350 



Journal of Agricultural Research 



Vol. IV, No. 4 



used as the nutrient medium. The average plate count for the first tube 

 in the 33 tests was 18.8, and 2.6 for the second tube (88 per cent effi- 

 ciency). When the three evident cases of leakage are left out of con- 

 sideration, the averages become 19.8 colonies per plate for the first tube 

 and 1.5 colonies per plate for the second tube (93 per cent efficiency). 

 In spite of the possible leakage and the fact that the results are all so 

 low that the experimental error due to possible contaminations in plat- 

 ing is high, the results show the Rettger aeroscopes to be fairly efficient. 



COMPARATIVE TESTS WITH SAND AND RETTGER AEROSCOPES 



Comparative tests were then made between the sand aeroscopes and 

 the Rettger aeroscopes constructed as described (p. 348), but set up 

 singly instead of in tandem. The openings of the two aeroscopes were 

 so placed that they were less than i inch apart, and 7 liters of air were 

 drawn through each at the same rate by two aspirator bottles of the 

 same construction. Thus, the filters had an opportunity to obtain air of 

 the same bacterial quality. In order to make the agar plates compar- 

 able, 5 c. c. of water were used to wash the sand from the sand filter. 

 One c. c. of each suspension was used in making the plates. The results 

 of these analyses are given in Table I. The averages of the 25 tests are 

 68 colonies per plate for the sand aeroscope and 30 per plate for the 

 Rettger aeroscope. When calculated on the per liter basis, these aver- 

 ages become 48.6 for the sand aeroscope and 21.5 for the Rettger aero- 

 scope. The same number of colonies per plate was obtained from each 

 aeroscope in six tests. There was i excess in favor of the Rettger 

 aeroscope, while 18 excesses were in favor of the sand aeroscope. Six 

 tests were omitted from this table because of bad check plates, but the 

 inclusion of these would not have affected the general results. 



Table I. — Comparative counts obtained with the Rettger and an early form of the 



standard aeroscope 



No. of test. 



3 

 4 

 5 

 6 



7 

 8 



9 

 10 

 II 



12 

 13 

 14 



No. of test. 



15 



16 



17 



18 



19 



20 



21 



22 



23 



24 



25 



Average 



Agar plate 

 counts. 



Rett- 

 ger. 



91 

 37 



53 

 iQ 

 35 

 23 

 IS 

 24 

 26 



Stand- 

 ard. 



216 



2^ 



21 



123 



96 



67 



58 



32 



118 



226 



91 



68 



Counts per 

 liter. 



Rett- 

 ger. 



34-3 

 7.8 

 65. o 

 26. 4 

 36.4 

 13-5 

 25. o 



16. 4 

 10. 7 



17. I 

 18.6 



21-5 



stand- 

 ard. 



154-3 

 22.8 



15-0 

 87.8 

 68.5 

 47.8 

 41.4 

 22.8 



84.3 



161. 4 



65.0 



3.6 



