THE EXAMINATION OP AIR 431 



Hesse's tube, thus accounting for the diminished number 

 of micro-organisms present. This theory was proved to 

 be correct by Laws, who examined the number and species 

 of bacteria found in London sewer-air ; these he found to be 

 the same as those in the external air, while those organisms 

 which were normal to sewage were found to be compara- 

 tively rare. 



Filtration of Air. — It has been found that cotton-wool 

 arrests in a trustworthy manner all organisms conveyed in 

 air which passes through it, so long as the wool is 

 moderately dry. Hansen has also found that the Pasteur - 

 Chamberland tube, which when wet will not permit the 

 passage of air, allows it to pass, and frees it from all 

 organisms, when dry. A sufficient number of bends of 

 narrow tube, whether wet or dry, are found also to reliably 

 sterilise air which does not pass through them at too great 

 a velocity. For bacteriological purposes cotton-wool is 

 ordinarily employed for filtration of air. Glass-wool, 

 powdered glass, sand, asbestos, sugar, and a number of 

 other substances, have been employed from time to time 

 to render air free from micro-organisms. The air sup- 

 plied to the Houses of Parliament is filtered through 

 cotton-wool. 



Examination of Air, — A large number of methods have 

 been described from time to time for the bacteriological 

 examination of air. Some observers simply expose plates 

 covered with nutrient medium to the air for a given time, 

 and then count and examine the various organisms as soon 

 as they have grown sufficiently well. Other investigators 

 have used various filtering materials, sugar, sand, etc. A 

 certain volume of the air to be examined is aspirated 

 through a tube containing one of these materials, which 

 is afterwards treated with sterile water; this is then examined 

 by the methods as given under ' The Examination of 



