrument required for the general investigation 

 of the electrical actions induced by light on fluid substances. 



This subject has occupied my attention for some time, and the com- 

 pleted investigation I hope to present to the Society. In the mean- 

 time the following description will give an idea of the method of in- 

 vestigation. 



A little consideration shows that the amount of current produced by 

 a definite intensity and quality of light acting during a short period of 

 time on a given sensitive substance in solution, is primarily a function 

 of the nature, form, and position of the poles in the cell relatively to 

 the direction in which the light enters, and the selective absorption, 

 concentration, and conductivity of the fluid. 



The diffusive action taking place in such cells complicates the effects 

 and is especially intricate when insoluble substances are formed. In 

 order to simplify the investigation in the first instance, poles that are 

 not chemically acted upon, and a sensitive substance yielding only 

 soluble products on the action of light, were employed. For this 

 purpose platinum poles and chlorous acid or peroxde of chlorine were 

 selected. 



The best form of cell had one of the poles made of fine platinum 

 wire fixed as closely as possible to the inner surface where the light 

 enters, the other pole being made of thicker wire placed deeper in the 

 fluid. 



As the action is confined to a very fine film where the light enters, 

 the maximum amount of current is obtained when the composition of 

 the fluid is modified deep enough to isolate temporarily the front pole 

 in the modified medium. Under these conditions the formation of 

 local currents is avoided, and the maximum electromotive force ob- 

 tained. 



In cells of this construction the amount of current is independent 

 of the surface of the fluid acted upon by light, so that a mere slit 

 sufficient to expose the front poles acts as efficiently as a larger surface. 

 This prevents the unnecessary exhaustion of material and enables the 

 cell to be made of very small dimensions. By means of such an ap- 

 paratus the chemical actions of light and their electrical relations 

 may be traced in many new directions. 



The amount and direction of the current in the case of chlorous 

 acid is readily modified by the addition of certain salts and acids, and 



