. 



ACTION OF THE ENZYMES. 107 



acetic acid and ethyl alcohol, or ethyl acetate and water, 

 have undergone complete decomposition the reaction comes 

 to a standstill. 



A reaction such as we have just spoken of, which can take 

 place from right to left as well as from left to right, is called 

 a reversible reaction. We indicate such a reversible reaction 

 as follows: 



CH 3 COOH +C 2 H 5 OH = CH,COOC 2 H 5 +H 2 0. 



Acetic acid Ethyl Ethyl acetate Water 



alcohol 



It can readily be seen that when equilibrium is estab- 

 lished in a reversible reaction the four substances reacting 

 with each other are present in the reaction mixture. The 

 characteristic feature of such a condition of equilibrium is 

 found in the fact that under the same external conditions 

 it is always the same no matter from which side it is reached. 

 In other words, it is immaterial whether chemically equivalent 

 amounts of acetic acid and ethyl alcohol or chemically 

 equivalent amounts of ethyl acetate and water are mixed 

 together. The condition of equilibrium reached in either 

 case is the same. 1 



Although we say ordinarily that when chemical equilibrium 

 has been established the reaction has come to a standstill, 

 this is in reality incorrect. When chemical equilibrium is 

 established between the two members of a chemical equation 

 it really means that the chemical changes are still going on, 

 only the amount of change in the one direction is exactly 

 counterbalanced by the reverse change in the opposite 

 direction. The reaction is therefore stationary. 



As has long been known, maltase acting upon maltose is 

 unable to bring about its complete analysis into dextrose. 

 The reaction comes to a standstill when some 85 percent of 

 the original maltose is split. HILL now tried the interesting 



1 See COHEN Physical Chemistry for Physicians. Translated by 

 MARTIN H. FISCHER, New York, 1903, p. 68. 



