546 PRINCIPLES OF CHEMISTRY 



double saline decomposition, the chloride of potassium may be converted 

 into all the other potassium salts, 4 some of which are of practical use. 

 The potassium salts have, however, their greatest importance as an 

 indispensable component of the food of plants. 5 



The primary rocks contain an almost equal proportion of potassium 

 and sodium. But in sea water the compounds of the latter metal pre- 

 dominate. It may be asked, what became of the compounds of potas- 

 sium in the disintegration of the primary rocks, if so small a quantity 

 went to the sea water ? They remained with the other products of the 

 decomposition of the primary rocks. When granite or any other 

 similar rock formation is disintegrated, there are formed, besides the 

 soluble substances, also insoluble substances sand and finely-divided 

 clay, containing water, alumina, and silica. This .clay is carried away 

 by the water, and is then deposited in strata. It, and especially 

 its admixture with vegetable remains, retain compounds of potassium 

 in a greater quantity than those of sodium. This has been proved 

 with absolute certainty to be the case, and is due to the absorptive 

 power of the soil. If a dilute solution of a potassium compound be 

 filtered through common mould used for growing plants, containing 

 clay and the remains of vegetable decomposition, this mould will be 



crystals varies according to the halogen, thus CsI 3 is black, CrBr 5 yellowish red.CsBrlj 

 reddish brown, CsBr 2 I red, CsCl 3 Br yellow. The crosium salts are the most stable, and 

 those of potassium least so, as also those which contain Br and I separately or together ; 

 for caesium no compounds containing Cl and I were obtained. The penta-haloid salts form 

 a smaller class ; among these salts potassium forms KC1 4 I, rubidium RbCl 4 I, caesium 

 CsI 5 , CsBr, CsCl*!, lithium LiCl^I (with 4HaO) and sodium NaClJ (with 2H 2 0). The 

 most stable are those salts containing the metal with the greatest atomic weight csesium 

 (see Chapter XL, Note 68). 



4 It is possible to extract the compounds of potassium directly from the primary 

 rocks which are so widely distributed over the earth's surface and so abundant in some 

 localities. Prom a chemical point of view this problem presents no difficulty ; for in- 

 stance, by fusing powdered orthoclase with lime and fluor spar (Ward's method) and 

 then extracting the alkali with water (on fusion the silica gives an insoluble com- 

 pound with lime), or by treating the orthoclase with hydrofluoric acid (in which case 

 silicon fluoride is evolved as a gas) it is possible to transfer the alkali of the orthoclase 

 to an aqueous solution, and to separate it in this manner from the other insoluble oxides. 

 However, as yet there is no profit in, nor necessity for, recourse to this treatment, as 

 carnalli te and potash form abundant materials for the extraction of potassium compounds 

 by cheaper methods. Furthermore, the salts of potassium are now in the majority of 

 chemical reactions replaced by salts of sodium, especially since the preparation of sodium 

 carbonate has been facilitated by the Leblanc process. The replacement of potassium 

 compounds by sodium compounds not only has the advantage that the salts of sodium 

 are in general cheaper than those of potassium, but also that a smaller quantity of a 

 sodium salt is needed for a given reaction than of a potassium salt, because the combin- 

 ing weight of sodium (23) is less than that of potassium (89). 



5 It has been shown by direct experiment on the cultivation of plants in artificial 

 soils and in solutions that under conditions (physical, chemical, and physiological) other- 

 wise identical plants are able to thrive and become fully developed in the entire absence 

 of sodium salts, but that their development is impossible without potassium salts. 



