392 VETEEINAEY HYGIENE 



at any distance below the superficial layers of the soil, 

 which explains the sterile nature of the subsoil. 



Organisms which produce Denitrif action are also known. 

 They result in the formation of gaseous nitrogen from 

 nitrates, ammonia, or organic compounds of nitrogen, and 

 this nitrogen is lost to the plant. 



Other organisms in the soil useful in agriculture, are 

 those previously alluded to as forming nodules on the roots 

 of certain leguminous plants, which enables them to utilize 

 the free nitrogen of the air. 



In the examination of soils for micro-organisms, it has 

 been found that few if any exist at a depth of 12 to 15 feet 

 from the ground, while from the surface of the ground to a 

 foot below, they are said to increase in number. 



Of Pathogenic Organisms there are two which belong to 

 the soil proper, viz., that of tetanus and malignant oedema. 

 Of organisms which do not belong to the soil, yet readily 

 flourish in, or at least are not killed by lying in the earth, 

 are anthrax, quarter-evil, and braxy in animals, enteric, 

 plague, and cholera in man. 



The various forms of fever known as malarial are in 

 the light of recent researches not soil diseases, but the 

 result of infection by insects the natural habitat of which 

 is the soil. The same perhaps may be said of African 

 Horse Sickness. It is probable that neither rain nor soil 

 play any other part in the production of these diseases 

 than in forming a suitable medium for the support of some 

 blood-sucking insect that is the means of inoculation. 



Soil diseases play a less important part in pathology 

 than was at one time believed ; miasma is a thing of the 

 past, and there are no poisonous malarial vapours ; there 

 are soils which we know to be dangerous, not on account 

 of emanations from them, but because they form the home 

 of insect pests capable of producing disease, and in this 

 group may be included the malarial and yellow fever 

 mosquito, and the blood- sucking fly which produces 'Tsetse' 

 disease, and allied diseases of the human subject. 



Tetanus. — It is clearly proved that the organism which 



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