Lavoisier’s achievement consisted in his recognition of the fact that weight is neither increased nor diminished in chemical changes, and in the elevation of this discovery, which has since been many times confirmed with ever-increasing accuracy, into the guiding principle of chemical investigation, the law of conservation of mass. This advance involved the introduction of the balance as the chief instrument of chemical research. Lavoisier’s great success depended, further, upon the fact that he chose the process of oxidation and reduction (the reverse of the reaction of oxidation) for study. Not only is oxygen the most active of chemical elements, if both intensity and variety of chemical behavior be considered, and far the commonest upon the earth’s surface, but also the most important chemical processes are reactions of oxygen.

The partial tearing off of oxygen from the carbon of carbonic acid and the hydrogen of water is the first step in the formation of all organic substances in the plant, and the recombination of oxygen with plant products the chief chemical activity of the animal. All this and much more Lavoisier recognized, and thereby revealed the true nature of another great phenomenon of nature. These investigations also disclosed, in the sequel, the chief source of all the energy which is available for the purposes of man.

It is only the energy stored up in the plant (originally the energy of the sunlight shining upon the green leaf of the plant and transformed by the action of chlorophyll) which is contained in all coal, wood, all kinds of oil, including petroleum, alcohol, in short every fuel. And it is exclusively by the union of the fuels with oxygen once more to form water and carbonic acid that this energy is liberated, as in the human body itself, and utilized by man.〖See Faraday on the “Chemical History of a Candle” in H. C., xxx, 86-170.〗 The resulting water and carbonic acid can then be used over again by the plant. The nature of this cycle of matter was clearly recognized by Lavoisier. This is the basis of nearly all our industry and commerce.

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