FOOD DIGESTION AND RESPIRATION 69 



of the reaction, is the greater the more molecules there are in the 

 way to be hit against. 



We saw above that the essential property of a catalyst is that 

 of increasing the rate of a reaction. Their importance is therefore 

 greatest when the reaction is, by nature, a very slow one, and this 

 applies in a marked degree to those of living systems. 



There are two ways in which catalysts act, according to whether 

 they are in true solution, forming homogeneous systems, or present f 

 in a distinct phase, as a solid or in colloidal solution, forming a 

 heterogeneous system. In the former case the effect is known 

 to be due sometimes to the formation, first of all, of a compound 

 between the catalyst and the components of the reacting system. 

 This compound is then decomposed rapidly with separation of the 

 products of the reaction and the reappearance of the catalyst itself. 

 The whole process takes place more quickly than the reaction 

 proceeds by itself. It must be admitted, however, that there are 

 cases of homogeneous catalysis to which this explanation is not 

 readily applied, and although the production of intermediate 

 compounds is not altogether excluded as a stage in the action of 

 enzymes, it does not concern us here, because our catalysts, the , 

 enzymes, are in colloidal solution and act, in some way, at the I 

 surfaces of contact with the solution containing the reacting 

 substances. Can we form any conception of what happens ? 



By the law of mass action, if we can increase the concentration 

 of the reagents, we shall increase the rate of the reaction. Further, 

 we have seen that if substances decrease surface energy, they are 

 concentrated by adsorption on the surface. What evidence is there 

 in the case of enzymes or other heterogeneous or insoluble catalysts 

 that adsorption plays a part ? Let us examine a fairly simple case, 

 that investigated by Faraday, where platinum surfaces were found 

 to bring about rapid combination between oxygen and hydrogen 

 gases. Strong evidence was brought in support of the view that 

 this was due to condensation of the gases on the surface of the 

 platinum, and in this way brought within range of the forces causing 

 their combination with each other (p., pp. 306 and 326). It was 

 shown by Faraday that the surface must be perfectly clean ; that 

 is, if another substance had already obtained possession, oxygen 

 and hydrogen were kept out to a great extent. Moreover, chemical 

 reaction with the platinum, forming intermediate compounds, was 

 excluded. No matter how the surface was cleaned, by mechanical 

 or by oxidising or reducing agents, the method was effective. 

 Chemical reaction between platinum and oxygen is also excluded 

 by the facts that nitrous oxide and hydrogen are caused to combine, 

 and that similar effects "are produced by most, if not all, solid 

 bodies." 



