it is not exactly true in any form; but it is
hardly too much to say that chemistry and molecular physics would
never have advanced to their present condition unless it had been
assumed to be true. Another immense service rendered by Dalton, as a
corollary of the new atomic doctrine, was the creation of a system of
symbolic notation, which not only made the nature of chemical
compounds and processes easily intelligible and easy of recollection,
but, by its very form, suggested new lines of inquiry. The atomic
notation was as serviceable to chemistry as the binomial nomenclature
and the classificatory schematism of Linnaeus were to zooelogy and
botany.

[Sidenote: In biology a like theory of molecularstructure.]

Side by side with these advances arose in another, which also has a
close parallel in the history of biological science. If the unit of a
compound is made up by the aggregation of elementary units, the notion
that these must have some sort of definite arrangement inevitably
suggests itself; and such phenomena as double decomposition pointed,
not only to the existence of a molecular architecture, but to the
possibility of modifying a molecular fabric without destroying it, by
taking out some of the component units and replacing them by others.
The class of neutral salts, for example, includes a great number of
bodies in many ways similar, in which the basic molecules, or the acid
molecules, may be replaced by other basic and other acid molecules
without altering the neutrality of the salt; just as a cube of bricks
remains a cube, so long as any brick that is taken out is replaced by
another of the same shape and dimensions, whatever its weight or other
properties may be. Facts of this kind gave rise to the conception of
'types' of molecular structure, just as the recognition of the unity
in diversity of the structure of the species of plants and animals
gave rise to the notion of biological 'types.' The notation of
chemistry enabled these ideas to be represented with precision; and
they acquired an immense importance, when the improvement of methods
of analysis, which took place about the beginning of our period,
enabled the composition of the so-called 'organic' bodies to be
determined with, rapidity and precision.[G] A large proportion of
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