LEAD.

505

Administration. —The usual mode of exhibiting it is mixed withtreacle : the dose usually stated in pharmacological works is one or twodrachms, but Alston gave much larger quantities ; his ljiodo of employingit as a vermifuge was the following:—The patient was well purged withsenna, and on the following morning one ounce of tin powder was givenin four ounces of treacle ; on each of the two following days half thisquantity was taken, and then the patient again purged. However, tinpowder is certainly much inferior to oil of turpentine as a remedy fortape-worm.

Order 24.—Lead and its Compounds.

Plum'bum. — Lead.

History. —This metal was known in the most remote ages of antiquity.It is mentioned by Moses [Job, xix. 23, 24.) The Greeks called itIluKi(]ooq ; the alchymists, Saturn.

Natural History. —It is found in the metallic state {native lead),combined with sulphur [galena), with selenium, with chlorine ( hornlead), with oxygen ( native minium), and with oxygen and an acid, formingan oxy-salt {carbonate, phosphate, sulphate, tungstate, molybdate, chromate,arseniate, and aluminate)

Preparation. —It is usually extracted from galena, which is roastedin reverberatory furnaces, by which it is converted into a mixture ofsulphate and oxide of lead, and afterwards smelted with coal and lime,the first to abstract oxygen, the second to remove the sulphur.

Properties. —It has a bluish-gray colour and considerable brilliancy.It may be crystallized by cooling in four-sided pyramids. It is malleable,but not ductile. Its sp. gr. 1P35. It has a peculiar odour whenhandled. It fuses at 612° F. and at a red heat boils and evaporates.Its equivalent is 104. By exposure to the air it attracts, first oxygen,and then carbonic acid, so as to form carbonate of lead.

Pure distilled water has no action on lead, if the gases (as air andcarbonic acid) be excluded; but if these be admitted, a thin crust ofcarbonate is soon formed. It is remarkable that the presence of mostneutral salts—sulphate of soda and chloride of sodium, for example—impairs the corrosive action of air and water. Hence, therefore, we caneasily comprehend the reason why leaden cisterns and pipes do not morefrequently give a metallic impregnation to water; and why rain-water ismore apt than spring-water to become impregnated with lead. Thelatter, however, by long keeping in leaden vessels, may also becomecontaminated with lead.

Characteristics. —If lead be dissolved in nitric acid, we may easilyrecognise its presence in the solution by the following tests:—Alkalies,their carbonates, sulphuric acid and the sulphates, and ferrocyanide ofpotassium, produce white precipitates ; chromate of potash and iodide ofpotassium occasion yellow precipitates; hydrosulphuric acid and thehydrosulphates form black precipitates of the sulphuret of lead ; lastly,a piece of zinc throws down metallic lead in an arborescent form.

The delicacy of these tests is, according to Devergie {Med. Leg. ii. 779),as follows:—