172 Mr. V. H. Veley on the Phases and 



velocity of the reaction between metallic zinc (containing *6 

 per cent, of lead) and the halogen acid increases until it 

 reaches a maximum ; these writers, however, apply again the 

 term " induction." Precisely similar results were obtained 

 by myself in the course of experiments on copper with diluted 

 nitric acid *; if the conditions were such that the reaction 

 commenced at all, then it started at first slowly. But these 

 two latter examples will be more especially discussed in the 

 sequel. 



Again, it is a familiar fact to photographers that the 

 action of light upon silver salts is less during the first period 

 of exposure than subsequently ; the researches of H. B. 

 Baker have shown that the chemical changes involved are 

 extremely complicated by the formation of intermediate 

 compounds. 



The proposition set forth of the production of intermediate 

 compounds can for simplicity be resolved into two general 

 cases : — 



(i.) In a chemical system compounds ab and cd unite to 

 form a third compound abed, which in its turn is resolved into 

 compounds ac and bd thus 



(i.) ab + cd=abcd. 



(ii.) abcd=ac + bd. 



(ii.) A compound abed decomposes into compounds ab and 

 cd, which subsequently react to form ac and bd, thus 



(i.) abcd = ab + cd. 



(ii.) ab + cd=ac + bd. 

 Both cases come under the category of concomitant varia- 

 tions, and the observations obtained in the course of experiments 

 are not susceptible of mathematical analysis until the several 

 amounts of reacting substances and products have been 

 accurately ascertained. I have dwelt upon this phase of 

 chemical change at some length, not indeed with the view of 

 pointing out that changes proceed slowly at first, for this is 

 patent to the merest tyro in experimental chemistry who 

 pours some dilute sulphuric acid upon metallic zinc ; but 

 rather I would wish to draw attention to the great importance 

 of the study of intermediate compounds which are either 

 generally overlooked or at best summarily dismissed as of no 

 concern f. It is to my mind idle to discuss as to what may 



* Proc. Roy. Soc. xlvi. p. 217. 



f The remark of Lord Rayleigh (B. A. Address 1884) is as pregnant 

 as ever : — "If I might without presumption venture a word of recommenda- 

 tion, it would he in favour of a more minute study of the simpler chemical 

 phenomena," 



