July 15, 191S 



Methods of Bacterial Analyses of Air 



361 



DUPLICATE SAMPLING WITH A REVERSIBLE ASPIRATOR 



These tests differed from those just given in that the ordinary aspi- 

 rator bottles were replaced by a reversible aspirator. A Y-connection 

 v\^as placed in the system so that the current of air could be drawn first 

 through one aeroscope and then through the other. Two liters of air 

 were usually drawn through each aeroscope before diverting the current. 

 In this way either 18, 24, or 30 liters of stable air were drawn through 

 each aeroscope, the larger volume being used at the beginning of the series 

 of analyses and the smaller toward the end, when it was discovered that 

 the plates were overcrowded with colonies. Table VII gives the data 

 gathered in this manner. The average numbers of colonies per plate 

 for the two series of 30 tests were 673 and 643. In this comparison the 

 individual results agree a little better than in the previous series, but 

 this is probably partly due to accident and partly to the influence of 

 overcrowded plates. If nine of the comparisons with the most over- 

 crowded plates are left out of the calculation, the averages become 276 

 for the one set of analyses and 226 for the other. Some individual 

 analyses present wide variations, but the averages show that fairly 

 comparable duplicate results can be secured with this type of aeroscope 

 when used in a series of analyses. 



Table VII. — Comparative counts made with two modified standard aeroscopes placed 



side by side 



Test No. 



3' 

 4- 

 5 

 6. 



7- 

 8. 



9 

 10. 



13- 

 14- 

 IS- 

 16. 



17- 

 18. 

 19. 

 20. 



23- 



24- 



25- 

 26. 



27- 

 28. 

 29. 

 30- 



Average . 



Number of 

 liters of air. 



24 

 24 

 30 

 30 

 24 

 30 

 30 

 30 

 24 

 30 

 24 

 24 



24 

 24 



18 



18 



Plate count. 



Sample. Duplicate. 



233 

 I, 092 



485 

 434 

 341 



204 

 12 



379 



1, 067 



2, 121 

 3,958 

 1.723 



316 



175 



43 



59 



513 



321 



472 



607 



868 



878 



179 



205 



I, 158 



147 



277 



341 

 318 



353 



643 



212 



1,187 



504 



463 



377 



186 



29 



106 



1,281 



2,863 



1,305 



4,445 



329 



190 



118 



28 



561 



102 



198 



1,409 



1,388 



894 

 123 

 119 

 674 

 283 

 146 

 404 

 115 

 154 



673 



Count per liter of air. 



Sample. Duplicate. 



49 

 228 



72 



71 



34 



2 



63 

 222 



354 

 825 



351 

 SS 



49 

 12 

 16 

 107 

 67 



131 



169 



241 



244 



50 



57 



322 



41 



77 



95 



144 



44 

 247 

 84 

 77 

 79 

 31 

 5 



18 

 267 



477 



272 



926 



91 



53 



33 



8 



117 



21 



55 



391 



386 



248 



34 



32> 



187 



79 



41 



112 



32 

 43 



150 



