184 PHYSIOLOGY 



Moreover, if we take only the earlier stages of the ferment action, it 

 is found that, with small proportions of ferment, equal amounts of 

 substrate are changed in successive intervals of time until about 

 10 per cent, has been hydrolysed. This is shown in the following 

 Table : 



2 PER CENT. LACTOSE WITH LACTASE 



Time Amount hydrolysed 

 ^ hour ..... 3-2 



f 6-4 



1 9-6 



2 hours 16-4 



3 . .... 20-8 



These results can be interpreted only by assuming that the first stage 

 in the reaction is a combination of ferment with substrate. It is only 

 this compound which represents the active mass of the molecules, 

 i.e. the molecules of substrate which are undergoing change. This 

 compound, as soon as it is formed, takes up water and breaks down, 

 setting free the hydrolysed substrate and the ferment, which is at 

 once ready to combine with a further portion of the substrate. In 

 such a case the velocity of reaction must be directly proportional to 

 the amount of ferment, and the same absolute quantity of substance 

 will continue to be changed in succeeding units of time. Supposing, 

 for instance, we had a load of bricks at the bottom of a hill which 

 had to be transferred to the top, and five men to effect the trans- 

 ference. The rate of transference would be directly proportional to 

 the number of men employed ; we could double the rate by doubling 

 the men. Moreover, the number of bricks carried in each unit of 

 time would be the same. Five men would carry as many bricks in 

 the second ten minutes as they would in the first, and so on. On the 

 other hand, the velocity with which the transference was effected would 

 be independent of the number that is, the concentration of the bricks 

 at the bottom of the hill. The active mass of bricks could be regarded 

 as that number carried at any moment by the transferring factor, 

 namely, the men. The equation of change would be < = KG, where 

 C is the concentration of the ferment. This concentration is always 

 being renewed, and kept constant by the breaking down of the inter- 

 mediate product, so that the rate of change would be continuous 

 throughout the experiment. 



On the other hand, when the amount of ferment is relatively large, 

 the rate of change, though at first very rapid, tends continuously to 

 diminish. This is shown by the following Table representing the rates 

 of change, during succeeding intervals of ten minutes, in a caseinogen 

 solution to which a strong solution of trypsin had been added 

 (Bayliss) : 



