90 BACTERIOLOGICAL ANALYSIS LESSONS 



XI. AGAR-AGAR. 



(1) Weigh out 10 grammes of agar-agar fibre, cut it up 

 finely, and allow it to swell in a very dilute solution of 

 acetic acid (3 to 4 cc. of glacial acetic acid to 500 cc. of 

 water) tor fifteen minutes. 



(2) Drain away the acetic acid. 



(3) Wash the agar-agar in distilled water, to remove all 

 trace of acid. 



(4) Add the washed agar-agar to 500 cc. of broth and 

 boil : it will dissolve in about fifteen to thirty minutes. 



(5) Neutralise carefully very little sodium carbonate is 

 required and boil again for a few minutes. 



(6) Allow it to cool somewhat then clear it by adding 

 the white of an egg, and heating it in the autoclave for a 

 half to three-quarters of an hour or even longer. 



(7) Now filter through a hot-water funnel. 



To hasten the filtration, if necessary, change the filter- 

 paper frequently, and always keep the unfiltered agar-agar 

 hot (see note p. 92). 



(8) Sterilise the filtered agar-agar in the autoclave for 

 twenty to thirty minutes at 120 C. : a single sterilisation 

 is sufficient. 



Agar-agar prepared in this way will often filter as 

 quickly as gelatine, and is exceedingly clear. 



To fill tubes with agar-agar : 



Melt the agar-agar in the autoclave and pour it into a 

 sterile separating funnel, and fill the tubes in the usual 

 manner. 



