The Romance of Modern Chemistry

CHEMISTRY AND AGRICULTURE the thumb is removed for a moment, it will be relit, showing that the gas which was collected was oxygen. So much, then, is fairly established, that the carbon dioxide of the atmosphere is taken in by the plant, and that the carbon is retained while the oxygen is given off. But chemists have not been able to dis- cover the actual chemical process to which the carbon dioxide is subjected in the mysterious laboratories of the plant leaf. It is, indeed, certain that water also is involved, so that the leaves may be said to feed mainly on carbon dioxide and water, a simple life diet which produces the most extraordinary results. When we speak of the carbon of the carbon dioxide being retained in the plant, we must not suppose that it is actually found in that form; it is no sooner extracted from the carbon dioxide than it passes into some form of combination with hydrogen and oxygen, probably formaldehyde in the first instance. As to the methods by which the living plant subsequently builds up more complicated pro- ducts, such as starch, sugar, and cellulose, we know very little. The experiment which convinced the scientists of three hundred years ago that vegetable matter could be pro- duced from water alone has been shown to be incomplete and inconclusive; but we must admit at the same time that water does enter very largely indeed into the com- position of living plants. Some succulent vegetables contain over 90 per cent, of their weight of water, and even trees felled in the driest period of the year will have as much as 40 per cent. If we suppose the plant’s supply of water completely removed, the remainder, which we may call the “dry material” of the plant, consists partly of combustible and partly of incom- 219