f light it is decomposed, the oxygen is sent off to make more carbonic
acid, and the carbon is retained to form a part of the tree. So long as
that tree exists in the form of wood, the carbon will remain unaltered,
but when the wood decays, or is burned, it immediately takes the form of
carbonic acid, and mingles with the atmosphere ready to be again taken
up by plants, and have its carbon deposited in the form of vegetable
matter.

[Give an instance of such change.

How do plants and animals benefit each other?

Describe the experiment with the glass tube.]

The blood of animals contains carbon derived from their food. This
unites with the oxygen of the air drawn into the lungs and forms
carbonic acid. Without this process, animals could not live. Thus, while
by the natural operation of breathing, they make carbonic acid for the
uses of the vegetable world, plants, in taking up carbon, throw off
oxygen to keep up the life of animals. There is perhaps no way in which
we can better illustrate the changes of form in carbon than by
describing a simple experiment.

Take a glass tube filled with oxygen gas, and put in it a lump of
charcoal, cork the ends of the tube tightly, and pass through the corks
the wires of an electrical battery. By passing a stream of electrical
fluid over the charcoal it may be ignited, when it will burn with great
brilliancy. In burning it is dissolved in the oxygen forming carbonic
acid, and disappears. It is no more lost, however, than is the carbon of
wood which is burned in a stove; although invisible, it is still in the
tube, and may be detected by careful weighing. A more satisfactory proof
of its presence may be obtained by _decomposing_ the carbonic acid by
drawing the wires a short distance apart, and giving a _spark_ of
electricity. This immediately separates the oxygen from the carbon which
forms a dense black smoke in the tube. By pushing the corks together we
may obtain a wafer of charcoal of the same weight as the piece
introduced. In this experiment we have changed carbon from its solid
form to an invisible gas and back again to a solid, thus fully
representing the continual changes of this substance in the destruction
of organic matter and the growth of plants.

CHAPTER III.

HYDROGEN, OXYGEN AND NITROGEN.

HYDROGEN AND OXYGEN.

[What is water composed of?

If analyzed, what does it yield?

How do plants obtain their hydrogen and oxygen?]

Let us now consider