68 INTRODUCTION TO GENERAL PHYSIOLOGY 



poi>ents present, it is found that there are twice the number of 

 molecules of the ester and water that there are of alcohol and 

 acid. Now, suppose that we begin with alcohol and acid, we find 

 that they combine to form ester until a certain amount has been 

 produced. The reaction then stops, and on examination we find 

 that the proportion of the four components is the same as in the 

 former case. When the reaction is accelerated by a catalyst, 

 similar conditions hold, since no energy is supplied by it. There- 

 fore the same catalyst may appear to have either synthesising 

 or hydrolysing powers, according to the substances on which it 

 acts (E., p. 190). 



The position at which no further change takes place is called 

 an equilibrium, or the reaction a balanced one, because what is 

 happening is that the two opposite reactions are both proceeding 

 at equal rates at this point. 



Our problem is, then, one of velocity of reactions. The funda- 

 mental law concerned is that of mass action, which states that the 

 rate at which a particular reaction takes place is proportional to 

 the mass of the reagents present in a certain volume. It follows 

 that the relative amounts of the original reagents and of their 

 products present in the final equilibrium is also proportional to 

 their original masses. Thus, if we increase the mass of water in 

 the example given above, we correspondingly increase the rate of 

 the hydrolytic reaction, and since the rate is maintained greater, 

 relatively to the opposite one, also in the equilibrium position 

 itself, there will be present finally a larger proportion of acid and 

 alcohol than if there were less water present to start with. When 

 the proportion of water is great, the synthetic reaction may be so 

 slow that it is difficult to detect it, and the final result may appear 

 to be one of complete hydrolysis. 



In stating the law of mass action correctly, it is necessary to 

 say that the rate of the reaction is proportional to the concentration 

 of the active masses of the reagents. The reason for inserting the 

 word "active" will be clear if we suppose that we are dealing 

 with a reaction brought about by hydrogen ions, and that the 

 acid used is a weak one. The active mass is not that of the 

 acid added, but that part of it which is electrolytically dis- 

 sociated. 



The word "concentration," or mass in unit volume, is necessary, 

 as can be seen by realising the kinetics of the process. The 

 reaction takes place because, as the molecules shoot hither and 

 thither, some of them hit those with which they enter into com- 

 bination. It is only a certain number of these encounters that 

 actually result in combination, but it is obvious that the number 

 of effective meetings that take place in unit time, that is the rate 



