BIOLOGY OF BACTERIA. 49

as culture-media and a variety of other substances, to a
temperature beyond that known to be the extreme limit
of bacterial endurance.

The presence of certain substances-—especially some
of the mineral salts—in an otherwise perfectly suitable
medium will prevent the development of bacteria, and
when added to grown cultures of bacteria will destroy
them. Carbolic acid and bichlorid of mercury are the
best known examples.

It is interesting to mention in this connection the
results of the experiments of Trambusti, who found it
possible to produce a tolerance to a certain amount of
bichlorid of mercury by cultivating Priedlandcr\s bacillus
upon culture-media, containing gradually increasing
amounts of the salt, until from 1—15,000, which inhibited
ordinary cultures, it could accommodate itself to 1—2000.

(/) .r-AV i'.v.—The action of the .r-rays upon bacteria
has been investigated by Bonome and (iros and others.
When the cultures are exposed to their action for pro-
longed periods, their vitality and virulence seem to be
slightly diminished. They are not killed by the .r-rays.

Some forms of the bacteria are never found except in
the tissues of diseased animals. Such organisms are
called parasites* The parasitic group really is divisible
into the purely parasitic and the occasionally parasitic
bacteria. Of the first division the tubercle bacillus may
be used as an illustration, for, so far as is known, it is
never found in other places than the bodies and dejeela
of diseased animals. The cholera spirillum illustrates
the second group, for, while it produces the disease
known as Asiatic cholera when admitted to the digestive
tract, it is a constant inhabitant of certain waters, where
it multiplies with luxuriance.

Bacteria which do not. enter the animal economy, or if
accidentally admitted do no harm, but live upon decaying
animal and vegetable materials, are called saprophyte

According to their products of metabolism, bacteria
are often described as—