Caustic soda tablets. [Illustration: Sample form]] [Illustration: FIG. 209.--Water filtering apparatus. That portion of the figure to the left of the vertical line is drawn to a larger scale than that on the right, in order to show details of Sprengel's pump.] METHOD.-- 1. Fit up the filtering apparatus as in the accompanying diagram (Fig. 209), interposing the wash-bottle with sulphuric acid between the filter flask and the force-pump (in the position occupied in the diagram by the central vertical line), and placing another screw clamp on the rubber tubing connecting the lateral arm of the filter flask with the wash-bottle. [Illustration: FIG. 210. Sterile test-tube brush.] 2. Filter the entire 2000 c.c. of water through the filter candle. 3. When the nitration is completed, screw up the clamps and so occlude the two pieces of pressure tubing. 4. Reverse the position of the glass tubes in the Wulff's bottle so that the one nearest the air pump now dips into the sulphuric acid. 5. Slowly open the metal clamps and allow air to gradually pass through the acid, and enter filter flask, and so restore the pressure. 6. Unship the apparatus, remove the cork from the mouth of the candle. 7. Pipette 10 c.c. of sterile broth into the interior of the candle, and by means of the sterile test-tube brush (Fig. 210) emulsify the slimy residue which lines the candle, with the broth. Practically all the bacteria contained in the original 2000 c.c. of water are now suspended in 10 c.c. of broth, so that 1 c.c. of the suspension is equivalent, so far as the contained organisms are concerned, to 200 c.c. of the original water. (Some bacteria will of course be left behind on the walls of the filter and in its pores.) Up to this point the method is identical, irrespective of the particular organism whose presence it is desired to demonstrate; but from this point onward the methods must be specially adapted to the isolation of definite groups of organisms or of individual bacteria. The Coli-Typhoid Group.-- 1. Number nine tubes of bile salt broth (_vide_ page 180), consecutively from 1 to 9. 2. To No 1 add 1 c.c. } of the original water sample 2 add 2 c.c. } before the nitration is commenced. 3 add 5 c.c. } 3. To the remaining tubes of bile salt broth add varying quantities of the suspension by means of suitably graduated sterile pipettes, as follows: No. 4 0.05 c.c. (equivalent to 10 c.c. of the original water sample). No. 5 0.125 c.c. (equivalent to 25 c.c. of the original water sample). No. 6 0.25 c.c. (equivalent to 50 c.c. of the original water sample). No. 7 0.5 c.c. (equivalent to 100 c.c. of the original water sample). No. 8 1.0 c.c. (equivalent to 200 c.c. of the original water sample). No. 9 2.5 c.c. (equivalent to 500 c.c. of the original water sample). 4. Put up each tube anaerobically in a Buchner's tube and incubate at 42° C.