NUTRIENT MEDIA 55 



gelatine is now filtered through a double-pleated filter- 

 paper previously moistened, and kept warm by means of 

 a hot- water funnel ; or, better, the filter can be placed 

 in an ordinary funnel in the steam steriliser until the 

 whole of the gelatine has run through perfectly clear. It 

 will be found best to return the first 100 c.c. of filtrate to 

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

 quite clear. 



The gelatine 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 

 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 into test-tubes at once. This is best done by melting 

 the gelatine at a low temperature, and pouring it into 

 a sterile separating-f unnel ; 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 

 with cotton-wool that has been heated in the hot-air 

 steriliser for about an hour at 120° C. The plugs, when 

 inserted, should be a little over an inch long, and should 

 fit as tightly as possible. The best way of plugging tubes 

 is to place a loose piece of wool on the mouth of the tube, 

 which is then pressed home with a penholder with a rotary 

 motion. The loose ends of the cotton-wool are then cut 

 or singed off, and each tube covered with tinfoil ; or the 



