in Ferric Salts and in Chromic Acid. 327 



and since the rate of the chemical reaction proper is propor- 

 tional to C m 



dtn, I (It = K a C m . 



Since a preponderance of either process tends to retard it and 

 to accelerate the other, it follows that condition must sooner 

 or later establish itself in which the rate at which the oxidizer 

 is supplied is practically equal to that at which it is used up. 

 Moreover, owing' to the exceedingly small volume of the dif- 

 fusion layer compared with that of the whole solution, this 

 balance will be established almost instantly. Thereafter we 

 can set, without any appreciable error, 



K 1 (C a -C ra ) = K s O ra , 

 whence 



C. K, + K„ 



O ~ K, 



a constant. 



That is, the concentration at the surface of the metal is pro- 

 portional to that in the solution, which was to be proved. 



The same reasoning must evidently hold for any reaction 

 between a solid phase and a dissolved reagent, provided, of 

 course, that the case is not complicated by secondary interfer- 

 ing effects, such as the formation of an insoluble product, or 

 the occurrence of further chemical reactions in the solution. 

 (See p. 329.) We may, therefore, conclude that obedience to 

 the equation for a reaction of thefrst order is to l>e expected 

 in all normal cases of this type, quite irrespective of the rela- 

 tive rates of. the diffusion process and the chemical reaction 

 proper. This feature of our experimental results is thus fully 

 explained. 



In the discussion of our results we have employed a hy- 

 pothesis which is neither a part of the diffusion theory nor a 

 necessary consequence of it, namely, the assumption that an 

 increase in acidity tends to accelerate the chemical stage of 

 the reaction. (See p. 315.) This may or may not be true in 

 general, but seems to apply here, for the mutual relation of the 

 velocities recorded in Tables VIII and XI (barring minor dif- 

 ferences explainable by experimental error) is in qualitative 

 agreement with what would be expected if increasing acidity 

 accelerated the chemical reaction, the effect of increasing vis- 

 cosity being to retard the diffusion process, as we have already 

 seen. 



Only one value, the one referred to above as exceptional, 

 fails to conform to this explanation. This is the velocity for 

 silver in chromic acid in the presence of 5 molar sulphuric 

 acid, which is unexpectedly low, as the following comparison 



