534 



Mr. A. Schuster. 



zinc exposed to the air for some time showed the negative leakage to 

 a much smaller extent, but the positive leakage was not affected, so 

 that old surfaces sometimes discharged a positive charge more easily 

 than a negative one. Other metals behave like zinc, but the less 

 oxidisable ones show the effect to a much smaller extent. 



Hallwachs draws the conclusion, to which indeed his experiments 

 seem inevitably to lead, that the ultra-violet light produces a chemical 

 change at the surface, which is of the same nature as the one going 

 on when surfaces are left lying exposed to the air. The explana- 

 tion which at first sight seems the most natural one, namely, that the 

 chemical change is an oxidation, would appear from an experiment 

 more recently communicated by him* not to be the true one, for 

 copper covered with a film of oxide behaves like clean metallic 

 copper. 



Shortly after the publication of these experiments, Righif showed 

 that a photo-electric air-cell can be constructed by taking two 

 parallel plates of different metals with an air space between them and 

 connecting them with each other. If one of the plates is of zinc, the 

 other of brass, and if the zinc plate is illuminated, a current tends to 

 flow from the zinc to the brass through the air. 



In the actual experiments, the brass plate was in the form of a 

 wire netting, so that the rays from an arc lamp could pass through 

 the grating and fall normally on the zinc plate, and the difference 

 of potential was observed by means of an electrometer. The energy 

 of ultra-violet light vibrations must therefore directly or indirectly 

 be converted into that of electric separation, and it is to my mind 

 natural to assume that a chemical action is set up in the first place 

 by the light, and that the observed electrical effects are therefore in 

 the first instance due to chemical action. The fact that the electro- 

 motive force of such a photo-electric cell is in the same direction as if 

 the plates were immersed in water, the zinc becoming the anode, is- 

 very suggestive. At present, however, we must not attach too great 

 an importance to it. I consider it quite possible that if the oxygen 

 of the air is replaced by hydrogen, the currents would still be in the 

 same direction. StoletowJ has examined the intensity of currents 

 sent from an outside electromotive force through an interval between 

 two parallel plates acting as electrodes, the kathode being illuminated 

 by ultra-violet light. I cannot at present attach any very great im- 

 portance to the experiments made with different gases. .We know 

 how very delicate the surface actions on which these currents depend 

 are, and how the greater part of the effect may in some cases be due 



* ' Wiedemann, Annalen,' vol. 37, p. 666 (1889). 



t Thil. Mag.,' vol. 25, p. 314 (1888). 



I ' Comptes Kendus,' vol. 107, p. 91 (1888). 



