PURIFYING WATER 79 



82. Hardness of Water. To the manufacturer ' ' pure " 

 water usually means water that can be used in boilers, 

 to produce steam. The most objectionable impurity from 

 his point of view is the "hardness" of the water. Hard- 

 ness is also harmful in water to be used in the house- 

 hold for bathing and for the washing of clothes. Water 

 is said to be "hard" if it does not wet the skin readily, 

 and if soap put into the water forms, at first, an in- 

 soluble scum. Suds, or lather, is not formed in hard 

 water until the hardness is removed by the soap; hence 

 hardness is defined as the soap-consuming power of a 

 water. Distilled water, water from melting ice, and rain 

 water form a lather almost immediately; hence they are 

 called " soft " waters. Some of the impurities that cause 

 hardness become insoluble, and settle, when the water is 

 boiled. In this case the hardness is said to be temporary. 

 Hardness that cannot be removed by 



boiling is called permanent hardness 

 (c/.226). 



If a boiler is filled repeatedly with hard 

 water, it becomes clogged with a deposit 

 "boiler scale " just as a kettle does (Fig. 65). 

 Iron is a conductor of heat (cf. 65) ; hence the 

 walls of a new iron boiler cannot become much The kettle fo Uned witb solids 

 hotter than the water in the boiler. But boiler deposited from boiling 



hard water. 



scale is so poor a conductor that a boiler con- 

 taining much of it needs to be heated very hot, or the water will not 

 boil. As a result of this over-heating, the " scale" is often broken up 

 into substances that act upon the iron, and weaken it. Boiler scale 

 has caused very serious explosions and loss of life. 



83. Purifying Water. Water may be purified 



(1) by distilling it; (2) by boiling it; (3) by filtering it. 



