But all this wonderful, this boundless, power over material laws is gained by these laws. He alters no property of matter, but he uses one property or another as he needs, and he uses one property to overpower another. It is by knowing that gravity is more powerful in the case of air than in the case of hydrogen gas, that he makes air sustain him as he floats, beneath a bag of hydrogen, above the earth; it is by knowing that it is more powerful in water than in air that he sails in iron ships; it is by knowing chemical affinity or repulsion that he makes the compounds or extracts the simple elements he desires; it is by knowing that affinity is force, and that force is transmutable into electricity, that he makes a messenger of the obedient lightning shock; it is by knowing that heat, itself unknown, causes gases to expand, that he makes machines of senseless iron do the work of intelligent giants. He subdues Nature by understanding Nature. He creates no property; he therefore performs no miracle, though he does marvels. By what means, then, does man bring one property, or law, into play instead of, or against, another? By one means only, that of changing the position of matter. This is Bacon's aphorism (Nov. Org. Book i. 4): "Man contributes nothing to operations except the applying or withdrawing of natural bodies: Nature, internally, performs the rest." In order to trace and recognize the truth of this fact, let us follow in rough and rapid outline the operations by which man effects his purposes. We will begin at the beginning, and suppose him to have only reached the stage when a knowledge of the effects of fire enables him to work with metals. He produces fire by friction--that is, by bringing one piece of wood to another, and rapidly moving the one on the other; or else by striking two flints on each other, which also is merely rapid motion and shock. He carries the wood to a hearth, he brings to it the lump of crude metal or the ore; he urges the fire by a blast of air--still his acts are only those of imparting motion. Then the fire acts on the metal, it excites some affinities and enfeebles other affinities, which result in removing impurities; it softens the purified metal. Then the workman lifts it on a stone, and by beating it with another stone--still motion--he moves its particles so that it assumes the form of a hammer, an axe, a chisel, or a file. Then by rubbing with a rough stone--still motion--he moves away some particles from the edge, and makes it sharp and fit for cutting. By plunging it in water when hot--still only motion--he tempers it to hardness. With the edge thus obtained he cuts wood into the forms he requires for various purposes, and by degrees he learns how to fashion other pieces of metal into other and more elaborate tools. Yet all this is done by no other means than giving motion to the material on which, or by which, he works. From tools he advances to machines, by