GENERAL PRINCIPLES 153 



acid), filter, and sterilize in the steamer. If filtrate is 

 not clear, cool to 6o C, add whites of two eggs per litre, 

 re-heat for half-an-hour in steamer, and filter through 

 Chardin paper in warm filter-jacket. 



Nutrient Agar. Add 1*5 per cent of powdered agar to 

 broth. Melt in the steamer at ioo C. for 1-5 hours. 

 Standardize, and replace in steamer for 20 minutes to 

 precipitate phosphates. Cool to 6o C, add two whites of 

 egg per litre, reheat for half an hour, filter through Chardin 

 paper by aid of a hot-water funnel or in a steamer, or filter 

 through glass-wool. Tube, and sterilize. Agar melts 

 between 90 and ioo C. and remains fluid down to 40 C. 



Glycerin Agar. Add 6 per cent of glycerin after 

 filtration ; tube, and sterilize. 



Glucose and Lactose Agar. To agar made with sugar- 

 free broth, add 2 per cent of glucose or lactose. If to be 

 tinted with neutral red, add before filtration 2 per cent of 

 a solution of neutral red (J per cent). 



Litmus Lactose Agar. Add to nutrient agar prepared 

 from sugar-free broth 1 to 2 per cent of lactose, and 

 sufficient litmus to give a good colour (about 5 to 10 c.c. 

 of a 1 per cent litmus solution per 100 c.c. of total medium). 

 Conradi and Drigalski's medhim is similar, plus nutrose 

 and crystal- violet. 



Peptone Water, Dissolve by the aid of heat 1 per 

 cent of peptones and J per cent of NaCl in distilled water. 

 Tube, and sterilize. For water investigations it is usual 

 to keep a stock solution ten times this strength. 



Glucose, lactose, sucrose, and mannite are used with 

 peptone water plus Durham's fermentation tubes. Mostly 

 in 1 per cent strength. 



MacConkey's Media are much used in water examina- 

 tions in this country. 



Bile-salt Litmus Glucose Peptone Water is made in single 

 strength, and in triple strength thus : Peptone, 20 or 

 60 grm. ; glucose, 5 or 15 grm. ; taurocholate of sodium, 

 5 or 15 grm. ; litmus solution (10 per cent sterile) 100 c.c, 

 and water to 1 litre. Put peptone, glucose, bile-salt, 

 and water in a flask and heat in steamer for 45 minutes. 

 Filter through Chardin's paper, add the filtered litmus 



