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ELEMENTS OK MATERIA MEDICA.
3. Emplastrum Saponjs, Ph . L. & Dub. (soap, lb. ss.; lead plaster,lb. iii.) This quantity of soap is said to be too much by one half. Thisplaster, spread on leather, is employed as a discutient and mechanicalsupport. When two parts of it are mixed with three parts of resinousplaster ( emplastrum resinee, Ph . L.), it constitutes the emplastrum saponiscompositum, vel emplastrum adhcerens, Ph . D., and which, when spreadby a machine on linen, is sold in the shops by the name of adhesiveplaster (Dr. Duncan, Edinb. Dispensatory).
Order 12. Compounds of Barium .
Bary tee Car bonas. — Car'bonate of Bary ta.
History. —The earthy base called baryta was discovered in 1774 bySclieele. It was at first termed terra ponderosa. In 1783 Dr. Witheringrecognised the native carbonate of baryta, which has been called, after itsdiscoverer, Witherite.
Natural History. — Witherite occurs in the lead mines of the northof England (as those of Anglesark, in Lancashire ), and in various otherlocalities. The mineral called baryto-calcite (a compound of carbonateof baryta and carbonate of lime) is found at Alston Moor, Cumberland.— [For some curious anecdotes respecting its discovery at the latterplace, consult Parkes’s Chem. Essays, vol. i. p. 324].
Preparation. —The native carbonate of baryta is sufficiently pure forthe preparation of the other barytic salts, and is the kind meant in thePharmacopoeia . Absolutely pure carbonate may be prepared by theaddition of a pure alkaline carbonate to a solution of some barytic salt;as the sulpliuret (obtained from the decomposition of the sulphate bysome carbonaceous matter), or the chloride; or it may be obtained byigniting (or boiling in water) finely powdered sulphate of baryta withthree parts of carbonate of potash, and washing away the resulting sul-phate of potash; or by decomposing sulphate of baryta by carbonate ofammonia.
Properties. —Native carbonate of baryta occurs massive, stalactiticand crystallized: the primary form of the crystals is the right rectangularprism. The sp. gr. of this mineral is 4'3. Heated before the blowiipeit melts into a white enamel, with the evolution of much light and theloss of carbonic acid. Artificially prepared, carbonate is a fine, tasteless,odourless powder. It is almost insoluble in both hot and cold water:4,304 parts of cold, or 2,304 parts of hot water, being required to dissolveone part of carbonate. It is more soluble in carbonic acid water.
Characteristics.— It dissolves with effervescence in hydrochloricacid, forming a solution of chloride of barium: the evolved gas, whencollected, is found to be carbonic acid ( vide p. 190). The hydrochloricsolution is not precipitated by ammonia, the liydrosulphurets, or theferrocyanides; but the soluble sulphates, phosphates, and carbonates,throw down white precipitates, which are, respectively, sulphate, phos-phate, oxalate, and carbonate of baryta: sulphate of baryta is insolublein both water and nitric acid. The salt (chloride of barium) obtainedby the evaporation of the hydrochloric solution, tinges the flame ofalcohol greenish yellow.