c matter, just as
do plants that are destitute of chlorophyl (such as fungi, broomrapes,
etc.). This is why they are only met with in living beings or upon
organic substances. The majority of these algae develop very well in the
air, and then consume oxygen and exhale carbonic acid, like all living
beings. If the supply of air be cut off, they resist asphyxia and take
the oxygen that they require from the compounds that surround them. The
result is a complete and rapid decomposition of the organic materials,
or a fermentation. Finally, there are even certain species that die in
the presence of free oxygen, and that can only live by protecting
themselves from contact with this gas through a sort of jelly. These are
ferments, such as _Bacillus amylobacter,_ or butyric ferment, and _B.
septicus_, or ferment of the putrefaction of nitrogenized substances.

[Illustration: FIG. 1.--ATMOSPHERIC DUST.]

These properties explain the regular distribution of bacteria in liquids
exposed to the air. Thus, in water in which plants have been macerated
the surface of the liquid is occupied by _Bacillus subtilis_. which has
need of free oxygen in order to live, while in the bulk of the liquid,
in the vegetable tissues, we find other bacteria, notably _B.
amylobacter_, which lives very well by consuming oxygen in a state of
combination. Bacteria, then, can only live in organic matters, now in
the presence and now in the absence of air.

What we have just said allows us to understand the process of
cultivating these organisms. When it is desired to obtain these algae,
we must take organic matters or infusions of such. These liquids or
substances are heated to at least 120 deg. in order to kill the germs that
they may contain, and this is called "sterilizing." In this sterilized
liquid are then sown the bacteria that it is desired to study, and by
this means they can be obtained in a state of very great purity.

The Bacteriaceae are very numerous. Among them we must distinguish those
that live in inert organic matters, alimentary substances, or debris of
living beings, and which cause chemical decompositions called
fermentations. Such are _Mycoderma aceti_, which converts the alcohol of
fermented beverages into vinegar; _Micrococcus ureae_, which converts
the urea of urine into carbonate of ammonia, and _Micrococcus
nitrificans,_ which converts nitrogenized matters into intrates, etc.
Some, that live upon food products, produce therein