728 A MANUAL OF BACTERIOLOGY 



The Bacteriology of Air 



Just as in water, the bacteria in the air vary consider- 

 ably at different times and seasons, under different 

 conditions, and in various localities. The species met 

 with are mostly saprophytes, consisting largely of chromo- 

 genic forms. A number of moulds occur (as spores), and, 

 in fact, ordinarily are in large excess, together with yeasts 

 and torulae. 



It is not easy for micro-organisms to become diffused 

 through the atmosphere ; they are incapable of a volun- 

 tary rising, and cannot be torn from a fluid or moist solid 

 medium by a strong current of air. The medium on 

 which they are growing must dry up completely and 

 crumble into fine dust before they can be distributed 

 through the agency of air-currents (but see p. 426). 



The number of organisms in the air varies with the 

 season, with rain, with altitude, with movement, etc. At 

 Montsouris, Miquel found in 1 cubic metre of air 49 

 organisms in winter, 85 in spring, 105 in summer, and 

 142 in autumn. After heavy rain the air is largely freed 

 from organisms. Frankland found at Norwich Cathedral 

 at an altitude of 300 feet 7 organisms in two gallons, while 

 on the ground 18 were found ; at the Golden Gallery at 

 St. Paul's two gallons of air contained 11 organisms ; in 

 St. Paul's churchyard the number was 70. On high 

 mountains organisms are nearly absent from the air, and 

 the same is the case at sea at a distance from land exceed- 

 ing about 100 miles. Organisms are much fewer in the air 

 of the country than in that of towns. At the entrance- 

 hall, Natural History Museum, South Kensington, Frank- 

 land found in the morning 30 organisms ; in the after- 

 noon, when many visitors were present, the number had 

 risen to 292, showing the influence of movement. By 

 keeping a volume of air absolutely still, enclosed in a box 



