BACTERIOLOGICAL EXAMINATION OF AIR. 



125 



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Fig. 58. — Petri's 

 sand filters. 



Petri's Sand-filter Method. — A glass tube open at both ends, and about 

 3j inches long and half an inch wide, is taken, and in its centre is placed a 

 transverse diaphragm of very fine iron gauze (Fig. 58, e) ; on 

 each side of this is placed some fine quartz sand which has 

 been well washed, dried, and burned to remove all impurities, 

 and this is kept in position by cotton plugs. The whole 

 is sterilised by dry heat. One plug is removed and a sterile 

 rubber cork c inserted, through which a tube d passes to 

 an exhausting apparatus. The tube is then clamped in an 

 upright position in the atmosphere to be examined, with 

 the remaining plug / uppermost. The latter is removed 

 and the air sucked through. Difficulty may be experienced 

 from the resistance of the sand if quick filtration be at- 

 tempted. The best means to adopt is to use an air-pump 

 — the amount of air drawn per 

 stroke of which is accurately 

 known — and have amanometer 

 (as in Fig. 38) interposed be- 

 tween the tube and the pump. 

 Between each two strokes of 

 the air-pump the mercury is 

 allowed to return to zero. After 

 the required amount of air has passed, the sand 

 a is removed, and is distributed among a number 

 of sterile gelatin tubes which are well shaken ; 

 plate-cultures are then made, and when growth 

 has occurred the colonies are enumerated; the 

 sand b is similarly treated and acts as a control. 

 The Sedgwick-Tucker Method. — A third and 

 better method is that of Sedgwick and Tucker, 

 whose apparatus combines the quahties of both 

 filter and culture tubes, whilst the employment of 

 finely granulated sugar as the filtering medium 

 removes the very obvious objections to the use 

 of sand. The apparatus consists of a glass tube 

 some 30 or 35 cm. long and 4 cm. wide, drawn 

 out at one end into a neck to retain a cotton 

 plug, whilst into the other end is fused a smaller 

 piece of glass tubing about 15 era. long and .5 cm. 

 wide. The surface of the larger portion is ruled 

 by a diamond into square centimetres to facili- 

 tate the counting of colonies (see Fig. 59). To 

 retain the filtering medium in position, a small 

 piece of tightly rolled fine-meshed brass-wire 



By permission, from Abbott's 

 *' Bacteriology." 



Fig. 59. — The Sedgwick-Tucker cloth is inserted into this narrow tubing about 



aerobioscope. 



5 cm. from its lower end. After cleansing and 



drying, the bore of the smaller part of the apparatus is filled with No. 50 

 granulated sugar, which is lightly packed by gently tapping, both ends are 



