COP
c op
{ 352 ■)
grains, which, fubftra&ed from the 294, leaves11.5 for the water. The lulphuric folutiondecompofed by metallic zinc gave 58 grains ofcopper. Another portion of the malachite wasdiffolved in nitric acid, and the precipitate byammonia was entirely rediffolved by an excefsof this alkali, fhewing therefore the total ab-fence of iron. Laltly, the quantity of oxygenunited with the copper is inferred to be thatwhich muft be added to the 58 and 29^ to makeup the 100, namely 12.5.
The native carbonat when pure has precifelythe fame compofition as the artificial, as Proufthas clearly Ihewn, and will be prefently men-tioned.
The compofition of the different fpecies of thisfamily of copper ores as far as it is hitherto knownis extremely fimple, the ore when feparated fromits matrix containing nothing but arfenic acid,oxyd of copper, water, and fometimes a portionof iron. The arfenic acid is belt eftimated byuniting it by ftronger affinity with fome othermetal where it makes an infoluble fait of aknown and invariable compofition. Lead is thebeft fitted for this purpofe, fo that when thenitrat or acetite of lead is added to arfeniat ofcopper, a foluble nitrat or acetite of copper isformed, together with a white infoluble precipi-tate of arfeniat of lead. By preliminary ex-periments Klaproth found that 100 grains offolid arfenical acid gave 297 grains of folid ar-feniat of lead after moderate drying, and hence1.00 grains of the arfeniat contain 33.66 ofarfenic acid. ‘
For the analyfis, the ore being firft diffolvedin nitric acid, the folution was accurately neu-tralized with carbonated potafh, and then ace-tited lead was added till all precipitation ceafed,a little of this fait remaining in excefs. Theprecipitate of arfeniated lead when waffiedand dried indicated the proportion of arfenicacid in the ore. The folution now containedacetic and nitric acids, united with oxyd ofcopper and potafh, and a little acetited lead;from which the lead was firft feparated by ful-phat of foda, that precipitated it in the formof an infoluble fulphat, after which a little ful-phuric acid was added to engage the copper,and this metal in its turn was feparated by po-lilhed iron in the ufual manner. The copperhowever is thus obtained in the metallic ftate,but in the ore it is an oxyd, and therefore one-fourth of the weight of the copper muff beadded for oxygen, that is (as in the a&ual ana-lyfis) for 40 parts of metallic copper, 50 of the
oxyd of copper muft be eftimated. It is neeef-fary to neutralize the nitrous folution at firftas the arfeniated lead is very foluble in acidsand therefore would not otherwife be properlycollected.
Where iron is fufpected in the ore, the fo-lution inftead of being finally precipitated bypolifhed iron may be decompofed by ammonia,and the copper feparated from the iron by anexcefs of the alkali.
As many acids form a white precipitate withthe folutions of lead, the powder in queftion isproved to be arfeniat of lead by digefting withlulphuric acid, which difplaces the former acid,and leaves it uncombined in the liquor; andarfenic acid is detected by various methodsmentioned under that article, particularly bygiving a brick-red precipitate with nitrat offilver, the arfenic acid being firft neutralizedwith foda.
Mr. Chenevix analyfes the arfeniats of copperin the fame general way, but with fome vari-ations.
The ore (previoufly heated moderately toexpel and eftimate the mere water) is diffolvedin dilute nitric acid and nitrat of lead pouredin. The folution (containing an excefs of acidand therefore holding fome of the arfeniatedlead diffolved) is then evaporated nearly to dry-nefs, and alcohol added to compleat the fepa-ration of the arfeniat of lead. After which thecopper is decompofed by potafh, and is obtainedin the ftate of the brown oxyd.
The compofition of this lingular ore, foundhitherto only in Peru , was afcertained nearly atthe fame time by Prouft, and by Rochfoucault,Beaume, Fourcroy and Berthollet , a committeenominated for the purpofe by the French Aca-demy. The experiments are inftruftive.
The ore thrown on hot coals burns with abeautiful blue and green flame, whereas mala-chite, which it mofi refembles in appearance,gives only a faint green flame. Diftilled per fe }malachite gives no other volatile product thancarbonic acid and water •, whereas the muriatgives in the fame procefs a quantity of oxygen gaswith only a very fmall proportion of carbonicacid, and a green liquor ftrongly fmelling ofmuriatic acid and proved to contain copper bygiving a fine blue with ammonia. The refiduein the retort was a brown oxyd of copper. Thiswhen expofed to air turns green, fhewing there-fore that it is not a fimple, but a muriated oxyd.Part of the muriat of copper therefore is vola-tilized, as it is well known that this lalt readily