28 THE NATURE OF THE ACTIVITIES OF MICROORGANISMS. 
looked upon the process as a purely chemical change; but event- 
ually Pasteur and others proved that fermentation is a physiological 
process, brought about only by the growth of yeast. 
For a long time there was no conception of more than one type of 
fermentation. But even in the days of Schwann it was recognized 
that there was another type of chemical changes which resembled 
the yeast fermentation in some respects. This was the sort of 
changes which occur in the digestion of food and which were known 
even in those early days to be due to certain materials present in the 
digestive fluids. As early as 1833 a substance called diastase was 
known which could convert starch into sugar, and in 1836 pepsin, 
causing the digestion of proteids in the stomach, was discovered. 
Although these processes were realized to be different from the 
fermentation produced by yeast, their general similarity led to their 
being called fermentations, and the active substance in each case 
was known as a ferment. , 
It very soon appeared that these two types of fermentation were 
different in some fundamental respects. Whereas alcoholic fermen- 
tations, produced by yeast, can be stopped by certain chemicals 
like glycerine, the other type of fermentation, due to digestive fer- 
ments, cannot be stopped by such materials. Moreover, the 
microscope shows that the second type of ferments does not contain 
any living bodies like yeast. Hence, while yeast is a living ferment, 
the digestive ferment cannot be regarded as living. But these latter 
ferments contain some substances which are very peculiar in their 
nature. Like living organisms, they are destroyed by high heat, and 
they act only at a moderate temperature. Unlike most simple 
chemical changes, these fermentations do not occur at high tempera- 
tures, but become impaired and stopped when the temperature 
rises slightly above 100 F. It has been found possible to isolate 
from the fermenting material (saliva, gastric juice, etc.) the ferment- 
ing body. From the digestive juices a substance can be obtained in 
the form of a powder which can be preserved indefinitely. It con- 
tains 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. 
