40 SCIENTIFIC THOUGHT. 



It was recognised by Young, and still more clearly by 

 Fresnel, that the medium which they supposed to be the 

 carrier of light could not have the ordinary properties of 

 either a solid, a liquid, or a gas. It offered apparently 

 no resistance to the motion of the heavenly bodies, its 

 waves were not like those which in air produced sound ; it 

 propagated its waves at a speed much greater than any 

 other velocity known at that time ; at the same time the 

 wave-motion was not that of a body possessing the pro- 

 perties of a gas i.e., an elastic, compressible fluid : it was 

 that of a body offering resistance to change of form rather 

 than to change of bulk. It was evident that the different 

 properties, which we see roughly assembled to constitute 

 the three forms of ponderable matter with which we 

 are practically acquainted, the solid, the liquid, and the 

 gaseous, cannot be assembled in any similar manner in 

 this imponderable substance, the ether. It was bound to 

 have inertia i.e., mass otherwise the laws of motion could 

 not be employed in dealing with it, and mathematical 

 thinking about it would be impossible. A more perfect 

 description of the elementary movements which con- 

 stituted light evidently required a minute experimental 

 study, and a closer mathematical definition of the dif- 

 ferent properties of matter, known popularly but not 

 very clearly under the terms compressibility, rigidity, 

 27. mobility, elasticity, viscosity, &c., and of the inter-de- 

 of elasticity, pendence of these clearly defined properties one on the 

 other. Just about the time when the vibratory theory 

 of light began to be seriously entertained by natural 

 philosophers, a beginning had also been made in this 

 study: the theory of elasticity had been founded in 



