POTASSIUM, KUBIDIUM, CAESIUM, AND LITHIUM 549 



may also be obtained from the chloride by a method similar to that by 

 which sodium carbonate is prepared from sodium chloride. 8 bis There is no 

 difficulty in obtaining any salt of potassium for example, the sulphate, 9 



quantities (as a sediment from wine) for medicinal use under the name of cream of tartar. 

 When ignited without access of air, it leaves a mixture of charcoal and potassium 

 carbonate. The charcoal so obtained being in a finely-divided condition, the mixture 

 (called ' black flux '), is sometimes used for reducing metals from their oxides with the 

 aid of heat. A certain quantity of nitre is added to burn the charcoal formed by 

 heating the cream of tartar. Potassium carbonate thus prepared is further purified by 

 converting it into the acid salt, by passing a current of carbonic anhydride through 

 a strong solution. KHC0 3 is then formed, which is less soluble than the normal salt 

 (as is also the case with the corresponding sodium salts), and therefore crystals of the 

 acid salt separate from the solution on cooling. When ignited, they part with thei* 

 water and carbonic anhydride, and pure potassium carbonate remains behind. The 

 physical properties of potassium carbonate distinguish it sufficiently from sodium 

 carbonate ; it is obtained from solutions as a powdery white mass, having an alkaline 

 taste and reaction, and, as a rule, shows only traces of crystallisation. It also attracts 

 the moisture of the air with great energy. The crystals do not contain water, but 

 absorb it from the air, deliquescing into a saturated solution. It melts at a red heat 

 (1045), and at a still higher temperature is even converted into vapour, as has been 

 observed at glass works where it is employed. It is very soluble. At the ordinary 

 temperature, water dissolves an equal weight of the salt. Crystals containing two 

 equivalents of water separate from such a saturated solution when strongly cooled (Morel 

 obtained KaCOgSHjO in well-formed crystals at 4- 10). There is no necessity to de- 

 ctcribe its reactions, because they are all analogous to those of sodium carbonate. When 

 manufactured sodium carbonate was but little known, the consumption of potassium 

 carbonate was very considerable, and even now washing soda is frequently replaced 

 for household purposes by 'ley' i.e. an aqueous solution obtained from ashes. 

 It contains potassium carbonate, which acts like the eodium salt in washing tissues, 

 linen, &c. 



A mixture of potassium and sodium carbonates fuses with much greater ease than the 

 separate salts, and a mixture of their solutions gives well-crystallised salts for instance 

 (Marguerite's salt), K 2 C03,6H2O,2Na2C0 3 ,6H 2 O, "Crystallisation also occurs in othe? 

 multiple proportions of E and Na (in the above case 1 : 2, but 1 : 1 and 1 : 8 are known), 

 and always with 6 mol. H-jO. This is evidently a combination by similarity, as in alloys, 

 solutions, &c. 



8 About 25,000 tons of potash annually are now prepared from KC1 by this method 

 ftt Stassfurt. 



9 Potassium, sulphate, KjSO^, crystallises from its solutions in an anhydrous condi- 

 tion, in which respect it differs from the corresponding sodium salt, just as potassium car- 

 bonate differs from sodium carbonate. In general, it must be observed that the majority 

 of sodium salts combine more easily with water of crystallisation than the potassium 

 salts. The solubility of potassium sulphate does not show the same peculiarities as that 

 of sodium sulphate, because it does not combine with water of crystallisation ; at the 

 ordinary temperature 100 parts of water 'dissolve about 10 parts of the salt, at 8'8 

 parts, and at 100 about 26 parts. The add sulphate, KHSO 4 , obtained easily by 

 heating crystals of the normal salt with sulphuric acid, is frequently employed in 

 chemical practice. On heating the mixture of acid and salt, fumes of sulphuric acid are 

 at first given off; when they cease to be evolved, the acid salt is contained in the residue. 

 At a higher temperature (of above 600) the acid salt parts with all the acid contained in 

 it, the normal salt being re-formed. The definite composition of this acid salt, and the 

 ase with which it decomposes, render it exceedingly valuable for certain chemical trans- 

 formations accomplished by means of sulphuric acid at a high temperature, because it ia 

 possible to take, in the form of this salt, a strictly definite quantity of rolphurio acid, 



