Page:Cyclopaedia, Chambers - Supplement, Volume 2.djvu/291

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tipper one Is broad, and divided into three acute fegments ; tiie under one is fomewhat fmaller, and is divided only into two. The flower is compofed of one petal, and is of the labiated kind. The upper lip is erc$: and hollow, of an oval figure and emarginated ; the under lip is broad and reflex, and is divided into three fegments, of which the middle one IS the Iargeft. Theftamina are four Tubulated filaments prefs- ed clofely under the upper lips of the flower; they ftand ex- panded, one pair of them are fhorter than the other., and the longeft do not reach to the verge of the flower. The an- therze are oblong and lateral. The gerrnen of the piftil is fquare. The ftyle is flender, and is of the length and fitua- tion of the ftamina. The ftigma is' acute and bifid, one of the fegments being fhorter than the other. The fruit confifts of four roundiih berries, which lie in the bottom of the cup, and each contain one feed. Linnai. Genera Plant, p. 280. PRASIUS, in natural hiftory, the name of a gem much ap- proaching to the nature of the emerald, but of a coarfer green, and wanting its bardnefs, and having in its green a caft of yellow.

It is the ftone which the antients called pra/ites; and when of a greater than ordinary admixture of yellow, the chryfo- prafus, and of which the gem diftinguifhed by later authors under the name of&ejmaragetoprajusj is only one of the varieties. The prajius, even in its molt perfect ftate, is much lefs beauti- ful than moll of the other gems : it is found of various fizcs, and not unfrequently confiderably large : it is feldom met jvith fmaller than a pea ; from that to the fize of a horfe bean is its molt ufual ftandard, from, this to the fize of a nutmeo- it is more rarely found ; and the larger fpecimens are coarfer and lefs frequent than thefe.

It is of various figures, but is never found in a columnar or cryftal-like form; this is declaring againft the fenfe of our dealers in gems, indeed, who frequently buy and fell columns of the fhape of fprig cryftal, under the name of the f/narag- doprafus ; but thefe are all truly no other than cryftals tinged to a coarfe and dead green, and without any mixture of yel- low ; fo that they are more properly pfeudfmaragdi, or baf- tard-emeralds, than anything of the prajius kind. The prajius is frequently of .an orbicular form, flatted on one fide and convex on the other, and often oblong or oval ; but more ufually it is of a perfectly irregular fhape, made up of a number of flat faces, as we often fee fome of the coarfer nodules of the debafed cryftals from the German mines ; and it is ufually of an obfcure or much lefs polifhed furface than many other of the native gems. Its colour is a dufky green, with a mixture of yellow, and often of bluifh ; and its moft valuable pieces are of the hardnefs of the garnet ; and its worft, which are truly profit' 9 are confiderably harder than cryftal; the columnar prafii, as they are called, are, on the contrary, often as foft, or fofter than pure cryftal. ft is like the other gems, found in the different fpecimens of all degrees of colour, from its ufual dufky green to the mere co- lour of water ; it is at times found alfo with more or lefs vel- lownefs in the different fpecimens, or in the different parts of the fame fpecimen, and has fometimes a milky caft among th green : authors have diftinguifhed it according to thefe its dif- ferent appearances, by four names.

1. The prajius or prafites, fimply fo called. This is the name given to the gem in its moft perfect ftate, in which it has confiderable fhare of yellow with the green, and the green is of a more pure and graffy colour.

2. The fmaragdoprafus. This is the name given it when there is lefs yellow in the colour and fome mixture of blue with the green.

3. The chryfoberyllus. This is the name given to the profit when it has the fame bluifh caft, with a greater admixture of yellow.

4. The chryfopraftus. This is the name given it when it has a large admixture of yellow, and its green is a pure but a very pale one.

And, finally, De Boot mentions another fpecics, as he calls it, of praftus, which was more opake than the reft, and had but little green, and a large admixture of white among the yel- low ; but this is in reality only a fpecies of ncphriticus. The fineft of the profit are found in the ifland of Ceylon ; but there are many of them in New Spain. We have them alfo from Silefia and Bohemia; but thefe are very poor ones ■taV/'s Hift. of Foft". p. 597.

PRECATORES, in church hiftory, a feet of heretics, who under the pretence of praying always, refufed to work. Hofm, Lex. Univ. in voc.

PRECEDENCY. See the articles Precedence, Cycl. and Cerfmonial, Suppl.

PRECIPITATION, (Cycl) in aflaying, is the feparation of any part of a compound body while melting in the fire, or when cooling from fufion from the reft of the mafs, in fuch manner that it finks to the bottom, while the remainder con- tinues at top and makes the furface. The heavy part thus precipitated from the reft, is called the regains of that body. ■J his is an operation which almoft always requires the addi- tion of fuch ingredients as ferve to take away the mutual con- nection and coherence of the parts of the body to be fepa- rated ; that is, fuch as have a menftrual virtue, and kee[ Suppl. Vol.11.

