NATURAL WATERS 65 



boiler plates greatly increases the consumption of fuel required 

 for the production of a certain quantity of steam. 



A distinction is often made between waters which contain 

 their calcium and magnesium as bicarbonates and those in 

 which the salts present are the sulphates. The former are 

 known as "temporarily," the latter as "permanently" hard 

 waters. By the removal of the excess of carbon dioxide from* 

 the former the calcium and magnesium carbonates are pre- 

 cipitated, while with the latter the salts are dissolved 7;er se, 

 and cannot conveniently be removed. The usual plan adopted 

 to effect the softening of temporarily hard water is to add lime, 

 CaHjOg, in sufficient quantity to combine with the free carbon 

 dioxide and that present as bicarbonates, when the precipi- 

 tate formed contains the calcium (and magnesium) carbonate 

 originally present, together with that formed from the added 

 lime : 



Ca(HC03)2 + CaH.O^ = 2CaC03 + 3H,0. 



On standing the precipitate settles out, and the clear liquid is 

 then almost free from calcium and magnesium and is " soft." 

 It is much improved both for washing and for steam-raising 

 purposes. 



In a drinking water the presence of calcium compounds, 

 except in excessive amounts, is not very objectionable — indeed, 

 is often advantageous, furnishing a portion of the lime neces- 

 sary for the building up of the hard parts (bones or shells) of 

 the animal. Moreover, in many cases water is delivered 

 through leaden pipes, and soft waters, especially if they con- 

 tain vegetable acids — e.g., from peat — attack and dissolve lead, 

 often to such an extent as to cause lead poisoning in those 

 who drink them. The presence of calcium sulphate renders 

 water incapable of this dangerous action upon lead, for the 

 metal becomes coated with a film of the very insoluble lead 

 sulphate, which protects it from further contact with the water. 



Of greater importance than the mineral matter in drinking 

 water is the amount and nature of the organic matter. 



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