BIOLOGY Or fiACTfiKL-L	53
is cultivated upon white of egg, it produces only the
green fluorescent pigment, while in pure peptone solu-
tion it grows with the production of blue pyocynnin
alone. His experiments prove a very interesting fact,
that for the production of lluorescin it is necessary that
the culture-medium contain a definite amount of a
phosphatic salt. Sometimes one pigment is soluble,
the other insoluble, so that the colony will appear one
color, the medium upon which it grows another. Some
organisms will only produce their colors in the light ;
others, as the Bacillus inycoides roseus, only in the dark.
Some produce them only at the room-temperature, but,
though growing luxuriantly in the incubator, refuse to
produce pigments at so high a temperature. Thus,
Bacillus prodigiosus produces a brilliant red eolor when
growing at the temperature of the room, but is colorless
when grown in the incubator. Colored lights seem to
have no modifying influence upon the pigment-produc-
tion. Rven if for successive generations the bacterium
be grown so as to be colorless, it speedily recovers its
primitive color when restored to its old environment, no
matter what the color of the light thrown upon it. Bac-
teria which have been robbed of their eolor by incuba-
tion, when placed in the normal environment produce
the original color, no matter what color the light they
receive. Some of the pigments—-perhaps most of them —
are formed only in the presence of oxygen.
4. Liquefaction of (*cla I in.— When certain forms of
bacteria are grown in gelatin the culture-medium is
partly or entirely liquefied. This characteristic is en-
tirely independent of any other property of the bacte-
rium, and is one manifested alike by pathogenic and
non-pathogenic individuals. Sternberg and Bitter have
shown that if from a culture in which liquefaction has
taken place the bacteria be removed by filtration, the
filtrate will retain the power of liquefying gelatin* .show-
ing that the property is not resident in the bacteria, but
in some substance in solution in their excreted products.