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tf                 PATHOGENIC BACTERIA.

piin-ent oders which seem dependent purely upon odor-
iferous principles dissociated from gases. Many of them
are extremely unpleasant, as the onion-like odor of the
tetanus bacillus. The odor does not have any direct rela-
tion to decomposition, but, like the colors and acids,
seems to be a peculiar individual characteristic of the
metabolism of the organism.

8.  Production  of Phosphorescence.—A Bacillus  phos-
phoresceus and numerous other organisms  have a dis-
tinct phosphorescence associated with their growth.    It
is said that so much illumination is sometimes caused by
a gelatin culture of some of these as to enable one to tell
the time by a watch.    Most of them are found in sea-
water, and are best grown in sea-water gelatin.

9.  Production of Aroma tics.—The most important of
these is indol^ which was at one time thought to be pecu-
liar to the cholera spirillum.    For the method of deter-
mining its presence, see "Dunham's Solution."   At pres-
ent we know that a variety of organisms produce it, and
that it and phenol, kresol, hydrochinon, hydroparacumaric
acid, and paroxy-phenylie-acetic acid are by no means

10.  Reduction of Nitrites.—A considerable number of
bacteria are able to reduce nitrites present in the soil or
in culture-media prepared for them into ammonia and
nitrogen.    To the horticulturist this is a matter of much
interest.    Winogradsky has  found a specific  nitrifying-
bacillus in soil, and asserts that the presence of ordinary
bacteria in the soil causes the reduction of no nitrites so
long as his special bacillus is withheld.

n. Peptonisation of Milk.— Numerous bacteria possess
Aepowerof digesting-peptoniziug—the casein of milk.
The process differs with different bacteria, some digesting
the casein without any apparent change in the milk"
some producing coagulation, some gelatinization of the
fluid In some cases the digestion of the casein is so