BACTERIOLOGIC EXAMINATION OF SOIL. 175 



amount of this soil is mixed thoroughly and the mixture 

 solidified upon the walls of an Esmarch tube. The col- 

 onies are counted with the aid of a lens. Fliigge found 

 in virgin earth about 100,000 colonies in a cubic centi- 

 meter. 



Samples of earth, like samples of water, should be 

 examined as soon as possible after being secured, for, 

 as Giinther points out, the number of bacteria changes 

 because of the unusual environment, exposure to increased 

 amounts of oxygen, etc. 



The most important bacteria of the soil are those of 

 tetanus and malignant edema, in addition to which, how- 

 ever, there are a great variety which are pathogenic for 

 rabbits, guinea-pigs, and mice. 



In the "Bacteriological Examination of the Soil of 

 Philadelphia,'' &avenel l came to the conclusion that 



1. Made soils, as commonly found, are rich in organic 

 matter and excessively damp through poor drainage. 



2. They furnish conditions more suited to the multi- 

 plication of bacteria than do virgin soils, unless the latter 

 are contaminated by sewage or offal. 



3. Made soils contain large numbers of bacteria per 

 gram of many different species, the deeper layers being 

 as rich in the number and variety of organisms as the 

 upper ones. After some years the number in the deeper 

 layers probably becomes proportionally less. Made soils 

 are more likely than others to contain pathogenic bacteria. 



In 71 cultures that were isolated and carefully studied 

 by Ravenel, there were two cocci, one sarcina, and five 

 cladothrices; all the others were bacilli. 



1 Memoirs of the National Academy of Sciences, First Memoir, 1896. 



