THE PREPARATION OF CULTURE MEDIA 



123 



in the accompanying figures. In most of the methods of filtration 

 commonly employed the fluid which is to be filtered is sucked through 

 the walls of the filter, either by a hand suction-pump or by some form 

 of vacuum-pump attached to an ordinary water-tap. 



The hollow candle-filter may either be firmly fitted into a cylin- 

 drical glass chimney and surrounded by the 

 fluid which is to be filtered, or else the candle 

 may be connected to the collecting flask 

 with sterile rubber tubing and suspended 

 freely in the fluid. Perfect filters of these 

 types will hold back any of the bacteria 

 known to us at present. 



Filters before use must be sterilized. 

 The candles themselves are subjected to 

 150° C. in the hot-air sterilizer for gne hour. 

 The glassware and washers necessary for 

 setting up the apparatus may be sterilized 

 by boiling. In order that filters may be re- 

 peatedly used with good result, it is neces- 

 sary that they should be carefully cleaned 

 from time to time. This is best done in the 

 following way: 



Filters through which fluids from living 

 cultures have passed are first sterilized in 

 the Arnold steam sterilizer. Their exterior 

 is then carefully cleaned with a fine brush. 

 Following this a five-tenths per cent solu- 

 tion of potassium permanganate is passed 

 through them and this again removed by 

 sucking through a five per cent solution of 

 bisulphite of soda. This last is washed out 

 by sending a considerable quantity of dis- 

 tilled water through the filter, which is then dried and sterilized by 

 heat. 



The suction necessary for filtration through these filters is usually 

 applied by means of the ordinary suction-pump attached to a running 

 faucet. 



Slanting of Media. — Solid media which are to be used in slanted form 

 in test tubes should be inclined on a ledge (easily improvised of glass 

 tubing) at the proper slant, after the last sterilization. Agar, the medium 



Fig. 25. — Kitasato Filter. 



