Colonies for a Satisfactory Soil Plate. 101 
TABLE IV. 
Colonies on 1-40,000 and 1-400,000 Bacterial Dilutions. 
(Colonies on 1-40,000 Dilutions number under 30.) 
DILUTIONS 
1-40,000 1-400,000 
29.0 Bose) ale’ 
26.7 sone eae 
26.0 aire aay 
25.3 SAEs ihe 
24.7 AEA idlgh 
23.0 SHO Rae 
22.3 ep ta sr 
20.7 3.3% AP 
20.7 207m 
20.3 3.0* 
20.0 3.0* 
19.0 3.0* 
17.7 ia Ose anes 
17.3 Les He 
16.0 3.0* 
16.0 DOR hate 
13.0 Malle ae 
Bei 2.0* 
11.3 3.3* 
11.0 OR ane 
10.0 2,0* 
Average of all 19. 
Average of * 19. 
Average of * ** 21. 
oor 
mmr 
hoe 
All figures are averages of triplicate plates. 
*Counts for 1-400,000 dilution are within 1.5 colonies of 0.1 of number on 1-40,000 dilution. 
* **Counts for 1-400,000 dilution are within 0.7 colonies of 0.1 number on 1 40,000 dilution. 
SUMMARY. 
1. These and other tests (of which these are representative) have 
shown that thirty is near the optimum number of colonies for a petri 
plate 100 mm. in diameter. Plate II. 
2. The averages of a sufficient number of plates carrying between 
10 and 100 colonies are satisfactory for computing bacterial numbers. 
