64 ELEMENTARY AGRICULTURAL CHEMISTRY 



the removal of the rock in solution giving rise to the cavea 

 and underground watercourses so common in these districts. 

 When such water is boiled the bicarbonates are decomposed, 

 the normal carbonater being again formed and precipitated : 



Ca(HC03)2 = CaCOg + H,0 + CO,. 

 Calcium bicarbonate 



In many cases the precipitated calcium or magnesium carbonate 

 forms a firmly adherent coating (" fur " or *' crust ") upon the 

 bottom and sides of the kettle or boiler. 



Calcium and magnesium sulphates are soluble in water, the 



former to the extent of about 1*7 grammes per litre. Waters 



containing calcium or magnesium compounds are known as 



"hard'' waters, and have a peculiar and well-known action 



on soap. The latter is essentially the sodium salt of a fatty 



acid — e,g., stearic acid, HCigHg^Og, the calcium and magnesium 



salts of which are insoluble in water. For water to form a 



lather with soap or properly exercise its cleansing power it is 



necessary that the water should contain some dissolved sodium 



stearate. When a small quantity of soap is dissolved in hard 



water, the calcium or magnesium present in the water, by 



double decomposition with the soap, gives a curdy, flocculent 



precipitate of the calcium or magnesium salts of the fatty 



acid^ e.g., 



2]SraC,3H3,0, + CaSO, = Ca(C,3H3P,), + Na.SO,. 

 Sodium Calcium Calcium Sodium 



stearate sulphate stearate sulphate 



The dissolved soap is thus removed, and more has to be dis- 

 solved before the proper cleansing action can be exerted. 

 Hence hard waters are unsuitable for domestic use, especially 

 for laundry purposes ; they involve the consumption of large 

 quantities of soap, and contaminate the articles washed with 

 the precipitated " lime " or " magnesia soaps." 



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

 deposit of calcium carbonate or of calcium sulphate upon the 



