9. Place the tubes for forty-eight hours in the incubator at 37° C., and eliminate any contaminated tubes. To prevent drying, 0.5 c.c. glycerine bouillon (see page 209) may be added to each tube between steps 8 and 9. 10. Cap those tubes of media which remain sterile with india-rubber caps and store for future use. ~Potato.~-- 1. Choose fairly large potatoes, wash them well, and scrub the peel with a stiff nail-brush. 2. Peel and take out the eyes. 3. Remove cylinders from the longest diameter of each potato by means of an apple-corer or a large cork-borer (i. e., one of about 1.4 cm. diameter). The reaction of the fresh potato is strongly acid to phenolphthalein. If, therefore, the potatoes are required to approximate +10, as for the cultivation of some of the vibrios, the cylinders should be soaked in a 1 per cent. solution of sodium carbonate for thirty minutes. 4. Cut each cylinder obliquely from end to end, forming two wedge-shaped portions. 5. Place a small piece of sterilised cotton-wool, moistened with sterile water, at the bottom of a sterile test-tube; insert the potato wedge into the tube so that its base rests upon the cotton-wool. Now plug the tube with cotton-wool (Fig. 111). 6. Sterilise in the steamer at 100° C. for twenty minutes on each of _five_ consecutive days. [Illustration: FIG. 111.--Potato tube.] NOTE.--The cork borer reserved for cutting the potato cylinders should be silver electro-plated both inside and out, and the knife used for dividing the cylinders should be of silver or silver plated. When these precautions are adopted the potato wedges will retain their white color and will not show the discoloration so often observed when steel instruments are employed. ~Beer Wort.~--Wort is chiefly used as a medium for the cultivation of yeasts, moulds, etc., both in its fluid form and also when made solid by the addition of gelatine or agar. The wort is prepared as follows: 1. Weigh out 250 grammes crushed malt and place in a 2-litre flask. 2. Add 1000 c.c. distilled water, heated to 70° C., and close the flask with a rubber stopper. 3. Place the flask in a water-bath regulated to 60°C. and allow the maceration to continue for one hour. 4. Strain through butter muslin into a clean flask and heat in the steamer for thirty minutes. 5. Filter through Swedish filter paper. 6. Tube in quantities of 10 c.c. or store in flasks. 7. Sterilise in the steamer at 100° C. for twenty minutes on each of three consecutive days. The natural reaction of the wort should _not_ be interfered with. NOTE.--It is sometimes more convenient to obtain "_unhopped_"[6] beer wort direct from the brewery. In this case it is diluted with an equal quantity of distilled water, steamed for an hour, filtered, filled into sterile flasks or tubes, and sterilised by the discontinuous method. ~Wort Gelatine.~--