156 EEVISIONS OF THE THEORY OF ELECTEOLYSIS. 



rather than to paraphrase them. Claiisius belongs to our own day. 

 The gray hair and benign expression of this beautiful old man 

 made a lasting impression on me as I saw him at the International 

 Electrical Congress in Paris in 1881. 



Grotthus conceived of the motion of the part-molecules as taking 

 place under electric force only by the progressive exchange of part- 

 ners in a polarized chain of molecules. Clausius rejected the fixed 

 equilibrium in the normal condition of an electrolyte, and assumed 

 some free part-molecules or, as Faraday named them, ions; and the 

 dissociation imagined by Clausius may be described as temporary, 

 or a continuous exchange of partners in chemical association. Wil- 

 liamson as early as 1851 declared " that in an aggregate of the 

 molecules of every compound a constant interchange between the 

 elements contained in them is taking place." This view Clausius 

 rejected. The assumption that only a few of the molecules in a 

 solution are dissociated into ions appeared to him to satisfy the facts 

 of electrolysis. 



The first attempt to answer the question as to the manner in which 

 the facts of electrical conductivity are reconciled with those of 

 general chemistry was contained in the theory of Grotthus; the sec- 

 ond attempt was made by Clausius in 1857, and thirty years later a 

 further answer was made in the work of Arrhenius. 



Another question relates to the intimate relation between the pas- 

 sage of electricity through an electrolyte and the simultaneous motion 

 of the ions, the ponderable attendant of the electrical transfer. The 

 law of Faraday paved the way for an answer to this question; the 

 reply of the present is largely the work of Hittorf and of Kohlrausch. 



The simplest statement of Faraday's discovery in electrolysis is 

 that the passage of a fixed quantity of electricity is always associated 

 with the transfer of a gram equivalent of the ion. The passage 

 through an electrolyte of 96,550 coulombs of electricity always 

 releases or deposits a gram equivalent of an ion; that is, a gram 

 molecule of an ion whose valence is one, or half a gram molecule of 

 an ion whose valence is two. This law demonstrates that there is a 

 fixed minimum, called by Helmholtz the atomic charge^ conveyed 

 by univalent ions, others conveying only integral multiples of this 

 charge. 



The researches of Hittorf on the migration of ions demonstrated 

 that the observed change in concentration at the electrodes during 

 electrolj^sis makes it necessary to assign to the positive and negative 

 ions different velocities. Hittorf's work, though of fundamental 

 importance, commanded but little attention at the time of its publica- 

 tion. In fact, it was opposed by the leaders of physical science and 

 was not accepted till thirty years later. 



