240 HISTORY OF CHEMISTRY. [LECTURE XII. 



not regard the conception of polyatomicity 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 NH 3 + H. 2 O. 5G 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, struc- 

 tural formulae, still his participation in the development 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 must necessarily 

 arise. Consequently, those who first distinguished the two 

 ideas from each other, took the first step towards considerations 

 of atomicity, and Dumas, Liebig, and Laurent must be men- 

 tioned in this connection. Whilst the different valency of the 

 elements was recognised by means of the phenomena of substi- 

 tution, 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 lecture 57 how 

 Williamson was led to advance the conception of the polyatomic 

 radicals. He employed it to explain the formation of chemical 

 compounds, inasmuch as the polybasic radicals possess the 



56 Annalen. 113, 324. 57 Compare p. 214. 



