LEAD. 651
tin in an iron vessel; pour it into an earthenware mortar heated a little above theßielting point of the metal; trituvate briskly as the metal cools, ceasing as soon as aconsiderable portion is pulverized; sift the product, and repeat the process withWhat remains in the sieve.— The Dublin College Orders of the purest Tin, anyrequired quantity. Liquefied by heat, let it be strongly agitated until it passesinto a powder, which when cold is to be shaken through a sieve.)—Tin may bereduced to powder by shaking it when melted in a wooden box, the inside ofwhich has been rubbed with chalk. The doses and uses have been above de-scribed.
Tin Filings (Slanni Limatura seu Basura Slunni) have also been used inmedicine.
Order XXV.— LEAD AND ITS COMPOUNDS.
1. PLUM'BUM.—LEAD.
History. —This metal was known in the most remote ages of antiquity. It ismentioned by Moses. (Job, xix. 23,24.) The Greeks called it ^.oAtß^; thealchymists, Saturn.
Natural History. —It is found both in the metallic state (Native Leud) andmineralized. It is met with combined with sulphur (Galena,) with selenium, withchlorine (Hörn Leacl,) with oxygen (Na/ive Minium,) and with oxygen and anacid, forming an oxy-salt (Carbonate, Phosphate, Sulphate, Tungstate, Molyb-date, Chromate! Arseniate, and Aluminate.)
Preparation. —It is usually extracted from galena, which is roasted in reverbe-ratory furnaces, by which it is converted into a mixture of sulphate and oxide oflead, and afterwards smelted with coal and lime; the first to abstract oxygen, thesecond to remove the sulphur.
Properties. —It has a bluish-gray colour and considerable brilliancy. It maybe crystallized by cooling in four-sided pyramids. It is malleable, but not ductile.Its sp. gt. 11-35. It has a peculiar odour when handled. It fuses at 612° F. andat a red heat boils and evaporates. Its equivalent is 104. By exposure to theair it attracts, first oxygen, and then carbonic acid, so as to form carbonate oflead.
Pure distilled water has no action on lead, 1 if the gases (as air and carbonic acid)be excluded ; but if these be admitted, a thin crust of carbonate is soon formed. Itis remarkable that the presence of most neutral salts—sulphate of soda and chlorideof sodium, for example—impairs the corrosive action of air and water. (Chris-tison, 7'reatise on Poisons.) Hence, therefore, we can easily comprehend thereason why leaden cisterns and pipes do not more frequently give a metallic im-pregnation to water; and why rain-water is more apt than spring-water to becomeimpregnated with lead. The latter, however, by long keeping in leaden vessels,may also become contaminated with lead.
Characteristics. —If the lead be dissolved in nitric acid, we may easily recog-nise its presence in the Solution by the following tests:—Alkalis, their carbonates,sulphuric acid and the sulphates, and ferrocyanide of potassium, produce whiteprecipitates (which are respectively hydrated oxide, carbonate, sulphate, and fer-rocyanide of lead;) Chromate of potash and iodide of potassium occasion yeüowprecipitates (chromate and iodide of lead;) hydrosulphuric acid and the hydrosul-phates form black precipitates {mlphuret of lead;) lastly, a piece of zinc throwsdown metallic lead in an arborescent form.
i For some obserralions on ihis snbject sne Mr. Tsylor's memöir iu tta Guy's Hospital Reports, vol. iii.