OPA
their Meetings, as often as there were Letters in their Names. Thus Martial,
Nxvia fex Cyathis, feptem Juflim bibatttr.
Rhoiegims defcribes a Angular kind of Onomamia : Thus 'Thcodotus, King of the Goth:, being curious to know the Succefs of his Wars againit the Romans, an Onomantical Jew order'd him to ihut up a Number of Swine in little Styes and to give fome of them Roman, to pothers, Gothic Names, with different Marks to diftinguifh them ; and there to keep them to a certain Day: which being come, upon infpecting the Styes, they found thofe dead, to whom the Gothic, and thofe alive, to whom the Roman Names were given : Upon which the Jew foretold the Defeat of the
The Word Onomantia is form'd from the Greek 'cvoua, Name and (uivrrAa, Divination. Indeed there is fomething fingular in the Etymology : for, in ftrictnefs, Onomancy Jhould fignify Divination by AJjes ; from ov@-, Afmtts, and uztmia. And to fignify Divination by Names, it fhould be Onomatamancy .
ONOMAIOPjEA, in Grammar, lie. a Figure of Speech, whereby Names and Words are form'd to the Re- femblance of the Din or Sound made by the Thing figni- fy'd ; as Tricqite-Track, from the Noife the Ladies make at this Game : And from the fame Source arifes the Buzzing of Bees, the Grunting of Hogs, the Cackling of Hens, the Snoring of People afleep, th eC/.i/iim; of Arms, £?c.
The furell Etymologies are thofe deduced by the Onoma- topaia. See Etymology.
The Word is form'd from the Greek 'ovofui. Name, and "zroiiy, jingo, I make, feign.
ONONYCH1TES, fomething that has the Hoof:, that is, the Feet of an Afs : A Name the Heathens, in the firft Century, gave the God of the Chriftians, becaufe they owned and adored the fame God with the yews. Now 'twas a Notion, (howfoever it had its Rife) as appears from Cor. Tacitus, Hifl. lib.*}, c.16. That the Israelites, much af- filed with Thirft, were led to a Spring by an Afs going to drink ; and that in gratitude for the Benefit, they worship- ped an Afs 5 and that the Chriftians did fo likewife. See 1'ertnll. Apol.
The Word is form'd from the Greek ov@-, Afs, and %lv% ,
Hoof.
ONTOLOGY, orONTOsoPHY, the Doctrine, or Know- ledge * Etite, of Being, in the General, or AbftracL See Ens.
Ontology coincides with what in the Schools we more u- fually call Metaphyficks. See Metaphysicks.
ONYCOMANCY, or as fome have ir, Onymancy, a kind of Divination by means of the Nails of the Fingers. See Nail.
The anrient Practice was to rub the Nails of a Youth with Oil and Soot, or Wax 5 and to hold up the Nails thus rubbed aoaintt the Sun. Upon them were fuppofed to appear Figures or Characters, which fhew'd the Thing requir'd.
Hence, our modern Chiromancers call that Branch of their Art, which relates to the Infpection of Nails, Onyco- mancy.
The Word is form'd from the Greek cVi/f, Nail, and fj.av7\'icr, Divination.
OPACITY, in Philofophy, a Quality of fome Bodies, which renders them Opake, i. e. impervious to the Rays of Light. See Lioht.
The Term Opacity is ufed in oppofition to Tranfparency. See Transparency.
Opacity, according to the Cartefians, confifls in this, that the Pores of the Body are not all ilrait, or direclly before each other ; or rather, nor pervious every way.
But this Doctrine is deficient ; for tho' it muft be al- low'd that to have a Body tranfparent, its Pores muft be firaight, or rather, open every way : Yet how it fhould happen, that not only Glafs and Diamonds, but even Water, whofe Parts are fo very moveable, fhould have all their Pores open and pervious every way ; and at the fame time, the finefl Paper, or the thinned Gold Plate, fhould exclude the Light for want of fuch Pores, is incon- ceivable. So that another Caufe of Opacity muft be found.
Now, all Bodies have vaftly more Pores or Vacuiries, than are neceffary for an infinite Number of Rays to find a free Paflage thro them in right Lines, without ftriking on any of the Parts themfelves. For fince Water is nineteen times lighter, i. e. rarer, than Gold ; and yet Gold it/elf is fo very rare, that Magnetic Effluvia pafs freely thro' it without any oppofition ; and Quickfilver is readily receiv'd within its Parts, and even Water itfelf by Compreffion, it muft have much more Pores than folid Parts : Confe- quently, Water muft have at lead forty times as much Va- cuity as Solidity. See Pore.
cm )
OPA
The Caufe therefore why fome Bodies are ofafa, does not contift in the want of reailinear Pores, Dervious every way ; but, either in the unequal Denfity of the Parts ; or in the Magnitude of the Pores, and being either empty, or fillM with a different Matter ; by means whereof the Rays of Light, in their PaiTage, are arretted by innumerable Re- fractions and Reflexions 5 till, falling at length on fome folid Part, they become quite extinct, and are utterly abforb'd. See Ray and Refraction.
