The Advance of Science in the Last Half-Century | Page 9

material substance, Democritic
atomism is at an end and Cartesian continuity takes its place.
[Sidenote: The real value of hypothesis; it predicates the existence of
units of matter.]
The real value of the new atomic hypothesis, however, did not lie in the
two points which Democritus and his followers would have considered
essential--namely, the indivisibility of the 'atoms' and the presence of
an interatomic vacuum--but in the assumption that, to the extent to
which our means of analysis take us, material bodies consist of definite
minute masses, each of which, so far as physical and chemical
processes of division go, may be regarded as a unit--having a
practically permanent individuality. Just as a man is the unit of
sociology, without reference to the actual fact of his divisibility, so
such a minute mass is the unit of physico-chemical science--that
smallest material particle which under any given circumstances acts as
a whole.[F]
The doctrine of specific heat originated in the eighteenth century. It
means that the same mass of a body, under the same circumstances,
always requires the same quantity of heat to raise it to a given
temperature, but that equal masses of different bodies require different
quantities. Ultimately, it was found that the quantities of heat required
to raise equal masses of the more perfect gases, through equal ranges of

temperature, were inversely proportional to their combining weights.
Thus a definite relation was established between the hypothetical units
and heat. The phenomena of electrolytic decomposition showed that
there was a like close relation between these units and electricity. The
quantity of electricity generated by the combination of any two units is
sufficient to separate any other two which are susceptible of such
decomposition. The phenomena of isomorphism showed a relation
between the units and crystalline forms; certain units are thus able to
replace others in a crystalline body without altering its form, and others
are not.
Again, the laws of the effect of pressure and heat on gaseous bodies,
the fact that they combine in definite proportions by volume, and that
such proportion bears a simple relation to their combining weights, all
harmonised with the Daltonian hypothesis, and led to the bold
speculation known as the law of Avogadro--that all gaseous bodies,
under the same physical conditions, contain the same number of units.
In the form in which it was first enunciated, this hypothesis was
incorrect--perhaps 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 Linnæus were to zoölogy 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