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 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 impossible to get them to combine together except on certain conditions.

The difference between combination and mixture is well known.  Shake sand and sugar in a bag for ever so long, but they will only mix, not combine or form any new substance even with the aid of electric currents; but place oxygen and hydrogen gas under proper conditions, and the gases will disappear, and water (in weight exactly equal to the weight of the volume of gases) will appear in their place.