White lead is made either, 1. by the reaction of the vapour of vinegar, and exhalations from decomposing stable manure, upon coiled plates of lead, or 2. by passing carbonic acid through a solution of subacetate of lead. Its composition is somewhat different according to the mode of preparation. The neutral carbonate, made by double decomposition between a soluble salt of lead and an alkaline carbonate, consists of one eq. of acetic acid, and one of protoxide of lead; the white lead of commerce is generally believed to be a compound of this neutral carbonate with variable proportions of hydrated oxide of lead.

Sensible and Chemical Properties. Carbonate of lead is in the form of a white powder or pulverulent lumps, heavy, inodorous, and nearly tasteless, insoluble in pure water, very slightly soluble in water containing carbonic acid,* dissolved with effervescence by nitric acid, blackened by sulphuretted hydrogen, reduced by heat to the yellow protoxide, and by heat with charcoal to the metallic state.

Impurities. The commercial carbonate of lead is very often adulterated; the most common impurities being sulphates of lime, baryta, and lead, and carbonate of lime.

Effects on the System

This salt, in consequence of its insolubility, is less apt than the acetates to irritate or inflame the alimentary mucous membrane; but it more readily affects the system at large, and is supposed by some to be the most noxious of all the preparations of lead. This proneness to act on the system may possibly, as suggested by Dr. Christison, be owing to the disposition which, from its weight and insolubility, the carbonate of lead has to adhere to the membrane, so that it is not carried off with the contents of the bowels, and is more exposed to absorption.

* To test the solubility of carbonate of lead, the following experiment was performed at my request by Professor Procter. A solution of neutral acetate of lead was precipitated by carbonate of potassa, and the carbonate of lead thus obtained was thoroughly washed. This was introduced into a bottle containing carbonic acid water, which was instantly corked. After standing for twelve hours, with occasional agitation, the liquid was filtered, exposed to the air so as to allow that portion of the carbonic acid to escape which had been retained by pressure, and subsequently boiled so as to drive off the remainder. No appreciable deposition took place, nor was (he liquid affected by iodide of potassium; but when a current of sulphuretted hydrogen was passed through it, the liquid was perceptibly darkened, and upon standing deposited a minute quantity of sulphuret of lead. This experiment proves that carbonate of lead is appreciably dissolved by water impregnated with carbonic acid, and is retained by the water after the carbonic acid has been driven off by heat.

Medical Uses

Carbonate of lead is never given internally. Externally, it is employed as a desiccant and antiphlogistic application. Some recommend it to be sprinkled in the state of powder upon excoriated surfaces; but it should be used in this way with caution. I have known it, sprinkled thickly upon an abraded surface on the leg, to produce severe inflammation, with much pain and swelling, and a superficial slough. There is some danger, too, from its absorption; for a case is on record in which death in a child resulted from the external use of the medicine. The best method of application is in the form of an ointment (Unguentum Plumbi Carbonatis, U.S., Br.), which is made by rubbing up eighty grains of very finely powdered carbonate thoroughly with a troyounce of simple ointment, previously softened by a gentle heat. In this way it may be used in ulcers, burns and scalds, excoriations of different kinds, and irritating cutaneous eruptions. A liniment, formed by mixing it with flaxseed oil to the consistence of cream, has been particularly recommended in burns. A case is reported by Dr. Kunkler, of Madison, Indiana, in which colica pictonum occurred in consequence of the application of carbonate of lead, in the form of common white paint, to an extensive scald of the arm; but this effect is very rare. (N. Am. Medico -chirurg. Rev., i. 605).