356 PRINCIPLES OF CHEMISTRY 



the specific gravity of the first is 0*899 ; that of the second, O925, at 

 that is, here also the boiling point rises with the increase of 

 molecular weight, and so also, as might be expected, does the density. 



Cases of isomerism in the exact sense of the word that is, when, 

 with an identity of composition and of molecular weight, the pro- 

 perties of the substances are different are very numerous among 

 the hydrocarbons and their derivatives. Such cases are particularly 

 important for the comprehension of molecular structure, and they 

 also, like the polymerides, may be predicted from the above-mentioned 

 conceptions, expressing the principles of the structure, of the carbon 

 compounds 42 based on the law of substitution. According to it, for 

 example, it is evident that there can be no isomerism in the cases of 

 the saturated hydrocarbons C 2 H 6 and C 3 H S , because the former is 

 CH.j, where methyl has taken the place of H, and as all parts of 

 the hydrogen atoms of methane ought to be supposed to have the 

 same relation to carbon, it is all the same which of them be subjected 

 to the methyl substitution the resulting product will only be ethane, 

 CH 3 CH 3 ; 43 and therefore the same argument applies in the case of 

 propane, CH 3 CH 2 CH 3 , where one compound only can be imagined. It 



42 The conception of the structure of hydrocarbon compounds that is, the expression 

 of those unions and correlations which their atoms have in the molecules was for a long 

 time limited to the representation that organic substances contained complex radicles 

 (for instance, ethyl C 2 H 5 , methyl CH 3 , phenyl C 6 H 5 , &c.) ; then about the year 1840 the 

 phenomena of substitution and the correspondence of the products of substitution with 

 the primary bodies (nuclei and types) were observed, but it was not until about the year 

 I860 and later when, on the one hand, the teaching of Gerhardt about molecules was 

 spreading, and, on the other hand, the materials had accumulated for discussing the 

 transformations of the simplest hydrocarbon compounds, that conjectures began to 

 appear as to the mutual connection of the atoms of carbon in the molecules of the com- 

 plex hydrocarbon compounds. Then Kekule and A. M. Butleroff began to express the 

 connection between the separate atoms of carbon, regarding it as a quadrivalent element. 

 Although in their methods of expression and in some of their views they differ from each 

 other, and also from the way in which the subject is treated in this work, yet the essence 

 of the matter namely, the comprehension of the causes of isomerism and of the union 

 between the separate atoms of carbon remains the same. In addition to this, starting 

 from the year 1870, there appears a tendency, which from year to year increases, to dis- 

 cover the actual spacial distribution of the atoms in the molecules. Thanks to the 

 endeavours of Le Bel (1874), Van't Hoff (1874), and Wislicenus (1887) in observing cases 

 of isomerism such as the effect of different isomerides on the direction of the rota- 

 tion of the plane of polarisation of light this tendency promises much for chemical 

 mechanics, but the details of the still imperfect knowledge in relation to this matter 

 must be sought for in special works devoted to organic chemistry. 



43 Direct experiment shows that however CH 3 X is prepared (where X = for instance 

 Cl, <fec.) it is always one and the same. If, for instance, in CX 4 , X is gradually replaced 

 by hydrogen until CH 3 X is produced, or in CH 4 the hydrogen by various means is 

 replaced by X, or else, for instance, if CH-.X be obtained by the decomposition of more 

 complex compounds, the same product is always obtained. 



This was shown in the year 1800, or there about, by many methods, and is the funda- 

 mental conception of the structure of hydr.ocarbon compounds. If the atoms of hydro- 



