same things that must be understood in either chemical or
alchemical laboratories. The general knowledge that certain
liquids vaporize at lower temperatures than others, and that the
melting-points of metals differ greatly, for example, was just as
necessary to alchemy as to chemistry. The knowledge of the gross
structure, or nature, of materials was much the same to the
alchemist as to the chemist, and, for that matter, many of the
experiments in calcining, distilling, etc., were practically
identical.

To the alchemist there were three principles--salt, sulphur, and
mercury--and the sources of these principles were the four
elements--earth, water, fire, and air. These four elements were
accountable for every substance in nature. Some of the
experiments to prove this were so illusive, and yet apparently so
simple, that one is not surprised that it took centuries to
disprove them. That water was composed of earth and air seemed
easily proven by the simple process of boiling it in a
tea-kettle, for the residue left was obviously an earthy
substance, whereas the steam driven off was supposed to be air.
The fact that pure water leaves no residue was not demonstrated
until after alchemy had practically ceased to exist. It was
possible also to demonstrate that water could be turned into fire
by thrusting a red-hot poker under a bellglass containing a dish
of water. Not only did the quantity of water diminish, but, if a
lighted candle was thrust under the glass, the contents ignited
and burned, proving, apparently, that water had been converted
into fire. These, and scores of other similar experiments, seemed
so easily explained, and to accord so well with the "four
elements" theory, that they were seldom questioned until a later