"Well, the coal is burned up and destroyed, is it not?"

"The carbon is burned and changed, but not destroyed. In this case,
the heat has caused the carbon to unite with the element oxygen
which exists in the air in the form of a gas, and a chemical
compound is formed which we call carbon dioxid. This compound is a
colorless gas. This element oxygen enters the vent of the stove and
the compound carbon dioxid passes off through the chimney. If there
is any smoke, it is due to small particles of unburned carbon or
other colored substances.

"As a rule more or less sulfur is contained in coal, wood, and other
organic matter, and this also is burned to sulfur dioxid and carried
into the air, from which it is brought back to the soil in rain in
ample amounts to supply all of the sulfur required by plants.

"Everywhere over the earth the atmosphere contains some carbon
dioxid and this compound furnishes all agricultural plants their
necessary supply of both carbon and oxygen. In other words, these
are the two elements that plants secure from the air. The gas,
carbon dioxid, passes into the plant through the breathing pores on
the under side of the leaves. These are microscopic openings but
very numerous. A square inch of a corn leaf may have a hundred
thousand breathing pores."

"Now, as we go on, I am especially anxious to get at this question
of supply and demand," said Mr. Thornton. "I think I understand
about iron and sulfur, and also that these two elements, carbon and
oxygen, are both contained in the air in the compound called carbon
dioxid, and that this must supply our crops with those two elements