“Yes, that is very clear.”

“There is only one pound of carbon in ten thousand pounds of ordinary country air.  Now, there are one hundred and sixty square rods in an acre, and since there are twelve inches in a foot and sixteen and one-half feet in a rod, it is easy to compute that there are nearly a hundred million pounds of air on an acre, and that the carbon in this amounts to only five tons.  A three-ton crop of corn or hay contains one and one-fourth tons of the element carbon; so that the total amount of the carbon in the air over an acre of land is sufficient for only four such crops; while a single crop of corn yielding a hundred bushels to the acre, such as we often raise in Illinois on old feed-lots or other pieces of well treated land would require half of the total supply of carbon contained in the air over an acre.  However, the largest crop of corn ever grown, of which there is an established authentic record, was not raised in Illinois, but in the state of South Carolina, in the county of Marlborough, in the year 1898, by Z. J. Drake; and, according to the authentic report of the official committee that measured the land and saw the crop harvested and weighed, and awarded Drake a prize of five hundred dollars given by the Orange Judd Publishing Company,—­according to this very creditable evidence, that acre of land yielded 239 bushels of thoroughly aid-dried corn; and such a crop, Mr. Thornton, would require as much carbon as the total amount contained in the air over an acre of land.”

“Well, that is astonishing!  Then there must be some other source of supply besides the air.”

“There is no other direct source from which plants secure carbon; but of course the air is in constant motion.  Only one-fourth of the earth’s surface is land, and perhaps only one-fourth of this land is cropped, and the average crop is about one-fourth of three tons; so that the total present supply of carbon in the air would be sufficient for about two hundred and fifty years.  But as a matter of fact the supply is permanently maintained by the carbon cycle.  Thus the carbon of coal that is burned in the stove returns to the air in carbon dioxid; and all combustion of coal and wood, grass and weeds, and all other vegetable matter returns carbon to the atmosphere.  All decay of organic matter, as in the fermentation of manure in the pile and the rotting of vegetable matter in the soil, is a form of slow combustion and carbon dioxid is the chief produce of such decay.  Sometimes an appreciable amount of heat is developed, as in the steaming pile of stable refuse lying in the barnyard, while the heat evolved in the soil is too quickly disseminated to be apparent.

“In addition to all this, every animal exhales carbon dioxid.  The body heat and the animal force or energy are supplied by the combustion of organic food within the body, and here, too, carbon dioxid is the chief product of combustion.

“Thus, as a general average, the amount of carbon removed from the atmosphere by growing plants is no greater than the amount returned to the air by these various forms of combustion or decay.  In like manner the supply of combined oxygen is maintained, both carbon and oxygen being furnished to the plant m the carbon dioxid.