20 Waterhouse — Electrical action of Light upon Silver. [No. L, 



have been exposed to sunshine, there is an apparent reversal due to the 

 loss of power in the light, and not to change of direction of the E. M. F. 

 As a rule nay observations with coloured glasses or the spectrum have 

 agreed with Becquerel's and Minchin's that no reversal of sign is produced 

 by any of the coloured rays. At the same time, I have found that in 

 some cases the blue rays appear to have a reversing tendency, as might 

 be anticipated from their very strong reversing action on certain forms 

 of sensitive photographic plates containing iodide or bromo-iodide of 

 silver. This point, however, requires much more complete investigation 

 with the aid of the spectroscope, and will be further considered when 

 dealing with the silver haloids. During the time I have been engaged 

 with these observations, the weather has been unusually changeable and 

 cloudj- for the time of year, and hence it has been difficult to compare 

 the results of observations on different days. For this reason it has 

 been impossible to give more than general indications of the amount 

 of deflection caused by the action of light in the cases recorded : exact 

 observations wordd have to be made with a standard light. 



It seemed desirable to commence the observations with experiments 

 on plain silver plates in different fluids. The plates used were not 

 quite pure, having been reduced from various silver residues, and were 

 about '974 touch. They were four inches long and one and a quarter 

 inch wide, and were usually cleaned with fine emery powder, or with 

 emery cloth immediately before and after use. It is, however, better to 

 make sure of the purity and cleanliness of the surface of the plates 

 for each operation by heating them red-hot and then immersing them 

 in dilute sulphuric acid. As facilities for doing this with thick plates 

 were not readily available, it has been omitted in all the following 

 observations. As a rule, the plates were immersed in the solutions to 

 a depth of from 2 to 2| inches, care being taken to avoid moistening the 

 upper unimmersed parts by capillary action or otherwise, and so exposing 

 them, to irregular currents from this cause. The plates were about 

 half an inch apart, being kept separated by two wooden blocks with a 

 dark ruby glass plate between them. 



I. Silver Plates in Water. 

 Distilled Water. 

 Distilled water being almost a nonconductor, the currents observed 

 were naturally exceedingly weak and could only be clearly seen with 

 strong sunshine. The deflection observed without the magnet varied 

 from "5 to 3 divisions of the scale, and in nearly all cases the exposed 

 plate was positive to the unexposed, and formed the anode or dissolving 

 plate of the couple. In some cases the exposed plat* became more 



