106 FRAGMENTS OF SCIENCE. 



discord. In short, as regards sound in general, we 

 have a very clear notion of the external physical pro- 

 cesses 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 

 rarefactions 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 ex- 

 perience 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 en- 

 quiries 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 experience 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 den- 

 sity, 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, 



