Conditions of Chemical Change, 171 



on the rate of formation of ethereal salts ; these writers gave*, 

 without apparently being aware of the fact, the same ex- 

 planation as Bunsen and Roscoe. 



Again, Hell and Urech (cf. supra) noticed these same 

 phenomena in their investigations on the bromination of the 

 acids of the acetic series ; they very cogently pointed out that 

 their results would be incomprehensible on the hypothesis 

 that the reaction consists in the simple substitution of hydrogen 

 by bromine, for in that case the maximum velocity should 

 be at the commencement of the reaction, when there is the 

 greatest mass of unaltered bromine present. These writers 

 seem to have been the first to give the probably true exp^na- 

 tion of this phase of chemical change, tracing it to the forma- 

 tion of an intermediate compound (in the particular case of 

 acetic acid to a body of composition C 2 F 4 2 Br 2 4HBr) f, 

 whereby a delay would of necessity be occasioned, and the 

 chemical change, though started, could only proceed at first 

 slowly. 



It was noticed also by Harcourt in some unpublished in- 

 vestigations, and confirmed by later experiments of my own, 

 that the evolution of gases from homogeneous liquids as a 

 process of chemical change % illustrates in a remarkable way 

 the same phenomena. These were especially observable in 

 the decomposition of potassium ferrocyanide with concen- 

 trated sulphuric acid, and in the reduction of nitric acid with 

 ferrous sulphate ; reactions in which intermediate substances 

 are undoubtedly formed, and, indeed, in the latter case the 

 progress of the change is rendered visible. In the course of 

 the experiments it was noticed that, if a liquid from which a 

 gas was being evolved was cooled and subsequently restored 

 to its former temperature, or if the superincumbent pressure 

 was suddenly increased from a third to one atmosphere, then, 

 though the evolution of gas was perfectly uniform before the 

 temporary alteration of condition, the phenomena of inert- 

 ness and acceleration were repeated in a greater or less degree. 



Simultaneously Spring and van Aubel§ noticed that the 



* Ann. de Chim. [3] lxvi. pp. 5-153. On the above point the authors 

 write as follows : — " Pour la concevoir il faut admettre une sorte d'inertie, 

 de resistance a vaincre qui retarde la combination . . . : cette acceleration 

 initielle semble done constituer un caractere assez general de ce genre de 

 reactions." 



t The writers do not bring forward much experimental evidence in 

 favour of the existence of this particular compound, but the formation of 

 unstable bromo-addition products, such as that of camphor for example, 

 is by no means infrequent among carbon compounds. 



X Phil. Trans. (1888) [A], p. 272. ~X 

 § Ann. Chim. Phys. [6] xi. pp. 505-534. 



