146 BIOLOGY AND TECHNIQUE 



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 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 one hour. The 

 glassware and washers necessary for setting up the apparatus may 

 be sterilized by boiling. In order that filters may be repeatedly 

 used with good result, it is necessary 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 solution 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 distilled 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 run- 

 ning 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 most frequently employed in this way, should 

 be left in this position until hard. 



Indicators. We cannot in this place go into details concerning 

 the theory of indicators. For this we may refer the reader to the 

 recent treatise by Clark on the "Determination of Hydrogen Ions." 7 

 According to Ostwald, indicators are acids or bases whose undis- 

 sociated molecules "have a different color from that of their dis- 

 sociation products." This conception has been somewhat modified 

 by more recent work, in that color changes are found to be associated 



''Clark, W. Mansfield, Determination of Hydrogen Ions, Williams and Wilkins 

 Co., Baltimore, 1920. 



