have a natural cycle which is amply sufficient to meet all
requirements for these elements as plant food, other elements have
no such cycle, and it is the chief business of the farmer to make
these elements circulate.

"Take carbon, for example. This element is well represented by hard
coal. Soft coal and charcoal are chiefly carbon. The diamond is pure
crystallized carbon, and charcoal made from pure sugar is pure,
uncrystallized carbon. This can easily be made by heating a lump of
sugar on a red hot stove until only a black coal remains. Now these
different solid materials represent carbon in the elemental form or
free state. But carbon may unite with other elements to form
chemical compounds, and these may be solids, liquids, gases.

"Thus carbon and sulfur are both solid elements, one black and the
other yellow, as generally found. If these two elements are mixed
together under ordinary conditions no change occurs. The result is
simply a mixture of carbon and sulfur. But, if this mixture is
heated in a retort which excludes the air, the carbon and sulfur
unite into a chemical compound called carbon disulfid. This compound
is neither black, yellow, nor solid; but it is a colorless, limpid
liquid; and yet it contains absolutely nothing except carbon and
sulfur."

"That seems strange," remarked Mr. Thornton. "Yes, but similar
changes are going on about us all the time," replied Percy. "We put
ten pounds of solid black coal in the stove and an hour later we
find nothing there, except a few ounces of ashes which represent the
impurities in the coal."