quoted. For instance, when that alchemist tells us, "Fire is the most
passive of all elements, and resembles a chariot; when it is drawn, it
moves; when it is not drawn, it stands still"--we may suppose he meant
to express the fact that a vast number of substances can be burnt, and
that combustion does not begin of itself, but requires an external
agency to start it.

Unfortunately, most of the terms which the alchemists used to
designate their Elements and Principles are terms which are now
employed to designate specific substances. The word _fire_ is still
employed rather as a quality of many things under special conditions,
than as a specific substance; but _earth_, _water_, _air_, _salt_,
_sulphur_, and _mercury_, are to-day the names applied to certain
groups of properties, each of which is different from all other groups
of properties, and is, therefore, called, in ordinary speech, a
definite kind of matter.

As knowledge became more accurate and more concentrated, the words
_sulphur_, _salt_, _mercury_, &c., began to be applied to distinct
substances, and as these terms were still employed in their alchemical
sense as compendious expressions for certain qualities common to great
classes of substances, much confusion arose. Kunckel, the discoverer
of phosphorus, who lived between 1630 and 1702, complained of the
alchemists' habit of giving different names to the same substance, and
the same name to different substances. "The sulphur of one," he says,
"is not the sulphur of another, to the great injury of science. To
that one replies that everyone is perfectly free to baptise his infant
as he pleases. Granted. You may if you like call an ass an ox, but you
will never make anyone believe that your ox is an ass." Boyle is very
severe on the vague and loose use of words practised by so many