144 AIR 



been passed through, the tube is replaced in its sterile box, and 

 conveyed to the laboratory. A file mark is made across the 

 centre, and the tube broken in half. The plugs of glass wool 

 and the sugar are pushed by a sterile wire into a flask containing 

 10 or 15 c.c. of liquefied nutrient gelatine. The contents are well 

 mixed, and a roll-tube preparation made by distributing the 

 gelatine over the wall of the flask. 



The plan recommended by Haldane of using flat-bottomed 

 flasks for the gelatine, and so making an ordinary plate prepara- 

 tion, is preferable, or still better the nutrient gelatine, after the 



a, plug of glass wool; b, plug of glass wool and finely powdered cane sugar; 

 c y plug of cotton wool. 



admixture of the sugar, may be poured out into Petri-dishes and 

 solidified in the ordinary way. 



In using sugar for air bacteriology, certain precautions must 

 be attended to. The sugar must be perfectly dry before it is 

 filled into the tubes, and its sterilization has to be very care- 

 fully done. If heated to too high a temperature it darkens, 

 coheres, and is useless. The particles of sugar must be of 

 suitable size. In practice, the sugar is somewhat coarsely 

 ground, and then passed through two wire sieves, one fine, the 

 other coarse mesh. In this way the fine powder and the larger 

 granules are rejected. The former would clog, and the latter 

 allow the passage of organisms between the particles. 



One great difficulty which the writer has experienced with 

 sugar-filters is that if the examinations are made upon moist 

 air, such as sewer air, the water in the air passing through the 

 filter rapidly moistens the sugar, and makes the particles cohere, 

 so that after a short time filtration becomes exceedingly slow. 



3. Examination of Air for the Presence of P articulate Con- 

 tamination. Fliigge and his pupils have shown that minute 

 particles or droplets are expelled from the upper respiratory 



