LECTURE XIII.] HISTORY OF CHEMISTRY. 269 



were so. Van 't Hoff has endeavoured to render the matter 

 clear by means of a geometrical conception as to the arrange- 

 ment of the atoms in space. This cannot, however, be more 

 fully entered into here. 



A considerable time ago, Rochleder 7G pointed out that 

 substances belonging to one particular class, very easily undergo 

 isomeric changes ; and he called these substances defective 

 because they are produced by the separation of certain atoms 

 from saturated substances. They are now commonly called 

 unsaturated compounds, and we shall consider them particu- 

 larly, as their study is a subject of great interest. 



In his paper on the theory of organic compounds, Couper 77 

 ascribed to carbon the capacity of bringing sometimes two, 

 and sometimes all four of its units of affinity into play ; hence 

 it was not difficult for him to explain the existence of such 

 compounds as carbonic oxide, ethylene, etc. Amongst others, 

 Wurtz 7S and Kolbe 79 fell in with this view. The latter derives 

 the unsaturated hydrocarbons from the type of carbonic oxide, 

 assuming in all of these substances one or several carbon 

 atoms which are active with two affinities. He writes C 2 O 2 



TT TT TT 



Carbonic oxide, C 2 p TT Ethylene, C 2 p TT Propylene, C 2 p H 



or C 2 . C 2 H 2 Acetylene. 



Kekule at first tried to explain the unsaturated substances 

 by the assumption of a denser arrangement of the carbon 

 atoms, 80 but afterwards, in his admirable and important investi- 

 gations of organic acids 81 he appears to have been of opinion 

 that, in these substances, the affinities of the carbon are not 

 fully satisfied, and that they contain free affinities or blanks. 

 This assumption became more probable both from Kekule's 

 own experiments and from those of Carius, 82 in accordance 



76 Ber. Wein. Akad. II, 852 ; 12, 727 ; also ibid. 49 (second part), 115. 

 77 Ann. Chim, [3] 53, 469 ; compare p. 254. 78 Le5ons de philosophic 

 chimique. 136. 79 Kolhe, Lehrbuch der organischen Chemie. I, 738 ; 

 2, 576. 80 Kekule, Lehrbuch der organischen Chemie. I, 166. 



81 Annalen. 117, 120; Supplementband I, 129, 338; Supplementband 

 2, 85 ; 130, i. 8 - Ibid. 124, 265 ; 126, 195 ; 129, 167, 



