SOFTENING HARD WATER. 149 



magnesia) ; the result of which is that they possess to a greater or 

 less extent the same peculiar property as the solution of bicar- 

 bonate of lime prepared as above described; viz., that, on boiling, 

 this compound is chemically affected by the heat in such a fashion 

 that ordinary " carbonate " of lime or magnesia is formed, which 

 being now insoluble in water precipitates in the solid form. 

 Transfer the solution just prepared to a flask, and heat it carefully 

 to boiling over a lamp ; solid particles will rapidly be deposited, 

 and in a short time almost the whole of the dissolved lime will be 

 thrown out of solution as carbonate of lime. When ordinary 

 spring water is boiled, exactly the same thing takes place; the 

 " furr " inside a kettle is principally the carbonate of lime thus 

 formed; in steam boilers the production of an internal solid 

 incrustation in this way is constantly taking place, and is a source 

 of great inconvenience in many ways, more especially in that it 

 prevents the heat reaching the water so readily, and thus leads to 

 waste of fuel, as well as damage to the boiler by the outer portion 

 becoming unduly heated. 



Water containing lime or magnesia dissolved is said to be 

 "hard;" when present as bicarbonate of lime or magnesia, the 

 hardness is mostly removed by boiling, the lime being thus 

 rendered insoluble and precipitating ; lime so removed constitutes 

 what is termed temporary hardness for that reason. Most water, 

 however, also contains more or less lime and other mineral matters 

 in forms not removed by simple boiling (chiefly as sulphate of lime 

 or dissolved gypsum) ; and the hardness due to this cause is con- 

 sequently termed permanent hardness. Hardness in the water 

 supply of a town is unobjectionable up to a certain limit; but 

 beyond that it is a serious evil, as very hard water wastes much 

 soap when used for washing purposes, and is much less well 

 adapted for cooking, making tea and coffee, and such like dietetic 

 purposes, than softer water. Some of the softest water supplied 

 to large towns and cities in Britain is that of Glasgow, derived 

 from Loch Katrine. Ordinarily, temporary hardness in water is 

 removed and the water softened by simply boiling the water and 

 allowing it to stand. 



Expt. 156. Other Ways of softening Hard Water. Water 

 rendered temporarily hard, by the presence of much bicarbonate of 

 lime therein, is sometimes softened on a large scale by various 

 methods before distributing it for town use : for the most part 

 these methods are substantially modifications of " Clark's process," 

 which consists in adding to the water just so much lime as will 

 suffice to convert all the bicarbonate of lime (and magnesia, &c.) 

 present, and all dissolved carbonic acid gas into ordinary carbonate 



