THE PREPARATION OF CULTURE MEDIA 



121 



edges of the cotton are made to adhere to the sides of the funnel by 

 allowing a thin stream of tap water to run over them, while smoothing 

 them against the glass with the hand. 



The medium, when poured into such a filter, should be poured along 

 a glass rod at first, to avoid running down the sides or bursting the filter. 

 After filtration has begun, the filter should 

 be kept as full as possible. The first liter 

 or so which comes through may not be clear, 

 but the filter gains in efficiency as the coag- 

 ulum settles into the fibers of the cotton, 

 and the first yield may be sent through a 

 second time. Filtration of agar or gelatin is 

 best done in a warm room with windows 

 and doors closed, and the filter covered with 

 a lid, to avoid too rapid cooling. The funnel 

 and filter should be warmed just before use. 



Filtering through Paper. Many media 

 may be efficiently cleared by filtration 

 through close filter paper without the aid of 

 coagula. 



The Tubing of Media. Most of the media 

 described in the foregoing section are used 

 in test tubes. In order to fill these tubes, 

 the media are best poured into a large glass 

 funnel to which a glass discharging tube has 

 been fitted by means of a short piece of 

 rubber tubing (see Fig. 20). Upon this is 

 placed a thumb cock. The plug is then re- 

 moved from the test tube by catching it be- 

 tween the small and ring fingers of the right 

 hand and the glass outlet is thrust deeply 

 into the test tube, in order to prevent the 

 medium from touching the upper portion 

 of the test tube where the cotton plug 

 will be lodged. About 7 to 8 c.c. is put in each test tube. 



Sterilization of Media. By Heat. Media which contain neither 

 sugars, gelatin, glycerin, nor animal serum may be sterilized in the auto- 

 clave at fifteen pounds pressure for fifteen minutes to half an hour. 

 Media which contain these or other substances subject to injury from 

 the high temperature, must be sterilized by the fractional method, 



FIG. 23. BERKEFFLD 

 FILTER. 



