CATALYSTS AND ENZYMES 131 



to exist, and in a few others they have been introduced as an 

 hypothesis to explain irregularities of reaction. In the majority 

 of cases there is no experimental proof for the existence of such 

 compounds. Further, as Ostwald has pointed out, the theory 

 of intermediate reaction fails entirely to account for the action 

 of negative catalysts, for here the indirect action must go more 

 slowly than the direct one. Accordingly the direct reaction would 

 run at its own undiminished rate by itself, for the quantity of 

 catalyst is too small to take all the substratum at the beginning 

 and compel its conversion by the slower rate. 



The conclusion may accordingly be drawn that although inter- 

 mediate reaction may and probably does occur, and increase 

 reaction velocity in a considerable number of catalytic reactions, 

 it is evident that this does not furnish a universal explanation 

 of catalysis applicable to all cases. 



III. Theory of Altered Solubility and Different Reaction-velocity of 

 the Substratum in the Catalyst. The outline of this theory was first 

 given by Faraday, who showed that all porous and finely divided 

 bodies, such as porcelain clay, wood charcoal, animal charcoal, 

 and some metals, notably platinum, possessed the property of 

 taking up different gases and condensing them, and ascribed to 

 this property the power such bodies were known to possess of 

 inducing or favouring chemical reaction, as, for example, in causing 

 the union of a mixture of hydrogen and oxygen at ordinary tempe- 

 ratures in the case of spongy platinum. Such an effect can obviously 

 only obtain in a heterogeneous system, that is, a system in which 

 there is discontinuity of structure and chemical composition, for 

 in a homogeneous system there is no opportunity for variation 

 in concentration of the components of the system. For example, 

 in the above case of spongy platinum, there are portions of the 

 system where on account of the presence of the platinum within 

 molecular distance of action, the hydrogen and oxygen can become 

 condensed in the metal or on its surface, and the action can 

 therefore proceed more rapidly than in those parts of the system 

 outside the range of action of the platinum where the reaction, 

 if it proceeds at all, can only proceed at the rate at which it takes 

 place in the absence of platinum. 



Similarly, in the case of a colloidal catalyst, such as most 

 enzymes are supposed to be, it may be supposed that the catalyst is 

 present in the form of ultra-microscopic particles suspended in the 



