20 GENERAL SCIENCE 



by the action of the heat on the calcium hydrogen 

 carbonate. 



Water containing calcium sulphate, or gypsum (CaS0 4 ), 

 is said to be " permanently hard/' because it can be 

 softened only by removing the water from the gypsum 

 by distillation or by adding some substance such as 

 borax, ammonium carbonate, or washing soda (Na 2 CO 3 ), 

 also called sodium carbonate. These substances are 

 cheaper than soap and they break up the gypsum and 

 form substances which will not act on soap. But these new 

 substances are hard on the clothes and also on the hands 

 of the laundry workers, so it is better to use "soft" water. 



17. Salts. A number of compounds which can be 

 classed as salts have already been mentioned. Salts are 

 formed by the interaction of a base and an acid. If, as 

 described in 13, just enough base is used to neutralize 

 the acid, a neutral salt is formed. Common salt and 

 sal ammoniac and gypsum are examples of neutral salts. 

 If an excess of a base is used with an acid, a basic salt 

 is usually formed. If an excess of an acid is used, an 

 acidic salt is usually formed. Cream of tartar is an 

 example of an acidic salt. 



There are great quantities of common salt and other 

 salts stored in the earth, which have been there for long 

 ages. Salt is taken from the earth in various parts of 

 the United States by drilling wells and pumping out the 

 salt water, by digging mines, and in California it can be 

 shoveled from the surface. The salt water or brine is 

 allowed to evaporate and then the salt will form in -crys- 

 tals. The largest salt mines are in Germany, where the 

 salt is about a mile deep. This salt was perhaps deposited 

 during the evaporation of an ancient inland sea, such as 

 the Caspian, or the Great Salt Lake in Utah. 



