14 



STERILIZATION BY FILTRATION 



Technique. Fill the vessel about three-fourths full of water and immerse the flasks 

 by means of the wire tray, put on the lid, pass a thermometer through the opening pro- 

 vided for the purpose, and light the gas. Watch the thermometer carefully until the 

 desired temperature is reached, then set the regulator in the manner to be described later 

 (Chap. IV.), and no further supervision is required. 



The regulator being once set for a given temperature will always work automatically 

 at that temperature until it is again altered, so that beyond lighting the gas and when 

 necessary pouring water into the bath no further manipulation is required. 



FIG. 9. Weissnegg's water bath. 



Note. When using the bath for the first time, it is advisable to set the regulator before- 

 hand by means of a blank experiment, thus avoiding accidental overheating of the 

 medium. Sterilization is then carried out as already described, the medium being 

 heated on six or eight consecutive days for an hour each time. 



SECTION III. STERILIZATION BY FILTRATION. 



The application of heat in some form is the usual method of sterilization 

 used in bacteriological work, but it sometimes happens that fluids have 

 to be dealt with which cannot be subjected to even a moderate degree of 

 heat without profoundly altering their nature. In order to sterilize such 

 a fluid, it is passed through a solid bougie, the pores of which are so fine 

 that while liquids and solids in solution pass through, micro-organisms are 

 retained. Pasteur in his early work utilized plaster plates as the filtering 

 medium, but as a result of Chamberland's researches porous porcelain 

 superseded plaster. 



Filters. 



The Pasteur- Chamberland filter consists of a porous porcelain tube or 

 bougie closed at one end but open at the other, and finished at the latter with 

 a nozzle of glazed porcelain. The unfiltered liquid traverses the pores of the 

 bougie from without inwards, and issues from the nozzle filtered and steri- 

 lized. The Pasteur-Chamberland bougies are made in two grades of porosity. 



