82 



SCIENTIFIC THOUGHT. 



as 1831 conceived this peculiar condition of matter to 

 be equivalent to a state of strain, could be represented 

 by the mechanical analogy of the strains existing in an 

 elastic solid. He had distinguished three distinct forms 

 of this elastic strain, and had identified these three 

 forms severally with electrostatic, magnetic, and galvanic 

 forces. He had not given a physical explanation of the 

 origin of these forces, but had merely used the " mathe- 

 matical analogies of the two problems (the electrical and 

 the elastic) to assist the imagination in the study of 

 both." l Maxwell now took a further step and pro- 

 ceeded to give a physical or mechanical description of the 

 nature of this state of stress, of the electrotonic state of 

 matter. With this object in view he conceives of a 

 medium which is capable of exerting force on material 

 bodies by being itself strained, and exhibiting the 



" suggests the idea that there may 

 be a problem in the theory of 

 elastic solids corresponding to every 

 problem connected with the distri- 

 bution of electricity on conductors, 

 or with the forces of attraction and 

 repulsion exercised by electrified 

 bodies. The clue to a similar re- 

 presentation of magnetic and gal- 

 vanic forces is afforded by Mr 

 Faraday's recent discovery of the 

 affection, with reference to polarised 

 light, of transparent solids sub- 

 jected to magnetic or electro- 

 magnetic forces." 



1 Quoted from Maxwell's paper 

 " On Physical Lines of Force," in 

 the 'Philos. Mag.,' 1861 (see ' Coll. 

 Papers,' vol. i. p. 453), in which 

 Maxwell applies Rankine's concep- 

 tion of molecular vortices to the 

 representation of magnetic pheno- 

 mena. He refers to his earlier paper 

 (1855) on (geometrical) " lines of 



force " in which he had " shown 

 the geometrical significance of the 

 electrotonic state," and had used 

 " mechanical illustrations to assist 

 the imagination, but not to ac- 

 count for the phenomena." " I 

 now," he says, " propose to examine 

 magnetic phenomena from a me- 

 chanical point of view, and to 

 determine what tensions in, or 

 motions of, a medium are capable 

 of producing the mechanical pheno- 

 mena observed. If by the same 

 hypothesis we can connect the 

 phenomena of magnetic attraction 

 with electro-magnetic phenomena, 

 and with those of induced cur- 

 rents, we shall have found a theory 

 which, if not true, can only be 

 proved to be erroneous by experi- 

 ments which will greatly enlarge 

 our knowledge of this part of 

 physics" (ibid., p. 452). 



