July 15, 1915 



Methods of Bacterial Analyses of Air 



355 



COMPARISON OF THE EFFICIENCY OP THE MODIFIED STANDARD AND THE RETTGER 



AEROSCOPE 



Since the previous work with the Rettger aeroscope had not given 

 conclusive results, another series of tests was carried out. As before, 

 two large aspirator bottles were used and 8 liters of air were drawn 

 through each filter at the rate of % liter per minute. The aeroscopes 

 were set up in tandem and all joints carefully sealed with paraffin. In 

 this case the suspensions were plated upon two different media, but 

 only those results secured by the use of plain nutrient agar will be dis- 

 cussed at this place, the other results being referred to later. (See p, 

 363.) Since the results given as plate counts from the two aeroscopes are 

 not comparable because the Rettger suspensions consisted of 5 c. c. and 

 the sand suspensions of 10 c. c. of physiological salt solution, they have 

 also been computed per liter of air. The latter figures are given in the 

 fourth and seventh columns of Table IV. In this series of comparisons 

 the Rettger suspensions were plated in duplicate and the sand suspensions 

 in triplicate. 



Table IV. — Co^nparative counts obtained with modified standard and Rettger aeroscopes 



Test No. 



3 



4 



S 



6 



7 



8 



9 



10 



II 



12 



13 



14 



15 



Average 



16 



17 



18 



19 



20 , 



21 



22 



23 



24 



Average 



Modified standard aeroscope. 



Plate count 

 of first 

 tube. 



171 

 88 



122 



125 



73 

 117 



91 



73 



46 



48 



45 



79 



"5 



132 



130 



97 



51 

 47 

 73 

 79 

 68 

 178 

 81 

 72 

 47 



Plate count 



of second 



tube. 



9-3 

 2-3 

 4. o 



2.6 



1-3 

 2. o 

 o 



2-3 



1.6 

 1-3 



1.6 

 1.6 

 2.6 



2-3 



1.6 



2-3 



1.6 

 .6 



I. o 



I. o 



Count per 

 liter. 



214 

 IIO 



153 

 156 



91 



146 



114 

 91 



58 



60 



56 



99 



144 



165 

 163 



64 

 59 

 91 

 99 

 85 

 223 



lOI 



90 

 59 



97 



Rettger aeroscope. 



Plate count 

 of first 

 tube. 



35 

 47 

 61 



59 

 69 

 62 

 27 



43 

 32 

 22 

 10 



27 

 27 

 63 



IS 



40 



69 

 131 



»7 

 74 

 31 

 44 

 15 

 13 



Plate count 



of second 



tube. 



1-5 



2-5 



5 

 o 

 o 



5 

 o 

 o 



I. o 



1-5 



5-8 



5-5 

 •5 



2. o 



•5 



•5 



2. o 



30 



1.8 



Count per 

 liter. 



29 

 38 

 37 

 43 

 39 

 17 

 27 

 20 



14 

 6 



17 



17 



39 



9 



25 



43 

 82 



54 

 46 



19 



28 



9 



36 



