THE MICROBES OF THE SOIL 279 



To ascertain the number of microbes in a given 

 sample of soil, the colonies produced on the gela- 

 tine-plates are accurately counted. This is per- 

 formed by the apparatus represented in Fig. 55, 

 which will be described in the next chapter. To 

 ascertain the characteristics of the microbes, further 

 cultivations must be made ; the microbes must be 

 transplanted into various media, and exposed to 

 different temperatures ; and they must be inocu- 

 lated into different kinds of animals. The labour 

 of separating each species and studying it in detail 



FIG. 55. WOLFFHUGEL'S APPARATUS. 

 (For estimating the number of Colonies in a Plate-cultivation.) 



would be extremely great ; hence microbian soil 

 examinations have largely been confined to the 

 determination of the number of microbes, and not 

 to the peculiar species. In determining the signifi- 

 cance of such examinations, we must bear in mind 

 the following facts: (1) The number of microbes 

 present in a soil does not necessarily indicate the 

 number of pathogenic forms. (2) Small samples 

 of soil may show marked variations in the number 

 of microbes ; this is owing to minor local influences. 

 (3) Surface soil always contains a larger number of 



