transformation. Most of these efforts were directed
toward changing the commoner metals into gold, and many fanciful ways
for doing this were described. The chemists of that time were called
_alchemists_, and the art which they practiced was called _alchemy_. The
alchemists gradually became convinced that the only way common metals
could be changed into gold was by the wonderful power of a magic
substance which they called the _philosopher's stone_, which would
accomplish this transformation by its mere touch and would in addition
give perpetual youth to its fortunate possessor. No one has ever found
such a stone, and no one has succeeded in changing one metal into
another.

~Number of elements.~ The number of substances now considered to be
elements is not large--about eighty in all. Many of these are rare, and
very few of them make any large fraction of the materials in the
earth's crust. Clarke gives the following estimate of the composition of
the earth's crust:

Oxygen 47.0% Calcium 3.5%
Silicon 27.9 Magnesium 2.5
Aluminium 8.1 Sodium 2.7
Iron 4.7 Potassium 2.4
Other elements 1.2%

A complete list of the elements is given in the Appendix. In this list
the more common of the elements are marked with an asterisk. It is not
necessary to study more than a third of the total number of elements to
gain a very good knowledge of chemistry.

~Physical state of the elements.~ About ten of the elements are gases at
ordinary temperatures. Two--mercury and bromine--are liquids. The others
are all solids, though their melting points vary through wide limits,
from caesium which melts at 26 deg. to elements which do not melt save in the
intense heat of the electric furnace.

~Occurrence of the elements.~ Comparatively few of the elements occur as
uncombined substances in nature, most of them being found in the form of
chemical compounds. When an element does occur by itself, as is the case
with gold, we say that it occurs in the _free state_ or _native_; when
it is combined with other substances in the form of compounds, we say
that it occurs in the _combined state_, or _in combination_. In the
latter case there is usually little about the compound to suggest that
the element is present in it; for we have seen that elements lose their
own peculiar properties when they enter into combination with othe