304 PRINCIPLES OF GENERAL PHYSIOLOGY 



weight arrives at the bottom with considerable kinetic energy. This teaches us 

 that the actual products of a catalysed reaction are not necessarily identical with 

 those obtained in the absence of a catalyst. 



The next point is that, within limits, we can vary the rate of fall by the 

 application of much or little oil. Although the catalyst does not affect the 

 position of the equilibrium point, the rate at which this is reached is directly 

 proportional to the amount of the catalyst present. Moreover, comparing the 

 relative efficiency of different amounts of oil, we note that small amounts produce 

 at first a much greater effect than the same amounts added after there is already 

 a considerable amount present. This is characteristic of adsorption and applies 

 to enzymes, the catalysts of living organisms, particularly. 



We may next note the difference between what is sometimes called " trigger 

 action " and catalysis. Suppose that the plane is, for convenience, raised to a 

 rather steeper position than before, and that the weight is prevented from sliding 

 down by the support of a catch of some kind. When the catch is removed, the 

 weight falls, but the amount of work done in moving the catch has no effect what- 

 ever on the subsequent process ; whether the trigger moves very stiffly or easily, 

 the weight descends at the same rate. The true catalyst, oil, exerts its action 

 throughout the whole of the descent, whereas the action of the trigger is completed 

 before the fall begins. Supersaturated solutions are cases of "trigger action." 

 They remain indefinitely as such until infected with a crystal, and then the rate of 

 crystallisation is independent of the amount of crystals added. The same fact is 

 exhibited in the case of supercooled acetic acid, as shown by B. Moore (1893) in 

 his experiments, in which a long tube was used. 



One more fact, the meaning of which will be appreciated later, is that in our 

 model the oil partially disappears by sticking to the glass, so that the whole of it is 

 not present on the weight at the bottom. In a certain sense we may say that it 

 has " combined " with some other constituent of the system. In some catalytic 

 reactions we meet with phenomena of this nature ; for example, in the chamber 

 process of sulphuric acid manufacture, the nitric acid, which acts as a catalyst, 

 slowly disappears, being used up in subsidiary reactions. 



It is held by some that a catalyst may actually start a reaction which was not in progio> 

 on account of chemical " friction." Our model, again, shows this phenomenon. The friction 

 between the weight and the glass may be so great that no movement appears to take place 

 until oil is applied. The question is rather of theoretical interest and may almost be said to 

 be one of words. The use of the word "friction" implies the possibility of movement, and 

 it may be said that the weight really does move, but is arrested again. There are also all 

 degrees of friction, with corresponding rates of movement, and the rate of a reaction may be so 

 slow that it is, in practice, impossible to say whether it is actually proceeding or not. 



The remark may be made here, that the number of reactions known to be capable of 

 catalytic acceleration is very large and increasing every day. 



Discussion of the mechanism of catalysis will best be deferred. It is probably of a different 

 nature in different cases. 



Before passing on to the subject of enzymes proper, a few words are necessary 

 with respect to reversible reactions in relation to catalysis. 



In the example chosen to begin with, namely that of the action of acid on the 

 ester system, we saw that the position of equilibrium is unaltered by the presence 

 of the catalyst. Now this position of equilibrium is due to the simultaneous 

 existence of the two opposite reactions of hydrolysis and synthesis, which are 

 proceeding at an equal rate at this moment. 



In order to see how the actual position of this equilibrium depends on the relative rate 

 of two opposite reactions, we may take a rough illustration, which must not be followed in 

 too great detail. Suppose that two people start to walk towards one another from two distant 

 places. Where they meet will clearly depend on the relative rates at which they \valk. 

 Supposing that their rates are the same, they will meet half way between the places from 

 which they start. Imagine that one of them is excited, " catalysed," so that, instead of 

 walking, lie runs. He will meet the other man before he has taken many steps from home. 

 It is also obvious that, if one ran, the only way by which the two could meet at the same 

 place ("equilibrium position") is that the other man runs also, and at the same rate. He 

 must be equally " catalysed " in fact. 



We see, therefore, that the catalyst, acid, must act on both the hydrolytic and 

 synthetic components of the reactions. That this is actually the case has been 





