206 Faraday. 



to the chemical affinities producing it ; that when it is deficient 

 in force it may be helped by calling in chemical aid, the want 

 in the former being made up by an equivalent of the latter; 

 that, in other words, the forces termed chemical affinity and 

 electricity are one and the same." 



In the interval between Faraday's earlier and later papers 

 on the cell, some important results on the same subject were 

 published by Frederic Daniell (b. 1790, d. 1845), Professor of 

 Chemistry in King's College, London.* Daniell showed that 

 when a current is passed through a solution of a salt in water, 

 the ions which carry the current are those derived from the salt, 

 and not the oxygen and hydrogen ions derived from the water ; 

 this follows since a current divides itself between different mixed 

 electrolytes according to the difficulty of decomposing each, and 

 it is known that pure water can be electrolysed only with great 

 difficulty. Daniell further showed that the ions arising from 

 (say) sodium sulphate are not represented by Na 2 and S0 3 , but 

 by Na and S0 4 ; and that in such a case as this, sulphuric acid 

 is formed at the anode and soda at the cathode by secondary 

 action, giving rise to the observed evolution of oxygen and 

 hydrogen respectively at these terminals. 



The researches of Faraday on the decomposition of chemical 

 compounds placed between electrodes maintained at different 

 potentials led him in 1837 to reflect on the behaviour of such 

 substances as oil of turpentine or sulphur, when placed in the 

 same situation. These bodies do not conduct electricity, and 

 are not decomposed ; but if the metallic faces of a condenser 

 are maintained at a definite potential difference, and if the 

 space between them is occupied by one of these insulating 

 substances, it is found that the charge on either face depends 

 on the nature of the insulating substance. If for any particular 

 insulator the charge has a value s times the value which it 

 would have if the intervening body were air, the number f 

 may be regarded as a measure of the influence which the 

 insulator exerts on the propagation of electrostatic action 



* Phil. Trans., 1839, p. 97. 



