3l6 REAGENTS AND PROCESSES 



plugs and tubes, or flasks, will not need to be baked, but may 

 be sterilized, together with the nutrient medium already poured 

 into them, by subjecting them for fifteen minutes to a tempera- 

 ture of 115° C. in the autoclav. At this temperature a single 

 sterilization suffices. 



A good artificial nutrient medium for yeasts is made by adding 

 0.05 per cent, of tartaric acid to a 10 per cent, solution of cane- 

 sugar. A filtered aqueous extract of malted barley also gives 

 good results. To prepare this, barley is germinated until the 

 plumule just begins to protrude; the barley is then dried and 

 ground up, and water is poured over it until there is about 

 twice as much water by volume as of the powdered malt. The 

 water should stand over the malt, with occasional stirring, for 

 about an hour, when it may be filtered off and sterilized. Steril- 

 ized grape juice is also an excellent nutrient medium for yeasts. 

 Cultures of yeasts grown in the above media may be made to 

 produce spores in about twenty-four hours if some of the 

 culture is transferred to the surface of sterilized bits of flower- 

 pot Vv^hich are half submerged in water and kept covered by a 

 bell-jar. 



Cohn's normal solution for the culture of bacteria is pre- 

 pared as follows: Dissolve in 200 gm. of distilled water i gm. 

 of acid potassium phosphate, i gm. of magnesium sulphate, 2 

 gm. of neutral ammonium tartrate, and o.i gm. of calcium 

 chloride. 



An infusion of meat for the culture of bacteria is prepared 

 by covering finely chopped lean beef with water and allowing 

 it to stand for twenty-four hours in an ice-chest, after which 

 it is to be filtered through a muslin bag, using pressure of the 

 hands to make the filtration more complete. The filtrate is 

 then cooked and again filtered, and neutralized by the gradual 

 addition of a solution of carbonate of soda. The solution should 

 be tested with litmus paper, and the addition of carbonate 

 of soda should cease as soon as neutralization is accomplished. 

 To this solution is added 0.5 per cent, of common salt. Ten 

 grams of peptone may be added to a liter of the infusion. 



