Page:Cyclopaedia, Chambers - Supplement, Volume 1.djvu/980

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"•cried. Then with a pair of bellows excite the 'fire for a ■ ■quarter of an hour, then put in the ore at feveral times ; nor will it be amifs to add to it fome fcales of iron ; but the ore mult be put in fo as to be above the coals, facing ■the hole, through which the blaft of the bellows is made ; ■but let it not touch the wall of <he furnace. _ FiU the fur- nace at leaft two thirds with charcoal, and let it be in pieces -of a moderate bignefs ; and after every portion of ore is put in, add a larger of charcoal above it. When all the ore is put in, continue to blow till all the fire is confumed ; -then pour water upon the foremoft bed, drop by drop, to -cool the lead gathered in it, and fee among the fcoriae if there be any lead lodged among them ; and if there be, fe- parate it, and weigh all together, and you will from this know the value of the ore, and what the fmclter may obtain from it. Cramer's Art of Affaying, p. 298. This metal, as in common ufe, always contains fome fil- ver, which is not in quantity enough to be worth the charges of feparating. The moft, common proportion is not more ■than two drams to a centner. This, though of little con- ference on any other occafion, is carefully enquired into ■by the affairs of the ores of metals, particularly in regard 'to 'the ores of filver, for the reparation of which from its ■■ore lead is employed ; and it occafions a very great error, when its computation is neglected, in eftimating the value of an ore from the affay. This encreafmg filver is therefore always fubtracted by the more accurate affayers ; and in order to do this with the lefs trouble, a large quantity of lead is granulated at once for this ufe, and mixed by fitting ; then the ufual quantity employed in the affaying an ore is to be ■tried alone in a copel, and the bead of filver it affords care- fully preferved. After this, when the bead produced from a filver ore is weighed ; that bead of filver, given by the lead, is to be put into the oppofite fcale among the weights, and then ■the weights balancing the bead, will, without more trouble, give the -Weight of the filver from the given quantity of •its ore. Cramer's Art of Allaying, p. 216. All the metals may be confidered in certain circumftances, as acting in the manner of menftruums on one another, and of thefe none has fo much power as lead. When lead is expofed to a moderate fire in an earthen veffel, the furface of the melted mafs is foon covered with fcoriae-, in form of -a thin flsin of many colours : when the fire is fo far increafed as to make the veflels quite red hot, this (kin melts a little, and is thrown to the fides of the veffel in form of a furfu- raceous matter, of various yellowim or whitifh colours, called litharge. After this the melted lead fmoaks, and fmall drops, refembling the firft fcoriae rife, and fwim like globules of oil upon the furface; thefe are one after another added to the li- tharge at the fides ; and if the fire be kept in this degree, the whole fubftance of the lead is finally turned into litharge. If copper he added to lead thus fmoaking, it caufes a much ■briflcer ebullition than before, and the copper burfis, and ■divides, and feems to become one homogene mafs with the had ; this mafs is brittle, and if made with equal parts of lead and copper, refembles tempered fteel when broken. Gold and filver melted thus with lead become brittle alfo, and gold becomes pale by thefmalleft admixture of it. Tin and lead melt together with a fire ftrong enough to melt the lead alone ; but when this is increafed, fo as to ■make the vefTels red, the tin immediately rifes up above the lead, in form of little whitifh dufty hillocks, fo that the whole looks as if afh.es had fallen into the veffel ; and the tin thus calcined, when cold, is variegated in colour, with white, yellow, and red. Lead, fo long as it keeps its me- tallic form, can never be made to join with iron, though urged by ever fo violent a fire.

The femi-metals are all eafily melted with lead by fire, and if added in too large a quantity they take away its mal- leability.

Lead and all its produces turn into glafs by a ftrong fire ; and this glafs, or in the place of it litharge melted with Hones, and other vitrifiable fubftances, makes them run into glafs with a much lefs fire than they otherwife would do ; and if a very large quantity of this glafs or litharge be added, they become fo far attenuated as to run through the fides of the vejlels. Lime ftdncs fuffer the fame change by means of this matter ; and the incombuftible ftones, with proper management, are fubject to the fame fate ; but un- lefs thefe procefTes are carefully conducted, the litharge or glafs of lead gets away alone through the veffel. Among the metals litharge facilitates the fufion of copper and iron by the fire, hut it confumes a confiderable quantity of them, and runs itfelf with what it poffefles into glafs ; and finally, the whole tin and its calx are with difficulty vitrified by glafs of lead, and they lofe nothing by it : but if gold and filver are very frequently melted with litharge, they will at length be found to have loft a little of their weight ; but this fmall quantity is not deftroyed, but may, by the proper operations, be again regained out of the litharge.

