For thousands of years, most people thought that air was the essential gas and that other gases were just contaminated forms of air. Once scientists learned that many specific gases could be distinguished from ordinary air, they began to realize that gases, like solids and liquids, take part in chemical reactions and represent an entire class of substances. But gases differ from solids and liquids in many important ways. For one thing, molecules of gas are normally very far apart. This means that gases can be compressed, or squeezed into a smaller space, without changing the molecules themselves. For all practical purposes, neither liquids nor solids can be compressed as gases can. And because gases have so much empty space between their molecules, they mix with each other very rapidly. Two different gases can mix thoroughly in less than a few seconds...