260 Prof. H. E. Armstrong and Dr. T. M. Lovviy. [June 11, 



Comparable with these are the two isodynamic forms of ?r-and 

 /?-bromonitrocamphor, for example : 



Normal neutral form. Pseudo acid form. 



In the solid state, both forms of 7r-bromonitrocamphor are stable : 

 when either form is dissolved in a liquid, isomeric change sets in ; 

 sooner or later, it may be only in the course of a few hours or even 

 days, a state of equilibrium is established, about 6 per cent, of the 

 material being present in the pseudo form and 94 per cent, in the 

 normal form.* The change, however, does not occur spontaneously 

 but is undoubtedly dependent on the presence of a catalyst, as equili- 

 brium is established with great rapidity if a trace of alkali be added ; 

 acids have only a slight, although definite, accelerating effect. In the 

 case of /?-bromonitrocamphor, solutions in benzene of the neutral as 

 well as of the acid form which have been kept during several days 

 without undergoing change, when transferred to another vessel, 

 have rapidly passed to a condition of equilibrium doubtless because 

 this vessel had been less successfully cleansed than that first used. It 

 can, therefore, scarcely be doubted that the change occurs within a 

 complex system one which, it is only reasonable to suppose, constitutes 

 an electrolytic circuit. The process is reversed when crystallisation sets 

 in ; if the evaporation of the solvent take place sufficiently slowly, the 

 whole of the material is converted into and crystallises out in the less 

 soluble form ; if, however, evaporation take place rapidly, the isomeric 

 change may lag behind the crystallisation and both forms may 

 separate. In the case of nitrocamphor, the normal form is the one 

 that separates from the solution ; but in the case of :r-and /3-bromonitro- 

 camphor, although the pseudo form is the minor constituent in the 

 solution, being much less soluble than the isomeride, it is one to 

 separate on crystallisation. 



The passage of the one form into the other in the case of saccharin, 

 for example, may be pictured as involving the occurrence of changes 

 such as are represented in the equation : 



NaOH /OH NaOH 



< 



1 CoH 4 



Supposing the stable form of lower potential to crystallise out, the 

 crystals, in almost every case, would contain a minute and variable 

 amount of the isodynamic form entangled, as it were, in the mass. In 



* Compare Lowry, ' Chem. Soc. Trans.,' 1898, p. 966 ; ' Chem. Soc. Proc.,' 1903, 

 p. 129. 



