170 PA THOGENIC BA CTERIA.

of bacteria in water is very simple, and can generally be
prosecuted without much apparatus. The principle de-
pends upon the equal distribution of a given quantity of
the water to be examined through a sterile liquid medium,
and the subsequent solidification of this medium in a

FIG. 47.—Heyroth's instrument for counting colonies of bacteria in Petri dishes.

thin layer, so that all the colonies which develop may
be counted.

The method, which originated with Koch, may be per-
formed with the Koch plates or with Petri dishes or
with Esmarch rolls. It is always best to make a num-
ber of these pi ate-cultures with different amounts of the
water to be examined, using, for example, o.oi, o. i, 0.5,
and i.o c.cm. added to a tube of gelatin, agar-agar, or
glycerin agar-agar.

The exact method must depend somewhat upon the
quality of the water to be examined. If the number of
bacteria per cubic centimeter is small, large quantities
may be used, but if there are millions of bacteria in
every cubic centimeter, it may be necessary to dilute the