146 E. F. ROEBER 



one, since there is an abundance of processes in which an 

 aqueous solution of sodium chloride is electrolyzed. We thus 

 come to the class of purely electrolytic processes. Of course, 

 Joulean heat and a certain electro-thermic effect is here not 

 absent, simply because it is impossible to avoid it. But it is 

 generally so small as to be neglected, and, in fact, there are 

 many cases in which it is necessary to make it small, since the 

 output would be decreased by increased temperature. 



One preliminary remark may be made on electrolytic 

 processes in general. An electrolytic cell represents an elec- 

 trochemical system, and it is mostly possible to arrange it 

 in such a way that when electrolysis begins we get the prod- 

 ucts which we want. But when electrolysis is continued 

 we have no longer our original electrochemical system and the 

 products of electrolysis may no longer be pure; we may now 

 get something we do not want. The first condition that must 

 be fulfilled for the successful working of an electrolytic process 

 on an industrial scale is to have the conditions good at the 

 start, to begin with pure solutions and to maintain them pure 

 during electrolysis. As F. Haber says, everything can be 

 accomplished with pure solutions ; nothing with foul solutions. 

 In many cases in which a process looked promising from the 

 experiments made on a small scale in the laboratory, it turned 

 out to be afterwards a failure on a large scale, because the 

 electrolyte soon became impure and its purification required 

 expense and the overcoming of difficulties which had not been 

 expected in advance. The same fundamental principle of 

 the working of electrochemical processes is expressed by D. 

 H. Browne in a slightly more general form when he asserts that 

 the keynote of success in electrolysis is to keep initially good 

 conditions permanently good. Close experiment will almost 

 always reveal the proper conditions under which certain work 

 can be done, but the continuance of these conditions is attain- 

 able only by continual vigilance. 



To return to the electrolysis of an aqueous solution of 

 sodium chloride, the products of electrolysis are caustic soda 

 and hydrogen at the cathode and chlorine at the anode. This 

 is what is wanted if our final products are to be bleaching 

 powder and caustic. But if we did not remove the caustic 



