116 BACTERIOLOGY 



in the autoclave. If the latter method be selected, the 

 pressure should not exceed one atmosphere and the time 

 of exposure be not over fifteen minutes. 



NUTRIENT AGAR-AGAR. 



The preparation of nutrient agar-agar by the beginner is 

 far too frequently a tedious and time-consuming operation. 

 This is due mainly to lack of patience and to deviation from 

 the rules laid down for the preparation of this medium. If 

 the directions given below for the preparation of nutrient 

 agar-agar be strictly observed, no difficulty whatever should 

 be encountered. Many methods are recommended for its 

 preparation, almost every worker having some slight modi- 

 fication of his own. 



The methods that have given us the best results, and from 

 which we have no good grounds for departing, are as follows : 



Prepare the bouillon in the usual way. Agar-agar reacts 

 neutral or very slightly alkaline, so that the bouillon may 

 be neutralized before the agar-agar is added. Then add 

 finely chopped or powdered agar-agar in the proportion of 

 1 to 1.5 per cent. Place the mixture in a porcelain-lined 

 iron vessel, and on the side of the vessel make a mark at 

 the height at which the level of the fluid stands; if a liter 

 of medium is being made, add about 250 to 300 c.c. more of 

 water and allow the mass to boil slowly, occasionally stirring, 

 over a free flame, from one and a half to two hours; or until 

 the excess of water i. e., the 250 or 300 c.c. that were 

 added has evaporated. Care must be taken that the 

 mixture does not boil over the sides of the vessel. From time 

 to time observe if the fluid has fallen below its original 

 level; if it has, add hot water until its volume of 1 liter is 



