59^ LABORATORY EXERCISES 



Rani his Medium for Moidds. 



Grams 



Cane sugar 70.00 



Tartaric acid 4 . 00 



Ammonium phosphate o. 600 



Magnesium carbonate 0.400 



Ammonium sulphate o 250 



Zinc sulphate 0.750 



Ferrous sulphate 0.075 



Potassium silicate 0.070 



Water 



Too complicated to be of much value. 



1500.00 c.'c. 



LESSON 5 



Potatoes as Medium. — Whole white potatoes are taken and washed with corrosive 

 sublimate i : 1000. They are then wrapped in filter paper and steamed in the 

 sterilizer about thirty minutes, the next day twenty minutes, the third fifteen 

 minutes. The potatoes are then cut in two by a knife heated in a Bunsen flame. 

 The cut pieces are laid in a large fiat glass dish on a circular piece of filter paper, the 

 glass dishes having been sterilized by corrosive sublimate. Inoculations are then 

 made on the surface of the potatoes. This method is especially useful for the 

 growth of glanders, and chromogenic bacteria. 



Potato Juice. 



Grated potato, grams 100 



Water, c.c 300 



Mix and put in ice chest over night; strain off 300 c.c. through a cloth. Cook 

 for one hour in water bath, filter and add 4 per cent, glycerin. Sterilize. Do not 

 neutralize as best growth of tubercle bacillus is obtained when the juice is acid. 

 Growth is rapid and luxuriant, but non-virulent (Archiv fiir Hygiene, XVI). For 

 culture in tubes with potatoes. Use knife designed by Ravenel, which is used in 

 the same manner as a cork punch (Fig. 210). The semi-tubular pieces of potato, 

 punched out, are beveled by a slant cut and placed in a test-tube which is laid 

 flat with flat side of the potato down to prevent warping; the whole is then sterilized 

 by the intermittent German process. After sterilization, it is sometimes advisable 

 to add glycerin soaked in a cotton plug, to the test-tube in order to prevent drying. 

 A specially designed test-tube (Fig. 211) is used so that the cut piece of potato 

 can be introduced at the top and the glycerin in the enlarged bottom. 



Glycerinated Potato. — i. Prepare ordinary potato wedges. 



2. Soak the wedges in a 25 per cent, solution of glycerin for fifteen minutes. 



3. Moisten the cotton-wool plugs at the bottom of the potato tubes with a 25 

 per cent, solution of glycerin instead of plain water. 



4. Insert a wedge of potato in each tube and replug the tubes. 



5. Sterilize in the steamer at ioo°C. for twenty minutes on each of five consecutive 

 days. 



