192 EXPERIMENTAL OBSERVATIONS ON 



tion to run in the opposed direction must be present and ever- 

 increasing in amount as the equilibrium point is neared, so as to 

 stop the reaction at the equilibrium point. Hence before the 

 equilibrium point is reached there must be a decrease in velocity 

 due to the tendency to reversion. 



Accordingly it is not safe to assume that, because a reaction 

 runs to 99 per cent, and over before equilibrium is reached, and 

 is therefore regarded as a complete reaction, it will run up to 

 99 per cent, with the same velocity as if there were no equilibrium 

 point and no tendency to reverse near that point. 1 



As pointed out above, Bayliss has shown that there is a ten- 

 dency, at least, to reversion in the case of caseinogen and trypsin, 

 and actual reversion has been shown with other enzymes, and even 

 in the case of the action of acids upon disaccharides E. F. 

 Armstrong and R. J. Caldwell have demonstrated that there is 

 a tendency to reversal indicated by the rotation of the plane of 

 polarised light beyond the maximum value corresponding to com- 

 plete hydrolysis. 



In fact, the retardation due to products of reaction which 

 causes the velocity in the later stages to fall off from the loga- 

 rithmic expression may in all cases probably be ascribed to the 

 tendency to reversion. The usual view that the drop is due to 

 removal of enzyme from the sphere of action by its combination 

 with one or more of the products of reaction is not incompatible 

 with this supposition. For just as it is supposed that, in order 

 that the action may proceed from left to right, it is necessary for 

 the enzyme to enter into some relationship or combination with 

 the substratum, so it must be supposed that some such relation- 

 ship is necessary with the reaction products, or one of them, in 

 order that the reaction may proceed in the opposite direction from 

 right to left. Nor is it any objection to the view that slowing by 

 the products of reaction is due to the tendency to the establishment 



1 Visser (quoted by Hamburger, Osm. Druck und lonenlehre, vol. iii., p. 97, 

 1904) found that the action of iiwertase upon cane-sugar was not quite 

 complete, as always 1 per cent, of the cane sugar was left. Visser deduced 

 a formula which gave a constant with his own results and those of Henri. In 

 this he first, as recommended above in the text, retained the reversibility 

 expression ; and secondly, introduced a variable for the alteration in intensity 

 of action of the enzyme throughout the reaction. The method suggested in 

 the text for making the second of these two corrections is different from that 

 of Visser. 



