NUTRIENT AGAR-AGAR 167 



and formed large, firm masses floating on and in clear 

 gelatine) , 



10. Filter through papier Chardin into a sterile flask. 



11. Tube in quantities of 10 c.c. 



12. Sterilise in the steamer at 100 C. for twenty 

 minutes on each of three consecutive days i. e., by 

 the discontinuous method. 



Nutrient Agar=agar. 



1 . Weigh a 2 -litre flask and note the weight or coun- 

 terpoise exactly. 



2. Measure out double strength meat extract, 500 c.c., 

 into the " tared" flask. 



3. Weigh out and mix 10 grammes of peptone, 5 

 grammes of salt, and 20 grammes of powdered agar, 

 and make into a thick paste with 150 c.c. distilled 

 water, and add to the meat extract in the flask; place 

 the flask in a water-bath. 



4. Arrange the steam can and water-bath as already 

 directed (for the preparation of gelatine) and figured. 



5. Bubble live steam (at 100 C.) through the 

 medium mass, for twenty-five minutes, by which time 

 complete solution of the agar is effected. 



6. Now weigh the flask and its contents; then (I035 1 

 grammes + weight of flask) minus (weight of flask and 

 its contents) equals the weight of water required to 

 make up the bulk of the medium to i litre. Add the 

 requisite amount (see preparation of gelatine, page 

 1 66, step 6). 



7. Titrate, and estimate the reaction of the medium 

 mass; control the result. Calculate the amount of 



1 This figure is obtained by adding together i litre of water (meat ex- 

 tract), looo grammes; 2 per cent, agar, 20 grammes; i per cent, peptone, 

 10 grammes; 0.5 per cent, salt, 5 grammes total 1035 grammes. Modi- 

 fications of the process as to quantities or percentages will nessitate corres- 

 ponding alterations in the calculated medium figure. The average weight 

 of a measured litre of 2 per cent, agar when prepared in this way, after 

 filtration, is 1010.5 grammes. 



