Of carbon, oxygen, hydrogen, and nitrogen.
1144. Whence do plants derive those substances?
From the air, the earth, and water.
1145. How do plants obtain carbon?
They obtain it chiefly from the air, in the form of carbonic acid gas. The carbon, of the carbonic acid gas, which is thrown out by the breath of animals, and by other processes in nature, is absorbed by the leaves of plants, and the oxygen which had united with the carbon to form the carbonic acid gas, is again set free for the use of animals.
1146. How do plants obtain oxygen?
They obtain it from the atmospheric air. But as they do not require a large amount of oxygen for their own use, they throw off the amount which is in excess, after having separated it from the other elements with which it was combined when taken up by them. From the humble blade of grass, to the stately tree of the forest, plants operate to purify the air, and to correct and counteract the corruption of the air, by the myriads of animals inhabiting the earth.
It has been generally stated that plants in rooms purify the air by absorbing carbonic acid by day, and releasing a part of the oxygen; but that, as the presence of light is necessary to produce this action, they do not restore oxygen to the air, by night, but, on the contrary, give off carbonic acid gas. Therefore it has been stated that plants in rooms by night are unhealthy. Mr. Robert Hunt, one of the ablest chemists of the present time, makes the following remarks upon this subject in his "Poetry of Science:"—
"The heavens declare the glory of God: and the firmament showeth his handy work. Day unto day uttereth speech, and night unto night showeth knowledge."—Psalm xix.