12 THE PEOGRESS AND SPIRIT OF 



the first and most able exposition of the doctrine. Partial 

 suggestions of it, both in England and Germany, had already 

 occurred ; but we owe to Mr. Grove its distinct enunciation 

 as a physical principle, and the illustration of this principle 

 by instances drawn from his own researches and those of 

 others, which give it all the characters of a new physical law. 

 His work, of which the third edition is before us, is remark- 

 able, not only for the bold and exact logic of its inductions, 

 but also for its clearness and simplicity of style; qualities 

 valuable in all scientific writings, and essential on subjects 

 like those here treated of. 



By the term co-relation, as applied to physical forces, Mr. 

 Grove means to convey the general idea of reciprocal pro- 

 duction; that is, that any force capable of producing 

 another, may reciprocally be produced by it. But the 

 principle here involved, as well as its wide scope, will be 

 better understood by taking co-relation to express generally 

 those relations of forces which render them mutually and 

 constantly convertible one form or manifestation of force 

 generating another, so as to bring together into the same 

 series of effects, physical actions and changes seemingly the 

 most remote and dissimilar. Thus, to take a familiar but 

 striking instance. The same single electrical current from a 

 voltaic battery is capable in its circuit of evolving heat and 

 light of creating magnets of producing mechanical force 

 of violently affecting the nervous and muscular organisa- 

 tion and of inducing, by decomposition or combination, 

 the most powerful chemical changes simply according to the 

 nature of the different material objects which the experi- 

 mentalist interposes in the circuit, so as to subject them to 

 this current of power. Here then (gravitation excepted) we 

 find all the great natural forces, of which we have present 

 knowledge, evolved from a single source ; and that source, 

 be it remarked, a chemical change of affinities, giving origin 



