ve 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.

It is only certain kinds of atoms that will combine at all with other
kinds; and when they do so combine, they will only unite in absolutely
fixed proportions, so that chemists have been able to assign to every
kind of element its own combining proportion. The substances that will
combine will do so in these proportions, or in proportions of any _even
multiple_ of the number, and in no other. Thus fourteen parts of
nitrogen will combine with sixteen of oxygen; and we have several
substances in nature, called nitrous oxide, nitric oxide, nitric
di-oxide, &c., which illustrate this, in which fourteen parts of
nitrogen combine with sixteen oxygen or fourteen nitrogen with a
multiple of s