416 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



Dumas acknowledged that Laurent had been the first 

 to suggest that chlorine not merely took the place but played 

 the part of hydrogen (Ann. Chim. Phys., 1857,49, 496). 

 Laurent claimed further (Comptes rendus, 1840, 10, 412) that 

 he had anticipated in all but the name Dumas's discovery of 

 the permanence of types. He had recognised from the first 

 that the law of substitutions was often incorrect ; when it 

 happened to be true this was simply because of the strong 

 tendency of the radical attacked by the chlorine to conserve 

 its type, and not to any inherent accuracy of the law. These 

 stable groupings of atoms he had described in his Thesis 

 (1837) as " fundamental radicals " before substitution, and as 

 " derived radicals " after substitution (ibid. 412 and 416) ; in 

 a book published after mYdeath they are described as FUNDA- 

 MENTAL NUCLEI and as DERIVED NUCLEI (Chemical Methods, 

 tr. Odling, 1854, p. 195). To render his theory more 

 intelligible, he translated his idea into a geometrical figure : 



" Let us imagine a four-sided prism, of which the eight 

 angles are occupied by eight atoms of carbon, and the 

 centres of the twelve edges by twelve atoms of hydrogen. 

 Let us call this prism \.\\Qform or fundamental nucleus, and 

 let us represent it by C 8 H 12 . 



" Let us suppose that chlorine, put in presence of this 

 simple prism, removes one of the edges or hydrogen atoms ; 

 the prism deprived of this edge would be destroyed, unless 

 it were supplied with some other edge, whether of chlorine, 

 bromine, zinc, etc. ; no matter what the nature of the edge,- 

 provided it succeeds in maintaining the equilibrium of the 

 other edges and angles. Thus will be formed a new or 

 derived nucleus similar to the preceding, and of which the 

 form may be represented by C 8 (H 11 C1) " (Chemical Method, 

 1854, 195 : compare Comptes rendus, 1840, 10, 416, where 

 a similar but more complex figure is described in an extract 

 from his Thesis, 1837). 



Addition-products could be derived from the fundamental 

 nucleus by adding pyramids to the ends of the prisms, to 



