xvii MOLECULAR ARCHITECTURE 413 



all the hydrogen of these substances is replaced by chlorine 

 in equal volume, without changing their fundamental 

 character, we must conclude : 



" That in organic chemistry there exist certain TYPES which 

 persist even ivhen in place of the hydrogen which they contain 

 one introduces equal volumes of chlorine, of bromine or of iodine" 



"That is to say that the theory of substitutions rests upon 

 facts, and on the most striking facts of organic chemistry " 

 (Comptes rendus, 1839, 8, 620-622; also Ann. Chem. 

 Pharm., 1839, 32, 117-119). 



Dumas (1840) on chemical and mechanical types, In a 



fuller memoir " On the Law of Substitutions and the Theory 

 of Types " {Comptes rendus, 1840, 10, 149-178 ; Ann. Chem. 

 Pharm., 1840, 33, 259-300), Dumas distinguished as belong- 

 ing to the same CHEMICAL TYPE " substances which contain 

 the same number of equivalents, united in the same way and 

 showing the same fundamental chemical properties" whilst, 

 following Regnault, he included under the heading of 

 MECHANICAL TYPES " substances having the same formula, 

 produced by substitution, but essentially different in their most 

 salient chemical properties" (Comptes rendus, 1840, 10, 158, 

 162). In the latter class he included a large number of cases 

 of oxidation, in which a neutral substance was converted 

 into an acid by substituting oxygen for hydrogen, etc. In 

 the same paper the system of naming substitution-products 

 by means of the prefix CHLORO-, etc., was formally set out 

 ( Comptes rendus, 10, 169). In both memoirs ( Comptes rendus, 

 8, 621 ; 10, 169) Dumas referred to the analogy between sub- 

 stitution without change of type in organic chemistry and 

 isomorphism in inorganic chemistry. The two phenomena 

 were equally fatal to Berzelius's theories, since, as Liebig 

 pointed out (Ann. Chem. Pharm., 1839, 31, 119), the replace- 

 ment of the hydrogen of acetic acid by chlorine without 

 change of type was no more remarkable than the replacement 

 of the manganese of the permanganates by chlorine in the 

 isomorphous perchlorates. Liebig, however, regarded both 

 cases as exceptions, rather than as examples of a general 



