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



and by which the resulting current is transmitted. The copper 

 and dissolved oxygen tend also to produce an opposing current, 

 for which the zinc plate would serve as ode ; but this current is 

 overcome and masked by the much stronger one of the other 

 pair, and is only manifested by diminishing the force in com- 

 parison with what it would have been, if a plate wholly indiffer- 

 ent to the oxygen was substituted for the copper. The foimula 

 of the force may be thus stated, 



z for 0-0 for H + H for 0- (c for 0-0 for H + H for 0) ; 



leaving a balance of affinities, after the elimination of equal and 

 contrary forces, of 



(<?— c) for 0. 



If this view be correct, the inconstancy of the water battery 

 must arise from the exhaustion of the dissolved oxygen on the 

 surface of the copper plate, this being the only element of which 

 the supply is deficient in quantity ; a conclusion fully warranted 

 by the above experiments with the resistance tube. And it is to 

 the abundant supply of this element in the DanielPs battery that 

 its superior constancy is owing. 



The substitution for the copper plate of any other conducting 

 substance having no affinity for either oxygen or hydrogen, would 

 destroy the negative portion of the formula {z—c) for 0; and 

 the deflection of the needle by a current passed through the re- 

 sistance tube would be then proportional simply to the affinity of 

 the zinc for bxygen. And thus the relative affinity for oxygen of 

 all conducting substances, having no affinity for hydrogen, might 

 be ascertained if such a substitute for the copper plate could be 

 procured. 



Now platinum forms no combination with, and consequently 

 exercises no affinity sensible to the galvanometer for, either 

 oxygen or hydrogen when developed in the nascent state on its 

 surface by electrolysis. Platinum might therefore be supposed 

 to furnish the required substitute for the copper plate. But 

 hei-e a difficulty arises, in consequence of the peculiar conditions 

 of surface which platinum ordinarily assumes. After an electric 

 cuiTent has passed through water, between two plates of platinum, 

 they are no longer similar in their electric relations, but are 

 capable, when closed in circuit, of creating a reactionary current, 

 until they gradually return to their ordinaiy state of inertness. 

 Though platinum, therefore, has no affinity for oxygen or hy- 

 drogen, it is in some way susceptible of different relations to them, 

 and is in fact extremely inconstant in its value as an electro- 

 negative substance. 



This property of platinum, which is equally shared by gold, 

 and partially by silver and other electro-negative bodies, has been 



