294 HISTORY OF CHEMISTRY. [LECTURE XIV. 



oil and aldehyde by the action of hydrochloric acid. 104 He 

 then turns the theoretical views advanced at that time to 

 immediate account in the synthesis of picoline and collidine, 

 which are obtained by the condensation of acrolein-ammonia 

 and of aldehyde-ammonia : 165 



2 C 3 H 4 ONH 3 = C G H 7 N + 2 

 4 C 2 H 4 ONH 3 = C s H n N + 4 



Kekule, a few years later, condensed two molecules of alde- 

 hyde so as to form crotonic aldehyde, 160 thereby throwing 

 light upon the chemical nature of the so-called acrylic aldehyde 

 already examined by Lieben. 107 This reaction was afterwards 

 studied by Wurtz, 168 who showed that the two aldehyde mole- 

 cules unite in the first place, without the elimination of water, 

 to form aldol, the aldehyde of /3-hydroxybutyric acid, and that 

 crotonic aldehyde is then formed from the latter by the loss of 

 water. The general character of this interesting reaction was 

 established, subsequently, by various investigations, and espe- 

 cially by the researches of Claisen. 169 



The idea of condensation has been greatly extended in 

 recent times, every reaction in which union of carbon to carbon 

 occurs amongst the molecules that act upon one another being 

 designated a condensation. The word thus became synony- 

 mous with synthesis, and lost all independent meaning and all 

 meaning corresponding to its etymology. It is due to this 

 that Baeyer's reaction for the formation of hydrocarbons from 

 aldehydes and benzene and its derivatives, and likewise Perkin's 

 method of forming unsaturated acids from aldehydes and the 

 salts of fatty acids, came to be designated as condensations. 



The idea of condensation has also undergone change in 

 another direction inasmuch as internal condensations have 

 been contrasted with the processes just mentioned, which 

 have in turn been called external condensations. By internal 

 condensations we now understand reactions in which a single 



164 Ibid. 97, 350. 165 Ibid. 155, 283 and 297. 166 Ibid. 162, 77. 

 167 Ibid. 106, 336; Supplementband. i, 114. 168 Jahresbericht 1872, 449; 

 1873, 474; 1876, 483; 1878, 612. 169 Annalen. i8o ? i; ibid. 218, 121. 



