SALTS OF SODA — THE EARTHS. 107 



time crumbling down to a fine powder, which is found 

 to be less caustic than it was before it underwent 

 this change. The reason of this is, that when the 

 carbonate of soda, by exposure to the air, parts with 

 a quantity of water, and consequently becomes much 

 lighter, it at the same time gradually combines with 

 a second dose of carbonic acid, which it obtains from 

 the air, and its caustic qualities become more fully 

 neutralized than they are in the common carbonate. 



223. The salts of soda are for the most part white, 

 and easily soluble in water ; and, like the compounds 

 of potash, are widely diffused throughout nature. 

 They are found, besides the sources already men- 

 tioned, in many rocks and soils : a great many plants, 

 more especially those which grow near the sea or in 

 salt marshes, contain large quantities of salts of 

 soda ; and a good many of the compounds which 

 this alkali forms with different acids are found native, 

 though in far smaller quantities than the nitrate. 



224. The sulphate of soda is also found native in 

 Spain and other countries, and is frequently met with 

 in mineral waters ; it is also called Glauber's salts, 

 and Cheltenham salts, as it is the principal ingredient 

 in the mineral springs of Cheltenham. Sulphate of 

 soda is left after the process for obtaining muriatic 

 acid (190): it is occasionally employed as a manure. 



225. The earth, or soil, consists of a mixture of 

 various ingredients ; but the greater part of it is com- 

 posed of certain substances which are called earths. 

 Some of these are bases, and resemble the alkalies ; 



