PROPERTIES OF WATER AND SOURCES OF SUPPLY. 3 



signs of pollution. These in the country usually 

 consist of other houses along its banks which may 

 discharge sewage, or farmyard refuse into it in a 

 crude or only partially purified state. In other 

 places manufactories may exist with trades' waste 

 flowing into the river. There may be boat traffic 

 on the river from which refuse will be sure to be 

 pitched into it ; surface and road drains, and ditches 

 may fall into it, delivering polluted waters. Even 

 sewage works may exist along its banks; in fact a 

 hundred and one things may be present to render 

 its use for drinking purposes a grave risk; when 

 chemically examined the elements composing a water 

 are revealed. Metals, for instance lead and copper, 

 in and above certain quantities render the supply 

 unfit. These are the inorganic substances and are 

 easy of determination, but when we pass on to the 

 organic matter the problem assumes a much more 

 complex and uncertain problem to deal with, and in 

 fact is far the most important question. Good water 

 always contains oxygen solution, and its absence 

 from water denotes pollution ; to oxygen and carbonic 

 acid the flavour of water is chiefly due, since distilled 

 water is insipid and undesirable for drinking pur- 

 poses. Ammonia and nitrogen, except as traces, are 

 to be regarded with suspicion, as they generally 

 denote some sort of organic pollution or other, as 

 also do in a somewhat lesser degree sulphur l and 

 chlorine. Mineral impregnations gained by the water 

 during its passage along the natural strata^ are not 

 usually detrimental to a domestic supply, although 

 for manufacturing purposes they are often very in- 



1 This does not apply to sulphurous medicinal springs. 



