156 RESEARCHES ON THE VOLATILE HYDROCARBONS. 



II 



On the Influence of C 2 H 2 upon the Boiling-points in Homologous Series of Hydrocarbons, and 



in some Series of their Derivatives; toith Critical Observations on Methods of taking 



It is well known that we are indebted to H. Kopp * for the discovery of certain 

 definite relations existing between the chemical constitution and some of the physical 

 properties of homologous liquid bodies. Of these, one of the most important is that 

 of a uniform difference between the boiling-points of the contiguous members of an 

 homologous series, corresponding to the uniform difference in their elementary consti- 

 tution. Kopp has shown by numerous examples, that, as a general rule, in those 

 series which are characterized by a common elementary difference of C 2 H 2 between 

 the members, in the order of the series, the corresponding difference of boiling-point 

 is about 19° C. ; hence, that the difference between the boiling-points of any two 

 members of such a series is x .19° for a difference of x C, H, in the elementary 



formulae. In the earlier observations on this subject, this relation between the 

 boiling-points and formulae was found so nearly constant in the different series exam- 

 ined, that any deviations from this apparent general law were referred, not unreason- 

 ably, to assumed inaccuracies in the determination of the boiling-points of the bodies 

 compared. But the more recent and extended generalizations of Koppf have led 

 him to point out several exceptional series, in which the boiling-point difference is 

 greater, and others in which it is less, than 19° for an elementary difference of C 2 H 2 . 

 That there are such exceptional series is confirmed in a very decisive manner by my 

 own observations, as I shall proceed to show. My determinations make the boiling- 

 point differences in some cases so much larger than those of other observers as to 

 leave no room for doubt on this point ; especially if the comparative value of these 

 determinations be duly estimated with reference to the more reliable character to 

 which the preparations are entitled, on account of the more efficient means which I 

 have employed for separating the liquids. Since Kopp first called the attention 

 chemists to this subject, different theories have from time to time been advanced by 

 Schroder, Lowig, Gerhardt, and others, and supported by laborious research and 

 observation. It will be interesting to examine some of these theories in the hgW 



of 



o 



♦ Annalen der Chemie und Pharmacie, 1842, XLI. 79, 169 ; 1845, LV. 177, etc. 

 t Annalen der Chemie und Pharmacie, 1855, XCVI. 2. 



