CHAPTER IV. 

 SOILS. 



Soil is the term used to denote that portion of the 

 earth's crust which by its condition or properties can 

 affect health. It is conveniently spoken of as composed of 

 two layers : (i) an upper or surface soil, and (2) a deeper 

 or subsoil layer. The upper layer contains the products 

 of the decay of animal and vegetable matter, constituting 

 mould or " humus." The subsoil layer is intermediate 

 between the upper layer and the underlying formations or 

 strata. Both layers are originally derived from these 

 deeper layers by " weathering," a geological term which 

 includes all those forces that make for denudation of 

 surface. 



Examination of a Sample. 



Ground Air. The amount of air in soil varies with its 

 porosity, and its state in regard to moisture. Thus a dry 

 porous soil may contain, if loamy, 70 per cent ; if loose 

 sand, 40 to 50 per cent. Such air is collected by aspiration 

 through a tube leading to a perforated bulb, sunk into the 

 soil, in which an opening has been made. The analysis of 

 ground air is made for : CO 2 (increases with depth, most 

 in summer and autumn, least in winter and spring, more in 

 impure porous soils ; varies from 4 to 8 per cent in January, 

 to 8 to 24 per cent in August) ; Moisture : 85 per cent ; 

 Oxygen : 21 to 18 per cent. The amount, in a quantity of 

 soil, may be estimated by filling a burette to the zero mark, 

 with sample of soil, and connecting the nozzle with that 

 of another burette containing 50 c.c. of water, by a piece 

 of rubber tubing. On opening the stopcocks and raising 

 the burette containing the water, the water flows into the 

 other burette, wetting the soil. The process is stopped 

 when the water reaches the zero mark, by closing the 

 stopcocks. The loss of water from the one burette is a 

 measure of the amount absorbed by the soil displacing the 

 contained air. 



