=Contamination of the Soil.=--The natural capacity of the soil to decompose and reduce organic matter is sometimes taxed to its utmost by the introduction into the soil of extraneous matters in quantities which the soil is unable to oxidize in a given period. This is called contamination or pollution of soil, and is due: (1) to surface pollution by refuse, garbage, animal and human excreta; (2) to interment of dead bodies of beasts and men; (3) to the introduction of foreign deleterious gases, etc.[11] _Pollution by Surface Refuse and Sewage._--This occurs where a large number of people congregate, as in cities, towns, etc., and very seriously contaminates the ground by the surcharge of the surface soil with sewage matter, saturating the ground with it, polluting the ground water from which the drinking water is derived, and increasing the putrefactive changes taking place in the soil. Here the pathogenic bacteria abound, and, by multiplying, exert a very marked influence upon the health by the possible spread of infectious diseases. Sewage pollution of the soils and of the source of water supply is a matter of grave importance, and is one of the chief factors of high mortality in cities and towns. _Interment of Bodies._--The second cause of soil contamination is also of great importance. Owing to the intense physicochemical and organic changes taking place within the soil, all dead animal matter interred therein is easily disposed of in a certain time, being reduced to the primary constituents, viz., ammonia, nitrous acid, carbonic acid, sulphureted and carbureted hydrogen, etc. But whenever the number of interred bodies is too great, and the products of decomposition are allowed to accumulate to a very great degree, until the capacity of the soil to absorb and oxidize them is overtaxed, the soil, and the air and water therein, are polluted by the noxious poisons produced by the processes of decomposition. _Introduction of Various Foreign Materials and Gases._--In cities and towns various pipes are laid in the ground for conducting certain substances, as illuminating gas, fuel, coal gas, etc.; the pipes at times are defective, allowing leakage therefrom, and permitting the saturation of the soil with poisonous gases which are frequently drawn up by the various currents of ground air into the open air and adjacent dwellings. =Influence of the Soil on Health.=--The intimate relations existing between the soil upon which we live and our health, and the marked influence of the soil on the life and well-being of man, have been recognized from time immemorial. The influence of the soil upon health is due to: (1) the physical and chemical character of the soil; (2) the ground-water level and degree of dampness; (3) the organic impurities and contamination of the soil.