THE EXAMINATION OF AIR 329 



THE BACTERIOLOGICAL EXAMINATION OF AIR. 



The air does not normally contain any characteristic 

 bacterial flora, as the organic matter required for their growth 

 is not found in the air to any considerable extent. Their 

 presence is due to the fact that they are blown about with the 

 dust by air currents and winds from surfaces where they exist 

 in a dried-up condition. Bacteria do not themselves unaided 

 rise into the air ; when air-currents are absent they always 

 sink under the influence of gravity to the ground, where 

 they always find better conditions for their growth and 

 development. Wherever the greatest quantity of dust 

 exists will be found the greatest number of bacteria ; there- 

 fore the air in the summer always contains a larger number 

 of bacteria than it does in the winter. A larger number 

 of bacteria is always found in the air of towns than in the 

 country. At high elevations, at the tops of hills or moun- 

 tains, the air is almost free from micro-organisms ; whereas 

 on plains and low-lying places bacteria are almost always 

 found in greater or lesser numbers. Again, the atmosphere 

 of the open sea far out from land is almost free from bac- 

 teria. By far the greatest number of micro-organisms are 

 found in the air of rooms and crowded public places, when 

 they are whirled up from the ground with the dust. 



The micro-organisms generally found in the air are the 

 spores of moulds, yeasts, and bacteria, particularly the 

 spores. Pathogenic organisms are sometimes found, par- 

 ticularly where a number of patients are collected together 

 for treatment. The tubercle bacillus has frequently been 

 found in the air of hospital wards containing phthisical 

 patients, whose sputa have been allowed to dry. 



A great number of researches have been made by' various 

 investigators as to the number of organisms found in the 

 air in various parts of the world, but the results, although 



