The Breath of Life by John Burroughs — Read

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 behavior 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 different impressions of the same thing. So a mechanical 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 carbon dioxide, but why it does so admits of no explanation. Herein again is where life differs from fire; we can describe combustion in terms of chemistry, but after we have described life in the same terms something -- and this something is the main thing -- remains untouched.