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.