1904.] Catalysis ~by Colloidal Metals and similar Substances. 359 



Bredig's results thus contradict the idea of a heterogeneous reaction 

 taking place in a stationary liquid. They, however, become intelligible 

 if we consider the influence of convection. Proceeding on lines 

 similar to those adopted by Nernst, we may assume each particle to be 

 surrounded by a film of adhering liquid, the thickness of which depends 

 very largely on the amount of motion taking place in the solution. 

 The thinner the film, the greater will be the flow of solute to the 

 particles, and the greater, therefore, the velocity of the reaction. 

 Now, in the solutions containing large quantities of the catalyser, 

 considerably more convection is produced than in those in which small 

 quantities are present, owing to the brisker evolution of bubbles of 

 oxygen gas, and in this fact the explanation of Bredig's result is 

 very probably to be sought. 



In accordance herewith, we should expect that in solutions containing 

 very small amounts of catalyser, and in which, consequently, the 

 evolution of oxygen is so slight that no gas-bubbles are produced, 

 no deviation from the law of proportionality between K and the 

 concentration of the catalytic particles would occur. 



This point has not been specially tested by Bredig, but by collating 

 results scattered over his paper, we may gather that the conclusion 

 drawn is correct. Thus, in Table 11, first part, p. 291, we have for a 

 concentration of N/30000 platinum an average value of K of approxi- 

 mately 0*016 min.- 1 , whereas, at the end of Table 13, p. 300, we have, 

 at a later date, with the same platinum solution, diluted to a concentra- 

 tion of N/300000 K, approximately equal to 0*0012 min.- 1 , i.e., only 

 slightly less than one-tenth of the former value. 



In agreement with this, Senter found that in his very dilute solutions 

 the velocity of the reaction was proportional to the concentration of 

 the catalyser, whereas in more concentrated solution it increased more 

 rapidly. 



The Experimental Results on Dependence of Reaction Velocity on Tempera- 

 ture Cannot be Reconciled with Nernst's Hypothesis unless Convection Plays 

 an Important Part. Bredig's results regarding the influence of tem- 

 perature cannot be reconciled with Nernst's hypothesis if we suppose 

 the particles and liquid to be stationary. In this case the only effect 

 of a rise of temperature would be to increase the diffusion coefficient 

 of the solute by about 2J per cent, per degree, and the result of doing 

 this can be seen from the following theorem : If concentrations in 

 a liquid are determined solely by diffusion according to Fick's law, 

 in such a manner that they arise out of a given initial state and a 

 condition not defined as a time-relation which is maintained uniformly 

 throughout the experiment (such as the one that the concentration is 

 permanently kept at zero on the surface of stationary particles of any 

 shape and distribution whatever), then the concentration at any point 

 may be expressed as a function only of the co-ordinates of that point 



