CONNECTION OF ELECTRICITY AND MAGNETISM. 253 



theory of ligbt, in which the mechanical attributes of a rigid elastic solid 

 are ascribed to the ether, is beset with many theoretical difficulties. It 

 appears that the new electro-magnetic theory of light answers better in 

 some cases, as, for instance, in the theory of reflection and refraction ; 

 whether it does so everywhere must be decided by its further elabora- 

 tion. This natural and unsonght-for connection of the new electro- 

 magnetic theory with the theory of light is, indeed, an important suc- 

 cess, which gives rise to the greatest hopes for the future. 



In his former papers. Professor Maxwell has already shown that the 

 same kind of influence which, according to his electro-dynamic theory, 

 neighboring elementary volumes of ether have upon each other, can 

 be imitated by a mechanism containing a system of rotatory elastic 

 spheres, every oue of which acts on its neighbors by means of a system 

 of friction-balls interposed between them. Such a mechanism may 

 appear rather too complicated for the structure of the ether ; nevertheless, 

 it is a result of great general importance that actions the laws of which 

 are congruent with those of electro-dynamic phenomena can be produced 

 by the x^lay of common mechanical forces. 



Eecently Prof. Clerk Maxwell has given, in his treatise, (p. Ill, ch. 6,) 

 a more general and abstract demonstration of the same result, derived 

 from Lagrange's (or Hamilton's) general i)rinciples of mechanics. He 

 takes for that end the electro- dynamic potential as expressing the vis 

 viva of all the known and unknown motions which are connected neces- 

 sarily with the motion of electricity in a conductor. Without any special 

 assumption about the nature of these motions, the laws of pondero-mo- 

 toric and induced electro-motoric force may be deduced from this con- 

 ception in its most general form. This shows again that electro-dynamic 

 forces are subject to the same general principles of action as common 

 mechanical forces. 



The two volumes lately published by Professor Maxwell contain not 

 only this new theory, but a very complete, methodical, and clear expo- 

 sition of all those parts of electric science which could be brought under 

 precise theoretical conceptions, and be developed mathematically. He 

 has done a great service by this work to every student of physics. 

 Hitherto we were obliged to search after the papers treating the dif- 

 ferent parts of this subject through a great number of scientific period- 

 icals and books. Besides, in relation to the original parts of the book, 

 principally those treating of the ultimate causes of electro-dynamic 

 action, I do not hesitate to say that I consider his method of forming 

 new theoretical conceptions, which are at the same time perfectly definite 

 in their quantitative determination and yet as general as possible, and 

 not more specified than is needed, or more than our present knowledge 

 of the facts allows, as really a model of cautious scientific progress. 



