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140                PA THOGENIC BA CTERIA.

the peculiarities of certain pathogenic forms enable us
to use special means, taking advantage of their eccentrici-
ties, for their isolation, and that the general methods are
in reality more useful for the non-pathogenic than for tlie
pathogenic forms.

All three methods depend upon the observation of
Koch, that when germs are equally distributed through-
out some liquefied nutrient medium which can be solidi-
fied in a thin layer, the growth of the germs takes place
in little scattered groups or families, called colonies, dis-
tinctly separated from each other and capable of trans-
plantation to tubes of culture-media.

Plate-cultures.—The plate-cultures, originally made
by Koch, require considerable apparatus, and of late years
have given place to the more ready methods of Petri and
Von Esmarch. So great, however, is the historic interest
attached to the plates that it would be a great omission
not to describe Koch's method in full.

Apparatus.—Half a dozen glass plates, about 6 by 4
inches in size, free from bubbles and scratches and
ground at the edges, are carefully cleaned, placed hr a
sheet-iron box made to receive them, and then put in

the hot-air closet, where
they are sterilized. The box,
which is tightly closed, al-
lows the sterilized plates to
be kept on hand indefinitely
before using.

A moist chamber, or double
dish, about 10 inches in di-
ameter and 3 inches deep, tlie
upper half being just enougli

FIG 23.~Complete levelling appa-   }            ^ ^ ]ower ^ ^

ratus for   pouring plate-cultures, as   .     **

taught by Koch.                           lt: to close over it, is carefully

washed.   A sheet of bibulous

paper is placed in the bottom, so that some moisture can
be retained, and a i : 1000 bichlorid solution is poured in
and brought in contact with the sides, top, and bottom