The habit of extending analogies beyond their legitimate application
was a source of confusion in the early ages of science. Most of the
superstitions of primitive religion, of astrology, and of alchemy,
arose from this source. A good example is the extension of the
metaphor in the words _generation_ and _corruption_: words
in constant use in scientific works until the nineteenth century
began. Generation is the production of a substance that before was
not, and corruption is the destruction of a substance, by its ceasing
to be what it was before. Thus, fire is generated, and wood is
corrupted, when the latter is burnt. But the implicit metaphor in the
use of the terms likens substances to the human body, their production
and destruction implies liability to disease, and thus prepares the
way for the notion of the elixir, which is first a potion giving long
life, and curing bodily ailments, and only after some time a remedy
for diseased metals--the philosopher's stone.

It will be seen that the theory of the mediaeval alchemist was that
matter is an entity filling all space, on which in different places
different forms were impressed. The elements were a preliminary
grouping of these, and might be present--two, three, or four at a
time--in any substance. No attempt was ever made to separate these
elements by scientific men, just as no attempt is ever made to isolate
the ether of the physical speculations of to-day. The theory of modern
physicists, with its ether and vortices, answers almost exactly to the
matter and form of the ancients, the nature of the vortices
conditioning matter.

The extracts from Book XI. bring us to another class of substances.
All compound bodies are classified as imperfect or perfect. Imperfect