PHYSICAL AGENTS 



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hold back the bacteria. It is used on a large scale in the 

 purification of water for sanitary or manufacturing reasons 

 (Fig. 102). Air is also rendered "germ free" in some surgical 

 operating rooms, " serum laboratories" and breweries by filtra- 

 tion. In the laboratory it is a very common method of steriliz- 

 ing liquids which would be injured by any other process. The 



Fig. 104. — Pressure filtration. A, cylinder which contains the filter 

 candle; B, cylinder for the liquid to be filtered; C, sterile flask to receive the 

 filtrate; D, air pump to furnish pressure. 



apparatus consists of a porous cylinder with proper devices 

 for causing the liquid to pass through either by suction 

 (Fig. 103), where the pressure will be only one atmosphere 

 (approximately 15 pounds per square inch), or by the use of 

 compressed air at any desired pressure (Fig. 104). The two 

 main types of porous cylinders ("filter candles," "bougies") 



