4. Tube or flask, and sterilise as for nutrient bouillon. ~Cohn's Solution.~-- 1. Weigh out and mix Acid potassium phosphate (KH_{2}PO_{4}) 5.0 grammes Calcium phosphate 0.5 gramme Magnesium sulphate 5.0 grammes Ammonium tartrate 10.0 grammes and dissolve in Distilled water 1000 c.c. 2. Tube, or flask and sterilise as for nutrient bouillon. ~Naegeli's Solution.~-- 1. Weigh out and mix Dibasic potassium phosphate (K_{2}HPO_{4}) 1.0 gramme Magnesium sulphate 0.2 gramme Calcium chloride 0.1 gramme Ammonium tartrate 10.0 grammes and dissolve in Distilled water 1000 c.c. 2. Tube or flask; sterilise as for nutrient bouillon. ~Plaster-of-Paris Discs.~-- 1. Take large corks, 2.5 cm. diameter, and roll a piece of stiff note-paper round each, so that about a centimetre projects as a ridge above the upper surface of the cork, and secure in position with a pin (Fig. 112). 2. Mix plaster-of-Paris into a stiff paste with distilled water, and fill each of the cork moulds with the paste. 3. When the plaster has set, remove the paper from the corks, and raise the plaster discs. 4. Place the plaster discs on a piece of asbestos board and sterilise by exposing in the hot-air oven to 150° C. for half an hour. [Illustration: Fig. 112.--Cork and paper mould for plaster-of-Paris disc.] 5. Remove the sterile discs from the oven by means of sterile forceps, place each inside a sterile capsule, and moisten with a little sterile water. 6. Sterilise in the steamer at 100° C. for thirty minutes on each of three consecutive days. ~Gypsum Blocks (Engel and Hansen).~-- These are in the form of truncated cones and for their preparation small tin moulds are required, each having a diameter of 5.5 cm. at the base and 4 cm. at the truncated apex. The height (or depth) of a mould is 4.5 to 5 cm. 1. Mix powdered calcined gypsum into a stiff paste with distilled water. 2. Fill the paste into the moulds and allow it to set and dry by exposure to air. 3. Remove the block from the mould and transfer it to a double glass dish of adequate size (7 cm. diameter × 7 cm. high). 4. Sterilise block in its dish for one hour in the hot-air oven at 115°C. 5. Carefully open the dish and add sterile distilled water to moisten the block and form a layer in the bottom of the dish 1 cm. deep. ~Wine Must.~--(Wine must is obtained from Sicily, in hermetically sealed tins, in a highly concentrated form--as a thick syrup--but not sterilised.) 1. Weigh out "wine must," 200 grammes, place in a 2-litre flask and add distilled water, 800 c.c. 2. Weigh out ammonium tartrate, 5 grammes, and add to the dilute must. 3. Place the flask in a water-bath regulated to 60° C. for one hour and incorporate the mixture thoroughly by frequent shaking.