I9I2.] BOGERT— CARBON COMPOUNDS. 265 



include the compounds under certain types, which in this way have scarcely 

 any other significance than that of type or example formulas. I am rather of 

 the opinion that the generalization should be extended to the constitution of 

 the radicals themselves, to the determination of the relation of the elements 

 among themselves, and thus to deduce from the nature of the elements both 

 the nature of the radicals and that of their compounds." 



The recognition of the quadrivalence of carbon atoms and their 

 power of uniting with each other, accounted for the existence and 

 combining vahie of radicals, as well as for their constitution. The 

 type theory therefore found a broader generalization and ampli- 

 fication in the extension of the valence hypothesis of Kekule and 

 Couper to the derivatives of carbon. 



While in years gone by, as has been said, the classification of 

 carbon compounds was mainly or exclusively according to the 

 source from which they were obtained, in modern times the classi- 

 fication has been based solely upon their structural relations and 

 entirely independent of their origin. 



One of the first to adopt this method of classification was Lowig, 

 in 1840. Gmelin, in 1848, arranged carbon compounds in his 

 " Handbook " according to the number of carbon atoms they con- 

 tained, and subdivided them on lines similar to those suggested in 

 Laurent's nucleus theory, as already mentioned. 



Schiel, in 1842, remarked upon the fact that alcohol radicals 

 form a simple and regularly graded series of bodies, of which the 

 properties as well as the composition exhibit corresponding regular 

 gradations, and he predicted the existence of other similar series. 

 Shortly afterward, Dumas pointed out that the fatty acids con- 

 stitute such a series. Gerhardt, in his " Precis de Chimie Organique " 

 (1844), collected a large number of such groups, gave to them the 

 name " homologous series," and distributed them under the general 

 divisions suggested by his type theory. This recognition of homolo- 

 gous series as the units in classifying organic compounds was a 

 great step in advance, simplified the classification enormously, and 

 was very fruitful in stimulating investigation to discover other simi- 

 lar series. 



The terms " fatty " and " aromatic " chemistry appeared about 

 1858. At first used in more restricted sense, they were gradually 



