6^5 SCIENCE PROGRESS 



between carbon and oxygen as in acetoacetic or oxycrotonic 



acid : 



0«- H 



••; | 



CH 3 . C ::: CH . CO,H 



or between carbon and nitrogen as in prussic acid : 



i-> C S N <-| 



1 H ' 



The first of these substances has been prepared in a green and 

 in a colourless form to which distinct formulae must obviously 

 be assigned ; the second substance does not present favourable 

 material for investigation, but a large number of its derivatives 

 have been separated in isomeric forms, one of them, ethyl di- 

 acetylsuccinate (Know, Anna ten, 1899, 306, 332-393), being known 

 in no less than five ; the third case still awaits detailed study. 1 



The isolation of labile isomerides in well-defined crystalline 

 forms has been so frequently observed in recent years as to 

 dispose once and for all of the idea that they can be regarded 

 as merely "phases" in the motion of a vibrating material. 

 Confirmatory evidence is supplied by the relative slowness with 

 which these substances change into one another, as is proved by 

 the fact that the isomeric forms of many typical diketones, such 

 as a-benzoyl-camphor : 



/OH 



/CH.CO.QHs /C = C< 



QH U / I $ QH ]4 /| \C 6 H S 



(Forster, Trans. 1901, 79, 987), and the substances investigated 

 by Knorr (loc. cit), can be separately recrystallised if care is 

 taken to carry out the operation quickly and at a moderate 

 temperature. The most conclusive evidence that the changes 

 in question are intermolecular and not intramolecular (a point 

 which Laar himself selected as illustrating the contrast between 

 his views and those of Butlerow) is, however, that which is 

 afforded by the necessity for a catalyst to establish a condition 

 of equilibrium between the various forms : a change which 

 thus requires the co-operation of a system of molecules must 

 obviously be of the former and not of the latter class. 



1 Michael and Hibbert {Ann., 1909, 364, 64-76) have during the present 

 year put forward evidence to show that prussic acid possesses a nitrite struc- 

 ture, H . C i N. 



