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THE AMERICAN NATURALIST [Vol. XLII 



fair velocity if a few drops of a mineral acid are added to 

 the solution, while the addition of a trace of a particular 

 enzyme (invertase from plant or animal) enormously in- 

 creases the rate of change, so that the whole 128 grams 

 of cane-sugar are soon hydrolyzed to hexose. The reac- 

 tion progresses quantitatively in the same sort of way as 

 before, giving a logarithmic curve of sugar-content. In- 

 deed the same graphic curve, Fig. 1, A, would represent 

 the facts if the value of n were reduced from many hun- 

 dred minutes to quite a few. 



The most striking point about this new state of things 

 is that the added body is not used by its action, but the 

 acid or enzyme is still present in undiminished amount 

 when the reaction is completed. 



Such actions were at first styled "contact" actions, but 

 are now known as catalytic actions, because we have 

 learned that the catalyst does not work just by contact but 

 by combining with the sugar to form an intermediate ad- 

 dition compound, and that this compound is then split up 

 by the water liberating the catalyst again, but freeing the 

 sugar part, not as cane-sugar, but combined with the water 

 to form two molecules of hexose. 



On many chemical reactions, finely divided metals such 

 as platinum and gold have a very powerful catalytic ac- 

 tion. Thus platinum will cause gaseous hydrogen and 

 oxygen to unite at ordinary temperatures, and will split 

 up hydrogen dioxide with the formation of oxygen. The 

 intermediate stages in this catalytic decomposition may be 

 summarily simplified to this— 



H a 2 + Pt = PtO + H 2 



and 



PtO + H 2 2 = Pt + 0, + H 2 0. 



Thus the reaction goes on and on by the aid of the ap- 

 pearing and disappearing ''intermediate compound" PtO 

 till at the end the rL0 2 is all decomposed and the platinum 

 is still present unaffected. 



The enzymes are the most powerful catalytic agents 



