CULTURE MEDIA 



91 



to 'a liter of the medium. After this has been ascertained, and 

 the amount added, the medium is thoroughly mixed and then 

 left covered and undisturbed while the water in the outer com- 

 partment of the cooker is boiled for an hour. It is well to have 

 boiling water at hand in another receptacle so that the supply 

 in the cooker may be replenished if it gets low, without chilling 

 the medium. The gelatin is now filtered through paper wet with 

 hot water, and should be kept warm during 

 filtration by means of a funnel-heater, or by 

 a steam bath, although these are not indis;^ 

 pensable. If it gets cold it may be poured 

 out of the funnel and warmed again in the 

 pan. A portion of the filtrate should be 

 boiled in a test tube over the flame for a 

 minute or two. It should then remain 

 (i) perfectly clear, (2) alkahne to litmus 

 paper, and (3) should sohdify oh cooling in 

 tap water. After filtration the medium is 

 filled into tubes and sterilized in streaming 

 steam by the fractional method, 20 minutes 

 at 100° C. for 3 consecutive days. Gela- 

 tin may be sterilized in the autoclave at 

 iio°C. for 10 minutes, but it should be „ ^ . ^ , 



' Fig. 34. — Apparatus for 



chilled in cold water at once after removal, filling media into tubes, it 



, , , . , , . . . i is held in a ring-stand sup- 



and even then its gelatmizing property port, 

 may be seriously impaired. 



In filling gelatin into tubes it is important that the medium 

 should not be spilled on the mouth of the tube or on the cotton 

 plug, as this accident causes the latter to be glued in position. 

 The filling apparatus indicated in Fig. 34 will be found convenient 

 for filling any sort of liquid medium into tubes, and with proper 

 care one may fill tubes rapidly without soiling the mouths of tubes 

 and their cotton plugs. 



Gelatin may be made from beef extract. The extract, peptone, 

 salt and gelatin are dissolved at a temperature below 6o°C.or 



