CUL 77 VA TION OF BA CTERIA . 125 



enough water added to bring the total amount up to 1000 

 c.cm. This liquid is called the meat-infusion. To it 10 

 grams of Witte's or Fairchild's dried beef-peptone and 5 

 grams of sodium chlorid are added, and the whole boiled 

 until the albumins coagulate. The reaction is then care- 

 fully tested, in order that whatever sarcolactic acid may 

 have been present in the meat may be neutralized by the 

 addition of a few drops of a saturated aqueous solution of 

 sodium carbonate. The solution is added drop by drop, 

 and the reaction frequently tested with litmus-paper. 

 When a neutral reaction, or, better, a faint alkaline re- 

 action, is attained, the mixture is well stirred, boiled 

 again for about half an hour to precipitate the alkaline 

 albumins formed, and filtered. The use of phenolphtha- 

 lein to determine the reaction of the culture-media is much 

 more reliable than litmus, and in many laboratories has 

 replaced it. The method of using it suggested by Timpe 

 is to continue the addition of the carbonate of sodium 

 solution until a drop of it produces a red spot upon phe- 

 nolphthalein-paper. Such a paper can easily be made by 

 using a solution of 5 grams of phenolphthalein to i liter 

 of 50 per cent, alcohol. The bibulous paper is cut into 

 strips, moistened with the solution, and then hung up to 

 dry. It keeps quite well. Acids do not change the 

 appearance of the paper, but small traces of alkali turn 

 it red. 



If it is necessary to be extremely accurate concerning 

 the acidity or alkalinity of the culture-medium, the 

 method of titration with phenolphthalein can be em- 

 ployed. For this purpose a small quantity of the culture- 

 fluid say 10 c.cm. receives an addition of a drop or 

 two of a weak alcoholic solution of phenolphthalein 

 (i : 300), and then drop by drop from a burette a dilute 

 soda solution is added until a faint rose color occurs, when 

 a simple calculation will show that if so much is required 

 to bring about the required color in the 10 c.cm., so much 

 more will be required for the total amount. The occur- 

 rence of the rose color marks the change from a neutral 



