plain man has long known that things consist of parts which remain,
under some circumstances, invisible. When he approaches an object from
a distance, he sees parts which he could not see before; and what
appears to the naked eye a mere speck without perceptible parts is
found under the microscope to be an insect with its full complement of
members. Moreover, he has often observed that objects which appear
continuous when seen from a distance are evidently far from continuous
when seen close at hand. As we walk toward a tree we can see the
indefinite mass of color break up into discontinuous patches; a fabric,
which presents the appearance of an unbroken surface when viewed in
certain ways may be seen to be riddled with holes when held between the
eye and the light. There is no man who has not some acquaintance with
the distinction between appearance and reality, and who does not make
use of the distinction in common life.

Nor can it seem a surprising fact that different combinations of atoms
should exhibit different properties. Have we not always known that
things in combination are apt to have different properties from the
same things taken separately? He who does not know so much as this is
not fit even to be a cook.

No, the imperceptible world of atoms and molecules is not by any means
totally different from the world of things in which the plain man
lives. These little objects and groups of objects are discussed very
much as we discuss the larger objects and groups of objects to which we
are accustomed. We are still concerned with _things_ which exist in
space and move about in space; and even if these things are small and
are not very familiarly known, no intellectual revolution is demanded
to enable a man to understand the words of the scientist who is talking
about them, and to understand as well the sort of reasonings upon which