470

Alkalies and Neutral Salts.

pakt I IT.

" Take of subcarbonate of kali, a pound ; crystals of tartarreduced to a very fine powder, two pounds and a half, or aquantity sufficient to saturate the kali ,■ boiling water, a gallon.To the subcarbonate of kali dissolved in the water graduallyadd the tartar; filter the solution through paper, evaporate,and set it aside that crystals may form as it cools."

In these processes the superabundant acid of the supertar-trate of potash is saturated by the potash of the subcarbonate,and a neutral tartrate obtained. The quantity of alkali re-quired for this purpose must necessarily vary, owing to thedegree of dryness of the subcarbonate employed. To obtainregular crystals, a very slow nearly spontaneous evaporationis necessary: and, therefore, this salt as found in the shops,and prepared on a large scale, is in the form of a white granu-lar powder, which is produced by the evaporation being con-tinued to dryness with frequent stirring.

Qualities. — This salt has a bitterish cool taste. Its crys-tals are tetrahedral prisms, terminated by dihedral summits;and in this state it is soluble in its own weight of water at 60°;but in the granular form, four parts of cold water are requiredfor its solution. When long kept in solution, its acid is de-composed, and its alkali remains in the state of a subcarbon-ate.' Alcohol also dissolves it readily. It is partially decom-posed by even the weaker acids and acidulous solutions, as oftamarinds and other acid fruits, which reduce it to the stateof supertartrate; and it is completely decomposed by lime-water, muriate of barytes, magnesia, nitrate of silver, and ace-tate and superacetate of lead. Its constituents, abstracting thewater of crystallization, are, in 100 parts, 58'69 of acid, and41-31 of alkali.*

Medical properties and uses. — Tartrate of potash is a valu-able purgative, operating easily and without griping; and evencorrecting the griping properties of some other substances, asof senna and the resinous purgatives, with which it is, there-fore, usually combined. The dose is from 5j to 3j in solution.

AQUA SUPERCARBONATIS POTASS^. Edin. Wa-ter of Supercarbonate of Potash.

"Take of water, ten pounds; pure subcarbonate of potash,one ounce ,■ dissolve, and expose the solution to a current ofcarbonic acid gas arising from carbonate of lime in powder,three ounces ; sulphuric acid, three ounces; and water, threepounds; gi-adually and cautiously mixed. The chemical appa-ratus of Dr. Nooth is well adapted for this preparation. Butif a larger quantity of the solution be required, an apparatus

» Murray's Chemistry, 2d. ed. iv. 029.

» Btnelius.