Conditions of Chemical Change. 169 



been handed down through a series of manuals on Chemistry, 

 the writers of which seem to consider that the phenomenon is 

 necessarily conditioned b}^ the agency of light. Bunsen and 

 Roscoe make a mental presentation of resistance to be over- 

 come before chemical change can commence. It is to be 

 supposed, though the writers do not state it in so many 

 words, that the term " induction " is based upon an analogy 

 of magnetic and electric phenomena, called by the same term, 

 which last just so long as the predetermining cause, namely 

 the vicinity of an electrified or magnetized body. 



The analogy is, however, incomplete in that though there 

 are chemical changes which last as long as the external cause 

 or condition, be it E.M.F., light, or heat, yet there are others 

 which, when once commenced, will proceed, so far as can be 

 judged, without the external agency. As the phenomenon is 

 not necessarily dependent upon sunlight *, and as probably it 

 is not entirely of a dynamical character, I have preferred to 

 adopt the term " Inertness " or " Reluctance." 



Bunsen and Roscoe farther showed that when a mixture 

 of hydrogen and chlorine gases, in which chemical change 

 has set in under the influence of light, is darkened and then 

 again exposed, the phenomena of inertness and initial accelera- 

 tion are repeated ; this observation, which has an important 

 bearing on the course of chemical change, seems to have been 

 quite overlooked by writers upon the subject. Pringsheim 

 (cf. supra) , repeating Bunsen and Roscoe's experiments, 

 traces the above phenomena to the purely chemical cause of 

 an intermediate reaction, basing the explanation on the fact 

 that a mixture of hydrogen and chlorine gases was less sus- 

 ceptible to actinic rays when dry than when moist (an obser- 

 vation confirmed by Dixon t). Pringsheim considered the 

 reactions to be as follows : — 



(i.) H 2 + Cl 2 + H 2 = C1 2 + 2H 2 , 



(ii.) Cl 2 + 2H 2 =2HCl4-H 3 0, 



without, however, bringing forward any experimental evidence 

 in support of the view, and, indeed, it is not probable that 

 the anhydride C1 2 would exist, as such, in the presence of 

 water. But the investigations of Pedler J supply the necessary 

 data, in that he has shown that chlorine in the presence of 



* Bunsen and Roscoe themselves also illustrate their results by others 

 obtained in the study of the bromination of tartaric acid, a chemical 

 change probably not -wholly conditioned by actinic rays. 



t Phil. Trans. (1893) [A], p. 144 (Bakerian Lecture). 



X Journ. Chern. Soc. 1890, pp. 599-625. 



