' Length of Time to Incubate Petri Plates. 109 
SUMMARY. 
Counts made after ten days’ incubation at 20° C. of petri plates, 
made from bacterial dilutions of soil, give reliable results as to the 
bacterial content of the soil, providing the number of colonies present 
per plate is small enough for all organisms to develop into colonies. 
The rapidity with which bacteria develop into colonies has been 
shown to vary with the soil, and to be influenced by soil temperature, 
moisture and aeration. 
Much of the lack of confidence in results obtained by the plate method 
is due to having too many colonies present per plate’ and not allowing 
sufficient time of incubation of the petri plates. 
