PRESIDENTIAL ADDRESS. 889 



way as before, giving a logarithmic curve of sugar-content. Indeed the same 

 graphic curve, fig. 1, A, would represent the facts if the value of n were reduced 

 from many hundred minutes to quite a few. 



The most striking point about this new state of things is that the added body 

 is not used up by its action, but the acid or enzyme is still present in undiminished 

 amount when the reaction is completed. 



Such actions were at first styled ' contact ' actions, but are now known as 

 catalytic actions, because we have learned that the catalyst does not work just by 

 contact but by combining with the sugar to form an intermediate addition 

 compound, and that this compound is then split up by the water liberating the 

 catalyst again, but freeing the sugar part, not as cane-sugar, but combined with 

 the water to form two molecules of hexose. 



On many chemical reactions, finely divided metals such as platinum and gold 

 have a very powerful catalytic action. Thus platinum will cause gaseous 

 hydrogen and oxygen to unite at ordinary temperatures, and will split up 

 hydrogen dioxide with the formation of oxygen. The intermediate stages in this 

 catalytic decomposition may be summarily simplified to this — 



H3O,, + Pt = PtO + IIjO and PtO + H,0, = Pt + 0. + H,0. 



Thus the reaction goes on and on by the aid of the appearing and disappearing 

 * intermediate compound ' PtO till at the end the HjO,, is all decomposed and 

 the platinum is still present unaftected. 



The enzymes are the most powerful catalytic agents known, and most of 

 them are specifically constituted to effect the hydrolysis, oxidation, reduction, 

 or splitting of some definite organic compound or group of compounds containing 

 similar radicals. 



Innumerable enzymes have in late years been isolated from the plant-body, 

 so that it would seem that there is one present to catalytically accelerate each of 

 the slow single changes that in the aggregate make up the complex metabolism 

 of the plant. 



The law of mass applies with equal cogency to catalytic reactions. If twice 

 the amount of acid is added to a solution of cane-sugar (or twice the amount 

 of enzyme) then the reaction velocity is doubled, and hydrolysis proceeds twice 

 as fast. As the catalyst is not destroyed by its action, but is continually being 

 set free again, the concentration of the catalyst remains the same throughout 

 the reaction; while, on the contrary, the amount of cane sugar continually 

 decreases. 



If the catalyst be present in great excess the amount of hydrolysis will be 

 limited by the amount of cane-sugar present, and as this is used up so the reaction 

 will progress by a logarithmic curve as in fig, 1, A. In this case b may represent 

 the amount of catalyst. If, on the contrary, there is a large amount of sugar and 

 very little acid or enzyme present, so that the catalyst becomes the limiting 

 factor, then we happen upon a novel state of things; for by the law of mass the 

 rate of hydrolysis will now remain constant for some time till the excess of sugar 

 is so far reduced that it in turn becomes a limiting factor to the rate of change. 

 In this case the velocity curve would consist of a first phase with a straight 

 horizontal line of uniform reaction -velocity leading into the second phase of a 

 typical falling logarithmic curve (see fig. 1, c). These conditions have been 

 experimentally examined by Horace Brown and Glendinning, and fully explained 

 and expounded by E. F. Armstrong in Part II. of the critical ' Studies in Enzyme 

 Action.' '■ 



Having now outlined the three fundamental principles of reaction-velocity, 

 the law of mass, and the catalytic acceleration of reaction-velocity, we are in a 

 position to consider the broad phenomena of metabolism or chemical change in 

 the living organism from the point of view of these principles of chemical 

 mechanics. 



' Pror, Rmj, Soc, vol. Ixxlii. 1904, p. ijll,. 



