538 



Mr. A. Schuster. 



Finally, the action of ultra- violet light, being probahly of the nature 

 of a chemical action, at once destroys the impediments which prevent 

 metals acting as kathodes. The principal difficulty of tie explanation 

 lies in the behaviour of white-hot platinum in hydrogec, but, as has 

 already been explained, the platinum seems to undergo a molecular 

 change in that case, which may either be due to or equivalent to a 

 chemical action. 



Properties of Gases in their Sensitive State. 



A gas may be put into a sensitive state when the molecules are 

 broken up into ions either by a discharge of electricity or by a 

 chemical action as in a flame. It is interesting to note the similarity 

 of behaviour of gases made sensitive in these two ways. 



In a footnote to the paper, in which the theory of electrolytic con- 

 vection is brought forward by Griese, he suggests that the power of 

 gases rising from flames to condense moisture, as shown by Aitken, 

 may be due to the same cause as its power to conduct electricity. 

 The question has been treated with great ability by R. v. Helmholtz, 

 whose untimely death at an early age science has recently had to 

 deplore. R. v. Helmholtz was led to the enquiry by the discovery of 

 an effort of electric discharges to act in the same way as solid nuclei 

 would in precipitating the aqueous vapour from a moist gas. It 

 would seem natural at first sight to ascribe this effect, as well as 

 those previously described by Aitken, to the actual presence of dust 

 particles. I must refer the reader to Helmholtz's paper,* in which 

 this hypothesis is finally found to be insufficient and in which the 

 action of both flames and electric discharges is ascribed to the 

 presence of ions in the gas. Messrs. Lenard and Wolff have recently 

 shown that when a metal, charged negatively, is exposed to ultra- 

 violet light, the space in front of the discharging plate acts on a 

 steam jet like dust particles would. They conclude from these re- 

 searches that the ultra-violet light does actually disintegrate the 

 surface of the conductor. Some of their experiments, however, 

 admit of a different interpretation ; the electric discharge itself, which 

 we know to take place under the circumstances, might be the cause 

 of the condensing power of the air. As far as the evidence goes, I 

 should say that the explanation given by Lenard and Wolf is the 

 more probable one, though it is not altogether proved as yet. But 

 even admitting the presence of dust particles thrown off from the 

 conductor, it is still an open question whether the disintegration of 

 the electrode is the cause of the electric discharge or whether it is 

 only one of the phenomena attending it. We should expect a con- 



* ' Wiedemann, Annalen/ vol. 32, p. 1 (1887). 

 f ' Wiedemann, Annalen,' vol. 37, p. 443 (1889). 



