Acceleration in Chemical Change. 273 



Second Categor>/. Case (1). — The formation of some third 

 substance which occnsions a dehiy. 



(1) The formation of an unstable bromo-addition ])roduci 

 in the bromination of organic acids, put forward by Hell and 

 Urech as the most reasonable hypothesis, but not established 

 by the isolation of such a compound in their inyestigations, 

 howeyer probable on a priori grounds. 



(2) It was formerly supposed that the so-called induction 

 period in the combination of hydrogen and chlorine, first 

 obseryed b}- Bunsen and Roscoe, was due to the intermediate 

 formation of chlorine monoxide (Pringsheim) or hypochlorous 

 acid (Becquerel, Gautier and Helier, and myself), but since 

 from more recent inyestigations "^ it appears that the presence 

 of neither of these substances purposely introduced produces 

 any measurable effect, another explanation has been proposed 

 in accordance with the equation 



C1.C1 + H0H + H.H = H.C1 + HCI + H0H, 

 though the possibility of the formation of an intermediate 

 compound ti'Cls ^HgO . -S'Hs, wherein ,u, ?/, and z are positiye 

 integers, is not excluded. 



Other examples might be cited under this last head, but it 

 is only intended to give a selection, while certain other ex- 

 amples might be discussed, such, for example, as the etheri- 

 fication of organic acids as observed by Berthelot and Pean 

 de St. Gilles, which has as yet been explained only in a general 

 and superficial manner. 



Case (2). The solution or retention of one of the products 

 of the reaction in the solvent. This affords a problem of 

 chemical equilibrium of the second order which, though the 

 subject of numerous observations, has not been the subject of 

 mathematical analysis like more direct problems. Jt is there- 

 fore proposed to discuss this more fully, and though the 

 observations were made for the most part several years ago, 

 yet the more recent development of this branch of chemistry 

 has induced me 'to recalculate my former results, and herein 

 I have to express my indebtedness to Prof. Wm. Esson for 

 kind assistance in the matter. More than forty years ago 

 Harcourt observed that the eA'olution of nitrogen gas from 

 aqueous solutions of ammonium nitrite starts at first slowly, 

 then proceeds to a maximum from which it decreases in 

 accordance with the law of mass action expressed by the 

 equation 



logA/(A-.i')=aT (5) 



This phenomenon could be reproduced by a temporary 

 * Mellor, Journ. Chem. Soc. Trans, pp. 1292-1301 (1902). 

 Phil. Mag. S. 6. Yol. 6. No. 32. Aug, 1903. T 



