460 



Messrs. S. E. Sheppard and C. E. K. Mees. [Dec. 20, 



Thus 



Wratten ordinary K = 0203 



Barnet ordinary K = 0-0260 (Q- 0249 -0-0254) 



For occasions of space the mean value of K with the extremes are 

 given, except where it seems desirable to show that the course of the 

 reaction is unchanged. 



Influence of Concentration on the Velocity. The constant as developed 

 above contains implicitly CFC the total concentration of the iron; 

 it should, therefore, be proportional to this experimentally. 



CFerrous oxalate. 



N/12-5 



N/12-5 



N/25 



N/50 



N/50 



Table VI. 



K found. 

 0-4011 

 0-4061 

 0-203 

 0-100 

 0-102 



Br = N/500 



The values for N/12'5 showed some tendency to decrease; this is 

 probably due to the rapid accumulation of bromide and other reaction 

 products, which do not diffuse rapidly, and so influence the velocity by 

 remaining in the reaction layer, a fact which is one of the main causes 

 of divergencies and difficulties in heterogeneous reactions.* A more 

 stringent proof of the proportionality is given later. 



On the diffusion theory of the development velocity, this pro- 

 portionality is simply the expression of the fact that the amount of 

 reducer diffusing into the reaction layer is proportional to the con- 

 centration (vide p. 457). Nernstf has^ pointed out that on this theory 

 it is not admissible to draw conclusions as to the order of the reaction 

 in heterogeneous systems, so that the otherwise probable theory, that 

 the reaction is mono-molecular with respect to iron, cannot be regarded 

 as strictly proven by this. 



Influence of Bromide on the Velocity. It is well known to photo- 

 graphers that bromide considerably influences the rate of development, 

 for which reason it is used as a restrainer. The influence of bromide 

 must be due to its effect on the purely chemical reaction 



+ 

 Ag + E = Ag + R. 



(met.) 



The effect on the expression K = y log -Q * p was studied. 



oo 



It was found that the values of K for low values of t increased till 



* Ostwald, ' Lehrbuch,' 2 to Auflage, 2te Bd., ' Chem. Kinetik.' 

 f W. Nernst, Theoretisohe Chemie,' 4te Auflage, p. 573. 



