112 AUTO-CATALYSIS AND INFECTION 



AUTO-CATALYSIS AND INFECTION 



In the course of any reaction the products formed may them- 

 selves in certain cases act as catalysts and alter the velocity of 

 the reaction. For example, in the hydrolysis of esters by water, 

 the process is at first so slow as to be inappreciable, but as the 

 process goes on, the hydrogen ions of the acid set free in the reaction 

 itself act as a catalytic agent upon the portion of ester still un- 

 decomposed and hasten the reaction. To such a process Ostwald 

 has assigned the term of " auto-catalysis," and has pointed out 

 that such a process may play an important role in biological pro- 

 cesses, and that the course of such auto-catalytic reactions bear 

 a close analogy to the phenomena of fever. 



If the substance produced acts as a negative catalyst it will 

 have only the effect of making the reaction run more slowly, 

 and as its effect will increase with rising concentration, the result 

 will be that the course of the reaction will resemble that of an 

 ordinary reaction, save that the tendency to run more slowly 

 as the equilibrium point is approached will be increased. 



When, however, the substance formed in the reaction acts as 

 a positive catalyst, the course of the reaction becomes markedly 

 changed in a most interesting fashion. 



For while the ordinary non-catalysed reaction, or a reaction 

 in which the concentration of catalyst lemains constant, as in 

 all those which we have previously considered, the velocity of 

 reaction diminishes steadily onward from the beginning, in an 

 auto-catalytic reaction, as the quantity of auto-catalyst increases 

 as the reaction proceeds, the reaction is correspondingly hastened. 

 Hence a reaction of this type may begin by being barely perceptible, 

 but gathering way as it proceeds like a descending avalanche, 

 may in the end become stormy or explosive. 



Examples of such auto-catalytic reactions are seen in the ten- 

 dency of many explosive substances to spontaneous explosion. Thus 

 if gun-cotton be not most carefully washed from oxides of nitrogen, 

 these products, present in too minute quantity at first to cause 

 any change, may set up a slow and at first inappreciable reaction, 

 which, slumbering at first, gradually increases in velocity and 

 finally fires off the gun-cotton. 



