Dr. E. J. Mills on the Atomic Theory. 115 



Resulting- symbols. Generating symbols. 



C 6 H 5 C1 + HC1 = C 6 H 6 + CI 2 , 

 C 3 H 2 C1 + HC1=C 3 H 3 + CI 2 , 

 C 2 H CI + HC1 = C 2 H 2 + CI 2 , 

 C 3 H C1 2 + 2HC1=C 3 H 3 + 2CF, 

 C 6 H C1 5 + 5HC1 = C 6 H 6 + 5C1 2 , 

 C CI + HC1 = C H + CI 2 . 



The passage from chlorine derivative to parent compound is 

 effected by the numerical exchange of CI for H, the validity of 

 which need not be disputed, though it is often much misunder- 

 stood. We then have four formulae— C 6 H 6 , C 3 H 3 , C 2 H 2 , C H 

 — between which to choose. Now the equational method, if I 

 may so term it, does not exactly choose between these, but takes 

 the least common multiple of them } and, on that ground, decides 

 on C 6 H 6 as the formula of benzol. The doctrine of replacement 

 in successive stages (as, for instance, the treble ethylation of 

 ammonia) is substantially identical in its symbolic exposition 

 with the equational method. Now this method is held to point 

 out that the " atom of benzol weighs C 6 H 6 . v Is it necessary to 

 indicate that the use of the process for finding the least common 

 multiple is not atomic, and that, if so, arithmetic should also 

 possess its atomic theory ? 



If the prejudices of education were less complete or less closely 

 intertwined than they are, I might hopefully ask atomic theorists 

 whether, as professedly inductive reasoners, they are still pleased 

 to accept these shadows for reality. My argument may have a 

 better fortune, perhaps, with those who sit loose to theory, or at 

 least will not voluntarily put on the shirt of Nessus. Accord- 

 ingly, it may be advantageous if, after proceeding from the con- 

 struction of symbols to equations, we visit the atomic theory in 

 a spot that is peculiarly its home, namely the province of iso- 

 merism. It is here that, we are told, the greatest victories of that 

 theory have been won, that it is of tie highest practical utility, and 

 where, without it, we can conceive no other guide to an explana- 

 tion*. The following special case, which, I believe, has no excep- 

 tional features, is selected from Kekule's Lehrbuch (vol. ii. pp. 257, 

 258), and relates to the well-known instances of the two isomers 

 C 4 H 4 O 4 , and the three isomers C 5 H 6 4 . These bodies, which 

 are known as fumaric and maleic acid, and itaconic, citraconic,and 

 mesaconic acid, respectively, unite each with the same quantity, 

 of hydrogen to form succinic acid, C 4 H 6 O 4 , and pyrotartaric 

 acid, C 5 H 8 O 4 . Each of the isomers combines also with the 

 same weight of bromine ; but, contrary to expectation (based on 

 the hydrogenating experiments), two dibromosuccinic acids, 



* British Association Report for 1870 : Transactions of Sections, p. 45. 



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