1 88 MICROBIOLOGY OF AIR. 



air in small bubbles through liquid gelatin. Practically all of the particles 

 will be retained and the number of colonies which develop may be counted. 

 This method is sometimes modified by drawing the air through a definite 

 volume of water, care being taken to insure sufficient contact of air and 

 water to remove all dust particles. A proportionate part of the water 

 is then plated and the number of organisms estimated. Air is sometimes 

 drawn through a filter made of sugar, sodium sulphate, or sodium chloride, 

 and this material then dissolved in water and plated. Sand, asbestos, 

 glass, etc., are sometimes used as air filters, then thoroughly washed, 

 and the wash water plated. 



Relative quantitative examination of the air is of more historical than 

 practical importance. It has been useful in the development of the germ 

 theories of fermentation and of disease and in overthrowing the theory 

 of spontaneous generation. There is so little ordinarily to be learned 

 by a study of the air flora that a comparison of plates exposed directly 

 will usually suffice. Where more accurate results are desired, one must 

 resort to one of the filtration methods discussed above. 



Qualitative determinations of the species of air organisms are not 

 often made. When necessary it may be done by simple examination of 

 the colonies developed on the plates or by animal inoculations made from 

 the water used in the air filter. It is sometimes necessary to vary the 

 composition of the medium used in order to favor the development of 

 certain types of organisms desired, for example, a higher percentage of 

 molds will be found and a more luxuriant development will take place 

 if wort agar or acid gelatin is used. 



NUMBER OF BACTERIA IN THE AIR. The number of bacteria in the 

 air is determined by a variety of conditions. The velocity of air currents 

 and the nature of the surface with which these currents will come in con- 

 tact, are probably most important. Bacteria are usually more abundant 

 on quiet days in the air of buildings than out of doors, but on windy 

 days the reverse is true. They are often more abundant in cities than in 

 the country. Fewer are found at high altitudes and over large bodies of 

 water. Frankland found that there are fewer in winter than in summer 

 They are washed from the air during rains. Bright sunlight destroys 

 many. The nature of the soil and the vegetation covering it has a marked 

 influence. The following figures from various authors are appended to 

 serve as an index to what may be expected in the air content of bacteria. 



