LECTURE XIV.] HISTORY OF CHEMISTRY. 293 



bases 159 may here be referred to. This method he afterwards 

 altered and considerably improved. 160 



In many of these investigations an idea was turned to 

 account which has already borne much fruit, and which will, 

 no doubt, also be of great service in future. I refer to the so- 

 called condensation processes ; that is, to the very frequently 

 occurring formations, both in nature and in artificial reactions, 

 of complex substances from simple ones, where several iden- 

 tical or similar molecules unite to form one molecule, usually 

 with the simultaneous elimination of hydrogen, water, ammonia, 

 etc. Gerhardt drew attention to reactions of this kind when 

 he formulated his theory of residues (p. 1 80), but it is only in 

 comparatively recent times within the last thirty years or 

 thereabouts that the necessary attention has been bestowed 

 upon these processes. Berthelot was probably the first who 

 closely studied such reactions, and he obtained valuable results 

 by doing so. Amongst these results are the syntheses, dis- 

 covered by him, of benzene C 6 H 6 from acetylene, of diphenyl 

 from benzene, of anthracene from toluene, 161 etc. In these 

 experiments he established, amongst other things, the fact, 

 which has since been frequently confirmed, that at a high 

 temperature several molecules of a hydrocarbon may unite to 

 form a new molecule with the elimination of hydrogen. 



Some time afterwards, Baeyer began to work at this subject. 

 He regards the difference between condensation and poly- 

 merisation as consisting in the fact that in the former the 

 molecules combine by virtue of union with carbon atoms, and 

 in the latter of union with oxygen or with nitrogen atoms. 162 It is 

 already clear to him that for purposes of synthesis, condensation 

 is alone of importance. He draws attention, besides, to very 

 important syntheses which have already been carried out, such 

 as the formation of mesitylene from acetone by Kane, 163 and 

 Chiozza's synthesis of cinnamic aldehyde from bitter almond 



159 Annalen. 66, 129 ; 67, 6 1 and 129 ; 70, 129; 73, 180 ; 74, I, 33, 117 ; 

 75> 356; 78, 253; 79, ii. 16 Berichte. 14, 2725; 15, 407, 752., 762. 

 161 Bull. Soc. Chim. [2] 6, 268. 162 Annalen. Supplementband 5, 79. 

 163 Ibid. 22, 278, 



