132 FRAGMENTS OF SCIENCE. 



and elasticity corresponding to the temperature of freezing- 

 water the velocity of sound in it is ten hundred and ninety 

 feet a second. It is almost exactly one-fourth of the ve- 

 locity in water ; the reason being* that though the greater 

 weight of the water tends to diminish the velocity, the 

 enormous molecular elasticity of the liquid far more than 

 atones for the disadvantage due to weight. By various 

 contrivances we can compel the vibrations of the air to 

 declare themselves ; we know the length and frequency of 

 sonorous waves, and we have also obtained great mastery 

 over the various methods by which the air is thrown into 

 vibration. We know the phenomena and laws of vibrating 

 rods, of organ-pipes, strings, membranes, plates, and bells. 

 We can abolish one sound by another. We know the 

 physical meaning of music and noise, of harmony and dis- 

 cord. In short, as regards sounds we have a very clear 

 notion of the external physical processes which corre- 

 spond to our sensations. 



In these phenomena of sound we travel a very little 

 way from downright sensible experience. Still the imagi- 

 nation 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 experience has 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 ex- 

 tend our inquiries from the auditory nerve 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 a 

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

 manifested itself by transplanting into space, for the pur- 



