648 PBINCIPLES OF CHEMISTRY 



This salt also separates ad a sediment from wine. When the plants, 

 containing one or more of the salts of potassium, are burnt, the 

 carbonaceous matter is oxidised, and in consequence the potassium is 

 obtained in the ash as carbonate, K 2 C0 3 , which is generally known 

 as potashe*. Hence potashes occur ready prepared in the ash of 

 plants, and therefore the ash of land plants is employed as a source 

 for the extraction of potassium compounds. Potassium carbonate ia 

 extracted by lixiviating the ash with water. 8 Potassium carbonate 



pressed thus two substances, A and B, do not react on each other of their own accord, 

 but the addition of a 'small quantity of a third particularly active substance, C, produces 

 the reaction of A on B, because A combines with C, forming AC, and B reacts on this new 

 compound, which has a different store of chemical energy, f orming-the compound AB or 

 its products, and setting C free again or retaining it. 



It may here be remarked that all tho 'mineral substances necessary for plants (those 

 enumerated at the beginning of the note) are the highest saline compounds of their 

 elements, that they enter into the plants as salts, that, the lower forms of oxidation of the 

 same elements (for instance, sulphites and phosphites) are harmful to plants (poisonous), 

 and that strong solutions of the salts assimilated by plants (their osmotic pressure being 

 great and contracting the, cells, as De Vries showed, (see Chapter I., Note 19) not only 

 do not enter into the plants but kill them (poison them) 



Besides which, it will be understood from the preceding paragraph, that the salts of 

 potassium may become exhausted from the soil by long cultivation, and that there may 

 therefore be cases when the direct fertilisation by salts of potassium may be profitable. 

 But manure and animal excrements, ashes, and, in general, nearly all refuse which may 

 serve for fertilising the soil, contain a considerable quantity of potassium salts, ana 

 therefore, as regards the natural salts of potassium (Stassfurt), and especially potassium 

 sulphate, if they often improve the crops, it is in all probability due to their action 

 on the properties of the soil. The agriculturist cannot therefore be advised to add 

 potassium salts, without making special experiments showing the advantage of such a 

 fertiliser on a given kind of soil and plant. 



The animal body also contains potassium compounds, wiiich is natural, since animals 

 consume plants. For example, milk, and especially human milk, contains a somewhat con- 

 siderable quantity of potassium compounds. Cow's milk, however, does not contain much 

 potassium salt. Sodium compounds generally predominate in the bodies of animals. 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 (Chapter VTEL, 

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

 on extensively in the east of Bussia (before the discovery of the Stassfurt salt), the ash 

 of grasses, and the green jportions 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, 

 for 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 called refined potash, or pearlash. This method of treatment cannofl 

 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, sttch as the 

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

 directly purified. The tartrate is most frequently employed, since it is prepared in large 



