and the
wood is transformed slowly.

A form of charcoal known as animal charcoal, or bone black, is
obtained from the charred remains of animals rather than plants, and
may be prepared by burning bones and animal refuse as in the case of
the wood.

Destructive Distillation. When wood is burned without sufficient
air, it is changed into soft brittle charcoal, which is very different
from wood. It weighs only one fourth as much as the original wood. It
is evident that much matter must leave the wood during the process of
charcoal making. We can prove this by putting some dry shavings in a
strong test tube fitted with a delivery tube. When the wood is heated
a gas passes off which we may collect and burn. Other substances also
come off in gaseous form, but they condense in the water. Among these
are wood alcohol, wood tar, and acetic acid. In the older method of
charcoal making all these products were lost. Can you give any uses of
these substances?

54. Matter and Energy. When wood is burned, a small pile of ashes is
left, and we think of the bulk of the wood as destroyed. It is true we
have less matter that is available for use or that is visible to
sight, but, nevertheless, no matter has been destroyed. The matter of
which the wood is composed has merely changed its character, some of
it is in the condition of ashes, and some in the condition of
invisible gases, such as carbon dioxide, but none of it has been
destroyed. It is a principle of science that matter can neither be
destroyed nor created; it can only be changed, or transformed, and it
is our business to see that we do not heedlessly transform it into
substances which are valueless to us and our descendants; as, for
example, when our magnificent forests are recklessly wasted. The
smoke, gases, and ashes left in the path of a raging forest fire are
no compensation to us for the valuable timber destroyed. The sum total
of matter has not been changed, but the amount of matter which man can
use has been greatly lessened.

The principle just stated embodies one of the fundamental laws of
science, called the law of the _conservation of matter_.

A similar law holds for energy as well. We can transform electric
energy into the motion of trolley cars, or we can make use of the
energy of streams to turn the wheels of our mills, but in all these
cases we are transforming, not creating, energy.

When a ball is fired from a rifle, most of the energy of the gunpowde