358 Dr. H. J. S. Sand. The Bole of Diffusion during [Nov. 22, 



which have a very powerful effect, an inactive film may be formed on 

 the particles by chemical action of the poison, whenever the concentra- 

 tion of the latter in the solution is greater than a certain definite value, 

 and the platinum would thus be completely poisoned as soon as this 

 value was exceeded by the small quantity necessary to 'form the 

 coating. Such an explanation is, however, manifestly not applicable 

 to weak poisons, such as acids and certain electrolytes, the action of 

 which begins at low concentrations, and is not complete until very 

 large values are attained. In this case it seems that the only explana- 

 tion compatible with Nernst's hypothesis would have to be sought in 

 the tendency of such substances to cause the coalescence and final 

 precipitation of the particles. The influence of alkalies in accelerating 

 the reaction would lie in their tendency to bring about further disinte- 

 gration of particles which had already undergone partial coalescence.* 

 In any case, however, as has already been pointed out, we may assume 

 that in solutions which are at all stable no particles occur of a diameter 

 greater than 0*5 /x. 



The Experimental Results on Dependence of Reaction Velocity on Concen- 

 tration of the Catalyser can only be Reconciled with the Idea of a Heterogeneous 

 Reaction by Recognising the Important Part Played by Convection Currents. 

 It is a piiori clear with regard to the relative role of diffusion and 

 convection that the latter must be seriously considered, owing to the 

 fact that solid particles suspended in a liquid are known to be in a 

 state of continual movement performing the so-called Brownian 

 motions. Besides, in the more concentrated solutions oxygen-bubbles 

 are given off, and cannot fail to stir the liquid. 



As a matter of fact, a brief examination of the results summarised 

 above suffices to show that they are not in agreement with Nernst's 

 hypothesis, if we suppose diffusion alone, without convection, to be 

 responsible for the neutralisation of changes of concentration. We may 

 even go further, and say that the fact that K increases more rapidly 

 than proportionally with the concentration of the catalyser cannot be 

 reconciled with the idea of a heterogeneous reaction at all, unless the 

 process taking place on the surface of one of the particles influences 

 that of its neighbours by some non-chemical means, such as the 

 production of convection currents. If we were dealing with a 

 stationary liquid, whatever the chemical mechanism of the reaction 

 may be, the amount of substance decomposed in a given time at the 

 beginning of the experiment would be proportional to the number of 

 particles, that is, to the concentration of the platinum. Now, the 

 constant K may be taken as a direct measure for this amount, as is 

 clear from the following relation arising from Equation 1, 



-f = CK w- 



* Bredig, loc. cit., vol. 31, p. 306, note 2. 



