1YPES OF FERMENTATION AND DECAY 31 
can be preserved indefinitely. It contains no living cells, is not 
alive, and clearly does not belong to the same class of bodies with 
the yeast plant. But it will cause the fermentation to take place 
when added to a fermentable substance. 
These discoveries led to a sharp separation of ferments into two 
different classes. On the one hand were those which, like yeast, 
were produced by organisms and were called organized ferments, 
and on the other were those which contained no organisms and 
were called unorganized ferments. These latter ferments re- 
ceived the name of enzymes, which name is now in most common 
use. 
What are enzymes? Over this question there has been not a 
little discussion. But in spite of it we know very little about them. 
They seem to be chemical bodies, capable of producing chemical 
changes in certain substances. But their action seems to differ 
from chemical actions in general in that the ferment itself is ap- 
parently not used up in the process. Whether this is strictly true 
may, from theoretical reasons, be doubted; but at all events, no 
direct evidence exists that they are used up, and everything indi- 
cates that they can act indefinitely. A very small amount of an 
enzyme may produce a very large amount of chemical change, and 
the enzyme does not appear to enter into the new chemical bodies 
in any degree whatever. In some respects the enzymes resemble 
living bodies, especially in their relation to heat, and in the fact 
that they are always produced by living organisms. But in other 
respects they are sharply marked off from the organized ferments. 
A long series of disinfectants like glycerine and alcohol, which kill 
the organized ferments, have no influence upon the enzymes. 
The latter do not increase by growth in the fermenting material, 
nor does their continued action depend upon their nutrition. The 
opposite is true of yeast and bacteria. The distinction between 
these two classes of fermentations has been kept clearly in mind 
in the development of our knowledge of fermentation in the last 
half-century. 
