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XLTV.—On the Theory of Isomeric Compounds. By Dr A. Crum Brown. 



(Read May 2, 1864.) 



In the following remarks I intend to confine myself to the consideration of 

 those compounds which have not only the same composition per cent., and the 

 same molecular weight, but also the same constitutional formula. Such com- 

 pounds may be termed absolutely isomeric. As the constitutional formula of few 

 substances is fully known, this class is of course a small one, or rather there are 

 few substances of which we can certainly say that they belong to this class. 



The following are the principal pairs of substances which are or have been 

 supposed to be absolutely isomeric. I shall first enumerate them, and then 

 proceed to discuss the nature of their isomerism. 



1. The hydrides of the alcohol radicals, and the so-called alcohol radicals, as 

 hydride of ethyl and methyl gas ; hydride of propyl and methyl-ethyl. 



2. Chloride of ethyl and the product of the action of chlorine on hydride of 

 ethyl. Also the chlorides of other alcohol radicals and the mono-chloro deriva- 

 tives from the corresponding hydrides. 



3. Chloride of vinyl and chloracetene. 



4. Fumaric and maleic acids. 



5. Two of the three acids citraconic, itaconic, and mesaconic. 



6. Bromo-maleic and isobromo-maleic acids. 



7. Bibromo-succinic and isobibromo-succinic acids. 



8. Active and inactive malic acid. 



9. Active and inactive aspartic acid. 



10. Two of the varieties of tartaric acid. 



11. The two series of bodies known as the compounds of ethylene and of 

 ethylidene. 



12. Lactic and paralactic acids. 



13. The two series of alcohols derived, the one by fermentation from sugar. 

 and the other by the addition of water to the olefines, as Berthelot's and 

 Friedel's propylic alcohol, Wurtz's hydrate of amylene, Erlenmeyer and 

 Wanklyn's ^ hexylic alcohol. 



To the same class belong of course the ethers derived from these alcohols. 



14. The isomeric acids and alcohols having the general formula C„Hi(„_ jQ, 

 as kressylic acid, and benzoic alcohol. 



I now proceed to inquire whether these pairs of substances are absolutely 

 isomeric or not. 



In order to do so, we must determine whether they are really different, whether 

 they are not perhaps identical ; and secondly, whether they are or are not 



VOL. XXIII. PART III. Q ^ 



