POTASS i TM. Krninit'M. CAMS i r:\r, AND LITIIII \M 537 



chloride itself is sometimes found at Stassfurt as sylvine. By a 

 method of double saline decomposition, the chloride of potassium 

 may be converted into all the other potassium salts, 4 some of 

 Avhich are of practical use. The potassium salts have, however, 

 their greatest importance as an indispensable component of the food 

 of plants/ 5 



When mixed with solutions of other salts the solubility of potassium chloride naturally 

 varies, but not to any great extent. The specific gravity of the solid salt is 1'99 that is, 

 less than that of sodium chloride. All the salts of sodium are specifically heavier than 

 the corresponding salts of potassium, as are also their solutions for equal percentage 

 compositions. If the specific gravity of water at 4 = 10000, then at 15 the specific 

 gravity of a solution of p p.c. potassium chloride = 9992 + 63'29j? + 0'226 p 2 , and therefore 

 for 10 p.c. = 10647, 20 p.c. = 11348, &c. 



Potassium chloride combines with iodine trichloride to form a compound KC1 + IC1 5 = 

 KIC1 4 , of a yellow colour, which is fusible, loses the iodine trichloride at a red heat, and 

 .gives potassium iodate and hydrochloric acid with water. It is not only formed by 

 direct combination, but also by many other methods ; for instance, by passing chlorine 

 into a solution of potassium iodide so long as the gas is absorbed KI + 2C1 2 = KCl,ICl.- ( . 

 Potassium iodide, when treated with potassium chlorate and strong hydrochloric acid, 

 also gives this compound ; another method for the formation of which is shown by the 

 equation KC1O 5 + 1 + OHC1 = KC1,IC1 3 + 3C1 + 3H 2 O. This is a kind of a salt correspond- 

 ing with KIOo (unknown) in which the oxygen is replaced by chlorine. If atomicity is 

 taken as a starting point in the study of chemical compounds, and if the elements are 

 considered as having a constant atomicity (number of bonds) that is, if K, Cl, and I 

 be counted as univalent elements then it is impossible to explain the formation of 

 such a compound, because, according to this view, univalent elements are only able to 

 form dual compounds with each other ; for example, KC1, C1I, KI, &c., whilst here they 

 are grouped together in the molecule KIC1 4 . 



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

 rocks which are, especially in some localities, so widely distributed over the earth's 

 surface. From 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 (in the 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. 

 Thus after treating with hydrofluoric acid, many fluorides are obtained in solution, 

 chiefly the fluorides of aluminium and potassium. If the solution be evaporated and 

 sulphuric acid then added, hydrofluoric acid is evolved and the metals are obtained as 

 sulphates. On adding ammonia to the solution of these salts, the aluminium is precipi- 

 tated as hydroxide, and the salts of ammonia and potassium remain in solution. The 

 ammonia salt is then decomposed by igniting, and the potassium sulphate is obtained 

 alone. However, as yet there is no profit in, nor necessity for, having recourse to this treat- 

 ment, as there still exist abundant sources 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 Leblanc's method. The replacement of potassium 

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

 sodium are in general cheaper than those of potassium, but also that a less 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 (39). 



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. 



