Salts. 141 



them all is, that a very small quantity of them, 

 added to an infusion of red or blue coloured ve- 

 getable substances, turns it green, or abolishes 

 the reddish cast of the purple. The strongest 

 alkali produces the strongest green. Alkalies 

 are hot and pungent to the taste, have a great 

 degree of detergency, and are very corrosive ap- 

 plied to animal substances. 



Soda and potassa are the oxides of peculiar 

 metals, sodium and potassum, which continue in 

 a ductile state in the ordinary heat of our atmo- 

 sphere. They have a near relation to each other, 

 yet they are distinguished by the names of the 

 mineral and vegetable alkalies, as before re- 

 marked. These oxides, when pure, are dry and 

 solid substances, of a white colour, They are so 

 caustic that they cannot be handled without in- 

 juring the skin. 



Soda has all the general qualities of a salt ; 

 it requires a strong heat to make it melt ; it is 

 easy, when combined with the carbonic acid, to 

 crystallize. These crystals, from the large quan- 

 tity of water they contain, which adheres loosely 

 to them, and i* generally about one half of their 

 weight, are liable to spontaneous exsiccation or 

 efflorescence. If the water is quite evaporated, 

 there remains a w r hite saline crust, which may 

 again be dissolved in water and crystallized. 



This salt has been the longest known of any : 

 fa is this we find recorded in history under the 

 name of nitre. We are told it is to be found in 



