10 VETEEINAEY HYGIENE 



depth, will considerably assist in purifying the water by 

 acting as a natural filter. 



It is evident that in deep porous soils, where rapid oxida- 

 tion and destruction of organic matter occurs, we may look 

 for water of a pure tj^pe, and the more we depart from 

 these conditions the more impure will the water be. Agam, 

 the presence of the salts of metals in great abundance in 

 the formation will affect the purity of a water, for lime, 

 magnesia, soda, potash, iron, and alumina, are very readily 

 acted upon by the water passing through the soil, and 

 rapidly taken up in solution. 



In gravel formations it is usual to find a pure water, 

 particularly away from towns. Where clay exists the water 

 is usually impure ; in alluvial formations it is generally 

 bad ; in limestone and chalk districts the water is often 

 organically pure, but contains a large amount of mineral 

 substances, as lime and magnesia, rendering it exceedingly 

 hard. Surface waters, especially from cultivated lands, 

 and marsh waters, are generally very impure from the 

 amount of organic matter they contain. 



During the passage of water through rivers or canals, the 

 principal impurities which obtain an entrance are sewage 

 and manufacturing refuse ; the latter is derived from bone- 

 boilers, bleach-works, tanneries, etc. The importance of 

 this class of impurity is considerable. 



The impurities found in wells are derived from surface- 

 washings, and soakage through soil impregnated with organic 

 matter ; from leaking pipes or cesspits containing animal 

 excreta, and the leakage finding its way into the soil to be 

 carried to the nearest well. Shallow wells near buildings 

 or stables, or close to manure-pits, are simply receptacles 

 for filth. 



Hardness of Water. — As previously mentioned, water as 

 it falls from the clouds possesses no mineral matter in 

 solution, excepting in the vicinity of the sea. Eain-water 

 readily combines with soap to form a lather, and from the 

 peculiar feel it imparts to the fingers is known as ' soft.' 



In its passage through the earth, salts of lime and mag- 



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