3. Dilution Bottles.—Bottles for use in making dilutions should preferably be of tall form, of such capacity as to hold at least twice the volume of water actually used. Close-fitting ground-glass stoppers are preferable, but tight fitting cotton stoppers may be used, provided due care is taken to prevent contamination and to avoid loss of volume through wetting of the stopper before mixing has been accomplished.
4. Petri Dishes.—Petri dishes ten centimeters in diameter shall be used with glass or porous tops[[211]] as preferred. The bottoms of the dishes shall be as flat as possible so that the medium shall be of uniform thickness throughout the plate.
5. Fermentation Tubes.—Any type of fermentation tube[[203]] may be used provided it holds at least three times as much medium as the amount of water to be tested.
II. MATERIALS.
1. Water.—Distilled water shall be used in the preparation of all culture media and reagents.
2. Meat Extract.—Liebig’s meat extract shall be used in place of meat infusion. Other brands may be substituted for Liebig’s when comparative tests have shown that they give equivalent results.
3. Peptone.—Armour’s, Digestive Ferments Company’s, Fairchild’s, or any other peptone which gives equivalent results may be used.
4. Sugars.—All sugars used shall be of the highest purity obtainable.
5. Agar.—The agar used shall be of the best quality and shall be dried for one-half hour at 105° C. before weighing. Much of the agar on the market contains considerable amounts of sea salts.[[221]][[225]][[228]] These may be removed by soaking in water and draining before use.
6. Gelatin.—The gelatin used shall be of light color, shall contain not more than a trace of arsenic, copper, sulfides, and shall be free from preservatives, and of such a melting point that a 10 per cent. standard nutrient gelatin shall have a melting point of 25° C. or over. Gelatin shall be dried for one-half hour at 105° C. before weighing.