are little prone to do. It seems that they are fickle to the last
degree in their individual attachments, and are as prone to break
away from bondage as they are to enter into it. Thus the oxygen
atom which has just flung itself into the circuit of two hydrogen
atoms, the next moment flings itself free again and seeks new
companions. It is for all the world like the incessant change of
partners in a rollicking dance. This incessant dissolution and
reformation of molecules in a substance which as a whole remains
apparently unchanged was first fully appreciated by Ste.-Claire
Deville, and by him named dissociation. It is a process which
goes on much more actively in some compounds than in others, and
very much more actively under some physical conditions (such as
increase of temperature) than under others. But apparently no
substances at ordinary temperatures, and no temperature above the
absolute zero, are absolutely free from its disturbing influence.
Hence it is that molecules having all the valency of their atoms
fully satisfied do not lose their chemical activity--since each
atom is momentarily free in the exchange of partners, and may
seize upon different atoms from its former partners, if those it
prefers are at hand.

While, however, an appreciation of this ceaseless activity of the
atom is essential to a proper understanding of its chemical
efficiency, yet from another point of view the "saturated"
molecule--that is, the molecule whose atoms have their valency
all satisfied--may be thought of as a relatively fixed or stable
organism. Even though it may presently be torn down, it is for
the time being a completed structure; and a consideration of the
valency of its atoms gives the best clew that has hitherto been
obtainable as to the character of its architecture. How