+ CaO,H0 = CaO,CO, KO,H0.
The clear supernatant fluid is removed by decantation, or by means of a syphon, into a clean silver or iron basin, and is rapidly evaporated till it flows tranquilly like oil; it is then either cast into cylinders in metallic molds, or is poured upon a cold slab, and solidifies on cooling. It may also be obtained in acute rhombohedrons, if allowed to crystallize from a hot concentrated solution; the crystals containing four atoms of water. A. solution of hydrate of potash being one of our most important chemical reagents, it is very essential that it should be obtained pure. When obtained in the method that has been described, it is apt to be contaminated with carbonic, sulphuric, hydrochloric, an I silicic acids, lime, alumina, oxides of iron, etc., any of which substances can be detected by the appropriate tests. Pure caustic potash is, however, perfectly soluble in alcohol, which does not dissolve any of the impurities. Hence, by form in•; an alcoholic solution of potash, and by evaporating it in a silver vessel till the whole of the alcohol is expelled, we obtain this substance in a state of purity. Hydrated pot ash, on solidifying after fusion, occurs as a hard, grayish-white, opaque body, with a crystalline fracture, which may be readily again fused into a colorless oily fluid, but which only volatilizes at a very high temperature. It is soluble in about half its weight either of water or of alcohol, and rapidly absorbs both carbonic acid and moisture from the atmosphere. It acts as a powerful caustic, and quickly destroys both animal and vegetable tissues, and hence its solutions can only be filtered through pounded glass or sand. Its affinities are so powerful that few vessels are capable of resisting its influence. Its solution must be preserved in glass bottles, into the composition of which no oxide of lead enters, as it has the property of dissolving this oxide out of the glass. Vessels containing silica (porcelain, earthenware, etc.) are decomposed, and platinum itself is oxidized when heated in contact with it. The principal uses of this substance arc thus briefly summed up by prof. Miller: "Potash decomposes the fixed oils, and converts them into soluble soaps; when fused with siliceous minerals, it displaces the bases, and combines with the silica, forming silicate of potash. Potash is extensively employed in the arts: to the soap-boiler and the glass-maker, it is indispensable; when combined with nitric acid, it enters largely into the manufacture of gunpowder; and, in greater or less quantity, it furnishes important aids to a variety of processes employed in the manu factures of the country. In the laboratory, potash is in constant use for absorbing acid gases, such as carbonic acid, and for separating the metallic oxides from solutions of their salts, since, owing to the powerful affinity of the alkali for acids, it readily decom poses the salts of all the metals which produce oxides insoluble in of Chemistry, 2d ed., vol. ii. p. 353.
The salts which potash forms with acids are for the most part readily soluble in water, and colorless, unless (as, for example, in permanganate of potash) the acid is colored. Most of them are crystallizable, and they all communicate a violet tint, charac teristic of potash, to the flame of spirit of wine and to that of the blow-pipe. 3lany of them occur in the animal and vegetable kingdoms, and the ashes of plants contain them in large quantity, We shall now briefly notice the most important of these salts. Car
bonate of potash is obtained by burning plants in dry pits, dissolving the ashes in water, evaporating till the sulphates, chlorides, etc., separate in crystals, and dim boiling the mother liquid to dryness in iron pots. See PoTAsit. The quantity of pure carbonate of potash contained in it is liable to great variation, and the process termed etlkalimetry has been invented, with the view of rapidly determining the amount of this salt, on which the commercial value of the pearl-ash depends. Different plants furnish varying quantities of this salt, and the leaves and young shoots arc the parts which fur nish it most abundantly. The potash is, of course, obtained by the plants from the soil, which, when capable of supporting vegetable life, always contains that substance; and does not exist in the plants in the form of carbonate, but in union with various organic acids (such as acetic, mall; tartaric, and other acids), which, by incineration, become decomposed into carbonates. The purified carbonate of potash, employed in pharmacy and for chemical purposes, is prepared from the crude salt by adding an equal quantity of cold water, agitating, and filtering. By this means, all the less soluble foreign bodies are got rid of. The solution is then boiled down to a small bulk, and allowed to cool, when the carbonate separates in small crystals, containing 20 per cent of water, and represented by the formula 2Aq. Carbonate of potash is extremely deliques cent, and is soluble in less than its own weight of water, but is insoluble in alcohol. It has an acrid, alkaline taste, and its reaction upon is strongly alkaline. It is a compound of great importance, both as a chemical re-agent, and as entering largely into the preparation of most of the other compounds of potash, and into the manufacture of soap and glass. Bicarbonate of potash (K0,00, + HO, is obtained in white rhombic prisms, by passing a current of carbonic acid gas through a strong solution of carbonate of potash. These crystals are permanent in the air, but are decomposed by heat; water and carbonic acid being evolved, and the simple carbonate left. This salt is much less soluble than the carbonate, requiring four parts of cold water for its solution, which Is nearly neutral to test-paper, and has a much milder taste than the preceding salt. It is li eployed largely in medicine for making effervescing draughts. of potash (formerly known as sal-polychrest) is obtained by dissolving in water the acid residue of bisulphate of potash (KO, SO. + IIO,S02) which is left in the retort in the preparation of nitric acid. This solution, on being neutralized with carbonate of potash, furnishes hard transparent crystals of this salt. From its extreme hardness, this salt is used in medicine (as, for example, in Dover's powder), for the purpose of finely comminutinm vegetable matters. The bisulphate of potash, from which the preceding salt is obtained, is the sal-enizam of the older chemists. Except that it is occasionally employed as a flux, it is of no special importance. _Nitrate of potash has been already described under the head NrrEft. Chlorate of potash (KO,C10.) occurs in white rhomboidal tablets of a pearly luster. It has a cooling taste like that of niter. It fuses at a gentle heat without decomposition; but on increasing the heat, it gradually gives off all its oxygen, and is converted into chloride of potassium, according to the equation: Chlorate of Potash. Chloride ofOxygen.