~Lactose Litmus Gelatine (Wurtz).~-- 1. Prepare nutrient gelatine (_vide_ page 164, sections 1 to 4). 2. Render the reaction of the medium mass -5. 3. Replace in the steamer at 100° C. for twenty minutes. 4. Clarify with egg as for gelatine. 5. Weigh out lactose, 20 grammes (= 2 per cent.), and dissolve it in the medium. 6. Filter through papier Chardin. 7. Add sufficient sterile litmus solution to colour the medium pale lavender. 8. Tube, and sterilise as for nutrient gelatine. ~Lactose Litmus Agar (Wurtz).~-- 1. Prepare nutrient agar (_vide_ page 167, sections 1 to 4). 2. Render the reaction of the medium mass -5. 3. Replace in the steamer at 100° C. for twenty minutes. 4. Cool to 60° C. and clarify with egg as for nutrient agar. 5. Weigh out lactose, 20 grammes (= 2 per cent.), and dissolve it in the medium. 6. Filter through papier Chardin, using the hot-water funnel. 7. Add sterile litmus solution, sufficient to colour the medium a pale lavender. 8. Tube, and sterilise as for nutrient agar. ~Glycerine Potato Bouillon.~-- 1. Take 1 kilo of potatoes, wash thoroughly in water, peel, and grate finely on a bread-grater. 2. Weigh the potato gratings, place them in a 2-litre flask, and add distilled water in the proportion of 1 c.c. for every gramme weight of potato. Allow the flask to stand in the ice-chest for twelve hours. 3. Strain the mixture through butter muslin and filter through Swedish filter paper into a graduated cylinder. Note the amount of the filtrate. 4. Place the filtrate in a flask, add an equal quantity of distilled water, and heat in the steam steriliser for sixty minutes. 5. Add glycerine, 4 per cent., mix thoroughly, and again filter. 6. Tube and sterilise as for nutrient bouillon. ~Potato Gelatine (Elsner).~-- 1. Take 1 kilo of potatoes, wash thoroughly in water, peel, and finally grate finely on a bread-grater. 2. Weigh the potato gratings, place them in a 2-litre flask, and add distilled water in the proportion of 1 c.c. for every gramme weight of potato. Allow the flask to stand in the ice-chest for twelve hours. 3. Strain the mixture through butter muslin, and filter through Swedish filter paper into a graduated cylinder. 4. Add 15 per cent. gelatine to the potato decoction and bubble live steam through the mixture for ten minutes. 5. Estimate the reaction; adjust the reaction of the medium mass to +25. 6. Cool the medium to below 60°C.; clarify with egg as for nutrient gelatine (_vide_ page 166). 7. Add 1 per cent. potassium iodide (powdered) to the medium. 8. Filter through papier Chardin. 9. Tube and sterilise as for nutrient gelatine. ~Aesculin Agar.~--(B. coli and allied organisms give black colonies surrounded by black halo.) 1. Measure out 400 c.c. distilled water into a tared 2-litre flask. 2. Weigh out Agar 15 grammes Peptone 10 grammes Sodium taurocholate 5 grammes