DIGESTION 317 



form of platinum black, greatly hastens the decomposition, and the 

 oxygen bubbles off. A colloidal solution of platinum, prepared by 

 passing electric sparks between two platinum electrodes immersed 

 in distilled water, and containing the metal in the form of ultra- 

 microscopic particles, is still more effective. The precise nature of 

 the surface effect is not entirely clear. One factor appears to be 

 an increase in the concentration of dissolved substances at the sur- 

 face, and the better opportunity for mutual action thus afforded to 

 the ferment and the substrate, as the substance acted on by the 

 ferment is termed. The great extension of the surface cannot be 

 the only factor in the catalysis ; otherwise any fine powder or sus- 

 pension would have a catalytic action. But kaolin, or fine sand, or 

 colloidal solutions of ordinary proteins or gelatin, have little, if 

 any, effect on the decomposition of hydrogen peroxide. 



(2) Enzymes may produce their effects by contributing to the 

 formation of bodies intermediate between the substrate and the end- 

 products. If the time required for the formation of a given quantity 

 of the intermediate compound and the time required for the decom- 

 position of this compound into the final products of the ferment 

 action are in sum less than the time required for the direct change 

 of the substrate into the end-products, the enzyme will clearly act 

 as a catalyser of the reaction. It has been shown that in the case 

 of certain inorganic catalysers this does occur. There is some evi- 

 dence that the ferment actually combines with the substrate, the 

 combination then breaking up to form the end-products. 



The Quantitative Estimation of Ferment Action. Since we have as 

 yet no certain method of freeing the digestive ferments from im- 

 purities, our only quantitative test is their digestive activity. And 

 since a very small quantity of ferment can act upon an indefinite 

 amount of material if allowed sufficient time, we can only make com- 

 parisons when the time of digestion and all other conditions are the 

 same. If we find that a given quantity of one gastric extract, acting 

 on a given weight of fibrin, dissolves it in half the time required by 

 an equal amount of another gastric extract, or dissolves twice as 

 much of it in a given time, we conclude that the digestive activity of 

 the pepsin is twice as great in the first extract as in the second. But 

 this does not permit us to say that the one contains twice as much 

 pepsin as the other. For it has been found that the amount of diges- 

 tion in a given time is not directly proportional to the quantity of 

 ferment present, but to the square root of the quantity of ferment 

 (Schiitz's law). This law was deduced by Schiitz for pepsin, but is 

 said to hold also for trypsin, steapsin, and ptyalin (Pawlow, Vernon). 

 To determine the amount of proteolysis the nitrogen of the protein 

 which has gone into solution may be estimated (p. 482). The 

 following table shows the results of one experiment : 



