NUTRITIVE SUBSTRATA. 41 



5 to 10 per cent, of sugar. Yeast decoction is an aqueous 

 extract of yeast (about 1 litre of yeast to 2 litres of water, 

 boiled under pressure), filtered, and either neutralised or 

 rendered slightly alkaline with sodium carbonate or lime. 

 For special research, compound liquids may be used containing 

 sugar and the salts necessary for nutriment and normal growth 

 of yeast, including potassium, magnesium, and calcium, 

 phosphoric and sulphuric acids. An admirable means for 

 preserving pure cultures of yeasts is the solution, first used by 

 Pasteur, consisting of a 10 per cent, cane sugar solution. 



Special nutritive liquids are also used for bacteriological 

 investigation. Cohn's solution has historical interest, and its 

 composition is as follows : 100 c.c. of water, -05 gramme 

 mono-potassium phosphate, 0-05 gramme tri-potassium phos- 

 phate, 0-5 gramme crystallised magnesium sulphate, and 



1 gramme ammonium tartrate. To-day a nutritive broth is 

 chiefly used, prepared by steeping finely chopped beef for 

 a few hours in water, and then boiling and filtering the liquor. 

 The liquid is generally neutralised with soda, or rendered 

 slightly alkaline, and after adding 1 per cent, of peptone and 

 0-5 per cent, of sodium chloride, it is again boiled, filtered hot, 

 and finally sterilised in flasks or test tubes. Such an extract 

 must obviously vary in composition, and in special cases resort 

 may be had to an artificial nutritive liquid, free from albumen. 

 We may quote that prepared by Voges and Proskauer, con- 

 sisting of 1 litre of water, 5 grammes sodium chloride, 



2 grammes disodium phosphate, 6 grammes ammonium 

 lactate, and 4 grammes of asparagin. 



A. Fischer's base consists of 0-1 per cent, of di-potassium 

 phosphate, 0-02 per cent, magnesium sulphate, and 0-01 per 

 cent, calcium chloride, dissolved in tap water. The solution 

 is then added to peptone, or peptone and sugar, etc., according 

 to the requirements of the particular species of bacteria. For 

 the development of lactic acid bacteria (from milk) O. Jensen 

 uses peptonised milk prepared by treating 1 litre of milk 

 with 10 c.c. of hydrochloric acid and 2 grammes of pepsin. 

 By keeping it in the thermostat and frequently shaking, the 

 precipitated casein is redissolved ; the liquid is then neutralised, 

 cleared with albumen, and sterilised at about 120 C. 



