"Take of impure carbonate of potassa, two pounds; water, one pint and a half. Dissolve the impure potassa in the distilled water, and filter; then pour the solution into a suitable vessel, and evaporate the water that the liquor may thicken; then stir assiduously with a spatula, until the salt concretes."

"A purer carbonate of potassa may be prepared from the crystals of bicarbonate of potassa heated to redness."

Subcarbonas Potassae, Edin. Subcarbonate of Potassa.

"Let impure carbonate of potassa be put into a crucible, and exposed to a red heat; then triturate it with an equal weight of water. Pour the solution, after the impurities have subsided, into a clean iron pot, and boil it to dryness; stirring the salt constantly towards the end of the boiling, to prevent it from adhering to the vessel."

1 Pharmacologicia,.

2 Kali praeparatum, P. L. 1787. Potassae Subcarbonas, P. Tr. 1824.

Potassae Carbonas e Lixive Cinere, Dub. Carbonate of Potassa from Pearl-ashes.

"Take of potashes coarsely powdered, cold water, each one part. Mix them by trituration, and macerate them in a wide vessel for the space of a week, with frequent agitation; then filter the solution, and evaporate it to dryness; and towards the end of the process assiduously stir the saline mass with an iron spatula. In this manner having reduced it to a coarse powder, preserve it in well-stopped vessels. Previous to dissolving the ashes in the water, if they be very impure, roast them in a crucible until they become white."

Syn. Soucarbonate de Potasse (F.), Kohlensaures Kali (G.), Unvollmaakle Kohlenstoffzuure Potasch (Dutch), Sotto-carbonato di Potassa (I.).

The potash, or pearl-ash of commerce, is a heterogeneous mass, consisting chiefly of carbonate of potassa, with small portions of sulphate of potassa, hydrochlorate of potassa, silicious earth, oxide of iron, and oxide of manganese, in various proportions. (See Part ii.) The above processes are intended to separate the carbonate of potassa in as pure a state as possible; and by following the directions of any of the Pharmacopoeias, it is obtained sufficiently pure for medicinal purposes; while the insoluble metallic salts, and the greater part of the silicious earth, are left on the filter when the solution is strained. The present process is improved by cold instead of boiling distilled water being used.

Qualities. - The salt obtained by the above processes is really a carbonate, being composed of one atom of each of its components. In its dry state it consists of carbonic acid 31.43, potassa 68.57, in 100 parts, or of 1 eq. of potassa = 47.15+1 of carbonic acid = 22.12, making the equivalent 69.27; but as a hydrate, the state in which it usually exists, it contains 16 per cent. of water. It is in coarse white grains, which are so deliquescent, that they soon attract from the air as much water as dissolves them, forming a fluid of the consistence of oil: thence the salt must be kept in well-stopped bottles. Its taste is acrid and urinous : it changes to green the vegetable blue and red colours, combines with oils, and forms soaps, and is decomposed by acids with effervescence.1 It does not decompose tartrate of iron, with which it may be, therefore, ordered in prescriptions.