il rCcT-^- m i It- • i ^ '^^< ^^T !^^ \:A r%^^ --5? ^ r ^ > JTr^-'^ f^ =p.^^^ b«7/ «;- ' &^^ \Bi r^ . ^' ^%^ -^A Y?l^' < & «cC fj i f^-^^ w^m 4 ■ 'CV— "w ""^"^ 'V %^ ^^. ^-v^^ ^_^'^ \^ ^ ^ :;>^ W^// ?/t: ^1 '^*& s4^r^£E ^ uBfiB^fei . w-^ l^^^^V^I f"V ^ \ , ■ ■ f^- ^5 1^^^^^-^ mm^^ r^ ^1 k r Robert E. Gross Colleftlon A Memonal to the Founder of the Business Administration Library " ' Los Angeles I FIRST PLATL OF A CHEMICAL LABORATORY LEV/ IS, V.'ILLIANT . ! ^'V- ]1'<\ Coinmcrcium Fhilosophico-Technicum; or, The Philosophical Commerce of Arts; designed as An Attempt to Improve Arts, Trades, and X'lanufactures . Vol.1. ... London, Printed for the Author, 1765. 4to. xviii, X, 646pp. , [7]ff . Vith frontispiece double-page engraved folding plate depicting a Chemical Laboratory, by P.C, Canot, and another folding engraved plate (tipped in); also 5 engraved plates (four of v.hich are folding), one of which, betw. pp.6 36/}' , is a rej.etition of the second plate before the title-page. Half light-brov/n morocco and marbled boards v.ith blcik leather label and gilt on spine. Boards slightly scuffed. Small marginal tear, pp. 547/8. A very good large-paper copy, with the i-rivilege preceding the title-page dated !76Z, tippid in, usually lacking. An important work of chemical and metallurgical interest. There are chapters on the history of gold, conversion of glass into porcelain, experiments on furnaces, history of colors [dyes and inks], history of Platina, &c. There is disagreement among the biblic^.raphers as to the date of the first edition of this v.-o?-"'" ^^veen states 763; Ferguson gives the dates 1763-66; Partington lists vol.1, 1766, vol.n, 1763, etc. We have been unable to trace a volume II in any of the important collections. A second volume may have been planned, but there is no indication that it was published. ALter checking the work bearing the 1763 title-page and the present one with the ! 765 date on the title-page, v/e find that they are identical in collation, format, etc. The only difference being the change of date and the addition of Vol.1 on the '765 title-page. The I-dgar Fahs Smith collection lists the title with the 763 date and belov that "another copy, 1765 . Apparently the present copy is a second issue of the first edition with a new title-page. Duveen: 335 . Partington: II, 762-4. Ferguson: II. 30. Lists the French translation &: a long note on Lewis. D ,N,B . Bio'^r. Universelle. Kress: 6M4 (ed. of 1763). Hanson, Bibl. of Leon. 75^-1775: 2857 (ed. of 1763). I 1 « 1^ GEORGE R. ^ ^ n 1 ,.,;>T1 hplnved William Lewis, or WHEREAS o^,r truayrind ^J^'^^^^^^^^^^ K,.gPn, in our County of 5«n7 MB has by^^^^ ^^ ,^^^^J^ rcprefented unto Us, .^'^^^'^^^'^'fXfti.lVaterials, for a Work, Years, engaged - makn.g Experiments, and co^^^ ^^^ ^^^ entitled « Commercium Philosophico ^ ^ ^„^ ,to " PfllLOSOPHICAL COMMERCE of ARl - X-'Vn crreatForvvard- .^ promotevfefulKnowledger andhatl.. engt^^^^^^^^^^^^^^ 111 . -11 u ,f xprvire to Uur bubieCts conceinea inyrr/^j, irc.wfj, humbly prelumeswill ^^^ / ^^[J^^J '° ^T " .Kat Kindof Knowledge on which ^X£!'B^Zi:^^^S^^^^ to g-t him our Royal SnceTnd iSuege for the fole Printing, ^^^^^^^^^^f^^^^^^^^^^ Work, for the Term of Fourteen Years, agreeable to J^e S mute m that L^^^^ .Tiade and provided: We are gracioufly pleaied to ^^^df/^^f^^^^^^J^^if ',2" ^e and do accoroingly, by thefe Prefents, as far as may be ^S'-JJ^bl.!^ "f /^^ Stauuein that c'afe made and provided, f-"! ^^^ --J ^^,\lf JJ^^^^^^^^ Lewis, his Executors, Adminiftrators, and Affigns, Our ^^y^^^^^'j^^^^ the fole Prinrirg. Publiihing, and Venning the laid Work fo the Term of Fourteen Years from the Date hereof; ftr.aiy f^^^ldd-ng all m^r Su^^^ jefls, within our Kingdoms and Dominions to ^^P""^ ^^"^^S''T ffoever iheflme, either in the like, any other Volmi^e or Volumes jh^^^^^^ or to Imporc, Buy, Vend, Utter, or Diftnbute any Cop'es th reof Rep in ed beyond the Seas, during the aforefaid Term of Fourteen Years^ Executors Confrnt and Approbation of tht faid William Lewis, his Heirs |Y,dTnd Adminiftrators, and AfTigns, under their Hands and Seals, ^l^^ ^ad and obtained, as they will anfwer the contrary at their Peril ; whereof the Com- milTione;, and'other Officers of our Cuftoms the M.ifter, Wardens, and Company of Stationers, are to take Notice, and that the fame may be em.red in the Rcgiller of the faid Co.^pany, and that due Obedience be lendered to Our Will and Pleafure herein declared. Given at Our Court at 5/. J^ms^s, the Eighth Day of F.^r^^ry. 1762, in the second Year of Our Reign, ^j His Majejifs Command. E G K E M N T. i " J '^ Tbfli^ lA^TMt . L COMMERCIUM Pbilofophko'Technicum ; OR, THE Philosophical Commerce OF ARTS: DESIGNED AS AN ATTEMPT TO IiMPROVE Arts, Trades, and Manufadures. By W. LEWIS, M. B. and F. R. S. Fiat Experimentum. Bacon. LONDON, Printed by H. Baldwin, for the AUTHOR; And Sold by R. Willock, at Sir Ifaac Newton's Head in Comhill. MDCCLXIII. T O T H E KING, SIR, TH E advancement of arts, trades, and raa- nufaaures, and the extenfion of commerce, are now become the more immediate objefts of Your Royal care ; as being the moft certam means of attaining Your darling wilhes, the rendermg Your people powerful and happy, and perpetuatmg the bleffings of peace. I have therefore pre- fumed, vvith all humility, to offer to Your MmhstVs protcaion, a work defigned to maprove and enlarge many of the arts «f P^^->. ^f "^^ P™" n,ote that kind oi knowledge on which they de- pend. The in,portance of the fubjed, and the honour, the never to be forgotten honour, wh.ch Your [ » ] Ymir Majestv was pleafed to do me, hy Your .^ tent:on to fon.e ledures and experiment! ml T Your command at K.w, for /ewCtt r ^ tion of chemi/lr,to the ■ improvemen't ol'/^'': arts as well as of philofophy, w^ I ho ^ f this ambition in ^ ^' ^^ ^ ^'°P^> ^^cufe Your Ma JEST y s Moft humble and devoted Subjea and Servant, Klngfton on Tliamej, vviiiiam Lewis. PREFACE. As all the arts, by which matter Is diverfly modified and accommodated to human ufes, have a neceflary dependence upon the properties or qualities of the bodies on which they are exercifedj enquiries into the pro- perties of different bodies, and the effed:s refulting from various applications of them to one another, become ap- parently of primary importance, as well for the illuftration and improvement of the prefent arts and the difcovery of new ones, as for the advancement of ufeful knowledge. The properties of bodies make the objedl of two fciences, natural philofophy and chemijlry ; which, though in many cafes fo clofely interwoven, and fo nearly allied, that per- haps no boundaries can be eftablifhed between them, ap- pear in others to have eflcntial and important differences. In the introdudlion to a work, of which enquiries into the properties and relations of bodies make a principal part, the neceffary precifion, in regard to matters of fad:, as well as of fcience, requires that we fhould endeavour to diftin- guifh them. a Natural iV PREFACE; Natural or mechanical philofophy feems to confider bodies chiefly as being entire aggregates or malTes ; as be- ing divilible into parts, each of the fame general properties with the whole J as being of certain magnitudes or figures,, known or inveftigable ; gravitating, moving, refilling, &Co. with determinate forces, fubjed: to mechanic laws, and re- ducible to mathematical calculation.. Chemistry confiders bodies as being compofed of. fuch a particular fpecies of matter; diflbluble, liquefiable, vitrefcible, combuflLble, fermentable, &c. impregnated with colour, fmell, tafle, &c. or confifling of diffimilar parts, which may be feparated from one another, or tranf- ferred into other bodies. The properties of this kind are not fubjedt to any known mechanifm, and feem to be governed by laws of another order.. To the grand. acTcive power, called cittraBlon, in the mechanical philofophy, what correfponds in the chemical is generally diftinguilhed by another name,., affinity. The mechanical attradlion obtains between bodies con- fidered each. as one whole, and between bodies of the fame as well as of different kinds. It obtains while the bodies are at fenfible diilances ; and the comparative forces, with which they tend together at different diflances, are objedls of calculation . When the attrafting bodies have come into the clofeft contadi we can conceive, they flill continue two diftindl bodies, cohering only fuperficially, and fepa=>- rable by a determinate mechanic force. The PREFACE. V The chemical attradlon, or affinity, obtains between bodies as being compofed of parts, and as being of a diffe- rent fpecies of matter from one another. It never takes place while the two bodies are at any fenfiblc diftance ; and when they are brought into the clofeft contadt, there is frequently neceflary fome other power, as fire, to excite their adlion upon one another. In proportion as this action happens, they are no longer two bodies, but one ; the affinity confining in the intimate coalition of the parts of one body with thofe of the other. The properties of this new compound are not in any kind of ratio of thofe of the compounding bodies, nor difcoverable by any ma- thematical inveftigation : two bodies, each by itfelf very eafily fufible in the fire, as lead and fulphur, fhall form a compound very difficult of fufion -, and two which cannot feparately be made to melt at all, as pure clay and chalk, (hall melt with eafe when joined together. As the chemical union, and the properties thence re* fulting, are exempt from all known mechanifm, fo neither can the bodies be feparated again by mechanic force. But a third body may have a flronger affinity to either of the component matters than they have to one another, in wliich cafe, on prefenting to the compound this third body, the former union is broken, and one of the firft bodies coalefces with the third, while the other is detached and feparated. Thus, when quicklime is diflblved in water, if we add to the tranfparent fluid a little vitriolic acid, the acid par- ticles unite with the diflblved particles of the lime into a a 2 new Ti PREFACE. new compound ; wliich, notwithflanding the pungent tafle of the one ingredient, the corrofive acidity of the other, and the folubility of both, proves infipid and indilToliible, and which therefore, feparating from the water, renders it at firft milky, and on ftanding fettles to the bottom, in form of powder or fmall cryftals, of the fame general properties with the native gypfums or plafter-of-paris ftones. If this powder be ground with inflammable matter, as powdered charcoal, no aftion happens between them, hov/ exquifitely foever they be mixed : the two powders con- tinue gyplum and- charcoal, and may be in great meafuro parted from one another by means of water, the charcoal powder remaining for a time fufpsnded in the fluid, while the heavier gypfum fettles. On expofing the mixture to a proper degree of heat, a llrong chemical affinity begins to take place : the acid quits the lime, and unites witli the inflamm.able principle of the coal, forming therewitli another new compound, common brimfl:one, which, like the former, proves infipid, and indifl"oluble in watery li-. quors, though in other properties remarkably different ; melting in a fmall degree of heat into a red fluid; in, a fome-^ what greater heat, if air is excluded, riflng into the upper part of the veiTel unaltered; on the admiffion of air chang-^ ing into a blue flame, with a fuifocating volatile acid fume,, which by air and moifl:ure returns into the original, inodo- rous, ponderous, vitriolic acid. By mixing the brimfl:one with iron filings, a freili tranf- pofition is produced; and as in the preceding cafe the adlion^ is excited by fire, fo in this it is excited by water. The. mixture^. PREFACE. vii mixture, kept pcrfedlly dry, continues unaltered for years : on being moiflened with water, it grows fpontaneoufly hot in a few hours, and if the quantity is large, it even burfts into flame, with fuch commotion, as has induced many to afcribe earthquakes and vulcanoes to this caufe. During this adtion, the acid is transferred to the iron ; and the inliammablc matter, before combined with it, efcapes into the air. The combination of the acid with the iron forms the green vitriol or copperas of the fhops ; a fait of a ftrong tafte, and of eafy folution in water, though the quantity of iron in it is very far greater than that of the inflammable matter by which, in the form of brimflione, the mifcibility of the acid with water was deflroyed. To the green folution of the vitriol, if fome vegetable aflies, or the earth called magnefia, be added, the iron falls to the bottom, conflderably altered, in form of ochre or ruH:, deprived of its attractive power to the magnet> and of all its metallic properties, which however are eafily reflored by expoflng it to the fire in mixture with a little charcoal powder. In room of the iron thus thrown out from the liquor, the acid attacks the vegetable earth or magnefia ; and though with one kind of earth, as we have icen above, it forms an infipid and indiflbluble concrete ; with both thefe earths it compofes a bitterilh fait which diflblves eafily, and which, at leaft when magnefia is made ufe of, is the fame with that of the purging mineral waters. If to the folution of this fait we add a volatile alcaline fait, the penetrating fmell of the alcali is fupprefled in an inftant, the acid uniting with the alcali into a new com- poundi. vm PREFACE. pound, and depofiting the earth which it had taken up before. From this compound, fixt alcaline falts abforb the acid, and fet at liberty the volatile alcali with all its original properties. Though the acid and fixt alcali, feparately, are very pungent and corrofive, and fo ftrongly difpofed to unite with water that they imbibe it from the air, yet the combination of the two has only a mild bitterifli tafle, and diflblves in water very difficultly and fparingly. After all thefe tranfpofitions, the acid may ftill be reco- vered pure, and made to pafs again through the fame and through a multiplicity of other combinations. From al- moft all its combinations it may be transferred to inflam- mable matter, and frona the inflammable matter to iron : from the brimflone, which it forms with the one, the acid may be obtained by burning with a proper apparatus j and from the vitriol, which it forms with the other, by diftil- lation. It is obvious, that in all thefe cafes, the acftion is not between bodies confidered as aggregates or maffes, but be- tween the infenfible and diflimilar parts of which they are compofed; that the feveral effe6ts can be regarded no otherwife than as Ample fadts, not reducible to any known mechanifm, not invefliigable from any principles, and each difcoverable by obfervation only ; and that the powers, on which they depend, are, fo far as can be judged in the prefent ftate of knowledge, of a difterent kind from thofe, by which bodies tend to approach or cohere with forces proportionate to their difl:ances, or to refifl or propel accord- ing PREFACE. Ix ing to their quantities of matter and velocities. It fecms of importance, that thel'e two orders of the aiFedions of bodies be kept diftindl, as many errors have arlfen from applying to one fuch laws as obtain only in the other. When Archimedes obferved, in the bath, that the bulk of a body, hov^^ever irregular, might be found, by plung- ing it into a velTel of water, and mcafuring the water which run over, or the fpace which the water occupied in the velfel more than it did before; and that gold has near twice the weight of filver under an equal bulk ; he con- cluded, that if gold and filver were mixed together, the quantity of each metal in the mixture might be found by calculation from the bulk of the mafs compared with its weight j and on this foundation, he is faid to have difco- vered a fraudulent addition of filver made by the workman in Hieros golden crown, at a time when the chemical methods of analyfing and allaying metallic compofitions appear to have been unknown. The mechanical philofophy has extended this way of inveftigation to many different mixtures, and computed tables for facilitating the operation; not aware, that though the method is demonflrably juft if the two bodies were joined only fuperficially, the cafe is otherwife when tliey are intimately combined together. The ad of com- bination, whether in bodies brought into fufion by fire, or in fuch as are naturally fluid, is truly chemical, and the laws of the mechanical philofophy have noplace in it.. There are inflances, fome of which will appear in the prefent volume, of bodies being dilated on mixture into a larg:eri X PREFACE. larger bulk than they had before ; and, contrarlwlfe, of two being contracted into lefs bulk than even one of them occupied by itfelf. To render the procefs anywlfe to be depended on, ac- tual mixtures of the refpedlive bodies ought firft to be made, in different proportions, and examined hydroftati- cally, that the quantity of contradlion or dilatation in particular cafes may be known and allowed for. By thus borrowing from both fciences, we are furnifhed with means of difcovering the proportions of the ingredients in many mixtures, provided the ingredients themfelves are known, with tolerable certainty : in feme mixtures, as of lead and tin, this method is more commodious, and perhaps more exadt", than any which pure chemiflry has afforded. In this manner the mechanical and chemical fciences concur, and require the afliftance of one another, in their own operations, and in almoft all the manual arts. In the greater number of the arts the chemical prevails, and many are no other than diredt branches of practical che- miftry, as the arts of dying and ftaining, the running down of ores, the refining and compounding of metals^ the making and colouring of glafs, enamel, porcelain, &c. making wines, vinegars, fpirits, &c. preparing indigo> fmalt, Prufuan blue, vermilion, lakes, and other colours for the painter. It is in thofe arts, and in thofe branches of arts, which are flriftly chemical, that the moft impor- tant and nioft numerous difcoveries are to be expedled •: ckemiftryiiaving hitherto been the leaft cultivated, though not PR E F A C E. xi not the leafl fruitful; and producing daily, not barely new applications of principles already known, but new faits or principles to be further applied. Having for feveral years employed myfelf at times in experiments relative chiefly to the chemical arts, and made proper difpofitions for continuing fuch enquiries, I pub- liflied in 1748 propofals for a very extenfive work, con- fiding principally of thofe experiments, and of informations received from workmen and others. The feveral articles were to be printed in a mifcellaneous manner, without re- gard to any one being connected with that which 'preceded or followed it. As nothing was to be admitted but ufe- ful or interefting fadls, it feemed of little importance in what order the fadts fliould be difpofed, provided, by means of proper indexes, the reader could readily have recourfe to fuch particulars as might occafionally be wanted. Some friends advifed an alteration in this plan, judging it would be of more utility to the publick if the fadls were methodized i and the moil: convenient method was thought to be, to give a complete hiftory of each art by itfelf in all its branches. The difficulty of fuch an attempt, and the impofiibility of executing it to any good purpofe by one hand, were apparent : nor would a limple detail of the manual operations of different workmen be anywife agree- able, either to the views, or the materials, with which I had engaged in the undertaking. Another way occurred of procuring fome degree of regularity, without departing from the original views, any otherwife than by rendering them more compreheniive. , b Many ^i PREFACE. Many of the arts have natural and ftrong connections with one another y \vorking upon the fame materials, for purpofes either different or nearly the fame; or producing limilar effeds upon different or limilar fubjedls. One property of fuch materials, or the produdlion of one effedl, may therefore influence feveral arts : a colour, which caik- be eafily fixed in animal and vegetable fibres, is equally of benefit to the woollen dyer, the filk dyer, the dyer of linen and cotton thread, and the callico printer: a colour which will bear fire, and unite with vitreous bodies in fufion, concerns equally the glafs maker, the enameller^ and the painter on porcelain.. The, difcoveries and improvements made in one art,, and even its common procefles, are generally little known to thofe who are employed in another, fo that the work- man can feldom avail himfelf of the advantages which he- might receive from the correlative arts, and an eflfedt wanting to the perfedlion of his own art may be adlually produced in another. Thus, though the dyer of linen cloth, and of linen and cotton thread, wants means of communicating to them a black dye that fliall endure wearing, the callico printer fixes both on linen and on, cotton a black as durable as can be wiflied for. To enquire therefore by experiment into tHe different, means of producing one effedl, and trace it through all, the arts in which fuch an effedl is required; to examine the chemical properties of one fubjedt-matter, and confider its ufes and applications in all the arts in which it is con- cerned; to proceed in this manner with the capital effedls,, and materials, fo far as my own experiments, and my op- portunities PREFACE. xia jjortunities of information, (hould enable mcj appeared to "be the moft rational and diredl means, not only of cfta- blifliing folid principles of the feveral arts as now cxcr- cifed, but of procuring an ufeful intercourfe and communi- cation of knowledge, of fupplying many of their defeds, cf multiplying their refources, of improving their pro- dudts, of facilitating and Amplifying fome complex ope- rations, and rejeding ufelefs ingredients in fundry com- pofitions, of enriching one art with the practices, mate- rials, and fometimes even with the refufe-matters of another. Such therefore is the plan which I have chofen to fol- low, and of this alteration I gave notice in an advertife- ment in 1761. I have the fatisfaftlon to find that the French academy •of fciences, who, with the advantage of penfions from the fovereign, and with the affiftance of experienced artifts in diiferent profelTions, have been engaged for near a century in a hiflory of arts which has but lately been begun to be publiflied, exprefs exad:Iy the fame fentiments with thofe on which I have proceeded. In the memoirs for 1763, the hifiorian of the academy, in giving notice of the pub- lication of that work, obferves that " an inconvenience to be feared is, the want of that knowledge, and of thofe general principles, which bind arts as it were together, and eftablilh between them a reciprocal communication of light. All the arts, for example, that employ iron, have common principles, but it would be in vain to cxped the knowledge thereof from thofe who exercife thefe arts, b 2 each xiy PREFACE. each of whom knows only the application of thofe prin- ciples to his own art. The farrier, the lockfmith, the cutler, know how to work iron; but each of them knows only the manner of working which he has learnt, and is perfedlly ignorant that the art of working iron has general principles, which would be infinitely ufeful to him in a great number of unforefeen cafes, to which his common pradlice , cannot be applied. — 'Tis only by bringing tha arts as it were to approach to one another, that we can make advances towards their perfecftion : we fhall thus put them in a condition of mutually illullrating one ano- ther, and perhaps of producing a great number of ufeful difcoveries : 'tis only by this means that we can know effedtually their true principles, and enable them to receiv-a afliftance from theory." It were to be wiflied, that convenience had permitted thefe reflections to have had their full influence in the ex- ecution of the work. Thehiflory is publiflied in detached and independent pieces, each containing a minute detail of the whole feries of operations of one art, with defcrip- tions and plates of all the inftruments made ufe of: it is defigned not only to fupply the philofopher with the know- ledge otherwife obtainable only among workmen, and to entertain the mind with the hiftory of human inventions, but likewife to enable perfons to exercife the refpedlive trades in places where workmen are wanting. It is obvious that this plan does not at all interfere with mine, and that the views of the two undertakings are ellen- tially different. It is not my defign to dwell upon de- fcriptions of common and merely manual practices, to give particular PREFACE. XV particular inftrudlions for the fctting up and condudling of common manufadtureSjOrhiftories of eftablKhments which muft vary with times and circumftances. The articles which make the main objects of my enquiries are founded on the invariable properties of matter ; and befides the confideration of arts in their prefent ftate, experiments for improving them, or refearches in that branch of knowledge from which their more valuable improvements mufl: ariic, make a principal part of the work. With- the advantages that refult from the abovemen- tioned alteration in the plan, the execution becomes far more difficult. What is fadl or otherwife in regard to the properties of bodies, or the efFedts refulting from different operations on them, is to be determined by experiment. In the mifcellaneous method, we have no occafion to enter upon any points where the determination of experiment has not been clearly obtained. But in a regular hiftory the cafe is otherwife : we fhall often be led into fubjeds with which we are not fufliciently acquainted, and though we thought we had materials in abundance, we flaall find fbme deficiency, greater or lefs, in almoft every page : there are numerous particulars, which are not m.ified in the de- tached fragments of knowledge, but whofc want is firiking when thefe fragments come to be joined and methodifed into one whole. From this caufe, and from the difficulty and tedioufnefs, in fome cafes, of obtaining the neceffary informations among different workmen, unexpedled delays, if we aim at making the hiffory tolerably complete, muft frequently happen in the publication ; nor will it be eafy^ on xvi PREFACE. on certain occafions, even to avoid errors : indeed in dl-* redt experimental enquiries, the effedt of the operation is fometimes fo much influenced by circumflances which are apt to pafs unheeded, that it is not to be vs^ondered if errors are to be found in the writings even of the moil ac- curate experimental chemiits. The hiftory of each fubjedl I have made as complete, as my prefent means of information, and the experiments I have hitherto made, will permit ; but much remains ftill undetermined and unexamined : if the work goes on, and the author lliould be able to execute it to the utmoft of his hopes, the publick is not to expedl that any article will be perfedl ; refearches in chemical knowledge we can ne- ver ho^e to make complete, every new acquilition fhewing new paths for our further progrefs in a province of un- bounded extent. Such material deficiences as occurred upon a review of the volume, and fome miflakes which I had been led into either by my own inadvertence or the authority of others, are taken notice of in the appendix. It is hoped that the fame regard for the publick, which influences the author, will prevail on thofe who may dif- cover any other miftakes of moment, to give him an ac- count of them, that they may be redlified in fome future publication. It is hoped alfo, that from the communica- tion of friends and artifts, much of what is wanting in one publication may be fupplied in the next. One of the principal obflacles to the profecution of chemical enquiries has liitherto been the want of a proper apparatus. PREFACE. x\'ii apparatus. I have therefore begun the work with cin at- tempt to remove this impediment; to procure, at a fmall expence, a commodious and eafily manageable fet of fur- naces, &c. which may be all worked under a common ehimnev,and fome in the middle of a room without offence, and with which mofl of the operations, that require thc^ aifillance of lire, may be performed, in the way of experi- ment, with great eafe, expedition, and fafety : if thofe, who have been accuflomed to the common larger and more cxpcnfive furnaces, fliould at firft be at any lofs in the ufe of thefe fmall ones, a very little pradtlce and attention will remove all difficulties, and convince the operator of the convenience which I have long found from them in expe- rimental purfuits, for which alone they are defigned. The ftrudlure and management of the large furnaces, kilns, 6cc. ufed in different bufineffes, are intended alfo to make part of this work, and mechanical contrivances of other kinds are like wife occafionally confidered, with a view chiefly to render them more fmiple, convenient, or effediual. In the prefent volume I have given an entire effay on the im- provement of the machines for blowing air into large furnaces, &c. by a fall of water, without moveable bellows, in virtue of the water carrying down air with it in falling through pipes ; and I have the pleafure of being Informed, by a foreign correfpondent, of a machine which he has conftrudled on the principles there eflabliflied, which an- fwers as well as can be defired. It would be needlefs to fpecify in this place the feveral matters contained in the volume ; a lift of them may be ften xviii PREFACE. feen in the table of contents. It is fufficient here to have explained the principles, and the views, with which the author has engaged in the work. How far thefe principles and thefe views have a jull: foundation, or may tend to the advancement of arts and ufeful knowledge, and whether this laborious and expenfive undertaking fliall be dropt or profecuted, is left to the determination of the publick. THE THE CONTENTS. J- Page r^ESCRIPTlO N of a portable furnace for making ■*-^ experiments i II. Hijlory of gold, and of the various arts and btfmejjes depending thereon. Secft. L Of the colour of gold, and the methods of rejloring its lujlre tvhen fuUicd ' og Sedl. II. Of the gravity of gold — 41 Seft. III. Of the duSlility of gold, and the arts depending on this property ' ■ ■ ^a 1 . Preparation of gold leaf • j. r 2 . Preparation of gold or gilt ivire r i 3 . The degree ofextenjion of gold in wire and leaf^ 9 4 . Application ofgoldivire and leaf on other bodicsb i Seft. IV. Of the effects of fire on gold 66 1. Of the melting of gold ■ 65 2. Of the alterations faid to be producible in gold by fire 70 Sedl. V. Of the mixture of gold with other metals 74 1 . Of the mixture of gold with mercury : gold powder, water gilding, ^c. y r 2. Of the mixture of gold withftlver, copper, &c. the alterations produced by different proportions of different metals, and the effeSls of flrong or continued fire on the mixtures 82 Sed:. VI. Of the aBion of acid and fulphureous bodies on gold', various folutions of it, and their properties 90 1. Gold with the nitrous acid • c\q 2. Gold ixith the murine acid 04 3. Gold with the vitriolic acid ——. g^ a 4. Gold u The CONTENTS. Page 4. Gold 'with cojnpound menjlma 95 5. General fi'operiies of folutions of gold 97 hi Separation of gold from acids by inflammable liquors — — . 101 7. Precipitation of gold by alkaline falts 104. 8. Precipitation of gold by metallic bodies 108 9. Gold with fulphureoirs bodies ■ — iii Seft. VII. Of the alloy of gold ; and the tnethods of judging of the quantity of alloy it contains, from the co- lour and iveight '^'i'-^'^'\'''.''^-'^"r « 114 1. Of the alloy of gold 114 2. Method of ejiimating thefncnefsofgoldfrofn. its colour — 119 3. €f eflimating the fnenefs of gold from its ■ gravity — ??5. S^a. VIII. Oftheafayingofgold ^ 127 T. Cupellation with lead 1 129 2. Parting with aqua fortis • 135 Sedl. IX. Of the refining of gold, andthefeparation of fmall portions of it from other metals — 144 1. Separation of gold from bafe metals: by tefiing with lead ^'^ '^ ■ ■ ,C . 1 44 2 . Separation of gold from filvcr by Aquafortis 1 47 3 . Purification of gold from fiher and bafe metals by cementation • -^— ^ 1 54-, 4. Refining of gold frOm fiver and bafe metals by antimony ~ r-: 1^6 5. Purification of gold from pla'tina, fiver, and bafe metals, by aqua regia 159 6. ExtraBio?! of a fmall portion of goldfrofn a iarge quantity offilver — 1 6 1 7. PiXtra£lio7i of gold from copper — 166 ''^%. Separation of gold from gilt works 1O8 Sed. X; Of tinging glafs and enamel by preparations of gold — 170 Sect. XI. 'the mineral hifiory of gold . 181 I. Of the fnatrices of gold, audits diffemination through different minerals 181 2. Separation The CONTENTS. iii Page 2. Separation of gold from earthy andjiony bodies by water i^o 3. Separatio7i of gold from earths andjlones by mercury irjo 4. Extrablion of gold intimately combined in the compojition of fands igr 5. Extraction of gold from the ores of other metals ■ 1 99 Seit. XII. Of the alchemical hijlory of gold 199 1. Oftheprodu£li/>ilityofgoldbyart 200 2. Of the dejlruulibility of gold — — 204 Se«3:. XIII. Lnit at ions of gold . 213 1 . Gold coloured metal - ' 2 1 3 2. Gold coloured pigments —— 221 3 . Gold coloured varnijlj or lacker 223 Addition to the hijlory of gold : of gold fowers on linen: of ex- tending the gold on brocades by pajjing them between rolls 226 III. Experiments of the converfon of glafs vejfels into porcelaine, and for ejlablijlmig the principles of the art 230 Sed. I. • Experiments of the fiiccejjive changes produced in green glafs by baking 233 Setft. II. Experiments of the quality ofthefubjiance into isthich green glafs is converted by baking 236 Se£l. III. Experiments of comparing the effeSis of dfj'erent kinds of materials on green glafs by baking 241 Sedt. IV. Experiments of the baking of different forts of glafs, and of bodies approaching to a vitreous naturez 4 5 1. Vitreous and femivitreous bodies compofed of earths, without metallic or faline additions 246 2. Metallic glafes 247 3. Glaffes prepared with faline additions 248 Se<5l. V, Objervations on the caufe of the change which green glafs undergoes from baking — 251 IV. Of the expanfion or contraBion of certain bodies at the time oj their paffng from afuid to afolidjlate [-— - 256 V. IV The C O N T E N T S. V. . Pags 0/ the blowing of air into furnaces by afall of water 267 Se and partly, for receiving fome bees-wax, which makes the rod pafs more freely, and preferves the gold from being rubbed off. The plate being properly fecured, one end of the rod, made fome- what fmaller than the reft, is puflied through fuch a hole as will admit it, and laid hold of by ftrong pincers called clamps, whofe chaps are toothed, fomewhat like a file, to keep the rod from flipping out by the violence neceflary for drawing it : the handles or branches of the clamps are bent upwards, and an oval iron loop put over the curvature, fo that the force, wliich pulls them hori- zontally by the loop, ferves at the fame time to prefs them together: to the loop is faftened a rope, whofe further [ 54 I further end goes round a capftan, or upright cylinder, with crofs bars, which requires the ftrength of feveral men to turn it. The rod, thus drawn through, is well nealed, then pafled in the fame manner through the next hole, and the nealing and drawing repeated, lefs and lefs force fufficing as it diminishes in thicknefs : when reduced to about the fize of a large quill, it is delivered in coils to the wire-drawer. The remainder of the procefs requires plates of a differ- ent quality; thofe of fleel, whether in a hard or a foft ftate, being now found to fret the wire, or to raife a bur upon its furface, and flrip off the gold. The plates for this part of the work are brought from Lyons in France : the holes are drilled in them here. They are formed of a metallic mafs, whofe compofition is kept a fecret, but whofe prevailing ingredient is plainly iron : I have begun an examination of this metal, and fhall communicate the refult of the experiments in one of the future numbers of this work. There are two forts of thefe plates i one of confiderable thicknefs, for the wire in its larger ftate j the other, only about half as thick, for the finer wire, where lefs force is fufficient in the drawing. There are confiderable differences alfo in the quality of the metal it- felf, not to be difi:inguiflied by the eye,, or any otherwife than by repeated trials : fuch of the thicker plates, as are found good, are valued at a high price. The Lyons plates, though brittle, have fufficient toughnefs to admit of the holes being beaten up, or contradled, by a few blows of a hammer; fo that when any of them have been widened by a length of wire being drawn through, they are thus reduced again to the proper dimenfions for preferving the gradation : the holes, after each beating up, are opened by a long flender inftrument, called a point, made of re- fined fteel; one end of which, to the length of about five inches. C S5 J inches, is round, and ferves as a handle ; the refl, about twice as long, is fquare, and tapered to a fine point. The firft holes being Ibonefl gulled, or fo far worn, as to be unfit for bearing further redudtionsj the next to them, grown likewife wider, fupply their places, and are therafelves fuccefiively fupplied by thole which follow ; whence, as each plate is furniflied with feveral more of the finall holes than are wanted at firft, it continues to afl-'ord a complete feries after a confiderable number of the larger has become unferviceable. Great part of the dexterity of the workman confifts in adapting the hole to the wire ; that the wire may not pafs fo eafily, as not to receive fufficient extenfion, or fo difficultly as to be broken in the drawing. For determining this point with greater certainty than could be done from the mere refiflance of the wire, he ufcs a brafs plate called a fize, on which is meafured, by means of notches likefleps cut at one end, the increafe which a certain length of wire fhould gain in paffing through a frelli hole : if the wire is found to flretch too much or too little, the hole is widened or contraded. As the exten- fion is adjufled by this inftrument, there are others for meafuring the degree of finenefs of the wire itfelf : Hits of different widths, made in thick, poliflied iron rings, ferve as gages for this ufe. The wire-drawers procefs begins with nealing the large wire received from the refiner : this is performed by- placing it, coiled up, on fome lighted charcoal, in a cylindrical cavity, called the pit, made for this purpofe, under a chimney, about fix inches deep, and throwing more burning charcoal over it : the pit having no aper- ture at bottom to admit the air, the fuel burns languidly, affording only fuflicient heat to make the metal red-hot, without endangering its melting. Being then quenched in water for the fake of expedition in cooling it, though the' r 56 ] the metal would doubtlefs be foftened more eftedlually if fufFered to cool leifurely, one end of it is palled through the firfl hole in the thick plate, and faftened to an upright wooden cylinder fix or eight inches in diameter : in the top of the cylinder are fixed two ftaples, and through thefe is pafTed the long arm of a handle, by which the cy- linder is turned on its axis by feveral men. In the con- tinuation of this part of the procefs, called degrofilng, the wire is frequently nealed and quenched, after every hole or every other hole, till it is brought to about the fize of the fmall end of a tobacco-pipe : and in this ftate it is cut into portions for the fine wire-drawer. In this laft part of the wire-drawing procefs, nealing is not needful ; but it is ftill as necefiTary as before to wax the wire at every hole. Much lefs force being now fufiicient for drawing it through the plate, a different inllrument is ufed : a kind of wheel, or circular piece of wood, much wider than the foregoing cylinder, is placed horizontally: in its upper fur face are fome fmall holes, at different diftances from the axis, and into one or another of thefe, according to the force required, is occafionally inferted the point of an upright handle, whofe upper end is received in a hole made in a crofs bar above. From this the wire is wound off upon a fmaller cylinder, called a rochett, placed on the fpindle of a fpinning-wheel ; and this laft cylinder being fixed on its axis behind the plate, the wire is again drawn through upon the firft ; and being at length brought to the proper finenefs, it is nealed to fit it for the flatting-mill. This nealing is per- formed in a different manner from the foregoing ones, and with much lefs heat; for if the wire was now n>«de red hot, it would wholly lofe its golden colour, and become black, bluifh, or white, as I have often experienced in different parcels of gilt wire. Being wound upon a large hollow copper [ 57 ] copper bobin, the bohin is let upright, Ibme lighted char*- coal or fmall-coal placed round it and brought gradually nearer and nearer, and fome more fmall-coal put in the cavity of the bobin j the wire being taretully watched, that as foon as it appears of the proper colour, it may be immediately removed from the heat. This is an operation of great nicety, and is generally performed by the mailer himfelf. The wire, though it in good meafure retains the fpringinefs which it had acquired in the drawing, and does not prove near fo foft as it might be made by a greater heat, is neverthclcfs found to be futiiciently fo for yielding with eafe to the flatting mill. The flatting-mill confifts of two rolls, turned in a latlie to perfect roundncfs, exquifitely poliflied, placed with their axes parallel one over another, fet by fcrews till their circumferences come almofl: into contad:, and both made to go round by one handle : the lowermoft is about ten inches in diameter; the upper commonly little more than two, though fome make it confiderably lar- ger, and indeed it would be more convenient if made as large, or nearly fo, as the lower : their width or thick- nefs is about an inch and a quarter. The wire, unwind- ing from a bobin, and pafling firft between the leaves of an old book, prefTed by a fmall weight, which keep it fomewhat tight, and then through a narrow flit, in an upright piece of wood called a ketch, which gives notice of any knot or doubling, is diredled by means of a fmall conical hole in a piece of iron, called a guide, to any particular part of the width of the rolls ; that if there fliould be any imperfedtion or inequality of the furface, the wire may be kept from thofe parts ; and that when one part is foiled by the pafTage of a length of wire, the wire may- be fhifted, till the whole width of the rolls is foiled, fo as to require being cleaned and poliflied anew with the I fine [ 53 ] _ , fine powder, called putty, prepared by calcining a mix- ture of lead and tin : the workmen value the rolls from the number of threads they will receive, that is from the number of places which the wire can thus be fhifted to: o^ood rolls will receive forty threads. The wire, flatted between the rolls, is wound again, as it comes through, on a bobin ; which is turned by a wheel, fixt on the axis of one of the rolls, and fo proportioned, that the motioa of the bobin jufl keeps pace with that of the rolls. The rolls, as well as the drawing plates, have been often procured from France; and it has been thought that the wire received from the French rolls an additional beauty and luftre; though it does not appear that the French have any durable advantage in this refpedt above the Englifli, or that thegloffinefs communicated by either is of any real advantage to the manufadlure j for it quickly goes off. The moft important point in their preparation is, the giving them that perfeft truth and equability of furface, which the flatting of fo fine wire demands. The internal part is formed of iron, and a plate of refined fleel is lapped round and v/elded over the iron : where the two ends of the fteel plate meet there is frequently an imperfedion, the jundlure being generally vifible acrofs the face of the roll. In rolls of great width, fome curious artifts have obviated the inconveniencies arifing from this caufe, byufing, inflread of a broad plate, along narrow bar of the fteel, and twifl:ing it round the roll in feveral circumvolutions, that the little inequalities, in hardnefs and folidity, happening at the junctures, might be in the direction of the ribbon that pafles between the rolls, and not tranfverfe to it. In the narrow rolls ufed for the flatting of wire, a pradice of this kind would be very difficult ; but the fame end might perhaps be an- fwered, and even more effectually, by calling the fteel, inftead [ 59 ] inllead of a ftrelght bar, into the form of a hoop or ring of afomewhat Icfs diameter than the fize of the intended roll ', then forging the hoop, on the round beak and flat of the anvil alternately, to procure it the requifite uniformity of its parts and the due extenfion ; afterwards placing it in a proper mould, fixing the axis in its due pofition, and running into the intermediate fpace fome •caft iron, which, from its known property of expanding, as it fets or becomes folid, will continue every where to fill the cavity, and irremoveably fix itfelf both to the hoop and to the axis. T/je degree of extenjion of gold in wire and leaf. The vaft extent, to which gold is apparently ftretched in the foregoing operations, has induced fevcral perfons to make experiments for determining itsexadt degree by meafuration and weight. In an experiment of Reau- mur's, forty-two fquare inches and three tenths of gold leaf weighed one grain troy ; and Mr. Boyle found that fifty and feven-tenths weighed but a grain. As a cubic inch of fine gold weighs 4902 grains, the thicknefs of the gold leaf examined by the one was the 207355th, and of that by the other only the 248532nd part of an inch. Dr. Halley found, that of fuperfine gilt wire fix feet weighed a grain : M. de Reaumur makes about four inches more go to the fame weight ; and Mr. Boyle is faid, if there be no error in the numbers, to have had gilt wire much finer than any of thefe. Allowing fix feet to make a grain, and the proportion of gold to be that commonly ufed by our wire-drawers j the length to which a grain of gold is extended on the wire, comes to be near 352 feet. In flatting, the wire is extended, according to M. de Reaumur, one feventh part of its length, and to the width I 2 of [ 6o ] ^ of one niaety-flxth of an inch : in fome trial« I have.