rOTASII.
273
Order 10. Compounds of Potassium.
HiSTOiiy. —Caustic alkaline solutions were probably known to theGreeks and Romans. We learn from Pliny {Hist. Nat. lib. xxviii. cap. li.),that soap was made in his time from tallow and wood-aslies ; and we maytherefore conclude that some method was known of depriving the alkalinecarbonate of its carbonic acid. Geber ( Invention of Verity, ch. iv.)describes the method of making caustic alkali. Black, however, in theyear 1756, first distinguished, chemically, the caustic alkalies from theircarbonates. Potash was formerly called kali, or the vegetable alkali.
Naturae History. —Potash in combination with acids is found inboth kingdoms of nature.
{a.) In the inorganized kingdom. —Potash is found, in the mineralkingdom, in combination with sulphuric, nitric, silicic, and perhaps car-bonic acids. As an ingredient of rocks, it is more abundant than soda.
[b.) In the organized kingdom. —In organized beings potash is met within combination with phosphoric, sulphuric, nitric, carbonic, and variousorganic acids. It occurs more abundantly in vegetables than in animals.
Preparation, {a.) Of aqum pot as see. —To prepare a solution of causticpotash, add fresh burned lime to a solution of the carbonate of potash.In the London Pharmacopoeia the proportions employed in the manufac-ture of liquor potassce are fifteen ounces of the carbonate, eight ounces oflime, and a gallon of boiling distilled water. The lime, when slackedand mixed with half a gallon of water, is to be added to the carbonate ofpotash dissolved in the remaining half gallon of water, The mixture isto be kept in a close vessel, and frequently shaken. When cold, thesupernatant clear liquor is to be poured off: this is the liquor potassce ofthe Pharmacopoeia .
In this process the lime abstracts carbonic acid from the carbonate ofpotash, and the potash thus set free dissolves in the water.
REAGBNTS.
Water-—
1 eq. Carb. Potash 701 eq. Lime .... 28-
. 1 eq. Potash . . .
1 eq. Carbon 0 . A 11 . 22
RESULTS.
Liquor Potassse (Ph. L.)
1 eq. Carbon e . Lime . .
50
It is best to avoid filtering the liquid, but to decant it. If, however, afilter be employed, it should be of cotton or linen, and the atmosphericair should be, as much as possible, excluded; as the solution readilydecomposes paper and woollen cloth, and abstracts carbonic acid fromthe air.
(b.) Of hydrate of potash. —If liquor potassse be evaporated to drynessin a clean iron vessel, and the residual mass fused and poured into moulds,we obtain the hydrate of potash of the Pharmacopoeia , and which wasformerly called potassafusa, kali pwrwm, lapis infernalis sive septicus, orcauterium potentiale.
(c.) Of potash with lime. —To render the potash less deliquescent it isrubbed with an equal weight of lime, by which is procured the mixturetermed potassa cum calce.
Properties. («.) Of hydrate of potash. —The solid sold in the shops