LIVING MATTER AND ITS PRODUCTS 159 



follows because in the equation P A K . P*, where n is greater 

 than unity, that as P A and P B diminish together P A must get rela- 

 tively smaller compared to P B , and with sufficient dilution tends to 

 become infinitely small relatively to P B . 



The other factors which affect the position of the point of 

 equilibrium and determine whether a reaction shall be practically 

 reversible or not are the value of C, the chemical energy involved 

 in the change from the one phase to the other, and the absolute 

 temperature of the reaction; for changes in either of these affect 

 the value of the constant K. 



The effects of changes in C and T upon the point of equilibrium 

 can best be understood by following out the energy changes as the 

 substances pass in the reaction from the one condition to the other. 

 The matter can best be understood by graphic illustration as in the 

 diagram on p. 158, in which are represented the energy changes at each 

 instant as the substance passes from the one form to the other according 



p 



to the simplest type of equation, that in which H = C - RT log p-'. 



*^A 



The base-line XX' represents the zero line of energy exchange, 

 ordinates above the base-line representing quantities of energy set free 

 by the reaction at any stage, and ordinates below, energy required in 

 order that the reaction may proceeed. At X the substance is all in the 

 form A and at X' in the form B ; at intermediate points fractions are 

 in the two forms proportionate to the distances from X and X'. The 

 horizontal line above or below XX' represents by its height above or 

 below XX' the amount of chemical energy set free (positive when 

 above XX' and negative when below) when a grni. molecule passes 

 from the form A to the form B. The curved line represents the 

 osmotic energy set free at each point in the reaction when a grni. 

 molecule changes form at the particular osmotic pressures present at 



that point. The height of this line above or below XX' is given 



p 

 numerically by the expression RT log p B , and the sign is reversed 



J^A 



in plotting it so that at each stage the difference in the heights above 

 XX' of the straight line and the curved line give the value of H, the 

 energy set free at that stage in the reaction, for a grm. molecule changed, 



p 

 according to the equation H = C - RT log pA Tracing now the value 



-^A 



of the amount of energy set free in the reaction at each stage, we 



p 

 have at X P E = 0, and therefore log p B = log 0= - oo . Therefore at 



'A 



