1. Measure out wort (prepared as above), 900 c.c., into a sterile flask. 2. Weigh out gelatine, 100 grammes (= 10 per cent.), and add it to the wort in the flask. 3. Bubble live steam through the mixture for ten minutes, to dissolve the gelatine. 4. Cool to 60°C.; clarify with egg as for nutrient gelatine (_vide_ page 164). 5. Filter through papier Chardin. 6. Tube, and sterilise as for nutrient gelatine. ~Wort Agar.~-- 1. Measure out wort (as above), 700 c.c., into a sterile flask. 2. Weigh out powdered agar, 20 grammes; mix into a smooth paste with 200 c.c. of cold wort and add to the wort in the flask. 3. Bubble live steam through the mixture for twenty minutes, to dissolve the agar. 4. Cool to 60° C.; clarify with egg as for nutrient agar (_vide_ page 167). 5. Filter through papier Chardin, using the hot-water funnel. 6. Tube, and sterilise as for nutrient agar. ~Peptone Water (Dunham).~-- 1. Weigh out Witté's peptone, 10 grammes, and salt, 5 grammes, and emulsify with about 250 c.c. of distilled water previously heated to 60° C. 2. Pour the emulsion into a litre flask and make up to 1000 c.c. by the addition of distilled water. 3. Heat in the steamer at 100° C. for thirty minutes. 4. Filter through Swedish filter paper. 5. Tube in quantities of 10 c.c. each. 6. Sterilise in the steamer at 100° C. for twenty minutes on each of three consecutive days. ~"Sugar" or "Carbohydrate" Media.~-- Formerly the ability of bacteria to induce hydrolytic changes in carbohydrate substances was observed only in connection with a few well-defined sugars, but of recent years it has been shown that when using litmus as an indicator these so-called "fermentation reactions" facilitate the differentiation of closely allied species, and the list of substances employed in this connection has been considerably extended. The media prepared with them are now no longer regarded as special, but are comprised in the "stock media" of the laboratory. The chief of these substances are the following, arranged in accordance with their chemical constitution: _Monosaccharides_ Dextrose (glucose), lævulose, galactose, mannose, arabinose, xylose. _Disaccharides_ Maltose, lactose, saccharose. _Trisaccharides_ Raffinose (mellitose). _Polysaccharides_ Dextrin, inulin, starch, glycogen, amidon. _Glucosides_ Amygdalin, coniferin, salicin, helicin, phlorrhizin. _Polyatomic alcohols_ _Trihydric_, Glycerin. _Tetrahydric_, Erythrite. _Pentahydric_, Adonite. _Hexahydric_, Dulcite, (dulcitol or melampirite), isodulcite (rhamnose), mannite (mannitol), sorbite (sorbitol), inosite.