In the state of active life the bacterides are killed by a temperature of from 70 to 80 degrees, while the spores require the application of a temperature of from 100 to 120 degrees to kill them. Oxygen of a high pressure, which is, as well known from Bert's researches, a poison for living beings, kills many bacteria in the state of active life, but has no influence upon their spores. In a state of active life the bacteriae are interesting to study. The absence of green matter prevents them from feeding upon mineral matter, and they are therefore obliged to subsist upon organic 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° 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.