536 



Mr. A. Schuster. 



duces a transference of electricity. I have shown that a discharge,, 

 whether disruptive or continuous, throws the whole enclosure into a 

 sensitive state. It has been suggested from various sides that some 

 of the phenomena described by me were due to illumination of ultra- 

 violet light. In order to decide the question, I have divided a vacuum 

 tube into two parts by means of a quartz diaphragm ; a discharge 

 passed through one part of the tube and on that side the effects pre- 

 viously described by me were again observed, but the quartz plate- 

 completely destroyed the effect in the other compartment, while all 

 observers agree that quartz is transparent to the radiations which 

 produce electrical effects. 



Because E. Wiedemann* finds (in accordance with Hallwachs' 

 experiment) that the leaves of an electroscope can be made to col- 

 lapse when they are illuminated by an arc lamp, it hardly follows, 

 as implied by him, that this is the explanation of the same effect 

 observed by me when there was no arc lamp nor any light except the 

 feeble glow of a discharge carefully screened off from the gold leaves 

 by a metallic plate. The above-mentioned experiment, which will be 

 described in detail in another communication, disposes of the question, 

 and shows that, independently of any ultra-violet radiation, the dis- 

 charge puts the gas into a sensitive state in which it becomes a 

 conductor. According to the theory of electrolytic convection, this 

 should be so, because the primary discharge supplies the necessary 

 ions. 



Discussion of Unipolar Effects. 



It is of interest to see whether we can trace any regularity in the 

 appearance of the unipolar effects, and whether there is any hope of a 

 general law which will allow us to predict when an electrode will be 

 positively, and when it will be negatively, unipolar. 



I am aware of the dangers of premature generalisation, but, at the 

 same time, such generalisations as a rule do no harm except perhaps 

 to the reputation of the author; they have, on the other hand, the 

 advantage of putting the issue clearly before the reader, and often 

 lead more quickly to a definite result than a mere statement of dis- 

 connected facts. 



As far as the facts go at present, I am inclined to draw the follow- 

 ing conclusion : — A free discharge of electricity between a negative ion 

 and the anode is possible. On the other hand, a considerable fall of 

 potential is required in order to produce an exchange of electricity 

 betiveen a positive ion and the kathode unless the electrode takes part 

 in a chemical action set up at its surface, in which case the rate of 

 exchange of charges at the kathode may become greater than that at the 

 anode. 



* ' Wiedemann, Annalen,' vol. 35, p. 219 (1888). 



