Every substance, however finely divided into molecules, if it is not a
simple substance, must therefore have, inside the _molecular_ structure,
a further _atomic_ structure. And in the case of unresolvable or
"elementary" substance, the molecule and the atom are not necessarily
the same. For though there is reason to believe that, the molecule of
these does consist, in some cases, of only one atom--in which case the
atom and the molecule are identical; in other cases, the molecule is
known to consist of more than one atom of the same element; and the
atoms are capable of being differently arranged, and when so arranged
have different _properties_ or behaviour, though their nature is not
changed. This property is spoken of by chemists as _allotropism_. No
chemist on earth can detect the slightest difference in _constitution_
between a molecule of _ozone_ and one _oxygen_; but the two have widely
different properties, or behave very differently. There is thus a great
mystery about atoms and their possible differences under different
arrangement, which is as yet unsolved. Those who wish to get an insight
into the matter (which cannot be pursued farther here) will do well to
read Josiah Cooke's "The New Chemistry," in the International Scientific
Series. The mind is really lost in trying to realize the idea of a
fragment of matter too small for the most powerful microscope, but
existing in fact (because of faultless reasoning from absolutely
conclusive experiments), and yet so constituted that it is
_practically_ a different thing when placed in one position or order,
from what it is when placed in another.

Turning from this mystery, as yet so obscure, to what is more easily
grasped, we shall hardly be surprised to learn, further, that every kind
of, atom obeys its own laws, and that while atoms of one kind always
have a _tendency to combine_ with atoms of other kinds, it is absolutely