114 BACTERIOLOGY. 



cotton plugs. It may be sterilized by either the inter- 

 mittent method with streaming 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 recom- 

 mended 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 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 litre 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 



