84 PHYSIOLOGY OF ALIMENTATION. 



bine to form sulphurous acid, which is oxidized to sulphuric 

 by the nitrogen tetroxide formed through the union of the 

 nitrogen dioxide with oxygen. The nitrogen dioxide, which 

 plays the role of a catalyzer in this reaction, unites first of all 

 with oxygen to form nitrogen tetroxide, but the nitrogen 

 dioxide is restored immediately in that the sulphurous acid 

 takes away the oxygen from the nitrogen tetroxide. The 

 nitrogen dioxide acts, therefore, only through its mere pres- 

 ence, appearing in an unaltered state at the end of the reac- 

 tion. Nor does it initiate a chemical reaction, for the oxida- 

 tion to sulphuric acid occurs whenever sulphurous acid is 

 J^xposed to oxygen. But the reaction,' which under these 

 ^^tvumstances occurs only very slowly, occurs very rapidly 



^1 Phen nitrogen dioxide is present. 



J| There are other facts, into a discussion of which we cannot 



^B go here, that speak against this conception of the action of a 

 %ferment, and seem to indicate that certain physical charac- 

 Iteristics of the ferment (or, in general, any catalytic agent), 

 such as its surface, electrical condition, etc., constitute the 

 real cause of its peculiar action. However this problem may 

 ultimately be settled, the fact seems fairly well established 

 that, except for a simple decomposition which is to be dis- 

 cussed later, the ferment appears in an unaltered state at the 

 end of the reaction. 



-* (6) In infinite time the amount of chemical change brought 

 about by a ferment is independent of the concentration of the 

 ferment. This means that a small amount of a ferment will 

 bring about as much chemical change as a larger one, pro- 

 vided unlimited time is given. In this regard, therefore, a 

 ferment differs from the ordinary constituent of a reaction 

 mixture. We know from the law of chemical mass-action of 

 GULDBERG and WAAGE that in any ordinary chemical reaction 

 the amount of the chemical change is proportional to the 

 concentration of the reacting substances. This does notfliold 

 in the case of ferments^ where the amount of ferment is in 

 nearly all cases exceedingly small when compared with the 



