THE PREPARATION OF CULTURE MEDIA 151 



is allowed to boil for 20 minutes and the reaction is adjusted to the desired 

 P 7/ (About 0.2 of a P H is allowed for change in the reaction during 

 sterilization. For pneumococcus work the optimus P// is 7.8; in the adjust- 

 ment, therefore, before sterilization, the reaction is set at P H 8.) The broth 

 is sterilized in the Arnold sterilizer for 20 minutes on three successive days. 

 For streptococcus work the final read ion should be P// 7.4. 

 Pepton-Salt Solution (Dunham's solution) : 



1. Distilled water 1,000 c.c. 



Pepton (Witte) 10 gms. 



NaCl 5 < ' 



2. Heat until ingredients are thoroughly dissolved. 



3. Filter through filter paper until perfectly clear. 



4. Tube or store flasks. 

 Sterilize by discontinuous method. 

 Nitrate Solution: 



1. Distilled water 1,000 c.c. 



Pepton 10 gms. 



Potassium nitrate 10 gms. 



2. Heat until ingredients are thoroughly dissolved. 



3. Filter through filter paper until perfectly clear. 



4. Tube or store flasks. 



Sterilize by discontinuous sterilization. 



Uschinsky's Protein-Free Medium. 13 To one liter of distilled water add: 



Asparagin 3.4 grams. 



Ammonium lactate 10 * ' 



Sodium chlorid 5 ' ' 



Magnesium sulphate 0.2 ' ' 



Calcium chlorid 0.1 ' 



Potassium phosphate 1.0 ' ' 



When these substances are thoroughly dissolved, add 40 c.c. of glycerin. Tube 

 and sterilize. 



Meat Extract Gelatin. Gelatin is best made in the Arnold and should 

 at no stage be autoclaved. 



One per cent pepton, 0.5 per cent salt, 0.5 per cent meat extract in one 

 liter of water are dissolved in the Arnold. When these ingredients have 

 been dissolved, 15 to 18 per cent of finest French sheet gelatin 14 are added, 

 and the mixture kept in the Arnold until the gelatin is completely dissolved. 



13 Uschinsky, Cent. f. Bakt., 1, xiv, 1893. 



14 The acidity and consistence of the different commercial gelatins vary con- 

 siderably and care should be taken in selecting a uniform and suitable brand, 

 such as Hesterberg '& gold label gelatin. It is advisable, when working during 

 the summer or in hot climates, to add 18 per cent. 



