POTASSIUM. 135 



alkali-metals ? which, in many respects, show a great resemblance to 

 each other in chemical and physical properties. For reasons to be 

 explained hereafter, the compound radical ammonium is usually 

 classed among the alkali-metals. 



JThejilkali-metals are all univalent^ they decompose water at the 

 ordinary temperature, with liberation of hydrogen; they combine 

 spontaneously with oxygen and chlorine ; their hydroxides, sulphates, 

 nitrafes^ phosphates, carbonates, sulphides, chlorides, iodides, and 

 nearly all other of their salts are soluble^ in water ; all these com- 

 pounds are white, solid substances, many of which are fusible at a 

 red heat. Of all metals, those of the alkalies are the only ones form- 

 ing hydroxides and carbonates which are not decomposed by heat. 



The metals themselves are of a silver- white color, and extremely 

 soft; on account of their tendency to combine with oxygen they 

 must be kept in a liquid not containing that element ^coal-oil) or in 

 an atmosphere of hydrogen. 



The metals may be obtained by heating their carbonates with carbon 

 in iron retorts, the escaping vapors being passed under coal-oil for 

 condensation of the metal : 



K 2 CO 3 + 20 = SCO 4- 2K. 



Occurrence in nature. Potassium is found in nature chiefly as a 

 double silicate of potassium and aluminum (granite rocks, feldspar, 

 and other minerals), or as chloride and nitrate. By the gradual dis- 

 integration of the different granite rocks containing potassium silicate, 

 this has entered into the soil, whence it is taken up by plants as one 

 of the necessary constituents of their food. 



In the plant potassium enters largely into combination with organic 

 compounds (tartaric acid, citric acid, etc.), and when the plant is 

 burned, ashes are left containing the potassium, now in the form of 

 carbonate. By washing such ashes (chiefly wood ashes) with water 

 and filtering, the insoluble matter (carbonates, phosphates, and sul- 

 phates of calcium and magnesium, silica, etc.) is left behind, whilst a 

 ,lye is "Obtained containing the soluble constituents, of which potassium 

 carbonate is the principal one, chlorides and sulphates of potassium 

 and sodium also being present in small quantities. 



/ By evaporation of this lye to dryness an impure potassium car- 



[ bonate is obtained, which is sold as crjj^e potash. 



^ Up to within twenty-five years ago the chief supply of potash was 

 obtained by this process, and the trees of thousands cf acres were 

 burned with the view of obtaining potash. To-day this mode of 



