CARBONATE OF POTASH.

299

of soda, it is impossible to determine what share the chlorate had in pro-ducing the beneficial effects said to have been obtained by what iscalled the saline treatment of these diseases.

It appears, then, that all the uses of this salt have been founded oncertain views of chemical pathology, some of which are now considereduntenable. It is very desirable, therefore, that some person, unbiassedby theoretical opinions, would carefully investigate its effects and uses,which I am inclined to think have been much overrated.

Administration. —The usual dose of it is from ten or fifteen grains tohalf a drachm. Dr. Wittman, in one case, gave 160 grains daily, with alittle hydrochloric acid immediately after it, to decompose it: the effectswere hot skin, headache, quick, full, and hard pulse, white tongue, andaugmentation of urine.

Potas'sm Car'bonas.—Carbonate of Pot'ash .

History. —It is probable that the ancient Greeks, Romans, andEgyptians, were acquainted with this salt. Pliny [Hist. Nat. lib. xiv.and xxviii.) describes some of the uses of wood-ashes, and mentions alye of them (' cineris lixivium). For a long period carbonate of potashwas confounded with carbonate of soda. Geber ( Invent. of Verity , ch. iv.),in the eighth century, describes the method of procuring it by the com-bustion of tartar. It has been known by various names ;. such as, salt oftartar, mild vegetable alkali, fixed nitre, and subcarbonate of potash.

Natural History. —Reuss (Gairdner, On Mineral Springs, p. 18)found carbonate of pptash in the waters of the Wuissokow, and in thechalybeate of Twer.

It is formed, during the combustion of inland plants, by the decompo-sition of the vegetable salts of potash (the acetate, the malate, and theoxalate, but principally the first). Hence it is procured in great abun-dance from wood-ashes. In some few cases it has been supposed to existready formed in plants, as in a fern referred to by Mr. Parkes [ChemicalEssays, vol. ii. p. 17), the expressed juice of which is employed by thepoor weavers of Yorkshire , in the cleansing of cloth at the fulling mills.

Preparation. —It is principally obtained from wood-ashes (cineresvegetabilium, seu cineres e lignis combustis). These are procured byburning wood piled in heaps on the ground, sheltered from the wind, or inpits.— [For an account of the proportion and composition of wood-ashes,see Berthier, Traite des Essais, t. l er . p. 259).] The soluble constituentsof the ashes are, carbonate, sulphate, phosphate, and silicate of potash,and chlorides of potassium and sodium. The insoluble constituentsare, carbonate and subphosphate of lime, alumina, silica, the oxides ofiron and manganese, and a dark carbonaceous matter. In America theashes are lixiviated in barrels with lime, and the solution evaporated inlarge iron pots or kettles, until the mass has become of a black colour,and of the consistence of brown sugar. In this state it is called by theAmerican manufacturers black salts [cineres clavellati crudi). The darkcolour is said by Dumas to be owing to ulmate of potash.

To convert this substance into the pot-ash of commerce [cineres clavel-lati calcinati), it is heated for several hours, until the fusion is complete,and the liquid becomes quiescent. It is then transferred by large ironladles into iron • pots, where it congeals in cakes. These are broken up,