- {Qen made by the workmen, the extenfion in length ap- peared lefs, but that in breadth fo much greater, thafc the fquare extenfion was atleaft equal to that affigned by Reaumur. Hence one grain of gold is flretched on the: flatted wire, to the length of above 401 feet, toafurface of above 100 fquare inches, and to the thinnefs of the 492090th part of an inch. M. de Reaumur carries the extenfion of gold to a much greater degree. He fays the wire continues gilded when, only one part of gold is ufed to 360 of filver; and that it may be ftretched, in flatting, one fourth of its length, and to the width of one forty-eighth of an inch. In this cafe,, a grain of gold muft be extended to 2900 feet, or upwards of half a mile, and cover an area of more than* 1400 fquare inches. He computes the thicknefs of the- golden coat, in the thinnefl: parts of fome gilt wire, to be- no more than the fourteen millionth part of an inch, fc^ that it 15 only about a hundredth part of the thicknefs of gold leaf. Yet notwithfl;anding this amazing tenuity, ifapiece- of the gilt wire be immerfed in warm aqua fortis, which, 'will gradually diflblve and eat out the filver, the remain-- ing golden coat will fliill hang together, and form, while the fluid prevents it from collapfing, a continuous opaque tube. To fucceed in this experiment, ' the aqua fortis muft not be very ftrong, nor the heat great ; for then the acid,, adling hafl;i]y and impetuoufly upon the filveri would difunite the particles of the gold. Whether any other metal can.be extended to an equal degree is not yet as clear ; for as it is the great value of gold which engages the workmen to endeavour as much as poflible to ftretch it to the largeft furface, the fame cfl:brts have not been made in regard to the lefs valuable metals s. [ 6i ] metals: to make a fair companTon, trial fliould be made of extending filver upon the furface of gold in the fame manner as gold is extended upon filver. It may be ob- ferved alfo, that as gold is near as heavy agiin as filver, or contains near double the quantity of matter under an equal volume; fo, if equal weights of the two metals be ftretched to equal extents, the filver will be little more than half the thinnefs of the gold ; and converfely,. if filver could be brought to equal tenuity with gold in regard to bulk, it would, in regard to quantity of matter, be near of double extenfibility. ^application of gold leaf and wire on other bodies. There are various methods of applying the gold,, thus extended, to cover the furface of other bodies. For laces and brocades, the riattcd gilt wire is fpun on threads of yellow filk approaching as near as may be to the co- lour of gold itfelf. The wire, winding off from a bobin,. twifts about the thread, as it fpins round ; and,, by means of curious machinery, too complex to be defcribed here, a number of threads is thus twifled at once by the. turning of one wheel. The principal art confiils, in fo regulating the motion, that the feveral circumvolutions of the flatted wire on each, thread may juil: touch one ano- ther.,, and form, as it were, one continued covering. It is faid that, at Milan, there is made a fort of flatted wire gilt only on one fide, which is wound upon the thread, fo that only the gilt fide appears; and that the prepai^ation of this wire is kept a fecret, and has been, attempted in other places with little fuccefs. There is alfo a gilt copper wire, made in the fame manner as the gilt filver: Savary obferves, that this kind of wire, called falfe gold, is prepared chiefly at Nuremberg ; and that the ordinances of France require it to be fpun, for its dlAindion C 62 ] diftinftion from the gilt filver, on flaxen or hempen threads. One of our writers takes notice, that the Chinefe, inftead of flatted gilt wire, ufe flips of gilt paper, which they both interweave in their fliuffs, and twifk upon fiilk threads : this praftice he inconflderately propofes as a hint to the BritiOi weaver. Whatever be the pretended beauty of the fluffs of this kind of manufadture, it is obvious that they mufl: want durability : the Chinefe themfelves, according to Du Halde's account, fenfible of this imperfec- tion, fcarcely ufe them any otherwife than, in tapeflrries, and fuch other ornaments, as are not intended to be much worn, or expofed to moiilure. Paper, wood, and other like fubjedls, are gilded, by fpreading upon them fome adhelive fubftances, and when almoft dry, fo as but jufl: to make the gold ftick, applying gold or gilt leaf, and preffing it down with a bunch of cotton, or the bottom of a hare's foot : when grown thoroughly dry, the fuperfluous orloofe gold is wiped off, and the fixed golden coat burnifhed with a dog's tooth, or with a fmooth piece of agate or pebble. Different kinds of adhefive matters are employed for this ufe : where refiftance to rain or moifliure is required, oil paints ; in mofl other cafes, a fize, made from cuttings of parch- ment or white leather, by boiling them in water. The compofition commonly ufed for oil gilding confifls of yellow ochre, finely powdered, and a fuitable quantity of drying oil, ground together till they unite into an uni- form mixture, of fuch a confiftence, that it may be freely laid on with the pencil, without fpreading beyond the part on which it is applied, and that it may fettle fmooth with a glofl!y furface. For gilding on wood, &c. with what is called water- fize, the parchment or leather fize above-mentioned is mixed with whiting, and feveral layers of the mixture "■^'''-""■"' i fpread [ h ] fpread upon the piece, one after another Is dry, fo as to cover the grain of the wood, and the iniperfedtions left by the tool, and form a perfedly fmooth furfacc for applying the gold upon : over this is commonly fpread fonie of the fame lize mixed with yellow ochre. Thefc compofitlons do not well admit of the gold being bur- nifliedi and therefore, where burnillied gilding is re- quired, another mixture, called gold fize, is either laid above thefe, or applied on the wood at firft. The gold- fize is compofed of tobacco-pipe clay, or bolar earths, ground with a fmaller proportion of ruddle and black lead, and tempered with a little tallow and oil olive. In thefe points there is little uniformity among the work- men, the fame end being obtainable by different means, among which we cannot perhaps diftinguifli any fuperio- rity in the effedl of one to that of another, and of which fancy or prejudice have often chofen the more com- pounded in preference to the more fimple. The principal caution, in regard to the gold-fize, feems to be, to ufe no more of the unftuous materials than is neceffary for procuring the due confiftence ; and to make a trial of the preparation previous to its being employed in any work of confequence. For fome purpofes, the gold is ufed In powder, which, from its being kept in fhells, is called fliell gold. This is prepared by grinding gold leaves, or gold-beaters frag- ments, with a little honey; and afterwards feparating the honey from the powdered gold by means of water. Gold may be reduced alfo by diffolving it in mercury, and evaporating the mercury in the fire, or by diffolving it in aqua regis, and precipitating with certain additions, of which hereafter, into a powder, more fubtle than can eafily be obtained by mechanical comminution. Gilt [ 64 ] Gilt letters or figures on paper maybe formed of fliell .gold, tempered with gum water : or the charadters may be drawn with a milky folution of gum ammoniacum, made in water, and gold leaf applied upon them whea almofl dry : if they have become quite dry, they may again be fufficiently moiftened for receiving the gold by breathing on them. For raifed letters, fuch as are (ecn. in fome ancient manufcripts, whiting, yellow ochre, or other earthy powders, are tempered with Itrong gum wa- ter, and the letters formed of this compofition, by a pen, or more commodioufly, by means of a type or ftamp, pre- vioufly oiled, as hinted in a pamphlet on drawing and painting in water-colours, publiflied ia 1731 ; when dried to a due degree of tenacity, the gold leaf is laid on. It the charaders are formed of hard bodies, as powdered glafs or cryftal, they may be covered with a burnifhed golden coat, by carefully rubbing them with a piece of' folid gold. On the covers of books, the gildings are deprefled beneath the furface, and cemented with whites of eggs. The part being rubbed v/ith this liquid, the gold leaf is applied all over it, and the letters or figures made after- wards by heated flamps or rollers, which, at the fame time that they form the cavities, prefs down and fix the «old in them ; while the gold, on the prominent or fmooth furface, adheres fo loofely as to be eafily wiped ofF. In the pofl:humous papers of Mr; Hooke, a method is defcribed of gilding live craw-fifh, carps, &c. without injuring the fifli: The cement for this purpofe is prepared, by putting fome Burgundy pitch into a new earthen pot, .and warming the veflel till it receives fo much of the pitch as will ilick round it; then ftrewing fome finely powdered amber over the pitch when growing cold, add- ing [ 65 ] ing a mixture of three pounds of linfeed oil, and one of oil of turpentine, covering the veflel, and boiling them for an hour over a gentle fire, and grinding the mixture, as it is wanted, withfo much pumice ilone, in fine pow- der as will reduce it to the confiftence of paint. The fifli being wiped dry, the mixture is fpread upon it, and the gold leaf being then laid on, and gently preffed down, the fifli may be immediately put into water again without any danger of the gold coming off, for the matter quickly grows firm in water. As the qualities of this cement excellently fit it for fome other purpofcs, it was thought worth wliile to infert the whole procefs. Drinking-glalles, gilt on the edges,, have of late been much admired : the befl of thefe are brought from Ger- many j thofe hitherto made in England, though equal in! beauty to the foreign, being greatly inferiour in the durability of the gilding. It is fuppofed that the German glafles are gilt by fire : and it is certain that gold leaf may be made to adhere firmly to glafs foftened by heat, and that the effedl may be promoted by the interpofition of fome vitrefcent bodies more fufible than the glafs itfelf:. a piece of glafs pipe being moiftened with a weak folution . of borax, then covered with gold leaf, dried, and heated to a full red heat, the gold was found cemented more ftrongly than that on the German glafies, fo as fcarce at all to be fcraped off with a knife; though in fome parts it appeared fpecky or full of fmall holes, probably from want of fufficient addrefs in the application of it.. But how firmly foever the gold may be thus cemented, it. would be very difiicult to gild the edges of a glafs in this method without dainaging the reft ; and a careful exami- nation of fome of the German glafies fliewed pretty plainly that the gold had been fixed on them by other means. The glafles had evidently been ground and poliHied; yet K the r 66 ] thepollfheven of the part under the gold had not fufFered any injury, which it doubtlefs would have done from a degree of fire fufficient to foften its furface, or from any vitreous intermedium melted to it. The gold could be fcraped oft' pretty eafily with a knife .5 and by fleeping for a little time in heated Ipirit of wine or oil, particularly in the latter, it became more eafily feparable. One fide being thus cleared from the gold, there appeared a fmear upon the glafs under it J and. this being cleaned off, there ap- peared a like fmearinefs between the gold and the glafs on •the oppofite fide j whereas, on viewing in the lame man- ner the glafs which I had gilt by fire, the furface of the ■gold next to it looked remarkably bright, without theleaft cloudinefs upon the glafs. From thefe obferyations it may be prefumed, that the gold is cemented to the German glafles on the fame principle with the foregoing gildings ; and that the only fecret confifts in finding a matter, which will adhere to glafs, (o as not to be eafily rubbed off. I have tried maftich and fome other refinous bodies rubbed warm upon the glafs, and feveral fpirituous varniHieSj but -all of them were attended with fome inconveniencies, par- ticularly with the grand one of not adhering fufficiently to the glafs. I recommend to the trial of the artifts concerned in this afi^air the harder oil varniihes j and fliall myfelf profecute the enquiry, and when fuccefsful, communicate the refuk. S E C T. IV. Of the effeBs of Fire on gold. I. Of the melting of gold. GOLD melts in a low white heat, and, when in fufion, appears on the furface of a luminous blui/h .-green colour. Though its expanfion by fmall degrees of jieat, as from freezing to boiling water, is lefs fhan -that [ 6; ] diat of mo ft of the other metals, yet hi fufion it feems to expand more than any of the others; jifing up with a more convex or elevated furface, as it becomes fluid; and fubfiding, and growing more concave or deprelTed, as it fets again or returns to folidity. From this property, it follows, that gold is lefs fit for receiving fliarp and' perfedt figures when cafl: into moulds, than filver, cop- per, lead, or tin, which do not Ihrink fo much, and' far lefs fo than iron or bifmuth, which expand in their- paflage from a fluid to a folid flate. . The workmen, for the melting of gold, chufe gene-- rally a black lead. • crucible, .on account of its being'- fmoother than the Heffian or other common forts, and. confequently lefs apt to retain any particles of the coftly metal: it is likewife much lefs liable to crack, may be ufed for feveral fufions, and does not require the precau- tions neceffary to be obferved where the others are em- ployed. When the gold is divided into fmall parts, as filings, though all the particles be brought- to perfedt fluidity, they do not eafily reunite into one aiafs, many of them- continuing frequently in diftind drops. This repugnance is judged to proceed from fmall atoms of dufl, or other extraneous matters, adhering to the furfaces of the par- ticles, and preventing their clofe contadl : the addition of certain fufible faline fubftances, which diflblve and- vitrefy earthy bodies in the fire, is found to remove the impediment, and to colled and unite the gold how- ever divided. The ufe of fluxes is abfolutely neceffary in thefe circumflances ; and from their apparent utility here, it has been thought that they were needful- in other cafes, and hence they are often employed where they, feetn to be little wanted. K 2 Bbrax, [ 6B 1 Borax, one of the mofl powerful dilTolvents of earthy matters, is, in this refpedl, one of the heft fluxes for gold J but the gold melted with it, however fine, is commonly obferved to have its colour made fomewhat paler. From what caufe this flight diminution of the colour proceeds, I have not been able to difcover : nor do the workmen find the diminution ^onfiderable enough to prevent their ufing borax more generally than any other kind of flux. Nitre, added to the borax, prevents this effeftj and gold previoufly made pale by borax has its colour reflored by melting it with an addition of nitre : hence this fait is ufefully employed where the gold is defigned for the high coloured fort of leaf, for gilding, or other purpofes where the high colour of the metal is a principal objedt. When gold is alloyed with copper, and the full proportion of the copper is to be preferved, nitre is never to be ufed, the bafe metals being fcorified or deftroyed by it : in this cafe it will be advife- able to add to the borax a little charcoal in fine powder, which will preferve the copper from being fcorified by the heat. There is another material point, in melting gold, the prefervation of its malleability ; which is very liable to be injured, either from an excefs, or deficiency, or too * fudden an abatement of the heat, occafjoning an undue arrangement of its parts at the time of its becoming folid. When the gold is made exceflively hot, and the mould, into which it is to be poured, is warmed but little or not at all, the metal almoft always contrads a degree of hardnefs and rigidity; whereas by duly pro- portioning the heat of the mould to that of the metal, its foftnefs and toughnefs may generally be fecured. The gold-beaters, to whom thefe qualities are of more im- portance than in any other art, heat the mould, as already obferved. t 69 3 obferved, till the tallow, which it is rubbed with, runs -and fmokes, without taking flame; and pour out the gold as foon as its furfacc appears of a bright green colour : the clearnefs of the colour ferves them as a mark both of the gold being of a proper degree of heat, and of its be- ing hne. Thofe who work in alloyed gold, judge alfo from the appearance of the furface, whether the metal is of fuch a heat, or iuch a difpofition, as to prove tough or eager when cold ; taught, by ule, marks which cannot ealily be defcribed. It is fuppofed by fome, that gently fliaking or ftriking the crucible, fo as to communicate a k>nd of undulatory motion to the fluid metal jufl: before it is poured out, contributes to its toughnefs. It is a general opinion among metallurgic writers, that fine gold, in fufion, is made brittle by the contad: of vegetable coals, not thoroughly burnt, or by their fume j and what is pretty Angular, that gold alloyed with copper is not fo fubjed: to receive this injury. But it is probable that the brittlenefs, afcribed to this caufe, depended ra- ther upon others : for the gold-beaters«, who leave their crucible open, do not find, that the toughnefs of the gold is at all diminilhed, either by the vapour of the charcoal, or by a coal in fubftance falling in ; though, if any fuch diminution happened, it could not be fup- pofed to efcape their notice. There appears to be little danger to the malleability of gold from any kind of fumes but metallic ones. When gold is made brittle by a fmall admixture of bafe metals, or by their fumes, its malleability may be reftored by melting it with a little nitre, which fcorifies and diflblves all the other metals except fllver and pla^ tina. The nitre (hould be thrown upon the gold jufl as it is going to melt; and the metal poured out as foon as it flows thin. A long continuance of the fufion is apt to t 70 1 to deftroy the effedl of the nitre, and render the gold as- brittle as it was before: for fo much of the nitre, as has- adled upon the bafe admixtures of the gold, is changed, by that adlion into an alkaline fait ; and the flightefc £iccefs of any inflammable matter is fufficient to revive- the fcorifi^d metallic particles from the alkali, and ren- der them again mifcible with the gold. Corrofive mer- cury-fublimate, thrown by a little afS time upon gold iri- fufion, with care to avoid its noxious fumes, anfwers the- fame end with nitre-, and is commonly preferred to it by the workmen : on what foundation the effecft of fublimate- depends, will appear hereafter. II. Of the alterations f aid to be producible in gold by Jire» The greateft degrees of artificial fire, continued for ar length of time, have not been obferved. to make anyr alteration in gold, Gafto Claveus, in an apology for the- alchemifts, printed in. the fecond volume of the Tbeatrum. ahymicum, relates, that he put an ounce of pure gold, in an earthen veiTel, into that part of a glafs-houfe furnace where the glafs is kept conftantly melted, and con- tinued it there in fufion for two months together; and Kunckel mentions a like experiment, made in the glafs^ furnace of the duke of Holfatia, in which the gold was expofed to the fire for almoft thirty weeks. Thefe ve- hement and continued degrees of heat it was found to. fupport, without fuffering any fenfible alteration of its, quality, or diminution of its weight; whereas the other metals,, platina and filver- excepted, are foon deprived by< fire of their metallic afpedt, and either diffipated in fumes,, or changed to an earthy or glaffy form. What common fire eff^edls in the bafe metals has beea faid to be effedted in gold by the more intenfe heat col- teited in the focus of large burning-glaffes. Mr, Hom- berg [ 71 ] ■-berg reports, in the memoirs of the French acddemy for the year 1702, that he expofed gold, on a piece of charcoal, to a burning lens, about thirty-three inches in ■diameter, whofe activity was further increafed by the interpofition of a fmaller lens placed at a proper difiance for contradling the focus into a lefs compafs : that this vehement heat produced a powdery matter on the furface of the gold, which, gathering together, formed a vitreous drop in the middle; and then run off to the fides : that the furface, now bright, became again gradually covered with a like duft, which in like manner vitrefied and run offj that frefli drops of glafs continued to be thus pro- duced; and that at the lame time great part of the gold -evaporated in fumes. This experinient, as Cramer very juftly remarks, does not feem to have been made with fufficient care, or carried to a fufficient length, to warrant the confequences that have been drawn from it. The purity of the gold ought to have been fcrupuloufly examined, which it does not appear to have been at all 5 and fuch part of it as remained unaltered after the operation, ought to have been further fubmitted to the fame treatment ; for if any part of the gold was really changed, the whole would doubt- lefs have fuffered the fame change from a continuance of the famecaufe. The author, fenfible of this, fays indeed, that if the gold be expofed long to the heat, it will at length be totally vitrified or evaporated : but he does not affirm that this adlually did happen, and feems only to have judged from the firft appearances that it would hap- pen. To attempt the revival of the glafs into gold again, ■which Cramer and Macquer require for the fatisfadory" proof of its having been produced from gold, was not perhaps to be expected from him j fince, according to his theory, the glafs confided only of the e^irtby part of the C 72 J the gold, the fpecificating principles of the metal being; fuppofed to have evaporated in the intenfe heat : yet, if he really believed that it proceeded from the gold at all, it is exti-emely flrange, that fo curious a chemift fliould pafs over a produdl fo extraordinary, and fo interefting to his favourite purfuits,, without the leaft examination, and even negledt to repeat and verify the fingular experiment by which it was obtained ^ efpecially as he had all poffible opportunities of profecuting the enquiry, the apparatus- being at his command, and the materials provided for him by royal munificence. From one of his fubfequent. papers published in the fame memoirs for the year 1707,, it appears, that this vitrification of the gold was called in queftion by one who had been witnels to the experi- ment; who took notice that fome a(hes, flying off from. the charcoal on which the gold was placed, fell from time to time upon the forface of the metal ; and hence judgedj^, that the little quantity of glafs obtained was no other than a vitrification of thefe afhes. An objed;ion fo well founded required furely a repetition of the procefs, and a more attentive obfervation of the phenomena; but the author anfwers only by another experiment, which appears alfo to be fingle in its kind, that when filver was expofed on a piece of charcoal in the fame manner, no vitrification happened; as if the afhes could not be cafu- ally accumulated on the metal, fo as toproduce a fenfible vitrification, in one inftance, without being fo in another. I have been the more particular in the account of this experiment, becaufe it has been relied on by many as an jindubitable proof of gold being alterable in its nature;/ and becaufe a due attention to what the author himfelF has thought fit to communicate, fliews it to be at befl too imperfed: for any ftrefs at all to be laid upon it. The want of a proper apparatus here is an infuperable obftacle z to [ n ] to my repeating the experiment: but it is faid that others have repeated it, and found ftrong reafons to be- lieve that Homberg was deceived. M. Macquer relates, that feveral perfons have expofed goUl to the focus of the fame burning lens, and even of others ftill ftronger, with- out ever being able to vitrefy it : and that, though the gold did indeed decreafe in weight, yet the diminution appeared to happen, not from any of the principles of the metal being feparated, but from minute globules of it forced off in fubftance ; many of which were catched on a piece of paper placed underneath, and found to be per- fedl gold unchanged* It is probable, that thefe globules were forced off from the liquefied gold, not by the adion of the heat upon the metal itfelf, but upon its veffel or fupportj for aU the common veflels, or other fubflances that can be employed for this purpofe, on being expofed fuddenly to a great degree even of artificial fire, crackle or fume, and throw up a part of their contents. The earlier chcmifts, finding gold to be proof againfl the vehemence of their fires, thought milder means might be more effedlual, for loofening the clofe union of its component parts, and producing changes in it fimilar to thofe producible in the bafe metals. Accordingly they expofed it, for feveral weeks or months together, to the immediate adion of a gentle fire or flame not much greater than that in which lead melts : by this me- thod it is faid to have been notably altered in its pro- perties, and to have affumed feveral new ones : Kunckel, in his Laboratoriiim Chymicum, afiirms that he has fucceeded in this experiment, nnd fays that the gold fwells up into a fpongy fubftance, like iron treated in the fame manner. The obfcure and imperfetl accounts given of the procefs prevent our being able to repeat it fo as to determine with L certainty f 74 ] certainty Its true effedt: but there appears as little reafoa to believe, in this as in the other cafe, that the gold fuf- fered any permanent change. The gold is required to be prevloufly prepared : if this preparation confifts, as it mofl probably does, in combining it with any other kind of matter that will abide with it in the fire, and reducing it into fubtile powder, a heat of no long continuance will occafion a remarkable alteration in itsafpecS, though its other properties remain entire. If gold leaf be divided by grinding it with an admixture of earthy powders, as calcined hartfhorn or chalk, or with faline ones of the more fixed and lefs fufible kind, as vitriolated tartar, and expofed for fixteen or twenty hours, to a moderate heat, fcarce fufficient to keep the veflel red hot; the gold wholly lofes its metallic brightnefs, and changes its yellow co- lour to a red or purple. On feparating, by means of wa- ter or acids, the foluble fait or earth, the remaining golden powder recovers by fimple fufion its proper metallic form ; a ftrong heat diverting it of thofe fuperficial appearances which a weaker one had induced. S E C T. V. Of the Mixture of gold with other metals, - TH E repugnancy or contrariety, which obtains In fundry inftances, between different metallic bodies made fluid by fire, and which Is no lefs flrongly marked than that betwixt oil and water, is no where obferved in regard to gold ; this metal uniting readily with all the other known'metallic bodies, and feeming to have a flrong, though not equal, affinity to them all. J I. Of [ 75 3 X. Of the mixture of gold with mercury : Gold poioder, watcr-gildingy &c. Mercury, in the greateil cold that obtains in our at- mofphere, adheres readily to gold, totally conceals its colour, communicating a filver whitenefs to every part it touches, and by degrees penetrates and diflblves it. Some of the chemifts fpeak of an animation of mercury, by which its adivity on gold is greatly increafed ; and Mr. Boyle relates, that he had himfelf prepared mercury, fo as to diflblve half or even equal its w^eight of gold leaf, and to produce, during tlite diflblution, a fenfible heat, fometimes confiderable enough to be ofFenfive to the hand : but an enquiry into this point belongs rather to the hiftory of mercury than of gold. In order to obtain a fmooth amalgam, or uniform mix- ture, of gold and common mercury, the union is expe- dited, by reducing the gold into thin plates or grains ; which are heated red hot, and in this ftate thrown into as much mercury, as will cover them, previoufly heated in another crucible, till it begins to fmoke : on ftirring them together with an iron rod, the gold foon diifolves and difappears. If the amalgam is defigned for arty nice iifes, it fhould be cleanfed, from any filth it may have contraded, by grinding it in a glafs, flone, or wooden mortar, with fome common fait and water, and occafion- ally renewing the water, till the amalgam ceafes to dif- colour it, and appears of a pure vivid brightnefs. When the proportion of mercury is large, fo that the mixture continues fluid when cold, a confiderable part may be fcparated by preflingit through foft leather, as the thicker kind of waih-leather or doc-ikin : fo much of the quickiilver may be fqucezed out, as to leave a butyra- ceous or confillent mafs, containing little more than one L 2 part f 76 ] part of mercury to two of gold, but ftill of a filver white- ■, nefs, as if there was no gold in it. The confiftent amal- gam grows foft on being warmed or worked between the fingers, and hardens on lying in the cold, whence it has been propofed as a proper material for making feals from impreflions in wax : the amalgam of gold appears however to have no advantage in this refped above thofe of the inferiour metals, as is weil known to fome impoflors^ who have fold amalgams of bafe metals, for this ufe, as curious preparations of gold. The mercury, drained off from the amalgam, fliould be referved for the likepur- pofes again, as the leather, though novifible imperfection happens in it, may have its pores fo far dilated by the preflure, as to fuffer fome fmall particles of the goki to pafs through with the mercury : this may be difcovered by evaporating a little of the quickfilver over the fire, which in this cafe will leave a yellow fpot on the bottom of the vefTel. Mercury, one of the moft volatile of the metallic bodies> is expelled from gold by a fire not fufficient to make the mixt red hot. If the amalgam is expofed haftily to this degree of heat, it is apt to fwell up and leap about, and part of it to be thrown over the velfel : if the fire is gentle at firft, and increafed by degrees, the mercury exhales quietly. The impalpable atoms, into which the gold had been divided by its diffolution in the quick- filver, continue difunited after the quickfilver has exhaled ; provided due care is taken in the regulation of the fire, and in fiirring and rubbing the matter, towards the end of the procels, fo as to expofe it equally to the heat, and prevent its running into lumps. By this method a powder of gold may be obtained, much finer than that prepared by the grinding of gold leaf, and which has likewife this advantage, for the purpofes of painting, that it [ 77 ] it admits better of being burnifhed. It is obvious, that for ufes of this kind, the mercury ought- to be pure, as well as the gold : for the lead or other bafe metals, with which mercury is too frequently impregnated, will be left behind, and difcolour the gold. If an amalgam of gold be fpread upon copper, and the mercury evaporated by fire, the gold will remain fixed all over the furface of the metal, anfd thus afford a firm and durable gilding. The workmen rarely chufe pure copper for gilding upon in this manner, but gene- rally mix with it about a Jeventh part of brafs, that is, of a compofition of copper and zinc : they fuppofe that this addition renders the copper lefs porous, and makes a lefs quantity of gold to ferve : whatever there maybe in, this notion, the brafs is plainly of ufe on another account, to facilitate the adhefion of the mercury j for mercury unites exceeding difficultly with pure copper, and much more eafily, as I have often found in experiments of amalgama- tion, with copper divided by zinc. The piece to be gilt being well' cleaned, fome mer- cury, Shaken with a little aqua fortis, is fpread upon it, till the furface appears all over white as filver : being then heated, and re-touched in thofe. parts which have efcaped the mixture, the amalgam" of gold is laid on : the heat, foftening the amalgam, makes it fpread the more freely j and the intervention of tlie mercury and aqua fortis occafions it to adhere more uniformly. The piece, thus covered with the amalgam, is placed on a convenient fupport, over a charcoal fire ; and examined from time to time, as the mercury evaporates,: that if any deficiencies appear, they may be, fupplied with a little more of the amalgam before the procefs is completed. If a thicker gilding is required, than can refult from fo much of the amalgam as is applied at once, the piece, after [ 78 ] after the firft quantity has left its gold fixed upon the fur- face, is rubbed afrelh with the mixture of mercury land aqua fortis, and more of the amalgam fpread upon it : after the evaporation of the mercury from this, another and another quantity may be applk* . ia theYame manner. The golden coat, left after thefe op; lations, is fometimes of a. pale dead colour; occafioned perhaps, partly, by im- purities in the mercury, and partly, by a little of the mercury itfelf left unevaporated. Whatever be the caufe, the workmen find a remedy in rubbing upon the piece, while warm from the fire, after the loofe particles of gold have been wiped off with a clean fcratch brufh, made of very fine brafs wire bound together,) a compofi- tion, called gilding wax, which being burnt off, fome more of it is rubbed on, and this application repeated till the gold appears of a proper colour. The gilding wax is compofed of bees-wax, red ochre or ruddle, verdegris, vitriol or allum, and fometimes other additions : the acid of the falts and the cupreous part of the verdegris feem to be the materials on which the effedl of the compound chiefly depends. I have been informed by an ingenious artift, that he has employed for many years a faline com- pofition without wax, and found it to anfwer extremely well: equal quantities of nitre, fal ammoniac, green vitriol, and verdegris in fine powder, are mixed together, moiftened with water, and applied upon the pieces which is then heated till the mixture fmokes, and quenched in urine. There are two principal inconveniencies in this bufinefs: one, that the workmen are expofed to the fumes of the mercury, and generally, fooner or later, have their health greatly impaired by them : the other, the lofs of the mercury j for though part of it is faid to be detained in cavities made in the chimney for that purpofe, yet the greateft part of it is loft. From fome trials I have made it [ 79 ] It appeared that both thcfe inconvenicncles, particularly the nrft and mod conliderablc one, might in good nieafure be avoided, by means of a furnace of a due conflrudtion. If the communication of a furnace with its chimney, in- ftead of being over the fire, is made under the grate, the afli-pit door or other apertures beneath the grate clofed, and the mouth of the furnace left open ; the current of air, which otherwife would have entered beneath, enters now at the top, and pafling down through the grate to the chimney, carries with it completely both the vapour of the fuel, and the fumes of fuch matters as are placed upon it : the back part of the furnace fliould be laifed a little higher above the fire than the fore part, and an iron plate laid Over it, that the air may enter only at the front, where the workman ftands, who will thus be efFedtually fccureJ ffom the fumes, and from being incommoded by the heat, and at the fame time have full liberty of introducing, in- fpedling, and removing the work. If fuch a furnace is made of ftrong forged (not milled) iron plate, it will be fufficiently durable : the upper end of the chimney may reach about a foot and a half highei- than the level of the fire: over this is to be placed a larger tube, leaving an interval of an inch or more all round between it and the chimney, and reaching to the height of ten or twelve feet, the higher the better. The exteirnal air, palling up between the chimney and the outer pipe, prevents the latter from being much heated, fo that the mercurial fumes will condenfe againft its fides Into running, quick- filver, which falling down to the bottom, is there catched in a hollow rim formed by turning inwards a portion of the lower partj and conveyed by a pipe at one fide^ into a proper receiver. .-wr»i ji t-:r/lii Mr. Hellot communicates, In the Memoirs of ihe FrencK ■ academy for the year I745> a method of making raifed figures [ 8o ] i^gures of gold on U'orks of gold or filver, found among the papers of Mr. du Fay, and of which Mr. du Fay hiin- felfhadfeen feveral trials. Fine gold in powder (fuch as refults from the parting of gold and filver by aqua for- tis, as defcribed hereafter) is direded to be laid in a heap on a levigating ftone, a cavity made in .the middle of the heap, and half its weight of pure mercury put into the cavity: fome of the fetid fpirit, obtained from garlick root by diftillation in a retort, is then to be added, and the whole immediately mingled and ground with a muller, till the mixture is reduced in tOj an uniform grey powder. The powder is to be ground with lemon juice to the conliftence of paint, and applied on the piece previoufly well cleaned and rubbed over with the fame acid juice : the figures drawn with it may be raifed to any degree by repeating the application. The piece is expofed to a gentle; fire till the mercury is evaporated fo as to leave the gold yellow,, which is then to be prefled down, and rubbed with the finger and a little fand, which makes it appear folid and brilliant : after this it may be cut and embellifhed. The authour obferves, that being of a fpongy texture, it is more advifeable to cut it with a chifel than to raife it with a graver ; that it has an imperfedlion of being always pale, and that it would be a defireable thing to find means of giving it colour, as by this method ornaments might be made of exquifite beauty and with great facility. As the palenefs appears to proceed from a part of the mercury retained by the gold, I apprehend it might be remedied by the prudent application of a little warm aqua fortis, which diflblving the mercury from the exteriour part, would give at leaft a fuperficial high colour: if the piece is iilver, it muft be defended from the aqua fortis by cover- ing it with wax. Inftruments or ornaments of gold, ftained by mercury, where the gold is connected with •;; fubftances [ 8i ] lubftances incapable of bearing fire, may be relloted to their colour by the fiime means. The foregoing procefs is given entirely on the authority of the French writer. I have had no experience of it iiiyfelf, but have feen very elegant figures of gold raifed upon filver, on the fiime principle, by a dificrent procedure. Some cinnabar was ground, not with the diflilled fpirit, but with the exprefled juice of garlick, a fluid remarkably tenacious : this mixture was fpread all over the polifhed filver J and when the firll: layer was dry, a fccond, and after this a third was applied. Over thefe were fpfead as many layers of another mixture, compofed chiefly of afphaltum and linfeed oil boiled down to a due confidence. The whole being dried, with a gentle heat, on a kind of wire grate, the figures were traced and cut down to the filver fo as to make its furface rough : the incifions were filled with an amalgam of gold, raifed to different heights in different parts according to the nature of the defign ; after which a gentle fire, at the fame time that it evapo- ' rated the mercury, deflroyed the tenacity of the gummy juice, fo that the coating, which ferved to confine the amalgam and as a guide in the application of it, was now eafily got off. The gold was then preffed down and em- belliflied as in the former method, and had this advantage, that the furface of the filver under it having been made rough, it adhered more firmly, fo as not to be in danger of coming off, as M. du Fay fays the gold applied in his way fometimes did. Theartifl however found the procefs fo troublefome, that though he purchafed the receipt for a confiderable fum, he has laid the prailice afide. Mercury and amalgams rubbed on iron do not at all adhere to it : there are however means of applying the mercurial gilding on this metal, as well as on copper and filver. If the iron be dipt in a folution of the blue vitriol of copper, or rubbed with the vitriol itfelf fomewhat M ' moiflened. [ 82 ] moiftened, it becomes immediately covered with a cupre- ous coat, and now receives the gilding in the fame man- ner as folid copper. II. Of the mixture of gold with Jilver, copper and other metals ; the alterations produced by different proportions of • different metals^ and the effedls ofjlrong or continued fire on the mixtures. All the metals, which melt eafier than gold, diffolve it in a lefs heat .than the gold would melt in ; and gold, brought into fufion, dilTolves in like manner thofe which are more difficultly fufible. It is particularly difpofed to unite with iron, of wKich> if the irOn be pure, it diflblves twice or thrice its own weight in a degree of heat very far lefs than that in which iron melts : if iHrred in fufion with an iron rod, it corrodes a part of the iron, and a large portion of the gold adheres to the inflrument: hence Cramer, Schl utter, and othei* writers on affaying, prudently caution againll the ufe, of an iron rod for the Itirring of melted gold. In virtue of this property, gold proves an excellent folder for the finer kinds of iron and lleel inftruments : a fmall thin plate of gold being wrap- ped round the parts to be joined, the gold is foon made to melt by a blow-pipe, and ftrongly unites the pieces to- gether, without any injury to the inftruojent, ,hqw.c,ver delicate..., ; 3'' "-■'i''-i-'J ■'- On copper, its apparent adion is much lefs conGderable j yet, when once it is united with this metal, the increafe offufibility is more ftrongly marked, mixtures of gold with a little copper being found to melt with lefs heat than pure gold itfeif. Hence jnixtures of this kind ferve as folders for gold : two pieces of 'fine gold are foldered by gold that has a fmall admixture of copper; and gold alloyed with copper is foldered by fuch as is. alloyed with more [ 83 ] ^ more copper : the workmen add a little filver as well as copper, and vary the proportions of the two to one ano- ther, lb as to make the colour of the folder correfpond as nearly as. may be, to that of the piece : copper alone, in tlie quantity rcquifitc to procurd-the due- fenfibility, would incline the inafs too much to its own colour. Silver, mixed with gold, dilutes its yellow colour more or lefs according to its quantity. Onc;twentieth or lefs of filver renders gold very fejifi.bly paler. fei and th« ad-, dition of a twentieth more ma;ke.s it fenfibly paler than the former proportion : but when the filver is increafed to a tenth or an eighth of the gold, fo fmall differences in the quantity fcarce occafion fenfible variations in the colour, and ftiU lefs fo when the filver exceeds the gold j a little gold not near fo much a'ffeding the colour of filver, as a little filver does that of gold. All the mixtures are very malleable, though fomewhat harder, firmer, and more fonorous, , than either of the metals feparately : in this refped, as in the colour, a little filyer affedls gold more than a little gold does filver. Copper, in fmall quantity, renders the gold fomewhat harder than filver does, and fomewhat heightens the co- lour, by fuperadding its own reddifhnefs to the gold yel- low j but if the quantity of copper is confiderable, the coppery hue prevails: ,a little gold mixed with copper makes no remarkable alteration either in its colour or dudVility. The high colour which a fmall proportion of copper communicates to gold, has been obferved in differ- ent circumftances, and given rife to fundry procefies for the exaltation of the colour of the noble metal. Some recommend for this purpofe the fuperficial application of verdegris, blue vitriol, or other preparations of copper ; which indeed are often ufed by the workmen, but whofe M 2 efFedt [ 84 ]^ effeft feems to be, not the heightening of the colour of the metal itfelf, but the removal of the fuperficial tarnifh or difcoloiation which alloyed gold is apt to receive from the fire ; and this tffcd: appears to proceed, not from the copper, but from the acid which thefe preparations contain. Others, for communicating a high- colour to the whole mafs, dire6l the gold to be melted with three or four times its weight of the highell coloured copper, the mixture to be granulated or flatted into plates, then boiled in weak aqua fortis, in order to feparate as much of the copper as the acid will extrad:, the remaining gold to be melted with frefh copper, and this procefs repeated feveral times. It is apprehended, that by this method only a fmall portion of the copper will be left in the gold, and that this little will be fo intimately commixt with it as to reftft the adtion of acids and of the air ; and that the gold will thus receive the admired colour, with- out being made much more liable to tarnilh, or to change its colour on waftiing or boiling, than fine gold. Platina, next to the two foregoing metals, injures the malleability of gold the leaft. Mixtures of gold with one twentieth of its weight of platina I have drawn into moderately fine wire : mixtures of it with one fourth its weight were forged into pretty thin plates : and a mixture of equal parts (which is as large a proportion of platina as can be eafily united with gold) was indeed brittle, but bore feveral ftrokes, and ftretched confiderably under the hammer, before it began to crack about the edges. With regard to the colour, fmall proportions of the platina, as one fixtieth, make little alteration: .in larger proportions, as one twelfth, it communicates, not its ovvji whitenefs, but a particular and remarkable dull hue, the compound approaching more to the colour of bad copper than of 5 gold ■ _ [ 85 ] gold: in the quantity" of one fourth and upwards, it gives a dull whitiflinefs. Iron or fteel, in very fmall proportion, render gold hard and eager, and on increafing the quantity of the iron, the mixt continues brittle : Ibme of thefe mixtures are of luch a degree of hardnefs and clofenefs, as adapts them for receiving a fine edge, and it is laid that they have been formed into razors. The colour of the gold is made pale by a fmall quantity of the iron : equal parts of the tw^o form a grey mafs : if the quantity of the iron is three or four times greater than that of the gold, the mixt proves of a white colour approaching to that of filver. All the reft of the metallic bodies givepalenefs, dullnefs, and brittlenefs, indifferent degrees, fome more than others in'tlie fame quantities. Of tin and lead the mofl minute proportions, even the vapours which rife from them in the fire, though not fuflicient to add to the gold any weight fenfible in the tendered: balance, make it fo brittle that it flies in pieces under the hammer ; though gold, contrari- "wife, mixed with a fmall proportion of the lead or tin, does not appear to injure their malleability. Something of the fame kind feems to happen in the mixtures of gold, with the metals which of themfelves are brittle, as zinc, bifmuth, and regulus of antimony ; a fmall proportion of thefe metals rendering gold extremely brittle, whereas, when the brittle metal is in large proportion, its fragility is diminifhed by the gold : thus Mr. Hellot obferves, in a paper on zinc publifhed in the French memoirs for the year 1735. that a mixture of three parts of zinc and one of gold does not break fo ealily as a mixture of equal parts of the two. Some of thefe mixtures, particularly one of equal parts of gold and zinc, bear a fine poli(h, and pro- bably, as the above mentioned author takes notice, would be [ 86 ] ^ \yc excellent for making fpecula, being lefs fubjeil to tarnilh in the air than the compofitions of which copper is thebafis. It