ACTION OF ENZYMES AND CELLS 181 



which on integrating yields 



1 



a -x 



kt + const., 



as before when t = 0, x = 0, hence - = const., and on subtracting the 

 two equations 



a x a 

 x 



or 



a (a-x) 

 and for the reaction constant k 



Jet, 



~t.a.(a-x) 



These reactions, where two substances undergo change in con 

 centration on the left-hand side of the equation of reaction when 

 the reaction is proceeding from left to right, are spoken of as bi~ 

 molecular reactions. The best known examples are where an ester, 

 containing two monad radicles, such as ethyl acetate, is saponified 

 by an alkali. The reaction is different from that with an acid, for 

 the alkali as well as the ester is changing its concentration during the 

 reaction, while in the hydrolysis by the acid, the hydrogen ion concen- 

 tration which affects the change in the ester remains constant, and the 

 water produced being merely added to the water of the solvent has no 

 effect on the progress of the reaction. 



Tri-molecular reactions and higher are rare, from the tendency of 

 the reaction to break down into stages. For a tri-molecular reaction, 

 in which three molecules react together on t.he left-hand side of the 

 equation, the equation of velocity, supposing the three substances 

 present in equi-molecular proportions at the outset, would be 



dx_ 



dt~ ( a " x > > 



The pursuit of these higher reactions would, however, only be an 

 exercise in mathematics, and so may be left on one side to pass on to 

 experimental work. 



