HYDROGEN. 81 



Water, H 2 O = 18. Hydrogen monoxide. Water is not found 

 in nature in an absolutely pure state. The purest natural water is 

 rain-water collected after the air has been purified from dust, etc., by 

 previous rain. Comparatively pure water may be obtained by 

 melting ice, since, when w r ater containing impurities is frozen par- 

 tially, these are mostly left in the uncongealed 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. 



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, containing large 

 quantities of carbonic acid, 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 

 drinking-water. There are, however, good drinking-waters which 

 contain more of total solids than the amount mentioned above. 



Most objectionable in drinking-water are organic substances ) espe- 

 cially when derived from animal matter, and more especially when 

 in a state of decomposition, because such decomposing organic matter 

 is frequently accompanied by living organisms (germs) which may 



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