, IV.,) moving their
straight or curved filaments, and _Spirilli_ (Fig. 2, VI.), rolled up
spirally. These varied forms are not absolutely constant, for it often
happens in the course of its existence that a species assumes different
shapes, so that it is difficult to take the form of these algae as a
basis for classifying them, when all the phases of their development
have not been studied.

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 organi