DRINKING WATER 



77 



80. Substances Dissolved in Natural Water. Lakes 

 and rivers are usually fresh because the salty substances 

 brought into them are also carried away; but the ocean 

 becomes more and more loaded with dissolved material. 

 The same is true of such lakes as Great Salt Lake and the 

 Dead Sea. The reason is that while these bodies receive 

 both water and dissolved substances, only the water 

 evaporates. The solids are left behind. Nearly 2.7 per 

 cent of sea water is common salt. 



Mineral waters contain so much solid material in solution that it is 

 usually perceptible to the taste. The most common substances in 

 mineral waters are salt, soda, potash, limestone, gypsum, and com- 

 pounds of iron and of sulphur. As many as 15 grams of solids are 

 sometimes present in one liter of mineral water. Salt springs and 

 wells furnish most of our table salt (cf. 108 and Fig. 84). 



COMPOSITION OF SOME NATURAL WATERS 



81. Drinking Water. By "pure water" different 

 classes of people mean different things, for each is thinking 

 of some impurity that is especially objectionable to him. 

 If the water is to be used for drinking, the chief impurities 



