almost inconceivably small residue in the receptacle has no perceptible
effect on the action of a small quantity of any substance already
reduced to the form of gas or vapour introduced into it. Dr. W. Crookes
has made many beautiful experiments on the behaviour of the molecules of
attenuated matter in _vacua_. The small quantity of vapour introduced
contains only a relatively small number of molecules, which thus freed
from all sensible restraint within the limits of the glass vessel used,
are free to move as they will; they are observed to rush about, to
strike against the sides of the vessel, and under proper conditions to
shine and become _radiant_, and to exhibit extraordinary phenomena when
subjected to currents of electricity. So peculiar is the molecular
action thus set up, that scientific men have been tempted to speak of a
fourth condition of matter (besides the three ordinary ones, solid,
liquid, and gaseous), which they call the ultra-gaseous or radiant state
of matter.

This marvel of molecular structure seems already to have removed us
sufficiently far from the idea of a simple inert mass, which might be
primordial and self-caused. But we have not yet done. Even imagining the
extreme subdivision[1] of the particles in one of Dr. Crookes' vacuum
globes, the particles are still water. But we know that water is a
compound substance. The molecule has nine parts, of which eight are
hydrogen and one oxygen--because that is the experimentally known
proportion in which oxygen and hydrogen combine to form water. 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
ha