NUTRITION 399 
daily on the effect of other substances which are present. It is not always 
evident just how a third body affects the rate at which one substance is 
converted into another in a chemical reaction, and so doubtless many 
effects of this sort pass unnoticed. But when the effect is pronounced, 
the third body is spoken of as a catalyst, and the effect of the catalyst 
on the reaction is known as catalysis. By such agents reactions, so 
slow as to be unnoticed, may be greatly accelerated and become evident; 
and others, which might be very rapid, are retarded, even until they are 
negligible. 
Enzymes. Among the catalytic agents (which are varied and not at 
all confined to living beings) are certain substances produced by organ- 
isms and called enzymes. These are widely different in their action, 
though they all seem to be of protein nature, so far as their chemical char- 
acter is made out. The great difficulty in doing this lies in the impossi- 
bility, up to date, of separating them from the other proteins of the cell 
and obtaining them in any certain state of purity. In general they act 
best within certain narrow limits of temperature, such as 30-45 C., 
and most are totally destroyed at such temperatures as 60-75 C. Small 
quantities of free acid or alkali may facilitate their action; while certain 
metallic ions, e.g. Hg, Cu, Ag, may retard or inhibit their ordinary 
effect, just as they " poison " a live cell. 
There seems to be a great variety of enzymes, each producing an ap- 
propriate effect upon certain foods; but others are known which have 
to do with reactions quite apart from the digestive changes. The di- 
gestive enzymes, then, are only part of a larger class of bodies, whose 
number and variety are only imperfectly known. 
Reversible action. The action of a number of enzymes is known to 
be reversible; i.e. they not only, under certain conditions, hasten the 
otherwise imperceptible decomposition of a particular substance into 
two or more simpler compounds, but also, under other conditions, ac- 
celerate the combination of the simpler substances into the more com- 
plex one. Indeed, it seems likely that the constructive action of enzymes 
may soon be shown to be as important as the destructive. This action 
would be of the greatest importance in the making of complex foods from 
simpler ones, such as the formation of starch from glucose, of cane sugar 
from glucose and fructose, of proteins from amido-compounds, etc. 
But the knowledge of this constructive action is yet very scanty. 
Carbohydrate enzymes. Diastase is one of the most important and 
widespread enzymes. It is found in practically all parts of plants, but 
