Influence of Lujlit on Electrical Tension in Metals. 331 



whole of the effect of illumination to the galvanic polarization 

 produced by the light. Blue light acted much more power- 

 fully than yellow; red light was the weakest; and consequently 

 Grove concludes that these phenomena are due to the chemical 

 and not to the thermal rays of the solar beam, since in the 

 latter case the colours would act in the reverse order. Paci- 

 notti states that, of two copper plates standing in sulphate or 

 nitrate of copper, the illuminated one, when exposed to the 

 rays of the sun, of a petroleum lamp, or to the radiation from 

 a heated iron plate, was always negative to the other. On 

 the other hand, when he placed two small insulated copper 

 boxes of different sizes, one within the other, and filled the 

 space between them and the inner one with water of diffe- 

 rent temperatures, the warm copper was positive to the 

 colder. Hankel found that warm copper was always nega- 

 tive with respect to cold copper ; when two well-polished 

 copper plates were plunged in water, and one was exposed to 

 sunlight, it became negative to the other ; and, on the other 

 hand, the more the plates were oxidized or covered mth salts, 

 the more did the illuminated one acquire the positive state. 

 One or two Yoita's piles were found to give opposite effects 

 according as they were exposed to luminous radiation or to 

 heat. Hankel also experimented with plates of silver, tin, 

 brass, zinc, and platinum ; but he describes only the effects of 

 exposure to light, and no experiments with heat. 



In all these experiments the electrical currents were pro- 

 duced simultaneously with certain chemical phenomena, and, 

 as is assumed to be the case by some of the above-named 

 physicists, resulted from them. But it appears that electrical 

 currents may be generated by the action of light without any 

 chemical action, in a manner analogous to that of thermo- 

 electric currents, for the production of which no chemical 

 action is requisite. 



Currents such as these, which are generated in a simple 

 metallic circuit by the action of light, have hitherto, as far 

 as I am aware, been observed and described only by Adams 

 and Day in the paper above mentioned. They state that, in 

 several of the pieces of selenium examined by them, a partial 

 illumination produced a difference of potential, in virtue of 

 which an electrical current was developed, and, further, that 

 at the points where the platinum connecting-vfires were fused 

 into the selenium, illumination produced a current opposite 

 in direction to what a thermoelectric current should have 

 been. 



I have endeavoured to connect this peculiar phenomenon 

 with some other metals ; and the experiments described in this 



