282 SCIENCE PROGRESS 



quantity of finely divided platinum is sufficient to cause com- 

 bination to take place at ordinary temperatures. Again, the 

 oxidations produced by hydrogen peroxide proceed in many 

 cases at a very slow rate, which can be enormously increased 

 by the addition of traces of iron or manganese. 



What, then, are the characteristics of such phenomena ? 

 And are there to be found in the living organism bodies acting 

 in a similar way to the platinum or iron in the above cases? 



The essential property of a catalyst is that given in the 

 well-known definition by Ostwald, in which it is stated to be 

 a body which accelerates (or in some cases retards) a chemical 

 reaction. The original definition also states that the chemical 

 reaction in question is one that, left to itself, proceeds at an 

 extremely slow rate. But it is obvious that it is theoretically 

 immaterial what the original velocity of the reaction is : any 

 foreign substance that changes the rate of the reaction is a 

 catalyst. At the same time the practically important cases are 

 those where the reaction is almost or quite inappreciable without 

 the assistance of a catalyst. A further characteristic of these 

 bodies is that they do not appear in the final products of the 

 reaction and usually are to be found unaltered at the end. 

 There are, however, exceptions to this last statement, as we 

 shall see later, owing to the instability of the catalyst under 

 the conditions of the reaction. 



At the outset it is important to be quite clear as to the 

 distinction between what may be called " trigger-action " and 

 catalysis. Let us suppose a block of some solid material resting 

 at the top of an inclined plane, which has a somewhat rough 

 surface, and is at such an angle that the block would, if left 

 to itself, slide down very slowly. This is prevented by a stop 

 of some kind which can be removed, say by pulling a trigger. 

 Now it is plain that it makes no difference either to the rate 

 at which the block slides down or to the energy developed in 

 its fall whether the trigger is very easily worked or is stiff 

 and needs the expenditure of much energy to move it. But let 

 us suppose that the block is slowly sliding and that we apply 

 a catalyst, in the shape of grease, to the inclined plane. The 

 block will now move faster and the rate will be proportional 

 to the amount of grease used, although the energy developed 

 will, as before, be unaltered. Here we see illustrated two 

 additional properties of catalysts — viz., that the amount of 



