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whieli it held in solution. Or sonactimes it arises Dom a comp0und being formed by the union or one bo h With an other, insolubie in the liquid that is the medium os action. The precipitate is allowed to subside, is usualty washed i thwater, and is driod. Froin the law of chemicat attraction, that quantity influences the force of assinity, it osten happens that tho precipitate either retalias in combination a portionos the substance by which it had beeu dissolved, or attracta a portion os the substance by which it is thrown down, and
this sonaetimes proves a Source of impurib, or os peculliarpo vers in medicinat preparations. When a substance, in paSSing to the solid state, assumes a regular geometrio forna, the process is named CRYSΤALLI-ZAΤION, and these figured masses are denominated Crystals. Their fornis are various, though nearly constant Mil regaritto each Si Stance ; they are usualty transparent, ha J, and have a regular internes structuro. The crystultiZation in happen in livo ways srom a state os solution. Is a saturnted
solution has heen prepared with tho aid of heat. the inc eased quantity of the solid, u hicli the hoat has enabled the liquidio dissolve, separatos as the temperature falis ; and the attraction os cohesion heing thus flowly exerted botu cen the particles, unites them so as to sorin crystias. Or, is a portion of the solvent be withdra evaporation, aud e e-cialty by slow ovaporation, the particios of the solid mitto flowly, and with a similis result. In both these Liniis of crystalliration froni a watery Solusetion, the crystalliZed substunce always Petreiiis a quantity of watCP, and s requently even a considerable proportion in iis composition. It is ossentiat to tho constitution of the crustat, iis ii 'ansparency, Structuro, and forna, aud is henco named the vator es Crystalligation. Sornu costias Iose it Dom
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OP THE GENERAL PRINCIPLEsmere expostare to the air, when they are salii to egloresce; osters attra et water, aud become humid, or deliquesce. Crystalligation is promoted by the mechanicat actiori of thuair ; likewise by ais0rding a nucleus, Whence it may COm- menco, and e ecialty a crystal of the substance dissolvod ;and with regarit to a sew substances, their astini ty to the sol vent requires to he diminished by the addition os anothed substance to ensile them to crystalliae. In Pharmacy, crystalligation is of importanςe, by enablingus to obta in Substances, especialty those helonging to the classos salts, in a pili e serm ; different Salis, even When preSent in the sanie solution, heing thus separated by their different tendencies to crystalliZation, acco ing aS they are more OPless solubie in the solvent, or have their solubility more OP
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sure to different liquids, uniess the duo allowance be made sorthe diiseretice in specilio gravit1 . This it is to be presumedivili osten he neglected, and lience tho Edinburgii College have rejected the use of me ures, and oven the proportionsos every liquid by weight. The use of measures, ho eve in apportioning liquids,. those at least whicli are not too dense, benig more easy aud convenient, Will probably alwVS be re- tesued ; and it is there re sanctioned by the Dublin and tho London Colleges, in the late edition of their Pharmacopoeias. The Dublin College ad it tho usual division of the wine gallion into eight pomails or pinis, the pound into si xteen cunces, and ille omice Dato eight drachnis. The London College distingitisti them, at the fame time, by particular appellations, whicli camiot be oh unded with those denotina the weighis, as represented with their symbols in the solio; ing table:
is liable to vary, not only according to the mobility and sp cisic gravity of the liquid, a circumstance of inserior importance, Since With regard to cach substance it remalias the fame, but also according to the thickness of the cdgo of tho Vesset, and the degree of inclination. The London Collum have theresere substituted this divi lan os mininas, Whicli ai I
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has heen attached to the terna, compriam the history both of Aliments and of Medicines. It is HSed, howeVer, more CO rectly, as opposta to the Materia Alimentaria; and in iliis limited sense may be defined that departinent of Medicine, Whicli describes the properties, and iuvestigates the offecison the living system, of those SubStances Whicli are employ-ed as remedies against discas -Substances whicli are notnecessa to the immediate support of the functions os lis i to repair the waste of the boh, or furnish matter ivlienceiis secretions are derived, but are more peculi j ad tedio excite actions in the System, or produce changes, witha view to the removat os morbid states. It includes the hi toxy of these substances, independent of the preparations towhicli they are subjected to fit illem for administrati0n, this hel ging to the depretinent os Pharmacy.
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ΤΗΕ subjects of inqui , in the study of the aclicles of the Materia Medica, may be compriZed under theiae Natu alHisto , their Chemicat History, and what may be more strictly denominated their Medicat Histo . The utility of ΝATURAE HIsΤORY in furnishing appropriate characters by whicli the productions os nature in ho distinguished froni cach other, is abundantly obvious; andiis application to the steticles of the Materia Modica is underthis potnt of vlew indispensabie. From Want of Such characters, many of the remedies described by the ancient phy-Sicians cannotino v be accitaralely ascerta ined; did ivo notpoSSeSS them, our observations would in the progress of timehe liable to the fame inconvenience ; and the accinite distinctions whicli the methods of natural histo assord, are at present necessary to discriminato belweeu substances Whichlime a near resemblance to each other, or to describo with accuracy ille remedies employed in disserent comitries. This subject has like vise been considered under a litior potat of vie v. Frona attention to the charactera of the articles of the Materia Medica, as they aro objects of natu alhistory, it has been supposed, that assistance may bo derive l
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PRELIMINARY OBGERVATIONs. 9su: the investigation of their virtues. In those artificiat sys tonas of classification, indoed, in whicli the avrangements arefovia ded entirely on a Riv leading discriminating characters, the natural alliances whicli exist among bodios are osten dis-x arded, and they are in no crae particularly traced ; the substances whicli are associaled being placed together moesyDona possessing these characters, though they may at thesanie time disser dely in the generat assemblage of their qualitios. But in those natural methinis of clamification in
whicli the arrangement is fio unded on the occurretice of anumber of characters taken from What is essentiat to tho substance, the gradations os nature are more Strictly oh-
served, and those hodies are anan ged together, Whicli, in their generat aspiratice, nature, and qualities, have a close resemblance. It is the prosecution of this natural methodihat has been supposed usesul in ascertaining the medicinalviriues of the productions os nature a Supposition uot un-reas ahle, Since, Where there exiSts a natural resemblauce
in structures and qualities, it is no improbabie inserenco that there may be a resemblance in medicinal powevs.
