52 VELOCITY OF REACTION, AND 



is capable of yielding is insufficient to cause the cleavage to occur. 

 Hence the reaction may cease at a different point with different 

 catalysts (see p. 66). 



The only point essential with the simple type of catalyst is 

 that it shall work towards the equilibrium point ; but it can cause 

 movement towards that point not occurring before, alter velocity 

 of reaction, and may fall short of reaching the equilibrium point. 



The experimental evidence with regard to the action of enzymes 

 is entirely in accord with the view expressed in this article. 



If, as has already been pointed out, the appropriate solutions 

 on which the digestive enzymes act be kept in sterilised condition, 

 not the slightest change is observable in any one case, no matter 

 how long the solution is preserved, but if the enzyme is added 

 its effect is apparent in a few minutes. 



Further, the nature of the reaction and of the products formed, 

 as well as the relative amounts of the latter, are often determined 

 by the nature of the catalyst added to the same medium. As, 

 for example, in the catalysis of proteid by pepsin, trypsin, acid, 

 and alkali respectively. Here, in all four cases, the products 

 and their amounts are different. Are we, then, to suppose that 

 all these different reactions to as many different equilibrium points 

 are running concurrently in any given proteid solution, but at 

 so slow a rate as not to be observable ? It is an interesting 

 theoretical speculation ; but it would appear more probable that 

 these different catalysts possess a specific affinity for attacking 

 some definite molecular groupings in the complex proteid molecule, 

 and in each case started a reaction which was not possible until 

 that particular catalyst was present in the solution. 



EQUATIONS FOE VELOCITY OF REACTION 



The various expressions deduced for the value of the energy 

 set free in the reaction give us the driving agent in the reaction, 

 but the unknown values of the resistances opposed to this, and 

 the amount of the effect of catalysts upon them, render the velocity 

 theoretically indeterminate from a knowledge of the energy set 

 free in the reaction only. We have hence, in order to obtain 

 formulae for the expression of the velocity of reaction under 

 different conditions, to introduce empirical constants to denote 



