viii.] CONCENTRATION CURRENTS 137 



able electrodes, the clay of which has been so shaped as to give 

 dissimilar surfaces of evaporation. The clay is moist, the air of 

 the room is dry, B is giving off water more quickly than A, 

 and is sucking water from A, so the electrode current is to your 

 left, as figured. I now bring down over the electrodes a bell- 

 jar with a bit of wet blotting paper sticking inside it, i.e., con- 

 taining wet air. The galvanometer spot is sharply deflected to 

 your right (by the checked or reversed water current). On 

 removal of the bell-jar the spot is sharply deflected to the 

 left (evaporation current to room air), and finally, when the 

 evaporation and deflection have become steady, I cover the 

 electrodes with a dry heated bell-jar, which at once, by accelerat- 

 ing the evaporation, causes a sharp deflection to the left. And 

 for the present I am not concerned to know whether or no the 

 concentration of saline solution plays a part in these evaporation 

 effects ; all I want to do is to show you that trifling altera- 

 tions of evaporation can give quite considerable electrical 

 effects. 



Si. Concentration current. The next experiment is intended 

 to remind you of the usual direction of a concentration current 

 a point which in most text-books and monographs appears to 

 be considered as too self-evident to be worth specifying. Two 

 amalgamated zinc rods dip into a 25 per cent, solution of zinc 

 sulphate, and are connected with the galvanometer. 



A drop of distilled water allowed to run down B into the 

 solution gives deflection to your right (current from B to A). 

 A drop of saturated zinc sulphate to B gives deflection to your 

 left (current from A to B). 



And of course dilution at A gives deflection to the left, con- 

 centration at A gives deflection to the right. In terms of the 

 ionic movements, this happens by reason of greater velocity 

 of the anion (which travels from the more concentrated to the 

 less concentrated solution, giving therefore current in the reverse 

 direction). With acids, e.g., HC1, and complex organic salts, 

 e.g., CHgCOOK, in which the kation travels faster than the 

 anion, the current is from more to less concentrated solution. 

 But with all our ordinary neutral salts, and with alkalies, the 



