306 MICROBIOLOGY OF AIR 



accurate results still may be obtained by drawing measured vol- 

 umes of 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 suffi- 

 cient contact of air and water to remove all dust particles. A propor- 

 tionate 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 precentage 

 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 cur- 

 rents and the nature of the surface with which these currents will come 

 into contact, 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 sun- 

 light destroys many. The nature of the soil and the vegetation cover- 

 ing it has a marked influence. The following figures from various 



