348 



Method of estiniatmg Bacterial Density 



Unlike the excessively variable sets, the sets with subnormal variance 

 cannot be identified individually in Cutler's data, and we have therefore 

 less evidence upon which to put forward a biological explanation of the 

 phenomenon; certain facts, however, concerning observations made in 

 the course of 1921, suggest that additional precautions in the preparation 

 of the medium, may be effective in ehminating the disturbing cause. 



The additional data were accumulated in the Bacteriological Depart- 

 ment^ in the summer and autumn of 1921 in the course of some work 

 on the relationship of bacterial numbers to nitrate content in the field 

 soil. In each of these experiments a series of some 45 samples of soil 

 were taken from a plot 9 by 15 feet in area and the bacterial numbers 

 in each sample estimated by the plate method using Thornton's agar 

 medium. The first experiment was carried out with the dunged plot 

 in Barnfield. The technique used was similar to that employed in 

 Cutler's work, five parallel platings being made of each sample and the 

 colonies counted after an incubation of seven days at 20° C. 



Of the 33 sets available, three show excessive variance, the remainder 

 are distributed as in Table XVI . 



Table XVI 



X- = 308 P = -381 



It will be seen that these agree well with the Poisson Series, and show 

 no sign of subnormal variation. 



A second experiment was carried out at Kingsthorpe Hall, Nor- 

 thampton. The soil is here of a markedly different type from the heavy 

 Rothamsted soil, being a light ferruginous loam. In this experiment the 

 technique was varied in that the colonies on each plate were counted 

 twice, after seven and twelve days' incubation. It will be sufficient to 

 compare the observed and expected values of the total, >S(x^), for different 

 groups of plates. 



1 The authors wish to acknowledge their indebtedness for the assistance rendered by 

 other Departments at Rothamsted in this work. 



