344 Journal of Agricultural Research voi. iv.no. 4 



is examined for bacteria by the usual methods employed for the bac- 

 terial analysis of liquids, which include direct microscopical examina- 

 tion, the plating of aliquot portions in a solid nutritive medium, and 

 methods of fractional cultivation in liquid media. When the liquid 

 filter is nutritive, it may be of such a nature as to harden on cooling. In 

 this case the bacteria are allowed to develop in this medium. 



(b) Drawing the air through porous solids, which act as filters. 

 These filters may be either soluble or insoluble. According to its nature, 

 the filter is either dissolved or washed in a sterile liquid and the latter 

 analyzed for bacteria as indicated under i . 



(c) The bacteria in the air may be allowed to settle upon the surface 

 of a solid nutrient medium and to develop into colonies wherever they 

 fall. In using this method, a measured quantity of air may be used or 

 the results may be expressed as the number of bacteria falling on a given 

 area in a given period of time. The method may also be varied by 

 using a sterile liquid instead of a solid culture medium, the liquid being 

 afterwards analyzed to determine the number of bacteria present. 



It is unnecessary to review the literature of air bacteriology before 

 the time of Petri (20, 21) and Miquel (17, 18) in detail, since these authors 

 have given us very full and complete accounts of the early work. As is 

 well known, this early work had to do very largely with the efiforts of 

 Pasteur to convince the adherents of the theory of spontaneous genera- 

 tion that their supposed cases of life without previous life were really 

 due to forms of life already existing in the air. Pasteur used primitive 

 forms of our present methods of bacterial air analysis in several instances. 

 Among these, he tried both soluble and insoluble granular solids as filter- 

 ing agents, such as guncotton and asbestos. In his later work he used the 

 vacuum-flask method of analysis for rough quantitative work. This 

 method has been further developed by Hansen (7). 



Miquel (17, 18, 19) also used several methods of analysis. Among 

 these he tried an "aeroscope" containing a glass plate coated with a 

 sticky mixture of glycerin and glucose which caught the bacteria, mold 

 spores, dust, and the like. Since this technique did not allow a distinc- 

 tion between living and dead organisms, he devised a method of filter- 

 ing air through water or other liquids and testing the liquids used for 

 bacteria by means of the method of fractional cultivation. When taking 

 samples at a distance from the laboratory, he used solid porous sub- 

 stances, such as glass wool, asbestos, and powdered sodium sulphate, 

 as filtering agents. 



Frankland (6) used finely powdered sugar, glass wool, and a mixture of 

 glass wool and glass powder as filtering agents. Hueppe (14) and Straus 

 and Wurtz (28) filtered air through liquefied nutrient gelatin and poured 

 plates from this suspension. Von Sehlen (25) used melted agar in a similar 

 way. 