It has been proved, by the experiments of different che- mifts, that lead contains a real running mercury. Kunkel and Becher both feparated mercury from it ; and Mr. Groffe, of the Paris academy, has juftified their experiments by

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producing the fame effect ; by means not only very different from theirs, but abfolutely oppofite and contrary. They fuppofe that the mercury contained in lead is fixed there, either by acids or by fulphurs, and therefore they ufed alkaline materials to difengage and fupport it. On the contrary, Mr. Groffe ufed only acids, and by means of thefe procured a pure rtunning mercury from this metal. His method was to diflblve thin plates of lead in fpirit of nitre, weakened with an equal quantity of water. He then obferved a grey coloured powder precipitate, which when tried on gold or copper, fhewed itfelf to be mercurial, and even in the pow- der he faw fmall globules of quickfilver. Mem. de l'Acad. des Scienc. 1733.

As there could be no poffibility of the acids forming mer- cury with lead, it is very plain that it muff have been ori- ginally there.

In the trying lead by the teft of the burning glafs, a piece of that metal laid on a charcoal very readily melts, and afterwards wholly diffipates itfelf in fumes ; if it be placed on a piece of ffone it fumes very much, and becomes at length a thin fluid refembling oil, or melted refin ; and this liquor, in cooling, fixes itfelf into a fort of glafs, which has this peculiar quality, that it is always compofed of plates, or fcales, in the manner of talc ; it is of a greenifh cr yel- lowifh colour, with fome reddifh fpots, and is very foft to the touch. If it be kept longer in the focus of the glafs it fpreads abroad upon the ffone, and by degrees penetrates its fubftance, and promotes its fufion. If the common calx of lead, red lead, or litharge, be ufed inftead of plain lead in this experiment, the event is the fame. The whole becomes a thin fluid matter, which in cooling affords a flaky glafs, like talc. Mem. Acad. Paris 1709.

If this talcky glafs of lead be expofed to the fame focus, laid on a piece of charcoal, it prefently melts, and re-affumes the form of lead ; and, if then taken away, and fuffered to cool, is no way different from common lead. If the calx of lead, or red lead, or litharge, be in the fame manner melted in the focus of the burning glafs on a piece of charcoal, they in the fame manner immediately re-affume the form of lead. It appears from the whole that there is in lead an oily inflam- mable matter, which is eafily driven off, either by the folar or by a common fire ; that the bafis of lead, which appears alone, when this is driven off, is a fcaly or talcky earth ; and that this oily matter of lead is no way different from the oils of vegetables, fince the oil of charcoal is able perfectly well to fupply its place, and to reftore this talcky earth to the form of lead again.

Lead is well known to be in its natural ftate a metal fo little fonorous, that it is even a proverbial expreffion ufed to any metal that is lefs fonorous than others, that it is almoft as mute as lead.

Mr. Lemery, however, once fliewed Mr. de Reaumur, and afterwards to the academy of fciences at Paris, a lump of lead, which had been left in one of his veflels, after fome chemical operation, which when ffruck, gave a very loud and acute found.

Mr. Lemery had no occafion afterwards to repeat the expe- riment, which had afforded him this remarkable lead, and left it to Mr. Reaumur to difcover the manner of doing it, who luckily hit upon it, without the difficulty he had ex- pected from the refearch ; and, it appears, that we have a thoufand times met with fonorous lead, without knowing it to be fo. This property is not the effect, of any particular operation, but is found in all lead, which after fufion has taken a certain form, and this a form which accident al- moft always gives it on that occafion. The common way of melting lead, in order to the calling it into moulds, is in iron ladles ; it very rarely happens, that all that is melted in the ladle is caff into the mould, but a fmall quan- tity remains, which foon forms itfelf into a mafs in the ladle, and this mafs is always foronous. Its figure is ufually either that of the fegment of a fphere or of a fpheroide, and it is plane or flat on one fide, and convex on the other: this is all it feems to have of regularity. The ladles are generally very irregular, and a thoufand other accidents occur, which prevent the lead's receiving any determinate form. The maffes are irregularly convex on the under fide, and on the upper there is commonly a protuberance, which is fomctimes in the center, but much more frequently on one fide. The mafles of this kind, examined by Mr. Reau- mur, were in general of about three inches diameter, and of about a third of an inch in thicknefs j thefe were all very fonorous. There are, doubtlefs, certain proportions of their thicknefs to their diameter, which may make them more or lefs fonorous, but all are confiderably fo : nor is the being caft in a ladle any effential accident to the rendering them fonorous, fince pieces of the fame fhape caft in fand are equally fo with thofe from the ladle ; and hence it will be eafy to caft lead into different moulds of earth, with all the varieties of fpheroida! figures, and thence learn what is the proportion that moft aififts the found. Poiftbly alfo figures of a different kind may prove fonorous ; but Mr. Reaumur tried one convex on each fide, and found it to have no found