186 THE BOOK OF NATURE STUDY 



driven along steadily under pressure and the amount received is 

 fairly constant from day to day, and the water travelling through 

 the pores of the pervious rock becomes admirably filtered and 

 purified. When the water travels very deep down into the rocks 

 it may emerge heated and form hot springs. 



The water of both deep-seated and surface springs passing 

 through the body of the rock comes into contact with any soluble 

 ingredients there may be, and takes them into solution. Carbonate 

 of lime, magnesia, or iron are thus taken up by water containing 

 carbonic acid; sulphate of lime, and certain other salts of iron, 

 magnesia, and alkalies are taken into solution by all waters, and 

 brought out by them where they issue as springs. Small quan- 

 tities of such salts give to spring waters their pleasantness, and 

 deprive them of the " flat " taste of rain water. Larger quantities 

 turn the water into strong solutions, and they are then known 

 as mineral springs and are named calcareous, magnesium, alkaline, 

 saline, chalybeate, etc., according to their prominent ingredients. 



The most important dissolved substances in ordinary springs, 

 which do not contain enough to entitle them to be " mineral 

 springs," are sulphate and carbonate of lime. Either of these 

 salts gives to the water the property known as hardness. A 

 water is soft if it dissolves soap and forms a lather with it. 

 Distilled water, rain water, or water flowing off areas of granitic 

 or siliceous rocks are soft. But a hard water forms a precipitate 

 with soap, and continues to do so until all the calcareous (or 

 magnesian) salts are extracted. The water has then become 

 soft, and will form a lather. Water containing carbonate of 

 lime precipitates this in kettles, boilers, or pipes when heated, 

 and especially when boiled, because the carbonate of lime is 

 only held in solution by the carbonic acid contained in the water, 

 and this acid is partly driven off by heating and completely 

 expelled by boiling. Hence boiling is a second method of 

 softening water hardened by carbonate of lime, and in conse- 

 quence of this property such water is said to be temporarily hard. 



On the other hand, if sulphate of lime is the principal salt 

 in solution, the water will not be softened by boiling, because 

 the sulphate is held in solution by water alone without the aid 

 of the carbonic acid. Hence it is called permanently hard water. 



