68 



SOIL AND SEWAGE 



to obtain a representative sample. Also in this way sufficient 

 soil will be obtained for a concurrent chemical examination. 



By means of this borer the exact 

 depth of the soil taken can be 

 ascertained. Owing to its length it 

 cannot be sterilized in the hot-air 

 oven, but it can be conveniently 

 and sufficiently sterilized by pouring 

 in methylated spirit and igniting. 

 After sterilization wrap the lower 

 portion in a sterile cloth and secure 

 with string. This plan is very con- 

 venient when a number of samples 

 have to be taken in one day, and 

 at, perhaps, a long distance from 

 the laboratory, since the borer can 

 be resterilized at once before each 

 sample is taken, it being only 

 necessary to carry a bottle of spirit 

 and a number of sterile cloths in 

 a metal box. The soil is removed 

 by a sterile spatula from the in- 

 terior of the borer to the sterilized 

 tin or other receptacle used for the 

 soil. 



The examination should be com- 

 menced as soon after collection as 

 possible. 



To estimate the total number of 

 bacteria, and for some other steps 

 of the examination, very extensive 

 dilution must be practised. As an 

 example of a convenient method 

 of dilution the following procedure 



Fig. 7. Fraenkel's Borer. is g iven : ther methods of dilution 



will readily suggest themselves. It 



is important to remember that owing to a number of inherent 

 difficulties (such as the difference of coherence of different soils) 



