BACTERIOLOGY OF THE SOIL 267 



air coming quietly from the lungs is pure and sterile. Even in active 

 disease processes of the throat this is true. In case the breath 

 comes violently, as in speaking, coughing, and sneezing, the reverse 

 is the case. In general it may be put down as an axiom that disease 

 germs cannot rise from a fluid, such as sewage. If they could it 

 would mean that they are lighter than air, which is not the case. 

 Sewer gas, as a rule, is a bearer of some pathogenic bacteria chiefly 

 cocci but in reality it is purer than generally supposed. The 

 spread of organisms from sewage only extends 3-6 metres into the 

 atmosphere and then only by the bursting of bubbles in the presence 

 of gas under pressure. This is of course in the absence of extra- 

 neous air currents as far as possible. 



FIG. 85. Sedgwick-Tucker aerobioscope. (Williams.) 



Bacteriology of the Soil. 



At least two forms of pathogenic bacteria are habitually found in 

 the soil. The tetanus bacillus, it is well known, exists in garden 

 earth, manure, and top soil generally. Dirt getting into wounds 

 is the most frequent cause of tetanus. Drinking water laden with 

 soil has been known to have in it tetanus bacilli, and if used in 

 an unsterilized condition in wounds or when a comparatively 

 feeble antiseptic, such as creolin, has been added, it may cause 

 tetanus. 



The bacillus of malignant oedema has also been isolated from 

 soil. Streptococci and colon bacilli, too, have been found in garden 

 soil. Typhoid bacilli may contaminate soil, but do not multiply 

 in it. In sandy soil 100,000 bacteria per gram have been found, 

 in garden soil 1,500,000 bacteria per gram, and in sewage polluted 

 soil 115,000,000 bacteria per gram have been determined. The 



