There is quite a lot of water in the world, either moving or lying on its surface, or soaked a little way into it, or in the air above it. There is so much of it about that we generally find it rather uninteresting, and do not consider that, so far as we are concerned, it is the most important chemical compound (or perhaps mixture of chemical compounds) in the universe. Just to touch on the physiological side first of all. We live as human beings because our frames carry a multiplicity of muscles, the so-called " meat " of animals, those engines which are capable of carrying on work within the body, and of enabling the body to do work external to itself; even whilst you lie back with your eyes closed listening to the rain, even if you drop right off to sleep, a vast number of little engines are consuming fuel within your body and keeping you alive. No one of these engines can use anything but liquid fuel, and the liquid fuel is produced by mixing other chemicals, either solid or liquid, with water, and effecting some chemical changes with a loss of potential energy which you utilise; and so you live.