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 sixteen oxygen, or a multiple of fourteen nitrogen combine
with sixteen oxygen, and so on.

See now where we have got to. When we had spoken of a tiny fragment of
primal matter--a drop of water, for instance--it seemed as if there was
no more to be said; but no, we found ourselves able to give a whole
history of the molecules of which the substance consists; and when we
had considered the molecule, we found a further beautiful and intricate
order of _atoms_ inside the molecule, as it were.

And there is no reason to suppose that science has yet revealed all that