IN LIVING MATTER 35 



that is, at the equilibrium point the products of the osmotic 

 pressures of the substances reacting upon the one side of the 

 equation raised in each case to a power corresponding to the 

 number of molecules entering into the reaction, bear a constant 

 ratio to the similar products of the substances on the other side 

 of the equation of reaction. 



All the other special cases previously considered can obviously 

 be deduced from this general equation of equilibrium. Otherwise 

 it is of little practical interest, for reactions more complicated 

 than those given under the special types are too difficult to deal 

 with experimentally. 



It may be pointed out that since the osmotic pressures are 

 proportional to the molecular concentrations of the reacting sub- 

 stances, in all the above equations the P representing osmotic 

 pressure may be replaced by a C representing the molecular con- 

 centration. This is the form in which such equations are usually 

 given, but since the energy changes which are responsible for 

 bringing about a reaction and establishing an equilibrium are 

 dependent upon the osmotic energies of the dissolved substances 

 it has been thought advisable to give the equations in the form 

 shown above. 



Before passing from the subject of equilibrium in solution to 

 that of velocity of reaction, and the effect of enzymes and other 

 energy-transformers upon reaction and velocity of reaction, it 

 may be profitable to consider briefly the conditions which deter- 

 mine whether a reaction is practically reversible or not ; that is 

 to say, which determine whether the equilibrium point shall lie 

 at an appreciable and practically measurable distance from either 

 extreme end of the reaction. 



In the first place, it is clear from the form of the general 

 equation of equilibrium that the osmotic pressure, and therefore 

 the molecular concentration, of none of the reacting substances 

 on one side can be zero, 1 unless the osmotic pressure of one or 

 more of the reacting substances on the other side also became 

 zero. But a zero value on both sides could only mean that sub- 

 stances taking part in the reaction are absent, and this condition 



1 For if any one factor in the product P A . P B . PC, &c., becomes zero, the value 

 of the whole product is zero, and this cannot be equated to any finite value on 

 the other side. 



