714 DR A. ORUM BEOWN ON ISOMERIC COMPOUNDS. 



valents of a diatomic atom differ from one another as chlorine does from bromine ; 

 and that the one can no more be changed into the other, than an atom of chlorine 

 can be changed into an atom of bromine ; or, 2. We may suppose that such a 

 change is possible. 



Our knowledge of facts is not as yet sufficiently extensive to enable us to 

 decide definitely between these two hypotheses, but it may be of some use to 

 examine their consequences. 



The second is, as yet, obviously too vague and indefinite to admit of this, I 

 shall therefore confine my remarks to the first. The principal advocate of this 

 hypothesis is Professor Erlenmeyer of Heidelberg. Professor Butlerow of 

 Kasan, has also published some speculations in the same direction ; and in 

 his paper on Organic Acids in the supplementary volumes of Liebig's " Anna- 

 len,' Professor Kekul£ of Ghent treats shortly on the same subject. The only 

 attempts, however, to apply this hypothesis in a definite way to the explanation 

 of particular cases of absolute isomerism are, as far as I am aware, 1. That of 

 Professor Kolbe, who applies a form of this hypothesis to the case of the isome- 

 rism of oxide of ethylene and aldehyde. I have already given my reasons for be- 

 lieving that these substances are metameric, and shall therefore not discuss the 

 point further here. 2. That of Butlerow (Zeitschrift, v. 301), who endeavours 

 by means of it to explain the isomerism of hydride of ethyl and methyl gas ; 

 and 3. That of KekulS (Ann. Ch. Ph., Supp. b. ii. Ill), in his explanation of the 

 isomerism of maleic and fumaric acids, and of citraconic, itaconic, and mesaconic 

 acids.* 



1 shall examine the 2d and 3d of these examples in detail, and think I shall 

 be able to show a certain degree of inconsequence in both. Professor Butlerow 

 argues, that in methyl gas the two atoms of carbon are combined by two affinities 

 of the same kind (secondary affinities), each being the affinity which in iodide 

 of methyl is combined with iodine. In hydride of ethyl, the two carbon atoms 

 are combined in the same way as in the other members of the ethylic series, 

 therefore, probably in the same way as in the members of the acetic series, one 

 of which is acetonitrile, which is cyanide of methyl, the one is therefore the free 

 affinity of methyl (secondary), the other the free affinity of cyanogen. These 

 must be different, because hydride of ethyl is not identical with methyl gas. 

 Butlerow indicates this, by calling the free affinity of cyanogen primary. We 

 have thus in methyl gas two primary affinities united together, and in hydride of 

 ethyl a primary united to a secondary. 



In this reasoning we have three assumptions, — 1. That the nature of the 

 carbon affinities is unchangeable ; 2. That the carbon atoms continue united to- 

 gether by the same affinities through a series of chemical reactions, such as the 



* I do not here notice the remarks of Professor Kolbe (Zeitschrift, vi. 13) on the same subject, 

 as his object is rather to prove the metamerism than to explain the isomerism of these bodies. 



