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. 97). Air is also rendered "germ-free" in some sur- 

 gical operating rooms, "serum laboratories" and breweries by 

 filtration. In the laboratory it is a very common method of 

 sterilizing liquids which would be injured by any other 



Fig. 99.— 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. 



process. The apparatus consists of a porous cylinder with 

 proper devices for causing the liquid to pass through either 

 by suction (Fig. 98), 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. 

 99). The two main types of porous cylinders ("filter 