is fuppofed' by many, that gold, melted with other metals, is always difFufed equally through their whole vo- lume, infamiich, that the quantity of gold, obtainable froni any part of the mixt, fliall bear exadlly the fame pro- porti-on to that part, as the whole of the gold does to the whole mafs. There appears, however, in many cafes, a fenfible inequality in the diftribution. M. Hellot, in his French tranflation of Schlutter's German work on the fmelting and allaying of ores and metals, gives an account of an experiment which clearly flievved this inequality : a quantity of filver, amounting to upwards of twenty pounds, containing about a fifty- lixth part of gold, .was melted in a crucible, and poured into cold water, in order to its being reduced into grains : by dipping at different times an iron ladle into the water, under the ftream of metal, he received a part of the firft running, a part of the middle, and a part of the laft : the three parcels, affayed feparately, were all found to differ in their content of gold. There is a curious experiment of Mr. Romberg's, re- lated in the French memoirs for the year 171 3, which, though I have not yet tried it, I fliall venture to infert oii account of its Angularity. Equal parts of gold and filver, melted together and reduced into fine grains, were put into a crucible, with a mixture of about equal parts of decrepitated fea fait and rough nitre under them : the crucible being kept in afmallfirc, in a wind-furnace, for about a quarter of an hour, and then fuffered to cool and broken, the gold was found in one lump at the bottom, .aad the filver above it in two pieces, with fome grains, wrapped [ 87 ] wrapped up in thefalts, which had not entirely melted : thefilver was perfedly pure, and without the Icalt mixture of gold, but the gold retained about a fixth part of filver. He repeated the experiment with diftcrcnt mixtures of the two metals, and found the filver to be always pure from gold, but that the gold retained a little of the filver, except in two inftances, in which this alfo was pure. He obfcrves, that unlefs the gold apd filver are nearly in equal quantities, the feparation does notfucceed; and that the only nicety in the procefs confifls in hitting the due point of fufion, for if the fire is too long continued, or the mixt made to flow thin, the two metals, .after they have parted from one another, mingle again together. We have already fecn. that mercury may be evaporated from gold by a gentle heat : there are fome other metallic bodies alfo, which may be totally difTipated from gold by fire, but with confiderable differences in regard to the circumftances of the feparation. Arfenic, though of itfelf very volatile, adheres fo flrongly to gold as not to be eafily expelled: if the mixture is urged haflily vv'ith a violent fire, a part of the gold is carried off by the arfenical fumes. Zinc, in open veffels, "burns, changes into white flowers, and throws up along witlrits own fumes a fmall portion of the gold, which tinges apart of the flowers of a yellowi(h colour inclining to purple: thefe flowers do not rife high, part of them forms about the furface of the mafs,. and when once they are formed, they refift the fire i fo that though the whole of the zinc, by fre- quent fl;irring and fl:rong fire, fliould be thus changed, yet, unlefs the proportion of gold be large, the noble metal remains divided and interfperfed among the flowers. In clofe veffels, or where the external air has no accefs, the zinc, by force of fire, may be totally made to fublime : it may be expelled alfo in an open crucible, by keeping the mixt [ 88 ] mixt covered with powdered charcoal, which, fo far as it reaches, prevents the above change of the zinc. The ditljpation of regulus of antimony from gold re- quires, on the contrary, not only an open vefTel and free accefs of air, but the artificial impulfe of a blafl; of air upon ■ the furface: if the fire is vehement, the crucible {hallow, and the air ftrongly impelled, the gold is volatilized by this metal more confideiably than by either zinc or ar- fenic; but with proper care, the regulus may be blown off without any fenfible lofs of the gold. Some have pro- pofed regulus of antimony inflead of mercury for gilding on copper J as alfo on earthen ware and glafs, on which the mercurial gilding cannot be applied: the regulus and gold, firfl melted together, are diredled to be ground into fine powder, which being fpread upon the piece, the whole is expofed to a ftrong red heat, fo as to evaporate the regulus. The inconveniencies of this method are, that the powder does not of itfelf adhere to the fnbjeft, and can fcarce be equably fpread, and that part of the gold is wafted : that glafs melts in the heat necelTary for the exhalation of regulus of antimony, and that copper is liable to be corroded by the regulus, and have its furface rendered uneven. The bafe metals in general, which calcine or change to an earthy form in the fire, fufFer the fame change when mixed with gold, though with fome differences in the degree of facility. If gold, mixt with a fmall proportion of thefe metals, be kept for a confiderable time in fufion, the bafe metal, gradually fcorifying, rifes to the furface, no longer mifcible with the gold. If the gold is in lefs quantity, and the fire infufficient to bring the mafs into fufion, thewhole mixt lofes by degrees its metallic afpeiir, and the gold remains intermingled among the calx of the hafe metal, in a more attenuated ftate than it can perhaps be . [ 89 ] be brought to by other means : by long continuance o( a moderate fire^ the calx acquires more or lefs of a purple hue, according to the quantity of thcgold and the natural colour of the calx of the metal it is mixed with. Tin., which when calcined by itfelf is neither vltrefcible nor fufible in the fire, and which cannot be perfectly vi- trefied by the moft adlive fubftanccs commonly made ufe of in this intention, is remarkably atfeded by the admix- ture of gold. Dr. Brandt relates, in the tranfaftions of the Swedilh accadcmy for the year 1753, that two parts of tin, and three of gold, being melted together, reduced into fine powder, and calcined only to an afli grey colour, the calx melted with cafe into a yellow glafs, at the bot- tom of which was found a metallic regulus. I fnall exa- mine this curious experiment on another occafion. Though gold, in the highly attenuated ftate Jnto which it is thus reduced by calcination with bafe metals, is by fome bodies otherwife aded upon than in its grofier form, as appears from its habitude to tin in the preceding paragraph, and to the marine acid in the following feilion, it is by no means divelled of its metallic properties, or changed into a calx. Mercury, which does not difiblve metallic calces, any more than unmetallic earths and flones, on being triturated with the compound powder, imbibes the gold ; and on this foundation, gold, blended with the bafe metals, may in fome cafes be advantageoufly extradled from them. If mixtures of gold and lead be continued in a fire fuffi- cient to keep them in perfedl fufion, the lead, calcining and rifing to the furface, changes into a fluid fcoria, eafily feparable from the gold by means to be defcribed here- after. Bifmuth alfo fcorifies and feparates in the *fame manner ; and both thefe metals, promoting the fcorifica- N tion. [ 90 ] tion^ or fufion of the calces, of the other bafe metallic bodies, promote their feparation from gold in the fire. S E C T. VI. Of the aSlion of acid and fidphureous bodies on gold; various folutions of ity and their properties. I. Gold with the nitrous acid. TH E acid fpirit extracted from nitre, whether in its concentrated ftate, or in the more dilute one, in which it is commonly called aqua fortis, has not been found to have any adlion on pure gold. Hence gold is freed, by this acid, from filver, copper, lead, zinc, mer- cury, and fuch other metallic bodies as the acid diflblves : but that this feparation may fucceed, the quantity of the inferior metal muft be conliderable greater than that of the gold, for otherwife its particles will be enveloped by the gold fo as to be entirely defended from the acid. When nitre in fubftance is mixed with certain bodies containing the vitriolic acid, as calcined vitriol, and the mixture made red hot, the acid of the nitre is extricated in yellowifli red fumes. If the impure gold is interlaid with fuch a mixture, and expofed to the fire along with it, in a clofe veilel that the fumes may be confined j the bafe metal will be in part corroded, though its quan- tity is far lefs than would be adled upon by the acid in its liquid itate, but in this cafe the acid penetrates only a little way into the mafs. Hence, for the purification of gold by this method, the operation muft be two or three times repeated, the metal being each time melted and reduced into thin plates, that frefh furfaces may be ex- pofed to the fumes : and in the procefs by aqua fortis, if the bafe metal does not amount to a certain quantity, more bafe metal muft be added. The method of con- ducing [ 91 ] dufting the operations will be defcribcd in the eighth and ninth Sedtions. As pure gold has been always found to refill the nitrous acid, and as gold divided by filver or other metals has not been obferved to be aded upon by that acid in the common procefles of allaying or refining; it has been univerfally laid down as an axiom, that the pure nitrous acid can in no cafe have any adlion on gold, and that, in whatever manner it be applied to mixtures of gold with other metals, it can dillblve only the inferiour metal, and will always leave behind the full quantity of gold which the mixt contained. Here it may be obferved, once for all, that as the mutual relations of bodies are multifarioufly modified by the circumftances in which the fubjedls are applied to one another, fundry bodies dif- covering flrong repugnancies in fome circumftances, and flrong affinities in others ; we never can infer, from the conllancy and uniformity of the adtion or inadtion of two bodies on each other in all the circumftances in which they have been applied, that their relations will be the fame in any other circumftances ; and confequently, un- lefs all poflible means of application were known and ex- perienced, no axiom, in regard to the chemical affedtions of bodies, ought to be admitted as univerfal. Though the affayer and refiner depend upon the abfolute indiflblu- bility of gold by the nitrous acid, yet there are circum- ftances, in which gold is difTolved by this acid in con- fiderable quantity. This curious and important difcovery was made by Dr. Brandt, and publiQied in the Swedifli tranfadtions for the year 1748. In order to part a mixture of gold and filver, amounting to about fifteen pounds, in which the propor- tion of the filver to the gold was at fixteen to three (in- cluding with the filver a little copper which it contained) N 2 he [ 92 ] he boiled it with frefli portions of ftronger and ftronger aqua fortis, in a glal's body, to which was fitted a head and recipient for collecting the acid vapours that arofe : this method (liould feem at firfl: to be a notable improve- ment on the common procefs, in which the vapours, that iflue plentifully during the a<5lion of the acid, exhale and are loft. Nearly all the filver and copper being diflblved, and the folution poured off from the gold, the next por- tion of aqua fortis was boiled down till the matter at the bottom looked like a dry fait; which being judged to have been lb much deprived of tbe acid, that there was not enough left to render the little remaining filver dillbluble by water, he added more aqua fortis; which, after boil- ing for fome time, appeared yellow, and was poured off into a feparate glafs, its yellownefs being looked upon as a mark of its having become exceeding llrong by the lofs of its watery parts in the procefs. This yellow aqua fortis he ufed afterwards for diffolv- ing ibme filver, when, to his ailoniflnnent, a confiderable quantity of gold was found at the bottom of the glafs, though the filver had before been very carefully purified from gold. This experiment was many times repeated, in the prefence of feveral affayers, and at a meeting of the Swedifli academy, and always with the fame event ; pure filver, which gave no mark of gold with common aqua fortis, precipating from the above yellow aqua fortis a fpongy lump of gold. In keeping, a part of the gold fepa- rated fpontaneoufly, in form of a brown powder: after it- had been long kept, and depofited much of its gold, it was found on an effay to contain more gold than filver, in the proportion of 19 to 12: in this ftate, a quantity of it fufficient to diffolve four parts of filver yielded during the diffolution one part of gold ; fo that the nitrous acid is capable of diffolving above one fourth part as much gold as C 93 ] as it is of filver. The nitrous fpirit made ufe of in this operation had been prepared from pure nitre, and the experiment itfelf affords a convincing proof, that it was by the pure nitrous acid that the gold was dilTolved ; for if the dillblution of this metal liad been produced, as might be fufped:ed, by means of an admixture of marine acid, the menftruum could not, in the above method of application, have diffolved the filver. The foregoing procefs differs from tliat commonly fol- lowed for the parting of gold and filver, in the veflel being clofe fo as to exclude the external air, and in the heat being continued at laft till the matter became dry, fo that as the watery parts of aqua fortis rife firfl in diflil- lation, the acid muft in this cafe have been greatly con- centrated. Though the applying a head upon the veffel may feem to be a very immaterial circumftance in regard to the diilblution of the metal, it is perhaps one of the mofl eflential, for both diffolution and precipitation are in many caies remarkably influenced by the admiffion or exclufion of air: after the gold has been diflblved, if the velfel be well fliaken, lb that air m.iy be copioufly in- troduced and mingled with the liquor, the gold, as Mr. Scheffer obferves, falls o^uickly to the bottom. The importance of this experiment, in the way of cau- tion to thofe concerned in the parting of gold and filver by aqua fortis, is apparent. It is probable, that gold has been often diffolved in aqua fortis, without being known to be fo j and that this was the true caufe of the de- ception of Becher and other chemifls, who report that they had feen filver tranfmuted into gold by diffolution in fome particular kinds of aqua fortis. Had Dr. Brandt's folution paffed into other hands than his own, it might poffibly have been looked upon as another inflance of thefe pretended graduating or tranfmuting menflruums. 5 II. Gold [ 94 ] II. Gold unth the marine acid. The pure acid of fea fait has no adion on gold, fo long as the gold retains its metallic form ; whether the metal be boilded with it in open or in clofe vefTels, orex- pofed in the fire to its fumes ; in which laft circumftance, this acid dillblves or corrodes all the other known metallic bodies, except platina. Hence, though there are feveral metallic bodies, as filver, which the marine acid in its liquid ftate does not diflblve or extrad from gold, yet gold may be purified from thofe metals by the fumes of this as well as of the nitrous acid. On this foundation, the brittlenefs, which a fmall admixture of lead or tin, produces in gold, is remedied, by repeatedly injeding upon it in fufion a little corrofive mercury-fublimate; the marine acid of the fublimate uniting with the lead or tin, and either volatilizing, or changing them into a fcoria, which Is thrown off to the fides of the veffel. Small pro- portions of moll: of the other metals are in like manner leparated from gold by fublimate ; the acid having lefs affinity to the mercury of the fublimate than it has to the others, and accordingly parting from the former to join itfelf to the latter. When gold is changed to the appearance of a calx, by precipitation from aqua regia with volatile or fixt alkalies, of which hereafter ; or by calcination in mixture with tin or bifmuth, as mentioned towards the end of the pre- ceding fedion, the pure marine acid, by the affifi:ance of a moderate heat, perfedly diflblves it. I have found that even a weak fpirit of fait will take up gold fo pre- pared, though in no great quantity; and that the gold does not precipitate from this as from the nitrous acid, but continues durably fufpended. III. Gold [ 95 ] III. Gold with the Vitriolic acid. The vitriolic acid, in whatever manner applied, has not been obferved to have any adlion in gold, or to pro- mote the adlion of other acids. Hence, as oil of vitriol diflblves filver by a boiling heat, filver and gold may be parted from one another by this acid, as cffedually, though not focommodioufly, as by the nitrous. If the compound be reduced into grains or thin plates, and boiled in about twice its w^eight of oil of vitriol to drynefs, the lilver will be fo far corroded, as to be eafily waflied off by a little more of the acid; or if the mafs, after the corrofion, he melted in a crucible, the gold will feparate and fubfide, the filver forming a fcoria above it. Gold may thus alfo be purified from feveral other metallic bodies : Mr. Scheffer fiiys, that this is the moft diredt way of fcpara- ting tin from gold. IV. Gold with compem:d menjlrua. Gold is faid to be difiblved by the marine acid mixed with a fmall proportion of fpirit of urine ; by a mixture of the vitriolic acid with the fame urinous fpirit; by a mixture of the vitriolic acid with a little fixt alkaline fait j by the vapour, which arifes during the effervefcence of the vitriolic acid with fixt alkaline fait, colledted by di- ftillation ; in a fpirit prepared by faturating the vitriolic acid with volatile alkaline fait, exficcating the mixture, diifolving it in twice or thrice its quantity of aquafortis, and diflilling the folution. In my experiments, not one of thefe liquors appeared to have any adlion on gold. The moft effedual menftruum of gold is a mixture of the nitrous and marine acids, called aq«a regia ; which, in a moderate heat, readily and totally diflblves it into a tranfparent [ 96 1 tranfparent yellow liquor. As this compound does not at all diflblve filver, the gold may be extraded by it from a mixture of gold and filver, in the fame manner as the filver is extraded by aqua fortis j and as the extraftion of the filver by aqua fortis requires the quantity of filver in the mixt to exceed that of the gold, fo the extradtioji of the gold by aqua regia requires the gold to exceed the filver : the two metals may be fo proportioned, that nei- ther aqua fortis nor aqua regia rtiall be able to diflblve either, till an addition is made to the quantity of one or the other metal. When the quantity of gold in the mixt amounts to fo much as a third part of the filver, aqua fortis leaves always a fmali portion of the filver undilTolved along with the goldj and in like manner, when the quantity of gold amounts to fo much as a third of the filver, aqua regia leaves a little of the gold undilTolved along with the fil- ver : when either metal is in fmall proportion, the other appears to be completely attrad:ed by its proper menflru- um. If the gold, remaining after, the parting with aqua fortis, be diflblved in aqua regia, the filver it had retained will feparate and be left undiflblved : and if the filver, remaining after the parting with aqua regia, be diflblved in aqua fortis, the gold it had retained will in like man- ner feparate. This experiment affords a method of deter- mining the prccife quantity of either metal retained by the other, and a proof of the erroneoufnefs of the opinion of fome writers, that fo much filver, as gold retains in parting, is actually tranfmuted into gold. Aqua regia may be prepared, by diflblving powdered fea fait or fal ammoniac in four times their weight of aqua fortis, or by diflblving nitre in four times its weight of fpirit of fait, or by mixing the pure fpirits of nitre and fea ..fait together. The firft is the method mofl commonly followed. C 97 } followed. Kunckel obferves, that by putting the gold into the aqua fortis firfi:, and then adding the fait by little and little at a time, lefs of the menftruum will fuffice than if the fait was previoufly diflblved in the acid j the conflid:, excited by each addition of the fait, promoting the diflblution of the gold : this method appeared upon comparifon to have a fenfible advantage above the other, whether the fal ammoniac direded by Kunckel, or com- mon fait was ufed: the common fUt is to be preferred j for fal ammoniac, efpecially when a ftrong heat is called in aid to haflen thefolution, is apt tooccafion fome fmall part of the gold to be diflipated during the effervefcence. A folution in water of common fait, nitre and alum, boiled with leaf gold to drynefs ; or the falts in fubftance, mixed with the gold leaf, and urged with a flight red heat for fome hours in a clofc vefTel ; corrode a confiderable quantity of the gold into a faline form fo as to be diflblved upon adding water. The mixture of thefe falts, from its ailing infenfibly and without effervefcence, has been com- monly czWed meKjiruum^ne Jlrepitu: it can be confldered no otherwife than as an impure aqua regia, ading only by virtue of the acids of the nitre and marine fait, which are extricated from their bafes by the acid of the alum. V. General properties of folutions of gold. Solution of gold, whether made in fpirit of fait, or in any of the foregoing aqU£ regi^, is of a bright yellow colour, refembling that of gold itfelf. It fl:ains the flcin of a deep purple colour, which cannot be waflied out j and gives a like durable ftain, though with fome varia- tions in the fpecies of the colour, to fundry animal and vegetable fubflances, as drefl"ed leather, ivory and bones, feathers, woollen cloth, filk, linen, cotton, wood : O to [ 98 ] to marble it imparts a violet or purplllh colour, which penetrates to a confiderable depth, but on the harder ftones, as agates, it makes little impreflion, communica- ting only a fuperficial brown tinge. The folution for thele purpofes, fhould be prepared in Kunckel's method, that the acid may be fully faturated with the metal, and have as little admixture as may be of the faline matter : it fhould be diluted with three or four times its quantity of water, and if the colour is required deep, the piece, when dry, is to be repeatedly moiftened with it. Animal fubftances fhould be previoully well cleanfed from their undluofity, and foaked for fome time in water : the others require no preparation of this kind. The colour does not take place till a confiderable time, fometimes feveral days, after the liquor has been applied, and on fome fubjedls it is more (low than on others : to haften its appearance, the fubje(fl fhould be expofed to the fun and free air, and occafionally removed into amoiflplace, or moiflened with water. When folution of gold in aqua regia is foaked up in linen cloths, and the cloths dried and burnt, the particles of gold remain blended in the brown coaly powder, which, being moiflened with a little water, and rubbed on filver well cleaned from any undtuous matter, gilds it, without the application of heat, or the intervention of any other body: this is a ready but not a frugal way of applying gold on filver. If the menflruum has been prepared with an addition of fea fait, nitre, or fal ammoniac, and the folution is fet in a warm place, in a vefTel flightly covered, fo as to keep out duft, without preventing the evaporation of the wa- tery part of the liquor ; the gold, combined with the fa- line matter, fhoots into yellow cryftals, commonly fmall and irregular. Solutions in the pure marine acid, and in [ 99 ] in mixtures of the pure acids of nitre and fea fait, are very difficultly made to cryftallize : in order to the cryftal- lization of thefe, the liquor {hould be evaporated till only about one half of it remains, and then fet it by in the cold with the addition of a few drops of pure fpirit of wine. The cryftals obtained from high coloured Saturated folu- tions are generally of a red colour, and fometimes, as is faid, of a deep ruby red/ On diftilling with a gradual fire a folution of gold made in ftrong aqua regia, an acid fpirit comes over, which, from its rifiiig in red fumes, and from its dillblv- ing filver, appears to be the nitrous acid. On continuing the diftillation, whitifli fumes fucceed, a mark that part of the marine acid begins to rife -, though, after the operation has been protraded till the refiduum becomes dry, the gold flill retains fo much of the acid as to be dilToluble in water : it appears to be chiefly, if not folely, the marine acid which thus remains combined with the gold ; on which foundation the nitrous acid, employed for the diflblution of the gold, may be nearly all recover- ed, and its place fupplied by an equal quantity of common water; the marine fpirit, though inefFedual for procuring the diflblution of the metal in its common form, being fufficient for keeping it diflblved. When the matter has jufl: become dry, it appears of a deep red colour: on further increaflng the fire, the acid is totally diflipated, and the gold remains in powder, extremely fubtile, and of its proper hue. This is a convenient method of obtain- ing a fine powder of gold : if the aqua regia has been made with an addition of nitre or fea fait in fubftance, the faline matter, left with the gold, may be feparated by water. The mofl: eligible aqua regia, for the above pur- pofe, is a mixture of the pure acids, or of the nitrous acid and fal ammoniac ; for thefe will be wholly diffipated by O 2 fire. [ 100 ] fire, and the gold alone left. If the powder is moiftened with afolution of borax, it may be applied with a pencil on glafs or porcelane, and by a fuitable heat durably fixed upon them. On infpiffating nearly to drynefs a folution of gold made in an aqua regia prepared with common fal ammoniac, abflrading from the refiduum feveral frefh quantities of the fame kind of aqua regia, and at length increafing the fire fomewhat haflily towards the end of the dilHllation j the acid carries over with it a portion of the gold, fuffi- cient to communicate a yellow or a red tinge; and a more confiderable quantity of the gold, united with the more concentrated acid, fublimes, of a deep red colour, into the neck of the retort, concreting partly into long flender cryflals, and partly into a firm fubftance clofely applied on the glafs : the cryflals lie fo loofe, that they are apt to fall down again on moving the veflel; though, if this fhould happen, after the matter has become cold, they may be eafily feparated again, the refiduum growing firm as it cools : both the cryflals and the compadl fublimate difTolve eafily in water, deliquiate in the air, and melt with a fmall heat. By adding to the refiduum more aqua regia, and repeating the diflillation feveral times, the whole of the gold may thus be made to rife. Common aqua regia, prepared with rough fal ammoniac, appears to volatilize the gold as effedlually, as any of the more operofe compofitions recommended for this purpofe by the chemical writers. The rough fal ammoniac mufl neceffarily be ufed, not fuch as had been purified, as it is called, by fublimation ; for Dr. Brandt obferves, that if the fal ammoniac be firfl fublimed with a fufficiently flrong heat, and then diffolved in fpirit of nitre, the aqua regia thus prepared will not make gold volatile. He finds, that when the gold has been difiblved, and the menflruum diftiUed [ 101 ] diftilled off, there remains in the retort a faline mafs, containing the gold; that on every frclh folution and diftillation with the fame kind of aqua regia, the matter increafes more and more in its weight, and looks like a foul dark brown fait very hard of fufion ; that the liquor which- diftils is clear as water, and that nothing of the gold fublimes. He obferves alfo, that an aqua regia made with nitre in fubftance and the acid fpirit of fea fait, and with fea fait in fubftance and the acid of nitre, have lefs effedl in volatilizing gold than that with rough fal ammo- niac above mentioned. Thougli many have expected, from this volatilization of gold, a refolution of it into diffimilar parts, it is not found to have fuffered any real change. If the diftilled liquor, or the cryftals, or the fublimate, be expofed to a heat gradually increafed, the acid rifes, without carrying with it any part of the metal, the gold being left entire behind. The menftruum is lefs difpofed to elevate the gold a fecond time, than it was at firft. VI. Separation of gold from acids by inflammable liquors. The very fubtile inflammable fluid, obtained from a mixture of vitriolic acid with vinous fpirits, commonly called aether, or asthereal fpirit of wine, poured into a folution of gold made in aqua regia or in fpirit of fait, floats diftindt upon the furface, being far lighter than the acid liquor and not at all mifcible with it. The asther, of itfelf colourlefs, quickly becomes yellow, and the acid underneath lofes proportionably of its yellownefs ; the SBther imbibing the gold, keeping it permanently dif- folved, and, when loaded with the ponderous metal, con- tinuing ftill to float upon the acid. Gold is the only one, of the known metals, which the aether takes up from acid, and hence this fluid affords a ready method of S diftin- [ 102 ] dlftlnguiihlng gold contained in acid folutions : whether afmall quantity of Ibme other metals may not, in certain circumflances, accompany the gold in this reparation from the acid, or whether very large quantities of fome metals will not defend a minute portion of gold from the aftion of the ffither, may deferve further enquiry ; though fuch experiments, as we have hitherto made, incline us to think that they will not. The aether imbibes the gold, though it lies only on the furface of the acid folution : neverthelefs, to haften the effed, and to fecure againft any particles of the gold efcaping its adiion, it is expedient to fhake them lightly together, the veffel being clofely ftopt to prevent the evaporation of this very volatile fluid. If the folution in the aether poured oiF from the acid, be expofed to the open air, the aether exhales in a few mi- nutes, leaving the gold behind j if kept for fome months in a flender glafs ftopt fo as that the aether may exhale exceeding flowly, the gold does not refume its proper form, but fhoots, as is faid in the Swedifli tranfadlions into cryftals of a tranfparent yellow colour, a long prif- matic figure, and an auftere tafte. EfTential oils, fhaken with folution of gold, imbibe the gold in like manner, and carry it up to the furface, but keep it diffolved only for a little time : the metal gradually feparates, and is thrown off to the fides of the glafs in bright yellow films, which 6n. ihaking the veffel fall to the bottom. The oil, though of itfelf colourlefs, continues coloured after the gold has parted from it, effential oils receiving from the pure acid, firft a yellow, and afterwards a reddifh hue. Hence where thefe oils are employed as a teft of gold in folutions, it is not the colour which the oil acquires, but the fepa- ration of the golden films, that is to be regarded. The oils [ 103 ] oils appear to be more fluggilh than the aether in taking up the gold, and hence require to be well fliaken with the folution. Redified fpirit of wine mingles uniformly with the acid folution, and does not, for a time, occafion any other apparent change than rendering its colour more dilute. When the folution of gold has been infpiffated to drynefs, the metal, with the acid that remains com- bined with it, diflblves in fpirit of wine : if the men- flruum was either the pure marine acid, or a mixture of the pure nitrous and marine acids, or a mixture of the nitrous acid and fil ammoniac, the infpiffated matter dif- folves totally in the vinous fpirit: if the aqua regia was made by diflblving fea fait in aqua fortis, or by dilTolv- ing nitre in fpirit of fait, the neutral faline com- pounds contained in thefe menflrua not being diffo- luble in vinous fpirits, remain perfedlly white after the extradlion of the gold. From all thefe mixtures, as from effential oils, the gold feparates by degrees, though lefs fpeedily. On flanding for feme days, efpecially if the glafs is but lightly covered, the metal is feen floating in fine bright yellow pellicles upon the furface. The addition of a little effential oil to the fpirit haftens the feparation of the gold. Here it may be obferved, that many of thofe, who have buficd themfelves in the purfuit of medicinal pre- parations from gold, have been greatly deceived in the refult of their operations, from not being acquainted with the above properties of the metal. Finding that ef- fential oils imbibe gold from aqua regia, and receive with the gold a high colour, and that redified fpirit of wine, by digeftion with the oil, diffolves it, and becomes im- pregnated with its colour J they imagined they had thus obtained an aurum potabikt or true tindture of the gold, which [ 104 ] which they fuppofed to be endowed with extraordinary medicinal powers; not aware, that the gold conflantly Separated in the procefs, and that the colour of the pre- paration was no other than that which concentrated acids produce with efl'ential oils however pale or colourlefs. Liquors containing a grofTer inflammable matter, as wine, vinegar, folution of tartar, are likewife found to extricate gold from aqua regia in its metallic form j with this difference from the preceding, that the gold, inflead of floating on the furface, falls here generally to the bottom. VII. Precipitation of gold by alkaline fait s. On adding to folution of gold a folution of any fixt alkaline fait or a volatile alkaline fpirit, in fufiicient quan- tity to fatiate the acid ; the mixture becomes turbid, and on flanding for feme hours, the gold falls to the bottom, in form of a brownifh yellow muddy fubflance, retaining fome of the faline matter, great part of which may be feparated by repeated wafhing with hot water. That the gold may precipitate the more freely, the folution fhould be diluted with three or four times its quantity of water, or more. The alkaline liquor fhould be added by degrees, in little quantities at a time, till the mixture, after the gold has fettled, appears colourlefs, and a frefh addition of the alkali occafions no further precipitation or turbidnefs. When gold has been thus totally precipitated by volatile alkaline fpirits, as fpirit of fal ammoniac, the addition of more of the fpirit renders the liquor again yellow, occafioning apart of the gold to be rediflTolved : by adding a large quantity of the alkaline fpirit, almoft all the pre- cipitate is taken up j and even when the precipitated gold has been wafhed from as much of the adhering faline matter [ 105 ] matter as water will eafily extraft, a confiderable part of it will flill diflblve in pure volatile fpirits, but not fo much as before the ablution : I have not obferved the whole of the gold to be taken up in either cafe, though fome report that they have found it to be fo in bothfe Pure fixt alkalies, added in large quantity after the preci- pitation, do not appear to redilTolve any of the gold. If the aqua regia has been prepared with fal ammoniac, or if the precipitation is performed with a volatile alkali, the unwadied precipitate explodes, on being heated, with a bright flafh and a fmart noife ; whence its name aurum fulminans. If the aqua regia has been made without fal ammoniac, and the precipitation is performed with a fixt alkali, the precipitated gold makes no explofion : gradu- ally heated, it changes its dull yellowifli to a bright pur- ple or purple-violet colour, and on further increafing the heat refumes its metallic afpedt. A volatile alkaline fait, either in the diflblvent or in the precipitant, ieems to be cfTentially neceflary to the fulmination. Aurum fulminans weighs about one fourth part more than the gold employed, three parts of gold yielding four of the fulminating powder : this I relate on the authority of Lemery, Kunckel, and other practical writers, for though I have often made the preparation myfelf, I have never examined the increafe ojf its weight. Part of the increafe proceeds from the volatile alkali j for on adding to the aurum fulminans a little vitriolic acid, the volatile fait rlfes in fublimation, fatlated with the acid : the remaining powder is found to be divefled of its fulmina- ting power. From the coalition of the volatile alkali with the nitrous acid in the menftruum refults an amuio- niacalnitre, afalt which of itfelf detonates on beint, heated: ty what power of mechanifm its detonating quality is fo remarkably increafed in the aurum fulminans, is unknown. P The f io6 ] The explofion of* aurum fulm'inans Is more vehement) than that of any other known kind of matter : it goes off in a lefs degree of heat' than any of the other explofwe compofitions ; and even grinding it fbmewhat fmartly iiv amortar is fufficient for making it explode. Some inftan- ces are mentioned in th,e..Breflau collections, and the ephemerides naturce curioforum, of a very fmall quantity burfting in pieces the marble mortar in which it was rubbed ; and an accident of the fame kind happened fome years ago to a Ikilful chemift here. The operator cannot be too much on his guard ift the management of fo dan- gerous a preparation. It has been reckoned, that a few grains of aurum ful- minans adl with as much force as feveral ounces of gun- powder : but the adions of the two are of fo different kinds, that I cariniQt apprehend in what manner their ftrength can be compared. The report of aurum fulminans is of extreme acutenefs, offending the ear far more than that of a much larger quantity of gunpowder, but docs no,t extend 'ta.fo great a diftance; feeming to differ from it as the found of a (hort or tenfe mufical firing from that of a long one or of one which is lefs flretched. In fome experiments made before the royal fociety and mentioned in the firft volume of Dr. Birch's hiflory, aurum fulminans clofed up in a flrong hollow iron ball and heated in the jfire, did not appear to explode at all ; while gunpowder (treated in the fame manner burfl the ball. On the other hand a little aurum fulminans, exploded on a metalline plate in the open air, makes an impreffion or perforatioa In the plate; an effed which gunpowder could fcarcely produce in any quantity. :' This remarkable effed: of aurum fulminans on the body which ferves for its fupport, has induced fome to believe that its adion is exerted chiefly or folely downwards. It [ lo; ] It appears however to a<5t in all diredlons : for a weight, laid upon it, either receives a like impreflion, oris thrown ofFi and in the collediions abovemcntioncd an account is given of a large quantity (fome ounces) which exploding from too great heat ufed in the drying of it, broke open the doors and fhattered the windows in pieces. Mr. Hellot found, that when a few grains of the powder were placed between two leaves of paper, and cemented to one of them by gum water, only the leaf which touched the powder was torn by the explofion, and the other fwelled out; and that when both were brought into clofe contadl with it, by preiTing them together, it tore them both i from whence he concludes, that the atflion of the aurum is greateft on the bodies which it immediately touches. Both this property, and the acutenefs of the report, may poflibly depend upon one caufe, the celerity of the expanfion : experiments have fhewn, that the refiftance of the air to bodies in motion increafes with the velocity of the body in a very high ratio ; and per- haps the velocity with which aurum fulminans explodes may be fo great, that it is refifted by the air as by a folid mafs. The explofion of this preparation does not appear to make any change in the gold. When the powder is fpread exceeding thin between leaves of paper, and flowly heated, the detonation, as Mr. Hellot obferves, is flight and fucceffive, the powder becomes purple, and appears of the fame quality with the precipitate above mentioned which has no fulminating power. When a quantity is made to explode at once, in a large vellel, or under a proper cover, for confining the particles violently di- fperfed, the gold is found in fine dufl, partly purplifli and .partly of its proper yelloi^ colour: jt is faid that vk^hen . lonoi. ■ ^ 2. the [ ro8 T the explouon is performed between filver or copper plates,- the revived gold adheres to and gilds fome part of their- furface. If aurum fulminans be wafhed with frefli portions of hot water, that. as much as poffible of the faline matter may be extrafted, its fulminating quality will be greatly diminifhed. If ground with oil of vitriol, which expels the nitrous acid, and unites with the volatile alkali, or boiled in a folution of fixt alkaline fait, which expels the volatile alkali, and unites with the nitrous acid, it no longer makes the leaft explolion, and the gold may be re- covered by fimple fulion. When mixed with fulphur, and expofed to a gentle fire, the fulphur gradually burns off, and leaves the gold in like manner' recoverable without danger of fulmination. In all thefe cafes, if treated with a flow fire, it geherally aflumes a purple colour before it returns to its mttallic form. ■ VIII. Precipitation of gold by metallic bodies, Ali, thfe metallic bodies that diflblve in aqua regi*, platifia excepted, precipitate gold from it; the acid part- ing from the gold, and diflblving a portion of the others in its room. Some of them precipitate it alfo when they are previoufly dilTolved in other acids, and even in aqua fegia itfelf. Iron, in certain circumftances, becomes covered with the gold which it extricates from the acid, particularly •where vinous fpirits have been mixed with the folution. A liquor prepared by boiling gold leaf in water with nitre, fea fait, and alum, till the matter becomes dry, and then digefting the mixt in rectified fpirit of wine, is faid to anfwer the beft for the gilding of iron in this way j tho* it' does not appear to have any different efFeds from thofe of other mixtures of fpirit of wine with folution of gold. A folu* C 109 ] A fblutlon of gold in common aqua regia being largely diluted with fpirit of wine, a polilhed iron, dipt in the mixture, became immediately coated with a fine golden pellicle : the gold fulution without the fpirit of wine corroded the iron, and raifed a fcurf upon the furface. Thefe mixtures Ihould be prepared only as they are wanted, for on (landing for a day or two the gold begins to feparate. Iron diflblved in the vitriolic acid, or common green vitriol difTolved in water, precipitates gold in form of a dufky brown-red powder. As the vitriolic folutions of^ iron do not precipitate from aqua regia arry known metal- lic body befides gold, this experiment affords a commo- dious method of purifying gold from the fmalleft admix- ture of other metals : the particular way of inanaging the procefs will be given under the head of refining gold, • in the ninth fedtion. On adding copper to a folutlon of gold in aqua regia diluted with water, the copper became inftantly of a blackifli red colour; and on {landing, the gold fell in ' fubtile powder^ of its proper metallic afpedt, and of a high reddiih colour, which probably proceeds from fome cu-- preous atoms intermixed : it is remarkable in this experi- ment, that the liquor, after the precipitation of the gold, . appears colourlefs as water, a proof that the- quantity of ■ copper, taken up in the place of the gold, muft be ex- tremely minute. Solutions of copper in the vitriolic acid, orof blue vitriol in water, produced no precipitation^ or turbidnefs in folution of gold. Copper or verdegris dif- ■ folved in vinegar occafioned the gold to feparate in bright films, which covered the fides of the glafs, forming an al- mofl continuous golden pellicle: this feparation however feems to depend, not fo much upon the copper, as on the inflammable matter of the vinegar. I A plate t no ] A plate of pure tin, put into a folution of gold largely diluted with water, changes the yellowifti colour of the liquor to a beautiful purple or red: by degrees, a powder of the fame colour flowly fubfides, and leaver the men» flruum colourlefs. Solutions of tin, made in aqua regia, have the fame effeft with tin itfelf, in regard both to the precipitation and the colour j and hence charaders, drawn on paper with a diluted folution of gold, not vi- fible when dry, become immediately red or purple on paffing over them a diluted folution of tin. With tlie undiluted folutions, no rednefs is produced : after the red powder has fallen from the diluted liquor, if the whole be fet in a moderate warmth till the water has ex- haled, the gold is taken up again, the liquor becomes yellow as at firft, and only a white powder remains, which appears to be a calx of tin. The red liquor, fet to evaporate before the gold was fallen, yields only a yellow raafs.i from which redlified fpirit of wine extracts the gold combined with the acid, leaving, as in the other cafe, a white calx of tin. Mercury, diflblved in the vitriolic, nitrous, or marine acids, is a precipitant for gold, as well as in its metallic form ; and in all cafes, a part of the mercury is apt to fall down along with the gold. When mercury in fub- ftance is ufed, and the iblution of gold largely diluted, the undifTolved mercury gradually imbibes the gold. On dropping a folution of filver into one of gold, both metals precipitate : the filver, parting from the nitrous, unites and falls with the marine acid, and the gold falls for want of it : the matter which feparates firfl is white, ■then the liquor grows opake and a dark coloured powder fubfides, which leaves the menftruum clear and capable -of diffolving filver. The fame double precipitation hap- pens. [ I»I ] pens, and on the fame principle, on mixing folutlon of gold with folution of lead in aqua fortis. IX. Gold ivith fulphureous bodies. Pure fulphur, whofe fumes corrode, and which in fufion diflblves and fcorifies mod metallic bodies, has no adion on gold. Hence the ufe of gold for fome mechanic purpofes, where other metals are in time deftroyed by fulphureous fumes j as in the touch holes of guns. And hence by fufion with this concrete, gold may be feparated from moil of the other metals. From filver and copper it may be extradled, on this principle, where the propor- tion of gold is too fmall to bear the expences of the other common methods of feparation : fome particular manage- ments and additions, however, are requifite, to render the procefs fuccefsfui; fee fedtion the ninth. Though gold refifts pure fulphur, it unites perfcdlly. with a mixture of fulphur and fixt alkaline fait, com- monly called hepar fidphuris. As foon as the hepar melts, it begins to dilfolve the gokly with a lucid ebullition : two or three parts of fulphur, and three of the alkaline fait, are fufficient for one of gold. Great part of the compound diflblves in water, fo as to pafs through a filter without any feparation of the metal : Stalil obferves, that this folution is lefs oftenfive in fmell than that of the hepar itfelf, but of a more -bitter naufeous tafte. - The addition of any acid to this folution, abforbing the alkaline fait, precipitates the gold united with the fulphur; which laft may be difllpated by fire, or more readily feparated by