868 PRINCIPLES OF CHEMISTRY 



the specific gravity of the first is 0-899 ; that of the second, 0'925, 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 restricted sense of the word that is, 

 when with an identity of composition and of molecular weight, the 

 properties 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 8 , because the former is 

 CH 4 , in which methyl has taken the place of H, and as all the 

 hydrogen atoms of methane must be supposed to have the same 

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

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

 CH 3 CH 3 ; 43 the same argument also applies in the case of pro- 

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



can be distinguished beyond a doubt. Without Dalton's law chemistry could not have 

 been brought to its present condition, but it cannot alone express 11 those grada- 

 tions which are quite clearly understood and predicted by the law of Avogadro- 

 (^erhardt. 



* 2 The conception of the structure of carbon 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 , <fec.) ; 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 

 1860 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 Kekuleand A. M. Butleroff began to formulate 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 rotation 

 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, &c.) it is always one and the same substance. If, for example, in CX 4 , X is gradually 

 replaced by hydrogen until CH 3 X is produced, or in CJElj, the hydrogen by various 



