74 A MANUAL OF BACTERIOLOGY 



hour or more), then neutralise or standardise. Cool to 

 50 C. 5 add the white of an egg and return to the steamer 

 for an hour and a half, then filter through an agar filter- 

 paper {"papier Chardin ") in a hot -water funnel or in 

 the steamer. By this treatment a litre of agar should 

 pass through the filter in two to three hours. If it does 

 not come through clear, add the white of another egg and 

 repeat the process. Agar requires well cooking, otherwise 

 a soft watery jelly results. 



If an autoclave is available, a quicker and better method 

 is, after neutralising and adding the white of an egg, to 

 place in the autoclave with a small beaker inverted over 

 the mouth of the flask, and heat to 134 C. (two atmo- 

 spheres pressure) for half an hour. Turn the gas out, and 

 allow to cool without opening the stopcock. When cool, 

 open, and filter through the special agar filter-paper in a 

 hot-water funnel ; the agar will pass through in about 

 ten minutes or a quarter of an hour. Fill into test-tubes 

 and sterilise. Solidify in the upright or oblique position 

 as required. Agar may also be kept in bulk. 



In the case of bar or stick agar, first steep the agar in 

 1 per cent, acetic acid for a quarter of an hour, then drain 

 and wash it thoroughly to remove the acid. The further 

 procedure is the same as detailed above. This yields a 

 very clear, pale product, and is perhaps preferable when 

 an autoclave is not available. 



Glycerin agar. Add 4 to 6 per cent, of glycerin to the 

 nutrient agar after filtration and proceed as before. 



Glucose, agar. One or 2 per cent, of grape sugar is 

 added to the nutrient agar after filtration. 



Litmus media. The addition of neutral litmus to the 

 various culture media is a useful method of demonstrating 

 the production of acid or of alkali by organisms. To 

 prepare the litmus solution take the lump litmus, powder 

 finely, and boil with distilled water so that a saturated 



