In the case of water samples examined during the progress of an epidemic, of new supplies and of unknown waters the search is extended to embrace other members of the coli-typhoid group; and on occasion the question of the presence or absence of Vibrio choleræ or (more rarely) such bacteria as B. anthracis or B. tetani, may need investigation. When pathogenic or excremental bacteria are present in water, their numbers are relatively few, owing to the dilution they have undergone, and it is usual in commencing the examination, to adopt one or other of the following methods: A. _Enrichment_, in which the harmless non-pathogenic bacteria may be destroyed or their growth inhibited, whilst the growth of the parasitic bacteria is encouraged. This is attained by so arranging the environment, (i. e., Media, incubation temperature, and atmosphere) as to favor the growth of the pathogenic organisms at the expense of the harmless saprophytes. B. _Concentration_, whereby all the bacteria present in the sample of water, pathogenic or otherwise, are concentrated in a small bulk of fluid. This is usually effected by filtration of the water sample through a porcelain filter candle, and the subsequent emulsion of the bacterial residue remaining on the walls of the candle with a small measured quantity of sterile bouillon. A. ~Enrichment Method.~ (Dealing with the demonstration of bacteria of intestinal origin.) _Apparatus Required_ (_Preliminary Stage_): Incubator running at 42° C. Case of sterile pipettes, 1 c.c. graduated in tenths. Case of sterile pipettes, 10 c.c. graduated in c.c. Case of sterile pipettes, graduated to deliver 25 c.c. Tubes of bile salt broth (_vide_ page 180). Flask of double strength bile salt broth (_vide_ page 199). Tubes of litmus silk. Sterile flasks, 250 c.c. capacity. Buchner's tubes. Tabloids pyrogallic acid. Tabloids sodium hydrate. Bunsen burner. Grease pencil. (_Later stage_): Incubator running at 37° C. Surface plates of nutrose agar (see page 232). Aluminium spreader. Tubes of various media, including carbohydrate media. Agglutinating sera, etc. METHOD.-- 1. Number a set of bile salt broth, tubes 1-5, and a duplicate set 1a-5a. 2. Number one flask 7 and another 8. 3. To Tubes No. 1 and 1a add 0.1 c.c. water sample. To Tubes No. 2 and 2a add 1 c.c. water sample. To Tubes No. 3 and 3a add 2 c.c. water sample. To Tubes No. 4 and 4a add 5 c.c. water sample. To Tubes No. 5 and 5a add 10 c.c. water sample. 4. Put up all the tubes in Buchner's tubes and incubate anaerobically at 42°C. NOTE.--The bile salt medium is particularly suitable for the cultivation of bacteria of intestinal origin, and at the same time inhibits the growth of bacteria derived from other sources.