The Romance of Modern Chemistry

INVISIBLE SUBSTANCES the vegetable world. The green leaves of plants, aided by sunlight, have the power of decomposing carbon di- oxide, liberating the oxygen, and using the carbon for their own consumption. In regard, therefore, to the pro- duction and consumption of carbon dioxide, the animal and vegetable kingdoms are complementary to each other. To the ordinary person it may appear rather a difficult matter to detect the presence of these odourless, invisible gases, but the chemist has discovered ready methods of recognising and distinguishing them. The properties of each gas have been carefully studied, and in almost all cases substances have been found which will behave in some characteristic manner when a particular gas is present, and remain unaffected when that gas is absent. One of these useful substances, employed to test for the presence of carbon dioxide, is lime water. When slaked lime is shaken with water, a little of it dissolves, the water becomes slightly alkaline, and the clear part free from sediment is known as lime water. Now when a mixture of gases containing carbon dioxide is shaken with lime water, or is bubbled through the lime water, the latter becomes quite cloudy, owing to the formation of chalk. No other gas behaves towards lime water in this peculiar manner, so that we are able to obtain visible proof of the presence of a gas which is itself quite invisible. It is sometimes very necessary to be able to detect the presence of carbon dioxide; for although the gas is not actively poisonous, yet it does not support life, and its presence in large quantity is very harmful. In all pro- cesses of fermentation, as, for example, in the brewing of beer, large quantities of carbon dioxide are produced. Further, this gas, being considerably heavier than air, has a habit of accumulating at the bottom of vessels and 44