156 RESEARCHES ON THE VOLATILE HYDROCARBONS. 
IL. 
On the Influence of C,H, upon the Boiling-points in Homologous Series of Hydrocarbons, and 
in some Series of their Derivatives; with Critical Observations on Methods of taking 
Boiling-points. 
Ir 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,H, 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 7.19° for a difference of # C,H, in the elementary 
formule. In the earlier observations on this subject, this relation between the 
boiling-points and formulee 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 Kopp} 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, H;. 
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 of 
chemists to this subject, different theories have from time to time been advanced by 
Schréder, Lowig, Gerhardt, and others, and supported by laborious research and 
observation. It will be interesting to examine some of these theories in the light 
* Annalen der Chemie und Pharmacie, 1842, XLI. 79, 169; 1845, LV. 177, ete. 
+ Annalen der Chemie und Pharmacie, 1855, XCVI. 2. 
