240 HISTORY OF CHEMISTIIY [LKCT. xn.

not regard the conception of poly atomicity in the way in which
it had been advanced by Williamson, otherwise he could not
have inquired why amid-acids with a monobasic radical do not
also exist, if these are referred, as was done by Gerhardt and
Kekule, to the type NH3 -h H20.56 There can, consequently,
be no discussion as to whether Kolbe had not recognised
the quadrivalence of carbon before Kekule. Even although
the former rendered unquestionable services with respect to
the origination of constitutional or, as Butlerow calls them,
structural formulae, still his participation in the develop-
ment of the notions as to the atomicity (valency) of the
elements and radicals is not of importance, because, as I
believe, he did not distinguish between molecule, atom, and
equivalent, and also, as follows from the foregoing, because
he had not then grasped the idea of the part played by the
polyatomic radicals in holding the molecule together.

The doctrine of valency was possible, and was bound to
ensue as soon as atom and equivalent were separated from
each other. If the atoms were not equivalent, the question as
to the valency of one when referred to another mast necessarily
arise. Consequently, those who first distinguished the two
ideas from each other, took the first step towards considera-
tions of atomicity, and Dumas, Liebig, and Laurent, must
be mentioned in this connection. Whilst the different
valency of the elements was recognised by means of the
phenomena of substitution, the theory of polybasic acids led
to the conception of the polyatomic radicals. Both views
remain side by side for a long time without exercising any
important influence upon each other, until a fusion of the two
took place at the hands of Kekule ; that is, until the valency
of the radicals was explained by that of the elements.

We have already seen in the preceding lecture67 how
Williamson was led to advance the conception of the poly-
atomic radicals. He employed it to explain the formation of
chemical compounds, inasmuch as the polybasic radicals

w Armalen. 113, 324, w Compare p. 214.