1865.] of the Series C w H 2 , !+2 . 175 



possible to isolate pure compounds from a mixture of many homologous 

 compounds by even repeated fractional distillations, and I have convinced 

 myself, whilst engaged for some time upon this subject, that it is much 

 easier to obtain one of the hydrocarbons free from homologues having lower 

 boiling-points, than to remove the last traces of higher boiling compounds 

 which adhere obstinately. This is the case even, if, after repeated fractional 

 distillations, a hydrocarbon is obtained which boils constantly between two 

 degrees, and from it a chloride is prepared which also boils constantly between 

 two degrees. The acetate which is derived from this chloride distils for the 

 most part between two degrees, but after each rectification a small quantity 

 of a liquid, possessing a higher boiling-point, remains behind ; and if, 

 finally, an acetate of a constant boiling-point has been obtained, the alcohol 

 prepared from it also contains a small quantity of a substance which raises 

 the boiling-point towards the end of the distillation. From a constantly 

 boiling radical, on the other hand, derivatives are obtained which have a 

 constant boiling-point, and which are colourless; whilst those from the 

 hydrides have generally first a yellowish colour, and leave dark residues 

 behind on distillation, and even if they are obtained colourless, after several 

 rectifications, they generally darken again after some time, and brown 

 flakes separate out. These observations, as well as the unpleasant odour 

 which all the compounds derived from the hydrocarbons from petroleum 

 possess, prove that they contain impurities which will certainly tend to raise 

 the specific gravities. Moreover, Dale has shown that the hydrocarbons 

 obtained from the acids of the series C n H 2n _ 2 O 4 also have a much lower 

 specific gravity than the corresponding petroleum hydrocarbons. 



Hence it appears highly probable that only one series of hydrocarbons of 

 the formula C n H 2/i+2 exists. 



Hydride of ethyl and ethyl are compounds possessing quite an analogous 

 constitution, and they cannot be fairly compared with alcohol and ether, in 

 which two groups C 2 H 5 and H, and C 2 H 5 , C 2 H 5 are separated by an 

 oxygen atom, whilst in the hydrocarbons all the carbon atoms are united 

 together in exactly the same manner, and thus only one graphical represen- 

 tation of these bodies is possible*. 



If, however, as the identity of the two series has not yet been strictly 

 proved, the view should be preferred that isomeric hydrocarbons exist, we 

 are compelled to admit also that isomeric chlorides, compound ethers, 

 alcohols, and defines exist, as these derivatives of the hydrocarbons show 

 exactly the same differences as are found in the hydrocarbons themselves, 

 and all these bodies must then have to be considered as absolutely isomeric. 



The cause of the isomerism so often observed in compounds belonging 

 to the aromatic series, as well as the cause of homology in these bodies, has 

 been much elucidated by the beautiful researches of Fittig on the benzol 

 hydrocarbons. He believes that similar relations to those which he has 



* Vide " On the Theory of Isomeric Compounds." By Dr. A. Crum Brown, Trans. 

 Roy. Soc. Edinb. vol. xxiii. part iii. p. 707. 



