190 HISTORY OF CHEMISTRY. 



But still the operations of the mind as well as 

 the information of the senses, ideas as well as facts, 

 are requisite for the attainment of any knowledge ; 

 and all great steps in science require a peculiar 

 distinctness and vividness of thought in the disco- 

 verer. This it is difficult to exemplify in any better 

 way than by the discoveries themselves. Both Davy 

 and Faraday possessed this vividness of mind ; and 

 it was a consequence of this endowment, that Davy's 

 lectures upon chemistry, and Faraday's upon almost 

 any subject of physical philosophy, were of the most 

 brilliant and captivating character. In discovering 

 the nature of voltaic action, the essential intel- 

 lectual requisite was to have a distinct conception 

 of that which Faraday expressed by the remarkable 

 phrase 33 , "an axis of power having equal and oppo- 

 site forces :" and the distinctness of this idea in 

 Faraday's mind shines forth in every part of his 

 writings. Thus he says, the force which determines 

 the decomposition of a body is in the body, not in 

 the poles 34 . But for the most part he can of course 

 only convey this fundamental idea by illustrations. 

 Thus 35 he represents the voltaic circuit by a double 

 circle, studded with the elements of the circuit, and 

 shows how the amons travel round it in one direc- 

 tion, and the cathwns in the opposite. He con- 

 siders 36 the powers at the two places of action as 

 balancing against each other through the medium 

 of the conductors, in a manner analogous to that in 



33 Art, 517- 34 661. 36 96. 38 917. 



