Pressure on Electrical Conduction and Decomposition. 437 



variometer as well as a silver voltameter, variations of current, 

 as well as total current, can be measured ; and, finally, the 

 quantity of gas liberated in the electrolytic tube under the 

 accumulating pressure exerted by it upon the liquid is deter- 

 mined by allowing it to burst in a stoppered eudiometer (of 

 special construction) over mercury, so that the whole of the 

 gas is collected. From the volume of gas thus obtained the 

 bursting or maximum pressure can be calculated. 



The principal results may be very briefly stated as follows: — 



1. When such an electrolytic tube containing dilute sul- 

 phuric acid is fixed in a vertical position, the lower electrode 

 connected with the positive and the upper electrode with the 

 negative pole of a battery, the evolution of gas appears to 

 become less and less, and the electrical resistance of the con- 

 tents of the tube increases, until, with an E.M.F. of 30 volts, 

 the needle of a galvanometer in circuit shows only a very small 

 deflection. This action appears to be due to the formation of 

 very dense sulphuric acid (H 2 S0 4 ?) at the + electrode at the 

 bottom of the tube, whilst the water from which it has been 

 separated forms a layer above it of so high resistance as to 

 almost absolutely stop the passage of the current. This con- 

 dition must not be mistaken for the cessation of electrolytic 

 decomposition produced by pressure. 



The strong acid is produced at the + electrode by the action 

 of the current, and owing to the small sectional area of the 

 tube, though it again mixes with the liquid above it by diffu- 

 sion, it does so less rapidly than it is separated by the current. 

 Moreover, under a sufficient pressure diffusive rate is lessened 

 as the length of the mean free molecular path is decreased. 



The above singular action affords a means of concentrating 

 sulphuric acid without boiling. 



2. When the poles are reversed, the dense sulphuric acid 

 is formed by the current at the upper electrode, and thus 

 becomes again mixed with the rest of the liquid in the 

 tube as it descends through the liquid, the mixing action 

 being facilitated by the bubbles of hydrogen rising from the 

 lower electrode. In this way the separating action of the 

 current is prevented, and my glass tubes (which burst at about 

 290 to 300 atmospheres) fail to withstand the pressure of the 

 gases liberated by the current. I sought to strengthen one 

 glass tube of special form by first coating it with pure silver 

 by Martin's method, and then thickly electrotyping it with 

 copper ; but it also burst. It is certain, therefore, that a 

 pressure of 300 atmospheres is insufficient to arrest the elec- 

 trolytic decomposition of dilute sulphuric acid. I am not yet 

 certain whether pressure exercises any direct influence on the 



Phil. May. S. 5. Vol. 20. No. 126. Nov. 1885. 2 I 



