THE SUBSTANCES OF THE EARTH. 13 



(H 2 S0 4 ), muriatic or hydrochloric acid (HC1), phospho- 

 ric acid (H 3 P0 4 ), silicic acid (H 4 Si0 4 ), carbonic acid 

 (CO,), nitric acid (HN0 8 ), etc. 



Bases are a class of substances whose nature is very 

 different from that of acids. A portion of them are 

 called alkalies, or alkaline sutstances, and have a hot, 

 sharp taste. 



Some of the bases are potash (K 2 0), sodium (Na), 

 lime (CaO), magnesia (MgO), oxide of iron or iron rust 

 (FeO), etc. 



Acids and bases have a strong attraction for one an- 

 other, and when united form a class of substances called 

 salts. They are so named because many of them have a 

 taste similar to that of common salt. 



They are called sulphates, chlorates, phosphates, 

 etc., as chlorate of potash (KC10 3 ), phosphate of lime 

 (CaOP 2 5 ), nitrate of soda (NaN0 3 ), etc. 



Gypsum, or land plaster, is sulphate of lime and water. 

 It is the result of a combination of sulphuric acid and 

 quicklime : 



H 2 S0 4 + CaO = (CaS0 4 + H 2 0). 



Phosphate of lime is produced by a union of phosphoric 

 acid and quicklime : 



2 H 3 P0 4 + CaO = CaOP 2 5 + 3 H 2 0. 



If a feather is dipped in hydrochloric acid and held 

 over an open bottle of ammonia, the ammonia, escaping 

 by evaporation, will unite with the acid and form a white 

 powder upon the feather, called ammonic chloride : 



NH 8 + HC1 = NH 4 C1. 



When the two parts of a rochelle powder are dissolved 

 and poured together, the acid of the one unites with the 

 base of the other, producing a salt, which remains dis- 



