MEASUREMENTS OF ELECTROLYTIC MIGRATION 5 



latter might be removed b}' assuming tlie existence of hydrated 

 ions. 



In general it is possible to obtain good results without casting 

 suspicion upon them by the use of membranes, as is shown by 

 the accurate measurements of Noyes and Sammet, Jahn's pupils 

 and others. For measuring very weak electrolytes such as 

 acetic acid, the use of some sort of diaphragm is almost indis- 

 pensable ; I used bundles of glass tubes of small bore, which 

 is cumbersome, but is otherwise free from criticism. The dia- 

 phragms which are least open to objection are cloth, cotton 

 batting, earthenware, and agar-agar. It is unfortunate that 

 it is just in the case of weak electrolytes where diaphragms 

 would be most useful that cataphoresis (electric endosmosis) is 

 most prominent. Solidified gelatine, intestinal membrane, fish- 

 bladder, gold-beaters' skin, membrane of an egg, and parch- 

 ment, are the diaphragms which exert the greatest influence 

 upon the migration ; the influence varies with the nature of the 

 electrolyte and its concentration. 



ERRORS INTRODUCED BY THE METHOD OF CALCULATION. 



There are several methods of calculation in use, although 

 only one of them is correct. Hittorf described this with the 

 greatest clearness in his classical researches ; yet of only six- 

 teen authors besides Jahn's pupils can I say with certainty that 

 their results were correctly calculated. In many cases the data 

 given in the papers are not sufficient to warrant a definite ex- 

 pression of opinion ; but it is safe to assert that of the authors 

 who used the analytical method, not more than half calculated 

 their results correctly. 



In measuring migration we are measuring the movement of 

 the electrolyte through the solv^ent ; hence the calculation must 

 be based upon a constant amount of solvent. If for example we 

 find that before electrolysis the vt grams of water in an electrode 

 compartment contains a certain amount of electrolyte, we must 

 determine the amount of electrolyte contained in w grams of 

 water after electrolysis. The favorite among the other methods 

 is to determine the amount of electrolyte in a constant volume 

 of solution before and after electrolysis ; it is evident that only 