adding a little copper for abforbing the fulphur. A like feparation may be obtained by adding copper or iron to the mixture of gold and hepar in fufion ; thefe metals precipitating the gold, and uniting with the hepar in its place. Mr. Hcllot recommends detonation with [ 112 ] tvlth nitre as the eafieft method of recovering the gold from the fulphureous mixture : the heft way of mana- ging this procefs appears to be, by making the matter red hot in a deep crucible, and dropping in the nitre, which Ihould be previoufly well dried and heated, by a very little at a time, as the addition of any confiderable quantity at once would occafion the deflagration to be fo ftrong as to force off" fome particles of the gold : this is the only in- convenience in the procefs, and it is not to be wholly avoided without great precaution ; for in many experi- ments of melting gold with nitre, when inflammable bodies had been mixed with the gold, I have almoft always obferved numerous globules of the metal thrown up about the fides of the crucible: when afrefti addi- tion of the nitre produces no further deflagration, the fire is to be increafed fo as to bring the whole into fuflon^ and the crucible being then fufFered to cool, the gold is found at the bottom of the ialine mafs, pure and of a high colour. A neutral ialt, compofed of fixt alkaline failt faturated with the vitriolic acid, being brought into fufion in 3 clofe crucible, with the addition of a little foot or pow- dered charcoal j the vitriolic acid and inflammable prin- ciple unite together, and form fulphur^ the fame with common brimftone, which remaining combined with the alkali, the compound proves a true hepar fulphuris : and Accordingly gold, melted with thefe ingredients, is diflbl- ved by them in the fame manner as by a hepar already jnade. Dr. Brandt gives an account of an experiment, from which he concludes, that gold, by being diflblved in the above mixture, and afterwards recovered from it, fuffer* a confiderable change. About a grain of gold and two hundred grains of lilver were mdted with the mixture. t 1^3 ] and precipitated by adding twice as much copper : the fcoria, containing the copper, was melted witii calx of lead, and the lead revived from the compound, that if any of the gold and filver fliould have remained in the fcoria, they might be imbibed by the lead : the precipi- tated mafs was cupelled with the revived lead, and thea parted by aqua fortis: the gold powder, which the aqua fortis left undiffolved, differed fomewhat in appearance from that which commonly remains in parting, and being melted with a pure white fixt alkaline fait, the gold turned out pale and almofl like filver. I have not yet repeated this experiment, and do not apprehend that it will bear any great flrefs to be laid upon it. It is more probable that the gold retained a part of the extraneous matter, than that it fuffered itfelf any real change. The author obferves that the crucible, in which the gold pow- der was melted, had a green tinge round its edge, and that the alkaline fait was coloured yellow, but that the gold after the fufion was found to be of its full weight; fo that a part of the gold might have been dilTolved and retained by the fait, and an equal quantity of other matter remained blended with the reft of the gold. The phofphorus of urine has been faid by fome to re- duce gold into a red mucilage. By digeftion or diftilla- tion in clofe veflels, as a retort and receiver, the phof- phorus appears to have no adlion on gold : this I relate on the authority of Mr. MargrafF, whofe experiments, in the Mtfcellanea Beroli?ienfia for the year 1740, have faved me the trouble of this examination : gold filings were digefted with thrice their weight of phofphorous for four weeks, and the fire being then increafed, part of the phofphorous fublimed, and part remained above the gold. In appearance like fine glafs : this laft grew moift on the admiflion of air, and difiblved in water, leaving the gold Q^ unchanged. [ iH J unchanged. Nor does gold appear to be afFeded by the fumes of phofphorus fet on fire. But the flowers or faline acid matter remaining after the inflammable principle of the phofphorus has been confumed, and the microcofmic fait or effential fait of urine, which contains this acid, being melted along with gold in a moderately flrong fire, manifeflly corrode the metal, and receive from it a purple tinge. SECT. VIL Of the alloy of gold ; and the methods of judging of the guan-* tity of alloy it contains, from the colour and iveight. I. Of the alloy of gold. GOLD, in its pure ftate, is reckoned too foft and flexible for the common purpofes of coins and utenfils ; and hence, to increafe its hardnefs, and render it better adapted to thefe ufes, it is allowed to be mixed with a certain quantity of inferiour metals ; which, in refpe the quantity of fait requifite is generally about a third of the aqua fortis. The clear part of the folution is to be poured off, and the remainder paffed through a double filter of paper : the undilTolved- ^ 2- matter; t 160 3 matter is to be waftied two or three times with water in the filter, and this liquor poured to the reft. For recovering the gold from the folution, Cramer direds two methods, diftilling off the menftruum, and precipitating the gold by mercury. But in either of thefe ways we cannot be certain of having the gold pure. For though it has been previoufly cupelled with lead, yet, if it contained any platina, it will retain the whole of the platina after the cupellation, and in fome circumftances as we have already feen, it will retain alfo a little copper: both the platina and copper will diflblve with it in aqua regia; mercury will precipitate the platina along with the gold; and the abftradtion of the menftruum will leave with it both the platina and copper. The purity of the gold is fecured by precipitation with common green vitriol. The vitriol is to be diflolved in cold water, the folution paffed through a filter, and added' in large quantity to the folution of gold : the quantity of vitriol, before its diflblution, fliould be ten or twelve times greater than that of the gold. As the precipitate falls ilowly, the mixture is to be fet by for twenty four hours or more : the liquor, then become clear, though of a deep colour, is to be poured off; the brownifh powder at the bottom, boiled in a little aqua fortis, then wafhed with water, and melted with the addition of a little nitre. Gold thus purified, appears to be perfe.dly fine; a point not obtainable by any other known means that can be pradtifed in the Way of bufinefs. Nor does the procefs feem to be fo expenfive as the imperfe(3: one by aqua fortis ; for there, three parts or more of filver being added to one of gold, at leafi: fix parts of aqua fortis are required for diffolving the filver ; whereas the gold, in the above procefs, may be diffolved by half that quantity of the menflruum : great part of the acid may like- wife [ i6i ] be recovered by diftillation from the liquor which remains after the gold has fallen, Kunckel is the firfl who has taken notice of this pre- cipitation by vitriol: but having ufed a vitriol which partook of copper as well as iron, he feems to have thought that the effeft depended on the copper, and recommends the bluefl: and moft venereal of the common forts of vitriol as the beft: accordingly moft of thofe, who have mentioned this procefs, dircdl blue vitriol or vitriol of copper. I have not found that blue vitriol pro- duces the leaft precipitation in folution of goldj fo that, by this mifapprehenfion in regard to the nature of the precipitant, Kunckel's difcovery was rendered ufelefs, till Brandt happily obferved, that green vitriol produces the effedl which had been afcrlbed to the blue. IV. Extract ien of a fmall portion of gold from a large quantity ofjilver. The moft advantageous method of feparating a fmall proportion of gold from a large one of iilver appears to be by means of fulphur, which unites with and fcorifies the iilver, without affedting the gold. But as fulphurated filver does not flow thin enough to fuffer the fmall particles of gold, difl"ufed through it, to reunite and fettle to the bottom, feme addition is necefTary for colledling and car- rying them down. In order to the commixture with the fulphur, fifty or lixty pounds of the mixt metal, or as much as a large crucible will receive, are melted at once, and reduced into grains by lading out the fluid matter, with a fmall crucible made red hot, and pouring it into cold water ftirred with a rapid circular motion. From an eighth to a fifth of the granulated metal, according as it is richer or poorer in gold, is referved j and the reft well mingled with an Y eighth [ i62 ] eighth of powdered fulphur^ which eafily adheres to the moid grains. The grains, enveloped with the fulphur;,, are put again into the crucible, and the fire kept gentle for fome time, that the filver, before it melts, may be thoroughly penetrated by the fulphur : if the fire was haftily urged, great part of the fulphur would be diflipated, without adling upon the metal. If to fulphurated filver in fufion, pure filver be added, the latter falls to the bottom, and forms there a diftindt fluid, not mifcible with the other any more than water is- with oil. The particles of gold, having no affinity to the fulphurated filver, join themlelvesto the pure filver, where- ever they come in contadl with it, and are thus tranf- ferred from the former into the latter, more or lefs per- fedly according as the pure filver was more or lefs tho- roughly diftufed through the mixt. It is for this ufe that apart of the granulated matter was referved. The fulphurated mafs being brought into perfedl fufion, and kept melted for near an hour, in a clofe covered crucible, one third of the referved grains is thrown in, and as foon as this is melted, the whole is well fiiirred^ that the frefli filver may be difiiributed through the mixt to colledl the gold from it ; the ftirring is performed with a wooden rod : an iron one would be corroded by the fulphur, fo as to deprive the mixt of its due quantity of the fulphur, and likewife render the fubfeqiient purification of the filver more troublefome. The fufion being con- tinued an hour longer, another third of the unfulphurated grains is added, and an hour after this, the i-emainder; after which the fufion is further continued for fome time, the matter being fiiirred at leafl: every half hour from the beginning to the end, and the crucible kept clofely covered in the intervals. 4 The X 163 ] The fulphurated filver appears in fufion of a dark brovvnifli colour. After it has been kept melted for a certain time, a part of the fulphur having efcaped from the top, the furface becomes white, and fome bright drops of filver, about the fize of peas, are perceived on it. When this happens, which is commonly in about three hours after the laft addition of the feveral grains, fooner or later according as the crucible has been more or lefs clofely covered, and the matter more or lefs ftirred, the fire mull be immediately difcontinued; for otherwife more and more of the filver, thus lofing its fulphur, would fubfide, and mingle with the part at the bottom in which the gold is colledled. The whole is poured out into an iron mortar greafedand duly heated; or if the quantity is too large to be fafely lifted at once, a part is firil laded out from the top with a fmall crucible, and the reft poured into the mortar. The gold, diffufed at firft through the whole mafs, is now found colledted into a part of it at the bot- tom, amounting only to about as much as was refervcd unfulphurated. This part may be feparated from the ful- phurated filver above it by a chifel and hammer j or more perfectly, the furface of the lower mafs being generally rugged and unequal, by placing the whole mafs, with its bottom upwards, in a crucible: the fulphurated part quickly melts, leaving unmelted that which contains the gold, which may thus be completely feparated from the other. The fulphurated filver is aflayed, by keeping a portion of it in fufion in an open crucible, till the fulphur is difli- pated, and then difTolving it in aqua fortis : if it fhould ftill be found to contain any gold, it is melted again, as much more unfulphurated filver added, as was employed in each of the former injedions, and the fufion continued about an hour and a half. Y 2 The [ i64 ] The gold, thus coUeded into a part of the filver, may be further concentrated into a fmaller part, by granulating the mafs, and repeating the whole procefs. The opera- tion may be again and again repeated, till fo much of the filver is feparated, that the remainder may be parted by aqua fortis without too much expence. The foregoing procefs, according to Schlutter, is prac- tifedat Rammelfberg in the lower Hartz. The prevail- ing metal in the ore of Rammelfberg is lead : the quan- tity of lead is at moft forty pounds on a quintal or hundred pounds of the ore : the lead, worked, off on a teft or concave hearth, yields about a hundred and ten grains of lilver, and the filver contains only a three hundred aad eighty fourth part of gold : yet this little quantity of gold, amounting fcarcely to a third of a grain in a hundred pounds of the ore, is thus colledled with profit. The author above mentioned confines this method of feparation tofuch filver as is poor in gold, and reckons parting with aqua fortis more advantageous where the gold amounts to above a fixty fourth of the filver : be advifes alfo not to attempt concentrating the gold too far, as a portion of it will always be taken up again by the filver. Mr. Scheffer however relates, that he has by this method brought the gold almoit to perfedl finenefs, and that he has likewife colleded all the gold which the filver contained; the filver of the laft operations, which had taken up a por- tion of the gold, being referved to be worked over again with a frefii quantity of gold-holding filver. The fulphu- rated filver is purified by continuing it in fufion for fome time, with a large furface expofed to the air; the fulphur gradually exhales, and leaves the filver entire : the particu- lar method of managing this operation will be given here- after in the hiftory of filver. Mr, C 165 ] Mr. Eller, in the memoirs of the Berlin academy for the year 1747* defcribes a procefs fonjewhat different from the foregoing ; which has been kept a fecret in a few hands; and from which, he fays. Saxony has for feveral years reaped confidcrable profits, by the fcparation of gold from gilt laces. The metal being granulated, a part of the grains is mixed with half their weight of litharge and an eighth of fandiver : this is called the precipitating mix- ture. The reft are mingled as above with powdered ful- phur, and expofed to a gradual fire till they are brought intofufion, which is known to be perfedl, when the fur- face, on lifting up the cover of the crucible, appears coloured, chiefly with red and yellow, and the colours come and go, as if fomething attrafted them. To every thirty- two ounces of the fulphurated metal, one ounce of the precipitating mixture is added, at three different times, at intervals of at leart five or fix minutes i after which, the crucible is covered again, and the fufion continued feven minutes. Part of the matter being now laded out, the reft is poured off, till a metallic mafs Ihews itfelf at the bottom : this is eafily difiinguifliable, -by its bright fiery afpeft, from the fulphurated mixt, which is of a leaden brown colour. The filver poured off, ftill containing a little gold, is treated in the fame manner a fecond, and a third time, ex- cept that in the third another precipitant mufl be ufed ; for that employed in the two firft, being partly compofed of filver not freed from its gold, would add to the fulphu- rated filver, now ahnoft entirely purified from gold, more gold. The precipitant is now a mixture of equal parts of pure copper and lead, melted together and reduced into grains. If aqua fortis fhould fiill difcover a little gold in the filver, which, never happens unlefs the filver contained a large [[ ;i66 ] a lage quantity at fifft, > the precipitation is repeated a fourth time. The feveral metallic mafles, thus precipitated, are to be granulated, fulphurated, and further concentrated by the fame precipitants as before j about an eighth part of lead being added, before the granulation, which is faid to ren- der the mixt more fufible, and promote the feparation of the gold. It is probable that bifmuth would anfwer better in this intention, as it forms both with the metals and with the fulphuramuch more fufible compound than lead does, and poffeffes alfo all the other properties of lead that appear to be here ^required. • - The matter which now fubfides is again granulated, mixed with a fixteenth of fulphur, kept in fufion about half an hour, the fcoria poured off, and the remaining mafs treated in the fame manner, without any precipitant a fecond or a third time. The gold being now fo far con- centrated as to exhibit a yellow colour, the mafs is melted with a fixteenth of copper, then granulated, the grains mixed with a fixteenth of fulphur, cemented for fomc time in a heat below ignition, after which, the fire being raifed, the matter is kept in fufion about fifteen minutes, and then poured out into a greafed and heated mould : the gold is found at the bottom, commonly of a brafs colour, and about the finenefs of eighteen carats: if too pale, the lafl operation is repeated with half the quantity of copper ; after which the gold is further refined by antimony as already defcribed. VII. ExiraBion of gold from copper. For Separating gold from large proportions of copper, as from the gilt clippings left by the button-maker, fome of our refiners have recourfe to cupellation or tefling with lead. But the long continuance of fire and great quantity of [ i67 ] of lead neceflary for fcorifying fo large a proportion of copper, and the difficult revival of the copper from the fcoria, render the procefs too expenfive for the produce of gold. Some have melted the gold-holding copper with about thrice its quantity of lead, and cafl: the mixture into cakes -, which being ranged in the higher part of a (loping canal, and moderately heated, it was expetSted that the lead, melting out and running down from the copper, would carry the gold with it. But though this procefs fuccceds for the reparation of iilver from copper, it is othcrwife in regard to gold : if the copper contains both gold and filver, only the filver melts out with the lead, the gold re- maining behind in the unmelted mafs of copper. Alonfo Barba gives a method which may in feveral cafes be pradlicable to advantage. The copper is calcined with fulphur, till it becomes pulverable, and the powder ground with quickfilver in the fame manner as earthy or liony bodies containing gold : the mercury imbibes the gold, without adling upon the calcined copper, which may. now be wafhed off with water. Many proceffes have been given for feparating gold from copper by precipitating powderss which are conipofed of very difcordant materials, as antimony, lead, fulphur, crocus of iron, mercury-fublimate, arfenic, vitriol, ver- degris, alum, nitre, fal ammoniac, wood aflies, quick- lime. Though thefe procelTes, a number of which is collefted by Swedenborg in the third volume of his regnmn fiibterraneum, are apparently fo injudicious, that an artift can have no inducement to make trial of them, yet they are not altogether without foundation: lead and fulphur, as Barba intimates, and as an experiment of Mr. Scheffer's has {hewn more fatisfaftorily, are the ufeful ingredients; and by means of thefe ingredients, gold may be extrafted from [ i68 ] from copper more advantageoully than by any other known method. ^i-; b The way of procedure is as follows. Some litharge, or any other calx of lead, is melted with about an equal quantity of fulphur, with which it unites into a fparkling mafs, of a femimetallic afpedt, and nearly of the fame quality with the common lead ores. The copper being brought into fufion, this mixture is thrown upon it, by a little at a time, till the quantity of lead becomes nearly equal to that of the copper : the copper abforbs the ful- phur from the lead, and the lead, being in a ftate of calx, remains uniformly blended with the fulphurated copper. A little powdered charcoal is then thrown in, and the whole well ftirred with an iron rod : .the lead is immedi- ately revived into its metallic form, and finking to the bottom carries the gold with it ; neither lead nor gold having any affinity to fulphurated copper. The effedl is the fame when the copper is firfl; fulphurated, and the litharge or calx of lead added to this mixture ; and pro- bably gold might be feparated in the fame manner from fulphurated iron. VIII. Separation of gold from gilt works. The folubility of gold and the indiffolubility of filver in aqua regia affords a principle on which gold may be feparated from the furface of filver, and on this founda- tion different proceffes have been contrived, of which the two following appear to be the beft. Some powdered fal ammoniac, moiflened with aqua fortis into the con- fiflence of a pafte, is fpread upon the gilt filver, and the piece heated, till the matter fmokes and becomes i>early dry: being then thrown into water, it is rubbed with a fcratch brufli, compofed of fine brafs wire bound together, [ i6g ] together, by which the gold cafily conies off. The other way is, by putting the gilt filver into common aqua regia, kept fo hot as nearly to boil, and turning the metal fre- quently till it becomes all over black: it is then to be wa(hed with a little water, and rubbed with the Icratch brufh, to get off what gold the aqua regia may have left. This laft method appears preferable to the other ; as the fame aqua regia may be made to ferve repeatedly till it becomes faturated with the gold, after which the gold may be recovered pure by precipitation with folution of vitriol, as direded in the fifth article of this fedion. For feparating gold from gilt copper, fome dire(3: a folution of borax to be applied on the gilt parts, but no where elfe, with a pencil, and a little powdered fulphur to be fprinkled on the places thus moiftened j the princi- pal ufe of the folution of borax feems to be to make the fulphur adhere ; the piece being then made red hot, and quenched in water, the gold is faid to be fo far loofened, as to be wiped off with a brufli. Others mix the fulphur with nitre and tartar, and form the mixture with vine- gar into a parte, which is fpread upon the gilt parts. Schlutter recommends mechanical means, as being ge- nerally the leafl: expenfive, for feparating gold from the furface both of filver and copper. If the gilt veffel is round, the gold is conveniently got off by turning it in a lathe and applying a proper tool, a fkin being placed underneath for receiving the fhavings : he fays it is eafy to colled into two ounces of fhavings all the gold of a gilt veffel weighing thrice as many pounds. Where the figure of the piece does not admit of this method, it is to be properly fixed, and fcrapers applied of different kinds according to its fize and figure, fome large and furnifli- ed with two handles, one at each end, others fmall and narrow for penetrating into depreffed parts. If the gold [ ^7^ J *r6ld cannot be got off by either of thefe ways, the file muft be had recourfeto, which takes ofFmore of the me- tal underneath than the turning tool or the fcraper, parti- cularly than the former. The gold fcrapings or filings may be purified from the filver or copper they contain by the methods defcribed in the preceding part of this fedion. The editors of the encyckpedie give a method of recover- ing the gold from wood that has been gilt on a water fize : this account is extradled from a memoir on the fame fubjedt prefented to the academy of faiences by M. de Montamy . The gilt wood is fteeped for a quarter of an hour, in a quantity of water, fufficient to cover it, made very hot : the fize being thus foftened, the wood is taken out, and fcrubbed, piece by piece, in a little warm water, with fhortfliff briftle bruQies of different fizes, fome fmall for penetrating into the carvings, and others large for the greater dilpatch in fiat pieces. The whole mixture of water, fize, gold, &c. is to be boiled to drynefs, the dry matter made red hot in a crucible to burn off the fize, and the remainder ground with mercury, either in a mortar,, or, where the quantity is large, in a mill, as defcribed hereafter, in the eleventh fedtion : fome clean fand isdireded to be added, which is faid to occafion the gold to be eafier laid hold of by the mercury. SECT. X. Of tinging glafs and enamel by preparations of gold. TH E tinging of glafs and enamels by preparations of gold appears to have been firfl attempted about the beginning of the lafl century, Libavius, whofe works compofe a valuable body of the chemical knowledge of his own time, conjectures, in one of his tradts entitled Alchymia, printed in 1606, that the colour of the ruby I proceeds [ 171 ] proceeds from gold, and that gold diflblved and brought to rednefs might be made to communicate a like colour to fadlitious gems or glafs. Neri, in his art of glafs dated i6i I, gives aprocefs on this principle, which he fays was found to fucceed : he diredls the gold to be diflblved in aqua regia, the menftruum to be evaporated or drawn off by diflillation, more aqua regia added, and the abftradlion repeated five or fix times : the remaining matter is to be calcined till it becomes purple, and then mixed with a proper quantity of the fined white or cryftal glafs. But though this procefs may be fuppofed to have fometimes proved fucccfsful, itdoubtlefs very often mifcarriedj info- much that the introdudlion of this defirable colour into glafs was very little known for many years after. Glauber, in the fecond part of his philofophical fur- naces publiflied in 1648, gives another method of pro- ducing a red colour by gold in a matter which is of the vitreous kind, though not perfefl: glafs. When powdered flint or fand is well ground with four times its weight of fixt alkaline fait, the mixture melts in a moderately ftrong fire, and when cold looks like glafs, but on account of its over-proportion of alkaline fait it runs into a liquid ftate on being expofed to the air : on adding this liquor to folution of gold in aqua regia, the acid, which held the gold diffolved, unites with the alkali which held the flint diflblved, and the gold and flint precipitate together, in form of a yellow powder, which by calcination be- comes purple : this powder being mixed with three or four times its weight of the alkaline folution of flint, the mixture dried, and kept melted in a fl:rong fire for an hour, a mafs is obtained, of a tranfparent ruby colour, and of a vitreous appearance, though flill foluble in •water or by the moifture of the air, on account of the jredund^nce of fait, Z z Boyle, [ 172 3 Boyle, in his treatife on the porofity of bodies, and in the appendix to his fceptical chymift publifhed in 1680, mentions an experiment, in which a like colour "was introduced into glafs without fufion. A mixture of gold and mercury having been kept in digeflion for fome months, the fire was at laft immoderately increafed, info- much that the glafs burffc with a violent explofion : the lower part of the glafs was found tinged throughout of a tranfparent red colour, which feemed, he fays, to emulate that of a not common x uby. About the fame time Caffius difcovcred the precipita- tion of gold by tin, and that glafs might be tinged of a ruby colour by melting it with this precipitate. I can give no further account of his experiments, having never had the good fortune to meet with this treatife. The procefs was foon after brought to perfedtion by Kunckel, who fays he prepared the ruby glafs in large quantity, and fold it for about forty Ihillings an ounce ; and that he made a chalice of it for the elector of Cologn, weighing no lefs than twenty four pounds, a full inch thick, and of an uniform fine colour through- out. He has no where communicated the procefs he followed, but fome ufeful obfervations relating to it are difperfed through his writings : he fays, that one part of the precipitate by tin is fufiicieht to give a ruby colour to twelve hundred and eighty parts of glafs, and a fenfible rednefs to upwards of nineteen hundred parts : that the fuccefs is by no means conftant, and that after long pradice, he ilill frequently failed : that oftentimes the glafs comes out of the fire colourlefs as cryftal, and receives its ruby colour on being afterwards expofed to a fmoky flame, infomuch that he imagines the difcovery of the ruby glafs did not arife from fimply melting the gold precipitate with glafs, but from the fubfequent foft- ening [ 173 ] cning and working of the glafs in the flame of a lamp, in the ufe of which Caflius was very converfant : that the addition of nitre and fal ammoniac calls fourth the colour, iind that the colour produced by fal ammoniac is more beautiful than that by nitre, but quickly difappears on a continuance of the fire. Orfchal, in a treatife entitled yj/^foi? vejlcy gives a pro- cefs, by which he fays he obtained a very fine ruby. He. diredts the purple precipitate, made by tin, to be ground with fix times its quantity of Venice glafs in very fine powder, and this compound to be exquifitely mingled with the fritt or vitreous compofition to be tinged : his fritt confifts of equal parts of borax, nitre, and fixt alkaline fait, and four times as much calcined flint as of each of the falts; but in what proportion the gold precipitate is to be mixed with the fritt, and in what manner the fufioa is to be performed, he does not mention. He reports thafhe had found the muddy matter, obtained in polilhing gold by a pumice flone, to impart likewife a ruby colour to glafs. Grummet, who had been operator to Kunckel in making the red glafs, publifhed a tra<5t in oppofition both to him and Orfchal, under the title oi fol non Jim vejle, in which he obferves, that the furnace ought to be fo con- ftrudled, that the operator may have full liberty of ex- amining the glafs in the fire, and of removing it as foon as it appears to have acquired the proper colour : he fays the enamellers obtain a ruby colour, by melting, with a large proportion of Venice glafs, the brownifli powder precipitated from fol ution of gold in aqua regia by fixt alkaline falts. But he imagines that the gold is no wife concerned in the produdlion of the colour. Venice glafs, and moft of the finer colourlefs kinds of glafs, have an ad- dition of manganefe, without which it would be very diffi- 5 cult [ 174 ] cult to render them perfed:ly void of colour: the man^ ganefe communicates at firft a purplifli hue, which on continuing the fire difappears, and at the fame time fup- preiTes or difcharges any other tinge that the glafs may be impregnated with: the addition of a little nitre revives the purplifli colour of the manganefe, and Grumm&t is of opinion that the colour with which glafs becomes tinged, by the admixture of preparations of gold, is no other than that of the manganefe extricated by the nitrous fait which the gold has retained in its precipitation. He affirms that the fame purplifli red colour will be obtained on melting Venice glafs with an eighth part of nitre, without any gold; that in a hundred repetitions of this experiment, it fcarcely fails once; and that neither nitre nor the gold- precipitate were found to give any thing of the admired colour to thofe kinds of glafs which have no manganefe in their compolition. The colours which manganefe imparts to glafs, it be- longs not to this place to examine : but that precipitates of gold will communicate, in certain circumftances, a purplifli red colour, I have feveral times experienced; having myfelf tinged of this colour fritts compofed of calcined flint, nitre and borax, without the addition of any manganefe or of glaflTes containing it. Though gold, diffolved in common aqua regia, exhibits its own yellow colour ; yet, when the menfl:ruum is feparated by fire to a certain point, or when the gold is precipitated by tin, or when it is precipitated by alkaline falts and afterwards moderately heated, or when the gold is barely divided by mechanical means into fubtile powder, and expofed for " fome time, in mixture with earthy bodies, to a flight heat, it aflTumes, in diiferent circumfl:ances, a violet colour, a purple, or a red verging to purple : in a fl:rong fire, thefe colours vaniih, and the gold melts into a mafs of its ori- ginal [ ^7S ] ginal appearance. All thefc colours I have Introduced into glals by preparations of gold ; and I have found them to be nearly as perifhablc in the fire when the coloured gold powder was thus diffufed through the glafs, as when cxpofed to the fire by itfelf : when the fire was raifed to any great degree, and the glafs made to flow thin, there was generally a button of revived gold colledled at the bottom. A folution of gold in aqua regia being infpifiated to drynefs in the bottom of a Florence flafk, and the heat further increafed till the gold refumed its proper colour, the lower part of the glafs was by this fimple procefs tinged purplifli : pieces of it being expofed to the flame of a lamp, they became in fome parts violet coloured, in fome of a bright purple, and in others purplifii red; and the parts which in one pofition looked violet orpurplifh, in another appeared red. A colour nearly of the fame kind is impreffed on glafs by gold leaf in feme ele«ftrical experiments ; a fadl which we are obliged to Mr. Franklin for the firfl: knowledge of. A narrow ftrip of gold leaf being placed between two flips of glafs, with both ends hanging out a little, and the glafs well tied round with filk thread, a flrong electrical explofion is made to pafs through the gold leaf. On ex- amining the glafs, the gold leaf, he obferves, will be found mifling in feveral places, and infliead of it a reddifh fl:ain on both the glaflTes, exaftly fimilaron both in theminuteft flroke, though fometimes fpread a little wider than the breadth of the leaf: the ftain appears to have penetrated into the fubfl:ance of the glafs, fo as to be proteded by it from the adlion of aqua regia. I have had this experiment feveral times repeated with plate glafs, and found it tinged, as above defcribed, in fome parts violet, in fome purplifh, and in fome reddiflij the colours could not be fcraped off, and ( 17^ ] and refifted aqua regia and fpirit of fait. If the elecflric explofion is made very ftrong, the glafs commonly flies in pieces, with fuch force, that it is neceffary for the operator to have his face fkreened from them. The preparation of gold which has been principally recommended for tinging glafs is CafTius's precipitate by folution of tin. To obtain this precipitate of the due Colour, a good deal of care is neceffary both in diffblving the tin, and in diluting the folutions. A mixture of two parts of aqua fortis and one of fpirit of fait is fuppofed to be the beft menflruum for the tin : into this mixture ■fome fine block tin, granulated, is to be let fall, grain by grain, waiting till one grain is diflblved before another is dropt in, that the diffoiution may go on flowly, with- out any heat or difcharge of fumes. The gold is dif- -folved in common aqua regia : and a few drops of this 'folution being mixed with feme ounces of pure water, as many drops of the folution of tin are added. If the mixture changes immediately to a clear bright purplifh red ■colour, the due degree of dilution has been hit ; if the colour appears dull, a greater quantity of water muft be added for the reft of the folutions. After the mixture has depofited its red matter, and become clear, a little more of the tin folution is to be dropt in, for difcovering, and precipitating, any gold that may ftill remain in it: the liquor being then poured off, the precipitate is wafhed and dried. Kunckel mentions another purple gold-powder, made nearly like that of Neri already mentioned, by infpiffating folution of gold to dry nefs, abftradling from it frefh aqua regia three or four times till the matter looks almoft like oil, then precipitating with ftrong alkaline ley, and wafh- ing the precipitate with water. By diffolving this powder in i ^77 ] in fpirit of fait, and precipitating again, it becomes, he fays, extremely fair, and in this ftate he direds it to be mixed with a due proportion of Venice glafs. Hellot defcribes a preparation which in mixture with Venice glafs was found to give a beautiful purple enamel. Equal parts of folution of gold, and of folution of zinc in aqua regia, are mixed together; and a volatile fpirit, prepared from fal ammoniac by quicklime, added to the mixture in fufficient quantity to precipitate the two metals. The precipitate is to be gradually heated, till it acquires a violet colour: it does not fulminate, making only a flight dull decripitation without any of its particles flying about. Though a purple, or a red colour approaching to that of the ruby, may by the foregoing means be baked upon glafs or enamels, and introduced into the mafs by fufion, the way of equally difFufmg fuch a colour through a quan- tity of fluid glafs is fliill a fccret. I was once, many years ago, fortunate enough to fuc- ceed, at a glafs-houfe, in a fmall pot of glafs, of which a falver was blown of a fine ruby red : the tinging mat- ter was the precipitate of gold by tin; the particulars of the procefs cannot now be recollected. I have fince tried the remainder of the fame preparation, with common flint glafs, with green glafs, with various fritts compofed of flint, borax, pure fixt alkaline fait, nitre, fal ammoniac. When flint was ufed, it was feveral times made red hot, and quenched in water, to render it more eafily pulvera- ble : both the flint and glafl^es were powdered in an iron mortar, and the powders well wafhed with diluted oil of vitriol, to extraft fuch particles of iron as they might have worn off" in the trituration , the gold precipitate was ground with the other ingredients, in agate or glafs mor- tars; its proportion was varied from an eight part to A a an [ 178 ] an eight tiundredtii of the vitreous materials; and the fire was continued, in a wind furnace, from fix to thirty hours. All the glaffes came out con fiderably coloured ; fome of a deep dullcy yellow ; fjme of a fine pale tranfpa- reiit yellow i fome of a brown colour, greatly refem- bling that which the glafs mentioned in the following^ page acquired under a muffle : fome appeared yellowifh or brownifh when looked down upon, and of a purple- violet or reddifli purple when held between the eye and the light: fome had fpecks and veins of a fine red; no one was neither red or purple throughout. Several of thefe glafles were melted again and again, by themfelves, and with the addition of more vitreous matter : fome were worked in the flame of a lamp : fome were laid in a mixture of powdered charcoal and foot, and made red hot in a clofe crucible ; and others being laid in the fame manner, the fire was increafed till they melted. The colours were by thefe means altered, but did not be- come uniform, or more approaching to the ruby colour than before: fome pieces, which had at firft: very con* fiderablc fpecks of a ruby lullre, loft them on a repetition of the fufion. At the fame time that thefe experiments were tried, the fame kinds of vitreous compofitions, mixed with dif- ferent metallic preparations, were expofed to the fire in different parts of the fame furnace, and were all found to receive beautiful and uniform colours, of which an ac- count will be given in their places. To what caufe the mifcarriage of thofe with gold was owing, whether the fuccefs, in regard to this metal, is influenced by the quan- tity of the matter, by the unfteadinefs of the heat in a fmall furnace, by the fufibility of the vitreous compofi- tion, by the metallic matter being ground with the ingre- dients before their expofure to the fire or added to them in [ 1/9 ] in fufion, by the continuance of the fire, by the fluid mat- ter being kept unmoved or llirred with an iron rod, by the crucible being covered or open, or other like circuni- llances, or whether the admixture of a little manganefe, though gold will certainly give a ruby colour without it, does not contribute to fecure the fucccfs, I have not yet difcovered. The proportion of the gold precipitate to the vitreous matter is perhaps of principal importance. Solution of gold, as we have feen already, produces no rednefs rvith tin unlefs diluted with a very large quantity of water, in which circumftance the whole mixture ac- quires that beautiful colour which we here want to tranf- fcr from the watery fluid into fluid glafs : it fliould feem therefore that the quantity of gold precipitate, for com- municating the admired colour to a certain volume of glafs, ought to be the fame with that, which communicated alike colour to an equal volume of water in the preci- pitation : a quantity extremely minute, and much lefs than that employed in any of my experiments. I have lately been favoured with fome pieces of glafs, in greatefl part colourlefs, with one or two large red fpots, feveral fmall flreaks of violet, and fome of a light brownifli yellow. The perfon from whom I received them informs me that he had •' found that in a heat not very fl:rong, under a muffle, the glafs becomes of an opake brown, and, if then polifhed, appears va- riegated like a fine pebble." I expofed a colourlefs piece to the flame of a lamp, impelled by a blow pipe, and on working it about, fometimes in the fmoke and fometimes in the flame, found it change to a true ruby red, perfedlly tranfparent, and free from veins of any other colour. Another piece, kept for two hours under a clofe muffle, in fuch a heat as made it jufl; foft enough to bend and receive an impreflion, became on the furface A a 2 green. r 180 ] green, brown and pale yellow in different parts, greatly re- fembling the coat of fome pebbles : in this flate, looked through againft the fun, it appeared of a beautiful ruby colour, and on breaking it, the internal part was found throucrhout of an uniform dark red when looked down upon, and of the ruby red when placed between the eye and the light. A large piece being continued under the muffle for four hours, its figure was found fcarcely al- tered, the coat was much thicker and beautifully veined with various colours, which were all loft in a glorious red when the piece was viewed between the light. All I have been able to learn in regard to the prepara- tion of this glafs is, that the quantity made at once is about fix hundred weight; that the tinging matter is mixed with the vitreous materials before they are put into the melting pot, the mixture being brought to the glafs- houfe in tubs; that the matter is not ilirred in fufion ; and that it is kept no longer in the fire than is necefi!ary for perfeding the glafs, which, as foon as fine, is cafjt into a kind of bricks.. Some imagine that this glafs has no mixture of calx of lead, of which a large proportioa is ufed in the compofition of the common flint glafs, and that the principal vitrefying ingredient is nitre : others judge it to be compofed of the fame materials as the common fort ; its weight feeming to be a proof of its containing lead, for it is found to be nearly of the fame fpecific gravity with flint glafs, which is greater than that of the glaffes made without lead in the proportion of above fix to five. This point I have determined in a more fatisfadlory manner: four hundred grains of the glafs, made red hot and quenched in water, were reduced into powder, and mingled with about twice as much black flux and a little alkaline fait : the mixture being melted in a crucible, and the" vefl^el fuffered to cool, a lump <^ [ i8i ] lump of metal was found at the bottom, weighing ninety grains. The metal appeared to be fomewhat /tiffcr than pure lead, and experiments convinced me that it contained fome tin and a little gold. s E c T. xr. The mineral hijlory of gold. GO L D is found in its perfedl metallic ftate j fome- times in maffes of confiderable magnitude ; more frequently in duft or minute grains, intermingled among earths and fands ; or in little drops and veins, bedded in different coloured flones, which flrike fire with fteel, and are not foluble in aqua fortis. It is never debafed into a true ore, as other metals generally are, by the coalition of arfenical or fulphureous bodies; though it is often very intimately combined in the compofition of fands and flones, and blended, in fmall proportion, with the ores of other metals. It is fcarcely ever free from fome admix- ture of other metals, particularly of filver: Cramer ob- ferves, that fuch as is found loofe in earths and fands generally contains more filver than what is lodged in a folid matrix. To fuch an admixture is apparently owing the palenefs of fome kinds of gold : and probably the Malacafiean gold, faid by Flacourt, in his hiftory of Ma- dagafcar, to be not only paler but much more fufible than that of Europe, and which has hence been fuppofed by fome to be in its own nature diftincft, is no other than a mixture of gold with a certain quantity of filver : it is faid to be of confiderably lefs value than the European gold, from which circumftance, omitted by Boyle and others who have quoted Flacourt's account, it may be prefumed that it is not regarded, upon tiie fpot, as being pure gold. The [ i82 ] The Brafils, die Spaniih Weft Indies, fome parts of the Eaft Indies and the coaft of Africa afford the largeft quan- tities of gold. Some parts of Europe alfo appear to be rich in this metal : the mines of the upper Hungary, which feem to be the moft confiderabie in this quarter of the world, have continued to yield gold for upwards of ten centLuies. Peru, Mexico, Chili, and other provinces of the Spanifli Weft Indies, abound with gold in a variety of forms. It is found both in the fands of rivers, and in mines ; inter- mixed with loofe earth, lodged in fiffures of rocks, and bedded in hard ftones ; at the furface of the earth, and at great depths ; in duft, in grains like the feeds of apples, called pepitas, and fometimes in mafles of an ex- traordinary fize. Reaumur reports that a piece was fhewn. to the French academy, which was faid to weigh fifty- fix marcs, or four hundred and forty-eight ounces; and Feuillee fays he faw one, in the cabinet of Antonio Por- tocarero, which weighed upwards of fixty-fix marcs or five hundred and twenty-eight ounces. Both thefe pieces were allayed, and found to be of different finenefs in dif- ferent parts of the mais : the firft was in one part twenty- three carats and a half, in another twenty-three, and in another only twenty-two : the fecond was in one part twenty-two carats, in another twenty-one, and in another but feventeen and a half. It is, however, rare to meet with maffes of the weight even of an ounce : the largeft in the Britifti mufeum weighs but fifteen penny- weight. Notwithftanding the extenfive diflemination of it through thofe provinces, yet the quantity of the gold, in proportion to the earthy and ftony matter mixed with it, appears to be in general exceeding fmall. According to Frezier in his voyage to the fouth fea, and Captain 4 Bretagh's [ '83 ] Eietagh'saccount printed in Harris's colledion, the common yield is no more than five or fix ounces of