12 VETEEINARY HYGIENE 



the water is urged as one explanation of protection ; while 

 the presence of humic acid in certain peaty upland waters, 

 is said to have a marked dissolving action on lead. 



Hard water has very little action on lead, for the pipe is 

 soon coated by a carbonate, and all further action ceases. 



Iron pipes are used as mains, to prevent water rusting 

 and corroding them they are coated with a vitreous glaze. 



In the Storage of Water galvanized iron cisterns are largely 

 used and are safe ; iron cisterns discolour the water but do 

 no harm ; zinc vessels may be acted upon ; slate vessels are 

 excellent but liable to leakage at the joints. Wooden 

 vessels for water storage are bad, and though largely used 

 as water-troughs they are Hable to contamination and the 

 growth of organisms. 



The Purification of Water for animals, does not bear such 

 an important position as it does in the case of human 

 beings, owing to the absence of such diseases as typhoid, 

 cholera, and dysentery. But for the highest degree of 

 health good and pure water is essential. 



The objects aimed at in the purification of water on a 

 large scale are freeing it from organic matter, and getting 

 rid of inorganic if it is in excess. The former is obtained 

 hj filtration, and is employed by all Water Companies who 

 obtain their supply from rivers, and other polluted sources. 

 The filter-beds consist of gravel of different degrees of 

 coarseness, on the top of which are layers of sand a foot in 

 thickness, the finest sand being on the top. 



For effective filtration the passage of the water through 

 the filter-bed must be slow ; to ensure this the depth of 

 water on the filtering layer is not more than two feet. The 

 action of this class of filter is very simple ; the organic 

 matter in the water forms a gelatinous deposit on the 

 surface of the sand, consisting of organisms of various 

 kinds. The gelatinous mass acts as a filter, refusing to 

 allow other bacteria to pass, and so long as the gelatinous 

 growth remains unbroken, filtration is effective. When the 

 slimy layer grows down too far into the sand, filtration 

 becomes slow and the surface has to be cleaned. 



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