166 PHYSIOLOGY 



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 concen- 

 tration is always being renewed, and kept constant by the breaking down 

 of the intermediate 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) : 

 VELOCITY OF TRYPSIN REACTION 



N 

 6 c.c. 8 per cent, caseinogen + 2 c.c. AmHO + 2 c.c. 2 per cent. 



trypsin at 39 C. 



1st 10 minutes K =0-0079 



2nd 0-0046 



3rd 0-0032 



4th 0-0022 



5th 0-0016 



7th 0-0009 



&c. &c. 



The cause of this rapid diminution in the velocity of change is probably 

 complex. One factor may be an auto- destruction of the ferment, which is 

 known to occur in watery solution. That this is not the only, or even the 

 chief, factor involved is shown by the fact that, when the action of trypsin 

 on caseinogen has apparently come to an end, it may be renewed by further 

 dilution of the mixture or by removal of the end-products of the action by 

 dialysis. It is evident that, in this retardation of the later stages of ferment 

 action, the end-products are concerned in some way or other, and the retarda- 

 tion can be augmented by adding to the digesting mixture the boiled end- 

 products of a previous digestion. The retarding effect of the end-products 

 resembles in many ways that observed in a whole series of reactions which 

 are known as reversible. 



As an example of such a reaction we may take the case of methyl acetate and water. 

 When methyl acetate is mixed with water, it undergoes decomposition with the forma- 

 tion of methyl alcohol and acetic acid. On the other hand, if acetic acid be mixed 

 with alcohol, an interaction takes place with the formation of methyl acetate and 

 water. These changes are represented by the equation : 



MeC 2 H 3 2 + HOH ^ MeOH + HC 2 H 3 O 2 . 

 methylacetate water methylalcohol acetic acid 



Each of these changes has a certain velocity constant, and, since they are in opposite 

 directions, there must be some equilibrium point where no change will occur, and 



