234 Mr. E. H. Cook on the Existence of 



it can vibrate at this rate (which seems to be admitted) why 

 cannot it transmit this vibration ? 



It cannot transmit it because its density is too great and its 

 elasticity too small we should be told. But if we reduce the 

 size of the atom we reduce the weight of it, and we reduce the 

 amount of energy necessary to throw it into vibration. In 

 fact, if we reduce the size 954,138,240,000 times, we should 

 reduce the weight and endow the atom with power of vibra- 

 ting. It appears, then, that the ability of a body to take up 

 vibratory motion depends upon the size of the atom ; and, given 

 that the atoms and molecules of bodies are sufficiently small, 

 they are capable of transmitting light and heat. In order to 

 compare the velocity with which light travels with that which 

 sound travels, let us take a few examples. In air we have 

 already seen that light travels with 888,500 times the velocity 

 of sound. In water we know that sound travels at about 

 5000 feet per second. The index of refraction of water is, 

 according to Brewster and Wollaston, 1*336 ; if the velocity of 

 light in air is 185,000 miles per second, the velocity in water 

 is 138,500. Thus the velocity of light in water is 146,300 

 times that of sound. The greatest velocity of sound in 

 any substance is through iron at 100° C, in which it is 

 17,500 feet per second. The substance with the greatest re- 

 fractive index is chromate of lead = nearly 3. The velocity of 

 light is therefore less in this body than in any. Let us com- 

 pare these two, and we find that the velocity of light in chro- 

 mate of lead is only 17,620 times as great as the velocity of 

 sound through iron. JSTow we have no doubt that the sound is 

 transmitted through the iron by the vibration of its molecules, 

 why may not the light be transmitted through the chromate of 

 lead by the vibration of its molecules ? Here, again, it is merely 

 a question of degree ; the one is a more rapid motion than the 

 other. There is another point which must not be overlooked in 

 comparing sound and light ; and that is, that the vibrations of 

 the molecules composing a light-wave oscillate in planes per- 

 pendicular to the direction of propagation of the wave, while 

 those composing a sound-wave oscillate in the direction of pro- 

 pagation. This difference, it seems reasonable to suppose, will 

 exercise an important influence on the relative velocities even 

 in a homogeneous medium. It is evident that it must, do so 

 in a medium whose elasticity is different in the two planes, 

 i. e. the plane of propagation and the plane of vibration. 



And in thus assuming that the particles of matter themselves 

 are capable of vibrating and propagating the undulatory move- 

 ment of light, are we making too great a strain upon them ? 

 Certainly not. We can form no notion, even the most remote, 



