680 PHYSIOLOGY OF DIGESTION AND SECRETION. 



to act by catalysis. Many similar catalytic reactions are known, 

 and the chemists have reached the important generalization that 

 in such reactions the catalyzer, platinum in the above instance, 

 simply hastens a process which would occur without it, but much 

 more slowly. A catalyzer is a substance, therefore, that alters 

 the velocity of a reaction, but does not initiate it. This idea is 

 illustrated very clearly by the catalysis of hydrogen peroxid. This 

 substance decomposes spontaneously into water and oxygen accord- 

 ing to the reaction H 2 O 2 = H 2 O 4* O, but the decomposition is 

 greatly hastened by the presence of a catalyzer. Thus, Bredig has 

 shown that platinum in very fine suspension, so-called colloidal 

 solution, exerts a marked accelerating influence upon this reaction; 

 one part of the colloidal platinum to 350 million parts of water may 

 still exercise a perceptible effect. Now, the blood and aqueous ex- 

 tracts of various tissues also catalyze the hydrogen peroxid readily, 

 and this effect has been attributed to the action of an enzyme (cata- 

 lase). The view has been proposed, therefore, that the enzymes of 

 the body act like the catalyzers of inorganic origin: they influence 

 the velocity of certain special reactions. Such a general conception 

 as this unifies the whole subject of fermentation and holds out the 

 hope that the more precise investigations that are possible in the case 

 of the inorganic catalyzers will eventually lead to a better under- 

 standing of the underlying physical causes of fermentation. It 

 should be borne in mind, however, that some of the best known of the 

 ferment actions of the body, such as the peptic or tryptic digestion of 

 protein, fit into this view only theoretically and by analogy. As a 

 matter of fact, albumins at ordinary temperatures do not split up 

 spontaneously into the products formed by the action of pepsin; 

 if we consider that the pepsin simply accelerates a reaction already 

 taking place, it must be stated that this reaction at ordinary 

 temperatures is infinitely slow, that is, practically does not occur. 

 At higher temperatures, however, similar decompositions of al- 

 bumin may be obtained without the presence of an enzyme. 



Reversible Reactions. It has been shown that under proper 

 conditions many chemical reactions are reversible, that is, may 

 take place in opposite directions. For instance, acetic acid and 

 ethyl-alcohol brought together react with the production of ethyl- 

 acetate and water: 



CH 3 COOH + C 2 H 6 OH = CH 3 COOC 2 H S + H 2 O. 



Acetic acid. Alcohol. Ethyl-acetate. Water. 



On the other hand, when ethyl-acetate and water are brought 

 together they react with the formation of some acetic acid and 

 ethyl-alcohol, so that the reaction indicated in the above equation 



