

POTASSIUM. 115 



19. POTASSIUM. 



K l = 39. 



General remarks regarding alkali-metals. The metals potas- 

 sium, sodium, lithium (rubidium and caesium) form the group 

 of the alkali-metals, which, in many respects, show a great re- 

 semblance in chemical and physical properties. For reasons to 

 be explained hereafter, the compound radical ammonium is 

 usually classed among the alkali-metals. 



The alkali-metals are all univalent; they decompose water 

 at the ordinary temperature with liberation of hydrogen ; they 

 combine spontaneously with oxygen and chlorine ; their oxides, 

 hydrates, sulphates, nitrates, phosphates, carbonates, sulphides, 

 chlorides, iodides, and nearly all of their other salts are soluble 

 in water ; all these compounds are white, solid substances, many 

 of which are fusible at a red heat. Of all metals, those of the 

 alkalies are the only ones forming hydrates and carbonates 

 which are not decomposed by heat. 



The metals themselves are of a silver-white color and ex- 

 tremely soft ; on account of their tendency to combine with 

 oxygen, they must be kept in a liquid not containing that ele- 

 ment (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 C0 3 + 20 = SCO + 2K. 



Occurrence in nature. Potassium is chiefly found in nature as 

 a double silicate of potassium and aluminium (granitic rocks, 

 feldspar, and other minerals) or as chloride and nitrate. By 

 the gradual disintegration of the different granitic rocks con- 

 taining silicate of potassium, this has entered into the soil, 

 whence it is taken up by plants as one of the necessary con- 

 stituents of their food. 



In the plant the potassium enters largely into combination 

 with organic acids (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, 



