Natural Waters. 71 



fatty acids. The dissolved soap is thus removed and more has 

 to be dissolved before the proper cleansing action can be exerted. 

 Hence hard waters are unsuitable for domestic, especially for 

 laundry, purposes ; they involve the consumption of large quan- 

 tities of soap and contaminate the washed articles with the pre- 

 cipitated "lime" or "magnesia soap." 



Hard waters are also unsuitable for steam-raising, since the 

 deposit of calcium carbonate or calcium sulphate (boiler scale) 

 upon the boiler plates greatly increases the consumption of fuel 

 required for the production of a certain quantity of steam. Cal- 

 cium carbonate alone forms a porous and non-adherent scale, 

 which is easily removed by "blowing off" the boiler. Calcium 

 sulphate forms a hard compact scale, which adheres very firmly. 



A distinction is often made between waters, which contain 

 their calcium and magnesium as bi-carbonates and those in which 

 the salts present are as 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 precipitated, while with 

 the latter the salts are in solution and cannot be precipitated by 

 the simple removal of carbon dioxide. 



The usual method of procedure to effect the softening of tem- 

 porarily hard water is to add "milk of lime" in sufficient quan- 

 tity to combine with the free carbon dioxide and that present as 

 bi-carbonates. The precipitate formed will be found to contain 

 the calcium and magnesium carbonates originally present, to- 

 gether with that formed from the added lime. On standing, the 

 precipitate settles out and the clear liquid is then almost free 

 from calcium and magnesium and is "soft." The milk of lime 

 should be added slowly and gradually and care be taken that no 

 great excess is used. Water so treated is much improved both 

 for washing and for steam-raising purposes. The "milk of 

 lime" is made by treating a quantity of quick lime with water 

 and after thoroughly stirring, the ' ' milk ' ' is then mixed with the 

 water to be purified. 



