MICROBES, OR BACTERIA. 111 
mented, and obtained from the milk of cows, sheep, 
or goats, skimmed or unskimmed, according to the 
kind of cheese desired. 
Sweet-milk cheese do not differ in their composi- 
tion from those of curdled milk. They consist of casein, 
albuminoid matter which encloses particles of butter: 
the liquid residue is the serum or whey, which con- 
tains lactic acid and mineral salts. 
Cheese, strictly so called, such as Gruyére and 
Roquefort, only differ from the foregoing because they 
have been exposed for a shorter or longer time to the 
action of the air, and of the microbes suspended in it. 
Cheese is first oxidized under the influence of the 
oxygen of the air; butyric and even alcoholic fermen- 
tation soon follows lactic fermentation, together with 
the disengagement of hydrogen and of putrid pro- 
ducts, when the action of the ferments which effect 
these transformations has gone on too long. 
In order to obtain the different kinds of cheese 
which come into the market, they are exposed to the 
weather, generally in holes which have been excavated 
in the rock for this purpose, on a bed of straw, or 
sometimes partially covered with it, until the cheese 
is ripe and has attained the desired quality. 
Butyric and ammoniacal fermentations lead us 
directly to the study of putrefaction; that is, the fer- 
mentation of dead organic matter. 
