332 The Photo- Electric Effect in some C<>mpou?ids. 



10 minutes. Other liquids (bromoform, benzene, &c.) had a 

 similar effect, but in these cases thorium B was dissolved to a 

 slight extent. 



I wish to record my thanks to Prof. Strutt, who suggested 

 the above research and to whom I am indebted for his 

 valuable assistance. 



KJ 



J<A 



XXXVI. On the Photo-Electric Effect in some Compounds. 



To the Editors of the Philosophical Magazine. 

 Gentlemen, — 



IX the Philosophical Magazine for December 1912, Dr. R. 

 S. Willows calls attention to a possible explanation of 

 an experiment which I made during an investigation of the 

 photo-electric effect in compounds (Phil. Mag. September, 

 1912). The experiment referred to is one in which it is 

 shown that moist zinc chloride has a well-marked photo- 

 electric effect, while dry zinc chloride is quite insensitive. 

 Dr. Willows suggests that the emission of negative electricity 

 by the moist zinc chloride may be identical with the effect 

 observed by Mr. A. E. Garrett some years ago, and quite 

 independent of illumination by ultra-violet light. The pro- 

 cedure in my experiments, however, was to insulate the 

 electroscope (connected with the zinc chloride plate), and then 

 to remove a shield which was interposed between the mercury 

 lamp and the apparatus. The leaf of the electroscope only 

 moved when the zinc chloride plate was illuminated. Hence 

 the effect is a direct result of illumination by ultra-violet 

 light. I have recently confirmed this observation with a 

 simpler and less sensitive apparatus. As before, there was 

 a well-marked effect when the moist salt was illuminated, but 

 there was no measurable effect in the dark — this may be due 

 to lack of sensitiveness. 



I did not look for any emission of positive electricity from 

 the illuminated substance. Had it been at all large, it would 

 probably have been noticed. With the type of apparatus 

 used in these experiments, it is always possible to explain an 

 apparent emission of positive electricity from the plate as an 

 emission of electrons to the plate from the surrounding elec- 

 trode, which is unavoidably illuminated by reflected light, 

 Special precautions would be necessary to avoid such spurious 

 effects. 



Yours truly, 



Cavendish Laboratory, A. Ll. HUGHES. 



Cambridge. 

 January 11th, 1913. 



