AGAR-AGAR 



131 



FIG. 66 



filter, which can be filled with water and kept hot over a Bunsen 

 burner (with a ring-shaped burner with many small openings). When 

 it is required on short notice a tolerably good agar can be prepared by 

 sedimentation, as follows: After the agar is entirely dissolved it is 

 placed in cylindrical vessels, which are thoroughly heated in the 

 steam sterilizer and left there when the flame is turned off. During 

 the very slow cooling of the agar the impurities fall to the bottom of 

 the vessel. When entirely cold the agar is caked out and the lower 

 stratum containing the impurities cut off and thrown away. 



6. After removal of the impurities by filtration or sedimentation 

 the agar is remelted and distributed into test-tubes, about 10 c.c. to 

 each. It is then sterilized by con- 

 tinuous sterilization in the autoclave 

 or steam sterilizer, and taken out 

 while hot and fluid. The tubes are 

 placed on an inclined plane, so that 

 the agar solidifies in a slanting 

 position. This yields the largest 

 possible surface for inoculation. 



A good 2 per cent, agar, the 

 preparation of which has been de- 

 scribed above, should be free from 

 impurities and transparent, but it 

 is never as entirely clear as a first- 

 class gelatin medium. Two per 

 cent, agar melts at 80 C., and re- 

 solidifies at 40 C. It can, therefore, 

 be kept in the incubator without 

 melting. Ten per cent, gelatin 

 melting at 25 C. cannot be kept in 

 the incubator. The transparency 

 and brilliancy of both gelatin and 

 agar may be improved by the 

 addition of the white of an egg before filtration. The egg is broken 

 and the white separated from the yolk. The albumen is mixed with 

 water and gradually added under constant stirring to the melted 

 gelatin or agar, the temperature of which, however, must not exceed 

 60 C. in order to avoid coagulation of the egg-albumen. After thor- 

 oughly mixing the media are heated in the steam sterilizer or on a 

 water bath. The egg-albumen coagulates and carries with it to the 

 bottom fine impurities, difficult to filter out without the coagulated 

 albumen. 



Both gelatin and agar frequently receive certain additions for 

 special purposes. Such additions may also be made to blood serum 

 or the latter may be combined with agar. The following culture 

 media are frequently used in determining definite characteristics of 

 certain bacteria when raised in pure culture. 



Double-walled hot-water funnel with cir- 

 cular gas burner and stand. 



