NUTRIENT AGAR-AGAR. 85 



in the steam sterilizer on three successive days, for 

 fifteen minutes each day the mouth of the flask or the 

 test-tubes containing it having been previously closed 

 with cotton plugs. 



NUTRIENT AGAR-AGAR. The preparation of nutri- 

 ent agar-agar by the beginner is far too frequently a 

 tedious and time-taking experience. This is due mainly 

 to lack of patience and deviation from the rules laid 

 down for the preparation of this medium. If the direc- 

 tions 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 has some slight 

 modification of his own. 



The methods that have given us the best results, and 

 from which we have no good grounds for deviating, 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 make a mark on the 

 sides of the vessel at which the level of the fluid stands, 

 add about 250 c.c. to 300 c.c. of water and allow the 

 mass to boil slowly, occasionally stirring, over a free 

 flame, for from one and one-half to two hours. Care 

 must be taken that it does not boil over the sides of the 

 vessel. From time to time observe if the fluid has 

 fallen below the mark of its original level ; if it has, 

 add water until its original volume is restored. At the 

 end of the time given remove the flame and place the 

 vessel containing the mixture in a large dish of cold 



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