P R E
P HE
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'would be decompofed by the water, as whenany nitrated folution is precipitated by bif-muth: or 3dly, when the whole of the newoxyd formed by the transfer of oxygen fromthe precipitated metal to the precipitant cannotbe diffolved by the acid prefent, as when nitratof copper is decompofed by lead, in which cafethe precipitate will be very largely alloyed bythe precipitant; or 4thly, when the affinities ofthe precipitant are fuch as more eafily to dc-compofe the acid than to abforb the oxygen ofthe metal already diffolved, as when nitrat ofcopper is decompofed by zinc, in which cafethe precipitate is partly reguline and partlyoxydated; and 5thly, where the affinities be-tween the two metals are fo ftrong that theprecipitate is largely alloyed with the precipi-tant, as when a folution of filver is decompofedby mercury.
Of thefe five cafes, the 3d is probably thatwhich much more contributes to' produce alarge alloy of the precipitated metal than eventhe affinity exifting between the two metals;thus Vauquelin found that 50 grs. of copper,diffolved in nitric acid even with excefs of acidrequired no lefs than 216 grs. of lead for its-compleat feparation, and the precipitate, in-ftead of weighing only 50 grs. amounted to128 grs. of which therefore 50 were copper,and 78 were lead. Yet thefe two metals havebut little affinity for each other. This incon-venience is corrected, though not entirely re-moved by adding a confiderable excefs of acid.
PRECIPITATE, •white, are all oxyds or
PRECIPITATE, red, [• faltsofM ercury,
PRECIPITATE, per fe } which see.
PRECIPITATE, purple, of'CaJftus. SeeGold.
PREEINITE.
Its colour is pale green, either whitilh/greyilh,or yellowifh. It occurs generally in mafs, butis fometimes cryllallized in oblique quadrilate-ral prifrns.
The cryftals are either fingle or aggregated;and are fmall or middle fixed. Externally theyare fmooth and fhining; internally they have agliftening pearly luftre.
The frathire is foliated or divcrgingly radi-ated; the fragments arc indeterminately angu-lar and wedge-draped. It is tranflucent, butfometimes is nearly tranfparent. It is fome-what harder than glafs, and is eafily frangible.Sp. gr. 2.6 to 2.94.
Befoie the blowpipe it boils and melts into a.yellowiAi brown enamel, but .it does not be-come gelatinous with acids, in which refpedt
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it differs from zeolite with which it has other-wife a confulerablo reiemblance.
The prehnite of the Cape of Good Hope hasbeen analyfed by Klaproth , and that of 1 raneeby Haffenffatz, with the following rcfults.,Ivlapr. Half.
43.83 — 50. Silex.
30.3} — 20.4 Alumine.
18.33 •—* 23.3 Lime.
5.60 — 4.9 Oxyd of iron.
1.83 — 0.9 Water,
o. —- Adatmefia.
99.98 100.0
It occurs in veins in primitive flaty gruns-tein, alfo in rocks of the molt recent lloetztrap formation. It was firlt difeovered byCol. Prehn at the Cape of Good Hope , andhas fince been found in Dauphine in Prance,in the valley of Eafili in the Tyrol, alfo inArthur’s Seat near Edinburgh , and in otherparts of Scotland .
PRINCE’S METAL. See Copper, Alloysof.
PROBIERSTEIN. See Kieselschiefer.
PROOF BOTTLE, an article of chemicalapparatus; for an account of which fee theAppendix .
PRUSSIAN BLUE, Berlinerblau , Germ.
PRUSSIC ACID, Blaufaure, Germ.
Pruffian Blue is a pigment of a beautifuldark blue colour which was difeovered aboutthe year 1710 by a cliemift of Berlin of thename of Dicsbach by the accidental mixture ofa folution of vitriol of iron with an alkaliwhich had been highly impregnated with ani-mal matter. This difeovery led to the prepa-ration of this fubltance as a pigment, but theprecife procel’s was long kept fecret till it wasmade known in -the Philofophical Tranfa&ionsfor 1724, by Dr. Woodward. This has beenadopted with feme variations by fucceedingchemifts.
The proccfs of preparing Pruffian blue asgiven by Macquer , in his valuable memoir onthe fubjeft, is the following: prepare a quan-tity of bullock’s blood by moderate drying, tillit is reduced to powder, mix four ounces ofthis with as much carbonat of potafli (madeby calcining equal parts of nitre and tartar)put the whole in a covered crucible with afmall hole at the top, and calcine it in a mode-rate fire till no more fmoke or flame iffuesthrough the hole, at which time the mafs willbe black and carbonaceous; then raife the heattill the whole is moderately red, and remove it.
2 i a