428 Mr. W. R. Grove on the Influence of 



the dark, or as zinc to copper. The platinum plates were now 

 reversed, that which bad 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 result obtained ; i. e. on re- 

 moving the brass cylinder there was a deflection of 12°, the 

 platinum exposed to light being positive to the sheltered one. 



This identity of electrical effect taking place with the reversed 

 plates seemed so strongly in favour of the impact of the solar 

 rays having an initiatory effect in producing a voltaic current, 

 that the only remaining point seemed to be to ascertain whether 

 it was due to light or heat, to the chemical or calorific rays of 

 the sun; yet the conclusion I then came to was erroneous, as 

 will presently be seen. 



In order to ascertain how far the effect was due to heat, I 

 arranged, in a room lighted by a small candle, the same appa- 

 ratus over a fire of asbestos heated by coal-gas, so that both 

 radiant heat and an ascending current of hot air impinged on 

 the side of the glass in which was the exposed platinum, while 

 the opposite side was entirely sheltered from the heat by a 

 metallic shelf on which the cell rested: this experiment was 

 continued until one side of the cell was uncomfortably hot to 

 the hand, while the other side was quite cool; but not the slightest 

 deviation of the galvanometer took place. 



I now repeated the former experiment with sunlight, changing 

 the liquid each time. In three successive experiments the de- 

 flections on the impact of light were in the same direction, the 

 exposed platinum being positive; but in a fourth the deflection 

 was in the reverse direction, the exposed platinum being negative : 

 in several subsequent experiments there was always a notable 

 deflection which ensued on the impact of light ; but it was some- 

 times in one direction and sometimes in the other. I ultimately 

 discovered that, in the deflection produced by light, the needle 

 of the galvanometer deviated in the same direction which it took 

 upon the first contact of the wires connected with the platinum 

 plates. The effect of light was therefore to increase the de- 

 flection occasioned by the polarization of the platinum plates ; 

 and this my subsequent experiments have, I think, fully esta- 

 blished. Although the experiment on the impact of heat seemed 

 "to show that the heating effect of the solar rays was not the cause 

 of the phfeuomena, yet it might well be that the solar rays absorbed 

 by the platinum black would produce a greater heating effect 

 at the actual point of contact of the platinum and liqviid than 

 any non-luminous heat would produce; and I was therefore anxious 

 to ascertain whether the different-coloured rays of light showed 

 any difference in their effects. To this end I procured three 



