162 PHYSIOLOGY 



momentum will carry it to the bottom. The amount of energy set free by the stone in 

 its fall will not vary whether the course be a uniform one, or whether it falls over a 

 precipice at one time and rolls down a gentle slope at another. It is evident that by 

 a mere alteration of the slope or, in the case of a chemical reaction, of the velocity of 

 part of its course, a change in the system may be initiated and brought to a conclusion 

 which without this alteration would never take place. 



Since the action of ferments, like that of catalysts, consists essentially 

 in the quickening up of processes which would otherwise occur at an in- 

 finitely slow velocity, it is possible that in their case also the formation 

 of an intermediate compound may be involved in the reaction. Light may 

 be thrown upon this question by a study of the velocity of the reaction 

 induced by the action of a ferment. 



It is well known that the velocity of a reaction dep3nds on the number of molecules 

 involved. As an illustration, we may take first the case of a reaction involving a 

 change in one substance. If arseniuretted hydrogen be heated, it undergoes decom- 

 position into hydrogen and arsenic. This decomposition is not immediate, but takes 

 a certain time, and the velocity with which the change occurs depends on the tempera- 

 ture. At any given temperature the amount of substance changed in the unit of time 

 varies with the concentration of the substance. If, for instance, one-tenth of the gas 

 be dissociated in the first minute, in the second minute a further tenth of the gas will 

 also be dissociated. Thus, if we start with 1000 grammes of substance, at the end 

 of the first minute 100 grammes will have been dissociated, and 900 of the original 

 substance will be left. In the second minute one-tenth again of the remaining substance 

 will be dissociated, i.e. 90 grammes, leaving 810 grammes. In the third minute 81 

 grammes will be dissociated, leaving 729 grammes. The amount changed in the 

 unit of time will always bear the same ratio to the whole substance which is to be 

 changed, and will therefore be a function of the concentration of this substance. Put 

 in the form of an equation, we may say that 0, the amount changed in the unit of time, 

 will be equal to KG, where K is a constant varying with the substance in question and 

 with the temperature, and C represents the concentration of the substance. The 

 equation < = KG applies to a monomolecular reaction. 



If two substances are involved, the equation will be rather different. In this case 

 the amount of change in a unit of time will be a function of the concentration of each 

 of the substances, and the form of the equation will be <f> = K(C T -f- C y ). In the case 

 of the unimolecular reaction, halving the concentration of the substance will halve the 

 amount of substance changed in the unit of time. In the case of a bimolecular reaction, 

 halving each of the substances will cause the amount of change in the unit of time to be 

 reduced to one-quarter of its previous amount. If now either a unimolecular or a 

 bimolecular reaction be quickened by the addition of a catalyser or ferment, and the 

 ferment enter into combination with one of the substances at some stage of the reaction, 

 it is evident that our equation must take account also of the concentration of the 

 ferment or catalyser. In the case of the catalytic effect of molybdic acid on the inter- 

 action between hydrogen peroxide and HI, there was definite evidence of a reaction 

 taking place between the molybdic acid and the peroxide, resulting in the formation 

 of an intermediate compound, namely, permolybdic acid. Erode has shown that the 

 interaction of the molybdic acid is revealed in the equation representing the velocity 

 of the reaction. Without the addition of molybdic acid the equation would be : 



4>=K(C H2 02 XC HI ) 

 After the addition of molybdic acid, the equation becomes : 



< = K(CH Oj + 7 C molybdic acid)C, r] , 



when y is another constant depending on the molybdic acid. If ferments act in a 

 similar way by the formation of intermediate compounds, this fact should be revealed 

 by a study of the velocity at which the ferment action takes place, 



