116 Dr. E. J. Mills on the Atomic Theory. 



C 4 H 4 Br 2 O 4 , and three dibromo-pyrotartrates (C 5 H 6 Br 2 O 4 ) are 

 formed. We have therefore to explain how it is that these facts, 

 assuming them to be true, are compatible with each other. Ke- 

 kule proceeds as follows : — 



" In accordance with the views relating to the atomicity of the 

 elements which we have previously unfolded on several occasions 

 (see especially § 1369), all the affinities of the atoms comprising 

 the molecule of succinic (or its homologue pyrotartaric) acid are 

 saturated ; these acids constitute, in a manner, closed molecules. 

 They contain two atoms of typical oxygen, that is, oxygen bound 

 on to carbon by only one of its two units of affinity. Two hy- 

 drogen atoms are united with the carbon only through the inter- 

 vention of these typical oxygen atoms. These two typical 

 hydrogen atoms are easily displaceable by metals ; but there are 

 two other oxygen atoms present which are attached to the carbon 

 by two units of affinity, aud consequently, in the language of the 

 type theory, belong to the radical. 



" Now, it is easy to see that, in addition to these two typical 

 hydrogen atoms, succinic has four, and pyrotartaric acid six> 

 more atoms of hydrogen. This hydrogen, which in the language 

 of type theory belongs to the radical, is, according to the theory 

 of the atomicity of the elements, directly combined with the 

 carbon — in fact, in such a manner that two hydrogen atoms are 

 always united with the same carbon atom. 



" Next, let us assume that two such hydrogen atoms are absent 

 (fehlen) from one or other of these two normal acids; we have, 

 on the one hand, the composition of fumaric and maleic acid, 

 the formula? of itaconic, citraconic, and mesaconic acid on the 

 other. But since in succinic acid two pairs of such hydrogen 

 atoms are bound to the carbon, two acids may evidently exist with 

 less hydrogen ; and similarly, in the case of pyrotartaric acid, we 

 can understand how there may be three isomers with less hydro- 

 gen, according to the absence of one or other of the three pairs 

 of hydrogen atoms, which in #he normal substance are directly 

 united with the carbon. 



"At that place in the molecule whence the two hydrogen atoms 

 are absent, two units of affinity of the carbon are left unsaturated ; 

 at that place there is, so to speak, a gap. Hence we can explain 

 the exceptional facility with which these substances unite, by way 

 of addition, with hydrogen or bromine. The free units of affi- 

 nity of the carbon make an effort to saturate themselves, and so 

 to fill up the gap. 



" If hydrogen be introduced into these unoccupied spaces, all 

 carbon atoms within the molecule are united to the same ele- 

 ment — hydrogen; there appears no ground for the existence 

 of differently modified normal substances as so obtained. In 



