346 Journal of Agricultural Research voi. iv, no. 4 



was the one which was tested in the present work. Weinzirl and Thomas 

 (31) as members of this subcommittee made a series of comparative 

 tests, using both the new standard method and Rettger's method, 

 securing results shghtly favoring the new standard method. 



A number of men, other than those mentioned in this review, have 

 contributed to the development of satisfactory methods of bacterial 

 air analysis, among whom are Sedgwick and Tucker (24), William (32), 

 Ficker (5), and others. 



The method which has been most used in determining bacterial 

 precipitation is the plate exposure method of Koch (15). He allowed 

 a layer of gelatin to solidify on the surface of a plate in the bottom 

 of a cylinder. The analysis consisted in exposing this layer of gelatin 

 to the air for a known period of time and then allowing the germs to 

 develop on the surface of the medium. Most investigators using the 

 plate exposure method have dispensed with the cylinder, which was 

 designed to prevent side currents of air from affecting the results, and 

 have used Petri plates alone. Koning (16) used a similar principle in 

 some of his analyses. He determined the precipitation per unit area 

 by exposing a known volume of a sterile liquid to the air and analyzing 

 the liquid afterwards. 



Koch's method (15) has also been modified so as to relate it to a 

 definite volume of air. Hesse (13) drew air slovvly through a long glass 

 tube lined on its inner surface with a layer of gelatin, upon which the 

 bacteria and molds were deposited and allowed to develop. 



Winslow (33) modified Hesse's method by substituting two bottles 

 with a layer of gelatin on the bottom of each for the long roll tube. 

 After drawing a liter of air into the system, the bacteria were allowed 

 to settle on the gelatin surface. Weinzirl and Fos (30) agree with 

 Winslow in regarding the method as unsuited for field work. 



PRESENT STUDIES OF METHODS 



TECHNIQUE 



The material to be analyzed was plated in duplicate — or in triplicate in 

 some cases — -within one hour after sampling. The medium used was an 

 agar made according to the formula now recommended by the Commit- 

 tee on Standard Methods for Bacterial Milk Analysis (i), except that 

 the acidity was usually between 1.2 and 1.5 per cent normal acid to 

 phenolphthalein. The plates were incubated in a constant-temperature 

 incubator for five days at 18° C. (later 21° C.) and then for two days at 

 37° C. Except in a few instances, check plates were made which were 

 designed to test out the sterility of the filter tubes, water blanks, and 

 Petri plates. In a few cases where the check plates contained more than 

 the occasional colonies which appear as a result of accidental contami- 

 nations, the entire results of the tests were discarded. 



