BACTERIOLOGICAL TECHNIQUE. 



213 



put in an ice-chest for from eighteen to twenty-four 

 hours. The infusion is strained, when cold, through a 

 fine cheese-cloth, and to the clear filtrate 1 per cent, of 

 peptone and 0.5 per cent, of sodium chloride are added. 

 The medium is then warmed for some minutes until the 

 peptone is dissolved, and then exposed to live steam 

 either without pressure in the Arnold steam sterilizer 

 (Fig. 20) for thirty minutes, or in the autoclave (Fig. 



FTG. 20. 



STERILIZING CHAMBER 



U A 



Arnold steam sterilizer. 



21) at one atmosphere of pressure for fifteen minutes, 

 or boiled over a free flame for ten minutes. While 

 still hot it is filtered through filter-paper or through 

 absorbent cotton, and the reaction is tested and suf- 

 ficient hydrochloric acid or sodium hydroxide added 



