LEA

LEA

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inftead of being engaged with lead, was fatu-rated partly by foda, partly by ammonia, andby due evaporation and cooling, cryftals of themicrocofmic fait, or the phofphat of foda andammonia were obtained. The other varieties ofthis phofphat were analyzed in the fame way.

With regard to the analyfis of the molybdatof lead the reader is referred to the article Mo-lybdena , and to Chrome , for the Chromat oflead. Tbofe ores in which lead is only a fmallpart of the metallic contents will be defcribedunder the other metals as filver, bifmuth, &c.

The compound of oxyd of arfenic, lead, andiron, examined by Lelievre and Vauquelin*,gave the following appearances : ioo partsroafted for half an hour, with a little fuet addedoccalionally to favour the evaporation of thearfenic, loft 38, and became black and magne-tical. The remaining 62 parts boiled in muri-atic acid made a red folution, giving out muchoxymuriatic acid gas, and cryftallized muriat oflead was depofited. This folution evaporated,rediflolved in water, and decompofed by fulphatof foda, gave 25 of fulphat of lead, equal to20.2 of lead or 22 of the oxyd. The refidue,faturated with ammonia, gave 39 parts of oxydof iron.

The analyfis of lead ores in the dry way isattended with much more lofs of the metalliccontents, efpecially when alkaline fluxes areufed, all of which a£t more or lefs on thisoxyd.

Lead is a metal of a blueifh-white colour,almoft filver-white when recently melted, butvery foon tarnilhes. It gives a peculiar fmellwhen rubbed or heated. Its fpecific gravity,according to Briflon, is 11,352. It is verymalleable, readily extending under the hammerinto very thin leaves, but its tenacity is lefsthan that of any other metal, for a wire T V ofan inch in diameter will break with a weightof 30 lbs.

Lead is the leaft fonorous of all the metals,giving when ftruck a very flat heavy found. Itmelts long before being red-hot. The meltingpoint of this metal has been varioufly given,owing to the known irregularity of the mercu-rial thermometers at very high temperatures.

Morveau gives it at 590* Fahr. Accordingto Mr. Crichton it is 6 12°. When flowlycooled it cryftallizes in quadrangular pyramids.Heated fully to rednefs it fmokes and fublimesin the open air, giving a grey oxyd, which fet-tles on the fides of the veffel that contains it,or if ia.-fcrge quantity mixes with the atmo-* Phil, Mag. vol xvi.

fphere around and collects in the chimnies ofthe furnaces where it is melted. Lead in allforms and combinations is poifonous whentaken in any quantity, and a frequent expofureto its vapour, or much handling it, graduallyproduces dangerous bowel-complaints, paralyticfymptoms, and other maladies.

Though the furface of lead at a commontemperature foon tarnilhes, this metal will re-main expofed to air and all weathers for a greatlength of time without further change, the ox-idated furface protecting the inner part fromdeftrudtion, and hence the durability of leadenroofs, pipes, &c. but yet in procefs of timethe whole is corroded throughout.

Water has but little direct aftion on this metaleither hot or cold, not being decompofed by it asit is by iron and fome other metals ; but it flowlyaffifts the action of the air, for lead will be cor-roded fooner in a damp than in a dry atmofphere.

When lead is melted in free expofure to theair, it becomes almoft immediately coveredwith a wrinkled pellicle of a dirty grey colour,and if this is skimmed off others form in fuc-ceflion, till the whole metal is changed into ayellowilh-grey oxyd, the weight of which wasone of the earlieft obfervations made on theeffeCt of calcination in increafing the weight ofmetals.

This grey oxyd, by a further continuance ofheat with conftant ftirring, paffes through vari-ous fhades of a greenilh-yellow to a deep dun-yellow, owing to a fucceffive abforption ofoxygen. The higheft ftate of oxygenation tobe produced by mere calcination, appears tobe that in which the oxyd has a beautiful highred colour, with more or lefs fcarlet, when itis called Minium or Red Lead , a fubftancewell known as a pigment, and efpecially as aflux in glafs-making, for which it is largely em-ployed.

Minium, however, cannot be made with anycertainty in the fmall way by mere calcinationin the air, however long this is continued, thecolour of the oxyd never rifing higher than a.dun yellow; it is only produced in manu-factories in the large way, with frequent ftir-ring. The procefs for making red lead is thusdefcribed by Dr. Watfon' as carried on in Der-bylhire.

The furnace is very much like a baker’soven with a low vaulted roof, and on eachfide of the furnace are two party-walls rifingfrom the floor of the furface but not reachingto the roof. In the interval between thefeEffays, vol. iii.