The Bacteriaceae are reproduced with amazing rapidity.  If the temperature is proper, a limpid liquid such as chicken or veal broth will, in a few hours, become turbid and contain millions of these organisms.  Multiplication is effected through fission, that is to say, each globule or filament, after elongating, divides into two segments, each of which increases in its turn, to again divide into two parts, and so on (Fig. 2, I. b).  But multiplication in this way only takes place when the bacteria are placed in a proper nutritive liquid; and it ceases when the liquid becomes impoverished and the conditions of life become difficult.  It is at this moment that the formation of spores occurs—­reproductive bodies that are destined to permit the algae to traverse, without perishing, those phases where life is impossible.  The spores are small, brilliant bodies that form in the center or at the extremity of each articulation or globule of the bacterium (Fig. 2, II. l), and are set free through the breaking up of the joints.  There are, therefore, two phases to be distinguished in the life of microbes—­that of active life, during which they multiply with great rapidity, are most active, and cause sicknesses or fermentations, and that of retarded life, that is to say, the state, of resting spores in which the organisms are inactive and consequently harmless.  It is curious to find that the resistance to the two causes of destruction is very different in the two cases.

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.]