ISOLATION AND PURE-CULTURE METHODS 



2 9 



Nutrient gelatin is employed extensively in the cultivation of 

 bacteria, but seldom with the fungi. It is made by adding 100 

 grams of gelatin to each 1000 cc. of bouillon. In the preparation 

 of this medium one must use as little heat as possible. It dissolves 

 readily in hot bouillon, and 

 filters much more quickly 

 than agar. The congealing 

 properties of gelatin are de- 

 stroyed by a long exposure to 

 a greater temperature than 

 1 00 C ., so that if the autoclave 

 is employed, then the gelatin 

 should be cooled promptly. 

 When sterilized, the periods 

 of steaming should not exceed FIG. 7. DOUBLE-WALLED METAL Box FOR 



ten or fifteen minutes. Gela- STORAGE OF GELATIN CULTURES; CON- 



NECTED WITH WATER SUPPLY. ( After Novy) 

 tin melts at a temperature 



above 35 C. and often lower, so that it must be stored in a cool 

 place ; and the cold water box of Novy (Fig. 7) may be used to afford 

 such a low temperature when the refrigerator is unsatisfactory. 



Starch jelly should become an important nutrient medium. It 

 can be obtained fairly pure, and in connection with synthetic salt 

 solutions it is valuable for slanting tubes. Commercial starches 

 generally contain resistant spore-forming organisms, and it is desir- 

 able to shake up the starch in a flask with 95 per cent alcohol for 

 one hour and dry rapidly on filter paper before using. A 10 per 

 cent mixture in the salt solution selected should be made. Rub this 

 up well before heating, and sterilize, if possible at from 90 to 93 

 C. on successive days. 



Vegetable products. Cylinders or slices of vegetables, such as 

 the sugar beet or potato, or even young stems or pods of bean, 

 prunes, squash, corn meal, etc., prove excellent media of different 

 types, most suitable for work with the fungi. The sugar beet is an 

 excellent general medium, rich in cane sugar. It is quite generally 

 obtainable during the autumn, and laboratories may then.be pro- 

 vided with a supply which will keep in a cellar over winter. The 

 potato is always available and offers an excellent starchy medium. 

 The stems or young pods of bean are rich in nitrogenous material , 



