CHEMICAL CHANGES IN LIVING MATTER. FERMENTS 179 



many cases the combination of gases can be hastened by increasing 

 the surface to which they are exposed, as by passing them over broken 

 porcelain or over powdered charcoal. This catalytic effect is certainly 

 connected with the power of a solid to condense gases at its 

 surface, and is therefore proportional to the extent of surface exposed. 

 Thus the efficacy of platinum in hastening the combination of 

 hydrogen and oxygen is in direct proportion to its fineness of sub- 

 division, and is best marked when the metal is reduced to ultra- 

 microscopic dimensions, as in the colloidal solution of platinum. 

 Every colloidal solution must be regarded as presenting an enormous 

 surface in proportion to the mass of substance in solution. There is 

 therefore a direct proportionality between the power of a substance 

 to condense a gas on its surface and its power to quicken the velocity 

 of chemical changes in which the gas is involved. The same process of 

 condensation occurs with dissolved substances. Just as the pressure 

 of a gas in immediate contact with a solid body is diminished, so the 

 osmotic pressure of a substance in solution is diminished at the surface. 

 There is therefore a diffusion of dissolved substances into the surface, 

 i.e. a concentration of dissolved substances at the surface of contact. 

 It was suggested by Faraday that the catalytic property of surfaces 

 was due to this condensation of molecules, and the consequent bringing 

 of the two sets of molecules within each other's sphere of influence. 

 Whether this is the sole factor involved is doubtful, since mere com- 

 pression of gases or increased concentration of solutions does not in 

 the majority of cases result in such a quickening of the velocity of 

 reaction as is brought about by the effect of the surface. 



It is possible that this condensation effect or adsorption may be 

 in every case combined with the second factor which we must now 

 consider, namely, the formation of intermediate products. If we 

 boil an alkaline solution of indigo with some glucose, the indigo is 

 reduced with oxidation of the glucose. The mixture therefore becomes 

 colourless. On shaking up with air the colourless reduction product 

 of the indigo absorbs oxygen from the atmosphere, and is re-trans- 

 formed into indigo. These two processes can be repeated until the 

 whole of the glucose is oxidised, and the process can be made continuous 

 if air or oxygen be bubbled through a heated solution of glucose contain- 

 ing a small trace of indigo. In this case the indigo does not add to the 

 energy of the reaction. It appears unchanged among the final products 

 and a small amount may be used to effect the change of an infinite 

 quantity of glucose. It therefore may be said to act as a ferment or 

 catalytic agent. Instead of an alkaline solution of indigo, we may 

 use an ammoniacal solution of cupric oxide for the purpose^of carrying 

 oxygen from the atmosphere to the glucose. This is reduced to cuprous 

 hydrate on heating with the sugar, but cupric hydrate can be at once 



