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ting an horizontal plane. Treat. Pract. Geom, p. 7-5. feq.

THEOGAM1A, eityopt*, in antiquity, a Sicilian feftival in honour of Proferpine, which Teems to have been inftituted in memory of her marriage with Pluto. Potter, Archaeol. Graec. 1. 2. C. 20. T. I. p. 402.

THEOMANTIA, QtopcuUa, in antiquity, divination by the fup- pofed infpiration of fome deity. For a particular account of which, fee Potter, Archaeol. Grace. 1.2. c. 12- T. i.p.298. feq.

THEOPHANIA, S^wn*, in antiquity, a feftival obferved by the Delphians upon the day whereon Apollo firft manifetted himfelf to them. Potter, Archaeol. Grax. 1. 2. c. 20. T. 1. p. 402.

THEOPHRASTA, in the Linnaan fyftem of botany, the name of a genus of plants, the characters of which are thefe : The cup is a fmall perianth ium, lightly divided into five obtufe Tegmenta, and remaining when the flower is fallen. The flower confiftsofa Tingle petal, in form of a bell, {lightly divided into five obtufe fegments. The ftamina are five tapering filaments, fhorter than the flowtjr. The anthers are fimple. The ger- men of the piltillum is of an oval figure. The ftyle is taper- ing, and fhorter than the flower. The ftigma is acute. The fruit is a very large globofe capfulc, containing only one cell. The feeds are numerous and roundtfh, and are affixed to every part of their receptacle, which is loofe. Linnm Gen. Plant. p. 66.

THEOPHRASTICS, a name given to the followers of Para- celfus, from his name Theophraftus.

THEOPNEUSTZE, Qiomivr^, an epithet given to enthu- fiaftical diviners. Pott. T. 1. p. 302.

THEOPROP1A, 0;.;7rpr>?na, a delignation given to oracles. See the article Oracle, Cyd.

THEORI, Qiogoi, in antiquity, an appellation given to thofe Athenians who performed the folemnity called Theoria. See the next article.

THEORIA, e^giK, in antiquity, a folemn annual voyage to Apollo's temple in the ifland Delos, performed by the Athe- nians always in the fame fhip in which Thefeus went. For the particularities of this naval procefiion, fee Potter, Ar- chaeol. GiiEC. 1. 2. C 9. T. I p. 284. feq.

THEORY {Cyd.)— Theory ofChemijiry. Under the Theory of this art are to be laid down all the general truths which the particular experiments of chemifts have hitherto demonftrated. Thefe are, on this occafion, to be taken for granted, and the whole body of fuch truths makes the univerfal Theory of che- miftry, for chemiftry is no fcience formed a priori ; 'tis no production of the human mind, or raifed by rcafoning, but collected a pojleriori from experiments ; it took its rife from various operations carnally made, and obferving thofe that had one and the fame uniform tendency, without any expectation of what followed ; and was only reduced into an art, by col- lecting and comparing the effects of fuch uncertain experi- ments, and noting the tendency thereof: So far then as a number of experiments agree to eftablifh any unqueftionable truth ; fo far they may be confidered as conftituting the Theory of cbemiftry. Such a Theory is neceffary to be pre- mifed to every art ; and fomcthing equivalent to this is pra- ctifed by evety artifan, in teaching his difdple how to proceed orderly in the exercife of his art ; and accordingly it would be impoflible to teach the practice of chemiftry to advantage, without having firft given fome fuch Theory. Thus it would he to little purpofe, to give a novice a parcel of rofemary, for inftance, and bid him, without any addition, diftil a water from it, which fhould contain the natural tafte and odour of the plant; unlefs he knew before-hand this general truth, that plants, expofed to a gentle heat, like that of the fummer's fun, do exhale their moft fubtle and volatile parts, which, being collected and condenfed by means of proper vcflels, appear in form of water, and are the thing required. Shaw's new Meth. of Chem. p. 3.

In the forming fuch a Theory of chemiftry, a direct ufe may indeed be made of the demonftrations in phyfics, as particu- larly in mechanics, hydroftatics, and hydraulics j fince the properties common to all bodies, and what farther affections certainly flow therefrom, have their place in chemiftry. One cannot, however, be too referved in this ufe, fince thofe fin - gular properties found in fome bodies, will, if applied to others, falfify the mechanical demonftrations, which might hold true every where elfe. Thus Galileo has fully demon- ftrated the law, by which an heavy body, let fall from on high, defcends in a fpiral or elliptic line, with a certain degree of acceleration to a point of the earth, perpendicular to the horizon of the point from whence it was firft let fall ; but if a loadftone be thus let fall, and in the courfe of its defcent enter the fphere of activity of another load-ftone, thedemonftration will not hold. So what Archimedes has fhewn, concerning bodies equiponderant in water, holds infallibly true, if con- fidered only in common cafes, but proves falfe in the inftance of gold ; which, tho' it fink in other fluids, is fufpended and diflblved in light aqua regia. Boerhaave, Chem. p. 3. Due regard being had to this rule, the difcoveries of natu- ralifts and mathematicians wil^ always be advantageous, never injurious to the art of chemiftry.

