144 BIOLOGY AND TECHNIQUE 



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 fore- 

 going 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. Upon this is placed a thumb cock. The plug is then 

 removed from the test tube by catching it between 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. are 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 

 autoclave 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 frac- 

 tional method, i.e., by twenty minutes' exposure in the live steam 

 sterilizer (Arnold, Fig. 8, page 83) on each of three consecutive days. 

 During the intervals between sterilizations, they should be kept at 

 room temperature or in the incubator, to permit the germination 

 of spores which may be present. Media containing animal serum 

 or other albuminous solutions which are to be sterilized without 

 coagulation, may be sterilized in water baths, or in hot-air chambers, 

 at temperatures varying from 60 to 70 C., by the fractional method. 

 In such cases five or six exposures of one hour on succeeding days 

 should be employed. 



By Filtration. It is often desirable in bacteriological work to 

 free fluid from bacteria. This is frequently necessary for the sterili- 

 zation of blood-serum or exudate fluids, or fOr obtaining toxins free 

 from bacteria. For these purposes a large variety of filters are in 

 use. Those most commonly employed are of the Chamberland 6 or 

 Berkefeld type, which consist of hollow candles made of unglazed 

 porcelain or diatomaceous earth. Both these types are made in 

 various grades of fineness, upon which depend both the speed of 



Pasteur and Chamberland, Compt. rend, de 1'acad. des sci., 1884. 



