FILTRATION METHODS 



865 



use any large piece of apparatus Andrewes uses a large metal aspirating syringe 

 of known capacity. By means of a side tap the aspirated air can be expelled 

 without disconnecting the syringe from the filter tube.] 



Whatever the form of aspirator used the air should always be aspirated 

 slowly and regularly so that the bubbles burst one by one in the liquid through 

 which it is passed. There are various other pieces of apparatus which can 

 be used for the same purpose. 



A. Methods based upon filtration. 



1. Filtration through insoluble substances. 1. Petri's method. Take a 



piece of glass tubing about 15 mm. in diameter and 10 cm. long and at each 



end arrange a pair of wire gauze plugs (B p B 2 , B 3 , B 4 , fig. 412) 



leaving a space of about 3 cm. between eaclTpair (C 15 C 2 ) and 



fill these two spaces with very fine sand previously heated to 



redness. Plug the two ends of the glass tube with wool and 



sterilize the apparatus in an hot air sterilizer. When it has 



cooled replace one of the wool plugs with a sterile perforated 



india-rubber bung, D, through which a piece of glass tubing, 



F, plugged with wool is passed. To use the apparatus, attach 



the end of the small glass tube, F, to an aspirator, take the 



wool plug out of the other end and slowly aspirate 100 litres 



of air. When the aspiration is completed the sand is mixed 



with sterile gelatin and a number of plates poured. The 



method is complicated and of little use in practice. 



2. Frankland's method. The tubes are similar to those 

 used by Petri but glass wool or asbestos is substituted for 

 the sand : this does away with the necessity for the metal 

 gauze. After the air has been aspirated, the filtering medium 

 is shaken up in a known quantity of broth and the tatter is 

 then used for sowing gelatin plates. This method though very 

 simple is not exact since organisms stick to the glass wool 

 or the asbestos and do not become suspended in the broth. 



II. Filtration through soluble substances (Pasteur). By 

 using soluble instead of insoluble substances the distribution 

 of the organisms in the gelatin is made more certain and 

 their enumeration is very accurate. Unfortunately the 

 method is not applicable when the atmosphere contains much Petri'l' sand mter 



moisture since in that case the filtering substances become !? r fch examina- 



. , ,. , , 5.. tion of air. 



moist, deliquesce and no longer act as a filter. 



Sulphate of sodium is ordinarily used as the filtering medium. The salt 

 is fused in an iron vessel, powdered and sifted and then introduced into a 

 glass tube of the shape shown in fig. 413. One end of the tube is plugged 



FIG. 413. Glass tube for soluble filters. 



with wool and beyond this is a constriction against which rests a small piece 

 of asbestos and then powdered sodium sulphate to a depth of about 8 cm., 

 the other end of the tube is drawn out and sealed in the flame. The apparatus 

 is sterilized in the hot air sterilizer. To use it, the powder is shaken down 

 against the asbestos plug by gently tapping the tube, the pointed end of the 

 tube is broken off and the other, plugged, end connected to an aspirator. 



3i 



