The Romance of Modern Chemistry

CHEMISTRY AND AGRICULTURE acre of forest, the winds of heaven secure a wonderfully rapid and even distribution of the carbon dioxide. To be strictly accurate, we must also bear in mind the fact that plants resemble animals in contributing to the contamination of the air; they, too, use up oxygen and breathe out carbon dioxide. In daylight, however, this process of plant-breathing is quite outbalanced by the reverse operation—a characteristic of plants alone —whereby they give out oxygen and purify the air. It is only when they are kept in the dark that the action of plants in giving out carbon dioxide becomes noticeable. Taken altogether, their services in purify- ing the atmosphere quite outbalance what they contri- bute to its contamination. The fact that a plant is really able, under the stimu- lating influence of light, to liberate oxygen from carbon dioxide may be demonstrated by a very simple experi- ment. A bit of a growing plant—a sprig of mint, for instance—is put in a glass tube, which is then filled with tap water and inverted in a dish also containing tap water. The latter is employed in preference to dis- tilled water in this experiment, because it is charged to some extent with carbon dioxide. This simple piece of apparatus is then exposed to sunlight for several hours. It will be noticed that gas bubbles are formed on the surface of the leaves, and that these frequently ascend and collect at the top of the tube. After a few hours have passed the gas which has collected in the tube may be examined. To do this, the thumb is put on the end of the tube while it is still under water, the tube may then be taken out and inverted, the gas in this way being brought to the mouth of the tube. If a glowing slip of wood is thrust into the gas while 218