CHEMICAL CHANGES IN LIVING MATTER. FERMENTS 181 



mediate bodies, certain simple analogies may help us to comprehend how a 

 factor which introduces no energy can yet assist the process. Thus a man 

 might stand to all eternity before a perpendicular wall twenty feet high. Since 

 he cannot reach its top at one jump, he is unable to get there at all. The intro- 

 duction of a ladder will not in any way alter the total energy he must expend 

 on raising his body for twenty feet, but will enable him to attain the top. Or 

 we might imagine a stone perched at the top of a high hill. The passive 

 resistance of the system, the friction of the stone, and its inertia will tend to keep 

 it at rest, even though it be on a sloping surface and therefore tending to slide 

 or roll to the bottom. If, however, it be rolled to the edge, to a point where 

 there is a sudden increase in the rapidity of slope, it may roll over, and having 

 once started its downward course, its 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 infinitely 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 depends 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 decomposition into hydrogen and arsenic. This decom- 

 position is not immediate, but takes a certain time, and the velocity with which 

 the change occurs depends on the temperature. At any given temperature 

 the amount of substance changed in the unit of time varies with the concentra- 

 tion 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 tempera- 

 ture, 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 X x C y ). In the case of the unimolecular reaction, halving the con- 

 centration 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 



