204 JAMES CLEKK MAXWELL 



endeavour to construct a mental representation of all the 

 details of its action, and this has been my constant aim in this 

 treatise." 



Let us sec, then, what were the experimental 

 grounds in Maxwell's day for accepting as true his 

 views on electrical action, and how since then, by the 

 genius of lleinrich Hertz and the labours of his 

 followers, those grounds have been rendered so sure 

 that nearly the whole progress of electrical science 

 during the last twenty years has consisted in tho 

 development of ideas which are to be found in tho 

 "Treatise on Electricity and Magnetism." 



The purely electrical consequences of Maxwell's 

 theory were of course in accord with all known elec- 

 trical observations. The equations of the Held ac- 

 counted for the electro-magnetic forces observed in 

 various experiments, and from them the laws of electro- 

 magnetic induction could be correctly deduced ; but 

 there was nothing very special in this. Similar equa- 

 tions had been obtained from the theory of action at 

 a distance by various writers ; in fact, Helmholtz's 

 theorv, based on the most general form of expression 

 for the force between two elements of current con- 

 sistent with certain experiments of Ampere's, was 

 more general in its character than Maxwell's. Tho 

 destructive features of Maxwell's theory were : 



(1) The assumption that all currents flow in closed 

 circuits. 



(2) Tho idea of energy residing throughout tho 

 electro-magnetic field in consequence of the strains 

 and stresses set up in the electro-magnetic medium 

 by tho actions to which it was subject. 



