Page:The New International Encyclopædia 1st ed. v. 12.djvu/54

* LEAD. aloue, but is praclked to a considerable extent iu combination with other processes. The preceding processes are all employed with galena ores. Lead carbonate is smelted in bla-t- lurnaees with suitable lluxes — limestone and slaj; from previous (jperations — the metallic lead draining into suitable slumps at the bottom of the furnace. Ix-ad sulphate is usually smelted in connection with other ores, generally galena, but it may be smelted alone by one of several methods. Tile lead obtained by any of the processes described contains imimrities such as copper, arsenic, antimony, zinc, iron, bismuth, tin. and sulphur, and their removal wlierc they exceed a certain proportion is necessary to produce marketable lead. Kefining proc'csses resolve themselves into the separation of copper by liquation or by means of zinc, and the extrac- tion of the other metals with the exception of bismutli and the precious metals by melting in an oxidizing atmosphere. In liquation the copper rises to the surface of the molten lead in the form of scum which is skimmed olF, and in the zinc process it combines with the zinc to form zinc scum, which is similarly removed. Oxida- tion of the other metals is brought about by melting and heating the lead, when they pass off one after another as the proper temperatures are reached. The melting is usually done in pots, but hearths and reverberatory furnaces are also employed. Lead containing silver may be re- fined by electrolysis. After being refined the molten lead is cast in molds into pigs and is then ready for the market. For a concise de- scription of the metallurgj' of lead, consult Schnable, Handbook of Metallurgy/ (London, isns). Phoperties of Le.vd. Metallic lead (symbol, Pb: atomic weight, 206.9) is a bluish-gray lus- trous metal that is exceedingly malleable and ductile, and has a specific gravity of about 11.4. It melts at 320° C. (608° ¥.). It is very soft, being easily cut by a knife and scratched by the nail, and readily receives impressions. It leaves a black streak on white jiapcr. and is an inferior conductor of both heat and electricity. Jletallic lead is extensively used for piping, both for gas and water. As lead is somewhat sohible in water, it is particularly desirable that caution should be taken to draw olf water that has been standing in pipes before using, especially when required for drinking. Owing to the fact that lead resists the action of acids, it has been extensively used in the construction of lead cham- bers in the manufacture of various acids, espe- cially sulphuric. It is also employed in the manufacture of shot. The alloys of lead with other metals have been sufficiently described un- der Alloy. Compounds of I.,ead. The two most impor- tant among the compounds of lead with oxygen are litharge and minium. Litharge is obtained wlien molten lead is heated at a moderate tem- perature in the air with constant stirring. It is used in the arts in the manufacture of flint glass, as a glaze for earthenware, for the prep- aration of lead salts, as a paint, and for dry- ing oils. Minium, which has been known since the time of Pliny, is a scarlet crystalline granu- lar powder, usually prepared by carefully heat- ing very finely divided pure litharge or white lead. It finds extensive use in the arts as a pig- 42 LEAD. nient, also in the manufacture of flint glass, as a cement for making steam-tight joints, and in the manufacture of secondary liatteries. ( See iliMCM.) Lead combines with carbon dioxide to form the carbonate, which is found native as ccrusite (q.v.). While lead, or basic car- bonate of lead, Pb(0H)..2PbC'„ is a white heavj- powder. It i.s extensively used in the arts as a jiigment and as a body for other colors in the manufacture of paints. It is produced artificially by the decomposition of basic acetates of lead liy means of carbon di- oxide. (See Paints.) lA<id ncrlnle, or sugar of lead. Pb(Cyi,0,).. 311.0. is made by adding lead oxide to acetic acid and gently lieating the mix- ture. It is used in medicine, in the arts, as a clarifying agent in the refining of sugar, and to a certain extent in chemical analysis. Lead sulphate, which is found native as anglesite, may be obtained artificially by precipitating a lead salt with sulphuric acid, yielding a heavy white ]iowder which is sometimes used as a substitute for white lead. The various lead chromates are colored yellow; thcj' are used as pigmenta and in calico-printing. Medicinal Uses of Lead CosiporNDS. A num- ber of lead compounds are employed in medi- cine as astringents and sedatives. Most of thi in are thus applied externally for weeping eczema and ulcerations. The acetate is given internally as a remedy for diarrhtea in typhoid fever, for gastric ulcer, for intestinal hemorrhage, etc. The astringent action for which jirincipally lead com- pounds are used, both externally and internally, is very powerful. While theyhave no efTect on the healthy unbroken skin, thev" speedily form a coat- ing of coagulated albumin on sores and ulcei and cause coagulation of albumin in the tissn. - and contraction of the small vessels. For their excellent sedative effects, lead salts are used in pruritus. Ordinary "lead water' contains lead in the form of sub-ocetatc. Lead earbonate (basic) is a 10 per cent, constituent of the official 'ointment of lead carbonate.' Lead iodide is a 10 ]M>r cent, constituent of the ointment bearing its name. Lead oleate is contained in ordinary lead plaster and in the 'Diachylon ointment' which is made from it. LeauPoisoning. Acute poisoning, which i^ but seldom met with, is due to the irritant action of lead salts on the alimentary canal. The treat- ment consists in administering an emetic (say, 20 grains of zinc sulphate), washing out the stomach, and then giving a dose of Epsom salt. Much more common and important is the chronic form of lead-poiMining. Chronic lead-poisoning may be due to drinking-water that has passed through new lead pipes (in course of time, a deposit of insoluble salts forms on the inner walls of pipes, and thus the danger of lead be- ing dissolved in the water is considerably di- minished), or water that has been kept for some time in lead cisterns. The amount of lead that m.iy thus be dissolved depends iiuich on the qual- ity of the water. Considerable amounts are dis- solved if the lead is exposed alternately to the action of air and of water. Another source of chronic lead-poisoning is in the often uncleanly habits of painters, plumbers, and workingmen engaged in the manufacture of lead compounds. .bsorbed for the most part by the kidneys, lead may cause pronounced s^Tiiptoms of an.fmia, gout, chronic inflammation of the kidneys, chron-