gold upon the caxon or fifty hundred pounds of tlie mineral : the richeft mines afibrd only ten or twelve ounces, and thofe which are hut jufi: rich enough to pay the charges of wprking of them, yield only two ounces on that quantity. It may be obfervcd in general, that the quantity of gold in minerals is more variable than that of other metals in their ores, and the profits of a gold mine more precarious ; this me- tal not being formed into any regular veins, or uniformly diftributed through any particular kind of earth or ftonc, but fcattered as it were here and there through different mineral bodies : when united with other metals in their ores, its proportion is by no means conftant, though in this cafe it is fubject to much lefs variation than when it is barely bedded in earths or ftones. Of the fource of the gold of the Eafl Indies and of Africa, we know but little. From Cape-coall; on the coafl: of Guinea we receive yearly between two and three thoufand ounces of gold duft, which is fuppofed to be colleded from the fands of rivers; and fome European traders aje faid to have been witneffes of the richnefs of the fands in certain parts of that coafl. In Hook's poft- humous papers an account is given of a pen'bn .having met with great quantities of gold in the fands of one of the rivers, the fand feeming to grow richer and richer as he advanced further up : in fome places he fays he gained fixty-three grains of gold from five pounds of fand, and he feems afterwards to have met with much more profitable fpots. Three or four hundred ounces, as I am informed, are colledled yearly from the fands of the Gambia, and caft into bars at James-fort, one of cue fettlements on that river. [184] .... It is faid, that the gold duft from Africa, in its pureil (late, is from twenty-one to upwards of twenty-two carats jinej but that the natives frequently mix with it filings of brafs. This admixture may be diftinguiflied by the •" hydroftatic balance, the fpecific gravity of brafs not being half fo great as that of the gold duft : in this way of examination great care muft be taken to make the water penetrate as perfedlly as poffible into all the interftices fo as to come into clofe contad with every particle. A little aqua fortis alfo, poured on the mixture, will immedi- - ately difcover the fraud, receiving from the copper in the brafs, a blue tindlure. It has been fufpeded, that if the gold be naturally alloyed with a little copper, this trial will prove fallacious, and that the natural copper will tinge the menftruum equally with that which is added by art : of this, however, there is no danger, the natural alloy not being in diftindl particles, but difFufed through each mafs or particle of the gold, fo as to be covered by the gold, and protedted from the acflion of the menftruum. There are feveral other means by which this abufe may be difcovered : if the duft be fpread thin on a piece of white paper, and moiftcned with any volatile alkaline fpirit, as that of hartftiorn, of fal ammoniac, or of urine, the fpirit will in a* few minutes diffolve fo much of the copper as to ftain the paper blue: ftale urine itfelf has a 'like effed, in an inferiour degree j and a folution of crude fal ammoniac, applied in the fame manner, produces a greenifli ftain. There are fundry European rivers which roll particles of gold with their fands, in no great quantity, yet fuch, that the neighbouring inhabitants, at certain feafons, find their account in colleding them. M. de Reaumur, in an eflay in the French memoirs for 171 8, drawn up from materials furniflied by the intendants of the feveral pro- vinces [ iS5 ] vinces In purfiinn^-eo^ rlif* orders of the duke of Orleans, gives an account of ten rivers or rivulets in the territories of Frai;ce that have gold 'iiixcd with their f^.nds in certain parts of their courfe : the Riiine, from Brifac dovi^n to Strafburg fparingly, fiom thence to Philipfburg more abundantly, and moft of all fo between Fort Louis and Germeflieim: the Rhone, in the pais de Gex, from the conflux of the Arve, from whi. h it is fuppofed to receive its gold, to about five leagues lower down : the rivulets of Ferriet and Benagues, which rife from the heights on the left hand of the defcent from Varilhere to Pahniers : the Ariege, aurigera^ about Palmiers, bA;low where it re- ceives the two foregoing rivulets : the G-'.ronne, fome leagues from Touloufe, below where it receives the Ariege: the Salat, which rifes, as the Ariege, in the Pyreneans:' the Ceze and the Garden, which come from the Cevennes ; and the Doux in Franche-Compte. The lait of thefe rivers is the pooreil, the gold having hitherto been colkdied from it rather in the way of curiofity than in a lucrative view : the greatell: quantities are obtained from the Rhine, not that this is really the richeft, but on account of its fands being the moft induftrioufly fearched, for fome of the others, particularly the Ceze and the Gardon, appear to be at leaft equal to it in richnefs. The quantity got from the Rhine, in an extent of near two leagues below Strafburg, is faid to amount to no more than four or five ounces in a year : this is the quantity brought to the ma- giftrates of Strafburg, who farm out the right of collect- ing it on condition of its being fold to them at a price confiderably under its value, fo that it may be prefumed that apart is otherwife difpofed of, and that the quantity really obtained is confiderably greater. There are many other rivers reported to yield gold, as the Tagus, the Danube, the Elb, the Oder, the Inn, the B b Sala, [ i86 ] Sala, &c. The Schwartz, in the county of Schwartzburg in uppper Saxony, is laid to be rich in this metal, and its fands to be worked with great profit: Stahl mentions a piece found in it as broad as a middling bean, though not fo thick, and fuppofes that the Sala receives all its gold from this river j the gold of the Sala being found only below vv'here the Schwartz enters it, and being lefs plenti- ful and in fmaller grains. It is generally reckoned that the "old particles, in all auriferous rivers, are wa(hed out by the ftream from fuch rich beds, and afterwards reft or fettle where the current is languid, or in places where they efcape its force: from what particular fources the gold in different rivers proceeds, does not feem to have been examined. The richeft parts of rivers, within the extent in which they roll gold, are thofe where their courfe is flow and umncerrupted, where they widen or change their direc- tion : the moft favourable feafon is when the water has fubfided after a flood. The appearance of the fand af- fords an ufcful mark for diftinguifliing the richeft fpots J the gold being always moft plentiful where the fand is reddifli or blackifti, or of a colour fomewhat differ- ent from what is feen elfewhere ; not that the red or black fands have any natural connexion with the gold, but on account of their being more ponderous than the white, fo that the fame caufe, which determines the par- ticles of gold, determines thefe alfo to fubfide. The black fand abounds with iron, being vigoroufly attradled by the magnet ; the red, viewed in a microfcope, appears as Reaumur obferves, a beautiful congeries of coloured cryftals, imitating all the gems known to the jeweller, among which thofe of the ruby and hyacinth teints are the moft common, whence the reddifh colour of the fand to the naked eye; the particles of gold are of irregular figures. [187] figures, but conftantly flat, fmooth, and with the edges rounded ofl*. Some gold has been difcovered in Britain, at different times, though hitherto in little quantity. Inftances are mentioned in Houghton's colled:ions, f'^om Camden, Sib- bald, and Gerard de Malines, oi gold iound in Scotland, about the head of the Clyde, in Crawford-moor; and Boyle fays he had from fome part of Scotland divers large grains of gold, taken up near the furface of the earth, over a lead mine, one of which clear from fpar, weighed two hundred and one grains. Mr. Boyle had alfo an Englifh tin ore, wherein there lay, in little cells, a number of fmall leaves or chips of gold : he obferves that though the tinmen, unable to feparatc them to profit, ufually melted both metals together, he was allured that one perfon advantageoufly employed his children to pick out the gold from the ore fkilfully broken. Some earlier writers mention alio gold foand in the tin ores of Corn- wall, and about the beginning of the prefent century a patent is faid to have been obtained " for feparating gold •* and filver from tin by precipitation in a reverberatory " furnace with fome peculiar fluxes": what fuccefs this fcheme met with, I have not learnt. Mr. Borlafe, in his natural hiftory of Cornwall, gives an account of fome tin ore abounding with a yellow matter, which was taken by the workmen for mundic: fome bits of the yellow matter, one of which was a vein as large as a goofe-quilf included in a ftone about the fize of a walnut, produced, on being melted, an ounce of pure gold : he mentions feveral other pieces of confiderable bulk, and gives a figure of one, found in 1756, weighing three hundred and feventy-fix grains. It is fuppofed to be chiefly the flream tin, (that is the tin ore found in detached pieces on the ijdes of hills) which contains gold; that all of this fort B b 2 contains [ i88 ] contains more or lefs of it ; and that both the gold and tin ore have been brought from elfewhere by torrents of water, and depofited where they arc now found like the gold fands of rivers. Boyle conjedures, that befides the grains of gold which lie detached among fands, there may be many particles lb minute and clofely fixed to the fand, as not to be per- ceivable by the eye, or feparable by the common me- thods of wafliing or picking; that many fmall portions of the metal may be incorporated alfo with the body of the fand, and that by fkilful management they might be ex- tradled. Experiments, he fays, confirmed him in this perfuafion : later experiments have verified it, and fhewn the exiftence of gold in fands to be even more extenfive than he feems to have apprehended. Many of the com- mon fands, particularly the yellow, red, black, and thofe of a black colour inclining to violet, appear to be rich in gold: Becher and Cramer pfefume that there is no fand in nature entirely free from it. Hellot relates, that in eleven efl'ays of one kind of fand, made by M. Lieberecht, by a procefs defcribed in the fequel of this fedtion, the yield of noble metal turned out conftantly from eight hundred and forty to eight hundred and forty-four grains on the quintal or 921600 grains, exclufive of what re- mained in the fcorias, which were ftill found to be rich ; that different parcels of fand, taken up at no great dift- ance from one another, differed in degree of richnefs, fome having afforded above a thoufand grains, others only three hundred and fifty, and others yielding none by the treatment which had fucceeded fo well for the reft; and that of the metal thus obtained from fands two thirds are commonly gold, and the reft filver. Yet notwithftanding the great richnefs of thefe fands, no means have hitherto been difcovered of availing ourfelves of [ i89 ] of the metal they contain, or of cxtrafting it to advan- tage in the large way. Becher indeed undertook to obtain gold with profit from the common fea fands, and entered into engagements with the Hates uf Holland for eftablirh- ing a mineral work on this foundation : but though expe- riments made on little quantities promifed very confider- able gain, and though one trial in large is likewife faid to have proved fuccelsful, yet, as he communicated the whole procefs to the commiflioners appointed to examine the affair, and as he has fliewn that fuch a work might be carried on more advantageoufly in Holland than in other parts of Europe, its never having been profecuted in Hol- land affords a flrong prefumption of its not being fuffi- ciently lucrative. The exifleuce of gold in fands is never- thelefs an interefting fadt, at lead to the philolbpher, and further examination may perhaps find means of making it turn to account. Though gold has been but lately difcovered, or expedled, in thefe minerals, in which it is fo common and fo plen- tiful, their unpromifing appearance having given little en- couragement to examine them; there are others, whofe flattering colour has raifed great expectations, but which have not been found upon experiment to yield any gold. The yellow pyrits or marcafites, and other minerals of a golden colour, or containing gold coloured fpecks, have by fome been regarded as ores or matrices of gold, and accordingly fubmitted to different operations, as fruitlefs as expenfive: their lofing their colour in the fire, or changing it to a yellow, foon difcovers that their tinging matter is not gold, but fulphur or a ferrugineous calx. There are indeed pyritge which contain gold, and in quan- tity fufficient to deferve notice: Henckel, in the 12th chapter of his pyrifologia, gives an account of fome aflays of minerals of this kind brought from the Hungarian mines. [ I90 ] mines, one of which yielded, on the centner or 1600 ounces, half an ounce of noble metal, of which one fifth was gold ai)d the reft filver : from the fame quantity of ano- ther he obtained no lels than fixty ounces of filver, and eight ounces and a half of gold : but the gold, as he obferyes, is by no means proper to the pyrites, or an effential part of its compofition, having been only cafually intermixed, as it is in ftones and other minerals in the gold mines. Many have been deceived alfo by fome of the talcs : of which there is one fpecies, naturally of a grey colour* which in a moderately flrong fire changes to a gold yel- low; and another, naturally of a glittering gold colour, which receives no change from moderate fire : both thefe bodies have a further refemblance to gold, in imparting a deep yellow tindlure to ftrong aqua regia. By repeated digeftion in frefli aqua regis, all the colouring matter may be extrafted, and the earthy part left white : but the fo- lution yields no gold, and is found to be no other than a folution of iron. Reaumur obferves that fpangles of the yellow talc are frequent in the fands of fome rivers, and that they may readily be diftinguilhed from gold, which they have often been taken for, by viewing them with a magnifying glafs ; the gold particles found in rivers be- ing conftantly fmooth, with the edges rounded, while the talky ones are rugged and fharp-edged, 'II. Separation of gold from earthy andjlony bodies by ivater. Gold intermixed with earthy bodies in fmall particles or duft, is feparated by wafliing with water, which carries off the lighter earth, and leaves the more ponderous metal behind : the great gravity of gold renders it bet- ter adapted to this way of feparation than any of the other anetals. There are fundry variations in the manner of condudt- condudling the proccfs, according to the quantity of the matter, the nature of the earthy body, and the convcni- ency of the phice ; many of which are minutely defcribed by Agricola in his treatife ^e re metallica : it will be furii- cient here to give a general idea of the manner of procedure, in an operation merely mechanical, and whofc fuccefs de- pends chiefly on manual dexterity acquired by practice. Where the quantity of matter is fmall, it is laid by a little at a time in a round fliallow difli called a huddle, or in an oblong veflel like a boat, which being gently fhaken backward and forward in a tub of water, the lighter part of the earth is taken up and waftied off, leaving with the gold fuch fund or fmall ftones as the mixt con- tained. By dextroufly repeating the agitation, that the whole may acquire as it were a kind of fluidity, the me- talline particles fink to the bottom, and the fand or ftones are thrown up to the top, and may be removed by the hand. At feveral of the gold mines of the Spanifh Weft In- dies, the gold is completely feparated by this fimple ope- ration. According to D'Ulloa's account, in his voyage to thofe parts, the earth, as it comes from the mine, is thrown into a refervoir, a ftream of water conducted on it, the whole ftirred together, and the muddy water let cfF into another and another refervoir : what the water leaves in the firft refervoir, and what it depofits in the others, is taken up in trays, or buckets with two handles, and agitated in freili waters, with an uniform circular motion, till the gold is colledied at the bottom. M. de Reaumur, in the French memoirs for the year 1718, gives a particular defcription of the method of wafliing the gold fands of the Rhine and fome other rivers. Aboard, five feet long, afoot and a half wide, with a ledge at each, fide, and at one end, is laid aflope, with [ 192 ] with this lafi: end on the ground, and the other raifed a foot and a halt: acrofs the board are nailed three pieces of rough cloth, about a foot wide, and at diflances of a foot 3 and at the upper end is placed a kind of baf- ket made of rods. The fand is fhovelledinto the bafket : water thrown on it wafhes the fand through, the ftones remaining : the earth and lighter parts of the fand are carried down to the bottom of the board, while the par- ticles of gold, and the heavy black and red fands already mentioned, are detained by the rough cloths, which, when they appear covered, fo as not to be able to detain more, are taken off and waflied in a tub of water, then nailed on again, and the procefs continued till a proper quantity of this richer fand is obtained. In fome places, inftead of cloths, ikins with the hair or wool on are ufed ; and in others, notches are made acrofs the board. The richer fand thus detained is put intoaveffel fome- what like a boat, which is gently fliaken in water, in the fame manner as the fan in winnowing corn, till the li^^hter grains rife to the top : thefe being carefully poured off with the water, the agitation is repeated fo long as any grains, of a different colour from the reft, are found to rife. No further feparation is to be expeded by this method, and the gold, of which fome particles begin now to be diftinguilhable by the eye, is extracted from the remaining matter by mercury, as defcribed in the fol- lowing article. Gold bedded in ftones may frequently be feparated from great part of the ftony matter on the fame principle, the ftone being previoufly reduced to powder. In the large way it is beaten in mills, under water, by large wooden ftampers armed at the bottom with iron, an iron grating being commonly fixed at one fide of the trough or pit, through which the finer parts are continually 5 wafhed [ 193 ] wartied off by the water. When a little quantity is to be powdered in a mortar, it fhould likewife be done by blows of the peftle, not by grinding: a blow only flat- tening the metalline particles, while triture wears and divides them, in part, to fuch tenuity, as not freely to fubfide in water. Stones of the hard flinty kind are pre- vioufly made red hot, and quenched in water; by which means they are rendered more eafily pulverable, and at the fame time many of the fmall particles of gold, melting in the fire, unite and form larger mafles. '- III. Separation of gold from earths andjiones by mercury^ Where the minutenefs of the particles of gold, and the weight of the matter with which they are intermixed, renders them infeparable by water, quickfilvcr is called in aid for imbibing and detaining the gold. The gold fands, freed by water, as in the foregoing article, from as much, of the lighter matter, as can be wafhed off by water with- out endangering a great lofs of the gold, are dried, and a fmall proportion, lefs than a hundredth part of their weight, of mercury poured on them : the whole is well kneaded up together, that the mercury may penetrate, as much as poffible, into all the interfticcs between the -grains ; it imbibes the atoms of gold it meets with, and the fand is afterwards wafhed off by water. In the Spanilli Weft Indies, at thofe mines where the gold is bedded in ftones, and requires quickfilver for •its feparation, the ftony matter is reduced to as fine pow'- Jer as pofllble, that every atom of the gold may be laid open to the mercury. The powder i^ foaked for fome ■time in a folution of common fait; the mercury fqueezed in through a linen cloth, fo as to fall like dew all over •the furface; and the mixture being well flirred and C c^'^^ 2- --^^ '■ kneaded. E 194 ] kneaded, a gentle heat is applied, by which the adivity of the mercury is fo far increafed, that the incorporation of the gold with it, which in the cold requires about thirty days, is faid by J. Hernandez, in an ell'ay on thefe mines, to be effeded by this method in five or fix days. Alonfo Barba, in the third book of his art of metals^ defcribes another method, which he fays he has pradtifed with great advantage: he puts the powder, with afuitable quantity of mercury and water, into a deep copper vefTel fixed in a furnace, and applies a fire fufficient to keep the water boiling : a fmall wooden mill affifts the ebullition of the water in giving motion to the earth" powder, which continually rifing and falling down again, is brought into frequent contad with the mercury at the bottom, fo as to give out its gold to the mercury in as many hours, as the common prdcefs without heat requires days. "When the gold is judged to be united with the mer- cury, the earthy powder is wafhed off by water, fo as to leave the amalgam clean. Where, the mercury has been kneaded up with the powder, a confiderable part of it is always divided into fuch minute globules as to be wafhed away along with the earth ; an inconvenience which in the other method does not happen or in a far lefs degree. The gold dull or filings, difperfed through the fweepings of the goldfmiths ihops, are recovered alfo-by amalgama- tion with mercury. Two broad iron bars, rounded at the ends, placed crofswife and fixed &n an upright axis-, are made to turn, by a handle at the top, on an iron plane fitted into the bottom of a tub. 1 he fweepings being put into the tub with a quantity of mercury and water, the powder, pafiing fucceffively under the iron bars, is ground and brought into contad with the quickfilver, which by degrees extrads the gold j while the water, which after a certain time is fuifered to run off through a fmall ^uill quiIl In the ficlV'or the tub, carries with it tfie rightcf earthy matter : fredi water is fupplicd till all the earth is thus vvadicd off. The operation might doubtlcrs be expedited by the ufe of heat, as in the above procefs ofBarba. ^ The gold being by thefe means transferred into the mercury, and the mixture wartied clean, as much as may be of the mercury is prefled out through leather, and the remainder forced off by fire. To colle(fl the exhaling mercury, a head and receiver are fitted to the iron pot in which the mafs is expofed to the fire: Barba advifes the pot to be lined with a mixture of clay and fand, to prevent the gold from adhering to or diffolving apart of the iron in cafe the fire fliould be raifed Co far as to make the gold melt. '- It is fcarcely to be expedled, that the greatefi: addrefs of the workmen can colledl, either by water or mercury, the whole of the gold diffufed through a large quantity of other matter i at leaft when the mercury is ufed, as it is in the large way. In fo fmall a proportion as a hundredth^ or a two hundredth part of the weight of the earthy pow- der. Reaumur, after having worked fome gold fand with mercury, in the method commonly pradifed by thofe who wa(h the fands of rivers, obtained, from the remaining fand, by treating it with a dftuble quantity of mercury, near as much gold as he had done the firfl time. IV. Extra5iion of gold intimately combined in the compojition of fands. For extradling gold from the ferruglneous fands men- tioned in the preceding part of this fedlion, the fand is to be made red hot and quenched in water, and the ignition C c 2 and [ 19,6 ] andextlndion repeated four times or oftener. The colour changes, from yellow, red,, or black, to a reddifh browm In the firft and fecond heating, the land yields a flight fmell, fomewhat like that of garlick, a mark of its con- taining arfenic : at the third time, the arefenical fmell i& fcarcely to be perceived, but on throwing into, the crucible a little tallow, or other like inflammable matter, it becomes^ jftronger than before: this remarkable circumftance is by no means peculiar to thefe fands, for there are feveral ar- fen.cal minerals which give out little of their arfenic in the fire till fome inflammable matter is added. The fand, thus calcined, is mixed with twice its weight of granu^ lated lead, and equal its weight of black flux, the mixture put into a crucible, and covered with fome fea fait dried, over the fire till it has ceafed to crackle. The crucible is placed in a good blaft furnace, the fire ftrongly excited, and the matter ftirredfrom time to time with an iron rod>i the fire mufl; be urged till the fcoria flows thin as water,, which is known by the rod coming out almofl clean,, •without the leafl: knob (licking to the end of it. The crucible is then fufl?ered to cool, and broken for getting out its <:ontehts : on the top is the common fait, in a diftinift cake; under this, a fliining black, compad, vitre- ous fcoria; and at the bottom, a lump of lead, eafily fepa- lable from the fcoria. The gold, contained in the fand, is now transferred intothe lead, and may be feparated by •working off the lead upon a cupel or left. Thefe fands contain alfo filver, which here accompanies the gold, and which may be parted, after the cupellation, either by aqua fortis or aqua regia, according as one or the other metal appears to prevail in the mixt. This is the procefs fol- lowed by M. Lieberecht in the alTays already given aa account ofl', As- [ ^97 ] As the black flux (which confifts of one part of nitre, and two or three of tartar, mixed together, and burnt ia a covered veflel to a bhick alkaline coal) is apparently too expenfive to be employed for any confiderable quantities of the mineral ; its place may be fupplied by a mixture of potaHi, or other iixt alkaline falts, with powdered charcoal. Four parts of potafli, three of charcoal, and thirty-two of lead, are fufficient for fixteen of the calcined fand. This mixture, as Hellot obferves, requires the fire to be con- tinued longer than the preceding to make the matter equally fluid; but when it is made fo, the yield is found to be in both cafes alike. The fame end may -be obtained alfo, by boiling the calcined fand in melted lead, without any inflammable or faline addition. For the vitreous matter, into which the lead is gradually converted, will diflblve the earth, and the unvitrefied part of the lead will receive and collect the gold J but a much larger quantity of lead is requilite ia this than in the other ways. In all the foregoing methods, though the quantity of gold obtained is conliderable, much flill remains in the fcoriae, from want, probably, of a thorough commixture of them with the lead : for whatever degree of fluidity the fcoriae are brought to, the particles of gold are too minute to fubfide by their own weight, and the lead can collecft them only from thofe parts which it comes in contait with. It may therefore be prefumed, that the reparation will be the more complete, as the commixture of the lead is the more perfecfl. Lead cannot be n.ixed perfedly with fands but in a vitreous or femivitreous form. By grinding the calcined fand with litharge or other calces of lead, and expofing the mixture to a moderately ftroiig fire, they may be intimately united into an uniform glafly- compound ; the fand being diflTolved by the vitrefied lead, nearly [ 198 ] nearly as fait Is dillblved by water. On adding to this compound a little powdered charcoal, or barely flirring it with an iron rod, the lead revives and falls to the bottom ; and as the fand had thus been in contadt with every part of it, the gold will probably be extradled from every part, or at leaft more effedlually than by the other methods. In this way it will be proper to mix fome alkaline fait with the litharge and fand j partly, to pro- mote the diffolution of fuch portion of the fand as floats on the top of the ponderous metallic preparation j and partly, to continue the fluidity of the fcoria after the re- vival of the lead. The crucible may be made of Stur- bridge clay, which feems to be one of the beft of the common materials, for refifl:ing glafs of lead in fufion. It appears to have been on a procefs of this kind, that Becher's propofition to the States of Holland was founded, for extrading gold with profit from common fands ; it is plain from his account of this affair in his minera arenaria, that he vitrified the fand with glafs of lead, or litharge, and he exprefsly mentions in one place the precipitation of the lead from the glafs by iron : he ufed alfo an addi- tion of filver, in great proportion, for imbibing the gold, and thus required a vaft capital for efliablifhing a work in large -, but where the lead is to be revived, the filver is' rather detrimental than ufefulj for the gold and filver contained in the fand are imbibed by the lead, and the additional filver occafions an enormous expence of aqua fortis for diflblving it in order to the feparation of the gold. Whether, with this rcdudtion of the expence, of which Becher himfelf feems to have had fome idea, the procefs might be pradlicable to advantage, or whether ibme earthy bodies might not be an ufeful auxiliary for promoting the fufion of the gold fands, may deferve fur- ther enquiry : one kind of earth is frequently obferved to bring [ '99 ] bring another Into fufion, tiiougli both arc of themfelves unf'ulible; and the earthy parts of different ores are made fluid in the furnace of the fmclter by the addition of other earths. V. 'Extraction of gold from the ores of other met ah. When gold is intimately combined with other metals in their ores, the org is to be run down in the fanij man- ner as the fame kind of ore without gold. The gold commonly melts out with the proper metal of the ore, from which it may be afterwards feparated by different procefles according to the nature of the metal. There are grounds to believe that mod metallic bodies, as ex- tradled from their ores, contain generally a portion of gold, though rarely fufficient to bear the expences of its fepa- ration. Zinc, arfenic, and mercury, are obtained from their ores by a kind of fublimation : hence if the ores of thefc contain gold, the gold is to be fought for, not in the me- tallic fubftance feparated, but in the remaining matter. There are fome other cafes alfo, in which the gold, in- ftead of accompanying the rrietal in its fufion, is thrown off in the flag: but the ores and flags of this kingdom have been fo feldom examined for gold, that at prefent I can fay little fatisfadlory on this fubjedl. SECT. xir. Of the alchemical hijlory of gold. SOME of the Greek writers, in the fourth and fifth centuries, fpeak of an art, as being then known, of tranfmuting the baler metals into gold ; and towards the end of the thirteenth century, when the learning of the eafl; had been brought hither by the Arabians, the fame 2 pretenfions [ 200 ] prctenfions begun to fpread through Europe. It has been iiippofed that this art, called alchemy, was of Egyptian original; and that, when the ancient Greek philolbphers travelled into Egypt, they brought back fomc of the alle- goric language of this Egyptian art ill underflood, which afterwards pafTed into their mythology. This is all that is known with certainty, or can be admitted with any fliew of probability, about the origin of an art, whofc hiftory and antiquity have been theobjeft of elaborate re- fearches, and treated with a profufion of erudition. Alchemy was the eariieft branch of chemiftry con- fidered as a philofophic fcience. In the other parts of chemical knowledge, fails preceded reafoning or fpecula- tion ; but alchemy was originally fpeculative. Such of the alchemical writers as are reckoned of mofl: authoiiiy as Geber, Hollandus, and others, declare, that we are not to hope for fuccefs in the practice of this work, without being previoufly well acquainted with the nature, eflence, and principles of metals; whence they were produced in the mines ; whence they receive their increafe ; how and to what ftate they have a natural difpofition to be brought, and would have been brought if it had not been for fomc impediment; and what thefe impediments are. The alchemifls fuppofed that nature, in all her works, aiming at perfedion in producing metals aimed at gold : that the imperfed or bafe metals failed of being gold, either from a redundancy or deficiency of fome particular element in their compofition, or for want of fufficient codtion, maturation, or depuration of their principles; and that art could corredt or remove thefe impediments, fo as to complete the work which nature had begun. They fuppofed the general principles of metals to be chiefly two fubftances, to which they gave the names of mercury ajid fulphur; and that c^both thefe there were diiferent i [ 201 ] different kinds, particularly of the latter; which they ad- mitted as many varieties of as there are metals j and which, in gold, they held to be pure, red, fixt and incombuftible, but of different qualities in the other metals. In thefe points there is no perfedl uniformity among the different alchemical philofophers, which indeed could not be ex- pelled in hypothefes on fo abftrufe a fubjedt, where ex- perience had afforded fo little light: fome have added a faline, fome an cartliy, and others an arfenical principle. They fuppofed that the pure mercurial, fulphureous, or other principles of which they imagined gold to be com* pofed, were contained, feparatcly, in certain other bodies. Thefe principles therefore they endeavoured to coiled, and to concodt and incorporate by long digeftions. In the many volumes written profeffedly to teach tlie proccfs at full length, the fubjefts from which the golden feeds are to be obtained, are wrapt in impenetrable obfcurity : thus much is plain, that the fuppofed adepts in this myfterious fcience do not all make choice of the fame fubjedls, or work upon them in the fame manner, their pradice beino- probably adapted to their particular hypothefes. By thus conjoining the principles of gold, if they could be fo procured and conjoined, it might be ex- peded that gold would be produced. But the alchemills pretend to a produd of a higher order, called the elixir, the medicine for metals, the tindure, the philofopher's ffone; which, by being projeded on a large quantity of any of the infcriour metals in fufion, (hould change them into fine gold; which, being laid on a plate of filver, copper, or iron, and moderately heated, (hould fink into the metal ; and change into gold all the parts it was applied on ; which, on being properly treated with pure gold, lliould change the gold into a fubftance of the iame nature and virtue with itfelf, fo as thus to be D d fi^fceptible [ 2C2 ] fufceptible of perpetual multiplication -, and which, by continued codtion, fhould have its power more and more exalted, fo as to be able to tranfmutc greater and greater quantities of tlie inferiour metals, infomuch that, ac- cording to its different degrees of perfedion, one part of it fliall be fufficient for ten parts, a hundred parts, a thou-, fand parts, twenty thoufand parts, two hundred and fe- venty two tlioufand three hundred and thirty parts of bafe metal. If thefe pretenfions were propofed as matter of fpecula- tion only, I believe no one, who has at all confidered the nature of metals, could hefitate in pronouncing them abfurd : they are inconfiftent even with the alchemical philofophy itfelf. But they are endeavoured to be fup- ported by arguments of another kind; by hiftorical rela- tions of the aftual tranfmutation of all the common me- tals, ftrongly attefted, not only by the alchemills them- felves whofe teftimony might be thought fufpicious, but likewife by perfons fuppofed to have been entirely un- prejudiced, who had been cafually favoured with fome quantity of the tranfmuting powder, or who had been witneffes to its aftonilhing operation and to the immenfe riches it had procured. In regard to thefe narratives I fhall only remark, that at a time when the tranfmutability of metals was generally believed, the circumftances cif certain princes might render it an advantage to them to be thought to have fuch an inexhauflible refource for wealth ; that fome perfons who, by methods which it was their intereft to conceal, had acquired fudden riches, might, in this art, find plaufible means of giving an account for them • that many of the fuppofed alchemifts have been con- vifted, and perhaps many others guilty, of impoflure j the gold, which they pretended to have" made, having fometimes [ 203 ] fometlmes been previoully concealed in the crucible, or in the materials, or at the end of th-; rod, with which the matter was iHrred in fufion, and fometimes intro- duced into the crucible by a confederate, when the fur- nace was covered, through an aperture communicating with another apartment. So many frauds and juggling ar- tifices are known to have been pradtifed on this oocafion, that the evidence of a fpedator can be of no force; and perhaps thofe, who were more than fpedators, were too much interefted to be admitted as evidences. I am very far, however, from cenfuring as impoftors all thofe who have declared themfelves convinced, from their own experiments, of the tranfmutabiJity of bafe metals into gold. Many experiments have been alledged, in which bale metals were made to yield fome portion of gold, and in which gold, treated with certain addi- tions, received an increafe: though the quantity ob- tained was rarely fuch as to bear the charges of the pro- cefs, it is reckoned fufHcient at leaft, in a philofophic view, to demonftrate the adlual tranfmutation, into gold, of a fubftance which before was not gold. Mod of thefe experiments are free from fufpicion of any fradulent de- lignj but there are ftrong reafons to fufpedl that the authors have been themfelves deceived by fallacious ap- pearances. Gold, as we have already feen, is now known to be far more frequent in metals and other minerals, than it was formerly fuppofed to bej and there is little wonder, if men of warm imaginations, biafled by a favourite hy- pothecs, have been led to believe that they produced gold when they extraded it from materials in which it was not imagined to prxexift. We have feen alfo, in a foregoing fedtion of this eflay, that the common method of parting filver from gold by aqua fortis does not com- D d 2 pletely [ 204 ] pletely feparate tlie filver j and it has often happened that when fine gold, melted with filver, was lubmitted to cer- tain operations, and afterwards parted, the portion of filver which the aqua fortis left in the gold, has been taken for an augmentation of the gold itfelf. Of this I knew a remarkable inflance in a procefs which was fome years ago referred to my examination, and whofe fuccefs wa& averred to have been fuch, that it was offered as a very lucrative operation, and a confiderable price demanded for the communication of it : the gold, after it had pafTed through the tedious procefs that was to enrich it, and hud been parted from the filver by weak aqua fortis, ac- cording to the diredlions, weighed indeed notably more tlian the pure gold employed : but on reducing it to its- former purity by folution in aqua regis, I found it reduced at the fame time to its former quantity. It is probable that many of thofe, who have been raoft fanguine in their expedlations of gain from alchemical operations, had no other foundation than thefe mifunderftood kinds of experiments ; which having once perfuaded them that they could make gold, they might naturally conclude that it could be made in any quantity. I fhall only further remark on this head, that if a part of the fub- ftance of any metal was by any operation tranfmutable into gold, a part more would be in like manner tranfmu- table by a repetition of the operation, and this fucceflive- ly, fo long as any part of the metal remained entire and perfedl, or fo long as it retained the properties which it had at firft : a procefs of this kind would be decifive, but fuch a procefs has not yet been made known. The deftrudion of gold is affirmed by the alchemifts to be more difficult than its produftion. This point alfa has been eagerly profecuted, not only on account of its being interefting as an objeS of philofophy, but on ac- count t 205 ] count likewKe of fome advantages expe(fled to refult from it, many having perfuaded themfelves that its de- ftrudlion or refolution would afford the fure foundatioa for its artificial produdtion. Divers experiments have been alledged, in which gold is faid to have beea deftroycd, or changed into a matter which was no longer gold, or refolved into diffimilar principles : but in thefe experiments, as in thofe of its produdtion, there was probably fome deception ; and many of them, as related by the authors themfelves, are apparently incon- clulive. Mr. Boyle gives an account of a procefs, by which he imagines part of the fubAance of gold to have been tranfmuted into filver. Into redified butter of antimony, that is a folution of the metallic part of antimony in the - marine acid, he poured as much fpirit of nitre as was fufiicient to precipitate the metal, and having diftilled off all that would come over in a fmart fire, he returned the liquor on the antimonial powder, and abftradled it again : of this menftruum, which is a kind of aqua regia, he had a great opinion, and gave it the name of menjiruum peracutum. Some gold was melted with three or four times its weight of copper, the copper extraded by aqua fortis, and the remaining gold powder being brought to its due colour by heat, a large proportion of the men- ftruum was poured on it : the gold dilTolved flowly and quietly, and there remained at the bottom of the glafs a confiderable quantity of white powder. The folution of gold being abftradted, and the gold again reduced to a body and difTolved a fecond time, it yielded more of the white powder, but not fo much as at firft. On melting thefe powders with borax, he obtained a white metal, which yielded to the hammer, and which, on being dif- folved in aquafortis, f]>ewed iifelf, by the odious bitter- nefs [ 206 ] nefs it produced, to be true filver. He fays, that even with good aqua regis, he could obtain from the very beft gold fome little quantity- of fuchawhite powder, but in fo very frnall proportion, that he never had enough at once to make him think it worth while to profecute fuch trials. It were to be wiflied that the ingenuous author had been more careful in afcertaining the purity of the gold made ufe of in thefe experiments, and noted the exad; quantity of filver obtained from it. Gold parted from filver or copper by aqua fortis, is by no means to be looked upon as being pure : nor is there perhaps any other method, as yet known, of perfedly purifying it from filver, than that by which the filver was feparated in the above expe- riments ; the difix>lution in aqua regia being in efFeft no other than a purification of the gold. Even aqua regia, when made with an under proportion of marine acid, will not produce a complete feparation ; this imperfedt aqua regia taking up, along with the gold, a little filver, feparable by a fecond diffblution. Mr. Boyle has given an account alfo of a very extraor- dinary experiment, under the title of the degradation of gold by an anti-elixir, which was publiQied in his own life time, and fince reprinted in 1739. The known cha- rader of; the author, the earneft defire he has fliewn in all his writings for the difcoveryof truth and the expofing of falfe pretences, have not only rendered the fail un- queftionable, but likewife induced many to adopt the confequences which he thought might be drawn from itj and to regard it as a proof of the real alterability of gold, and as ftrongly favouring the alchemical dodlrine of the tranfmutability of metals. I iliall therefore infert the ac- count of the experiment in the author's words, and fub- join a few remarks ; bmenting that it is not in my power to [ 207 ] to enquire more fatisfadtorily into fo curious a fad by a repetition of the experiment. The fubftance, by wliich the apparent degradation was produced, was a powder of unknown compofition, communicated to Mr. Boyle by a ftranger, and its quantity not fufficient for more than a fingle trial. There was fo very little of it, that he could fcarce fee the colour of it, fave that, as far as could be judged, it was of a darkifli red : the quantity was eftimated at an eighth or a tenth part of a grain. The gold had been formerly Englifli coin, and to be fure of its goodnefs, he caufed it to be, by one whom he ufually employed, cupelled with lead, and afterwards quarted with refined filver and purged aqua fortis. Two drams of this gold, thus purified, were weighed out, and put into a new crucible firfl: carefully "nealed, and the gold being brought into fufion, without addition, he put into the well-melted metal the little parcel of powder with his own hand, continuing the fire about a quarter of an hour, that the powder might have time to difFufe itfelf every way into the xnetal : the well-melted gold was then poured out into another crucible, which had been gradually heated to prevent its cracking. But though, from the firfl fufion of the metal to the pouring out, it had turned in the crucible like ordinary gold, fave that once, as the affiflant obferved, it looked for two or three minutes almoft: like an opal : yet when the mat- ter was grown cold, though it appeared on the balance that it had not loft any thing of its weight; yet, inftead of fine gold, there was a lump of metal of a dirty colour, and as it were overcafl: with a thin coat almoll: like half vitrefied litharge : to one fide of the crucible there ftuck a little globule of metal, that looked not at all yellowifh,. but like coarfe filver; and the bottom of the crucible was overlaid with a vitreficd fubftance, v^'hereof part was 4 of [ 208 J of a tranfparent yellow, and the other of a deep brown inclining to red : in this vitreiied fubflance there were plainly perceived flicking at leaft five or fix globules that looked more like impure filver than pure gold. Having rubbed this odd metal upon a good touchftone, whereon there was likewife rubbed a piece of coined filver and a piece of coined gold, the mark left by it on the flone was notorioufly more like that of the filver than of the gold. Having knocked the little lump with a hammer, it was found brittle, and flew in feveral pieces. Even the infides of thofe pieces looked of a bafe dirty colour, like that of brafs or worfe, for the fragments had a far greater refemblance to bell-metal than either to gold or filver. One dram being carefully weighed out, and put on an excellent new and well-nealed cupel, with about half a dozen times its weight of lead; though it turned very well like good gold, yet it continued in the fire above an hour and a half, which was twice as long as was expedled, and yet almofl: to the very laft the fumes copioufly af- cended, which fufficiently argued the operation to have been well carried on ; and when at laft it was quite ended, the cupel was found very fmooth and entire, but tinged with a fine purplifh red; and, befides the refined gold, there lay upon the cavity of the cupel fome dark coloured recrements, which were concluded to have proceeded from the deteriorated metal, not from the lead. But when the gold was put again into the balance, it was found to weigh only about fifty-three grains, and confe- quently to have loft feven ; which yet was found to be fully made up by the recrements abovementioned, whofe weight and fixity, compared with their unpromifing ap- pearance, did not a little puzzle the ingenious author, efpecially becaufe he had not enough either of them, or of leifurc to examine their nature. The ill-looked mafs, before [ 209 ] before it was divided for the cupelling, was weighed in water, and inftead of weighing about nineteen times as much as a bulk of water equal to it, its proportion to that liquor was but that of fifteen and about two thirds to oncj fo that its fpecific gravity was lefs by about three and a third than it would if it had been pure gold. From this experiment the author concludes, that an ope- ration very near, if not altogether, as ftrange as that which is called projedlion, and in the dilliculteft points much of the fame nature with it, may be fafely ad- mitted : for the experiment plainly fhews that gold, though confeflcdly the mofc homogeneous and leafl mu- table of metals, may in a very fliort time be exceed- ingly changed, both as to malleablenefs, colour, homo- geneity, and what is more, fpecific gravity; and all this by an inconfiderable proportion of injedted powder, not amounting, on the modefteft eftimate, to a nine hundred and fiftieth part of its weight. He adds, that there is a ftill flranger eifeft of this admirable powder, v/hich he has not mentioned becaufe he muftnot do it. On this hiftory it may be remarked, i. That little de- pendance can be had on the conjedlure of the weight of the powder, as it might pofTibly not be all diftinguiflied on the paper it was wrapt in, and as different kinds of bodies have different weights under equal volumes. 