seems to me unscientific to include this condition in their 

 definition. Oppenheimer gives practically Ostwald's definition 

 of a catalyst, as referred to above, with the addition of the 

 proviso that an enzyme is a " catalytically active substance 

 which is produced by living cells." But the origin of a sub- 

 stance does not concern us when investigating its properties, 

 and, as Bredig points out, this fact does not come into con- 

 sideration any more than the fact of the heart being composed 

 of living cells affects the discussion of its properties as a 

 pumping-engine. 



As catalysts, then, the function of enzymes is to accelerate 

 the velocity of reactions, so that the study of enzyme action in 

 the main becomes one of velocity of reaction. Our attention 

 must, accordingly, be directed first of all to this question. But, 

 as will be seen, it is necessary, before we can profitably discuss 

 the general laws of reactions as catalysed by enzymes, to 

 consider shortly how far these reactions are to be regarded as 

 reversible. In other words, is a synthetic process capable of 

 acceleration by the same catalyst that accelerates the corre- 

 sponding analytic one? and in what way is the position of 

 equilibrium affected by the presence of a catalyst ? 



REVERSIBILITY 



If we add ethyl acetate to water we find, on examination of 

 the mixture after some time, that, in addition to ethyl acetate 

 and water, there are present the products of hydrolysis of the 

 former, viz. ethyl alcohol and acetic acid. By analysis of the 

 solution at various intervals of time, we find, moreover, that a 

 state of equilibrium is arrived at. In this condition, provided 

 that no change takes place in other conditions, such as tempera- 

 ture, the relative proportions of ethyl acetate, acetic acid, and 

 alcohol are always the same. Again, suppose that we start 

 from the other end, so to speak, and take acetic acid and alcohol, 

 instead of ethyl acetate, we find that the same relative pro- 

 portion of the three bodies is present when equilibrium is 

 established. We have to deal, therefore, with two opposite 

 reactions. A little consideration will show that, as Van 't Hoflf 

 points out, these two reactions must be regarded as both pro- 

 ceeding simultaneously, but at varying rates in proportion to 

 the concentration of the reacting bodies, until in the equilibrium 

 position they possess the same velocity. In fact, suppose we 