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others in a ftate "of dilTolution. For inftance, the regulifl e part of antimony, and mineral fulphur, diflblve each other mutually, and conflitute crude antimony ; nor can they be feparated from each other by fire alone without deflroyihe*th| regulus; but if you add iron, copper, filver, &e. which are more thoroughly penetrated by fulphur, and are thus reduced to the ftate of ore, then the regulus of antimony is freed of. its fulphur, and finks to the bottom, as it is heavier than the additional bodies, when joined to the fulphur. Such a precipitation by fufion happens in vitrifications, fcorifi- cations, and coppelings, while one part of the body turns intodiofs, otfioria, and the other metallic part, if there be any, keeping ftill its metallic form, is colle&ed at the bottom of the fphcrical veflel. Therefore filver and gold, which are hardly fubjecr to a perfe& vitrification, conftantly remain and lhew themfelves in their own form; and on this account, tho' they were in ever fo fmall a quantity in a coppel, they always fhew themfelves very clearly to the eye when the fcir'm are abforbed; whereas fo fmall a regulus of the other metals would have been as it w^re buried and hidden under fo great a quan- tity of fioria.

Nor is precipitation by fufion lefs neceffary to obtain a'moft all the other metals, which on this account are called lefs perfect:, unlefs perhaps you except a very fmall quantity of native me- tal, which neverthelefs can fcarce be properly called pure. Befide, they are all to be had either in form of earth, or of that of a folid ore. In the firft cafe you may make a glafs by a bare fufion : in the fecond, if the fulphur and arfenic, which, together with the metallic part, conflitute an ore, are diffipated in roafting; the ore deftiture of the oily phlbgifton, becomes glafs in a pure fire, which glafs may be mixed with unmetallic ftones and earths ; but by adding a phlogifton or inflammable principle to it, this metallic glafs is again reduced to its metalline form; and fo long as it keeps under this form, it cannot be united with the glafs of the other fpecies, hut finks to the bottom of it, except only a very final! quantity of it, which is detained by the clammincfs of the glafs. The precipitating body in this cafe, therefore, is truly the phlogif- ton, or inflammable principle : any body that takes away the connection, by the removal of which a precipitation is made, is properly called the precipitating body. Cramers Aft! p. 1 87. Precipitation of folid bodies from fluid menftrua, is per- formed either by extracting, or evaporating over a gentle , fire the diflblving menftruum out of the diflblved fixed "body, or by adding fuch a body, as is greedily diflblved by that menftruum : as if one metal diflblved by an acid is precipi- tated hy another metal, or by an alkaline fait: for inftance, filver diflblved inaqua fortis is precipitated by copper, copper by iron, iron by zink, and all metals and femi-meals, either part- ly or entirely by pot-afhes, volatile, and urinous falts. A precipitation is alfo made by pouring on a folut'ion, fuch things as either cannot diflblve the body in hand, whether alone, or joined to a menftruum that contains the faid body ; or diflblve it in another manner; or in a Iefler quantity than if the menftruum had been ufed pure. In the firft cafe a to- tal precipitation is performed, as may be feen in the precipi- tation of filver out of aqua fortis, by means of fpirit of fait. In the fecond a great deturbation and precipitation is made, but a fecond folution foon follows, as it happens when iron be- ing diflblved in aqua fortis, you add to it in a proper manner a liquor perfectly free from alkaline fixed fait ; but then there remains ufually a certain part which is not perfectly diflblved a fecond time. In the third cafe there is but a partial preci- pitation made. You have an inftance of this, if mercury dif- folved in aqua fortis, and the menftruum thoroughly faturated with it, is precipitated either by common fait, or fal armo- niac, or by their acid fpirit. A precipitation is alfo fometimes made only by adding a large quantity of fair water to dilute ; fuch is that made on the regulus of antimony diflblved in fpirit of common fait, or in aqua regia, when a large quan- tity of cold water is poured on this folution ; for thefe men- ftrua do not diflblve this femi- metal, unlefs concentrated. AH thefe precipitations are promoted greatly by a gentle heat, by means of which the precipitating body enters more eafilv into the menftruum, and a confiderable quantity of water is next neceffary to dilute with, except in the precipitations 3 of the firft kind ; for moft commonly the more concentrated dif- folutions aflume the confidence of a parte fo foon as the pre cipitatlng body is added to them, which hinders this from mixing equally with the folution. Id. ibid. p. 196. Boerhaave makes the following obfervations on the different manner in which precipitation is performed by feveral different agents.

Thus 1. By water poured on oils diflblved in alcohol, where the liquor turns milky.

2. By water poured to folid refins diflblved in alcohol, where alfo the liquor turns milky.

3. By water in the diftillation of oily fpirits, if any water run after the fpirit is drawn off.

4. By acids on acids ; thus iilver and mercury are precipitated out of fpirit of nitre, in which they had been diflblved by adding fpirit of fait.

5. By metals with metals, and other bodies. Thus for ex- ample, dilute an ounce of filver diflblved in fpirit of nitre,

2 Z z with