Mr. I. B. Cooke on the Measurement of Chemical Affinity. 89 



As no such product is formed, it may legitimately be assumed 

 that the mutual affinity of these two substances has no part in 

 causing the current of the cell. In the same manner, when 

 water is decomposed between two zinc plates, as the hydrogen is 

 given off without combining with the zinc, we may conclude that 

 no affinity between these two elements is concerned in modifying 

 the strength of the current. The only possible affinities, there- 

 fore, whose balance of forces can constitute the current of a zinc 

 and copper pair in water, may be thus enumerated: — 



1st. Zinc for the oxygen of the water. 



2nd. Copper for the oxygen of the water. 



3rd. The hydrogen and oxygen of the water for each other. 



4th. The oxygen dissolved in the water, for the other elements 

 of the circuit. 



The only apparent chemical product formed by the action of 

 this simple circuit is the oxide of zinc. But as no hydrogen is 

 evolved from the copper, it must either be absorbed in some 

 way, or the water is not decomposed by the zinc. On the latter 

 supposition, the action would be merely local, the zinc combi- 

 ning directly with the dissolved oxygen in its o^vn neighbour- 

 hood, and no current would be circulated. The action must 

 therefore be as follows : — First, the zinc attacks the oxygen of 

 the water, liberating its hydrogen to combine with the oxygen 

 of the next particle of water ; and thus by successive combina- 

 tions and decompositions, hydrogen travels on to the copper 

 plate, when it finally combines with the oxygen held in solution ; 

 and in addition to the oxide of zinc, water also is a product of 

 the action. -«■> 



The most perfect type of electromotive apparatus would pro- 

 bably be produced, if two solid conductors, having a powerful 

 mutual affinity, could be arranged in a fluid electrolyte of which 

 they should be themselves the constituent ions. The two con- 

 ductors would be equally active as electromotors, and the current 

 resulting would be the measure of their combining energy. 

 The nearest approximation to this type at present known, is 

 probably the gas battery of Grove, where the two ions, oxygen 

 and hydrogen, unite to form the electrolyte water; the pla- 

 tinum plates being merely requisite to serve as odes or door- 

 ways for the combination of the elements, and for the passage 

 of the current affinity. All batteries have more or less resem- 

 blance to this type. The water battery of zinc and copper 

 differs from it principally in the substitution of the metal zinc 

 for one of the ions of the electrolyte, viz. hydrogen, for which 

 it has no affinity, but to the affinities of which its own have 

 much resemblance. The copper serves as the ode by which 

 the other ion, viz. the dissolved oxygen, contributes its action, 



