of one kind are identical in every respect.

We now know that many compounds exist which are formed by the union of
the same quantities by weight of the same elements, and, nevertheless,
differ in properties; modern chemistry explains this fact by saying
that the properties of a substance depend, not only on the kind of
atoms which compose the minute particles of a compound, and the number
of atoms of each kind, but also on the mode of arrangement of the
atoms.[3] The same doctrine was taught by Lucretius, two thousand
years ago. "It often makes a great difference," he said, "with what
things, and in what positions the same first-beginnings are held in
union, and what motions they mutually impart and receive." For
instance, certain atoms may be so arranged at one time as to produce
fire, and, at another time, the arrangement of the same atoms may be
such that the result is a fir-tree. The differences between the
colours of things are said by Lucretius to be due to differences in
the arrangements and motions of atoms. As the colour of the sea when
wind lashes it into foam is different from the colour when the waters
are at rest, so do the colours of things change when the atoms whereof
the things are composed change from one arrangement to another, or
from sluggish movements to rapid and tumultuous motions.

[3] See the chapter _Molecular Architecture_ in the _Story of
the Chemical Elements_.

Lucretius pictured a solid substance as a vast number of atoms
squeezed closely together, a liquid as composed of not so many atoms
less tightly packed, and a gas as a comparatively small number of
atoms with considerable freedom of motion. Essentially the same
picture is presented by the molecular theory of to-day.