PHYSIOLOGICAL 365 



been attained. In fermentations carried on for practical purposes 

 the product is gradually removed as it is formed, else what has been 

 effected would often be undone. If the products of a fermentative 

 action are removed as they are formed, and new fermentable material 

 is provided, the enzyme goes on without being used up or enfeebled. 

 This is often called the inexhaustibility of enzymes. 



We referred in passing to spongy platinum, which has curious 

 properties. If the metal in this state soaks up a mixture of oxygen 

 and hydrogen at ordinary temperature, the two gases are brought 

 very closely together. There is an enormous internal surface in the 

 interstices of the metal, and on that surface the two gases are 

 condensed and union takes place. So far as is known, nothing 

 happens to the platinum, but it acts as a catalyst; and something 

 like this happens with ferments. It may be that the colloidal nature 

 of the ferment (with innumerable ultra-microscopic particles in 

 suspension in a fluid) supplies a suitable surface on which a reaction 

 takes place. But it is also possible that the ferment enters into 

 temporary union with the substance that is being changed, and then 

 gets free again to combine with more. As yet we do not know. But 

 this is certain, that the vital changes that go on in our body and 

 within plants and animals depend in great part on ferments, which 

 are able to act very rapidly within small compass, which are 

 able to do a great deal, though present in minimal quantity, which 

 can go on and on without being exhausted by the changes that they 

 accelerate. It has often been a source of wonder that living creatures 

 can act and react so rapidly; part of the answer is to be found in 

 the abundance of different kinds of ferments. 



Let us take a glimpse at their great variety. There are those, like 

 the pepsin of the stomach, which break down the large protein 

 molecules that form an important part of our food. Others, like the 

 ptyalin made by the salivary glands of the mouth, change starch 

 into sugar. The sweetbread or pancreas forms a ferment that digests 

 proteins, another that digests starchy food, and a third that changes 

 fat into fatty acids and glycerine. There are ferments that work 

 by taking up water, and there are others that split up water and 

 liberate oxygen. Some enzymes break complex substances down, but 

 others build up. It is not too much to say that life, whether in animal 

 or plant, is quite inconceivable without ferments or enzymes. 



They are all-important, but the difliculty is to understand how 

 they work. Sir William Bayliss made a comparison which may help 

 a little in regard to catalysts in general. Suppose a brass weight at 

 the top of an inclined plane of polished plate-glass. If the glass is 

 tilted at the right angle the weight slides down. But if the bottom 

 of the weight is well oiled, the sliding down will be much more 

 rapid. The action of a catalyst is like the action of the oil. In both 

 cases there is a removal of some resistance to change — the friction 



