companion, and hence, in the language of chemistry, forming an
unstable compound. Again, in the third compound, though all hands
are clasped, yet one pair links oxygen with oxygen; and this also
must be an unstable union, since the avidity of an atom for its
own kind is relatively weak. Thus the well-known properties of
hydrogen peroxide are explained, its easy decomposition, and the
eagerness with which it seizes upon the elements of other
compounds.

But the molecule of water, on the other hand, has its atoms
arranged in a state of stable equilibrium, all their affinities
being satisfied. Each hydrogen atom has satisfied its own
affinity by clutching the oxygen atom; and the oxygen atom has
both its bonds satisfied by clutching back at the two hydrogen
atoms. Therefore the trio, linked in this close bond, have no
tendency to reach out for any other companion, nor, indeed, any
power to hold another should it thrust itself upon them. They
form a "stable" compound, which under all ordinary circumstances
will retain its identity as a molecule of water, even though the
physical mass of which it is a part changes its condition from a
solid to a gas from ice to vapor.

But a consideration of this condition of stable equilibrium in
the molecule at once suggests a new question: How can an
aggregation of atoms, having all their affinities satisfied, take
any further part in chemical reactions? Seemingly such a
molecule, whatever its physical properties, must be chemically
inert, incapable of any atomic readjustments. And so in point of
fact it is, so long as its component atoms cling to one another
unremittingly. But this, it appears, is precisely what the atoms