As we can
(in the present state of our knowledge) divide no farther, we call these
ultimate fragments of simple or elementary substance _atoms_.
[Footnote 1: As to the possibility of _indefinite_ subdivision of
matter, see Sir W. Thomsons's lecture, _Nature_, June, 1883, _et seq._]
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.
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