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128                 PA TJ1OGEN1C BACTERIA.

will. It is prepared as follows : To 1000 c.cin. of meat-
infusion or to 1000 e.cni. of water containing 2 grains of
beef-extract in solution, 10 grams of peptone, 5 grams of
salt, and 100 grams of gelatin (uGolcl label" is the best
commercial article) are added, and boiled for about an
hour over a moderately hot flame. Double boilers are
very slow, and if proper care is exercised there is little
danger of the gelatin burning. It must be stirred occa-
sionally, and the flame should be so distributed by wire
gauze as not to act upon a single point of the bottom of
the kettle. At the end of the hour the albumins of
the meat-infusion will be coagulated and the gelatin
thoroughly dissolved. Giinther has shown that the
gelatin congeals better if allowed to dissolve slowly in
warm water before boiling. The liquid is now cooled
to 60° C. and neutralized—/. e. alkalinized. As the gela-
tin is itself acid, a relatively larger amount of the sodium-
carbonate solution will be needed than was required for
the bouillon. When the proper reaction is attained, as
much water as has been lost by vaporization during the
process of boiling, intimately mixed with the white of
an egg, is added, well stirred in, and the whole boiled
for half an hour, then filtered.

If the filter-paper be of good quality and properly
folded (pharmaceutical filter), and if the gelatin be prop-
erly dissolved, the whole quantity should pass through
before cooling too much. Should only half go through
before cooling, the remainder must be returned to the
pot, heated to boiling once more, and then passed through
a new filter-paper. As a matter of fact, gelatin generally
filters readily. A wise precaution is to catch the first few
centimeters in a test-tube and boil them, so that if a
cloudiness shows the presence of uncoagulated albumin,
the whole mass can be boiled again. The finished gel-
atin is at once distributed into sterilized tubes, and then
sterilized like the bouillon by the fractional method.

Of course, the gelatin or any other culture-medium can
be kept en masse indefinitely, but should a contaminating"