32 Britain's Heritage of Science 



difficulty presented itself. The phenomena of light seemed 

 to be explained in a satisfactory manner by giving to the 

 aether the properties of ordinary incompressible elastic 

 bodies, though certain circumstances might have roused the 

 suspicion that we had not got hold of the whole truth. Yet 

 the essential points seemed so well accounted for by the 

 investigations of Green and Stokes, that there was every 

 reason to believe that outstanding difficulties would be 

 satisfactorily solved, without abandoning the substance of 

 the theory. It was quite clear, nevertheless, that the medium 

 invented to explain the properties of light, could not account 

 for the electrical effects 



It is here that Maxwell's genius saw the solution : the 

 problem had to be inverted. It was not the question of 

 whether a medium adapted to account for the comparatively 

 simple phenomena of light could explain electrical action, 

 but whether a medium constructed so as to explain electrical 

 action could also explain the phenomena of light. In 

 formulating the essential properties of the medium which 

 could produce the electrical effects, Maxwell had to fit a 

 mathematical mantle on the somewhat crude skeleton of 

 Faraday's creation. The task was formidable, and the 

 manner in which it was carried through stands unequalled 

 by any achievement in the whole range of scientific history, 

 both as regards its intellectual effort and its final results. 

 Only one of its successes need here be recorded. A quantity 

 of electricity may be measured either by its electrostatic, 

 when it is at rest, or by its electrodynamic effect, when it 

 is in motion. Looking separately at the two manifestations 

 of electricity, we are led to two different units in which it 

 can be measured, the so-called electrostatic and electro- 

 magnetic units. The time of propagation of an electro- 

 dynamic effect through space was proved by Maxwell to be 

 equal to the ratio of these two units. It could be calculated, 

 therefore, from purely electric measurements, and it turned 

 out to be exactly equal to the velocity of light. Hence 

 luminous and electrodynamic disturbances are propagated 

 with the same velocity, and we must conclude that their 

 nature is identical. There was, after the publication of 

 Maxwell's work, really nothing more to be said for the older 



