540 PRINCIPLES OF CHEMISTRY 



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

 which sodium carbonate is prepared from sodium chloride. There 



considerable quantity of potassium compounds. However, cow's milk contains but little 

 potassium salts. Sodium compounds generally predominate in animal bodies. The 

 excrement of animals, and especially of herbivorous animals, on the contrary, often con- 

 tains a large proportion of potassium salts. Thus sheep's dung is rich in them, and in 

 washing sheep's wool salts of potassium pass into the water. 



The ash of tree stems, as the already dormant portion of the plant (Chap. VIII. 

 Note 1), contains little potash. For the extraction of potash, which was formerly (before 

 the discovery of the Stassfurt salt) carried on extensively in the east of Russia, the ash 

 of grasses, and the green portions of potatoes, buckwheat, &c., are taken and treated 

 with water (lixiviated), the solution is evaporated, and the residue ignited in order to 

 destroy the organic matter present in the extract. The residue thus obtained is com- 

 posed of raw potash. It is refined by a second dissolution in a small quantity of water, 

 because the potash itself is very soluble in water, whilst the impurities are sparingly 

 soluble. The solution thus obtained is again evaporated, and the residue ignited, and 

 this potash is then calle refined potash, pearlash. This method of treatment cannot 

 give chemically pure potassium carbonate. A certain amount of impurities remain. 

 To obtain chemically pure potassium carbonate, some other salt of potassium is gene- 

 rally taken and purified by crystallisation. Potassium carbonate crystallises with diffi- 

 culty, and it cannot therefore be purified by this means, whilst other salts, such as the 

 tartrate, acid carbonate, sulphate, or nitrate, &c., crystallise easily and may thus be 

 directly purified. The tartrate is most frequently taken, all the more as it is prepared 

 in large quantities for medicinal use under the name of cream of tartar. When 

 ignited without the access of air, it leaves a mixture of charcoal and potassium car- 

 bonate. The charcoal is then obtained in a finely-divided condition (this mixture is 

 called ' black flux,' and is sometimes used for reducing metals from their oxides, by 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. KHCO 3 is then formed, which is less soluble than the normal salt 

 (as is 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 their water 

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

 properties of potassium carbonate distinguish it with sufficient distinctness 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 

 (880), 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. 

 There is no necessity to describe its reactions, because they are entirely 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 

 sodium salt in washing tissues, linen, &c. 



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

 separate salts, and a mixture of their solutions gives salts of a fine crystalline form for 

 instance (Marguerite's salt), K 2 CO;;,6H 2 O,2Na 2 CO3,6H 2 O. Crystallisation also pro- 

 ceeds in other multiple proportions of K and Na (in the above case 1 : 2, but 1 : 1 and 1 : 3 

 are known), and always with 6 mol. H 2 O. This is evidently a combination by analogy, as 

 in alloys, solutions, &c. 



