the
waste products of animal life, which would normally be dangerous to
health, into harmless mineral residue. They are really the scavengers of
the earth's surface, not actually carrying off garbage, but rather
transforming it, and, in the process, not merely destroying it, but
changing it so as to make it available for plant-food. It is through the
agency of bacteria that the air, which is being continually overloaded
with carbonic acid from the lungs of animals, is reduced and taken up by
plants so that an equilibrium is maintained. Otherwise, the atmosphere
would be more and more vitiated with carbonic acid and organic vapors,
and every one would die as if shut up in an air-tight room. But, because
of bacteria, neither is the surface of the earth overloaded with waste
organic matter nor do streams, however much polluted, continue to flow
without some improvement being traced in their quality.

In some of the ordinary manufacturing processes, bacteria are
all-important, as in making vinegar, wines, cheese; in fact, in any of
the fermented food products. In agriculture, they are entirely
responsible for supplying an adequate amount of food material to growing
plants. Fresh manure is not suitable for plant-food and would be of no
value on the fields or in the garden except as improved and modified by
bacterial action. One of the greatest discoveries of their importance
recently made has to do with the way in which peas and beans are able to
absorb nitrogen from the air through the agency of bacteria. One knows
that plowing under a crop of peas or clover enriches the soil, and that
peas or clover make the best growth for this purpose. The reason is that
these plants, through the activity of bacteria, are able to absorb
nitrogen from the air and afterwards to convert it into food material.

But with all these good qualities a few bacteria, gone bad, perhaps, are
associated with diseases, and by a series of experiments, chiefly those
of a Frenchman named Pasteur and of a German named Koch, and of their
followers, it has been ascertained that certain bacteria, and those
only, will cause certain diseases. These diseases, that is, these caused
by bacteria, are generally spoken of as epidemic or contagious, of which
typhoid fever and cholera are examples.

All contagious diseases cannot at present be definitely associated with
bacteria, probably for the reason that the methods employed to find the
bacteria have not been ad