436 Assistant-Prof. J. W. Clark on the Influence of 



occupying about the same volume as when combined to form 

 the electrolyte). 



III. The conduction of mercury. 



Of these three classes, it will be noticed that the liquids in 

 I. and II. are composed of complex molecules which, accord- 

 ing to Clausius, are in a state of continuous motion varied by 

 collisions with one another which cause dissociation*. Mer- 

 cury, on the other hand, is a metallic conductor, and a liquid 

 which is usually regarded as being composed of simple or 

 monatomic molecules, which are therefore incapable of under- 

 going dissociation ; but by the application of a sufficient 

 external pressure its molecules may be brought nearer together, 

 or, in other words, the lengths of the molecular mean free 

 paths may be reduced. 



The investigation to which I now wish to refer relates 

 to the decomposition of dilute sulphuric acid under an ex- 

 ternal pressure which it is my object to make appreciable 

 in comparison with the molecular forces of the liquid itself, for 

 it is only when that condition is fulfilled that any direct results 

 are to be anticipated such as those which are here sufficiently 

 suggested by the questions : — Is Faraday's law independent 

 of pressure ? Do the conduction and decomposition of an elec- 

 trolyte always vary together? Can electrolytic action be stopped 

 by pressure ? It was to obtain a reply to such questions that 

 I turned from the region of speculation to the surer ground of 

 experiment, and in the first instance set myself to measure 

 the electrical resistance of acidulated water, and the amount of 

 gas liberated from it by a known current under pressure. 



The method of experiment in outline is as follows : — The 

 dilute sulphuric acid to be electrolyzed is hermetically sealed 

 in a short glass tube (0*1 centim. in internal, and 0'7 centim. 

 in external diameter), through the ends of which pass thin 

 platinum-wire electrodes, of which the upper wire is usually 

 so encased in glass that only the portion in the electrolyte is 

 exposed : a precaution found to be necessary in order to avoid 

 continuous recombination. This roughly describes the "elec- 

 trolytic tube" in which the pressure upon the dilute sulphuric 

 acid is generated by the gases liberated by the current as they 

 accumulate in the (determined) volume of the tube which is 

 unoccupied by the liquid. When the circuit includes a gal- 



* If volatility and a large coefficient of expansion for heat may be 

 regarded as a probable indication of a long mean free molecular path, it 

 does not seem impossible, on the hypothesis of Clausius, that under the 

 influence of a sufficiently great external pressure, such liquids as condensed 

 gases (C0 2 , NH 3 , S0 2 , (fee, fused HgCl 2 ), might be rendered good elec- 

 trical conductors, or at least have their conductivity increased. 



