LIVING MATTER AND ITS PRODUCTS 161 



progress in this direction is the mirror reflex in the base-line of the one 

 given for the opposite direction (X to X'). For if the value of C was 

 positive in passing from A to B, it will be negative in passing from B 



p 

 to A, and value of - RT log p A will be positive at the X' end and 



*B 



p 



negative at the X end, precisely as - RT log p 8 was positive at the 



*A 



X end and negative at the X' end. Hence the same equilibrium point 

 is reached from whichever condition, A or B, we choose to travel. The 

 positions of the lines AA' and - AA' and of BB' and - BB' illustrate 

 the effects of a small and a large value of C either positive or negative* 

 and it is evident that a small value increases the distance of the equili- 

 brium point from the end point, and hence increases the reversibility 

 or incompleteness of the reaction. 1 



We learn accordingly that for a reaction of type P B =K . P A 

 the smaller the chemical energy involved in the change from A to 

 B, the more does the reaction become practically reversible. The 

 same statement is also true, within certain limits, for other types 

 of reaction. 



Now the value of the chemical energy is not measurable ex- 

 perimentally, for heats of reaction as usually measured do not give 

 either C or H of our equations, but instead the heat of reaction for 

 a grm. molecule changed at varying values of P A and P B - This 

 figure, which is the only experimental datum we possess, gives us 

 an integration of a small fraction of H at each stage throughout the 

 process. 



However, the heat of reaction must vary somewhat in the same 

 manner as C, and a small value of heat of reaction indicates a small 

 value of chemical energy, and a large heat of reaction a high value 

 of chemical energy. 



Using this criterion as the best available, we find that experi- 

 ment bears out the above conclusion. In all the typically rever- 

 sible actions, such as the formation of esters, the polymerisation 

 and hydrolysis of carbohydrates, and such reactions as we> have 

 seen above are reversible by enzymes, the heat of reaction is ex- 

 cessively low; so low indeed that it cannot be measured experi- 



1 The shape of the curved line varies with the expression for the value 

 of the osmotic energy, so that the effect of changes in low values of C is com- 

 plicated to follow. But high values of C will always land the equilibrium 

 point upon the asymptotic portion of the curve close to one or other of the 

 two end points. 



11 



