430 Mv. W. R. Grove on the Influence of 



of homogeneity in the liquid, and the other from the state of 

 surface of the platinum ; the latter would most probably be the 

 current affected by light. 



As the general effect of light was to increase the deflections 

 occasioned by polarization, whatever direction this assumed, it 

 seemed probable that the exclusion of one of the plates from the 

 light, winch I had commenced with in the hope of obtaining 

 currents initiated by light, was unnecessary, and that the observed 

 effects would be rather increased by exposing both plates to light. 

 I therefore arranged two platinized plates in a cell without a 

 porous diaphragm, inclined to each other like the letter V, but 

 without contact, and allowed the light to impinge on the interior 

 surfaces, or those opposed to each other ; but, to my surprise, the 

 effect was very trifling, the needle deviating only one or two de- 

 grees, and that in a sluggish and irregular manner. 



When, however, the two plates were arranged parallel, the 

 one shading the other from the light, as good deflections were 

 produced as with the porous cell, — the more so if the back or 

 shaded plate was of polished, and the front plate of platinized 

 platinum. 



Why light should produce a greater augmentation of the 

 current when impinging on one than on the two plates, I cannot 

 well understand, and therefore will not attempt any hypothetic 

 explanation, but leave it for further experiment. 



In all the experiments I have made on the subject of this paper, 

 the most marked effect upon the galvanometer is produced when 

 the polarization causes a small permanent deflection of from 5° 

 to 10°; when the polarization of the plates is extremely slight, 

 the effect of light is very feeble ; and when the polarization is 

 considerable, so as to deflect the galvanometer to 20° or 30°, the 

 increased force required to produce a small increase of deflection 

 is too great to afford notable results. 



I have used the term polarization, having no better word to 

 indicate the feeble currents which are always observed when two 

 i;)latinum plates immersed in a liquid are connected with a deli- 

 cate galvanometer. The electrical currents Avhich would ensue 

 if the plates were polarized by connecting them with a voltaic 

 battery and then detaching them, would be far too powerful for 

 the delicate indications which I have been examining. There 

 can be no doubt, at least to those who adopt the chemical theory 

 of the voltaic pile, that both these classes of polarization are 

 due, when one homogeneous liquid is employed, to slight depo- 

 sits on the plates, either of films of gas, or of some substance 

 which acts chemically on the liquids ; and the effect of light 

 would seem simply to be an augmentation of the chemical action 

 taking place at the surface of the electrodes, which is the locus 



