634 SCIENCE PROGRESS 



of pairs of compounds undergoing reciprocal isomeric change 

 with greater or less velocity, according to the ordinary laws of 

 chemical action ; Laar substituted for this the idea of an inces- 

 sant wobble taking place entirely within the molecule, uncon- 

 trolled by any of the ordinary chemical laws, and (if the 

 expression can be permitted) physical rather than chemical in 

 character. This point he made clear by repudiating the analogy 

 of dissociation, and substituting for it Maxwell's idea of molecular 

 vibrations giving rise to the production of spectra. In Laar's 

 opinion, then, quinone monoxime and nitrosophenol, or the two 

 forms of prussic acid, were merely phases in the motion of a 

 vibrating molecule, and the attempt to prepare either of them 

 separately was as hopeless as if one were to try and collect 

 sodium atoms in the different attitudes involved in the emission 

 of the Dj and D 2 lines. 



Does Tautomerism Exist? — Laar's ingenious and highly 

 speculative theory attracted a considerable amount of notice, 

 and served a useful purpose in calling attention to a large 

 number of cases in which isomeric compounds were either not 

 capable of separate existence or were converted into one another 

 with extreme readiness ; but it was not by any means generally 

 accepted, and was not destined to survive for long the ordeal of 

 experimental verification. Jacobson(Zter. 1887, 20, 1732 footnote ; 

 1888, 21, 2628 footnote) was one of the first to repudiate both 

 the theory and the name by which it was described. 



The word " tautomerism " is based on Laar's view, which (I 

 believe) is not shared by most chemists, that the molecules of 

 compounds whose chemical behaviour is represented by two 

 structural formulae differing in the point of attachment of a 

 hydrogen atom, never assume a definite constitution, but exist 

 in a constant state of oscillatory change. The majority of 

 chemists would explain the observations in question in this 

 way, that the known forms of such compounds are to be 

 represented by a definite grouping of atoms which in certain 

 reactions passes over into an isomeric grouping by a rearrange- 

 ment of bonds consequent upon the displacement of a hydrogen 

 atom. 



This process Jacobson proposed to call " desmotropy" or "bond- 

 shifting"; the term " merotropy " has also been used to 

 express the same idea (see, for instance, Michael, " Ueber 

 Desmotropie und Merotropie," Ann., 1908, 363, 20 et seq.) } leav- 



