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ons of the three metals. The lead is pro-portioned partly to the copper and partly to thefilver. It ffiould not be more than four timesthe weight of the copper, otherwife it willrender the whole mafs fo fufible that inftead ofthe lead and filver alone melting out, much ofthe copper will alfo mix with it, or if the heatbe at all too much raifed, the whole loaf ofliquation will melt down, and the procefs multbe begun again. On the other hand the leadfhould not be lefs than 2§ times the copper,otherwife a great part of it, (and along with itpart of the filver) will remain in the loaf afterheatimr.'

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The above therefore are nearly the extremelimits of the relative quantities of the lead andcopper, and the exafit proportion of lead isdetermined by that of the filver contained in thecopper, which muff be previoully found by afmall allay.

It is found that copper which contains morethan about three quarters of a pound of filverin ico lbs. will not yield all its filver by a fingleprocefs of eliquation, which is doubtlefs becaufeit will not bear alloying with fufficient lead toextract the whole, for in thefe circumftances,according to Cramer, every half ounce of filverrequires 17 lbs. (or 544 times its weight) oflead for its extraction, allowing four times asmuch lead as copper. Hence the refpectiveproportions of the three metals in a loaf ofliquation would be 544 parts of lead, 136 ofcopper, and 1 of filver. Thefe are nearly thofethat are aCtually employed, except that a lefsproportion of lead is found fufficient, being tothe copper only as 11 to 3 inftead of 12 to 3.

The whole procefs of obtaining pure filver byliquation is managed in the following way ac-cording to the accurate account given byDuhamelff It confifts eflentially of five diffe-rent operations. The firft is the melting to-gether the lead with the filver-holding impurecopper in due proportions, and calling themixed alloy into the proper moulds to form theloaves of liquation. The fecond procefs is theliquation , properly fpeaking, or the melting outfrom thefe loaves by a regulated heat, the leadand filver, and leaving the copper. The thirdis a fecond roafting of the porous loaves in orderto melt out a.portion of lead and filver, andother matters which remain after the foregoingoperation.. Tire fourth is the working off thelead on a large cupel of teft to convert it intolitharge, and leave the filver; and the fifth isthe further purification of the filver by a fecond

’ Crameri Docimafia praftica: proceffus 48.

refining with lead. Thefe two laft proceffeshave already been deferibed when treating ofthe other methods of refining filver. Thereare befides other operations performed on thecopper to refine it, and on the fcorire to obtainall the metal that may adhere to them, but we{hall here only deferibe the three firft proceffes.

The fufion of the metals is performed in ahigh blaft furnace, the bottom of which is linedby a mixture of charcoal rammed hard and laidin an inclined direction fo as to convey themetal when melted into a feparate receptacleadjoining. The crude black copper is pre-vioufly broken into fmall pieces when hot, andaffayed for the proportion of filver. Either leador litharge is ufed in the mixture, if the latter,it is mixed with charcoal to promote its reduc-tion. The furnace being previoully heated foreight or nine hours and filled with charcoal,a balket-full of the fcorice arifing from the firftreduction of the copper ore is thrown in, whichfoon melts and lines the furnace with a vitrefeentglazing that protects its walls from the violentadion of the blaft. Soon after, about 20 lbs.of lead are thrown on the hot coals, and withit about half the copper intended for one loafof liquation: 30 lbs. of litharge are thrownupon this, and with it a fmall bafket of charcoalto reduce it. The reft of the copper andlitharge are then added, and lead over thewhole. The heat excited by the blaft foonmelts together thefe ingredients, reducing thelitharge, and the whole falls down into the fmallwell intended for the melted metal. Whenenough is colle&ed for a fingle loaf it is tapped,and the metal is poured into an iron mouldprevioully heated, and lined with a thin coatingof clay. To withdraw the loaf from the mould,a ftrong iron bar bent at top is let into the hotmetal about two inches, and when cold fervesas a handle to draw up the loaf, and is knockedaway when the loaf is taken out.

The heat required in this operation is lefsthan that at which copper melts, fo that thereduction of the litharge and mixture of themetals goes on very fpeedily. In a well mana-ged furnace, feven loaves, each of about 35olbs.,are caft in an hour. Litharge is not ablolutelyneceffary, but on the whole is preferable toufing lead alone as it rs a powerful flux beforereduction, and there is always a great quantityof it produced in the fubfequent refining of thelead and filver. In eftimating the proportionof the metals, 120 lbs. of litharge are taken asequivalent to ico lbs. of lead.

1 l.’Art Ue Liquation, Mcra.de l'Acad. fo* 1788.