Maxwell. 269 



returned to the question of the process by which electric action 

 is propagated through space. In this memoir he proposed to 

 replace Poisson's equation for the electrostatic potential, 

 namely, 



by the equation 



according to which the changes of potential due to changing 

 electrification would be propagated outwards from the charges 

 with a velocity c. This, so far as it goes, is in agreement with 

 the view which is now accepted as correct ; but Kiemann's 

 hypothesis was too slight to serve as the basis of a complete 

 theory. Success came only when the properties of the inter- 

 vening medium were taken into account. 



In that power to which Gauss attached so much importance, 

 of devising dynamical models and analogies for obscure physical 

 phenomena, perhaps no one has ever excelled W. Thomson*; 

 and to him, jointly with Faraday, is due the credit of having 

 initiated the theory of the electric medium. In one of his 

 earliest papers, written at the age of seventeen,! Thomson 

 compared the distribution of electrostatic force, in a region 

 containing electrified conductors, with the distribution of the 

 flow of heat in an infinite solid : the equipotential surfaces in 

 the one case correspond to the isothermal surfaces in the other, 

 and an electric charge corresponds to a source of heat.J 



* As will appear from the present chapter, Maxwell had the same power in a 

 very marked degree. It has always been cultivated hy the " Cambridge school " 

 of natural philosophers. 



t Camb. Math. Journal, iii (Feb. 1842), p. 71 ; reprinted in Thomson's Papers 

 <JH Electrostatics and Magnetism, p. 1. Also Camb. and Dub. Math. Journal, 

 Nov., 1845 ; reprinted in Papers, p. 15. 



\ As regards this comparison, Thomson had been anticipated by Chasles, 

 Journal de 1'Ec. Polyt. xv (1837), p. 266, who had shown that attraction accord- 

 ing to Newton's law gives rise to the same fields as the steady conduction of heat, 

 both depending on Laplace's equation v' V = 0. 



It will be remembered that Ohm had used an analogy between thermal conduction 

 and galvanic phenomena. 



