84 ACTION OF LIGHT RAYS 



reversal of action within and without the cell, and as it is not yet 

 sufficiently appreciated an example illustrating it may be adduced. 

 Taking the heats of combustion as determined by Thomsen, 

 the following equation gives the heat of reaction in the change 

 between glucose and maltose (K=100 grm. calories): 



C 6 H 12 6 + C 6 H 12 6 <i> C 12 H 22 O n + H 2 0-33K, 

 ( Glucose) + ( Glucose) ^=i> (Maltose), 



6737K + 6737K ^ > 13,507K -33K. 



Thus the reaction is apparently a slightly endothermic one, as 

 it runs from glucose into maltose (a rule which holds for all the 

 similar reactions), but observe that as the reaction runs in this 

 direction the osmotic energy of the system diminishes, for every 

 two molecules of glucose passing out of the system are replaced by 

 only one of maltose. Hence for every grm. molecule of glucose 

 passing into maltose a definite amount of osmotic energy is set free, 

 which can be converted into chemical energy, and hence make an 

 apparently endothermic reaction run without added energy from 

 without. The amount of osmotic energy set free by the disappear- 

 ance of the molecules of a constituent varies with the pressure at 

 which it disappears, and therefore increases with the concentration 

 of the solution, 1 and hence it came about that the first successful 

 experimental proof of reversibility was given by Croft Hill 2 by the 

 use of exceedingly concentrated sugar solutions, and that all such 

 conjugations occur in concentrated solutions. Local concentration 

 will have a like effect, and in living cells concentrations on surfaces 

 and interfaces will produce such a result. 



In many of the reactions of inorganic chemistry the differences 

 in totals of chemical energy on the two sides possess a high magnitude 

 at ordinary temperatures as, for example, in the reaction between 

 hydrogen, oxygen, and water. Here the changes in osmotic energy 

 are too insignificant to produce an appreciable effect, and so the 

 reaction runs practically completely to one end or phase. 



But in the type of reaction with which we are here dealing of 

 conjugation or cleavage where the chemical energy change is rela- 

 tively small, the osmotic change becomes a powerful factor. 



In the green cell of the living plant the formaldehyde can be 

 condensed on an interface and there conjugate, although general 



1 The thermodynamic proof of this is given in a later chapter. 



2 Journ. CTiem. Soc., vol. Ixxiii., p. 634 (1898); vol. Ixxxiii., p. 578 (1903). 



