before Christ formed a theory of the transformations of matter, which
is essentially the theory held by naturalists to-day.

These philosophers taught that to understand nature we must get
beneath the superficial qualities of things. "According to
convention," said Democritus (born 460 B.C.), "there are a sweet and a
bitter, a hot and a cold, and according to convention there is
colour. In truth there are atoms and a void." Those investigators
attempted to connect all the differences which are observed between
the qualities of things with differences of size, shape, position, and
movement of atoms. They said that all things are formed by the
coalescence of certain unchangeable, indestructible, and impenetrable
particles which they named atoms; the total number of atoms is
constant; not one of them can be destroyed, nor can one be created;
when a substance ceases to exist and another is formed, the process is
not a destruction of matter, it is a re-arrangement of atoms.

Only fragments of the writings of the founders of the atomic theory
have come to us. The views of these philosophers are preserved, and
doubtless amplified and modified, in a Latin poem, _Concerning the
Nature of Things_, written by Lucretius, who was born a century before
the beginning of our era. Let us consider the picture given in that
poem of the material universe, and the method whereby the picture was
produced.[2]

[2] The quotations from Lucretius are taken from Munro's
translation (4th Edition, 1886).

All knowledge, said Lucretius, is based on "the aspect and the law of
nature." True knowledge can be obtained only by the use of the senses;