354 HISTORY OF CHEMISTRY. {LECTURE XVI. 



aromatic compounds possessed for the twenty years preceding. 

 There are besides, however, other important investigations in 

 organic chemistry which require to be mentioned. 



A matter of general importance was the introduction of the 

 idea of tautomerism or desmotropy, which was brought forward 

 by Laar, 125 in 1885, on the strength of some experimental obser- 

 vations and remarks by Zincke. 126 Laar applies the term tauto- 

 meric to a compound when two or more structural formulae 

 can be advanced in explanation of its interactions. A very 

 well-known example is furnished by aceto-acetic ether, which 

 reacts sometimes as if it should be represented by the ketone 

 formula CH 8 .CO.CH. 2 .COOC 2 H 5 , and Sometimes as if it should 

 be represented by the enol formula CH 3 .C(OH) :CH.COOC 2 H 5 . 

 There are numerous investigations dealing with substances of 

 this kind, of which there are a large number. Some of the 

 best known of these investigations are those of Claisen, 127 of 

 W. Wislicenus, 128 and of Knorr. 129 Opinions are still widely 

 divergent, with regard to the questions as to whether a desmo- 

 tropic substance is to be considered as a mixture of two or 

 more compounds (Laar), or whether the forms are continuously 

 passing into one another by means of oscillations (Kekule) or 

 shifting Unkings (Knorr 13 -), or finally whether one form is stable 

 under certain conditions while another is stable under different 

 conditions. 



The systematic and, theoretically, almost completed 

 nation of the sugar group has already been referred to 

 The uric acid group, which so long resisted elucidation and 

 synthesis, is now completely cleared up, 131 and this is chiefly 

 due to Emil Fischer's synthetical investigations. 132 



The hydrogenised aromatic compounds have likewise been 

 referred to already (p. 351), but the terpenes have not been 



125 Berichte. 18, 648 ; 19, 730. 12G Ibid. 17, 3030. Annalen. 291, 25. 

 128 Ibid. 291, 147. 129 Ibid. 293, 70. 13 Ibid. 279, 1 88. 1:J1 Compare 

 further, Grimaux, Ann. Chim. [5] u, 356; and 17, 276; Horbaczewsky, 

 Monatshefte. 3, 796 ; 6, 356 ; 8, 201 ; Behrend and Rosen, Annalen. 251, 

 235. 132 Berichte. 30, 549, 559, 1839, 1846, 2220, 2226, 2400, 3009 ; 

 31, 104, 431, 542, 1980, 2546, 2550, 2619, 2622 ; 32, 435. 



