14 Waterhouse — Electrical action of Light upon Silver [No. 1, 



was positive, whereas if exposed to the rays of a petroleum lamp, or of a 

 heated thick iron plate it was negative, as were also the other metals. 

 (Cimento, XVIII, p. 363.) 



In 1875, Hankel published a series of observations on this subject 

 (JTied. Ann., I, 1877) in which he showed that the electrical behaviour 

 of the metals under the influence of light depended very much on the 

 condition of their surfaces ; consequently, in such observations it is 

 necessary to consider separately each state of surface. His observations 

 were made on copper in different states, tin, brass, zinc, platinum and 

 silver. With regard to the latter, he records that when two plates of 

 fairly pure silver were immersed in filtered tap water, the plate ex- 

 posed to the light of white clouds was negative. When the plates 

 had been left a day in the water the rays of the setting sun still gave 

 a pretty strong negative impulsion. Platinum plates coated with silver 

 were slightly positive with white or blue light, while red light pro- 

 duced no effect. Silver plates coated with platinum, (old platinised 

 silver battery plates) which were slightly negative when coupled in 

 circuit with plain platinum, were found to be very sensitive to light, 

 and the exposed plate was positive. With coloured glasses the action 

 was strongest under blue glass, but was also quite strong under yellow 

 and red glasses ; gaslight also produced a pretty strong deviations 

 of the galvanometer needle, and it was found that the action under 

 dark red and blue glasses was stronger than under a light green 

 which was much more transparent. 



In 1878, Professor Dewar published a preliminary note on " Experi- 

 ments in electric photometry," (Proc. Boy. Soc, XXVII, 1878, p. 364) 

 in which he dealt principally with the construction of the best form of 

 cell for the general investigation of the electrical actions induced by 

 light on fluid substances. He found that the list of substances that 

 may be proved to undergo chemical decomposition by light, was very 

 extensive, some of the most active being the ferro- and ferri-cyanides of 

 potassium and the nitroprusside of sodium, tartrate of uranium and a 

 mixture of selenious and sulphurous acids in presence of hydrochloric 

 acid. The complete paper does not appear to have been published. 



In 1876, M. Egoroff published a note (Gomptes Rendus, Acad. Franc., 

 LXXXII, 1876) on a differential electro-actinometer for the purpose 

 of determining the absorption of the ultra-violet rays by different 

 media. The instrument consisted of two of Becquerel's electro-acti- 

 nometers placed one above the other and arranged so that the current 

 of one might be neutralised by the other. In some preliminary observ- 

 ations with iodised silver plates he found that the intensity of the 

 cutrent was proportional to the width of the opening through which 



