106 FKAGMENTS OF SCIENCE. 



as regards sound in general, we have a very clear notion 

 of the external physical processes which correspond to 

 our sensations. 



In the phenomena of sound, we travel a very little 

 way from downright sensible experience. Still the 

 imagination is to some extent exercised. The bodily 

 eye, for example, cannot see the condensations and rare- 

 factions of the waves of sound. We construct them in 

 thought, and we believe as firmly in their existence as 

 in that of the air itself. But now our experience is to 

 be carried into a new region, where a new use is to be 

 made of it. Having mastered the cause and mechanism 

 of sound, we desire to know the cause and mechanism 

 of light. We wish to extend our enquiries from the 

 auditory to the optic nerve. There is in the human 

 intellect a power of expansion I might almost call it 

 a power of creation which is brought into play by the 

 simple brooding upon facts. The legend of the spirit 

 brooding over chaos may have originated in experi- 

 ence of this power. In the case now before us it has 

 manifested itself by transplanting into space, for the 

 purposes of light, an adequately modified form of the 

 mechanism of sound. We know intimately whereon 

 the velocity of sound depends. When we lessen the 

 density of the aerial medium, and preserve its elasticity 

 constant, we augment the velocity. When we heighten 

 the elasticity, and keep the density constant, we also 

 augment the velocity. A small density, therefore, and 

 a great elasticity, are the two things necessary to rapid 

 propagation. Now light is known to move with the 

 astounding velocity of 186,000 miles a second. How is 

 such a velocity to be obtained ? By boldly diffusing in 

 space a medium of the requisite tenuity and elasticity. 



Let us make such a medium our starting-point, 

 and, endowing it with one or two other necessary 



