430 APPLIED BACTERIOLOGY 



quantity of dust exists will be found the greatest number qf 

 bacteria ; therefore 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 mountains, 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 bacteria. 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 

 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 



