60 THE CULTIVATION OF BACTERIA. 



chopped up very fine and covered with 1 litre of sterilized 

 water, and put into an ice-box for twenty-four hours, after 

 which the aqueous extract is obtained by nitration through 

 muslin by pressure, sufficient water being added if necessary 

 to make up the original liter. To this filtrate 10 grams of 

 peptone and 5 grams of sodium chloride are added, and the 

 whole is cooked on a water-bath or in an enamelled iron 

 kettle for a half-hour, after which sufficient of a saturated 

 solution of sodium carbonate is added drop by drop to give 

 the mixture a slight alkaline reaction. This, after cooking 

 for fifteen or twenty minutes, is filtered through absorbent 

 cotton several times into test-tubes and sterilized by steam 

 for twenty minutes on three successive days. 



The addition of 5 per cent, neutral glycerin to this bouillon 

 makes an excellent liquid culture-medium for tubercle bacilli, 

 and is highly recommended by Roux. 



Any standard beef-extract, such as Liebig's, Armour's, 

 Wyeth's, etc., may be used in making this bouillon, instead 

 of the meat itself, 5 grams of the extract being added to 

 1 liter of water, the rest of the process being the same. 



II. The Most Commonly Used Solid Culture -Media. 



1. Gelatin. 



Nutrient gelatin is prepared as follows : A meat-infusion, 

 as described for bouillon, is prepared and put into a large 

 Bohemian flask of 2 liters capacity ; to this meat-infusion are 

 added for each litre : 



Chloride of sodium, 5 grams ; 



Peptone,. 10 " ; 



Best quality gelatin 100 " . 



The flask, loosely closed with a plug of absorbent cotton, is 

 cooked over a water-bath until all the gelatin is dissolved. 

 This will take from an hour and a half to two hours. After 

 this the mixture, which will be found to be decidedly acid, 

 is neutralized by means of a solution of sodium carbonate 1 



