1S93.] Water!) on se — Electrical action of Light upon Silver. 13 



proportional to the intensity of the chemical action exerted on the 

 substance and consequently to the active lnminons intensity ; they only 

 shewed whether this luminous intensity was greater or less in one cir- 

 cumstance or in auother. 



With this instrument Becquerel observed the effect of different 

 rays of the spectrum on silver iodide and violet subchloride, and found 

 that in both cases the maximum of action was in the green about D 

 •f E ; but while with the chloride the action decreased on both sides 

 of this point, and ceased at A and H, with the iodide that had already 

 been exposed there was a second maximum in the indigo blue about 

 G|H, and thence the action decreased to P in the ultra-violet. In 

 neither case was any reversed action observed in the red rays, as ob- 

 served with sensitive papers, but that might be due to the fact that in 

 one case the sensitive surface was in water and in the other in air. 

 Becquerel has not recorded any corresponding observations with silver 

 bromide. 



About 1840, Robert Hunt repeated Becquerel's experiments with 

 many modifications, and the results he obtained (Phil. Mag., XVI, 1840), 

 completely confirmed them. More careful trials with the spectrum on 

 plates of different metals made later showed that every ray of the 

 spectrum produces an electrical disturbance. The rays, however, at the 

 least refrangible end, produce a deflection of the needle in one direction, 

 whilst the most refrangible rays set up a disturbance in an opposite 

 direction. There are many indications of a condition analogous to 

 polarity in the action of the prismatic rays. (Researches on Light, p. 

 295.) Hunt also remarks that " This action is only to be regarded 

 as one of the evidences of chemical disturbance, exciting electrical 

 currents ; yet at the same time, it opens the question of the identity 

 of the agent producing this disturbance and electricity." 



In 1858, Grove (Phil. Mag., XVI., (4), p. 426.) recorded that he 

 had succeeded in obtaining a deflection of the galvanometer needle by 

 allowing a beam of light suddenly to impinge on a daguerreotype plate 

 in a trough of water, the plate being connected with one pole of the 

 galvanometer and a gridiron of silver wire in front of the plate with the 

 other. In experiments with platinum plates he came to the conclu- 

 sion that the action of light was always in the direction of the polarisa- 

 tion current, though further experiments by Becquerel and others have 

 shown that this is not the case. 



In 1863, Pacinotti found that when pairs of plates of copper, zinc, 

 iron or lead were immersed in solutions of certain salts of the same 

 metals, the exposed plate was always negative, but with plates of silver 

 immersed in a solution of nitrate of silver the plate exposed to sunshine 



