ISOMERIC CHANGE 627 



is clear that the chemical metamorphoses of such a substance 

 must proceed sometimes in the sense of one chemical grouping, 

 and sometimes in the sense of the other, according to the nature 

 of the reagent and the conditions of the experiment. As an 

 example of such a two-sided chemical structure, one might 

 probably adduce the cases of cyanic acid, hydrocyanic acid, and 

 so forth. From this point of view it appears both unnecessary 

 and impracticable to determine whether cyanic acid is a carbi- 

 mide or the hydroxide of cyanogen, and whether prussic acid is 

 a nitrite or an isonitrite. ... It is scarcely necessary to add that 

 the ideas which I have here developed are an application to 

 the principles of chemical structure of the dynamic theory, the 

 foundations of which were laid by Berthollet." 



For this condition of dynamic equilibrium between isomers, no 

 new name was proposed by Butlerow ; but it may conveniently 

 be referred to as dynamic isomerism, a name which has the 

 merit of summarising — without adding anything to — the descrip-- 

 tive phrases which have been quoted above. 1 



Reversible Isomeric Change as an example of Mass-action. — It 

 is evidence of the essential soundness of Butlerow's judgment 

 that he was content to explain the properties of substances like 

 cyanic and hydrocyanic acids by extending to these more difficult 

 problems the laws that had been established by the study of 

 more commonplace phenomena. For him, the hypothetical 

 equilibrium between cyanic and isocyanic acids was no 

 mysterious or even peculiar action, but a normal type of 

 chemical change differing in no essential feature from a large 

 range of well-established equilibria. His reference to Berthollet 

 shows that he wished to represent the dynamic isomerism of 



1 A remark in a recent text-book to the effect that " The Hantzsch and Herr- 

 mann view of desmotropy has recently been revived by Lowry in a special case 

 under the name (borrowed from Bischoff) ' dynamic isomerism,' " can scarcely be 

 passed over without comment. The phrase "dynamic isomerism" was in reality 

 not borrowed from Bischoff, but was an adaptation of the adjective "isodynamic" 

 which had been proposed by Armstrong in 1889 (Morley and Muir's edition of 

 Watts' s Dictionary, article "Isomerism") as a substitute for Berzelius's " meta- 

 meric." So far as the theory itself is concerned, a complete statement of its 

 essential features was given by Butlerow in 1877, and it is therefore hardly correct 

 to suggest that in calling attention to his conceptions I was reviving the views 

 advocated by Hantzsch and Herrmann ten years later. The discovery of a 

 phenomenon is necessarily of greater importance than the attachment to it of 

 a new label, or the correction of a subsequent error, and in this respect no later 

 worker is in a position to dispute Butlerow's claim to a unique position in the 

 development of the theory of dynamic isomerism. 



