PRESIDENTIAL ADDRESS* 681 



' adsorption-compound.' Moreover, the suggestion that the union is a chemical 

 one tends to deprive the conception of chemical combination of its characteristic 

 quality, namely, the change of properties. Dye-stuff and charcoal are chemi- 

 cally unchanged by adsorption. 



The origin of adsorption from surface tension is easily'' able to explain why 

 it is less as the temperature rises, as we find e.xperimentally. As we have just 

 seen, surface tension diminishes with increase of temperature. 



Let us next consider what will happen if tlie liquid phase contains in solu- 

 tion a substance which lowers surface tension and is also capable of entering 

 into chemical reaction with the material of which the other, solid, phase con- 

 sists. For example, a solution of caproic acid in contact with particles of 

 aluminium hydroxide. On the surface of the solid, the concentration of the acid 

 will be increased by adsorption and, in consequence, the rate of the reaction 

 with it will be raised, according to the law of mass action. Further, suppose 

 that the liquid phase contains two substances which react slowly with each 

 other, but not with the solid phase. They will be brought into intimate contact 

 with each other on the surface of the solid phase, their concentration raised and 

 the rate of their interaction increased. One of the reagents may clearly be the 

 solvent itself. But in all these cases the rate of the reaction cannot be 

 expressed by a simple application of the law of mass action, since the active 

 masses are not functions of the molecular concentrations, but of the surface of 

 the phase boundaries. The application of these considerations to the problem 

 of the action of enzymes and of heterogeneous catalysis in general will be 

 apparent. That the action of enzymes is exerted by their surfaces is shown, 

 apart from the fact that they are in colloidal solution, by the results of 

 experiments made in liquids in which the enzymes themselves are insoluble in 

 the usual sense, so that they can be filtered off by ordinary filter paper and the 

 filtrate found to be free from enzyme. Notwithstanding this insolubility, 

 enzymes are still active in these liquids. The statement has been found, up to 

 the present, to apply to lipase, emulsin, and urease, probably to trypsin, and 

 the only difficulty in extending it to all enzymes is that of finding a substrate 

 soluble in some liquid in which the enzyme itself is not. That adsorption is a 

 controlling factor in the velocity of enzyme action has been advocated by myself 

 for some j^ears, but it is not to be understood as implying that the whole action 

 of enzymes is an 'adsorption phenomenon,' whatever may be the meaning of 

 this statement. The rate at which the chemical reaction proceeds is controlled 

 by the mass of the reagents concentrated on the surface of the enzyme phase at 

 any given moment, but the temperature coefficient will, of course, be that of a 

 chemical reaction. 



The thought naturally suggests itself, may not the adsorption of the reacting 

 substances on the surface of the enzyme suffice in itself to bring about the 

 equilibrium at a greater rate, so that the assumption of a secondary chemical 

 combination of a chemical nature between enzyme and substrate may be super- 

 fluous? I should hesitate somewhat to propose this hypothesis for serious con- 

 sideration were it not that it was given by Faraday as the explanation of one of 

 the most familiar cases of heterogeneous catalysis, namely, the union of oxygen 

 and hydrogen gases by means of the surfaces of platinum and other substances. 

 The insight shown by Taraday into the nature of the phenomena with which he 

 was concerned is well known and has often caused astonishment. Now, this 

 case of oxygen and hydrogen gases is clearly one of those called ' catalytic ' by 

 Berzelius. The fact th,at the agent responsible for the effect did not itself 

 suffer change was clear to Faraday. I would also, in parenthesis, call attention 

 to the fact that he correctly recognised the gold solutions which he pi'epared as 

 su.spensions of metallic particles, that is. as what we now call colloidal solutions. 

 Although the systematic investigation of colloids, and the name itself, were due 

 to Graham, some of the credit of the discovery should be given to the man who 

 first saw what was their nature. Adsorption, again, was accurately described 

 by Faraday, but without giving it a name. 



I confess that there are, at present, difficulties in the way of accepting con- 

 centration by adsorption as a complete explanation of the catalytic activities of 

 enzymes. It is not obvious, for example, why the same enzjTne should not be 

 able to hydrolyse both maltose and saccharose, as it is usually expressed. 



