30 THE NATURE OF THE ACTIVITIES OF MICROORGANISMS. 
duces butyric acid in butter; the lactic fermentation, which often 
causes the souring of milk and various other products, and which is 
responsible for the ripening of cream; the acetic fermentation, which 
produces acetic acid and forms vinegar; the proteolytic or peptonizing 
fermentation, which renders soluble certain insoluble proteids, an 
example of which is found in the ripening of cheese; the oxidizing 
fermentation, which causes the oxidation of organic matter, as in 
the fermentation of tobacco; the nitrifying fermentation, which con- 
verts ammonia into nitrates or nitrites; the denitrifying fermentation, 
which converts nitrates into nitrites or simpler compounds by 
depriving them of oxygen. Then there are the phenomena of putre- 
faction and decay, which are endless in variety and which lie at the 
bottom of continued soil fertility. 
There is a much longer list of the unorganized ferments, or 
enzymes, derived from both plants and animals. Some of the 
most important are the following : 
Diastase, found in both plants and animals, which changes 
starch to sugar; inulase, which has a similar action upon inulin; 
invertase, trehalase, rafinase, melizitase, lactase, which act upon 
sugars, changing their chemical formula; or, as the chemist says, 
inverting them. Emulsin, myrosin, erythrozym, tannase, lotase, 
and some others, which act upon chemical substances called gluco- 
sids; pepsin, trypsin, from animal digestive juices, and galactase 
from milk, together with bromelin, papam, and vegetable trypsin, 
from plants, which act upon proteids, causing them to change 
into simpler compounds. These are called proteolytic enzymes. 
Lipase acts upon fats, splitting their chemical molecules; rennet 
acts upon milk, causing it to curdle; thrombase is in the blood and 
is the immediate cause of blood clotting; cytase is an important 
enzyme, acting upon several parts of a plant cell and causing the 
cell structure to disintegrate; pectase causes the formation of 
vegetables jellies from materials in vegetable cells; urase brings 
about the ammoniacal fermentation of urea. All of the actions 
above mentioned are the result of chemical decomposition in the 
fermenting body, generally accompanied by the absorption of water. 
There is another class of enzymes, called oxidases, which cause 
