1904.] Catalysis ~by Colloidal Metals and similar Substances. 357 



With regard to this result, it must be observed that a colloidal 

 solution of hsemase does not afford the most suitable example from 

 which to draw theoretical conclusions, as nothing whatever is known 

 regarding the size of the active particles, their nature and their number. 

 More definite results can be obtained from a criticism of Bredig's 

 experiments, for in them we are dealing with platinum, or gold, 

 substances of known properties, the absolute quantities of which are 

 stated for every experiment. Moreover, although we are without 

 definite information regarding the shape, size, and number of the 

 particles, yet we are safe in assuming that in those colloidal solutions 

 which were at all stable, no particles occurred of a diameter greater 

 than 0*5 /*.* In order to carry out any calculations, the diffusion 

 coefficient of hydrogen peroxide is also requisite. So far as I am 

 aware, this number has not been determined. If we take as represen- 

 tative of diffusion coefficients of non-electrolytes at 25, the follow- 

 ing values determined by Scheffer for molecules heavier than 

 that of hydrogen peroxide : 



Urea, 0'94 [1 + (25 - 7'5) 0'025] 10~ 5 = 1-35 x 10~ 5 cm 2 /sec. ; 

 Chloral hydrate, 0'64 [1 + (25 - 9) O025] 10~ 5 = 0'86 x 10~ 5 cra 2 /sec. ; 

 Mannite, 0-44 [1 + (25 - 10) 0-025] = 0*60 x 10~ 5 cm 2 /sec., 



we see that we shall probably not make a very large error, and not 

 take a too high value, if we assume for hydrogen peroxide a diffusion 

 coefficient of 10~ 5 cm 2 /sec. 



Summary of the Available Experimental Results. It will be convenient 

 to preface this discussion by a summary of the available results. 



In neutral and acid solutions Bredig found that the reaction pro- 

 ceeded as one of the first order, that is, it was expressed by the 

 equation 



Jlog2=K (1), 



in which C is the initial concentration of the hydrogen peroxide, C its 

 concentration after the time t, and K a constant. The same equation 

 holds for hsemase, according to Senter's experiments. In alkaline 

 solution, no general formula was found by Bredig to express the 

 results. The constant K is greatly influenced by the addition of 

 certain substances (poisons) ; it varies more rapidly than proportionally 

 with the concentration of the catalyser, and increases geometrically 

 with change of temperature, rising to its threefold value every 20. 



On the Relative Size and Available Surface of the Particles in Wholly and 

 Partially Poisoned Solutions. I do not propose to enter into a discussion 

 of the effect of poisons more deeply than is required for the primary 

 objects of the present paper. It seems that two explanations are 

 compatible with Nernst's views. Firstly, in the case of substances 

 * Loc. cit., vol. 31, p. 272. 



