536 PRINCIPLES OF CHEMISTRY 



porphyry, ifcc., 1 are formed of such crystalline silicious rocks as these. 

 The oxides entering into the composition of 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, as has been already 

 mentioned, granite contains felspar, quartz, and mica. Potash and 

 soda, (fee., are generally found in these component parts of rocks. Thus 

 a felspar (orthoclase) in granite contains from 8 to 15 percent, of potas- 

 sium, whilst another variety (plagioclase) 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 10 per cent, of potassium. As 

 has already been mentioned, and will be further explained, 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 car- 

 bonic acid. It is evident that in the formation of these friable forma- 

 tions from the primary rocks by the action of water, the compounds of 

 potassium, as well as the compounds of sodium, must have been dis- 

 solved by the water (as they are soluble in water), and, therefore, the 

 compounds of potassium must be accumulated together with those of 

 sodium in sea water. And, indeed, compounds of potassium are always, 

 as we have already pointed out (Chaps. I. and X.) found in sea water. 

 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, KMgCl 3 ,6H 2 O, occurs at Stassfurt. This carnallite 2 

 is now employed as a material for the extraction of potassium chloride, 

 and of all the compounds of this element. 3 Besides which, potassium 



1 The origin of the primary rocks has been mentioned in Chap. X. Note 2. 



2 Carnallite belongs to the number of double salts which are directly decomposed by 

 water, and it only crystallises from solutions which contain an excess of magnesium 

 chloride. It may be prepared artificially by mixing strong solutions of potassium and 

 magnesium chlorides, when colourless crystals of sp. gr. 1'60 separate, whilst the Stass- 

 furt salt is usually of a reddish tint, owing to traces of iron. At the ordinary tempera- 

 ture sixty-five parts of carnallite are soluble in one-hundred parts of water in the pre- 

 sence of an excess of the salt. It deliquesces in the air, forming a solution of magne- 

 sium chloride and leaving potassium chloride. 



3 The method of separating sodium chloride from potassium chloride has been 

 described on p. 72. On evaporation of a mixture of the saturated solutions, sodium chloride 

 separates ; and then, on cooling, potassium chloride separates, owing to the difference of 

 the variation of their solubilities with the temperature. The following are the most 

 trustworthy figures for the solubility of potassium chloride in one hundred parts of 

 water (for sodium chloride, see Chap. X. JJote 18) : 



10 20 J 40 J 60 100 



32 85 40 40 57 