2. If no miflake was made in weighing the metal after the fufion, the quantity of powder mufl have been greater than was imagined; for the lump of metal appeared to weigh as much as the gold employed, exclufive of the vitrified fubftance which the bottom of the crucible was overlaid with, and of the metallic globules that adhered to it. If thefe globules had been picked out, or feparated by pounding and wafhing the pieces of the crucible, and weighed with the lump, it is fcarcely to be fuppofed that E e the [ 2IO ] the author, Co minute in his details, would have omitted fuch a circumflancej and befides, repeated experience has convinced me, that when gold, from any admixture, has contraded a vitreous coat, and any fmall grains of it ftick about the crucible by means of this coat, the grains can- not by thefe methods be completely coUeded. 3. That the gold employed was fine, and that its fpecific gravity was nineteen, was only fuppofed. The author himfslf appears to have fufpedled its purity, for he fays that to remove fcruples on this head he caufed a dram and a half of it, which had been purpofely referved, to be melted in his aififtant's prefence, and found it fine and well-coloured i but furely the colour of the mafs cannot be admitted as a fufficient criterion of its purity. Indeed it could not have been entirely pure; becaule though it had been fo at firft, yet the procefs of parting, however carefully per- formed, would have left fome filver in it. 4. Admit- ting the gold to have been perfedly fine, and no error to have happened; it will not follow that the gold was degraded or altered in its nature, or that the experiment gives any countenance to the pretenfions of the alchemifls. Gold is ftrongly afl^eded, in many of its properties, by the admixture of very fmall proportions of certain other bodies : a quantity of tin, lefs even than that which the foregoing powder is fuppofed to have amounted to, renders <^old brittle. It is plain that at leaft fifty-three parts in lixty of the gold in the above experiment were in like manner debafed only by the coalition of another matter with it, and that this extraneous matter was feparable by fimple cupellation with lead. The hiftory affords no rea- fon to believe that the remainder of the gold might not alfo have been recovered, by fuitable management, from the ponderous fixt recrement: for gold has frequently been found debafed and difguifed by fubflances which reMed [ 211 ] refifted cupellation, and which have afterwards been fe- parated by other proceflcs. An inftance of this has been already mentioned in the ninth fedlion, and a more re- markable one is given by Romberg in the memoirs of the French academy for 1693 : a quantity of gold, after cupellation, quartation, fulion with antimony and the difflation of the antimonial metal, and repeated fufions with nitre, continued quite brittle though of a high colour : by treating it again with antimony and lead, and working off the fuperadded metals on a cupel,' it loft its colour alfo, and became grey, but by further repetitions of the fufion, both its colour and malleability were at length reftored. Juncker reports from Borrlchius and Ofiander, that on grinding for a length of time, in a glafs mortar, aa amalgam of one part of gold and four of mercury, with diftiiled water, there feparates daily a black matter, which may be colledied by pouring off the water and fuffering it to fettle : that after the triture has been con- tinued for Tome weeks, the water yields, on being evapo- rated, fome granules of a cryftalline fait: that the black powder yields, on fufion, a green glafs j and that the metal is thus refolved or deftroyed. But mercury alone by con- tinued triture or agitation, is changed into a fimilar powder, of which a part abides fixt in a confiderably ftrong firei: the vitrefication probabl-y fJrdceeded from fome particles abraded from the glafs mortar; and the faline matter, the proportion of which is acknowledged to have been very minute, either prsexifted in the water or was extraded . from the. glafs. Borrichius himfelf affords a flrong prefumption that the blaok powder pro- ceeds rather from the mercury than from tlic gold: after fome days, he obfcrves, the amalgam grows ftiff, and the reparation more fparing, and therefore frefh mer- ■ E e 2 cury [ 212 ] cLiry is direded to be added. I have continued the trituration of an amalgam of gold, alniofl inceflantly, for more than a week, and afterwards recovered the gold entire. Kunckel imagines, that when glafs is tinged red by Caffius's. precipitate or other preparations of gold, the particles of the metal are not barely diftufed through the glafs, but refoived into their elementary parts, fo as na longer to be reducible into gold again. He might indeed < fail of recovering the gold ; but though no means were known of feparating fo fmall a proportion of it from fo large a quantity of vitreous matter, it furely could not follow that the gold was deflroyed. Some other procefles, propofed for the deftrudion of gold, have been already taken notice of. It h?s been {hewn, that the vehement heat, coUedled in the focus of large burning glaiies, and the long continued adion of a' gentle heat, do not, fo far as can be judged from thefadls as yet known, make any real change in it ^ and that the much boafled volatilization of gold does certainly not deftroy it, fmce the volatilized gold may with eafe be reflored to its fixity and all its former qualities. Upon the whole, both the producibility and deftru6li- bility of gold continue flijl problems in chemiftry. I know of no experiment from which the poflibility of either can be inferred; and to demonftrate their impoffi- bility is beyond the reach of experiments. But though thofe, who have laboured the moft ardently in thefe purfuits, inftead of acquiring, have generallv exhaufted riches, and may on good grounds be prefum.ed to have miffed of their principal aim ; yet juftice requires us to add, that their labours have not been altogether ufclefs, and that many valuable difcoveries, relative to different fubjeits, have refulted frgm their enquiries. It is. [ 213 ] is to be regretted, that their affedkd myfterioufnefs, and peculiar mode of philofophizing, have rendered their writings fo forbidding, that many ufeful fails, fcattered through them, lie ftill unknown. SECT. XIII. Imitations of gold. I. Gold coloured tnetal. FROM the general eflimation of the colour of gold, which has attracted the notice of the moft barbarous nations, the communication of the fame admired colour to metals of low price, for ufes where the other qualities' of gold are not required, becomes an important objedl. The produdlion of a kindred colour, by artificial compofition, in the common metal brafs, affords a principle for this imitation. Erafs is prepared by melting copper with the mineral called calamine, from which it receives an increafe of one third or one half its weight. The matter, which the copper imbibes from the calamine, is found to be zinc, of which that mineral is properly an ore; and accordingly zinc itfelf, melted with copper, communicates a colour of the fame kind. According to the purity of the zinc and copper, the proportions in which they are mixed, and the intimacy of their union, the compound metal proves more or lefs malleable, and approaches more or lefs to the colour of gold. Thofe who have given receits for making a gold coloured metal, differ greatly in the proportion of the two in- gredients; fome diredling the zinc to be taken only in a fifth or a fixth part of the weight of the copper, and others in an equal weight or more. From a fet of ex- periments. [ 214 ] periments made to determine this point It appeared, that there is fome foundation on both fides ; and that, both ■with the fmalleft and the largeft of thefe quantities of zinc, the metal proves more like gold than with the intermediate proportions. One part of zinc and three of copper formed a com- pound of a brafTy yellow colour, but rather brighter than common brafs, and at the fame time more brittle: when broken, its texture appeared partly fibrous, and partly grained. On increafing the quantity of copper to four, five, fix, eight, and ten times the weight of the zinc, the metal proved more and more tough, of a fine grain without any appearance of fibres, and its yellownefs more and more mixed with a reddifh caft, like that of gold alloyed with copper. The beft coloured mixture, obtained by an augmentation of the copper, confifled of five parts of copper to one of zinc : even this, however, differed ereatly in colour from fine gold. A mixture of ten parts of copper and one of zinc looked like gold fomewhat worfe than ftandard, and hammered extremely well. On diminifhing the proportion of copper, from thrice, to double, equal, and two thirds of that of the zinc, the colour was improved much more than by an augmentation of it, the compounds proved much more brittle, and wholly of a fibrous texture, without any , appearance of grains : they broke over fhort on trying to bend them, and fell in pieces under the hammer. Equal parts of copper and zinc, or a little more zinc than copper, feemed to produce the finefl colour : thefe mixtures, in the mafs, had a near refemblance to pure gold ; though flrokes drawn with them on a touchftone were remarkably paler, looking alaiofi: white when compared with thofe of the gold. And indeed all the compufitions I have examined, whether r 215 ] whether made by myfelf or others, how nearly foever they approached to gold in the mafs, were very different from it on the touchltone. The colour of thsfe compounds is improved by a fmall admixture of certain other metallic bodies. Cramer ob- ferves, that when copper is melted with a fourth or a fixtli of zinc, and a little pure tin ; the compound metal, well cleaned and laid in the air for fome days, acquires on the furface the colour of fine gold : this teint, though merely fuperficial, is not the lefs valuable; for though it fliould be dilcharged by cleaning, the piece foon recovers it again, avery frefh furface tarniiliing as it were to a like colour. Geoffroy relates, in the French memoirs for the year 1725, that on trying different metals, iron feemed to have the beft effed : equal parts of copper and zinc beino- brought into fuiion, he threw in fome iron filings, amounting to an eighth part of each of the other metals : the mixt turned out of a beautiful yellow colour, and a fine fmooth grain, not at all fibrous, as mixtures of copper and zinc in this proportion by themfelves always are, yet very brittle : on repeating the experiment with a fourth more of zinc, the proportions being ten parts of zinc, eight of copper, and one of iron filings, the metal proved of a grain like the former, but more compadr, harder, brighter, and in colour flill more like gold. He fays the conimixture of the iron with the other ingredients requires a particular management, which I cannot find that he has any where communicated. It has been faid, that the mixtures of zinc and copper may be made tough, by injedting upon them in fufion a little mercury-fublimate, as alfo by nitre, fal ammoniac, borax, and different kinds of inflammable bodies : but thefe additions, as I have often found from ex'perience, and as Pott alfo obferves in a difiTertation de zincoy, 4 will [ 2l6 ] will not anfwer. The great brittlenefs has been generally imputed to the lead, of which the common forts of zinc are fuppofed to partake in a greater or lefs degree; and hence it has been fuppofed neceffary to previoufly purify the zinc, by cementation andfulion with fulphur, which abforbs and fcorifies the lead without adting upon the zinc : fome forts of zinc may cjoubtlefs require a tteatment of this kind, but fuch, as has been ufually brought from the Eafl: Indies, does not feem, when prepared in this manner, to give lefs brittlenefs than when unprepared. It is certain, however, that copper impregnated with zinc, by cementation and fufion with calamine, prove* more malleable, than when melted dired;ly with as much of common zinc itfelf as it imbibes from the calamine j on account, perhaps, of the commixture being in the firfl: cafe more equal and perfedt. By the procefs with calamine, copper cannot eafily be made to receive the full quantity of zinc neceffary for producing a good gold colour : by combining the two procefles together, that is by making the copper firfl: into brafs, and then melting it with a fuitable quantity of zinc, a metal may be obtained of a belter quality than by either method fingly. A very ingenious artift, who now prepares a gold- coloured metal in great perfedtion, has a fine kind of brafs made on purpofe for this ufe. An enquiry into the preparation and improvement of brafs will make a fbparate article in one of the future numbers of this work. A good deal of addrefs is requifite in melting the copper and zinc together; for the heat neceffary for the fufion of copper occafions the zinc to burn and flame, and a con- fiderable part of it to be diffipated, fo that the remaining copper is defrauded of its due proportion. If the two metals are put into the crucible at firfl:, and the fire gradually raifed, greatell: part of the zinc v/ill be burnt off before [ 217 ] before the copper melts: if the copper be firfl melted by itfelf, and the zinc heated and plunged into it, a flrong commotion enfues, though the difllpation is much lefs confiderable than in the other cafe, the zinc being quickly imbibed by the melted copper and in fome meafure protedled and retained by it : if the copper and zinc be brought feparately into fufion, and one poured into the other, anexplofion happens, and great part of the mixture, in my experiment above two thirds of it, is thrown about in drops, to the great danger of the bye-flander. The union appeared to fucceed befl, and with leaft lofs of the zinc, when fluxes, containing inflammable matter, were added: I have generally uled a mixture of black flux and borax ; to which may be fubftituted a cheaper compo- fition of twelve parts of green glafs in fine powder, fix parts of potafh, two of borax, and one of powdered char- coal. The flux is firfl: to be brought into fufion in the crucible, and the copper and zinc dropt into it : as foon as thefe appear perfedlly melted, they are to be well flirred together with an iron rod, and expeditioufly poured out. The fame flux ferves repeatedly for the melting of feveral frefli quantities of the metal. There are many receipts for making a gold coloured metal, from verdegris a preparation of copper, and tutty a preparation or ore of zinc: the diff'erence, above taken notice of, in the effedt of zinc itfelf and of its common ore upon copper, induced me to try, whether, in this form ot combination alfo, fome ufeful variation might not happen. One of the befl: of thefe receipts feems to be that among Hooke's papers publiflied by Derham; in which eight parts of diftilled verdegris (that is, verdegris purified by iblution in difl:illed vinegar and cryfl:allization) and four parts of Alexandrian tutty, with two of nitre and one of borax, are direded to be mixed with oil to the confifl:ence F f of [ 2l8 3 of pap, then melted In a crucible,andpouredinto a flat mould firll well warmed. The perfon, who communicated this receipt to Dr. Hooke, fays that the metal will not only appear, but work, like coarfe gold ; that he fold it as dear as filver ; and that the king of Poland had a fervice of it, only mixing fifteen ounces of gold with a hundred of the compound metal. I tried this procefs with verdegris, which I had myfelf purified, by diflblving it in diftilled vinegar, and evaporating the filtered folution to drynefs : a large proportion of the verdegris remained undiflxilved ; and this refiduum,on being melted with black flux, yielded a brittle pale coloured metal almofl like bell-metal : from whence it might be prefumed, that the copper, in the infpiflated matter, was rendered purer than ordinary by the fepa- ration of this extraneous metal. On melting it with ehoice tutty, and the other ingredients, the refult was a very fine metal, which bore the hammer well ; but it was rather a fine brafs than a true gold coloured metal, its colour having lefs refemblance to gold than that of the mixture of equal parts of common copper and zinc already mentioned. Tutty and calamine contain zinc in a flate of calx j, and hence, in the ufe of thefe, inflammable additions are eflTentJally- neceflary, for reviving the zinc into its me- tallic f^rm. Some of the earlier writers diredt for this purpofe fubftances of a yellow colour, as turmeric, rhu- barb, faffron, aloes, which are ftill ufed, as I am inform- ed, by feveral workmen, who do not feem to have con- fidered, that thefe kinds of fubftances can be of fervice no otherwife than as they furnifh an inflammable matter, and that common charcoal anfwers the fame end. Two ways have been recommended, for giving a gold colour to copper, and at the fame time preferving its malleability, without the addition of any zinc, or of fubftances C 219 ] fubftances containing it; the one, as is fald, by Hom- berg (for, though given as from him by fome late repu- table writers, I cannot find it among his papers in the French memoirs) the other by Vigani. In Romberg's method, the copper is to be amalgama- ted with pure quickfilver, the amalgam boiled in river water for two hours, the quickfilver diftilled off in a re- tort, and again poured back and abftradted once or twice : the remaining copper, being now fufed, is find to appear of a beautiful gold colour, and to be more dudlile thaa common copper, fo as to' be well fitted for the finer machines and uteniils. The great difhculty of amalga- mating copper by the common methods feems to have prevented this procefs from coming to a fair examination. This difficulty I have furmounted in different ways : one of the eafieft and moft expeditious of which was, by diffolving the copper in aqua fortis, and, when the menflruum would take up no more, pouring the folutioa into an iron mortar, along with fix times as much quick- filver as there was of copper, and fome common fait, and then grinding them well together with an iron peflle ; the diffolved copper is extricated from the acid by the iron, in a very fubtile form, and falling in this flate intoi the quickfilver, is readily imbibed by it. This amalgam was ground and wafhed with water till it became per- fedlly bright, ajid the mercury was then diffilled off: the remaining copper, melted in a crucible, had, as was indeed expeiSed, no degree of yellownefs, and appeared exadlly of the fame colour as at firft. As no fenfible alteration was thus produced, a repetition of the trouble- forae operation was judged unneceffary. Vigani's procefs carries with it flrong marks either of error or referve; yet from the general charader of the author, and the favourable reception he met with in this F f 2 country. 1 [ 220 ] country, I fliould not perhaps be held excufed if I did not take fome notice of it. Copper is to be melted in a crucible, an equal weight of powdered fulphur fprinkled en it, the fufion continued till the fulphur is all burnt off, and the metal afterwards flatted into plates. A quantity oforpiment, auripigmentum, is to be melted and quenched in vinegar, and the fufion and extindlion repeated twenty- four times. The materials being thus prepared, fome bean meal is to be placed in the bottom of a crucible, above this nitre and tartar, then fome auripigmentum, on this fome of the copper plates with more auripigmentum over them: in this order of Gratification we are to proceed till the veflel is full, and then to invert into the mouth another crucible having a hole in its bottom. A moderate heat is to be continued fo long as any flame or fumes appear, after which the fire is to be raifed fo as to bring^ the matter into fufion, and continued in this ftate for an hour. It is not to beexpedled that this procefs can afford the dudliie gold coloured metal which the author promifes from it; fororpiment, in virtue of the arfenic of which it largely partakes, tinges copper, not yellow, but white. As Vigani throws a veil over fome of his preparations, though commonly but a thin one, I have been led to fuf- pe^,der, it is well mixe4 ; with half or a third its weight of fal ammoniac and the fame quantity of flowers of fulphur. With regard to the proportion of thefe ingredients, pradical writers differ not a little, and indeed they admit of great latitude, for I have j fucceeded equally with very different proportions : very little of any of them is retained by the tin in the fubfe- quent part of the. operation. The powder is put into a matras, or round glafs \yith a ihort neck, which is placed in [ 223 ] aiand-batli, and the fire increafcd by degrees, fo as to keep the land at lalfl red hot for fome time. 1 he fire being then fuiFered to decay, and the veflel broken when cold, a faline matter, con fifting chiefly of fal ammoniac, is found in its upper part : under this is a dark red mafs, wliich proves to be cinnabar, or a combination of mercury and lulphur : at the bottom is the aurum mofaicum, a fparkling, gold coloured, flaky mafs, weighing about a twelfth part more than the tin employed. The gold coloured talcs, formerly mentioned, have too much flexibility and elafticity to be reduced into powder of fuflicient iinenefs for the purpofes of painting: but there is one imitation of gold, for which powders of much fine* nefs are not required, and for which the talcs are better adapted than any other material I know of, on account of their refiftance to fire. A kind of glafs,- with gold coloured fpangles diffufed through its fubftance, has been much admired, and the preparation of it kept afecret : this ap- pearance may be communicated by the yellow talcs, by mixing them well with powdered glafs and bringing the mixture into fufion. III. Gold coloured varnijlo or lacker. Silver, coated with a tranfparent gold coloured varnifh, is made to refemblegold fo exadly, as wholly to fuppjy the placcof gold infome of the works called gilt. The bafis of the varnifh, or what gives adhefivenefs and glofiinefs to the colouring matter, isafolution of lac made in fpirit of wine. Lac or lacca is a fubftance coIled:ed by certain infeds in the Eaft Indies : it is found incruftated on fticks or branches of trees, in brittle mafi^es of a dark red colour, which being reduced into fmall grains, and freed from part of the colouring matter by infufion in water, are fold under the name of feed lac. It is in this ftate that the lac 4 is- [ 224 ] is to be ufed for varniflies : what is called fliell lac, or the grains formed into plates by melting them in boiling water, docs not anfwer fo well. The fpirit muft be highly redlified, or freed as much as poflible from any admixture of phlegm or water, for other- wife it will not dilTolve the lac. The nioft convenient and expeditious way, of preparing the fpirit for this intention, is by adding fome dry potafli or other fixt alkaline fait : the fait imbibes, and diffolves in, the watery part of the fpirit, and forms therewith a diftinft fluid at the bottom, from which the fpirituous part on the top may be poured off. More or lefs of the fait will be required according as the fpirit is more or lefs phlegmatic : if the firft quantity, after fianding for fome hours and occafionally fhaking the veffel, wholly diffolves, more muft be added and the agitation renewed. The fpirit being thus dephlegmed, fome feed lac, reduced into fine powder, is added to it, in the proportion of about three ounces to a pint : the veflel being fet in a moderate warmth for twenty-four hours and frequently fhaken, a part of the lac diflblvesj and the fpirit, now tinged of a reddifli brown colour, is ftrained off from the undiffolved part, and fet by for a day or two to fettle. The digeflion ihould be performed in a wide mouthed veffel, covered fo as to prevent the exhalation of the fpirit : the undiffolved lac foftens into a vifcous mafs, fo as fcarce to be got out through a narrow aperture. In different portions of the foregoing folution, poured off clear after the ftraining and fettling, fome gamboge and annotto are diffolved feparately. Gamboge is a yellov/ juice, iffuing from certain trees in the Eaft Indies, and exficcated into maffes by the fun's heat : Annotto is arti- ficially prepared from the red fkins of the feeds of an Ame- rican tree, by fteeping and agitating the feeds with water till 4' [ 225 ] till their colouring matter is transferred into the liquor : on boiling the flrained liquor, the colouring matter is faid to be thrown up to the furface in form of fcum, which is afterwards exficcated by itfelf, and formed into mafles, which, as brought to us, are moderately hard and dry, of a brown colour on the oufide, and a dull red within. Both thefe fubftances dilfolve very readily in the fpirit : the gam- boge communicates a high yellow colour, and the annotto, a deep reddifh yellow. The folution of the gamboge is mixed witii about half its quantity of that of the annotto, and trial made of the mixture on fome filver leaf: if the colour inclines too much to the yellow or the red, more of the one or the other liquor is added, till the true golden colour is obtained. There are fundry other materials, from a due mixture of which a like colour may be produced, as turmeric, fiffi'on, dragons-blood, &c. The filver leaf being fixed on the fubjecfl, in the fame manner as gold lea^V by the interpofition of proper gluti- nous matters ; the varnifli is fpread upon the piece with a brulh or pencil. The firft coat being dry, the piece is a^ain and a^ain waflied over with the varnifh till the colour appears fufliciently deep. What is called gilt leather, and many pidlure frames, have no other than this counterfeit gilding. Wafhing them with a little rediified fpirit of wine affords a proof of this ; the fpirit diffolving the varnifli, and leaving the filver leaf of its own whitenefs. For plain frames, thick tin foil may be ufed inflead of filver. The tin leaf, fixed on the piece with glue, is to be burnifhed, then poliflied with emery and a fine linen cloth, and afterwards with putty applied in the fame manner : being then lackered over with the varnifli five or fix times, it looks nearly like burniflied gold. The fame varnifli, made with a lefs proportion of the colouring materials, is applied alfo on works of brafs ; both G g for [ 226 ] for heightening the colour of the metal to a refemblancCi with that of gold, and for preferving it from being tar- nifhed or corroded by the air. Addition to the History of GOLD; SINCE the foregoing flieets went to the prefs, a new manufailure has been fet on foot in London, for embellilliing linen with flowers and other ornaments of o-old leaf. The linen looks whiter than moft of the printed linens ; the gold is extremely beautiful, and is faid to bear washing well. I have fee n a piece, which I was credibly informed had been waQied three or four times, with only the fame precaution* .as are ufed for the finer printed linens, and on which the gold continued entire and of great beauty. The Venetians have carried on a large trade, to the Levant, in a kind of brocade called damafquete, which, though it has only about half the quantity of gold or filver as that rhade among us, looks far more beautiful. The flatted wire is neither wound clofe together on the filk threads, nor the threads ftruck clofe in the weaving ; yet, by pafling the ftuff betwixt rolls, the difpofition and management of which is kept a fecret, the tifTue or flower is made to appear one entire brilliant plate of gold or filver. The French miniftry, ever vigilant for the ad- vancement of arts and commerce, judged this manufac- ture important enough to deferve their attention ; and accordingly, for contriving the machinery, they engaged the ingenious M. Vaucanfon, known throughout Europe for [ 227 ] for his curious pieces of mechanifm, who, in tlve me- moirs of the academy for the year 1757, lately printed, gives an account of his fuccefs, and of the cflablHhment of fuch a manufadlure at Lyons. The lower roll is made of wood, thirty-two inches ia length and fourteen in diameter; the upper one of copper, thirty-fix inches long and eight in diameter: this laft is hollow, and open at one end, for iniroducing iron heaters. For making the rolls cylindiical, he lias a particular kind of lathe, wherein the cutting tool, which the moll dex- trous hand could not guide in a flreight line through fuch a length as thirty-fix inches, is made to Aide, by means of jifcrcw, on two large ftcel rulers, perfedly flreight, and capable ot being moved at pleafure, nearer, and always txadly p:;rallel, to the axis of the roll. He firft difpofed the rolls nearly as in the common flatting mill. In this difpofition, ten men were fcarcely fufficient for turning them with force enough to duly extend the gilding; and the collars, in which the axes of the rolls turned at each end, wore or gulled fo fad, that the prelTure continually diminifhed, infomuch that a piece of fluff of ten ells had the gilding fenfibly lefs ex- tended on thelafl part than on the firfl. He endeavoured to obviate this inconvenience by fcrewing the rolls clofer and clofer in proportion as the fluff pafled through, or as the .wearing of the collars occafioned more play between them; but this method produced an imperfedlion in the fluff, every turn of the fcrew making a fenfible bar acrofs it. To leffen the attrition, each end of the axes, inllead of a collar, was made to turn between three iron cylin- ders called fridion- wheels : but even this did not anfwer fully, for now another fource of unequal preffure w;as dif- covered. The wooden roll, being compreflible, had its diameter fenfibly diminilhed : it likewife loft its round- G g 2 nefSg [ 228 3 nefs, Co that the preffure varied in different points of its revolution. On trying different kinds both of European and Indian woods, all the hard ones fplit, the foft ones warped without fplitting, and, of more than twenty rolls^ there was not one which continued round for twenty- four hours even without being worked in the machine. Thefe failures put him upon contriving another method of preiTing the rolls together, {o that the force fhould always accommodate itfelf to whatever inequalities might happen. The axis of the copper roll being made to turn between fridlion wheels as before, that of the wooden one is prefled upwards by a lever at each end furnilhed with a half collar for receiving the end of the axis. Each lever has the end of its fhort arm fupported on the frame of the machine, and the long arm is drawn upwards by an iron rod communicating with the end of the fhort arm of another lever placed horizontally : to the long arm of this lafl lever is hung a weight, and the levers are fo propor- tioned, that a weight of thirty pounds preffes the rolls together with a force equivalent to 17536 pounds, which was found to be the proper force for the fufficient exten- fion of the gilding. By this contrivance four men can turn the rolls with more eafe than ten can turn thofe •which are kept together byfcrews; and the fame weight adling uniformly in every part, the prefTure continues always equal, though the wooden roll fhould even become oval, and though the ftufF be of unequal thicknefs. A piece of cloth,, of about two ells, is fowed to the beginning and end of the fluff, to keep it out to its width when it enters and parts from the rolls, which could not be done by the hands for fear of burning or bruifmg them ; as it would take too much time to fow thefe cloths to every fmall piece of an ell or two, a number of thefe is fowed together. The fluff is rolled upon a cylinder, which is ■'■* [ 229 ] IS placed behind the machine, and its axis prefTed down by fprings to keep the fluff tight as it comes off. Four iron bars, made red hot, are introduced into the copper roll, which in half an hour acquires the proper degree of heat, or nearly fuch a one as is ufed for the ironing of linen : the wooden roll is then laid in its place, and the machine fet to work. If more than thirty ells are to be paffed at once, the wooden roll muft: be changed for ano- ther, for it will not bear a longer continuance of the heat without danger of fplitting, and therefore the manufadlu* rer ftiould be provided with feveral of thefe rolls, that when one is removed, another may be ready to fupply its room : as foon as taken off from the machine, it fhould be wrapt in a cloth and laid in a moid place. The principal inconvenience, attending the ufe of this machine, is, that the heat neceflary for extending the gilding, though it improves the brightnefs of white and yellow lilks, is injurious to fome colours,, as crimfon and green. A double preflure will not fupply the place of heat J and the only method of preventing this injury, or rendering it as flight as pofTible, appeared to be, to pafs the duff through with great celerity. iir, E X- III. EXPERIMENTS of the converlion of Glass Vessels into Porcelain, and for eftablifning the prin- ciples of the art. HAVING many years ago diftilled fome wood foot, with a ftrong fire, in a green glafs retort fet in fand, I obferved great part of the bottom of the retort, after the operation, to be remarkably changed : it was quite opake, of a black colour on the infide where the foot had been in conta£t with it, and whitifh on the outfide where it refled upon the fand : it had no longer the brittlenefs of glafs, but broke with difficulty like the better kinds of flone ware : its internal fubftance was white like porcelain; and not of aglafly fmoothnefs, but of a fine fibrous texture. This fingular change, in a body fuppofed fo little fuf- ceptible of alteration, was attributed to the vapours of the foot having penetrated into the fubftance of the glafs : fundry pieces of the fame kind of glafs were therefore intermixed with another quantity of foot, in an iron pot, to which was adapted a head with a receiver, and the diftillation conduded in the ufual manner, till nothing more could be forced out from the foot in a ftrong fire : on examining the pieces of glafs, fome, in the middle of the matter, feemed fcarcely at all altered ; others, about the fides and bottom of the pot, were changed in part nearly in the fame manner as the bottom of the retort had been. 4 This C 231 ] . This change of glafs has doubtlefs happened often, without being attended to. Neumann is the firfl: vviiter by "whom I find it taken notice of: in diflilling milk in a glafs retort, he obferved that the bottom of the vefTel acquired the appearance of porcelain, which he attributes to the fine white earthy matter of the milk forced into the glafs by the heat. M. de Reaumur was led to the fame difcovery by analo- gical reafoning, as I have already mentioned in the notes on Neumann's chemical works, where a further account of this affair is promifed. Reaumur, having had large experience of the effedls of inflammable and earthy bodies on iron, by baking, in the converfion of forged iron into fteel, and in the foftening of cafi: iron, applied the fame procefs to common glafs, and thus difcovered the new porcelain, which he calls porcelain by tranfmutation, porcelain by revivification, or porcelain of glafs. The glafs was cemented, or baked, in crucibles, firft with the foot, powdered charcoal^ and other fubflances employed in the experiments on iron : it became opake, externally dark-coloured or black, but internally of a fine white colour. Other materials were then made trial of, in hopes that fomc one might be found, which fliould occafion the furface to be as white as the internal part : among the fubftances tried in this view, of which he gives no paiti- cular account, he judged white fand and plalter-of-paris, or rather a mixture of the two, to anfwer the beft. He diredts vefiels of common green glafs to be filled and fur- rounded with this mixture, in large crucibles or cement- ing pots, fuch as are commonly ufed for the baking of earthen wares; the crucibles to be covered and luted, and fet in a potter's furnace : the fame fire, which bakes the common wares, changes the glafs vefiels intovefit:Is of the new porcelain. Heobferves, that this porcelain may- be [ 232 ] be made at a very cheap rate, as the glafs maker can form veflels more expeditioufly than the potter, and as it is happily the very coarfefl green glafs that yields the linefl porcelain : That it is eafily diftinguifhed from all the other forts of porcelain by the texture which it exhibits on breaking, as it has nothing of the granulated appear- ance of the other porcelains and earthen wares, any more than the gloffy fmoothnefs of glafs and enamels, the fur- face of the fradure being compofed of fine fibres like filken threads : That in beauty it is inferiour to the Chi- nefe, but equal to many forts that are held in efleem ; that in utility, and every effential quality of porcelain, it is equal to the beft, and that in fome refpedls it is fupe- riour to all that have hitherto been made : That it fuffers no injury from being fuddenly heated or cooled, bears a vehement fire without melting or altering its figure, and hence, befides its ufe for ornamental vafes, promifes to make excellent vefTels for the chemill". The charader given of this porcelain by Reaumur, and the valuable qualities he afcribes to it, rendered it an objedl of more importance than it had appeared at firft, and engaged me in a further examination of it. That the enquiry might be carried on with fome regularity, it was divided into five heads, (i) To trace the gradual pro- grefs of the change from the ftate of glafs to that of perfect porcelain, and to difcover whether a continuance of the procefs would be produdive of any further changes. (2.) To determine the qualities of this kind of porcelain, and how it differs from other porcelains and from glafs, in thofe properties which regard the application of it to common ufes. (3.) To compare the eftedts of different cementing materials on the fame glafs, and (4.) the effedls of the fame materials on different kinds of glafs. (5) To afcertain, as far as might be, the caufe of the change, or the [233 ] the true principle on which it happens. The following is the general refult of the experiments I have hitherto made upon thefe fubjedls. SECT. I. Experiments of the fuccejfive changes produced in Green Glafs by baking. IN order to determine the progrefs of the vifible change produced in glafs by baking, and theeffedls of different degrees or a different continuance of the heat ; a num- ber of pieces of common quart bottles were furrounded with white fand, in crucibles, which were placed in a wind-furnace, built on purpofe for experiments of this kind, confifting of feveral chambers one over another, with proper apertures in the middle for the afcent of the flame and heated air through each. The crucibles were left open, that fome of the pieces might be taken out from time to time, for difcovering how the change went on : and that the effcdls of the procefs might be feen in its full extent, the fire was flowly raifed, and continued for upwards of forty hours. Such pieces as were taken out before they became red hot, did not appear to have fuffered any change, though they had been kept for feveral hours in a heat very little below ignition. In a low red heat the change did indeed take place, but exceeding flowly; thofe which had been expofed for feveral hours to fuch a heat being very little altered. In a ftrong red heat, ap- proaching to whitenefs, jufl: not fufficient to make the glafs melt, the change went on pretty faft : after an hour's continuance of this degree of heat, the glafs had ac- quired the appearance of porcelain toaconfiderable thick- nefs ; and in two hours longer, the thickeft pieces, of the H h bottoms [ 234 ] bottoms of the bottles, were found fully changed through- out. In thofe pieces which were flowly affedted by a weak heat, and thofe which were more hafhily aded upon by a moderately iliong one, the progrefs of the change itfelf v/as, for the moft part, nearly in the fame manner. The green glafs became firft of a bluifh colour on the furface, and in this flate, when held between the eye and the light, it appeared lefs tran (parent than before, and of a yellowiih hue. After this it was found changed a little way on both fides into a white fubftance, exter- nally flill bluith ; and as this change advanced further and further within the glafs, the vitreous part in the mid- dle approached more and more to yellow : the white coat was of a fine fibrous texture, and the fibres difpofed, nearly parallel to one another, not longitudinally as might be ex- pedled from the diredlion given to the parts of the glafs in blowing it into veffels, but croflwife to the thicknefs of the piece. By degrees, the glafs became throughout white and fibrous, the external bluiflinefs at the fame time going ofi'", and being fucceeded by a dull whitifh or dun colour : the fibres were for the moft part regularly and uninterruptedly arranged from each fide to the middle, where the fibres from the two fides, meeting together, formed a kind of partition : along this jun6lure, there were in fome pieces confiderable cavities here and there; others were. perfectly folid. juThe pieces which were continued in the fire for any confiderable time beyond this period, and thofe which ■ were afterwards returned to it along with frefh fand, fuf- fered a frefii change, which proceeded, like the firft, from the furfice to the center. The fibres became divided or cut into grains at the outer ends, and by degrees they ■were thus fuccefllvely divided through their whole length ; the whole internal part of the porcelain afluming a gra- nulated [ 235 I nulated texture, not ill refcmbling that of the commoti porcelains. Thofe which were longer and longer expofed to the fire, received more and more alterations. The grains, at firfl fine and of fome degree of glollinefs, grew larger and duller; and at length, through fundry gradations, lefs re- markable and lefs uniform, wliat had once been glafs, and afterwards a compadl hard porcelain, became a porous friable fubllance, like a mafs of white fand ilightly cohering. During the change of the glafs into a fibrous porcelain, it generally preferved the fmoothnefs of its furface, and the fand freely parted from it : in the fubfequent changes, part of the fand baked together upon the furface, and ilrongly adhered, not to be got off, and not greatly differ- ing from it in appearance; I have fometimes been at a lofs to diflinguilh the matter which had been glafs from the fand that furrounded it. In ibme pieces the fandy coat was parted from the inter.