Vegetables, which are largely starch and carbon, absorb carbonic-acid gas from the atmosphere, which contains thirty-three grains of carbon in every square inch. If the world were all dry land, and covered with dense vegetation, all the carbon would be extracted from the atmosphere in twenty-three years. This carbonic acid is restored to the atmosphere by the waste and decay of vegetable matter, by the exhalations from animals, and by combustion.
The leaf of a living plant draws in carbonic acid gas from the air, and gives off the oxygen contained in it, retaining only the carbon. The roots drink in moisture from the soil; and out of the carbon and water contained in it the plant forms starch, sugar, fat, and other substances.
The animal takes the starch, sugar, or fat into its stomach, and draws in oxygen through its lungs. New chemical combinations are thus formed, undoing the work of the plant, and sending back to the air, from the lungs and skin, both the starch and oxygen, in the form of carbonic-acid gas and water. The same material is constantly circulating, - now floating in invisible air, now forming the substance of the growing plant, now of the moving animal, and now diffusing itself through the air ready to go its round again. It forms part of a vegetable to-day; tomorrow it is in a man's backbone; a week hence it may have passed through another plant and into another animal.
In burning coal, we cause its carbon to unite with the oxygen of the air and to disappear as carbonic acid gas. The carbon returns to the atmosphere from which it may have been taken millions of years ago when it was appropriated by the growing plants, which, in the form of vegetable matter, were afterwards buried beneath the surface of the earth only to reappear ages subsequently in the form of fuel. The earth itself breathes out carbonic acid, sometimes with water, sometimes alone. It sparkles in the springs of Carlsbad and Seltzer; it kills man and beast in the terrible "Valley of Death" in the island of Java.
In this way is supplied the loss of that which is daily buried by the shell fish and coral insects in the limestone formations and coral growths. These rocks contain, chained down in seemingly everlasting imprisonment, two fifths of their weight of carbonic acid.