nto
organic compounds, hydrogen can link itself with only a single bond to
any other element--it has, so to speak, but a single hand with which
to grasp--while oxygen has capacity for two bonds, nitrogen for
three (possibly for five), and carbon for four. The words monovalent,
divalent, trivalent, tretrava-lent, etc., were coined to express this
most important fact, and the various elements came to be known as
monads, diads, triads, etc. Just why different elements should differ
thus in valency no one as yet knows; it is an empirical fact that they
do. And once the nature of any element has been determined as regards
its valency, a most important insight into the possible behavior of that
element has been secured. Thus a consideration of the fact that hydrogen
is monovalent, while oxygen is divalent, makes it plain that we
must expect to find no more than three compounds of these two
elements--namely, H--O--(written HO by the chemist, and called
hydroxyl); H--O--H (H2O, or water), and H--O--O--H (H2O2, or hydrogen
peroxide). It will be observed that in the first of these compounds the
atom of oxygen stands, so to speak, with one of its hands free, eagerly
reaching out, therefore, for another 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