Page:Cyclopaedia, Chambers - Volume 2.djvu/483

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further and further back, among the preceding Signs of Stars ; at the rate of about 50 Seconds each Year : which retrograde Motion is call'd the FreceJJion, Recejfion, or Re- trccejfion of the Equinoxes.

Hence, as the fixed Stars remain immovable, and the Equinoxes go backward ; the Stars wilt feem to move more and more Eaflward with refpect thereto ; whence the Lon- gitudes of the Stars, which are reckon'd from the firfl Point of dries, or the Vernal Equinox, are continually in- crcafing. See Longitude and Star.

Hence it is that the Conflellations have all chang'd the Places affign'd them by the antient Astronomers : In the Time of Hipparchus and the oldeft Astronomers, the E- quinoctial Points were fix'd to the firit Stars of Aries and Tibra ; but the Signs are now no longer in the fame Points ; and the Stars which were then in Conjunction with the Sun when he was in the Equinox, are now a whole Sign, or 50 Degrees, to the Eafl thereof: Thus the firft Star of Aries, is now in the Portion of the Ecliptic call'd 'Taurus ; and the firfl Star of Taurus now refides in Ge- mini ; and Gemini is advanced into Cancer, &c. See Sicn and Constellation.

The Equinoxes will have made their Revolution Weft- ward, and will be return'd to Aries again ; or the ConStel- lations will have made theirs Eaflward, and will again fall into their former places, with regard to the Equinoxes, in 25815 Years, according to Tycho ; in 259-0, according to RiccioluS, and in 24800, according to CaJJini.

The Antients, and even fome among the Moderns, have taken the Equinoxes to be immoveable ; and afcribed that Change of Distance of the Stars here-from, to a real Mo- tion of the Orb of the fixed Stars ; which they fuppofe to have a flow Revolution about the Poles of the Ecliptic : fo as that all the Stars perform their Circuits in the Eclip • tic, or its Parallels, in the fpace of 25920 Years ; after which, they fliould all return again to their former places.

This Period the Antients call'd the Platonic, or great Tear; and imagin'd that at its Completion, every thing would begin again as at firfl : and all things come round in the fame Order they have already done. See Platonic Year.

The Phyfical Caufe of the FreceJJion of the Equinoxes, Sir Jfaac Newton demonstrates, does arife from the broad fpheroidat Figure of the Earth ; which again arifes from the Earth's Rotation a-round its Axis. See Earth.

PRECIOUS, or Pretious Stone, call'd alfo Gem, and Jewel, is a Stone extraordinarily hard, durable, tranfpa- rent, and of a beautiful Colour, or Water. See Stone and Gem.

Of thefe we may distinguish three Kinds -. i°. Such as are entirely tranfparent ; which again may be divided into fuch as are either colour-left, as thetDiamond ; or colour'd, as the Emerald : Which Divifion of colour'd Gems may be fubdivided into thofe of one Colour, as the Ruby ; and thofewith feveral, as the Amethyjl. 2 . Brilliant, or mi- ning, as the Bohemian Granate. 5 . Semi-tranfparent, as Opal.

Bifhop Wilkins divides precious Stones into more and tefs tranfparent. The lefs tranfparent he distinguishes by their Colours : into red, as the Sardius and Cornelian ; pale flefliy Colour, like that of a Man's Nail, as the Onyx ;' blueilti, as theTurquois; pale purple, as the Chalcedony; and thofe of various Colours, as Opal and Cat's Eye.

The more tranfparent he diflinguiflies into fuch as are colourlefs, as the Diamond and white Saphir ; and colour'd which are either red, as the Ruby, Carbuncle, and Gra- nate ; yellow, as the Chryfolite, and Topaz; green, as the Emerald, Smaragd, and Beryl; blueifh, as the Saphir; and purple or violaceous, as the Amethiit and Hyacinth.

Dr.JVoodward divides precious Stones fomewhat more precifely, into opake, femi-opake, and tranfparent. Again, iff, the opake are either of one Colour, as the Turqu'ois j or of various Colours, as Lazuli and Jafper. 2d, Semi-opake, either have their Colours permanent, as the Agar, Chal- cedony, Onyx, Sardonix, Cornelian, and Beryl; or their Colours vary, according to the Pofition of the Light, as the Oculus Cati, and Opal. 3d, Tranfparent Stones are either with Colours; as the Topaz and Jacinth, yellow, or par- taking thereof; Granate, Rubv and Amethyft, red ; Sa- phire, Water Saphire, and Aqu'e marine, blue; and Eme- rald, or Cryfolite,^ green, or partaking thereof: or without Colours, as the Cryftal, Pfeudo-Diamond, white Saphire and Diamond. '

The natural Hijlory, Characters, Properties, Sic. of each Stone; fee under its proper Article : Diamond, Corne- lian, Ruby, Turo,uois, Onyx, Emerald, Cryso- lite, &c.

