NUTRIENT GELATINE 59 



the filter, ■when the filtrate will be found generally to run 

 quite clear. 



The gelatme'thus prepared should be perfectly clear and 

 of an amber-yellow colour, and should not become cloudy 

 on heating. Nutrient gelatine should not be heated more 



Fig. 13. — Apparatus foe Filtering Media. 



than necessary, since by so doing it may lose the property 

 of setting when cold. If the medium becomes turbid after 

 sterilisation, it is probably due to the mixture being too 

 alkaline. Dilute lactic acid should always be used to reduce 

 the alkalinity, and not the mineral acids, such as sulphuric 

 or hydrochloric. The gelatine medium, prepared as above, 

 can be sterilised on three successive days in the steam 

 steriliser, for fifteen minutes on each occasion, or, better, 

 run off mto test-tubes at once. This is best done by melting 

 the gelatine at a low temperature, and pouring it into 

 a sterile separating-funnel ; or a more convenient method 

 to use after a little practice is a 100 c.c. pipette. The 

 chemically clean test-tubes are placed in a rack, and from 

 5 to 15 c.c. of the media run in without soiling the edges of 

 the tubes. When all the tubes are filled, they are plugged 



