IN LIVING MATTER 39 



B we choose to travel. The positions of the lines AA' and - AA' and 

 of BIT 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 equilibrium 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 

 experimentally, for heats of reaction as usually measured do not 

 give either C or H of our equations, but instead the heat of reac- 

 tion 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 

 reversible actions, such as the formation of esters, the polymerisa- 

 tion and hydrolysis of carbohydrates, and such reactions as we 

 have seen above are reversible by enzymes, the heat of reaction 

 is excessively low ; so low indeed that it cannot be measured 

 experimentally, since the effects obtained fall within the limits 

 of experimental error. 



Thus the heats of reaction for the formation of the following 

 esters from their constituent alcohols and acids in rational calories 

 are: ethyl formiate, 6 K; ethyl acetate, 20 K; ethyl-hydrogen 

 sulphate, 49 K ; amyl- hydrogen -sulphate, 2K; and ethyl 

 oxalate, 38 K. These amounts are scarcely measurable in the 

 volumes of fluid in which they occur, and on account of the slow- 

 ness with which the reactions take place, and in comparison with 



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 complicated 

 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, 



