CUL TIVA TION OF BA CTERIA. 135

adding a few drops of clean water to each tube before
sterilizing. It is not necessary to have a special small
chamber blown in the tube to contain this water; only
a small quantity need be added, and this will not touch
the potato, which does not reach the bottom of the
rounded tube.

A potato-juice has also been suggested, and is of some
value. It is made thus : To 300 c.cm. of water 100 grams
of grated potato are added, and allowed to stand on ice
over night. Of the pulp 300 c.cm. are expressed through
a cloth and cooked for an hour on a water-bath. After
cooking, the liquid is filtered and receives 4 per cent, of
glycerin. It may or may not need neutralization. Upon
this medium the tubercle bacillus grows well, especially
when the reaction of the medium is acid, but loses its
virulence.

Milk.—Milk is useful as a culture-medium. As when
the milk stands the cream which rises to the top is a
source of inconvenience, it is best to secure from a dairy
fresh milk from which the cream has been removed by
a centrifugal machine. It is placed in sterile tubes and
sterilized by steam by the intermittent method. The
opaque nature of this culture-medium often permits the
undetected development of contaminating organisms.
A careful watch should therefore be kept upon it lest it
spoil.

Litmus Milk.—This is milk to which just enough of
a saturated -watery solution of pulverized litmus is added
to give a distinct blue color. Cow's milk is inclined to
be acid in reaction, and a small • amount of sodium car-
bonate may be necessary to give it a distinct blue. The
use of litmus is probably the best method of determining
whether bacteria by their growth produce acids or alka-
lies.

The watery solution of litmus, being a vegetable in-
fusion, is likely to spoil; hence it should always be treated
like the culture-media and sterilized by steam every time
the receptacle in which it is kept is opened.