MATERULS EXISTING IN THE SOIL. 43 



compounds of the same gas with metals and other elemen- 

 tary bodies, some of which possess alkaline and others acid 

 properties. These substances arc usually termed the earthy 

 or inorganic constituents of plants. They are named Potash, 

 Soda, Lime, Magnesia, Oxide of Iron, Oxide of Manganese, 

 Silica, Chlorine, Sulphuric Acid, Phosphoric Acid. 



43. Potash forms the greater part of the well-known 

 alkaline substance sold by the grocer as Salt of Tartar, and 

 also of the potashes used by some bleachers in this country. 

 It is a compound of oxygen with a curious inflammable metal, 

 Potassium, which, when thrown upon water, decomposes 

 it, and unites with one of its elements, oxygen, while the 

 other element, hydrogen, is separated, and burns with a 

 beautiful flame. The same decomposition and production of 

 flame are witnessed even when the metal is placed on a plate 

 of ice. By simple exposure to the air also, it loses its 

 metallic brilliancy by uniting with oxygen. In all these 

 cases the same compound, oxide of potassium or potash, is 

 formed. Potash exists in considerable quantities in the 

 ashes of land plants, especially in those of the common 

 bracken,* and of the wormwood. It is obtained by washing 

 the ashes with water; the potash, being soluble, is dissolved 

 out, and when the water is boiled to dryness in an u'on pot, 

 it is obtained united with carbonic acid, forming what is 

 termed carbonate of potash. It is in this way that the potash 

 of commerce is prepared, and that substance, separated from 

 various impurities, is termed pearl ash. Potash is also con- 

 tained in considerable quantity in the ashes of several kinds of 

 sea- weed: thus in the ashes of some sea- weed from the 

 mouth of the Clyde, there was found so much as 22 per 

 cent of that substance.! By dissolving carbonate of potash 

 (potash or pearl ash) in water and boiling it with some quick- 

 lime, you can separate the carbonic acid from it, and obtain 

 a solution, which, when decanted from the sediment that is 

 formed (carbonate of lime) and boiled to dryness in a covered 

 vessel, will yield pure or caustic potash. 



44. Soda is a substance closely resembling potash in its 

 chaiacters. It is met with in the washing soda of the 

 grocer, in the soda ash and barilla of the bleacher, in the 



• The pteris aquilina of the botanist. 



t By Dr. Godechens, of Hamburg, in the laboratory of Professor 

 Will at Giessen. 



