Lond. Sub-carbonas Potassae Impurus, Edin. Lixivius Cinis1, Dub. Impure Carbonate of Potassa. Impure Subcarbonate of Potassa. Potashes. Pearl-ashes.

Syn. Carbonate alkalinule de Potasse (F.), Koloensuares Kali, Pottasche ( G.), Potash (Dutch), Potaske (Dan.), Potaska (Swed.), Potassa del Commercio (I.), Cenizas claveladas ( S.), Marra Ooppoo ( Tarn.), Hindee loonoo ( Cyng.), Jhar ka nemuck (Duk.), Kshara-lavana (Sans.).

This substance consists chiefly of subcarbonate of potassa, mixed with some other salts. It is known in commerce by the name of potash; and is brought to us principally from the Baltic and America.2 The manipulation of the process by which it is prepared differs in different countries; but the general features of it are every where the same. The dried stems and branches of plants are set fire to, and reduced to ashes; which are lixiviated by pouring over them, in proper vessels, hot or cold water, so as to dissolve the alkaline matter they contain. The impregnated solution, drawn off from the ashes, is then boiled to dryness in iron boilers, and leaves behind a solid saline mass, coloured brown by a small portion of vegetable inflammable matter, and which generally becomes moist. This is the potash of commerce. After the colouring matter is destroyed, and a portion of the water dissipated by calcination in a reverberatory furnace, it assumes a spongy texture, with a bluish or greenish colour, and is then denominated pearl-ashes.

Those vegetables only which grow at a distance from salt water are employed to obtain this product. Herbaceous plants yield the largest proportion, and shrubs more than trees. Kirwan remarks, that although fumitory produces more of this salt than any other plant, and next to it wormwood, yet, that 1000 parts of the ashes of wormwood yield more potassa than the same quantity of the ashes of fumitory, in the proportions of 748 and 360. It has been said that it was lately obtained in great abundance from the herbaceous part of the potato, cut down just as the fruit is beginning to form: 40,000 lbs. of the dried stems, it has been stated, will yield 2200lbs. of impure potassa; but the trials in this country have not confirmed these statements.3 It is generally supposed, that at least the greater part of the potassa is contained ready-formed in the vegetables; but this is somewhat doubtful, and perhaps in living plants the base only of potassa exists as an element, and is oxidized so as to form the alkali during the combustion.

Such is the conjecture of Dr. Murray1; and the same may take place during the spontaneous decomposition of plants where much water is present, for potassa can be obtained by the evaporation of dunghill water.2

1 Of these three appellations, that of the Dublin College is the least exceptionable, because it does not convey an erroneous idea of the nature of the substance, which is the case with the others.

2 Brunswick makes about 80 cwts. of potassa yearly.

3 Phil Mag. vol. i. p. 340.

The pearl-ash of commerce is still a very compound mass, containing, besides the subcarbonate of potassa, sand, with which it is often adulterated to a great extent, sulphate of potassa, chloride of potassium, oxide of iron, and oxide of manganese; to the last of which, according to Scheele, it owes its bluish or greenish colour. Different parcels of pearl-ash must undoubtedly contain different quantities of potassa; and thence no accurate standard of the proportion of the ingredients can be fixed. The following table, drawn up by Vauquelin, shows the comparative value of samples from different countries examined by him. The quantity of each was 1152 parts.3

The proportion of real alkali in any quantity of pearl-ash may be ascertained in the following manner:-Pulverize 100 grains of the pearl-ash, and digest in successive portions of hot water as long as anything is dissolved; filter to separate all the insoluble matter; and add sulphuric acid, according to Mr. Faraday's alkalimeter, in successive small quantities, until litmus paper shows that the whole of the

1 Murray's Chymistry, 2d ed. ii. 193.

2 See Birch's Experiments, Phil, Trans, for 1780, p. 345.

3 Annales de Chimic, xl. 284, alkali is neutralized. The quantity of the diluted acid contained in each division of the tube, which has been used indicates one grain of potassa.1 The alkalimeter is a tube 9 1/2 inches long, and 3/4 of an inch in diameter. It is graduated by pouring into it 1000 grains of water, and marking the point to which it reaches with a file or diamond: divide the space occupied by the water into 100 equal parts; and write the words in the marginal cut opposite to the numbers 23.44-48.96- 54.63 and 65. To use the tube, prepare a diluted acid of 1.127 sp. gr. at 60°, by mixing one measure of concentrated sulphuric with eight measures of distilled water: pour as much into the tube as will reach the mark 48.96 : this quantity should neutralize 100 grains of potassa; so that, by proceeding as above described, we readily ascertain the quantity of real potassa present in the specimen of pearl-ashes.

The pearl-ash of commerce is not sufficiently pure for medicinal use; and therefore it is used only for pharmaceutical purposes.

Carbonate of potassa is also procured by calcining bisulphate of potassa with charcoal in a reverberatory furnace: a sulphuret is formed, which is decomposed by roasting; the sulphur is dissipated, the potassium is oxidized, and the potassa combined with carbonic acid.

Officinal preparation.-Potasses Carbonas, L. E. D.