178 PHYSIOLOGY 



are converted by acids into the corresponding monosaccharides, a 

 ferment such as invertase acts only on cane sugar, and has no 

 action on maltose or lactose, each of which requires a specific ferment 

 (rnaltase, lactase) to effect their ' inversion.' But we find many 

 examples of a restricted action even among inorganic catalysers. 

 Thus potassium bichromate will act as the catalyser for the oxida- 

 tion of hydriodic acid by bromic acid, but not for the oxidation of 

 the same substance by iodic acid. Iron and copper salts in minute 

 traces will quicken the oxidation of potassium iodide by potassium 

 persulphate, but have no influence on the course of the oxidation of 

 sulphur dioxide by potassium persulphate. Tungstic acid increases 

 the velocity of oxidation of hydriodic acid by hydrogen peroxide, but 

 has no effect on the velocity of oxidation of hydriodic acid by bromic 

 acid, and these examples may be multiplied to any extent. One 

 cannot therefore regard the limitation of action of the ferments as 

 justifying any fundamental distinction being drawn between the 

 action of this class of substances and catalysts. 



Whereas the influence of most catalysers on the velocity of a reac- 

 tion increases rapidly with rise of temperature, in the case of 

 ferments this increase occurs only up to a certain point. This point 

 is spoken of as the optimum temperature of the ferment action. If the 

 mixture be heated above this point the action of the ferment rapidly 

 slows off and then ceases. This contrast, again, is only apparent. 

 The ferments are unstable bodies easily altered by change in their 

 physical conditions, and destroyed in all cases at a temperature con- 

 siderably below that of boiling water. Thus ferment actions, like cata- 

 lytic actions, are quickened by rise of temperature, but the effect of 

 temperature is finally put a stop to by the destruction of the ferment. 

 The same applies to those inorganic catalysers whose physical state is 

 susceptible, like that of the ferments, to the action of heat. Thus the 

 colloidal platinum ' sol ' exerts marked catalytic effects on various 

 reactions, e.g. on the decomposition of hydrogen peroxide and on the 

 combination of hydrogen and oxygen. The reaction presents an optimum 

 temperature, owing to the fact that the colloidal platinum is altered, 

 coagulated, and thrown out of solution when this is heated to near 

 boiling-point. We may therefore employ either class of reactions in 

 trying to form some conception of the processes which are actually 

 involved. 



Very many theories have been put forward to account for this 

 action of catalysers or of ferments. Many of them are merely tran- 

 scriptions in words of the processes which actually occur, and fail 

 to throw any light on their real nature. The essential phenomena 

 involved fall directly into two classes. In the first class we must 

 place those which are determined by the influence of surface. In 



