194 CHEMICAL DISCOVERY AND INVENTION 



retain their opposite electric charges, they are prevented from 

 travelling far from each other, and the minute quantity which 

 may be supposed to escape, according to calculations by Professor 

 Nernst, is too small to be recognisable by chemical tests. 



The theory of electrolytic dissociation is still beset with a few 

 difficulties which, however, will probably disappear in time, as a 

 certain proportion of them are due to experimental inaccuracies 

 and to complications which are attributable to the formation of 

 compounds, possessing electrolytic properties, between the 

 dissolved substance and the solvent. 



Another kind of difficulty is met with in the case of the com- 

 pounds which are known as " amphoteric " electrolytes. These 

 are substances which are capable of behaving as acids or bases 

 according to the nature of the substance with which they are 

 associated. Amino-acetic acid or glycine, NH 2 .CH 2 .COOH, and 

 amino-succinic or aspartic acid, COOH.CH(NH 2 ).CH 2 .COOH, 

 are examples of these compounds. These two compounds 

 contain the group NH 2 in virtue of which they are feeble bases, 

 while they both contain the carboxylic group CO. OH, which 



confers acid properties. In either case, acting as both acid and 



+ 



base, they must give rise to the hydrion H and hydroxidion HO, 

 the latter resulting from the ionisation of the hydrated form of 

 the substance ; amino-acetic acid becoming OH.NH 3 .CH 2 . 

 COOH. From this if the substance acts as base, the ions formed 



are NH 3 .CH 2 .COOH and OH, while if it acts as acid the ions are 

 OH.NH 3 .CH 2 .COO and H+, or the corresponding anhydrous 

 ion NH 2 .CH 2 .COO~. Combination may take place between 

 two molecules of the compound, the one acting as base, the 

 other as acid, or the acid and basic portions of one and the same 

 molecule may act on each other. In any case, according to the 

 theory, all these ions must exist together in an aqueous solution 

 of glycine, and the experimental enquiry into the electrical 

 conductivities of such solutions is therefore difficult, but there 

 is no reason to suppose that they offer any obstacle to the accept- 

 ance of the hypothesis. 



Industrial processes based on electrolysis are now common. 

 Such are, for example, the modern methods by which such 

 metals as sodium and aluminium and zinc are obtained, copper 

 is refined, and electro-plating with silver and gold (a long- 

 established method) with nickel, zinc, and brass, In addition 



