ed nor
destroyed, it is evident that it may assume many different forms. Thus
the falling water may turn the electric generator and produce a current
of electricity. The energy lost by the falling water is thus transformed
into the energy of the electric current. This in turn may be changed
into the energy of motion, as when the current is used for propelling
the cars, or into the energy of heat and light, as when it is used for
heating and lighting the cars. Again, the energy of coal may be
converted into energy of heat and subsequently of motion, as when it is
used as a fuel in steam engines.

Since the energy possessed by coal only becomes available when the coal
is made to undergo a chemical change, it is sometimes called _chemical
energy_. It is this form of energy in which we are especially interested
in the study of chemistry.

~Matter.~ Matter may be defined as that which occupies space and possesses
weight. Like energy, matter may be changed oftentimes from one form into
another; and since in these transformations all the other physical
properties of a substance save weight are likely to change, the inquiry
arises, Does the weight also change? Much careful experimenting has
shown that it does not. The weight of the products formed in any change
in matter always equals the weight of the substances undergoing change.

~Law of conservation of matter.~ The important truth just stated is
frequently referred to as the law of conservation of matter, and this
law may be briefly stated thus: _Matter can neither be created nor
destroyed, though it can be changed from one form into another._

~Classification of matter.~ At first sight there appears to be no limit to
the varieties of matter of which the world is made. For convenience in
study we may classify all these varieties under three heads, namely,
_mechanical mixtures_, _chemical compounds_, and _elements_.

[Illustration: Fig. 1]

~Mechanical mixtures.~ If equal bulks of common salt and iron filings are
thoroughly mixed together, a product is obtained which, judging by its
appearance, is a new substance. If it is examined more closely, however,
it will be seen to be merely a mixture of the salt and iron, each of
which substances retains its own peculiar properties. The mixture tastes
just like salt; the iron particles can be seen and their gritty
character detected. A magnet rubbed in the mixture draws out the iron
just as if the salt were not there. On the o