in a capsule (diameter 0.0008 to
0.0001 millimeter).

These little organisms, when observed in a fluid like blood, sputum,
etc., are found to present very active movements, although provided
with no organs of locomotion.

This Brownian motion is possessed by almost every minute particle of
matter, organic and inorganic, and is not due to any inherent power of
the individual. They are almost omnipresent, abounding in the air, the
earth, the water, are always found in millions where moist organic
matter is undergoing decomposition, and are associated with the
processes of fermentation--in fact, they are essential to it. The
souring of milk succeeds the multiplication of these germs. Certain
varieties are pigmented, and we observe colonies of chromogenic cocci
multiplying upon slices of boiled potato, eggs, etc., presenting all
the colors of the rainbow. All of these germs are not the cause of
disease. Certain species, however (termed pathogenic), are always
associated with certain diseased conditions.

The _bacteria-termo_--micro-bacteria--are slightly elongated, and
inasmuch as they multiply by division, frequently appear coupled
together, linked in pairs, and in chains. They are generally found in
putrefying liquids, especially infusions of vegetable matter. They
possess mobility to a remarkable degree. Observing a field of
bacteria-termo under the microscope, they may be seen actively engaged
in twining and twisting. A flagellum has been demonstrated as attached
to one or both extremities. This is too minute to be generally
resolved, even if it is a common appendage.

_Desmo-bacteria_ (or bacilli) are rod-like organisms, occurring of
various lengths and different thicknesses. In a slide of the bacillus
of tuberculosis and anthrax, we notice at intervals dots which
represent the spores from which, as the rods break up, future bacilli
are developed.

Then we have _spiro-bacteria,_ the spirilla and the spirochetae; the
former having short open spirals, the latter long and closely wound
spirals. The _spirillum, volutans_ is often found in drinking water,
and in common with some other specimens of this class is provided with
flagellae, sometimes at both extremities, which furnish the means of
rapid locomotion. The spiro-bacteria multiply by spores, although
little is at present known of their life history. They frequently are
attached together at their extremities, forming zigzag chains.

We have seen that bacteria