Carbonic acid gas is composed of a union of carbon and oxygen gas. If, then, the carbon is used by the plant, what happens to the oxygen? [Illustration: Fig. 13. Jar of Elodea in water, giving off bubbles of oxygen gas in the sunlight.] You must have noticed bubbles rising from the “pond scum” and water-plants when they are in the sunlight, the little bubbles sometimes coming up in a quick, regular succession from the leaves and stems. Let us collect this gas and test it to find out what it is. This is more easily done if the plants are living in glass jars, where you can see them and get at them readily. A very good plant to use is the common Canadian water-plant (~Elodea~), which you can buy in aquarium shops if you cannot get it from the ponds for yourself. Place a handful of this plant in a tall, glass jar filled with fresh water, and cover it with a glass funnel, so as to collect the bubbles as they rise. See that the funnel is well under the water and support over it a test tube full of water, as in fig. 13. Place the jar in as bright sunlight as possible, when you should see the bubbles beginning to come off quite quickly. As the bubbles rise in the tube A, the water is forced out till the whole vessel is filled with gas. Then place your thumb over the mouth of the tube of gas, and remove it quickly from the water. Test it by plunging into it a splinter of wood which has been burning, but just blown out, so that it is still glowing. If you plunge it quickly enough into the tube, it should catch fire and burn brilliantly. Now this is the test for oxygen gas, so that we have proved that the tube was full of oxygen. This oxygen is the part of the carbonic acid gas which is given off by the plant as it uses the carbon and frees the oxygen it does not need. You will find that the gas bubbles are given off much more rapidly when the plant is placed in bright sunshine than when it is shaded, and that when the plant is in darkness the bubbles stop altogether. This seems to show us that the sunshine must assist the plant to split up the carbonic acid gas, and we will find out more about this later on (~see~ p. 25). We have now found that carbon forms a large part of the plant body, that plants cannot grow in air in which there is no carbonic acid gas, and that in getting the carbon from the carbonic acid gas, they split it up and give off the oxygen. So that we see that =plants use the carbon in the air as well as the salts dissolved in the water of the soil as raw materials, with which they finally build up their food=. We must now try to find what food substance it is that they build up from these raw materials. CHAPTER VI. THE FOOD MANUFACTURED BY THE PLANT You will remember that much of the food provided in the nurse leaves consisted of starch, and that the baby plants use this food as they grow.