Chemiftry, as now conceived, is an affemblage of very different parts, which antiently fubfifted feparate, or at leait

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had a fubfifter.ee prior to each other, as the preparing of metals for human ufes, the attempts of tranfmuting the bafer metals into gold, the preparing of medicines, &c. If we are to trace the antiquity of chemiftry, as an art that teaches to convert other metals into gold, or to procure an univerfal remedy for all difcafes, the refearch will not carry us far back ; but as it relates to the difcovery of metals in the mine, and the digging, fcparating and purifying them, it chal- lenges even the higheft antiquity.

The art of metals is, no doubt, of very early ftand'mg. To find, procure, fufe, refine, and render malleable, and apply metal to ufe, is even of antedeluvian origin, and was attributed by the antients to their gods. Mofes, the oldeft author ex- tant, in hisganealogy of the patriarchs, relates that Tubal Cain, the eighth from Adam, was the inftructor of every ar- tificer in brais and iron, to prepare the inftruments and uten- fils of life ; and 'tis apparent nothing of this could be effected without the knowledge of metallurgy. This account given us by Mofes, is furprifingly feconded by prophane hiftory and fables ; for Diodorus Siculus, who lived in the time of Czefar, when /Egypt was become a Roman province, had a fair op- portunity of fearching into the antiquities of the ^Egyptians ; and he relates, as the rcfult of his enquiry, a very antient tra- dition of one Hephaiftos, whom that people hold the firft in- ventor of al! arts and operations about metals, and every thing elfe that undergoes the fire; together with their ufes ; which art he delivered down to pofterity, fo far as it might be of fer- vice to mankind. This Hephaiftos of the ./Egyptians and Greeks, is the fame with the Mulciber and Vulcan of the La- tins, to whom the fame art or invention is afcribed ; and the Vulcan of the Latins we generally fuppofe to have been the Tubal Cain of the Hebrews, here mentioned by Mofcs as the inventoF of the laid art.

It appears from Homer, Hefiod, Orpheus, and all the moft antient writers, that Vulcan had the art of working brafs and iron; and that, living under mount ./Etna, he was employed in forging arms for the gods and heroes. As Diodorus Siculus relates the ftory, he was the fon of Jupiter and Juno, and the firft king of ./Egypt, and that he was afterwards preferred to be a god, for having invented fire, and taught men the ufe of it. 'I he /Egyptians, as the fame Diodorus obi'erves, adored their god Vulcan as the inventor of the whole art and applica- tion of metals ; fo that the account of Mofes and Diodorus perfectly agree, and prove the chemiftry of metals almoft coceval with mankind. Shaw's Chem. p. 10. THEOXENIA, Qto&Hx, in antiquity, a feftival in honour of all the gods, and celebrated in many cities of Greece, but efpecially at Athens. Potter, T. 1. p. 402. THEOXINI Malagma, the name of a lort of cataplafm, good

againft pains of the feet. THERAPIDION, in botany, a name given by fome authors to the common oifter-green, or fea-laver, a plant of the tremella kind. See the article Tremella. THERMASM A, a word ufed by fome of the antients to exprefs any thing that warms the body, and by others particularly for a warm fomentation, prefcribed by Hippocrates for removing pains in the fide, and giving eafe in pleurifies. THERMOMETER {Cyd.)~ Dr. Martin has made ufeful re- marks on the conftruction and graduation of Thermometers, and has reduced the moft remarkable of thefe inftruments, which have been employed in different parts of Europe, for obferving the changes in our atmofphere, to one general ftandard, fo as to enable us to compare the feveral obferva- tions together. See his Phyfical and Medical Eflays. THERMOPOLIUM, a name for a fort of public houfes among the antients, in which hot liquors were fold, in the manner of our coffee-houfes. THESEIA, ©who:, in antiquity, an Athenian feftival, in ho- nour of Thefeus, and celebrated on the eighth day of every month. For the ceremonies of this folemnity fee Potter, Ar- chaeol. Gra^c. T. 1. p. 404. THESIS, {Cyd.) inmufic, denotes the fall of the hand in beat- ing time. The Latin writers call it deprejfto. Brojfard. THESMOPHORIA, ©«rpopo,;i« 9 in antiquity, a feftival in ho- nour of Ceres, which was celebrated by many cities of Greece; but efpecially the Athenians obferved it with great devotion and pomp. ' For the ceremonies of this folemnity, fee Potter, Archied. Graec. T. 1. p. 403. feq. THESMOTHETZE, ©ir^S/]*., in antiquity, an appellation given to fix of the nine Athenian arduous ; the firft and chief of the nine was called, by way of eminence, archon; the fe- cond in dignity was called bajileus, the third polemarehus, and the other fix Thefmotbeta ; for an account of whofe power and jurifdidtion, fee Potter, Archaeol. Graec. T. 1. p. 77. THESPHATA, ©ec-fola, in antiquity, an appellation given to

oracles. See Oracle, Cyd. THESPIANA, the name of an antidote, intended for inter- nal abfeefles. THETA,©, among the antients, oneof the Greek letters. Itwas ufed as a mark on the ballots of judges, by which they con- demned the pcrfon to death, it being the firft letter of the word ©«v«laf, death. Whence it had the epithet of n'tger and \nfe- h'x, thus:

O multum ante alias infelix litera Tluta.

4 . THETES,