68 ELEMENTARY AGRICULTURAL CHEMISTRY 



In thickly populated and manufacturing districts the rivers 

 are contaminated with the sewage and trade effluent of the 

 towns and villages, and thus often become foul and evil- 

 smelling. This is to be deplored both on account of the 

 annoyance and injury to health which they cause, and also 

 because of the serious loss to the community of the valuable 



combined nitrogen and other manurial constituents contained 

 in the sewage. 



The amount of suspended matter in river water varies 

 enormously, depending upon the rainfall, the character of the 

 surrounding soil, and other circumstances. Soft waters or 

 those containing carbonate of soda are often muddy, while 

 hard waters tend to deposit their suspended clay and become 

 clear. The very muddy nature of many South African 

 streams is believed by the writer to be due to their containing 

 sodium carbonate. In some cases, the quantity of suspended 

 matter is very great, and the dense, muddy river-water, if it 

 overflows the banks, deposits upon the soil a layer of finely- 

 divided particles of materials brought down from higher up 

 the valley. The sediment is often rich in plant food and 

 forms an important fertiliser. In some few places in England 

 — e.g.f on the Humber and Trent — land is systematically 