The Medicinal Virtues of precious Stones, or Gems in the general; fee under Gem.

The Origin and Formation of precious Stones, fee under Stone.

For the Art of engraving on precious Stones, fee E„,

SHAVING. '"'

The Art of cutting them, fee under Lapidary.

PRECIPE, or Praecipe quod reddat, a Writ of arc... Diverfity, both as to Form and TJfe. See Entry ar, Ingressus. '"

It extends as well to a Writ of Right, as to other Writs of Entry and Poflefflon ; and is fometimes call'd a Writ of Right, clofe, as wlien it iffues out of the Coutt of Chancerv dole ; fometimes a Writ of Right, patent, as when it if lues out of Chancery, patent or open, to any Lord's Court tor any of his Tenants deforced, againft his Deforcer. See

PRECIPITANT, inChymiilry, a Term apply'd, oanv Liquor which being pour'd on a Diffolution, feparate' what is there diflol red, and makes it precipitate, i.e fall to the bottom of the Veffcl. See Dissolution.

Thus Oil of Tartar, and the Volatile Spirit of Sal Ar moniac, are Precipitant!, with regard to the Diffolution ot Gold in Aqua Regain; and common Water is a Trecipi tant, wiih regard to the Diffolution of Jalap in Spirit „f Wine. See Precipitation.

Precipitant, is alfo ufed in Medicine, for a Remedy which feparates and precipitates any heterogeneous Matter contain'd in the Mafs of the Blood ; and by this me ins abates any irregular Fermentations, Effervefcences, or the like Diforders, which that Matter had excited.

Among the Number of Precipitants, are rank'd Harts- Horn, Crabs-Eyes, Ivory, Bezoard, Barks of Oak, and Guaiacum, Iron, Quinquina, Chalk, He.

PRECIPITATE, in Chymiftrv, aSubflance which ha- ving been diffolvedin a proper MenStruum, is again Sepa- rated from its Diffolvent, and thrown down to ihe bottom of the Veffel, by the pouring in of fome other Liquor. See Precipitant.

The Chymilts make various Precipitates of Mercury which are of various Colours, as the Precipitants vary ; viz. White, Red, Tellow, Green, Sec. See Mercury.

Ihe white Precipitate, is prepared of Mercury diffolved- in Spirit of Nitre, and precipitated wilh Salt- Water, or Spi- rit of Salt, into a white Powder.

It in lieu of the former Precipitants, hot Urine be pour'd on the Diffolution, we have a pale Rofe Colour.

To make the red Precipitate, they take the Diffolution ot Mercury made in Spirit of Nitre, evaporate all the Hu- midity over a gentle Fire, till nothing remains but a white Mais ; which by increasing the Fire, they rubify or raife to a red Colour Green Precipitate is made with Mer- cury, Lopper, and acid Spirits Tellow precipitate with

Mercury, and Oil of Vitriol : But !hefe three lafl are im- properly call'd Precipitates; becaufe not procured by Pre- cipitation. See Precipitation

PRECIPITATION, an Operation in Chymiffry, being a kind of Separation, whereby a Body diflblv'd in any Li- quor isdetach'd there-from, and falls down to the bottom of the Veffel. See Operation. Precipitation is either Spontaneous or Artificial. Spontaneous Precipitation, is when the Particles of the diffolved Body, feparate of themfelves from their Dif- lolvenr.

Artificial Precipitation, is when fome other Body, call'd a 'Precipitant, is added to procure this Separation'. S;e Precipitant.

There is alfo a total Precipitation, wherein the diffolv'd Parts are all feparated, and funk to the bottom; and 1 partial Precipitation, wherein the Parts diffolv'd are {till fufpended in the Fluid ; and don't fall down.

Theory of Precipitation.

To account for the Operation of Precipitation : It may be obferv'd, that a fluid Menflruum may be made to fuf- tain a Body fpecifically heavier than itfelf, either by ma- king the Refinance, arifing from the Cohefion of the Parts of the Fluid, equal to the Excefs of foecific Gravity of thofe Bodies above that of the Menftruum. See Men- struum.

Or by the heavy Body's being join'd to fome lighter one ; fo that the two together only make one whole, equal in weight to the Fluid.

In the firit cafe, the Refiflance, we know, is flill propor- tional to the Surface of the Corpufcles ; fo that the Sur- face being diminifh'd, the Refinance is weaken'd: the Proportion therefore of the Tenacity of the Menflruum, to the Gravity of the Corpufcles being thus deflroy'd, a Pre- cipitation mull enfue.

Precipitation, then, may be effeaed two ways, on this Foundation ; viz. either by the dropping in a Liquor fpe- cifically lighter, or Specifically heavier; in the former Cafe, the Gravity of the Menflruum, which is always proportional to the compound Gravities of both, will by this Mixture be- come lighter : Thus, the Menftruum being diluted, the Force

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