306 PRINCIPLES OF GENERAL PHYSIOLOGY 



A point of significance, in view of the mode of action of catalysts, is that they 

 exercise a powerful action even when present in very minute amounts. This is 

 not surprising when we remember that they usually reappear at the end of the 

 reaction in an unaltered state, and are therefore ready for further work. We have 

 already seen that the position of equilibrium under a particular kind of catalyst is 

 not affected by the amount of this catalyst present. What is observed is that 

 the time taken to attain equilibrium is shorter with the larger concentration of 

 catalyst. This fact is a useful criterion in deciding whether, in a particular case, 

 we have to deal with a catalytic process or with one in which the constituents 

 enter into combination in molecular proportions. In the latter case, of course, the 

 amount of product will depend on the amount added of the reagent whose nature 

 we desire to test. Although it is not a matter for surprise that the final effect of 

 a catalyst should be independent of its amount, it is striking to note how very 

 minute are the quantities which are able to produce considerable results. 



For instance, Erode (1901, p. 289) found that the reaction between hydrogen peroxide and 

 hydriodic acid was appreciably accelerated by the presence of 1 gram molecule of molybdic acid 

 in 31,000,000 litres. Again, a preparation of invertase, which probably consisted only to a 

 small percentage of the active catalytic agent, was found by O'Sullivan and Tompson (1890) 

 to hydrolyse 200,000 times its weight of cane-sugar. These facts will serve t impress ujmn 

 the reader the point that the amount of chemical eneri/y which a catalyst is capable <,f 

 supplying to .a reaction is negligible, and the assumption cannot be used to explain any of 

 the phenomena. This will be referred to again. 



Heterogeneous Systems. We shall find presently that the particular catalysts 

 of especial interest to us are in the colloidal state. We have, indeed, seen already, 

 in our first typical examples, that lipase and emulsin act in liquids in which they 

 are completely insoluble. It is therefore necessary to consider briefly the 

 mechanism of reactions in systems of more than one phase. The theory of these 

 reactions is due chiefly to Nernst (1904). They may be said to take place in three 

 stages. Suppose that the catalyst is present in the form of solid particles, and 

 that the other components, which are to be brought into reaction, are in true 

 solution. In order that they shall be influenced by the catalyst, it is obviously 

 necessary that they shall diffuse to it, since it is not uniformly distributed through- 

 out the system. The rate of diffusion is the first factor. If these solutes lower 

 surface energy, as practically all solutes do, they will next be concentrated by 

 adsorption at the interface between the catalyst and the solution. Adsorption is 

 the second stage. The third stage is the chemical reaction proper. It is clear 

 that the increased concentration on the surface will, in itself, hasten the reaction 

 by mass action, and this was, in fact, the explanation suggested by Faraday 

 (1839, 1, p. 184) for the effect of platinum in causing combination of oxygen and 

 hydrogen. Whether all cases of heterogeneous catalysis can be explained on these 

 lines is doubtful. In certain cases of catalysis in homogeneous systems, as we 

 shall see later, there is an intermediate compound formed between the catalyst and 

 the reagents ; but it is certain that this does not apply to all cases ; indeed, it 

 appears to be exceptional. 



Faraday's views on the possibility of the close approximation of oxygen and hydrogen on 

 the surface of platinum being sufficient to cause their molecules to enter into combination led 

 to a long discussion with De la Rive, who held that there is an intermediate formation of some 

 oxide of platinum. With our knowledge of Faraday's wonderful insight into the mechanism of 

 natural phenomena, we may well be inclined to think that he was most likely on the right side in 

 this case. Kohlrausch remarked, " Er riecht die Wahrheit," "he smells the truth" (see 

 Tyndall's "Faraday as a Discoverer," 1870, p. 55). 



In the discussion of this question,' it is to be remembered that there is reason to belirvi- 

 that it is during the actual process of condensation itself that the molecular stresses result in 

 unusual chemical activity (see Hardy's note to the paper by Drury, 1914, p. 175). 



In all heterogeneous reactions, the rate of the reaction, as measured, is 

 naturally that of the slowest member of the series. Adsorption is a rapid process, 

 when the substances are in contact, so that the rate of the reaction will be either 

 that of diffusion or of the chemical component of the reaction. When the catalyst 

 is in colloidal solution, the length of the way to be passed over by diffusion is very 

 short, since the active substance is almost uniformly distributed ; so that the 

 chemical reaction itself, unless of great rapidity, controls the rate of the whole 