-ial matter by a number of fine cavities refembling a dotted line : in others, they were clofely applied together. Such were the general effecfls of continued cementation in many repetitions of the experiment, though not with- out variations in fome particulars. Sundry pieces became throughout white, and almofl: opake, and fome blue, before they contrafted any fibrous coat, which afterwards proceeded in the fame manner as in the others. Some pieces, being broken in different parts after they had been changed to a fibrous ftate, inftead of the uniform tranf- verfe difpofition ot the fibres, had feveral prominences on the furfaces of the fradlures, from which the fibres iffued as rays in all diredlions. After the fine granulated ffate, which fucceeded to the fibrous on a continuance of the ce- mentation, fome pieces becftme porous, while in others the H h 2 grains [ 236 ] orains formed a kind of clofe plates, and the mafs proved very compadt. In fotne, the texture was clofe and even throughout, without any diftinguifhable grains, fibres, or plates. Of fonie of thefe variations, the probable caufes will appear in the following fedion : others depended per- haps upon the nature of the glafs employed. Notwithftanding thele, and other lefler differences, the general appearances, and the gradation of the vifible change proportionably to the degree of baking, are fo ftrongly marked, that, from the texture of the porcelain on breaking, we can always judge with certainty of its quality, or of the degree to which it has been baked. Reaumur has alfo taken notice of fomething of this kind, finding the porcelain fometimes turn out granulated in- ftead of fibrous: but his experiments do not feem to have been carried far enough to difcover the foundation of this difference j to difcover, that the different kinds of texture regularly fucceed one ^another from the continued adtion of one caufe, that they are all at the command of the workman, and that they are accompanied with remark- able differences in the intrinfic qualities of the porcelain, " '"'^ " SECT. II. Experiments of the quality of the Jubjlance into which Green Glajs is con'verted by baking. f\~^ H E porcelain into which glafs is converted by bak- J^ ing, whatever its fuperficial colour be, is, in its inter- nal fubftance, always white ; and its whitenefs is frequently aiot inferiour to that of the internal part of China ware. Its furface is unhappily the part which is leafl: beautiful. All the thick pieces were quite opake : feveral thin ones had a degree of tranfparency, refembling that of China ware. Jn this refpeft confiderable differences appeared : a very thin [ 237 ] thin coat of fibrous or granulated porcelain upon the glafs gave opacity j of the pieces of a clofe fmooth texture, fome, tho' pretty thick, were femitranfparent, and others, tho' thinner, were opake. In the fibrous ftate of the porcelaine, it is confiderably hard; much more fo than the glafs it was made from, and than any of the common kinds of porcelain. It freely and plentifully ftrikes fire with fteel, which green glafs does but in a low degree. It cuts common glafs, as indeed one piece of glafs in fome meafure will another j but nei- ther any of the common kinds of glafs, nor the file which cuts them, make any mark on the fibrous porcelain. Even when the change is fcarcely vifible on the furface of the glafs, the external part is found fenfibly harder to the file than the internal. It perfectly refifts both acid and alkaline liquors, neither permitting them to tranfude through it, nor being at all corroded by them. It bears vicifiitudes of confiderable degrees of heat and cold, fo that veffels of it may be plunged at once, without any danger of their cracking, from freezing into boiling water. It may be likewife fet on burning coals, with much lefs precaution than any of the porcelains or earthen wares ufed for containing liquids. In a moderate white heat, it melts, fo as to be eafily drawn out into long (lender firings, which appear femitranf- parent, and, on breaking, prove not fibrous as before, but of a vitreous fmoothnefs like white enamel. Some of the melted pieces were confiderably bright or gloffy, fome had no gloffinefs, and all of them proved fofter than before the fufion, feeming, though very compad:, to be little^ harder than common green glafs. It does not however melt near fo eafily as the glafs itfelf. When the cementation has been continued no longer than till [ 238 ] till only the external part of theglafs was changed, I have often obferved, that on haftily raifing the fire, the un- chano-ed glafs has melted and run out, leaving a cavity in the middle with a cruft of porcelain on each fide. When the porcelain has been fo far baked, that the fibrous texture has difappeared, and a coarfe granulated one come in its place, it proves far fofter than before : it now neither ftrikes fire with fteel, nor cuts glafs j but is itfelf cut with eafe both by common glafs and by the file. With this imperfedlion, it acquires an advantage of greater refiftance to fire : the longer the cementation was continued, the fufibility feemed always to be more and more diminiflied. A piece of the concave bottom of a common green bottle, which had its fibres only in part changed into grains, ftood the melting of a lump of brafs, of about two ounces, without anywife altering its figure, or fufixring any other apparent change, than that the thin edges were rounded off, and covered in fome parts with a green glazing, which feemed to have exuded from the mais. Some pieces of the fame cemented glafs being put into a fmall crucible, into which another was inverted and clofely luted, and the whole urged for two hours or more in a fca-coal fire vehemently excited by bellows, the pieces melted together into a very fpongy mafs, of an almoft pearly whitenefs and .fome brightnefs, intermixed in different parts with a green glafs, exad:ly refembling the glafs employed, and which probably was no other than a part of it, that had efcaped unchanged in the cementation, though not diftinguilhable by tlie eye, till thus fpued out from the lefs fufible porcelain, and col- lected in its cavities. Pieces which had acquired through- out a fine bright grain, were likewife in an intenfe fire, made to melt or foften into lumps, which generally proved 4 Spongy : [ 239 ] ipongy: but thofe, in which a large coarfe grain had fuccceded to the fine one, could Icarccly be made to fof- ten at all, whether expofed to tne fire in crucibles, or in contad: with a burning fuel, Thefe unfufible pieces, though a continuance of the baking with a moderate heat would have rendered them more and more porous and friable, on being haflily urged with an intenfe fire became remarkably more compad than they were before ; fome of them feenied fuperior in folidity to any kind of ware I know of. It may hence be prefumed that the fmooth texture ob- ferved in feveral pieces after the baking, fo clofe and compact that neither grains nor fibres can be dillinquiflied, proceeds from their having undergone a greater degree of fire than the others. Several of the pieces, which ac- quired this appearance in the baking, had in part begun to melt, and others had not: perhaps the former received their compadtnefs from an augmentation of the heat, in the earlier period of the cementation, and the latter in the fubfequent ftages when their fufibility had been greatly diminiflied -, and probably the femitranfparency of fome of the compad: pieces, and the perfed opacity of others, proceeded from the fame caufe. It is plain that the refiftance to fire, which Reamur makes a ge- neral property of this kind of porcelain, belongs to it only in certain ftates ; and that the velTels, which he found to bear the vehement heat of a forge, could not be of the fibrous porcelain he defcribes, but fuch as had been baked confiderably beyond that period. From the foregoing general refults of a great number of experiments, a particular detail of which could add little cither to the inftrudion or entertainment of the reader, it follows, that this porcelain, though little adapted for ornamental purpofes, on account of its want of beauty on [ 246 ] on the outfide, is very well fitted for many kinds of ufe- ful veflels. Green glafs mortars, or planes for levigation, may be advantageoufly changed into this hard porcelain, greatly exceeding the hardnefs of any kind of marble, and no ill fubflitute for agate or porphyry. Mortars and levi- gating planes of this kind feem for fome purpofes particu- larly excellent, as for the grinding of enamels, which, though they fliould wear off fome of the matter of the baked glafs, as well as of marble or alabafter, yet cannot be fo much injured by the little they receive from the for- mer, which is analogous to their own compofition, as by the greater quantity they receive from the latter, which is of very different quality. Glafs veflels, intended for bearing the fire, may, by converting them into porcelain, be made capable of bear- ing it in a much greater degree, and rendered much lefs liable to crack. As veflels of this porcelain have in thefe intentions manifefl: advantages above glafs, they have ad- vantages alfo above all the . ther porcelains and earthen wares. The unglazed earthen wares permit faline liquors to foak through them ; thofe which are glazed have their glazing corroded by acids ; the compadl fl:one wares, and thofe in general which have clay in their compofition, as all the common wares neceflarily have, are corroded and partly diflTolved by certain acids in a boiling fl:ate, parti- cularly by the vitriolic: while the porcelain from glafs is neither afi^ed:ed by thefe caufes, nor fo liable to be in- jured by haflily expufing it to the fire. I know of no material, fo proper, and fo commodious, for evaporating veflfcls, or others, that fhall be proof againfl: all kinds of faline liquors. There are fome vefi!els alfo, which may be figured more perfedly, and with lefs expence, in this kind of porcelain [ 241 ] porcelain than in any other. Long-necked matrafies for inftance could Tcarcely at all be formed by the potter, of that equal thickiiefs, and internal fmoothnefs, to which they are expeditioufly blown by the glafs-maker. The above account of the qualities of this porcelain in its different ftates, points out a caution to be obfcrved in the procefs, efpecially where mortars are to be changed, or in other cafes where great hardnefs is required i viz. to difcontinue the baking at the period of the greateft hardnefs ; for otherwife the matter foon becomes foft again, and even fofter than the glafs was at firft. It feems to have acquired its full hardnefs as foon as it has become white and fibrous ; and hence perhaps it may for moft purpofes be advifeable to difcontinue the operation as foon as the glafs appears covered with a moderately thick white coat. Fragments of the fame kind of glals, put in along with the velfel to be changed, and uccaliunally t.iken out and broken, will ferve to inform the operator how the change goes on. Of wide-mouthed vcflels, feveral may be placed within one another, with fand between them. In all cafes, care mufl be taken to apply the heat, as equally as may be, all over the crucible or pot containing the veffels, that the change may be as uniform as polhble throughout : no particular contrivances however are necef- fary for this ufe, the fame caution and the fame furnace, employed for baking the common fine wares, being fuffi- cient for the baking of glafs. SECT. in. Experiments of comparing the effeSls of different kinds of ma- terials on Green Glafs by baking. PIECES of green glafs bottles were furrounded with powdered charcoal, foot, and fundry kinds of earthy bodies, in feparate crucibles, which were all covered and I i luted. [ 242 ] luted, and placed in a wind-furnace : the fire was gradu- ally railed fo as to make the crucibles of a moderately flrono- red heat, and continued in this flate for fix or feven hours. The fire being then fufFered to decay, and the crucibles taken out and examined, the glafs was found in all to have become porcelain. In the upper chamber, mofl remote from the aftionof the fire, the pieces were in general fibrous, and fome of the thick ones not changed throughout : in the lower chamber, more immediately expofed to the fire, where the matters intermixt with the glafs were the fame as in the upper one, the fibres had in mofl of the pieces difappeared, and given place to grains. There did not feem to be any differences, that could be afcribed to the quality of the cementing matters, in the internal colour, hardnefs, texture, or the regular fucceflion of the changes ; though, in external appear- ance, the differences were very confiderable. All the pieces, which had been furrounded with foot, with charcoal, or with mixtures of the two, were exter- nally of a deep black colour : where fmall proportions of foot or charcoal were mixed with white earths, the por- celain turned out of a brown colour, deeper or lighter, according as the inflammable ingredient was in greater or lefs quantity. Judging that the dark colour, which the footer charcoal communicate, might be burnt off by fire affifted by the adion of the air, I put fome of the black pieces into a crucible, which was placed open in a blafl furnace, and excited the fire, for above an hour, to as great a degree as the porcelain feemed capable of bearing without beginning to melt : the colour refifled this heat, continuing as deep a black as at firft. The coloured clays, boles, ochres, powdered red bricks, and the fands which burn red, gave likewife different {hades of brown, inclining more or lefs to blackifh, red- difh. [ 243 ] difh, or yellowlfli : thefe colours alfo refiiled the joint adion of fire and air, equally with the preceding. The browns and blacks were on lome pieces very gloH'y and tolerably beautiful. Different white earths gave different fliades of whitiffi, greyifh, or brownifh ; but none of them gave a pure white, nor a whitenefs equal to that of the internal part of the porcelain. It is dirticult to diffinguifli precifely the efted: of particular earths, in this refped, from that of the degree of fire or otiier circumftances in the procefs : for of pieces of the fame bottle, which had bt n furrounded and baked with the fame earth, fomc turned out ma- nifeflly whiter than others. White fand, calcined flint, andgypfum, feemed in general to give the grcatefl white- nefs, and tobacco-pipe clay the gieateft brightnefs or gloflinefs, though this laft, baking together in a lump upon the porcelain, made the furface in fome parts ruugli. In this experiment, and in feveral repetitions of it, the furface of fome of the pieces proved rough like fhagreen, thatoffome wrinkled like fhnvelled leather, and of others buffered or full of blebs. Thefe appearances fecm to have depended more upon the fire having been too ffrong or too haftily raifed, fo as to make the gl.us foft or ready to melt, than on any particular quality of the materials with which it was furrounded ; thou-h it appeared alfo that fome materials difpofe to thefe imperfedtions more than others. Pieces of one and tRe fame glafs bottle having been baked, fome with tobacco-pipe clay, and others with quicklime, and with bone affics, in the fame degree of fire, and for the fame length of time, the por- celain with the clay proved almoft every where fmooth and poliflied as the glafs was atfirff, while thofe with the lime and with the bone afli were all over wrinkled. I i 2 From [ 244 1 From the different effeds of different materials on the furface of the porcelain, I have endeavoured to add fome embellifliment to this kind of ware. By covering the outfide of the glafs veffel to be changed with pow'dered charcoal, or with a mixture of powdered charcoal and white find, and the infide with white fand alone, the por- celain veffel, refulting from the cementation, has proved of a deep black or a bright brown colour without, and of a tolerable whitenefs within. By covering different parts of the glafs with different powders, as charcoal, white fand, white clay, lime and coloured earths, I have ob- tained, in one piece, variegations, not inelegant, of differ- ent fliades of black, brown, grey and whitifh, and with gloffy and wrinkled veins. The above colours, with the bluifli caft which the glafs exhibits before the change is completed, are all that I have obferved glafs to receive by the procefs of baking. As the adion of foot and charcoal on iron, in the con- verfion of iron into Iteel by baking, is promoted, and in fome refpeds varied, by the admixture of a little fea fait, and of the faline aihes of wood, I made trial of the fame compofition for the baking of glafs; and as the inflamma- ble ingredients in this mixture could not fail to give a black colour to the porcelain, I tried at the fame time, in another crucible, wood aihes alone, which had been calcined in a ftrong fire, to burn out all remains of their inflammable matter, and reduce them to perfed white- nefs. The fteel-making mixture did not anfwer fo well as the foot or charcoal by themfelves : the glafs did indeed become porcelaine, but of a bad quality, all over bliflered, with many cavities, and fome of them very large, in the internal part. The wood afhes, inllead of changing the glafs [ 245 ] glafs into porcelain, melted and united with it into one femivitreous lump. I tried likewife colcothar, or the red calx of iron, which remains from vitriol after the acid has been expell- ed by fire. Pieces of green glafs being furroundcd with this powder, and baked for feveral hours in tlic upper chamber of the wind-furnace, the glafs and colcother were all found to have run together into a black mafs, externally rough, internally fomewhat fmooth and caver- nulous, of confiderable hardnefs fo as to flrike fire freely with fleel. It is pretty remarkable, that a metallic fub- flance fo refradlory in the fire, fliould be fo greatly difpofed to melt with green glafs. SECT. IV. "Experiments of the baking of different forts of Glafs, and of bodies approaching to a nitreous nature. GLASS confifts of earthy or ftony fubftances, or metallic calces, brought into fufion and tranfpa- rency in a ftrong fire. Pure unmixed earths cannot be made to vitrefy by any known degree of fire ; but fre- quently one kind of earth is made vitrefcible by mixing with it a certain proportion of a different one, which fepa- rately is as unfufible as the ether: thus clay and chalk, though each by itfclf is altogether unfulibie, yet whea mixed together in due proportions, melt and form a truly vitreous compound : in feveral of the experiments I have been giving an account of, the crucibles were found partly vitrefied, not on the outfide which had been immediately expofed to the fire, but en the infide, whi.h had been in contad: with earths of a different kind from thofe of which the crucible was compofed. The feveral forts of glafs in common ufe are prepared however on another principle i [ 246 ] principle J from fand, calcined flint, or pebbles, mixed with certain metallic or faline bodies, by which the earth is brought into fufion more eafily than by the addition of other earths. Some glafles of each of thefe kinds were cemented in the fame manner as the green glafs bottles in the preceding experiments ; in hopes, that by purfuing the effedts of the procefs upon a variety of bodies, though nothing fliould refult of pradical utility, the nature of the change, philufophically confidered, might at lead be illuftrated. I. Vitreous bodies compofed of earths, without metallic or /a^ line additions. Pieces of crucibles, which from vehemence of fire had melted into a femitranfparent glafly ftate, were fur- rounded with bone alh ; which was here made choice of as being the moft indifpofed to vitrefy of all the earthy bodies I know of. As this kind of glafly matter is very hard of fufion, the crucible containing it was placed in a blaft: furnace, and the fire ftrongly excited by the bellows for feveral hours, that the matter might undergo as great a degree of heat as it could bear without melting : the fuel was fea-coal, coaked or charred as for the drying of malt, which I find to be a very convenient fuel where bellows is ufed, being very durable, and giving a firong heat, without fmoke. The crucible being grown cold, the pieces were found of their original vitreous appearance and without any change in their colour or tranfparency. Nor have I obferved that any compofitions of mere earths, whether brought to a perfedly vitreous or only to a femi- vitreous ftate, received any alteration from this procefs. China ware, which is reckoned a mixture of two diff'erent xarths fcmivitrified, was aUb found to rcfift it : the glazing of [ 247 ] of the ware foftened, fo as that the powder it was fur- rounded with partly adhered, but in other refpeds there was no fenfible alteration. I tried likewife fome of the more limple tranlparcnt and reniitranlparent ^ones, as cryflal and flint ; which, by long cementations with dif- ferent materials, received no other change than the dimi- nution of hardnefs and tranfparency which fimple heat produces in them. II. Metallic Glajfcs, Pure glafs of lead, furrounded with fand, and baked for many hours in a moderate red heat, fuifered no per- ceptible alteration, except where fome of it had melted off and diflblved a part of the land. Common flint glafs, in which the flint or fand that makes its balls is vitrefied chiefly by an addition of calx of lead, proved alfo unfub- duable by cementation : it became rough and brownilli ontheoutfide, and internally fomewhat cloudy, but gained nothing of the appearance of porcelain by long continued and repeated bakings. I cemented likewife fome glafl*es tinged with metallic bodies, as the common blue glafs tinged with the prepa- ration of cobalt called zafi^re, blue and green glafl'es with copper, and the ruby glafs already mentioned in the tenth fedtion of the hiflory of gold. All of them retained their vitreous appearance, and fuftered very little change even in their colour : the ruby glafs grew fomewhat darker, and one of the copper glafles more dull, but the blue glafs with zaflre did not appear to have received any alteration. The bone aflies, with which all thefe glaflfes were fur- rounded, adhered to them pretty firmly, probably from the furface having been foftened or partly melted by the heat : on the zaffre glafs and the ruby glafs, the earthy cruft remained white as at firft: on all the copper glafles 5 it [ 248 ] it was reddifii ; on that particularly, which had loftcon- fiderably of its beauty in the procefs, the bone a(h was tinged of a line flL-fli colour. This glaft. had been prepared from twenty-four parts of green glals, four of borax, and one of the powder feparated by agitaiion from an amal- gam of copper. III. Glajjls prepared with faline additions. All the glafles that could be changed into porcelain were fuch as had been brought to their vitreous flate by means of faline bodies ; thou-h fome of this clafs relifted the operation, and in thofe which did become porcelain, there were confiderable differences, in regard both to the facility of the change, and the quality of the porcelain itfelf. Green glafs bottles, compofed of fand and the faline aflies of wood, anfwered much the beft j and the French bottles better, in point of colour, than the English. One of our common quart bottles, and a French quart bottle, being furrounded with the fame fand, and baked in the fame fire, for the fame length of time, the porcelain from the French bottle turned out, in feveral repetitions of the experiment, m.nifcftly the whiteft, tho' in other refpecSs no material difference was obferved. It is probable that the difference in colour proceeded from the French glafs being made with a whiter fand than the Englifli : it is faid that the fand ufed for green glafs in France retains its whitenefs, in great meafure, after ffrong calcination; while tlat of our glafs-houfes burns reddifii. The vials, in which Hungary water is brought from France, are very dithcultly converted into porcelain, and the porcelain they afford is lefs white and lefs compadt than that of the common bottles. The vials are much more fufible than the bottles, probably from their having a larger [ 249 ] larger admixture of fallne matter : they begin t6 melt nearly as loon as the fire is railed high enough to change them; and how carefully foevcr the procefs is managed, apart of tiie infide commonly runs out, and the fand they are furrounded with bakes into a hard cruft upon the furface. The lower portion of one of thefe vials having been ce- mented with a mixture of fand and gypfum, a part of it appeared changed throughout into a pretty hard porcelain, partinto afubllance refembling the mixture baked to- gether, and a part feemed fcarcely changed at all : there were many large cavities, and the glafs, which had run out from them, coated apart of the mixture with a greea glazing. In Ibme other trials the change was more equal, but I have never obtained from thefe vials a porcelain fo uniform, or fo hard, as from the Lommon bottles. Glafs tubes, of a pale green colour, were atfe(5ted nearly in the fame manner as the Hungary water vials : they feemed to be fomewhat lefs difpofcd to melt, and the fand did not fo llrongly adhere to them : from whence it may beprefumed, that this kind of glals has a larger propor- tion of faline matter than the common bottles, but lefs than tlie Hungary vials. The common pale green glafs retorts and receivers did not aniwer well. A piece cut off from the bottom of a retort, and feveral circular fegments of receivers, were placed within one another in a large pot, with fome bone afli between and furrounding them, and cemented in a wind-furnace for feveral hours. They all became brown- ish, rough and flirlv^led on the furface, in fome parts blif- tered, and in fome extremely thin as if part of the glafs had melted off. They were femitranfparent, nearly in the fame degree as the finer forts of ftone ware. They eafily broke, and appeared internally white, not fibrous or granulated, but of afmooth glaffy furface, K k Common [ 25<^ ] Common window glafs appeared to fuffer the fame change as the glafs bottles, becoming opake, and inter- nally both fibrous and granulated according to the con- tinuance of the fire : but it was greatly difpofed to blifter, and part of the glafs generally melted out. The finer fort of glafs called crown glafs, and looking- glafs plates, did not become porcelain at all. In a mode- rate heat they grew wrinkled and Ihri veiled on the furface, and lefs tranfparent than before, but fiill remained glafs; on repeating the cementation with a ilronger heat, they partly melted and inxied with the furrounding powder, but did not appear to have fufiered any other alteration. A glafs which I had prepared myfelf from calcined flint and pure fixt alkaline fait, remained alfo unchanged,, ia the fame heat, and furrounded with the fame powders^ by which common green bottles had been turned into per- fed: porcelain. Green glafs itfclf, melted with an addi- tional quantity of fait, amounting to about a ninth part of its weight, and then cemented with fand, continued like- wife unchanged. The foundation of thefe remarkable dififerences may be prefumed, from this laft experiment, to depend chiefly on the different quantities of fait in the feveral forts of glafs. The vitrification of fand with wood-aflies is influenced not a little by the adlion of the two earths in one another; fo that, though vehement fire reduces 'the mixture into glafs, yet the quantity of faline matter in the aflies is much lefs than would be fufiicient for the vitrification of the two earths feparately. In the other coarfe forts of glafs, a larger proportion of the afhes, and confequently of faline matter, is ufed, or fome alkaline fait itfelf is added^ to render the mafs more fufible. In the fine glaffes, the quantity of fait is flill larger, the vitrification being effeded almofl wholly by this ingredient. It [ 251 ] It feems to follow, from the whole, that bodies of the glafly kind arc changeable into porcelain by baking, only in fo far as the vitrcfcent flate has been procured by means of faline fubftances ; that thofe which hold the lealt fait are the mod cafily changed, and that the porcelain they afford is the moft: perfect ; and that thofe, in which the proportion of fait is large, are very difficultly, if at all, made to undergo this change. Agreeably to thefc remarks, M. de Reaumur obferves, that the very worft glafs makes the befl: porcelain j and fuggefls, that in order to the per- fection of this kind of ware, it may be ncceflary for the glafs-maker to acquire a habit of blowing veiTels from more refractory forts of glafs tJian thofe which are com- monly worked. Perhaps the fame end might be obtained more advantageoufly on another principle, which the fore- going obfcrvations point out, viz. by torming the glafs of certain earthy compofitions more difpofed to vitrefy than thofe commonly employed ; fo that a very little fait fhall be fufficient for the vitrification in the furnace of the glafs-houle, and that the glafs they afford, inflead of being more refradiory, fliall be even more fufible than the common green glafs. The different efFeds of cementation on different kinds of glafs may perhaps afford fome light into the caufe of the change which coarfc glafs undergoes, and fome ufeful chara- 'goirtg^ confifl's. in its' 'having ^\ air veffel, or rcfervoir for the air, at the bottom. An oval wooden.tub, near {ovtn feet high, and three or four feet wide, is inverted, and its lower edge let into the ground five or fix inches. The lower [ 272 ] lower ends of the two upright pipes enter into the top of the tub, and under each pipe is a kind of fmall tool which the water falls on. The water loaded with air, daHiing againft: the ftool with great velocity, rebounds, and its air is difengaged : a pipe communicating with the top of the tub carries the air to the furnace, while the water runs out at a hole in the lower part ; a fufficient height of water being kept in the tub, above this hole, to prevent any air from efcaping by it. III. A funnel and pipe without air holes, inferted into an air 'ueJfeL M. Mariotte, in his treatife du mouvement des eaux, gives an account of another contrivance for blowing fire by a fall of water, which Belidor fays, from the infor- mation of a friend who travelled in Italy, is ufed in the Tiburtine mountain near Rome, and near Salan on the laic de Guarde. A wooden or tin pipe, fourteen or fifteen feet high, and one foot in diameter, has its lower end fixed into an air vefiel or inverted tub, as in the preceding article, from one fide of which a blaft-pipe goes tapering to the furnace. The upper end of the large upright pipe is contradled to an aperture of three or four inches, into which is fitted a funnel, whofe neck exadly fills it. Into the funnel there falls a ftream of water, from the height of ten, fif- teen, or twenty feet j which we may prefume to be daflied into drops in its fall, and to pufli down air before it on the fame principle as in the machine of Tivoli already ^mentioned. This [ 273 ] This inftrument promifes to be more efFedual than ei- ther of the preceding, though in this country it can be of little ufe, fohigh a fall of water being rarely to be pro- cured, at lead in thofe places where fnielting furnaces are eftablifhed. IV. A funnel and pipe ivlth air holes t inferted into an air vcJJ'el. At Lead hills in Scotland. In N°. 576 of the Philofophical Tranfadlons, In the year 1745, Mr. Stirling defcribes a machineereded in Scot- land, for blowing air into the furnaces in which lead ores are fmeltcd ; and for conveying frefli air into the works, fo as to favc the trouble and expence of the double drifts and fliafts, and the cutting of communications between them. A ftream of water runs into a wooden funnel, fo as to keep it always nearly full : the height of the funnel is five feet, and the diameter of its throat three inches and a half. The neck of the funnel is inferted into aa upright pipe, whofe diameter is five inches and an half, and its length fourteen, fifteen, or fixteen feet : imme- diately under the throat of the funnel, four air 'holes are made in the pipe, at equal diftances round it, about an inch and a half wide, floping downwards from the outfide to the infide. The lower end of the pipe enters into a wooden tub, clofe at top, but without a bottom, fix feet high and five and a half wide, funk into a pit dug in the ground, and well rammed about with clay ; in the middle of the tub, direftly under the Ripe, is a flat ftone about two feet high, for the water fb fall upon ; and into the top of the tub is fixed a wooden pipe for carrying off the air, com- N n municating [ 2/4 ] ^ municating at the further end with an iron one which enters the furnace: for regulating the blaft, a fmall hole is made in fome convenient part of the pipe, which is flopt with a pin, or opened, according as the blaft is re- quired more or lefs ftrong. The hole in the lower part of the tub, by which the wafte water pafles out, is about five inches fquarcj and one fide of the pit, where the water runs off, is a little lower than the furface of the fi:one, fo that the water can never rife high enough in the tub to cover the ftonej though it is fuppofed to continue always a confiderable height above the top of the hole. Though this machine is faid in the Tranfadions to be fufficient for the fmelting of harder ore than any in Lead- hills where it was erected, I have been informed by a perfon concerned in thofe works, that it has fince been found not to anfwer fo well as could be wifhed, and that accordingly it has been laid afide, and its place fupplied by the common bellows. In Danphiny in France. The blowing machines ufed in Dauphiny for the forges and fmelting furnaces have a great refemblance in their general flrudture to the foregoing. They are defcribed by Swedenborg in the fecond volume of his regnum fubter- raneum, but with little exadtnefs : a more accurate de- fcription and figures of them, taken from the papers left by Reaumur, are inferted in the art des forges & fourneatix Afer, publilhed laft year by the diredion of the French academy. The upright pipe is generally between twenty-five and twenty-fix feet high : it is compofed of two pieces of fir, hollowed, and joined together by iron work. In- flead of a diftind; refervoir or funnel on the top, apart of C ■'-IS ] of the pipe itfelf is hollowed fo as to perform the fiunc office: at tlie top it is twelve inches and a filth in dia- meter (Englilh meafure) from thence it grows narrower to the depth of nearly thifty-four inches, where its widtli is only about three inches and three quarters : immedi- ately below this part, called the choak, its cavity widens to nearly eight inches and a half, and this vvidtii it pre- ferves throughout the reft of its length. Under the choak are ten air holes, fix of which are in one horizontal plane, at equal dillances from one another, and the reft about three inches and three quarters lower down : all the holes are cylindrical, near two inches in diameter, and cut at fuch an obliquity, that the orifice of the upper ones is on the infide of the pipe eight inches, and on the outfide, only five inches below the choak. The tub or air veflel, which receives the lower end of the pipe, is five feet and a half, or alittie more, in depth, and nearly as much in width : the pipe enters into it about feventeen inches : about the middle of its height is a flatflooe or iron plate, fupported by crofs bars of wood. The air pafTes off, as already mentioned, through a pipe inferted into the upper part of the tub, and the water through a hole at the bottom : on the outfide of this hole is fixed a wooden frame, with an upright Aider, by which the aperture for letting out the water may be oc- cafionally increafed or diminifhed. The blafi: is regu- lated, and the air fufi^ered to efcape when it is not wanted, by a hole in the blowing pipe, to which is fitted a valve or a ftopper. One of thefe machines is faid to be fufiicient for the forge or iron finery, and two or three for the furnace in which the iron ore is run down. N n 2 In [ v^ ] In Foix in France, In the county of Foix, the blowing machines, as de- fcribed by Reaumur in the art des Jorgis dho^t quoted, are confiderably different from the foregoing. The pipe is redangular, and the part above the choak divides into three funnel-{haped partitions. On the top is a refervoir- or ciftern of water; and two of the partitions, clofe on all fides, pafs up above the furface of the water, for carrying down air,, and thus fupplying the place of the lateral air holes : the water enters into the third partition, which is only the fpace between the iv/o foregoing, and which has but two fides, formed by the two oppolite fides of the others. The author makes the principal difference of thefe ma- chines from thofe of Dauphiny to conlift in this difpo- lition of the upper part : but the plate, annexed to his defcriptlon, fliews another, which is, perhaps, more ma- terial to the eftedt of the inftrument. The whole height of the pipe, including that of the water in the refervoir on the top, is, according to the fcale, twenty or twenty- one feet, and the choak or narrow throat, is almoft down at the middle of this height ; fo that the water iflues through the choak with a velocity which it acquires from a prefl'ure of about ten feet, which is greater than in the machine of Dauphiny in the proportion' of about eleven to fix : the quantity of water feems alfo to be much lefs in proportion to the width of the pipe, the great pref- fure probably occafioning it to fpread, foas to fill a larger bore than it could do when falling with lefs velocity. Two pipes, divided in the fame manner at the top, are fed by one refervoir : the lower ends of the pipes enter into one large oblong box, from which the air and water j)afs out as in the foregoing machines. C 277 ] At St. Pierre in Languedoc. Mr. Bartiies, in a curious paper printed in the third volume of the memoirs of the correfpondentsof the French academy, gives a minute defcription, though in fome parts not io clear as could be wilhed, of a blowing ma- chine at the forge of St. Pierre on the river Obriou, which he looks upon as one of the mofl: perfedl of the inftrumcnts of tliis kind. Its general ftrudture is nearly the fame with that of Foix, but the height of water above the choak much lefs. The upright pipe is fquare, about nine feet high, and fomewhat more than feven inches wide. Into its top arc iiifertcd, at oppoiite fides, two pyramidal air pipes, widening upwards, and paffing up obliquely through a bafon of water four feet high. The fpace included be- tween the pipes, at their lower end, under the bafon, is a kind of hopper, into which the water enters through two apertures in the bottom of the bafon : to each of thefe appcrtures is fitted a pitlon or floppcr, hung to the end of a lever, by which it is raifed more or lefs, accord- ing as more or lefs water is required. Two of thefe in- ftruments are furniflied with water from one bafon; and the lower ends of both enter into one air vefl"e], which is near five feet high, about fix and a half long, near three and a half wide, at one end, and not quite two at the other. The ftones, for the water to fall upon, are fomewhat lefs than four inches and a half diftant from the pipes : the water runs off through two redlangular apertures at the bottom, each about eight inches and a half wide, and near fix inches high : the pipe which carries oif the air, is an inch and a quarter in dianjeter at the fmall end where it enters the furnace. The IT 278 ] The obfcure part of the defcription relates to the hop- per, and the ap^i'tures by which the water is difcharged t'rum.it into the perpendicular pipe. The hopper feems to be divided into two upright partitions; and there are " two horizontal redangular openings, through which the water rfins into' the two hoppers, each of them about feyen ;iaicnes and a" half long, and in width five inches arid a half, meafured on the level of the bottom of the refervoir, which width is reduced to four and a half at the extremity of the air pipes, where the hopper alfo terminates." The author obferves that in this machine, the water, ifiuing from the hopper, is neceflarily reduced into drops. To fatisfy himfelf more fully of this particular, he took a tin veffel, eight inches and a half fquare and fix and a half high : in the middle of the bottom he cut a rectan- gular opening, about an inch and a tenth long, and eight tenths wide : to the two long Aides of the flit he foldered two tin plates, inclined to one another, and a third acrofs them. Thefe apertures, he fiays, reprefent thofe of the iiiachine when the ftoppers are drawn up; and water put into this vefiel came out always, during the whole time of its running iri flreams which flruck againft and crofied one another, and which, after fpreading, were re- duced into drops. In tliis illuftration of the machine, • though it feems clear, there mufl: be fomething which efcapes my appre- henfion. Having cut an aperture of the above dimen- fions in the bottom of a veffel, I fitted to each of the longer fides a plate half the width of the aperture, both of • which plates were moveable, and kept at different inclinations by means of the third plate which paffed acrofs the middle of the two. The veffel being filled with V^^ater, I could not obferve, as indeed was expedted, the leaft [ 279 1 leaft crofling of the flreams that run through it : on the contrary greatefl part of tlie .water iifued in two oppofite diredlions, horizontally, from between the ends of the plates. SECT. ir. Experiments and obfervatlonsfor the improvement of the fore- going machines, and for ejlablijlnng their principles of aSlion, I. Of the quantity of ivater they require, and the quantity or force of the air they afford. THE quantity of water may be eftimated with fuffi- cient exadtnefs, from the height of the water in the funnel or bafon on the top, and from the width of the choak or throat of the funnel, through which it is prefled by the force of a column of that height. Dcfaguliers found, by an experiment often repeated, that the quantity of water running througla a fquare inch hole, twenty-five inches under the furface, ' is' five tons and a fifth in an hour, the ton containing two hundred and fifty-two gallons. The quantities difchargedthrougii equal holes at different depths being as the fquare root's of the depths, and the quantities through different holes at equal depths being as the areas of the holes ; it will appear on calculation, that in the machine at Lead-hills, whofe funnel is five feet high, and its throat three inches and a half in diameter, the expence of water is fomewhat more than fevcnty-feven tons in an hour, or near three hundred and twenty-four gallons in a minute; and that in the machine of Dauphiny, where the height of water in the funnel is only about half as great, and the bore of the throat a little wider, the quantity of water is about two hundred and fixty-fix gallons in a minute. Perhaps the real quantity of water may be fomewhat lefs than this -5 calculation [ 28o ] calculation gives, as the refiftanqe of the comprefled air may occafion fome retardation of the motion. Of the other machines, the defcriptions are too imperfed: or ob- fcure for any computation to be made from them. The water, iffuing from the narrow throat of the funnel with great velocity, is faid to fpread fo as to fill the wider bore of the pipe, and to become frothy from the mixture of air with it. The jet thus enlarged may be conceived as confiding of a multitude of flender ftreams or drops, the intervals between them being occupied by air, which is continually fupplied through tlie air holes, andpuflied down by the fucceeding drops or ftreams. It has therefore been reckoned, that the volume of air which pafiTes down the pipe mufi: be as much greater than that of the water, as the tranfverfe area of the jet, when fpread and reduced to drops in the pipe, is greater than when it pafi"ed through the throat of the funnel. Circles being to one another as the fquares of their diameters, the area of the pipe of the Lead-hills machine will be to that of the funnels throat as eighteen to twelve and a quarter : the volume of air, according to the above principle, being to that of the water in the fame proportion, and the quan- tity of water nearly 324 gallons in a mimate, the quantity of air in a minute fhould be about four hundred feventy- five gallons and a half, or 134000 cubic inches, or feventy- feven cubic feet and a half. In the fame manner, the machine of Dauphiny will be found to yield about 1080 gallons, or upwards of 304000 cubic inches, or 176 cubic feet of air in a minute : fo that by this way of reckon- ing, the Dauphiny machine, with near a fourth lefs •water than that of Lead-hills, fhould produce more than a double quantity of air. But tho' this method of computation appears fpecious, it is not perhaps to be much depended on ; air, in diiFer- 4 ent i [ 28i ] ent circumftances, occupying very different volumes, in virtue of its great compreflibility ; nor is it certain that the bores of the pipes are fufficicntly filled, fo as to carry down the full quantity of air. It may be prcfumcd, that the air, intermingled in the jet, is always in fome degree comprcfTed by the water ; fo that the interftices between the dreams or drops contain more air than equal fpaces of the atmofphere. It may be judged however from the above comparifon, that the wider the pipe is, in propor- tion to the funnel's throat, provided the water running through the throat will fpread through the whole extent of the bore of the pipe, the more air. will be carried down. .