TO DETERMINE   THERMAL  DEATH-POINT.  179
Sternberg suggested a method in which the time should
remain constant (two hours' exposure), and the object be
the determination of the exact dilution of the reagent
required to destroy the bacteria. " Instead of subjecting
a few of the test-organisms attached to a silk thread to
the action of the disinfecting agent, a certain quantity of
the recent culture—usually 5 c.cm.—has been mixed
with an equal quantity of a standard solution of the
germicidal agent, „ . . and after two hours' contact one
or two ose-fuls would be introduced into a suitable nutri-
ent medium to test the question of disinfection."
A very simple and popular method of determining the
germicidal value is to make a series of dilutions of the
reagent to bo tested; acid to each a couple of loopfuls of
a fresh liquid culture*, and at varying intervals of time
transfer a loopful to fresh culture-media. By a little
ingenuity this method may be made to yield information
as to both time and strength.
When it is desired to secure information concerning
the progress of the germicidal action of reagents, body-
fluids, etc., especially in the unusual and interesting
cases in which the material subjected to the test may
exert a restraining action for a time only, or bring about
destruction of some or many, but not all of the germs,
the use of the Petri dish can be introduced.
For example, it is desired to determine whether a
blood-serum is gennieidal or not. Into about 5 c.cm. of
the serum contained in a test-tube, two or three loopfuls
of any desired bacterium, in liquid culture, are added.
The tube is well agitated and immediately one loopful is
transferred to a tube of melted gelatin, distributed
through it, and poured into a Petri dish. After one
minute the operation is repeated, in five minutes again,
and so on as often as is desired.
The dishes are stood away until the bacteria develop
into colonies, which are then counted with a Wolfhugel
apparatus* On the first dish there may be 100 colonies;
on the second, 80; on the third, 50; on the fourth, 20; ou