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 differ greatly in appearance from the elongated dot of the bacterium proper, to the elongated rod or cylinder of the bacillus, and the long spirals of spiro-bacteria.  It is unfortunate that they are not sufficiently constant in habit to always attach themselves to one or the other of these genera.  The micrococcus has a habit of elongating at times until it is impossible to recognize him except as a bacterium; while bacilli, again, break up until their particles exactly resemble micrococci.

Bacteria cannot exist without water; certain forms require oxygen, while others thrive equally well without it; some thrive in solution of simple salts, while others require albuminoid material.