585 



CHAPTER XIII 



POTASSIUM, RUBIDIUM, CESIUM, AND LITHIUM. SPECTRUM ANALYSIS 



JUST as the series of halogens, fluorine, bromine, and iodine, correspond 

 with the chlorine contained in common salt, so there also exists a cor- 

 responding series of elements : lithium, Li =7, potassium, K=39, 

 rubidium, Rb=85, and cesium, Cs=133, which are analogous to the 

 sodium in common salt. These elements bear as great a resemblance 

 to sodium, Na=23, as fluorine, F=19, bromine, Br=80, and iodine, 

 1=127, have to chlorine, Cl=35'5. Indeed, in a free state, these 

 elements are, like sodium, soft metals, which rapidly oxidise in moist 

 air, and decompose water at the ordinary temperature, forming soluble 

 hydroxides, having clearly -defined basic properties, and the composi- 

 tion RHO, like that of caustic soda. The resemblance between these 

 metals is sometimes seen with striking clearness, especially in com- 

 pounds such as salts. The corresponding salts of nitric, sulphuric, 

 carbonic, and nearly all acids with these metals have many points in 

 common. The metals which resemble sodium so much in their reac- 

 tions are termed the metals of the alkalis. 



Among the metals of the alkalis, the most widely distributed in 

 nature, after sodium, is potassium. Like sodium, it does not appear 

 either in a free state or as oxide or hydroxide, but in the form of salts, 

 which present much in common with the salts of sodium in the manner 

 of their occurrence. The compounds of potassium and sodium in the 

 earth's crust occur as mineral compounds of silica. With silica, SiO 2 , 

 potassium oxide, like sodium oxide, forms saline mineral substances 

 like glass. If different other oxides, such as lime, CaO, and alumina, 

 A1 2 O 3 , combine with these compounds, there is formed glass, or a 

 glassy stony mass, which is distinguished by its great stability. It is 

 such complex silicious compounds as these which contain potash 

 (potassium oxide), K 2 0, or soda (sodium oxide), Na 2 O, and some- 

 times both together, silica, SiO 2 , lime, CaO, alumina, A1 2 O 3 , and other 

 oxides, that form the chief mass of rocks, out of which, judging by 

 the distribution of the strata, the chief mass of the accessible crust 

 (envelope) of the earth is made up. The primary rocks, like granite, 



