SODA— COMMON SALT. 103 



used as a manure, and will therefore come before us 

 again on a future occasion (898). 



213. Sulphate of potash is a soluble salt, readily 

 formed by acting on any of the ordinary salts of the 

 alkali, such as the carbonate or nitrate, by sulphuric 

 acid; it is chiefly used by the manufacturers of alum 

 (257). 



214. Salts of potash are met with in a great many 

 plants in small quantities; they are likewise almost 

 always to be found in the soil, and potash is by no 

 means uncommon in stones; indeed, it is evident that 

 the salts of potash found in the soil must have been 

 derived from the gradual breaking down and decom- 

 position of stones and rocks containing compounds of 

 potash. 



215. The second alkali, soda, is in its general cha- 

 racters similar to potash; it is a very caustic, solid, 

 white substance, has a powerful attraction for acids, 

 and is consequently never found in a pure and sepa- 

 rate condition, but always in combination with an 

 acid, or some other substance. The most abundant 

 source of soda is common salt, which exists in very 

 large quantities in sea-water, and is likewise found 

 native in the earth, when it is called rock-salt, to 

 distinguish it from the salt obtained by the evapora- 

 tion of sea-water, and called sea or bay salt. 



216. Common salt has already been stated to be a 

 chloride of sodium (189), a compound of chlorine with 

 a white brilliant metal called sodium. It used for- 

 merly to be called a muriate of soda, because it was 



