WATER. 149 



obtained by melting ice, since, when water containing impurities is 

 frozen partially, these are mostly left in the uncongeaieu water. 



The waters of springs, wells, rivers, etc., differ widely from each 

 other ; they all contain more or less of substances dissolved by the 

 water in its course through the atmosphere or through the soil and 

 rocks. The constituents thus absorbed by the water are either solids 

 or gases. 



Solids generally found in natural waters are common salt (sodium 

 chloride), gypsum (calcium sulphate), and carbonate of lime (calcium 

 carbonate) ; frequently found are chlorides and sulphates of potassium 

 and magnesium, traces of silica and salts of iron. Gases absorbed 

 by water are constituents of the atmospheric air, chiefly oxygen, 

 nitrogen, and carbon dioxide. One hundred volumes of water con- 

 tain about two volumes of nitrogen, one volume of oxygen, and one 

 volume of carbon dioxide. 



Water is said to be hard when it contains so much of salts of cal- 

 cium and magnesium that the formation of lather by soap is delayed 

 because these salts form insoluble compounds with the soap. Water 

 containing but little of inorganic matter is said to be soft. 



When the hardness is caused by metallic sulphates or chlorides the 

 water is called permanently hard, while it is termed temporarily hard 

 when the metals are present as carbonates, dissolved by carbonic acid. 

 On boiling such water carbon dioxide escapes, the carbonates of the 

 metals are precipitated, and the water is rendered soft. 



Mineral waters are spring waters containing one or more sub- 

 stances in such quantities that they impart to the water a peculiar 

 taste and generally a decided medicinal action. According to the 

 predominating constituents we distinguish bitter waters, containing 

 larger quantities of magnesium salts ; iron or chalybeate waters, 

 containing carbonate or sulphate of iron ; sulphur or hepatic waters, 

 containing hydrogen sulphide ; effervescent waters, strongly charged 

 with carbonic acid ; cathartic waters, generally containing sodium or 

 magnesium sulphate, etc. 



Drinking-water. A good drinking-water should be free from 

 color, odor, and taste ; it should neither be an absolutely pure water, 

 nor a water containing too much of foreign matter. Water containing 

 from 2 to 4 parts of total inorganic solids (chiefly carbonate of lime 

 and common salt) in 10,000 parts of water and about 1 volume of 

 carbon dioxide in 100 volumes of water, may be said to be a good 



