544 PRINCIPLES OF CHEMISTRY 



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, Si0 2 , 

 potassium oxide, like sodium oxide, forms saline mineral substances 

 resembling glass. If other oxides, such as lime, CaO, and -alumina, 

 A1 2 3 , combine with these compounds, glass is formed, a vitreous 

 stony mass, distinguished by its great stability, and its very slight 

 variation under the action of water. It is such complex silicious com- 

 pounds as these which contain potash (potassium oxide), K 2 O, or soda 

 (sodium oxide), Na 2 0, and sometimes 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 direction of the strata, the chief 

 mass of the accessible crust (envelope) of the earth is made up. The 

 primary rocks, like granite, porphyry, Ac., 1 bis are formed of such crystal- 

 line silicious rocks as these. The oxides entering into the composition 

 X)f these rocks do not form a homogeneous amorphous mass like glass, 

 but are distributed in a series of peculiar, and in the majority of cases 

 crystalline, compounds, into which the primary rocks may be divided. 

 Thus a felspar (orthoclase) in granite contains from 8 to 15 per cent, of 

 potassium, whilst another variety (plagiocla'se) which also occurs in 

 granite contains 1-2 to 6 per cent, of potassium, and 6 to 12 per cent, 

 of sodium. The mica in granite contains 3 to 1 per cent, of potassium. 

 As already mentioned, and further explained in Chapter XVII., 

 the friable, crumbling, and stratified formations which in our times 

 cover a large part of the earth's surface have been formed' from these 

 primary rocks by the action of the atmosphere and of water containing 

 Carbonic acid. It is evident that in the chemical alteration of the 

 primary rocks by the action of water, the compounds of potassium, as 

 well as the compounds of sodium, must have been dissolved by the 

 water (as they are soluble in water), and that therefore the compounds 

 of potassium must be accumulated together with, those of sodium in sea 

 water. And indeed compounds of potassium are always found in sea 

 water y as we have already pointed out (Chapters I. and X.). This 

 forms one of the sources from which they are extracted. After the 

 evaporation of sea water, there remains a mother liquor, which contains 

 potassium chloride and a large proportion of magnesium chloride. On 

 cooling this solution crystals separate out which contain chlorides of 

 magnesium and potassium. A double salt of this kind, called carnallite t 

 tu The origin of the primary rocks has been mentioned in Chapter X., Note. 2. 



