PREPARATION OF NUTRIENT AGAR-AGAR 129 



During boiling it is well to filter, from time to time, a 

 '' few cubic centimeters of the gelatin into a test-tube and boil 

 it over a free flame for a minute or so; in this way one can 

 detect if all the albumin has been coagulated i. e., if the 

 solution is ready for filtration. 



Gelatin should not, as a rule, be boiled more than ten or 

 fifteen minutes at one time, or be left in the steam sterilizer 

 for more than thirty minutes ; otherwise its property of 

 solidifying may be impaired. 



As soon as the preparation of the gelatin is complete, 

 whether it is retained in the flask into which it has been 

 filtered or decanted into sterilized test-tubes, it should be 

 sterilized, the mouth of the flask or the test-tubes containing 

 it having been previously closed with cotton plugs. It may 

 be sterilized by either the intermittent method with stream- 

 ing steam or by a single application of steam under pressure 

 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 modification 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 

 9 



