318 Dr. A. Schuster. Experiment* on the 



public. I shall avoid as much as possible all suppositions and hypo- 

 theses which cannot be put to the test of experiment ; but it seems 

 necessary to start with some assumption, in order to avoid too great a 

 vagueness in the subsequent explanations. The assumption which I 

 shall make is this: In a gas, the passage of electricity from one 

 molecule to another is always accompanied by an interchange of the 

 atoms composing the molecule. I have been much struck with the 

 results of some experiments recently made by Mr. L. J. Blake in 

 Professor von Helmholtz's laboratory ; investigating the vapours 

 rising from the surfaces of warm liquids which were kept charged to a 

 high potential, he found that these vapours were entirely unelectrified. 

 Different liquids were tried such as pure water, water containing 

 salts in solution, and mercury. Now, if it is found, that the vapour 

 actually rising from an electrified liquid is not electrified, can we 

 imagine that a molecule striking an electrified surface in its rapid 

 motion should carry away with it any part of the electricity, or that 

 one molecule should be able to communicate electricity to another in 

 an encounter ? Assuming that we cannot, we are led at once to the 

 supposition that the discharge in gases is accompanied by a breaking 

 up of the molecules. But this does not seem to me to be quite 

 sufficient to account for all phenomena, and especially not for those 

 involving stratifications. There is reason to think that the formation 

 of unstable compounds, such as ozone, plays an important part in the 

 glow discharge. It is very likely essential to the production of 

 stratifications. 



The test of any theory must be found in the numerical results it is 

 capable of giving. Hitherto, however, the qualitative phenomena 

 have not, in my opinion, been sufficiently separated from the great 

 number of disturbing effects to allow us to give a decisive value to 

 quantitative measurements. Though I shall not discuss the few 

 measurements which I have been able to make, I shall point out 

 where numerical results can be most easily obtained by the theory 

 and tested by experiment. I do not pretend to explain all the phe- 

 nomena of the discharge, but I shall attempt to show that there is no 

 distinct contradiction between the theory, in favour of which I argue, 

 and the facts ; that is more than can be said of any other theory that 

 has been proposed. I shall try to prove in the present paper that the 

 molecules are, in all probability, broken up at the negative pole. 



Experiments with Mercury Vapour. 



According to the kinetic theory of gases, the molecule of mercury 

 vapour consists of a single atom, which is incapable of vibration. 

 Mercury has a very brilliant spectrum, which proves that the theory 

 is incomplete in some important point. It is well known, on the 

 other hand, that the theoretical conclusion receives support from the 



