282 Change of Density of the Electrolyte at the Electrodes. 



Ar other confirmation is given by the behaviour of potassium 

 sulphate during electrolysis. After a current has been passed 

 in the same direction through potassium sulphate, it is found, 

 on testing with litmus paper or by cautiously adding litmus 

 solution, that the bottom strata of liquid have become alkaline 

 and the top strata acid, the solution being originally neutral. 

 It follows from this that the alkaline solution produced at the 

 kathode is denser than the electrolyte, which itself is denser 

 than the acid solution produced at the anode. Therefore, 

 according to theory, the streaks, if seen at all, should descend 

 from the kathode and ascend from the anode. This is fully 

 borne out by experiment. Starting with freshly-heated pal- 

 ladium electrodes, on putting on the current we immediately 

 see the streaks descending from the kathode ; and after a 

 reversal of the current other streaks may simultaneously be 

 detected rising from the anode. 



The best conditions for seeing the streaks seem to be : — 



(1) a horizontal electrode of small surface*, 



(2) a strong current, 



(3) no evolution of gas. 



The conditions (2) and (3) are incompatible with the use of 

 platinum electrodes in dilute sulphuric acid : it is quite pos- 

 sible, however, that even in this case matters might be so 

 adjusted that some trace of the formation of the streaks could 

 be detected. In the case of platinum electrodes in solutions 

 of sulphates of metals which do not act readily on water, such 

 as zinc, copper, iron, &c, the metal will be deposited and no 

 gas evolved at the kathode, and thus streaks of the weakened 

 solution can be seen at that electrodef. 



A direct proof that the streaks were formed of concentrated 

 acid might be thought possible, in the case of sulphuric acid, 

 on consideration of the following circumstances : — 



(1) When strong sulphuric acid is dropped into dilute sul- 

 phuric acid, the strong acid sinks to the bottom of the con- 

 taining vessel and remains undifTused for a considerable period. 



(2) If collecting-vessels are placed under the electrodes and 

 an arrangement fitted up by which the current may be perio- 

 dically reversed, say every minute, the streaks will be formed 

 at the electrodes and will stream into the collecting-vessels. 



* A vertical electrode may be used with advantage if it is well below 

 the surface of the electrolyte and if it is suspended by a horizontal wire, 

 so that the view of the electrolyte above and below it is not interrupted. 

 An angular electrode is very convenient, as the liquid flows most readily 

 from points. 



t Christiani noted the streaks with platinum electrodes in ferrous 

 sulphate. 



