solutions of varying percentages by weighing out the quantity of
carbolic acid required for each and dissolving in 100 c.c. of pure
distilled water in an accurately standardised measuring flask. The
solutions must be prepared freshly as required each day.

~Environment.~--

_a. General._--

Close the windows and doors of the laboratory in which the investigation
is carried out, to avoid draughts. Flush over the work bench and
adjacent floor with 1:1000 solution of corrosive sublimate. Caution the
assistant, if one is employed, to avoid unnecessary movement or speech.

_b. Contact Temperature_, ~15-18 deg. C.~--

This is the temperature at which contact between the germicide and the
test germ takes place, and is of importance, since some germicides (_e.
g._, Phenol) appear to be more powerful at high temperatures. 18 deg.
C.--practically the ordinary room temperature--is a temperature at which
the multiplication of B. coli is a comparatively slow process, but
variation of a degree above this temperature or of two or three degrees
below is of no moment. If the room temperature is below 15 deg. C. when
the experiments are in progress, arrange a water-bath regulated at 18
deg. C. for the reception of the tubes containing the mixture of germ and
germicide; if above 19 deg. C. immerse the tubes in cold water, to which
small pieces of ice are added from time to time to prevent the
temperature rising above 18 deg. C.

_c. Relative Proportional Bulk of Test Germ and Germicide_, ~50:1.~--

Five cubic centimetres is a convenient amount of germicidal solution to
employ, and to this 0.1 c.c. of the emulsion of test germ should be
added.

_d. Bulk of Sample Removed from Germ + Germicide Mixture at Each of the
Time Periods_, ~0.1 c.c.~--

This is sufficient to afford a fair sample of the germ content of the
mixture, and at the same time is insufficient to exert any inhibitory
action when transferred to the subculture medium.

_e. Subculture Medium._ ~Bile Salt Broth.~--

A _fluid_ medium is essential in order to obtain immediate dilution of
the germicide carried over; at the same time it is advantageous to
employ a selective medium which favours the growth of the test germ to
the exclusion of organisms likely to contaminate the preparation, and
if possible one which affords characteristic cultural appearances.

Bile Salt Broth (page 180) combines these desiderata; it permits only
the growth of intestinal bacteria, whil