364 



Journal of Agricultural Research 



Vol. IV. No. 4 



for catching the precipitated bacteria. Other liquids might be used in 

 the same way. 



The satisfactory nature of this technique was demonstrated by the 

 results secured in 34 comparative tests between this technique and the 

 plate-exposure technique. The results are given in detail in Table IX. 



Table IX. — Comparison between the exposed-plate and the exposed-pail methods of meas- 

 uring bacterial precipitation 



a These analyses were omitted from the averages because of contaminated check plates. 



In tests 1-12 and 23-34 the Petri plates were exposed for five minutes, 

 but this length of time was abandoned because of the overcrowding of 

 the plates. In the other cases, the numbers given were secured by 

 adding the results secured by exposing four different plates consecu- 

 tively for I X minutes each. In this way the same time of exposure 

 was secured without overcrowding the plates. 



The average result secured in these tests was 8 colonies per sq. cm., 

 secured from a 5 -minute exposure where this was determined by exposed 

 Petri plates. Where the pail method of exposure was used, the similar 

 figures were 78 colonies per sq. cm. In the most favorable cases the 

 plate-exposure method gave only one half of the numbers secured in 

 the other way, while in the least favorable cases the plate-exposure 

 method gave only i colony to 32 colonies which appeared on the plates 



