=Where does the plant find this carbon?= You may know that there are three chief gases in the air: oxygen and nitrogen, which are the important parts for our breathing, and a little carbonic acid gas, which you may remember is breathed out by animals and plants (~see~ p. 6), and is made of carbon joined with oxygen. As there was no carbon in the food solution, and the plant was surrounded by air containing carbon and oxygen in the invisible form of gas, the idea is suggested that perhaps it is from the air that the plant gets its carbon. Now let us see if this is true by trying the effect of removing the carbonic acid gas from the air in which the plant is growing. To do this we must set up an apparatus which will allow only air freed from carbonic acid gas to surround the plant. Such an apparatus is shown in the figure 12. The plant is grown in the closed bell jar D, which stands over the dish C filled with lime-water, which prevents carbonic acid gas entering through the cracks between the foot of the jar and the table. All the air which enters the jar D must come first through jar A, which is filled with a solution of caustic potash that has the power of absorbing the carbonic acid gas, and then through jar B with lime-water. You can draw plenty of air through jar D for the use of the plant by sucking at the indiarubber tube G, which must be carefully shut with a clamp when you stop the current. The bell-jar D will now be filled with air which is quite free from carbonic acid gas, and the small quantity which is breathed out by the plant itself will be absorbed by the lime-water in dish C. Place the whole in a light or sunny position, and change the air every day or two in the way you filled it, that is by drawing at G so that the fresh air comes in through A and B, and is free from carbonic acid gas. [Illustration: Fig. 12. Apparatus used to keep a plant without any carbonic acid gas. A, jar of caustic potash, B, jar of lime water, which absorb the carbonic acid gas, through which all the air entering jar D must pass; C, basin of lime water to absorb any of the gas given out by the plant growing in D; G, indiarubber tube which can be closed or attached to a siphon to draw air through D.] If you keep the plant growing under these conditions for some time you will find, in comparison with another quite similar plant growing in the open near it, that its growth is very slow. The leaves it forms are smaller, and finally its growth almost ceases. Further, if you test the leaves of the plant growing out in the air for starch (see pp. 24 and 25), you will find that they contain plenty, but that the leaves on the plant in the bell-jar are empty of starch. Now all healthily growing green leaves contain starch, so that this is a good proof that something is seriously wrong with the plant, which has been deprived of the supply of carbon in the air. This shows us that ~plants use the carbonic acid gas in the air~ for their growth.