138 EXAMINATION OF AIR, WATER, AND SOIL. 



From 0.1-0.2-0.3 c.c. of this solution are added to 10 c.c. 

 of bouillon in each of three test-tubes. Add from 1 to 3 

 c.c. of the water to be examined to each tube, and place in 

 the incubator. The only bacteria which will develop in this 

 medium are the typhoid and colon bacilli. These are then 

 plated and separated (see Bacillus typhosus). 



Abbott suggests the use of chemical coagulants, like alum 

 or iron, which, by precipitating as hydroxides, drag down the 

 bacteria. The precipitate is then examined. Or large quan- 

 tities of water are passed through a Pasteur filter, and the 

 accumulations on the filter brushed off and examined. The 

 water may contain only a few typhoid bacilli, and it may be 

 necessary to make numerous examinations before their pres- 

 ence is detected. 



Examination of the Soil. 



Soil to the depth of four feet always contains bacteria, 

 especially when it contains much organic matter. The upper 

 strata of virgin soil contain the greatest number. From the 

 surface down there is a gradual falling off in the number, 

 until at the depth of four feet they disappear entirely. In 

 cultivated soil and in soil in which fertilizers have been used, 

 bacteria are present in great numbers. In inhabited locali- 

 ties the upper strata contain many varieties. Sandy soil con- 

 tains fewer bacteria than a clay soil. Most of the organisms 

 found in soil are non-pathogenic. The pathogenic bacteria 

 are of the anaerobic variety, such as the bacillus of malignant 

 oedema and the tetanus bacillus. The nitrifying bacteria 

 contained in soil are of value. They decompose the organic 

 matter and convert it into suitable food for the higher plants. 

 That, as stated before, is the reason the farmer turns under 

 several crops of clover before sowing his grain. 



Ravenel, in an exhaustive article on this subject, comes to 

 the following conclusions : 



1. Made soil, as commonly found, is rich in organic matter 

 and excessively damp through poor drainage. 



2. Made soil furnishes conditions more suited to the multi- 

 plication of bacteria than virgin soils, unless the latter are 

 contaminated by sewage or offal. 



