Precipitate the caseinogen by the addition of acetic acid and filter. To the filtrate add a few drops of 1 per cent. aqueous solution of ferric chloride. _Purple coloration_ = salicylic acid. 6. _Sodium Carbonate or Bicarbonate._ To 10 c.c. of the milk in a test tube add 10 c.c. of alcohol and 0.3 c.c. of a 1 per cent. alcoholic solution of rosolic acid. _Brownish_ color = pure milk; _rose_ color = preserved milk. [Illustration: FIG. 211.--Milk-collecting bottle and dipper in case.] Quantitative.-- _Collection of Sample._-- The apparatus used for the collection of a retail mixed milk sample consists of a cylindrical copper case, 16 cm. high and 9 cm. in diameter, provided with a "pull-off" lid, containing a milk dipper, also made of copper; and inside this, again, a wide-mouthed, stoppered glass bottle of about 250 c.c. capacity (about 14 cm. high by 7 cm. diameter), having a tablet for notes, sand-blasted on the side. The copper cylinder and its contents, secured from shaking by packing with cotton-wool, are sterilised in the hot-air oven (Fig. 26). When collecting a sample, 1. Remove the cap from the cylinder. 2. Draw out the cotton-wool. 3. Lift out the bottle and dipper together. 4. Receive the milk in the sterile dipper, and pour it directly into the sterile bottle. 5. Enter the particulars necessary for the identification of the specimen, on the tablet, with a lead pencil, or pen and ink. 6. Pack the apparatus in the ice-box for transmission to the laboratory in precisely the same manner as an ordinary water sample. "Whole" milk may with advantage be collected in the sterile bottle directly since the mouth is sufficiently wide for the milker to direct the stream of milk into it. ~Condensed milk~ must be diluted with sterile distilled water in accordance with the directions printed upon the label, then treated as ordinary milk. _Apparatus Required_: Case of sterile capsules (25 c.c. capacity). Case of sterile graduated pipettes, 10 c.c. (in tenths of a cubic centimetre). Case of sterile graduated pipettes, 1 c.c. (in tenths of a cubic centimetre). Flask containing 250 c.c. sterile bouillon. Tall cylinder containing 2 per cent. lysol solution. Plate-levelling stand. Case of sterile plates. Tubes nutrient gelatine or gelatine agar. Tubes of wort gelatine. Tubes of nutrient agar. Water-bath regulated at 42° C. Bunsen burner. Grease pencil. METHOD.-- 1. Arrange four sterile capsules in a row; number them I, II, III, and IV. 2. Fill 9 c.c. sterile bouillon into the first, and 9.9 c.c. bouillon into each of the three remaining capsules. 3. Remove 1 c.c. milk from one of the bottles by means of a sterile pipette and add it to the bouillon in capsule I; mix thoroughly by repeatedly filling and emptying the pipette. 4. Remove 0.1 c.c. of the milky bouillon from capsule I, add it to the contents of capsule II, and mix as before.