LIVING MATTER AND ITS PRODUCTS 157 



total ethereal extract and free fatty acid, were shown to be due to 

 unaltered soaps dissolved out by the ether. 



IV. The most general case of equilibrium in solution is that 

 where an indefinite number of substances react together, and the 

 equation, as demonstrated on p. 150, becomes 



P^F^.etc 

 P^.Pt.PS.etc." 



which ma be written 



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

 sures 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 pro- 

 portional to the molecular concentrations of the reacting substances, 

 in all the above equations the P representing osmotic pressure may 

 be replaced by a C representing the molecular concentration. 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 



