160 PHYSIOLOGY 



two gases be passed over heated platinum. The same reaction, namely, 

 the combination of sulphur dioxide with oxygen, may be quickened by the 

 addition of a small trace of nitric oxide, and this fact is made use of in the 

 manufacture of sulphuric acid on a commercial scale by the ordinary lead- 

 chamber process. Hydrogen peroxide and hydriodic acid slowly interact 

 with the formation of water and iodine. This reaction may be quickened 

 by the addition of many substances, among which we may mention molybdic 

 acid. 



There is moreover a specificity in the action of catalysers, though not 

 so well marked as with ferments. Whereas all the disaccharides are con- 

 verted 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 (maltase, 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 oxidation 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 oxida- 

 tion 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 reaction 

 increases rapidly with rise of temperature, in the case of ferments this in- 

 crease 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 considerably below that of boiling water. Thus ferment 

 actions, like catalytic 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 c 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, coagu- 

 lated, 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 transcriptions in 



