632 SCIENCE PROGRESS 



Tantomerism — A False Hypothesis. — The observations dis- 

 cussed in the preceding pages of this article form substantially 

 a complete record of the development of the theory of isomeric 

 change from the earliest times to the present day, and it 

 would scarcely be necessary to add anything further if it 

 were not for the fact that an alternate hypothesis — advanced 

 by Laar in 1885 and almost immediately shown to be false — 

 attracted so much attention and left such a substantial trail 

 behind, that it is hardly possible to conclude without referring 

 both to the theory and to the experimental facts which brought 

 about its downfall. 



The significance of this theory will be seen most clearly if 

 reference is first made to the observation of Baeyer on isatin 

 and indoxyl (Ben 1883, 16, 2188). Isatin, like prussic acid, 

 was found to be capable of yielding two isomeric ethyl- 

 derivatives, 



/CO x /CO v 



C 6 H 4 < >C.OEt C fi H 4 < >CO 



Ethyl-isatin Ethyl-pseudo-isatin 



the former prepared directly from isatin, the latter by oxidation 

 from ethylindol — 



C 6 h/ >ch. 



\NEt/ 



As only one form of the parent substance was known, Baeyer 

 suggested that ethyl-pseudo-isatin was derived from a labile 

 isomeride which reverted to isatin whenever attempts were made 

 to prepare it — 



CH/ 7 NcO h> QH 4 / N:OH 



L \nh/ J 



/COs 

 *\n ^ 



Pseudo-isatin Isatin 



This suggestion differed from the explanation which Butlerow 

 had given in the case of prussic acid by denying the existence 

 of reverse action both in the liquid and in the solid state; 

 but it was evidently only a limiting case, such as Butlerow had 

 himself foreseen, and was in no way incompatible with the 

 views he had advocated. 



Laar's paper, " Ueber die Moglichkeit mehrerer Struktur- 

 formeln fur dieselbe chemische Verbindung " {Ber. 1885, 18, 



