THE BREATH OF LIFE 



which their flight is recorded suggests a shower of 

 shooting stars. Oxygen is found to be made up of 

 atoms of several different forms. 



m 



The " free path " of molecules, both in liquids 

 and in gases, is so minute as to be beyond the reach 

 of the most powerful microscope. This free path in 

 liquids is a zigzag course, owing to the perpetual 

 collisions with other molecules. The molecular be- 

 havior of liquids differs from that of gases only in 

 what is called surface tension. Liquids have a skin, 

 a peculiar stress of the surface molecules; gases do 

 not, but tend to dissipate and fill all space. A drop 

 of water remains intact till vaporization sets in; 

 then it too becomes more and more diffused. 



When two substances combine chemically, more 

 or less heat is evolved. When the combination is 

 effected slowly, as in an animal's body, heat is 

 slowly evolved. When the combustion is rapid, as 

 in actual fire, heat is rapidly evolved. The same 

 phenomenon may reach the eye as light, and the 

 hand as heat, though different senses get two dif- 

 ferent impressions of the same thing. So a mechani- 

 cal disturbance may reach the ear as sound, and be 

 so interpreted, and reach the hand as motion in 

 matter. In combustion, the oxygen combines rapidly 

 with the carbon, giving out heat and light and car- 

 bon dioxide, but why it does so admits of no ex- 



200 



