STERILIZATION OF CULTURE MEDIA. 59 



receptacle, R, is provided for the liquid to be filtered, and a pump for 

 compressing air is attached to it by a rubber tube. Instead of this 

 pump, water pressure may be used indirectly by attaching a strong 

 bottle to the water supply and allowing it to fill slowly with water, 

 and at the same time to force out the air through a tube connected 

 with the filtering apparatus. For this purpose the bottle, having a 

 capacity of a quart or more, should be provided with a rubber stop- 

 per through which two short tubes are passed. One of these is con- 

 nected with the water supply and the other with the filter. Of 

 course this is only practicable when a water supply with sufficient, 

 pressure is available. 



FIG. 32. 



As a rule, filtration cannot be substituted with advantage for ster- 

 ilization by heat in the preparation of culture media. Albuminous 

 liquids pass through the filter with difficulty, and the process of 

 sterilization by discontinued heating will usually prove more satis- 

 factory than filtration, which requires extreme precautions to pre- 

 vent accidental contamination of the filtered liquid. Moreover, the 

 filter may change the composition of the medium passed through it 

 by preventing the passage of colloid and albuminous material in so- 

 lution. Thus, in an attempt to separate blood corpuscles from the 

 serum by filtration through a Chamberlain filter, the writer obtained 

 a transparent liquid which did not coagulate by heat i.e., the albu- 

 minous constituents of the serum did not pass through the filter. 



