84 Human Physiology. 



finger, and a granite obelisk, alike grow larger and smaller 

 with every change of temperature, just as the liquid mercury in 

 the thermometer rises and falls. It is proved by experiment 

 that a granite obelisk, exposed to the rays of the sun, morning, 

 noon, and night, bends towards its shadow. And this absolute r 

 necessary separation of the atoms of matter is proved no less by 

 elasticity. When I bend my knife blade, or a piece of glass r 

 the atoms on one side are pressed nearer together; those on 

 the other are drawn apart; but repulsion on one side and 

 attraction on the other restore them to their former position. 

 If atoms touched each other, nothing could be elastic. 



It is not easy to believe that when a bullet is crushed against 

 a wall, or a chisel is driven through a bar of iron, there is no 

 contact of atoms that the diamond does not touch the glass 

 it cuts, nor the diamond dust the diamond it polishes; but 

 the fact is undoubted. In every case there is a struggle of 

 forces, but the atoms are never in actual contact. 



It is the fact of matter existing in distinct atoms, and equally 

 distinct molecules, each holding its position by forces of attrac- 

 tion, repulsion, and polarisation; each returning to its position 

 when for an instant jostled out of it, like soldiers in a well- 

 drilled military corps, that gives to bodies vibratory and 

 sonorous qualities. When I strike a rod of steel, or ring a 

 bell, I send through it a series of rapid vibrations of all its- 

 myriads of atoms, which are communicated to the atoms of the 

 atmosphere, and thence to my nerves of hearing. Were matter 

 solid, there could be no sound, for all sound is first produced 

 by vibrations of elastic bodies, and elasticity depends upon the 

 absolute separation of the atoms of matter from each other. 

 They are forced together and then fly apart, settling to rest 

 after a multitude of oscillations, or they are drawn asunder, 

 and then fly together with a similar result. Observe, in the 

 same way, the rebounds of an india-rubber ball, or the wonder- 

 ful elastic power of a cord of the same material. 



