42 METHODS OF STERILISATION 



6. Connect the other end of the bulb tube with the 

 delivery tube from the gas reservoir. 



The gas in its passage through the dry sterile cotton- 

 wool in the bulb of the filter tube will be freed from 

 any contained micro-organisms and will enter the 

 flask in a sterile condition. 



(b) Porcelain Filter. The sterilisation of liquids by 

 filtration is effected by passing them through a cylin- 

 drical vessel, closed at one end like a test-tube, and 

 made either of porous "biscuit" porcelain, hard-burnt 

 and unglazed (Chamberland system) , or of Kieselguhr, 

 a fine diatomaceous earth (Berkefeld system), and 

 termed a "bougie" or "candle" (Fig. 34). 



NOTE. In selecting candles for use in the laboratory avoid those 

 with metal fittings, since during sterilisation cracks develop at the 

 junction of the metal and the siliceous material owing to the un- 

 equal expansion. 



In this method the bacteria are retained in the pores 

 of the filter while the liquid passes through in a germ- 

 free condition. 



It is obvious that to be effective the pores of the 

 filter must be extremely minute, and therefore the rate 

 of filtration will usually be slow. Chamberland filter 

 candles possess finer channels than Berkefeld candles 

 and consequently filter much more slowly. To over- 

 come this disadvantage, either aspiration or pressure, 

 or a combination of these two forces, may be employed 

 to hasten the process. 



Doultons white porcelain filters it may be noted are 

 as efficient as the Chamberland candles and filter 

 rather more rapidly. 



Apparatus Required. 



1. Separatory funnel containing the unfiltered fluid. 



2. Sterile filter candle (Fig. 34), the open end fitted with a 

 rubber stopper (Fig. 34, a) perforated to receive the delivery tube 

 of the separatory funnel, and its neck passed through a large rubber 

 washer"(Fig. 34, b) which fits the mouth of the filter flask. 



3. Sterile filter flask of suitable size, for the reception of the 

 filtered fluid, its mouth closed by a cotton- wool plug. 



