158 Jourtial oj Comparative Neurology and Psychology. 



to cause the formation and destruction of photosynthetic sub- 

 stances, such as those mentioned above- The inversion of the 

 sense of reaction, therefore, cannot be due to a photosynthetic 

 process. May it not be due to the effect of light on the chemical 

 equilibrium of some other substance ? 



One of the more important results of recent investigation ir, 

 physical chemistry is the establishment of the fact that substance^ 

 in chemical equilibrium are dynamic and not static, as had for- 

 merly been supposed. It, for instance, alcohol be added to acetic 

 acid, it is well known that water and ethyl acetate will be formed; 

 but it is also true that if water be added to ethyl acetate, the form- 

 ation of alcohol and acetic acid results, that is, the former reaction 

 is reversed. When the reaction in both of these cases has reached 

 a state of equilibrium, there is a certain amount of each of the 

 following substances present: Alcohol, acetic acid, ethyl acetate, 

 and water, and this amount remains constant; but the reaction 

 continues; alcohol and acetic acid react to form ethyl acetate and 

 water just as fast as ethyl acetate and water react to form alcohol 

 and acetic acid. These reactions are expressed as follows : 



C2H,OH+HOOC.CH3^CH3 COO QH^ + H^O. 



This indicates that two reactions are taking place simultaneously 

 in opposite directions. The relative amount of substance indi- 

 cated in the two members of an equation representing equilibrium 

 in chemical reaction, depends upon the nature of the substances 

 and the environment, i. e., the temperature, pressure, etc. If, for 

 instance, the temperature of compounds in equilibrium be raised, 

 the equilibrium will be destroyed and the reaction in one direction 

 will take place faster than that in the other. When equilibrium 

 is again restored the relation of the amounts of the different sub- 

 stances will no longer be the same as it was at the lower tempera- 

 ture. If the temperature is lowered, the rate of motion will 

 increase in the opposite direction. Jones ('02, p. 514) states this 

 as follows: "The effect of a rise in temperature is to favor the 

 formation of that system which absorbs heat when it is formed. . . 

 Increase in pressure diminishes the volume and, therefore, favors 

 the formation of that system which occupies the smaller volume." 



Reversible chemical reactions were formerly supposed to be 

 quite exceptional, but it is now known that they are not. Jones 

 ('02, p. 481) writes: "We must regard chemical reactions in 

 general as reversible." 



