340 GREAT MEN OF SCIENCE 



electric and magnetic forces appear in the equations, was also 

 derived from a thought of Faraday's, according to which 

 light and electro-magnetic forces might 'by no means im- 

 possibly' be phenomena of one and the same ether.^ This 

 idea of the unity of the ether became the great fundamental 

 idea in Maxwell's theory as expressed by his equations. The 

 fact that it is a theory (summary of knowledge), and not 

 simply an hypothesis (supposition), was only somewhat 

 dimly indicated in Maxwell's time by Faraday's discovery of 

 the electro-magnetic rotation of the plane of polarisation 

 of light and - more obviously - by the appearance, as the 

 result of Wilhelm Weber's researches, of the velocity of light 

 in electrical phenomena and measurement. 



Maxwell's equations summarise, in a manner astonishing 

 even to the mathematician, the fundamental results of whole 

 series of researches. They contain Coulomb's two laws with 

 the essentials of the potential theory, Oersted's discovery and 

 Ampere's researches. Ohm's law, Faraday's law of induc- 

 tion; all this being supported on Faraday's conception of lines 

 of force, which is thus for the first time shown to be in full 

 harmony with the older potential theory of Laplace. They 

 further contain - as a hypothetical addition made by Maxwell 

 - the existence of electric waves, which, starting from 

 the electric oscillations already treated of by W. Thomson 

 (Lord Kelvin), are propagated in free space with the 

 velocity of light, are transverse like light waves, and should 

 be refrangible like the latter in material media. So mag- 

 nificent a summary in a few equations had hitherto never 

 been known. 



As regards their power of application, the sense of the 

 equations, in which besides the Cartesian co-ordinates of 

 space, time appears as an independent variable, is as follows: 

 let the electric and magnetic forces at an initial moment 



^ Tyndall collected several remarks of Faraday's in this connection, in 

 Faraday as a Discoverer, London, 1870, p. 154 ff. 



