NUTRIENT AGAR MEDIA 39 



porate with the medium by stirring. Steam in the Koch steriliser 

 for one hour at 100'. Filter hot through " papier Chardin." 

 Run into tubes or flasks, and autoclave for fifteen minutes at 

 115°, or sterilise by intermittent heating in the Koch steriliser for 

 thirty minutes on three successive days. 



Nutrient agar {Ravenel's method). — If dry peptone is employed, 

 or when it is necessary to prepare a stock of agar at short notice, 

 the method devised by Dr Mazyck P. Ravenel of the University 

 of Pennsylvania is recommended.^ The whole process, with the 

 exception of the time the meat is digesting need not exceed two 

 hours, and the result is a remarkably clear jelly. 



The method of preparation is as follows : — To make i litre, take 



A. Chopped meat . . . 500 grammes. 



Water ..... 500 c.c. 



Mix and place in a cool place over-night, then strain through a 

 towel or muslin. 



B. Agar-agar ... 12 grammes (i-2 per cent.). 



Water . . . 500 c.c. 



Put into autoclave, run up to two atmospheres (134° C.) put out 

 flame, allow to cool until below 100'' C. before opening. Let the 

 solution of agar cool still further to about ys" C. : then add B to 

 A. Add 10 grammes (i per cent.) of dried peptone and 5 grammes 

 (•5 per cent.) of common salt : bring to boiling point and boil for 

 five minutes, neutralise and filter. The filtration is rapid, no hot 

 water funnel being necessary if the filter paper is wetted with 

 boiling water immediately before using. The clarification of the 

 medium is effected by the coagulation of the albumen in the meat 

 water on the raising of the temperature after the addition of the 

 salt and peptone. Hence solution A must not be added to B 

 until cool enough to avoid premature coagulation. 



16. Glycerine agar — 



To nutrient agar prepared as above add 6 per cent of glycerine 

 after filtration, and before final sterilisation in tubes. 



17. Glucose {or lactose) agar — 



To nutrient agar prepared in the manner above described, 

 add 2 per cent of glucose or lactose at the same time as the salt 



18. Agar gelatine — 



To nutrient bouillon prepared as above, add 5 per cent, of 

 gelatine, and -8 per cent of agar. The temperature of sterilisation 

 ' Journal of Applied Microscopy, vol. i.. No. 6. 



