160 PHYSIOLOGY 



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 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 cata- 

 lytic 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 subdivision, 

 and is best marked when the metal is reduced to ultra-microscopic dimen- 

 sions, 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 in- 

 volved. The same process of condensation may occur with dissolved sub- 

 stances. In all cases where the presence of a substance in solution diminishes 

 the surface tension of the solvent, there is 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 compression 

 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 je-transformed 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 containing 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 re-formed 

 by shaking up the cuprous solution with air. It has been thought that many 

 or all of the catalytic reactions occur in the same way by two stages, i.e. 



