The most important of the salts of the protoxide of lead are-1. The carbonate which occurs native as a beautiful mineral in transparent needles or fibrous masses, and which is prepared under the name of lehite lead on a large scale as a pig Ment by a process to be subsequently described. The carbonate is insoluble in water, unless it is largely charged with carbonic acid. It is quickly blackened by exposure to hydrosulphurie acid (sniphureted hydrogen), either in the form of gas or in solution, and this is a serious drawback to the use of the lead salts as pigments. 2. The sulphate (Pb0,S02), which occurs native in white prismatic crystals, and is formed as a heavy white precipitate on adding sulphuric acid or a soluble sulphate to a soluble lead salt. 3. The nitrate (PhO,N06), which is formed by dissolving lead or its protoxide in dilute nitric acid. 4. The chromates, of which the principal are the neutral chrofnate chrome yellow (PhO,Cr02), and the dichromate or orange chrome. These are much used as pi-oments, and in calico-dyeing. 5. The acetates. The ordinary or neutral acetate Baq.) is prepared on a large scale by the solution of litharge in distilled vinegar. and evaporation, when the salt is obtained in four sided prisms, or more com monly in a mass of confused minute white crystals,. which at.21.27 Jose their water ot crystallization. From its appearance, and from its sweetish taste, it derives its common name of sugar of lead. It is much used both in medicine and in the arts. Basic acetate of lead, regarded by some chemists as a diacetate, and by others as a triacetate, and commonly known as Goulard's extract, is prepared by boiling a solution of sugar of lead with litharge, and adding alcohol, when the salt separates in minute transparent needles It is the active ingredient of Goulard 'water, which is imitated by the liquor plumb diacetatis and of Goulard's cerate, which is imitated by.the ceratum plumbi cone positum of the London pharmacopoeia.
The best tests for solutions of the salts of lead are the formation of a black sulphide with hydrosulphuric acid or hydrosulphate of ammonia, insoluble in an excess of the reagent; of a white insoluble sulphate with sulphuric acid, or a soluble sulphate; of a yellow chromate with chromate of potash; and a yellow iodide with iodide of potas sium: All the salts of lead, insoluble in water, are soluble in a solution of potash. Before the blow-pipe on charcoal, the salts of lead yield a soft white bead of the metal, surrounded by a yellow ring of oxide.
Its use in Afedicine.—The most important compound of lead in the materia medics is the acetate of lead, which is administered internally as an astringent and as a sedative. It is of service as an astringent, especially in combination with opium, in cases of mild English cholera, and even of Asiatic cholera, and in various forms of diarrhea. It will frequently check the purulent expectoration in plithisis, and the profuse secretion in bronchitis. In the various forms of hemorrhage—as from the lungs, stomach, bowels, or womb—it is employed partly with the view of diminishing the diameter of the bleed ing vessels, and partly with the object of lowering the heart's action, and by these means to stop the bleeding. The ordinary dose is two or three grains, with half a grain
of opium, in the form of a pill, repeated twice.or thrice daily. If given for too long a time, symptoms of lead-poisouing (q.v.) will arise.
Xining, Smelting, etc.—Lead was largely worked by theRomans in Great Britain, and pigs with Latin inscriptions have been frequently found near old smelting-works. The mining of lead in England was formerly regulated by curious laws; sonic places, such as the King's Field, in Derbyshire, having special privileges. It was the custom in this district not to allow the ore to leave the mine till it was measured in the presence of an official called a bar-master, who set aside a 25th part as the king's cope or lot. Up to a comparatively recent period, persons were allowed to search for veins of the ore without being liable for any damage done to the soil or crops.
Lead ore is pretty generally distributed, but by far the largest supply of this metal is obtained from Great Britain and Spain, the former country yielding some 75.000 tons per annum, and the latter probably an equal supply. Nearly a fourth of the total British produce is procured from the Northumberland and Durham district; where there exists, at Allenheads, one of the largest mining establishments in the world. Scotland and Ireland furnish only a very small quantity.
With the exception of a little from the carbonate of lead, all the supplies of this metal are obtained from the sulphide of lead or galena (q.v.). The lead ore, when taken from the mine, is broken up into small pieces, "botched," and washed, to separate impurities, by means of apparatus described under METAILLTEGY. Sulphide of lead, when tolerably pure, is smelted with comparative ease. It is first roasted in a reverbera tory furnace. From 20 to 40 ewts. of galena are put into the furnace at it time, either with or without lime. In about two hours the charge becomes sufficiently roasted. During the process, the larger portion of the ore (Pbb) takes up four equivalents of oxygen, and becomes sulphate of lead (PhO,S0,), a little oxide of lead (PLO) is also formed, while another portion remains unaltered as sulphide of lead. After it is roasted, the ore is thoroughly mixed together. and the heat of the furnace suddenly raised. This causes a reaction between the unchanged and the oxidized portion of the ore, and reduces much of the lead, sulphureous acid being at the same time evolved. In the third stage, lime is thrown in and mixed with slag and nnreduced ore. When this becomes acted on, the whole of the lead is practically separated from the ore, and is then run off at the tap-hole.
In some districts, the roasted ore is smelted on a separate ore-hearth called the Scotch furnace, where the heat is urged by bellows. Peat and coal are used as the fuel. This is a slower mode of smelting than the last, but yields a purer lead.