In the vegetable Lingdom especially, this natural assinitI has been industriousty tracod and applied to this purp0se.
Those vinet les whicli agree in their generat structure, habit, and appear maee, are thyOWn into What are named Νatural Orders or Familics; and experience has site via, that the individuals composing many of these natural Orders baxea remat Eable similarity in their effecis on ille system. Iullie subdivisions of the order, this analogy is not lcss striking, the disserent species having in gestierat similar virtuos. Ii, there re, a ne v species of any of these genera be discu-Vered, the discoverer may infer Ruth somo probabili ty apriseidii, that it Will possess viriues simitor 10 1hose of the genus to ivllicli it l, longs.
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MΛTERIA MEDICA. I liis cstcrion of the virtuos of medicines, though u doubi edly so far just, is however liable to many exceptions. Alany natural ordei A are composed os vegetabies, whicli, though they a ce in structure, have the most various and opposite qualities ; and even in those in whicli iliore is thogreatest similarity, there are important disserencos in thep perties of many planis arranged under them. Even in ille subdivision of the genus, there is osten a remain te di feren ce in the properties of the species ; and what sussiciently potnis out the deficiency of this method, different paris of the sanae plant have frequently Opposite poWEPS.Τet it is to he admitted, that with ait these exceptions, Na- tu alisis have osten been led by such analogies to just conclusions respecting the viriues of planis ; and in Stu ingthe vegetable part of the Materia Medica, attention is un- ubtedly due to these natu ut distinctions. Λ part of the Natural History of Medici es, of not lessimportance than their generio and speci sic characters, is the accurate description of the d sensibie qualities. Such descriptions afford the most obvious method of distinguiShing them, and in many cases also the most Easy and certain criterion os thei, purity aud perfection. Λ knowladge of these qualities is not less necessa in Ieading totheir proper administration, Since, Dum the peculiar qualities oriaste, flavour, Specific gravity, ox consistenco in mySubstance, Ono seriai may be better adapted to iis exhibition
It has also been imagined, that the sensibio qualities of medicines, particularly their taste anil smeli, afford indications os ilicir peculiar po vera, and experience to a certatu extent consimiis this supposition. In the vegetabie kin omespecially, it has beUn soland, sint substances which are in ripid and inodoxous xarely posscss any impol tant medicis 3I
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viriue, und a number of such substances have justly besin discarduit froni practice froni attention to this circumStance :their insipidity having led to suspicion of their activity, and occasioned a more strict examination of the evidence On whichtheir Supposed virtves ivere said to be establisted. on the0ther haud, planis possesSing much Odour or lane, are in generat active remedies ; and those whieli resemble cach otheran these qualities, have osten the sanie generat medicinalpo verS: thus astringenty is indicated by a styptic taste; bit
error in these indications, that they do not admit of veryeXtensive accumte application. The disserent iastes anilodours are so litile reducibie to preciso definition or description, that fetu generat xules can be formed froni them ; and eveu to the fetu that have been dolivered on this subjeci, there Me many exceptions. The most activo vegetable Sub StanceS too, have not these properties more poculiar than many others comparativeb inert, and henco it is not ostenthat much assistance can he derived freni this criterion os the viriues of planis. The CHEMIcAL HISTOAY of the articles os the Materia Medica forins another important generes object os investigation.
The opinion Scems to have he n early ad ted by those who cultivaled chemisto with a vieis to iis application to medicine, that those substances whicli act in a similar man ner on the living body must be composed of the fame principies, and that there re chemicat analysis may be a Successisl method os investigating thola modices virtuos;- opinion not altogether vitreasonable. The properties of any compound depend on iis chemicat composition ; they originatu froni that composition, and are alteIed by every varia-
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tion whicli it sussors. The medicinal powers of Quin sui Stauces must, in common with their other qualities, dependon the fame cause; and it is not vitreason te to preSume, that where similar po vers exist, they arise honi similarity of composition, et ther with regard to the constituent principies, or to the peculiar mode in whicli these are united. Consding in the justness of these conclusiotis, the Chemisis, about the beginning of the 17th centu , bestomedmuch labour on the analysis of the different vegetabies used in medicine. Above 500 planis ivere analyZed ; but this I bour led uot to a single useful result; and had even the analysis been per med with ait thoso essenties precautions, whicli it was impossissile that the state of Chemisto at that period could have hunished, the nature of it was such, thatit coiud inord no usesul information. The planis subjectedio analysis mere exposed to heat, and the producis collected ; but as these pxoducis do not pre-exist in the veget te,
but are sormed by neis combinations of iis elementS, and as lthese elements are in est vegetabies nearly the SamC, no Con-
t les afforded the fame producis; stud is the experiment
ta the physician would bo obta ined. Similar proximate iprincipies of different planis, though possessed of different limedicinal po vers, would give similar resulis ; or is any din l
discoVer on what chemicat principio, or si aut peculi rity os lcombination, ille medicinal viriues of any active vegetable ν