SOIL AND SEWAGE 69 



numerical estimations are only relatively accurate, and in any 

 case the same method should be used throughout for each 

 investigation : 



Accurately weigh a small sterile glass-stoppered bottle con- 

 taining 100 c.c. of sterile water. Quickly weigh in I gramme 

 of the soil (previously well mixed together) into the bottle, 

 using a sterile spatula to add the soil. With a little practice 

 I gramme can be quickly and sufficiently accurately added. Mix 

 very thoroughly by repeated shaking, if necessary breaking up 

 the soil by a pointed sterile glass rod. Call this solution 

 Dilution A. Allow the soil particles to settle, then add I c.c. 

 or more, according to the suspected contamination of the soil, to 

 a sterile flask containing 99 c.c. of sterile water. Mix thoroughly 

 and label Dilution B. Varying quantities of Dilutions A and B 

 are used for the examination. 



To estimate the number of aerobic organisms make gelatine 

 plates from these dilutions. Thus, o ! 2, 0*5, I *o c.c. of Dilution B 

 are convenient amounts to add to the gelatine tubes. For the 

 number of organisms developing at 37 C. use, in the same way, 

 agar plates. 



To estimate the number of spores, present as such, add 

 varying amounts of the dilutions to gelatine tubes. Heat to 

 So C. for ten minutes, then plate, incubate, and count in the 

 ordinary way. 



For B. colt enumerations various fractions of the dilutions are 

 added to tubes of lactose bile salt media. These are incubated 

 at 37 C., and those in which acid and gas are produced are used 

 to inoculate solid media, and the organism isolated exactly in 

 the same way as for the isolation of this bacillus from water. 



Streptococci and spores of B. enteritidis sporogenes are 

 examined for by methods identical with those used for water. 



SEWAGE 



The bacteriological examination of sewage is not, at the 

 present day, a procedure widely practised, although for special 

 purposes it is valuable. It is the only way the potential harm- 

 fulness of an effluent can be measured. Chemical standards 



