T H F 



THEORETIC, Thkorf.tical, or Theoric, fom\ed 

 from S..--. /A or.onlrml,lju, fomething relating to theory, 

 or tliat t'.Tii.ii..ilcs in fpeculatio... In which fenle it Hands 

 oppofcd to hraBical. 



The Iciences are ordinarily divided yMo theoretical, asthco- 

 lofry, philofophy, &c. ; md praflical, as medicine, law, &c. 



Set SllKSCK. ,• 1 • 



TiiKORETir, Theoretiais, is an appellation peculiarly given 

 to an ancient fed of phylicians, contradiftingui(hed by it from 

 the empirics. See EMflKiC. 



Theoretic plivficiaiis were fiich as applied themfelves to a 

 careful lludy and confideration of what relates to health and 

 difeafes ; the principles of the human body, and its ftrufture 

 and parts, with their actions and ufes ; whatever befals it, 

 cither naturally or pretcrnaturaUy ; the differences of dif- 

 eafes, their nature, caufes, figns, indications, &c. ; the tex- 

 tures, properties, &c. of plants, and other medicines, &c. 

 In a word, the liieoretic phyficians were fuch as proceeded 

 in their judgment and praftice on the fool of reafon, in op- 

 pofition to the empirical phyficians, who proceeded wholly 

 on experience. See Medicine. 



Theoretical Arithmetic and Philofophy. See the fub- 

 llantivcs. 



THEORI, Swfoi, in Antiquity, an appellation given to 

 thofe Athenians who performed the foleumity called thcoria. 

 THEORI A, Siifix, a folemn annual voyage to Apollo's 

 temple, in the ifland of Delos, performed by the Athenians 

 always in the fame (hip in which Thefeus went. For the 

 particularities of this naval proccflion, fee Potter Archasol. 

 Grncc. lib. ii. cap. 9. torn. i. p. 284, feq. 



THEORIC lAo'St.\,\n Ancient Authors, was what was 

 raifcd, by way of tax on the people, to defray the expences 

 of theatrical reprefentations, and other fpeftacles. 



There were particular quedors and treafurers of the theoric 

 money. By a law of Eubulus, it was made a capital crime 

 to pervert tlic theoric money to any other ufe ; even to em- 

 ploy it in the occafions of war. 



THEORICAL Astronomy, is that part of aftronomy 

 which conliders the true llrufturc and difpofition of the hea- 

 vens, and heavenly bodies ; and accounts for their various 

 phenomena therefrom. Sec Astronomy. 



It is thus called, in oppofition to that part which con- 

 fiders their apparent ftrufture, or their difpofition as viewed 

 by the eye, which is cA\cA fpherical aftronomy. 



The feveral parts of theorical aftonomy, fee under Sys- 

 tem, Sln, Star, Planet, Earth, Mook, Satellite, 

 and Comet. 



THEORY, a doftrine which terminates in the fole fpe- 

 culation, or confideration, of its objeft, without any view to 

 the praftice or apphcation of it. 



To be learned in an art, &c. the theory fuffices ; to be a 

 mafter of it, both the theory and praftice are i-equired. Ma- 

 chines, many times, promife very well in the theory, yet fail 

 in the praftice. 



We fay, theory of the moon, theory of the rainbow, of 

 the microfcope, the camera obfcura, the motion of the heart, 

 the operation of purgatives, &c. 



Theories of the Planets, &c. are fyftems or hypothefes, 

 according to which the aftronomers explain the reafons of 

 the phenomena or appearances of them. See Syste.m. 



Tjieory, in AJufic, in the hands of a mere mathematician 

 is f onfined only to ratios and the philofophy of found. ( See 

 Harmonics.) I5ut among praaical muficians, the theory 

 of harmony or compofition is connefted with the combina- 

 tion of agreeable founds, ^nd the praftice and performance 

 pi real mufic. 



Theory, Atomic, in Chemifiry, the means of explaining 

 9 



THE 



the compofition and dccompofition of chemical bodies, br 

 confidering their ultimate atoms or particles as peculiar anif 

 diftinft elementary fohds, never changing in their figure, 

 weight, or volume, under any circumltances. 



It would be difficult to conceive the exiftencc of any com- 

 pound, without fuppofing it to have originated by union, in 

 fome way or other, of particles of its elcmentaiy conftituents ; 

 but the prevalence of a doftrine, which has been generally 

 advanced by mathematicians, -ufz. the infinite divifibility of 

 matter, has never allowed philofophcrs to conclude that the 

 circumftance of compounds being made up of particles, muft 

 neceffarily limit the proportions in which the elements com- 

 bine. If the elementary bodies be conceived infinitely divi- 

 fible, the molecules, or compound particles, may be con- 

 ceived infinitely fmall, and the number of mean compounds 

 exifting between any two given extremes may be alfo con- 

 fidcrcd Infinite. 



If fuch were the nature of elementary matter, and no 

 other caufes interfered, there could be no limitation to the 

 proportions in which fimple matter would combine. Thisj 

 however, is contrary to faft; as it is a faft known from the 

 carlicft dawnings of chemical knowledge, that bodies are 

 limited in the proportions of their elements ; the moil 

 ftriking of thefe fafts being the mutual faturation which 

 takes place between an acid and an alkah, and the uniform 

 proportions afforded in the analyfis of many native com- 

 pounds. 



Philofophers were always fatisfied to confider this faft of 

 the limitation of the proportions of bodies as one of the hidden 

 fecrets of nature, as difficult to conceive as the nature of the 

 atti aftion by which their elements were held together. Ber- 

 thoUet appears to have been the firll to attempt this arduous 

 tafk, in his ingenious work, entitled " Chemical Statics." 

 He fuppofes that the particles of bodies, when brought within 

 the fphere of attraftion, combine without controul till the 

 compound affumes fome definite form, by which it is with- 

 drawn from the fituation in which it was formed. He fup- 

 pofes the chemical affinity of bodies to be diftinft from that 

 power on which their cohefion depends, and alfo that power 

 by which they tend to an elaftic ftate. 



Hence he concludes, that every folid compound is deter- 

 mined by the cohefion which takes place at fome limit in the 

 proportion of its elements : fuch he fuppofes to be the cafe 

 with falts and other cryftallizable compounds. On the other 

 hand, he fuppofes the hmitations of the proportions of the 

 elements of gafeous compounds to arife from the elaftic 

 form which they aflume in certain ftages of combination. 

 This hypothefis was fupported by fo many ftriking fafts, 

 that it was thought by fome to explain in general the caufe 

 of limited proportions. All agreed, that whatever might 

 be the true theory, the caufes pointed out by Berthollet had 

 confiderable influence in the compofition and decompofition 

 of bodies, but they faw at the fame time numerous cafes in 

 which this hypothefis failed to explain the fafts. 



Chemifts have, from the earlieft times, been acquainted 

 with thofe points of limitation which we call mutual fatu- 

 ration, and have been long familiar with thofe limited aug- 

 mentations of their proportions, called by fome dofes and 

 by others particles. Among the oxyds of metals, which 

 had been little examined before the time of Lavoifier, it was 

 found, that inftead of having an infinite number of means 

 between the loweft and higheft ftages of oxydation, only a 

 certain number of oxyds of each metal could be formed, in 

 which the ratio of the metal to the oxygen is uniform. 

 Many of the falts in the fame way are formed by limited 

 dofes of acid. Some of the fafts in the latter have been 

 explained on BerthoUet's hypothefis, while its application 



to 



