these compounds contain not more than three or four elements, of which
carbon is the chief; but their number is very great, and the diversity
of their physical and chemical properties is astonishing. The
ascertainment of the proportion of each element in these compounds
affords little or no help towards accounting for their diversities;
widely different bodies being often very similar, or even identical,
in that respect. And, in the last case, that of _isomeric_ compounds,
the appeal to diversity of arrangement of the identical component
units was the only obvious way out of the difficulty. Here, again,
hypothesis proved to be of great value; not only was the search for
evidence of diversity of molecular structure successful, but the study
of the process of taking to pieces led to the discovery of the way to
put together; and vast numbers of compounds, some of them previously
known only as products of the living economy, have thus been
artificially constructed. Chemical work, at the present day, is, to a
large extent, synthetic or creative--that is to say, the chemist
determines, theoretically, that certain non-existent compounds ought
to be producible, and he proceeds to produce them.

It is largely because the chemical theory and practice of our epoch
have passed into this deductive and synthetic stage, that they are
entitled to the name of the 'New Chemistry' which they commonly
receive. But this new chemistry has grown up by the help of
hypotheses, such as those of Dalton and of Avogadro, and that
singular conception of 'bonds' invented to colligate the facts of
'valency' or 'atomicity,' the first of which took some time to make
its way; while the second fell into oblivion, for many years after it
was propounded, for lack of empirical justification. As for the third,
it may be doubted if anyone regards it as more than a temporary
contrivance.