EXAMINATION OF SOIL. 447 



verized to dust, these and many others return to the atmo- 

 sphere as a result of the action of wind or of other agencies. 

 The bacteria brought down from the air do not penetrate 

 the deeper layers of the soil. They are retained in the 

 surface layers inasmuch as the earth is a good mechanical 

 filter. For this reason, the number of bacteria in the earth 

 diminishes with the depth. At a depth of about 6 feet the 

 number of bacteria has greatly decreased. Sometimes the 

 soil will be sterile; at other times, only a few hundred bac- 

 teria will be present. At a depth of 9 to 12 feet the soil is 

 practically sterile. Not only is it impossible for the bacteria 

 to penetrate from the surface downwards, for any great 

 distance, but it is also impossible for many organisms to 

 survive for any length of time under such conditions. This 

 fact was unconsciously recognized by man, ages ago. Thus, 

 in times of epidemics, as that of the Black Plague in 

 London, in 1665, bodies were ordered to be buried at a depth 

 of not less than 6 feet. 



Burial experiments made with cadavers of hogs and 

 other animals, into which large quantities of pure cultures 

 were injected or in which diseased tissue was placed, have 

 shown that at a depth of 3 to 4^ feet many bacteria are 

 destroyed within a month. This was the case with pure 

 cultures of the typhoid, cholera and Friedliinder germs. 

 In a typhoid spleen the Eberth bacillus was found alive on 

 the 96th day. The tubercle bacillus was alive and virulent 

 on the 95th day but was dead in 123 days. The tetanus 

 spores died out between the 8th and 12th month, and 

 anthrax spores remained virulent at the end of 11 months. 

 The hog erysipelas bacillus was alive after 8 months. The 

 bacillus of green pus and the Micrococcus tetragenus died 

 between the 1st and 4th month. In only two instances 

 could the pathogenic germ be detected outside of the 

 cadaver. The disease-producing organisms, present in a 

 cadaver, are destroyed in time when buried in the soil. 



