the mixture is placed in a dry test tube and
carefully heated in the flame of a Bunsen burner, as shown in Fig. 3, a
striking change takes place. The mixture begins to glow at some point,
the glow rapidly extending throughout the whole mass. If the test tube
is now broken and the product examined, it will be found to be a hard,
black, brittle substance, in no way recalling the iron or the sulphur.
The magnet no longer attracts it; carbon disulphide will not dissolve
sulphur from it. It is a new substance with new properties, resulting
from the chemical union of iron and sulphur, and is called iron
sulphide. Such substances are called _chemical compounds_, and differ
from mechanical mixtures in that the substances producing them lose
their own characteristic properties. We shall see later that the two
also differ in that the composition of a chemical compound never varies.

[Illustration: Fig. 3]

DEFINITION: _A chemical compound is a substance the constituents of
which have lost their own characteristic properties, and which cannot be
separated save by a chemical change._

~Elements.~ It has been seen that iron sulphide is composed of two
entirely different substances,--iron and sulphur. The question arises,
Do these substances in turn contain other substances, that is, are they
also chemical compounds? Chemists have tried in a great many ways to
decompose them, but all their efforts have failed. Substances which have
resisted all efforts to decompose them into other substances are called
_elements_. It is not always easy to prove that a given substance is
really an element. Some way as yet untried may be successful in
decomposing it into other simpler forms of matter, and the supposed
element will then prove to be a compound. Water, lime, and many other
familiar compounds were at one time thought to be elements.

DEFINITION: _An element is a substance which cannot be separated into
simpler substances by any known means._

~Kinds of matter.~ While matter has been grouped in three classes for the
purpose of study, it will be apparent that there are really but two
distinct kinds of matter, namely, compounds and elements. A mechanical
mixture is not a third distinct kind of matter, but is made up of
varying quantities of either compounds or elements or both.

~Alchemy.~ In olden times it was thought that some way could be found to
change one element into another, and a great many efforts were made to
accomplish this