Hence Cork, Paper, Wood, &c. are opake; while Glafs, Diamonds, £S?t\ are pellucid. For in the Confines, or join- ing of Parts alike in Denfity, fuch as thofe of Glafs, Water, Diamonds,^, among themfelves ; there arifes no Refrac- tion or Reflection, by reafon of the equal Attraction e- very way. So that fuch of the Rays of Light as enter the firft Surface, pafs firaight thro' the Body; excepting fuch as are loll and abforb'd, by finking on folid Parts. But in the bordering of Parts unequal in Denfity j fuch as thofe of Wood and Paper, both with regard to themfelves, and with regard to the Air or empty Space in their larger Pores, the Attraction being unequal, the Reflections and Refractions will be very great; thus the Rays will be unable to pafs thro* fuch Bodies, being continually bandied about, till they become extinct. See Attraction, fci?c.
That this Interruption, or Difcontinuity of Parts, is the chief Caufe of Opacity, Sir Ifaac Newton argues to ap- pear hence, that all opake Bodies immediately begin to be tranfparent, when their Pores become fill'd with a Sub- flance of equal, or almoft equal Denfity with their Parts; Thus Paper dipt in Water or Oil, the Stone call'd Ocidus Mundi fteep'd in Water, Linnen Cloth dipt in Oil or Vine- gar, and other Subftances foak'd in fuch Fluids as will in- timately pervade their little Pores, become more tranfpa- rent than before. On the contrary, the moft tranfparent Subftances, by emptying their Pores, or feparating their Parts;, may be render'd very opake : Thus Salts, or wet Paper, or Octtlus Hundi, by drying ; Horn, by fcraping ; Glafs, by pulverizing or flawing ; und Water itfelf, by being beat into Bubbles or Froth, are render'd Opake.
Indeed, to render Bodies opake and colour'd, their In- terftices muft not be lefs than of fome determinate Size : For the moft opake Bodies that are, if their Parts be very minutely divided, as when Metals are divided in Acid Menftruums, become perfectly tranfparent. See Colour, Transparency, &c.
OPAL, a precious Stone, of various Colours. See Pre- cious Stone.
In it are feen the Red of the Ruby, the Purple of the Amethyft, the Green of the Emerald j befides Yellow, and fometimes Black and White.
When the Stone is broke, moft of thefe Colours difap- pear ; which mews that they arife by Reflection from one or two principal ones.
Its Form is always either round or oval ; its prevailing Colour white. Its Diverfity of Colours makes it almoft of equal Value with a Saphir or Ruby.
Tavemier fays, perhaps fomewhat too positively, that there are Mines of Opal in Turkey. Other Authors, antient and modern, fay, Cyfrus t Arabia, Egypt,, Bohemia, and Hun- gary, produce it ; whence it js diftinguiih'd into two kinds, Oriental and Occidental. 'Tis too foft to bear poli/hing with Pewter or Lead ; fo that they are obliged to polifh it with Tripoli.
Pliny among the Antients, and Forta and Albertus 'Magnus among the Moderns, are very copious on the Virtues of the Opal; forfooth, becaufe it has the Colours of all the 0- ther precious Stones, ir muft have all their Virtues too. The Antients call'd it Faideros, from its promoting Love and Good-Will. Fliny and Salinas mention a Species of Opal, call'd £xecot;tahtha, which had fixty Colours.
Jrtifcial Opal. In the Fhilofoph. TranfaB. TAr.Colepreffe gives us an account of the manner of counterfeiting Opal, as practifed at Harlem. He fays, the Counterfeit is very lively, and thinks it only perform'd by the Degrees of Heat, which produce the Colours. When the Compofition is melted, tney take out fome on the Point of an Iron Rod, which being cool'd either in the Air or Water, is co- lourlefs and pellucid j but being put into the Mouth of the Furnace on the fame Rod, and there turn'd by the Hand for a little fpace, hath its little Bodies fo variously pofited in ivarious Parts of the fame Piece, that the Light falling on them, being varioufly modify'd thereby, repre- fents the fevcral Colours feen in the natural Opal. He adds, the Colours may be deftroy'd and reftored, according to the various Motions of its Particles by Heat.
OPALIA, orOpicoNsivA, Feafts celebrated at Rome, in honour of the Goddefs Ops. Varro fays, they were held three days after the Expiration of the Saturnalia.
According to Macrobius, they were held the nineteenth of December, which was one of the Days of the Saturnalia. He
adds,
