THE PROBLEMS OF SOLUTION 



109 



and shortly afterwards by a somewhat different 

 method the present writer, showed how to 

 render these molecular processes visible, and how 

 to watch the motion of the ions as they drift 

 through the solution under the action of the 

 electric forces. 



One apparatus, as improved by Nernst, for 

 this purpose is represented in Fig. 26. 

 Let the solution of a colourless salt be 

 first placed in the tube and a heavier 

 coloured solution then run in below, so 

 that a fairly sharp line of demarcation 

 is produced between them. The solu- 

 tions should be of the same molecular 

 concentration, the same conductivity, 

 and the denser solution must, of 

 course, be placed below the lighter. 

 Let us take, as an example, the case 

 of solutions of potassium bichromate 

 and potassium carbonate, which fulfil 

 the necessary conditions. The colour 

 of the former salt is due to the acid 

 part, the bichromate ion, which has 

 the chemical composition represented 

 by Cr207 ; the potassium ion is colour- 

 less. When a current of electricity is 

 passed across the junction between the 

 liquids, the colour boundary is seen to 

 move, and, from the rate at which it creeps along 

 the tube, the velocity of the bichromate ion under 

 a given electric force can be determined. 



The conductivity of a salt solution, made 

 solid by the addition of gelatine or some similar 

 substance, is nearly the same as that of the 

 liquid solution without the jelly, and this fact 



Fig. 26. 



