POTASSIUM. 255 



The fusibility of an alloy is invariably greater than that of its least fusible 

 constituent, and may be greater than that of its most fusible constituent. Thus 

 an alloy of 2 parts of tin, 3 of lead, and 5 of bismuth fuses at 91 C., while 

 tin alone melts at 228, lead at 325, and bismuth at 260 C. 



The conductivity of alloys for heat and . electricity is less than that of the 

 pure metals. The color of alloys is generally a modification of the predom- 

 inating ingredient, but instances are known where the color of alloys has no 

 relation to its constituents. For instance, German silver is perfectly white 

 although it contains a considerable portion of red copper. 



21. POTASSIUM (KALIUM). 



K' = 39 (38.86). 



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 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. 



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 hydroxides, sulphates, 

 nitrates, phosphates, carbonates, sulphides, chlorides, iodides, and 

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

 pounds are white, solid substances, most 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, such as coal-oil, which is not acted on by 

 them, 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 = 3CO + 2K. 



At present most of the alkali metals are obtained by the electrol- 

 ysis of the fused hydroxides, the metal and hydrogen being liberated 

 at the negative, oxygen at the positive pole : 

 KOH == K 4- H + O. 



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, 



