ISOMERIC CHANGE 635 



ing Laar to demonstrate, if he could, the concrete reality of his 

 hypothesis of " tautomerism." In view of the close resemblance 

 between this theory and Baeyer's conception of " pseudo- 

 merism," 1 it is open to question whether an additional name 

 was really necessary, but the explanatory statement quoted 

 above is broad enough to include all the cases of dynamic 

 isomerism which involve the migration of hydrogen atoms, 

 and if the new label had remained undisfigured it might 

 have provided a useful title by which these phenomena 

 could be selectively described. Unfortunately, Jacobson's sug- 

 gestion was immediately followed by one of those foolish 

 attempts to attach a new meaning to an old word which 

 inevitably lead to confusion, owing to the fact that whilst some 

 are content to read the gloss, others will continue to decipher 

 the original text of the palimpsest. Hantzsch and Herrmann 

 {Ber. 1887, 20, 2802), although rejecting Laar's theory, proposed 

 to annex the title he had devised for it, and to use it as a 

 general description for compounds which could react in two or 

 more different ways ; at the same time they proposed to alter 

 the meaning of Jacobson's word by extending it to isomeric 

 changes not involving the transference of hydrogen but 

 limiting it to those cases in which isomeric forms could be 

 crystallised out — a matter that is entirely dependent on the 

 accident of success or failure in experimental work and has 

 no theoretical significance whatever. The unfortunate result 

 of these suggestions has been that no definite meaning now 

 attaches to either word unless accompanied by some further 

 statement of the sense in which it is used. The only safe rule 

 in all such cases is to adhere strictly to the original significance, 

 and therefore, in opposition to the lax usage now in vogue, to 

 confine the use of Laar's word to the phenomenon of " tauto- 

 merism" in the sense in which he understood it. 



Does tautomerism, then, exist ? Most of the cases that Laar 

 quoted as illustrations involved the oscillation of a hydrogen 

 atom between oxygen and oxygen as in quinone monoxime or 

 nitrosophenol : 



r 



CHCH 

 OC CNO<- 

 CHCH 

 — H 



1 The name is due to Laar {Ber. 1886, 19, 730). 



