330 APPLIED BACTERIOLOGY 



of interest, are of little practical importance. The number 

 of organisms present in the air is largely determined by 

 the amount of moisture present, there being a much larger 

 number of bacteria in dry than in moist air. The air of 

 sewers has been shown to be remarkably free from micro- 

 organisms by Carnelley and Petri, and more latterly by 

 Laws and Arthur. All these observers obtained, roughly 

 speaking, half the number of organisms from the sewer- 

 air that they found in the external air. From this fact 

 it can be argued that these organisms were derived from 

 the outside air, the damp walls of the sewer acting like a 

 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 comparatively 

 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, is 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. 



Examination of Air. — A large number of methods have 

 been described from time to time for the bacteriological 



