d by micro-organisms to the lethal
effect of germicides. But it frequently happens that the bacteriologist
has to determine the relative efficiency of "disinfectants" from the
standpoints of the sanitarian and commercial man rather than from the
research worker's point of view. In pursuing this line of investigation,
it is convenient to compare the efficiency, under laboratory conditions,
of the proposed disinfectant with that of some standard germicide, such
as pure phenol. In so doing, and in order that the work of different
observers may be compared, conditions as nearly uniform as possible
should be aimed at. The method described is one that has been in use by
the writer for many years past, modified recently by the adoption of
some of the recommendations of the Lancet Commission on the
Standardisation of Disinfectants--particularly of the calculation for
determining the phenol coefficient.

This method has many points in common with that modification of the
"drop" method known as the Rideal-Walker test.

~General Considerations.~--

These may be grouped under three headings: Test Germ, Germicide, and
Environment.

1. _Test Germ._--~B. coli.~

As disinfectants are tested for sanitary purposes, it is obvious that a
member of the coli-typhoid group should be selected as the test germ. B.
coli is selected on account of its relative nonpathogenicity, the ease
with which it can be isolated and identified by different observers in
various parts of the world, the stability of its fundamental characters,
and evenness of its resistance when utilised for these tests; finally
since the colon bacillus is an organism which is slightly more
resistant to the lethal action of germicides than the more pathogenic
members of this group, a margin of safety is introduced into the test
which certainly enhances its value.

B. coli should be recently isolated from a normal stool, and plated at
least twice to ensure the purity of the strain; and a stock agar culture
prepared which should be used throughout any particular test. For any
particular experiment prepare a smear culture on agar and incubate at
37 deg. C. for 24 hours anaerobically. Then emulsify the whole of the
surface growth in 10 c.c. of sterile water. Transfer the emulsion to a
sterile test-tube with some sterile glass beads and shake thoroughly to
ensure homogenous emulsion. Transfer to a centrifuge tube and
centrifugalise the emulsion to throw down any masses of bac