The coal that is dug out of the earth consists mainly of carbon, which, in bygone ages, was extracted by plants from the air. Moreover, it would be of no use to us if we did not have the oxygen of the air in which to burn it. Neither could we smelt metallic ores without oxygen. All our forests and all our crops draw far more of their solid substance from the air than from the soil. Fuel and water are valuable sources of power, but so is the moving air that drives sailing ships and windmills, and the atmospheric pressure that helps to operate suction pumps. It is the moisture of the air that feeds our streams and, directly or indirectly, waters all plants that grow upon the land. Lastly, it is the atmospheric oxygen that we breathe that keeps us from very speedily becoming incapable of using any of the other resources of Nature. Air and water together contain, in their oxygen, nitrogen, hydrogen, and carbon, all the major constituents of our foods in unlimited abundance. It is tantalizing to think of the slow and roundabout way in which these things are wrought into edible shape--and the prices we have to pay for them. No less tantalizing, when coal is scarce and costly, is the thought that every vagrant breeze is laden with the carbon dioxide from which the chemistry of living plants so readily extracts the chief element of fuels. The total carbon dioxide of the atmosphere amounts to something like 2,200,000,000,000 tons, equivalent to 600,000,000,000 tons of carbon. We have spoken of the utility of the air as a source of power. It is, perhaps, even more useful as providing an easy means of storing and transmitting power. The engineer stores up energy in a mass of air by compressing it. When the air subsequently expands it gives up its energy, and, in so doing, may be made to perform a variety of useful tasks. By a somewhat analogous process energy is applied to creating a vacuum, in order that the ordinary pressure of the atmosphere may be made available for doing a particular piece of work. The suction pump, the siphon, and the vacuum cleaner furnish examples of this process; and so do such familiar operations as sucking beverages through a straw and filling a medicine dropper. From crude types of bellows, with which, from remote antiquity, air was compressed for the purpose of blowing fires, have been developed a host of wonder-working appliances of the present day, such as the air brake, the pneumatic tube, the compressed-air locomotive, diving apparatus, the caisson, certain kinds of refrigerating machinery, and a long list of pneumatic tools. To cap the climax of ingenuity in this field, methods involving both the compression and the expansion of air have been discovered whereby this invisible, elusive substance may be changed to a visible liquid and a visible solid; a process having extremely valuable applications, as we shall presently see.