«: ^:'-:l::'r:':ihi)-: Mr. Barthes, the only perfon I know of who has ex- amined thefe machines philofophically, and endeavoured to improve them, gives a method, in the memoir above- quoted, of comparing the proportional quantities or forces of the air in different blowing machines, on another principle. From confiderations too abftradted to be here particularized, he deduces a general rule, that the produce of air will be in all cafes in proportion to the quantity and velocity of the water : fo that the quantity of water and height of the fall being given in two machines, and the volume or force of the air afforded by one of them being meafured by experiment, the volume or force of the air in the other may be determined by the rule. Ac- cordingly he made feveral experiments of this kind in two machines j meafuring the force of the air, when the water in the bafon was at different heights, by the weight which the blaft ading on the arm of a balance, was ca- pable of raifing. Taking one of thefe experiments for a flandard, he computed by the rule what the refults of the others ought to have been ; but the experiments and calculations agreed ill together. And indeed the rule O o doei [ 282 ] does not not feem to be applicable but in circumftances, which can fcarcely be expected to occur -, for it fuppofes the machines to be all perfect, and every drop of the water to have its utmoil: effe^, or to carry down with it as much air as it is capable of doing j which cannot be admitted to be the cafe in any of the blowing machines yet conftruded. In the art des forges are mentioned fome obfervations of Reaumur of the quantity of air afforded by the wooden bellows. He finds that thofe ufed at the iron furnaces yield 98280 cubic inches, or upwards of five cubic feet of air at every ftroke ; and, including the two bellows, which aft alternately, 240 ftrokes in a quarter of an hour; which, on a reduftion of the French meafures to the Englifh, make 1 301 896 cubic inches, or upwards of 753 cubic feet, in a minute: this quantity exceeds that which the foregoing calculation gives for the machines of Dauphiny above four times, and therefore four of the machines fliould fcarcely be able to fupply the iron furnace with fo much air as the wooden bellows. does ; whereas two or three are faid to be fufiicicnt. Again, the bellows of the iron finery and forge was found to give two thoufand fifty-one cubic inches and a third at each flroke, and four hundred and twelve ftrokes in a quarter of an hour; whence the quantity of air in a minute is 458247 cubic inches, or fomewhat more than 265 cubic feet : this is greater than the calculation of the water machine, in the proportion of about three to two, tho' one of the water machines is found to fupply the office of the bellows. It is not to be fuppofed, that the quantity of air, which furnaces requrire, is confined, to any fuch precife limits, as that two bellows, from their being found to anfwer fufficiently for one kind of furnace, or even for one indi- vidual fujnace, can be concluded to yield quantities of 3^,0 b' aif [ 283 ] air exadly or nearly equal. Tlic above differences are perhaps as little as can be ex,pected in comparifons of this kind where the effedls compart;d are lb indeterminate. As to the water machines, it is plain, that the quan- tity of air carried down cannot be greater, than the fpaces between the drops or divided ftreams in the pipe can contain; and that though the air in thefe fpaces mud be confidercd as being comprefled to a certain degree, yet it cannot be fuppofed comprefled into two thirds of its na- tural volume, which would be neceflary for making the calculations of the wooden bellows and the blowing ma- chine to agree, becaufe fuch acondenfation would require the weight of a column of water of eleven or twelve feet, or the third part of fuch a column as is equivalent to the prelTure of the atmofphere ; whereas in the Dauphiny machine, though the air was prefled down with the full force of the column of water above the choak, the height of this column is lefs than three feet, and could not con- denfe it more than one twelfth part. In what manner Reaumur computed the air of the wooden bellovv^s, we have no account; it is probable that he judged, as others have done in the fame cafes, from their capacity J fuppofing the whole quantity of air they contained to be delivered at every ilroke. If fo, we can lay no ftrefs on the computation, for neither the wooden nor the leather bellows deliver their full contents of air ; a confiderable fpace remaining full of air when the bellows are clofed ; and this fpace containing confiderably more air than an equal volume of the atmofphere, on ac- count of the air being condenfed in it by the preflure of the bellows. I have been informed by a judicious work- man, that the bellows of the iron finery retains commonly a third, and fometimes half of its air ; and that when lined O o 2 with [ 284 ] with wood, fo that as little vacant ipaceas pofiible might be left, he found it to blow much ftronger than before. The ftrength of bellows is beft judged from the force of the blaft itfelf; and this force may be determined, in the method recommended by Mr. Barthes, already men- tioned, by the weight it is capable of raifing. He found that in the blowing machine of St. Pierre, defcribed at the end of the preceding fedion, the force of the blaft ifluing from a hole of an inch and a third in diameter, raifed the arm of a balance loaded with a weight of twenty-five ounces and a half. He gives fonie other ex- periments, cf comparing the proportional diminution of its force according to the diminution of the height of the water; which 1 fhall here infert in the original French meafures, to avoid unnecefl'ary fractions. The above force of twenty-five ounces and a half is the maximum of this machine, produced by the full quantity of water in the bafon, or a height of forty-eight inches above the choak : with a height of forty one inches, the weight raifed was twenty-two ounces; with a height of thirty-two inches, nineteen ounces ; with a height of twenty-eight inches and a half, feventeen ounces and a quarter ; with twenty- four inches and a half, fifteen ounces and a quarter; with nineteen inches, twelve ounces, and three eighths : with fixteen inches and two thirds, ten ounces and a quarter; and with a heighth of thirteen inches and a half, eight otinces and three quarters. i'- It may be obferved, that in fome of thefe experiments the water mufl have been employed to difadvantage; and that by increafing the height of the water much further than the above limits, in the fame machine, we could not expedt to produce proportional iiugmentations of the force of the blaft: for if a certain quantity of water, run- ning with a certain velocity through the choak, be fup- pofed [ 285 ] pofed to fill the bore of tlic pipe; a Icfs quantity, with a lefs velocity, mufl leave a vacancy, which will fufFer part of the air to efcape ; and a greater quantity, with a greater velocity, mull: have fome part of it fpcnt ineffedtually, for want of fufficient room to fpread. Some experiments men- tioned hereafter afford a clear proof of this. The force of the air may be determined in an cafier and more fimple method, by means of a glafs pipe, open at both ends, with one end fixed in a bafon of water. The bafon may be hung in the upper part of the tub or air-vefiTel of our water machines, and the glafs pipe let into it through a hole in the top, what fpace may remain between the pipe and the hole being properly clofed : the prefigure of the air on the furface of the fluid in tlie bafon, forces part of it up into the pipe; and this afcent will always be the meafure of the power or denfity of the air. Water is here greatly preferable to the quickfilver ufed in the fame intention on other occafions, as it dif- covers fmaller variations in the force ; for being fourteen times lefs ponderous than quickfilver, an equal preflure forces it fourteen times higher in the pipe : the whole afcent of quickfilver, by the prefigure of the air in bellows, is fo fmall, as frequently not to exceed that part of the pipe which is inferted into the tub. Infiead of a glafs pipe, a copper or iron one may be ufed ; and the afcent of the water meafured, either by occafionally dippino- a rod in it, or by means of a hollow copper ball, or other floating body, with a ftem fl:anding out of the pipe, and a proper weight below to keep it upright. It muft be obferved, that the height of the water in the pipe is to be efl;imated from the furface of the water in the bafon : whence the pipe ought to be of fmall bore in proportion to the bafon, that the water may not fall confiderably in the bafon by the lofs of that which rifes in the pipe. 5 Dr. [ 286 ] Dr. Hales found that a fmith's bellows raifed a mercu- rial gage about an inch, fo that it would have railed a water-gage about fourteen inches. The twenty-five ounces and a half, raifed in M. Barthes's experiment by the blaft of the machine of St. Pierre from an aperture of an inch and a quarter bore, Englifli meafure, are equiva- lent to theafcent of water in the gage pipe forty or forty- one-inches. I have been informed, that the pipe by which the air is difcharged into our iron furnaces is at lead of an inch and a half bore ; and that the air, with this aperture to pafs off by, ought to be of as great denfity as it can be reduced to by the human breath in a con- fined fpace; which is fuch as to raife the water in the gaoe about fifty inches •, in which cafe it is comprefTed into near an eighth part lefs volume than it commonly occupies in the atmofphere. But the quality of the fuel and other circumflances occafion fuch variations in this refpedt, that no general flandard can be laid down. I have been affured, that a charcoal fire will be excited as flrongly by fuch a blafl as raifes the gage thirty-fix inches, as a fire of coaked pitcoal will be by one of fifty inches. II. Objervations on the airvejfel. The ftrufture of the air vefTel, or tub at the bottom, is in great meafure independent of that of the refl of the inftrument ; the fame air vefTel ferving equally for dif- ferent kinds and fizes of thefe machines, while the per- fedion of the other parts confifls in their adjuflment and proportion to one another. The office of this vefTel be- ing only to ferve as a refervoir for the air, and to fuffer the wafle water to pafs off, no great care feems to be needful for regulating its dimenfions; and as the flone, ■which is placed in it under the pipe, ferves only to receive and [ 287 ] and fupport the fall of the water, or to occafion the water to be dafhed into fmall particles, that the air may be the more effedlually extricated, its diftance from the pipe feems alfo to require no exadt adjuftment. There are however Ibme particulars, in regard to the fize of this veilel, and the difpofition of fome of its parts, which ap- pear to deferve attention. The gage, mentioned in the preceding article, will be an ufeful addition to it ; (liewing at all times by infpec- tion the force of the blaft, and thus enabling the work- man to judge whether it is fufficient for the purpofes intended, and giving him notice of any failings or imper- feftions that may have happened in the machine ; as whether any air efcapes through the joints or crack, or whether the choak or throat of the funnel is obftrudled by ftones or other matters brought by the ftrcam. All the writers I have met with, who give any account of thefe kind of blowing machines, fcem to fuppofe the water within and without the air veflel to be upon a level. But as the air in the air veflel is fo far comprefled, as to be able to raife the water in the gage to a confiderable height, it muft neceflarily adl with equal power on the water below it ; and if this water can pafs off freely at the bottom, it muft be depreflTed as much as that in the gage pipe is raifed. The water within and without the veflel is exadtly in the fame fltuation with that in the bafon and pipe of the gage ; excepting only that the former receives a continual fupply within, which pafles off" as faft on the outfide. The excefs of the height of water on the outflde of the veflx:), above that of the water within, appears to be the very power by which the air is comprefled and driven into the furnace. ■ To be further fluisfied of this depreflion of the water, I ufed, for the air veflel of a fmall machine, a tall glafs, without [ 288 ] without a bottom, feven or eight inches of its lower part being immerfed into a tub full of water. As foon as the machine began to play and the gage to rife, the water within the glafs funk lower than that in the tub on the outfide; and the depreflion of the water and rifing of the gage were, as nearly as could be judged, equal, and kept pace with one another. In a little time the water was forced quite out of the glafs, and the air following it rofe in bubbles to the top of the tub. The bottom of the air veflel ought therefore to be funk at lead as much below the level where the external water pafTes off, as the gage is expedted to rife; for otherwife, before the air is fufficiently comprefled to raife the gage to the due height, it will force all the water out below, and in part efcape itfelf by the fame aperture. Hence the depth of the air vefTel, in any of thefe machines where the water has a free pafTage at the bottom, gives a power which the force of the blaft in that machine can never be made to exceed : thus at Lead-hills, the water being only of the height of two feet from the bottom of the veifel to the level of the bank where it runs off, the air can never be compreffed further, than to be able tofupporta column of two feet of water, or to raife the gage to that height; whereas in the machine of St. Pierre, the com- preffure is about two thirds greater. The finking of the water in the air .veflel may indeed be prevented, by making the aperture at the bottom, through which the water is difcharged, of fuch afize, that the preflure of the air may be able to drive through it no more water than is received at the top. But fuch an adjuftment would be apparently very difficult ; and tho' it iliould be exadly hit, yet, if the quantity of water received ■was not always the fame, it would fcarcely be poflible to avoid [ 289 J avoid either a dcpreflion or elevation of the water in the air veffel. Though the depth of water be fufficient to refift the preflure of the air, it will be ealily conceived, that if there was no folid body to fupport the fall, the great force of the ftream, falling from Ibch a height, would pufh down or daili about great part of the water in the bottom, fo that the air would get at the hole, and in part make its elcape with the water. It may be prefumed that even the drops of water, rebounding from the ftone, and falling down again, have a like effedt, though in a lower degree : for drops falling through the common atmof- phere into water, carry air with them, which afterwards rifes in bubbles, as may often be obferved in heavy rains ; and it is not to be fuppofcd that the drops fliould not here alfo carry into the water fome of the compreffed air, which furrounds them and is entangled between t]:em. Though part of the air, which thus pafles into the water, doubtlefs rifes again in bubbles, as appeared in ufinp- the glafs air vefTel above mentioned ; yet part may alio be puOied fo low, as to efcape through the hole, and difcover itfelf by bubbles in the water on the outfideof the vcflel, which I feveral times obferved before the water was driven entirely out of the glafs. Mr. Barthes likewife takes notice of air being thus carried down into the water by the drops, or introduced into the cavities which they form in falling. In order to prevent it, he recommends making a partition acrofs the tub, at the level of the ftone, with only a hole at one fide, and this in the part moft remote from the pipe through which the waterfalls : the rebounding drops are received upon the board, and run off gently through the hole in the water underneath. P p The [ 29° ] The inconvenience may be prevented alio, as effedually, and with more advantage in other refpedts, by mailing the air veffel of a very confiderable depth below the fur- face of the flone : it may be funk feveral feet into the ground below the level of where the outward water runs off, fo as to have always a column of water in the veffel, of any height required, or of a height which fhall fecure againft any air paffing down to the bottom. This ftruc- ture would free the workman from any care about in- creafmg or diminifliing the aperture, or regulating tiie height of the water. For if the deep veflel has an aper- ture in its lower part, large enough to difcharge all the water that can fall into it through the pipe in the top, or, for the greater fecurity, a good deal larger, its mag- nitude being of no inconvenience; if this veflel is iunk in a pit of water up to the level of the ftone, or to a cer- tain height above it ; and if the pit has a drain fufficient to carry off what more water it may receive : we maybe fure that the water will be always high enough in the veffel, becaufe the preffure of the water on the outfide will keep it fo ; and that the preffure of the air within the veffel will always keep it below the furface of the ftone. The air extricated from the water is always moift : when let off at a little way above the ftone, I have often obferved it to leave drops like dew on any folid body oppofed to it. A fmall degree of moifture may perhaps be of no difadvantage ; but fuch a degree as this muft doubtlefs be injurious, and render the air of lets effi- cacy for animating the fire. In the water machines of Dauphiny, inclined plates are faid to be placed at the entrance of the pipe which carries off the air, to keep back the watery drops. M. Barthes propofes letting the air off into another veffel, in which [ 291 ] which fponges are to be hung for imbibing Its moiflure, and in the bottom of which a coik. is to be fixed for oc- cafionally letting off the water that drops from the fpon- ges. I apprehend the intention may be more effcd:ually anfwered, by making the air veffelofa confiderable height above the furface of tlie water : for though the air at the bottom is neceffarily loaded with moifture, yet in rifing to the height of four or five feet, fo much of the water fcparates and falls down, as to leave the air feemingly of fufficient drynefs. The veffel might be made as high as the pipe itfelf : nor would this large fize be of any incon- venience in regard to the blalt, for as foon as it is filled with air of a certain denfity, the blaft: will continue of the fame force as from a fmall veffel. The joints (hould be well fecured to prevent the efcape of any air through them : the flone for receiving the dalh of water, fliould be placed near as much below the level of where the water runs off, as the gage is expedled to rife ; and the pipe fhould reach as low as within five or fix inches of the ftone. It would perhaps be of fome advantage to have the furface of the Hone a little concave, fo as to occafion the watery drops to be rather daflied backwards towards the ftream, than thrown upwards through the cavity of the veflel. III. Experinwits of air fajjing doiun through pipes 'with falling water. Water running through a crane. In the running of water through a fiphon or common crane, when the fucking pipe on the long leg of the crane was ftopt, the water, as it iffued from the extremity, filled the bore : on opening the fucking pipe, the column of water appeared lefs than the bore, P p 2 ■ Judging C 292 I Judging that the motion of the water muft be re- tarded in this laft circumftance, I nieafured by a pen- dillum the times in which equal quantities of water run through the crane in both cafes; and found, in many trials, that the quantity which took the time of a hundred fwings of the pendulum to run in when the fucking pipe was open, run in ninety-three, and fometimes ninety- two, when it was ftopt. As thefe differences feemed to proceed from air intro- duced into the water 'through the lateral pipe; I tried to m?.ke this air fenfible, by railing the veffel which re- ceived the water from the crane, and keeping the nofe of the crane immerfed in it. As often as the fucking pipe was opened, air bubbles arofe in the water of the receiver, and frefh bubbles fucceeded while it continued open ; but fo long as it was kept ftopt, no air bubbles were feen. To collecft the air, a calk without a bottom was funk nine or ten inches in a tub of water, and the nofe of the crane inferfed into a hole made in the top of the cafk : into another hole in the top was fitted a fmall pipe for giving vent to the air; and within the cafk was fixed an inverted mortar for the Aream to fall on. So long as water was kept running through the crane with the fucking pipe open, a fenfible blafl iffued from the blow- ing pipe of the cafk, and a burning coal expofed to it was excited in the fame manner as by a common bellows : the fucking pipe being flopt, no blafl was perceived, nor was any motion produced in the flame of a candle ap- plied to the orifice. It appears therefore that water, running down through an upright pipe, and filling its bore, admits air to enter through a lateral pipe : that after this admifilon, the width of the column of water contrafts, the introduced ^ ^1 '^ air r 293 ] aj'r occupying part of the cavity of the pipe; and that this air pafles down on the outfide of the water, or in a feparate column, not intermixed with it fo as to ren- der it frothy. Water defcendlng through an oblique fife ivith lateral afertures. I VARIED the foregoing experiment by taking, inflead of the crane, a leaden pipe, about ten feet long and three quarters of an inch bore. Several holes were made, at intervals, in the length of the pipe, and fmall tubes fixed into them like the fucking pipe of the crane. The pipe being laid allope, its upper end was turned up perpendi- cularly, and a funnel fitted to it, which was fupplied with water by a cock in the bottom of a refervoir : the other end of the pipe, which the water iflued from, was infertcd into the air veflel ufed in the preceding experiment. The lateral tubes being flopt, and the cock fo turned as to let the water run fafl: enough to keep the funnel always full, no air iflued from the blowing pipe. On opening the tubes, a confiderable blaft was perceived; the water pafled flower through the pipe, fo that the fame ftream made the funnel run over; and on pulling out fome of the tubes, and looking in through the holes, the column of water vsras very vifibly Icfs than the bore of the pipe. The tubes being fl:opt again, the blafl: ceafed, and the fl:ream did no more than keep the funnel full. A fmall variation in the circumfl:ances of this experi- ment made a very material difference in the efl^edl. The fupply of water having been diminiflied, fo as to rife only a little way above the throat of the funnel, a pretty ftrong blaft ifl"ued from the blowing pipe though all the lateral tubes wereclofely flopt ; and when the tubes were open, inflead of air pafling in by them, a blaft pafled out [ 294 ] ^ ' out from them, the air veflel in this cafe yielding none j fo that here the air mufl have been introduced at the top and pafled down the funnel, and afterwards efcaped where it firll found a vent. To be further fatisfied in this point, I repeated the experiment with a fomewhat different ap- paratus, in the following manner. Water falling through ajunnel. ' The glafs receiver of an air pump, about two feet high, open at both ends, had its lower end immerfed about feven inches in a vefTel of water, and fupported at a proper diftance above the bottom for the free pafTage of the water under the edges. A brafs plate being preffed clofe on the top, with leather between, a glafs funnel, about twelve inches deep, and above half an inch diame- ter in the throat, was fixed into a hole in the plate; and into another hole was fitted a fmall blowing pipe. A flopper being introduced into the funnel, till the Vv-ater it was filled with had become perfedly quiet, and then cautioufly removed, the water run in aftream, which falling into that in the receiver, produced air bubbles : but no blaft iffued from the pipe ; and when the pipe was ftopt, the water in the receiver did not fink lower than the level of that in the outer vefi"el, whereas, if any air had entered with the water, and been comprefled in the receiver, it mufi: have forced a proportional quantity of the water out below. .The funnel was then fupplied from a pipe, by which the water was made to dafh againft one fide of it. By this means the fluid received a fpiral motion, and twirling round the funnel, left a large vacuity in the middle, reaching down fometimes to the funnel's throat. The ftream, as it run through, was alfo twilled; a fenfible blail [ 295 ] blart: iflued from the air pipe; when the pipe was ftopt, the water in the receiver was forced lower and lower, and was foon driven entirely out, abundance of air bubbles fol- lowing it into the water in the outer veffel. When the funnel was kept entirely full: though the Aream was diredled as before againft its fide, there were little marks of any air being carried down. And whea the funnel was near empty, the effedts were alfo incon- iiderable; the vacuity in the middle of the fpiral circum- volutions of the water feeming to reach to the bottom, fo as to fuffer the air to efcape upwards tiirough the hollow column of water. Water falling fro7n a confiderabk height Into a funnel •with a pipe. A LEADEN pipe, fix feet high and an inch and a half in diameter, was inferted into an air veffel, with the water gage already defcribed. Into the top of the pipe was fixed a tin funnel, whofe throat fitted clofe to itj and into the funnel a dream of water was let fall, from a refervoir five feet above, in quantity fufficient to keep the funnel run- ning over. This apparatus reprefents Mariotte's blowing machine defcribed in the third article of the preceding fedion. The water, divided by the fall, puflied down abund- ance of air with it : a ftrong blaft ifl\ied from the blow- ing pipe, and the gage rofe high. On raifing up the funnel a little, the ftream that ifl'ued from it appeared all frothy : as often as the funnel was lifted up, the gage funk, the air, which liad been driven in by the dafli of water, efcaping between the funnel and pipe : on letting down the funnel clofe, the gage immediately rofe again. Inftead C 296 1 Inflead of a fall of five feet, a ftream was diredled into the funnel from only about half that height. The gage flill rofe confiderably, though not fo high as before. It is obfervable, that in the circumftances of thefe ex- periments, a twirling motion communicated to the water' in the funnel impeded the carrying down of air, the gage always finking on the water receiving fuch a motion ; whereas, in thofe of the preceding article, it feemed to be by the twirling of the water that the air was pufhed down. It appears therefore that there are two ways of making air pafs down with water through a funnel, one by di- redling the ftream againft the fide of the funnel, the other by letting it fall from a great height : that in the one cafe the air enters between the fpiral circumvolutions which the water forms in the funnel, and in the other between the drops into which a coniiderable part of it is reduced by the fall; that we cannot avail ourfelves of both ways at once, the one impeding the effedl of the other j and that in either cafe the air holes under the throat, fo necef- fary in other machines, can have no place, as they give a vent to the air brought down from above. Water falling from a funnel through a pipe with air holes. The fix-foot pipe, ufed in the foregoing experiment, continuing fitted into the air vefTel, it? upper orifice was widened, that the fmall end of a funnel-fliaped copper pipe, of the fame bore with the preceding funnel, might hang freely in it, without touching the fides. The funnel pipe reached up to the refervoir, and was kept always full, that the water might receive little or no air but at the vacuity between the nofe of the funnel and the leaden Dine. 2 In f 297 1 In this fituatlon, the quantity of air was much Icfs than in the preceding : the water fell through the funnel ia a flream not at all frothy, and the gage rofe but a little way. I widened the aperture of the leaden pipe, to let in more air, but flill the gage continued low. Into the orifice of the funnel I inferted a fmallerpipe, whofe diameter was one inch, and whofe area was of confequence lefs than half of that of the leaden pipe. The blafl: was now flrong, and the gage rofe higher than when the water fell from an equal heighth into the low funnel of the foregoing article. I tried funnels confider- able fmaller, and found the gage ftill to rife high : but at laft, with one of a quarter of an inch diameter, it did not rife at all, and no blaft could be perceived. One of the funnels which anfwered beft being properly fixed, with two or three inches of its neck hanging free within the wider pipe, I made feveral variations in the manner of admitting the water and air, with a view to com- pare the effedts of different ways of admiffion. The fun- nel being full, and gently fupplied fo as to keep the water in it as fteady as poffible, the height of the gage was marked: on giving a circular motion to the water, or letting it fall from a height, the gage always funk, even a flight twirl or dafli fenfibly affeding its height. The fpace between the nofe of the funnel and the pipe was ftopt, fo that no air could enter but at the top : the fun- nel being now full, and the water quiet, the gage fcarcely rofe at all; on twirling the water, it rofe confiderably, and when the water fell from a height, it rofe further, though not fo high as the ftandard mark. It appears therefore that there are two general methods in which water may be made to carry down air, one in which it receives the air at the top, and the other through lateral apertures; and that the circumftances, which con- Q^q tribute _ [ 298 ] tribute to the effed in one cafe, impede it in the other: That water, being at reft in a funnel, and then fuffered to run through, carries little or no air with it ; that when made to twirl round in the funnel, it carries a confiderable quantity; and that when itfalls from a height, fo as to be in great part daflied into drops, it pulhes down con- llderably more : That running through a pipe with lateral apertures, perpendicularly or obliquely, it receives air through the apertures, even when its motion is flow; that when the pipe is of equal bore throughout, the quan- tity of air thus received is not great; but that, when the pipe is contradled to a certain degree in the part where the apertures are, the quantity of air is greater than that intro- duced through the funnel without air holes : That air brought down from the top of the pipe or funnel prevents the introdudion of frefli air through the lateral holes,, which in this cafe, inftead of receiving more air, difcharge that already received. Finding that the two general methods, by which air is made to pafs down with a ftream of water, could not be united in one machine ; and that the pipe and funnel, with apertures for the entrance of air about or under the throat of the funnel, have the greateft effed ; I pro- ceeded to examine the moft proper form and difpofitioa of thefe. IV". Experiments andobfervationsjor regulating theJlruSlure of the fumiel and pipe. ^ Experiments with funnels and pipes of different heights. The water, as already obferved, paffing through the narrow throat of the funnel, is afterwards en^ iarged into a jet which fills the bore of a wider pipe. Tha [ 299 ] The quantity of air Introduced apj)cars to depend upon the degree of this enlargement, and on the quantity of water that runs through In a given time. The greater the height of water above the narrow- throat, the greater velocity will the jet receive, and the more it will be difpofed to fpread and be enlarged. The length of the pipe does not appear to be of fo much im- portance: It Ihould feem fufficient if the pipe Is of fuch length, that the preffure of water In It may be able to refift the compreffed air In the air veflel, and that after part of Its power has been fpent in overcoming that force, it may ftlll have velocity enough left to run down as faft as It can be fupplled from the funnel. In order to attain to fome determinate proportions, the following trials were made. A leaden pipe, feven feet high, and an Inch and a half in diameter, being fitted Into an air veflel, as In the fore- going experiments, funnel-fliaped pipes of different heights were fupported over it, fo as that the fmall end of the funnel might hang freely in the orifice of the leaden pipe, and leave fpace enough for the entrance of air all round. For the greater fecurlty of the throat being of the fame area in all the funnels, one and the fame copper pipe ferved as a throat for them all : the funnels being formed by Infert- ing this pipe into larger tapering ones of diflferent heights. The funnels were always kept full, and the water conveyed into them as gently as poflible, fo as to produce no dafh- ing or ivvlrling motion. A funnel of one foot high had very little effedl : the rifing of the gage in the air veflel was inconfiderable, and the flream of air from the blowing pipe was but jufl; to be felt : on opening fome holes made in the upright leaden pipe under the throat of the funnel, the jet of water appeared not fpread, but rather contrafted, and did not fill Q^q 2 the [ 3°^ ] the bore. With funnels of two and three feet, the gage rofemore, and the jet fpread, though it To the bottom of the funnel, enlarged as above men- tioned, was fitted a pipe fix feet high, and in width four inches by feven and a half. The lower end of this pipe was inferted into the head of a large cafk without a bottom, which was fet in a tub above three feet deep, with three fupports under the lower edge of the cafk to procure a fpacc between it and the bottom of the tub for the water to pafs freely off. About eight inches under the orifice of the pipe, around board, for the water to fall on, was hung by three cords, which pafifed up through the head of the cafk and ■were fecured by pegs. At one fide a tin vefTel full of water was fupported in the flime manner; and through a faucet, over the middle of this veflfel was inferted a glafs tube thirty four inches long. At the other fide was the blaft pipe, about three quarters of an inch in diameter. ' The machine being thus prepared, we proceeded to the trial of it, expedling that the two flreams from their floping diredtion towards one another, would crofs and be dalhed into drops, and carry down abundance of air. But in the effedl: we Were greatly difappointed : the blafi: was weak, and the gage rofe to no confiderable height, ■whether the wedge was dropt down or drawn up, fo as to fufter the water to pals in lefs or greater quantity, in thin [ 3^9 ] thin or in thick flieets : in continued trials and va- riations of the apertures for three or four days, the gage was not once obfcryed to rife fo high as,.ten inches, A good deal of air indeed cicaped through the junctures of the pipe and of the air veilc], but not near;enpugh tp make up the expected quantity. ,^;o\jd if,„ j] jurfvv oj -^IcJ The wedge anfwering fo ill, it was laid afide; and in its place was introduced a leaden vertel, of the fame (hape with the funnel's throat, and of fiach a fizc, as to rell againft the fides of the aperture by its upper edge, and hang fix or itwtw inches dowp in the wider part of the pipe : in the fides and bottom of; this velfel were made feveral holes, about two tenths of an inch in diameter. With this alteration I had the pleafure to find, that though air ruQied out from the joints eyen more plentifully than before, yet the blaft fron?, ; the blowing pipe was llrong, and the- water in the gage pipe rofe tp the top and run over. .....>... j., r,,,..;, i, :.. /;..,t I tried to meafure the quantity of water neceffary foe pro- ducing -this effedl for -a certain itme.* iTihe refc/voir being filled t'o the depth" of ifourteeii inches, -.the gage rofe-^s before, and continued high for^four minutes ; after which it begun to fink faft, the water in the refervoir havijfig tlien become too;low;t6 kedp the pipefiuU, though it con- tinued 'to run for a cofifiderai^k ,t^me. Joi^ger, ^From the dimenfions of the r^ferivoir already meutiyned it will ap^^ peac;;.that if.allthe. Winter, .h^4 jfu" PttVui-i ihfS.foqrrni;^ nutes it 1 would have latpounted jo, near tvvo hundred gallons in one minute ; 'bufat .le^irt a fourth of it remained after that period, fo thatirthevejtpence ^-oyilji.flot exceed a hundred and .fifty gallon:§iin ;a i^niaufe. •, We cqyld not expedl any great accuracy, in itlbisi ,de;;^rmination, ibecaufe asij-the ibeightiof the, waftdr conjtinually decreafed ' iin .-the refervoir, its velocity-liktWife' dacrcafed, \o that if a; due . - . i quantity m [ 310 J quantity run in the lafl minute, a fuperfluous quantity muft have run in the firft. The leaden cullender being taken out, and the whole throat left vacant for the ftream, the gage ftill rofe to the top; but the cxpence of water was now more than dou- ble to what it was before. Thele trials, though not carried to fuch a length as I could have wifhed, fatisfied me, and thofe who affifted at them, that much more air is to be obtained, by divi- ding theftream by means of a cullender, than by any other methods that have been tried ; and that with fuch a ma- chine as is above defcribed, a ftream of a hundred and fifty gallons at moft in a minute is fufficient to produce a continued blaft, from a pipe of three quarters of an inch bore, of fuch ftrength as to fupport a column of water of three feet or more. To afford as much afllftance as poffible to thofe who may be defirous of eredting machines of this kind, I fhall here colledl into one view the moft material particulars which my experiments have difcovered with regard to the perfedlion of their ftrufture, and form from them a defcrip- tion of fuch a machine as promifes to be the moft effec- tual. The bottom of the refervoir of the water {hould be about fourteen feet above the level of the ground : we need not be very felicitous about procuring a greater height, for though a greater would be of fome advantage, yet this advantage appears to be much lefs confiderable than has been commonly imagined. - In the channel by which the water is conveyed, are to be placed gratings of different fizes, as already mentioned, and before the aper- ture a finer grating, which may be either a perforated iron plate or a wire fieve, to ferve as ftrainers for keep- ing back fuch matters as would obftrud the apertures jf • I which ■I . i.L.ltul [ pi ] which the water is afterwards to pafs through. The ftream fliould enter at one fide, or be fo managed, that the water in the refcrvoir or funnel may not be agitated by it, or put into a fpiral motion, which our experiments have (hewa to be very injurious. In the bottom of the refervoir is to be made a round hole, for admitting the upper end of what we have hi- therto called the funnel, but which may here be more conveniently a cylindrical pipe, of copper or of cart: iron, five or fix inches in the bore, and feven feet long. To the end of this pipe is to be fitted a cullender, about a foot long; with the holes triangular, of half an inch eacli fide ; and fix or feven ftrips from top to bottom,, at equal diftances, preferved without holes, for admitting air to pafs down to the lower ftreams. All the holes ftiould be direfted downwards, that the rtireams may not be- forcibly projeded againfl: the fides of the pipe which is to receive them, fo as to have their velocity too much diminifhed. If there are fix of the perforated fpaces in the cullen- der, the number of holes in each may be twenty, fo that the whole number will be one hundred and twenty. The fide of each of the triangular holes being half an inch, the area of each will be the eighth part of a fquare inch, and the fum of their areas will be fifteen fquare inches. The quantity of water running through one aperture of fuch an area, at the depth of feven feet and a half under the furface, comes out on calculation about fix hundred and twenty-two gallons in a minute; but th« real quantity will doubtlefs be much lefs than this, . on account of the great fridion of the water in pafling. through a number of fmall holes, and of the refiftance of the air, which increafes in a very high ratio accord- ing to the increafe of the velocity and enlargement of the iurface: it is in part to make up for thefe retardations, that [ 312 ] tli'at the pipe is dircdted to be made fo high , The furface of the water is here above thirteen times greater than if it pafled all through one circular aperture. ■':■-'" Both the pipe and the cullender fliould have a flanch or rim round their orifices, and be fecured to one ano- ther by fcrews paffing' through the rims of both, with a plate of lead between them to make the jundlure ti^ht, as commonly pradifed in joining iron pipes for water works. This way of joining them admits the cul- lendei* -to be "taken off ^nd cleaned, when a diminution of the efFedl of the machine fliews the holes to be choak- ed up, which however, it is apprehended, will feldom, if ever, happen. As the holes will permit more water to run through, than may at all times be wanted, "it is proper to have fqme contrivance for occafionally clofing a part of them. This .may "be effedled by means of a thin copper pipe, open at both ends, as high as the cullender, and of fuch width as jufl to drop into it. It will be eafily conceived, tljat when this regifler is let entirely down, the lateral holes will be covered, and the water admitted only to thofe in the bottom ; and that by raifing it fur- ther and further, more and more of the lateral holes will be uncovered. The regifter is to be hung by a wire to a crofs ba:r over the refervoir, by which it may be raifed or lowered; and a fcale or divided board may beadjufled againft the upper part of the wire, for fhewing the height of the regifter, or the number of holes clofed by it. The mod commodious and effedlual way of admitting air to the water appears to be that of our iirft experiments, viz. hanging the throat of the funnel, in this cafe the cul- lender, within the wider receiving pipe, for by this means the air is admitted freely and uniformly all round. This lafl: pipe ihould likewife be of iron or copper, twelve 5 inches [ 3^3 ] inches In diameter, and fpread out at top to the width of fixtccn or eighteen inches, that a large ipace may be left round the cullender : this fpace Ihould reach three or four inches above the uppermoft perforations of (he cullender, to prevent any of the water from being dafhtd over the top. A pit is to be funk in tlie ground, not lefs than fix feet deep. In this is to be placed an air veflcl, made of wood lined with lead, without a bottom, three or four feet in width, and ten or eleven high. The vcfiel fliould be fup- ported on feet, of a proper Itrength, with futhcient fpaces between them for the water to pafs freely out : this way is preferable to the common one of placing the lower edge of the veflel on the bottom of the pit, and cutting an aperture in the fide, becaufe the height of the aperture is fo much taken off from that of the veflel. The refer- voir being fourteen feet above the ground, and the upper pipe and cullender reaching down eight feet, only fix feet remain below the cullender -, fo that the air veflel, having funk fix feet in the ground, will reach nearly up to the cullender, and almoft the whole height of the undcrmofl: pipe will be included within the vefl!el. This pipe may be above nine feet long, three feet or more of it going down into the pit; which three feet are here an entire gain in the height of the fall, for the pipe in the other machines comes at mofl: no lower than the level of the ground where the water runs off" on the outfide. This height is gained, in virtue of the com- prefled air in the veflTel pulhing down the water below, as already fliewn in the fecond article of this fedlion : it may be always as great as the height to which the water is intended to rife in the gage. At the diftance of five or fix inches under the orifice of the pipe is to be placed the concave iron plate or flone for the water to fall on. In the top of the air vefl"el is to be fixed the gage and the blowing pipe. S s Such [ 3H 1 Such Is the general conflruftion of the blowing ma- chine, which proniifes to be particuhirly ufeful in cafes where water is Icarce, or where the want of a natural fall renders it necellary to raife, by very expenfive means, the great quantities requifite for working the common bellows. It is prefumed, that one of thefe ma- chines will be fufficient for the iron forge, and for fundry other purpofes where the quantity of air is not required to be very great 3 that it will be lefs expenfive, on account of the durability of its materials, and the fim- plicity of its ftrudture, than any kind of bellows now in ufe; and what is of principal importance, that much lefs water will ferve for working it. In cafes where one of the machines cannot fupply air enough, as for the large iron fmelting furnace, two pipes may be ufed, both fed by one refervoir, and entering into one air vefl'el, as pradifed in fome of the inftruments defcribed in the firfl leftion. The ufing of two pipes appears more eligible than enlarging the bore of one ; for air cannot be fo freely introduced into a large body of water, though divided into ftreams by the cullender, as into two fmaller ones of equal quantity. It may be obferved, that the blaft will be ftronger in a denfe flate of the atmofphere, than when it is more rare or expanded, a greater quantity of air being then intro- duced under an equal volume. If therefore the quantity of water has been adjufted fo as to raife the gage "to a pro- per height when the air was light, it will frequently hap- pen that the fame quantity of water fliall raife it higher, and confequently, if no greater height is required, that a part of the water may be faved. As the gage of our ma- chine difcovers by infpeftion thefe variations in its effefl-, the regifler affords convenient means of regulating its power, and increafing or diminiftiing the quantity of water. 2 VI. [3>5] VI. History of Colours. PART I. Of Black. BLACK, a colour in many cafes the moft important, and in its ufe the moft extcnfive, of all thofe which art is concerned in preparing or applying, is chofen as the firfl; article of an experimental hiftory of colours; which will be occafionally continued in the profecution of this undertaking. The practices of the workmen in one branch of colour- ing are generally little known to thofe who are employed in another; the feveral methods of applying even one colour, on different kinds of bodies, being theobjedls of fo many diflindt arts, each of which has its own rules of working, peculiar to itfelf, and eftabliflied by long cuftom. Of the arts of communicating a black colour to diffe- rent fubjed:s, there are fome which have made great ad- vances towards perfeftion, whilft others remain far more imperfedt, in regard not only to the difpatch and facility of the execution, but likewife to the beauty and duration of the colour. Thus woollen and fdk are both dyed of a permanent deep black, but with this difference, that what the woollen dyer effefts by three or four dippings of the cloth in his dying liquor, the filk dyer fcarcely obtains from twenty or thirty dips; whereas, on the contrary, the dyer of linen and cotton thread, however he prolongs the operation, or repeats the dippings, is unable to communi- cate to the thread a blacknefs that fhall endure wearing- Thus alfo the printer fixes upon paper an ink which con- T t tinues [ 3i6] tinues unaltered for ages, and which Is not perhaps capa- ble of being changed by any natural agent that the paper itfelf can refiftj while the common writing inks foon lofe of their colour both on paper and on parchment, in- fomuch that records, of no very long ftanding, have become almoft entirely obliterated. In the prefent hiftory, I fhall endeavour to trace, as far as my opportunities of information will enable me, the preparation, produdlion, and communication of black co- lours, through all the profeffions in which they are con- cerned i that the artift, confined by his employment to particular views, may be made acquainted with the me- thods, by which fimilar efFefts to thofe which he pro- duces, or wants to produce, are obtained in other arts, or in arts which in other refpedls differ from his own. Ex- periments, while they ferve as a fure teft for afcertaining the refpedlive fadls, will often contribute at the fame time to enlarge and render them more extenfive ; and likewife afford means of diffinguifhing, in fome complex opera- tions, the circumftances or materials effential to fuccefs,. from the fuperfluous or injurious ones, which ignorance or chance perhaps at firff introduced, and which prejudice or cuflom have continued. By thus examining and comparing the different me- thods, by which a fimilar colour is obtained or produced, and by which the feveral tinging materials are applied on different fubjefts, I flatter myfelf that many of thefe arts,, however disjoined among different fets of workmen, will be found to have natural and flrong connexions, fo that an effective and ufeful commerce may be eftablifhed. among themj that they will not only tend to illuftrate,. but mutually to improve one another; and that in many- cafes the pradlice of one art may be abridged or facilitated, its imperfedions remedied, and its deficiences fupplied, by meansj [ 317 ] means, which could fcarcely ever be thought of by a pcr- fon converfant in that art Tingly, but which a general knowledge of the others may be expcded to fuggeft. o SECT. I. General obfervations on black colours. F. black, as of other colours, there are many (hades or varieties; different bodies, truly and fimply black, or which have no fenfible admixture of any of the reft of the colours, as black velvet, fine black cloth, the feathers of the raven, &c. appearing, when placed together, of teints very fenfibly different. 2. One and the fame body alfo affumes different degrees of blacknefs, according to the difpofition of the fenfible parts of its furface ; and in this rcfpedl, there is not, per- haps, any other colour, which is fo much affedled by an apparent mechanifni. Thus black velvet, when the pile is raifed, appears intcnfely black, much more fo than the filk it was made from ; but on preffing the pile fmooth, it looks pale, and, in certain pofitions, fliev.s fomewhat even of a whitifli call:. 3. This obfervation is agreeable to the phyfical theory, which afcribes the blacknefs of bodies to the luminous rays, that fall upon them, being in great part abforbed, or ftifled in their pores. When the furface is compofed of a multi^ tude of loofe filaments, or fmall points, with the extremi- ties turned towards the eye, much of the light is ftifled in the interftices between them, and the body appears dark : when the filaments are preffed clofe, or the furface fmooth- ed and poliflied, more of the light is reflected from it, and the intenfity of the blacknefs is diminiftied ; though the beauty may be improved by the gloffinefs which refults from the fmoo thing. . Tt 2 4. There «iV [318] 4. There Is one cafe however, in which a high polifh may, on the fame principle, produce blacknefs, in bodies otherwife even white. We find that fpecula of white me- tal or of quickfilvered glafs, which refledl the rays of light to one point or in one diretflion, look always dark, unlefs. when the eye is diredlly oppofed to the refledled rays. 5. As the abforption of the luminous rays, except in the cafe jufl: mentioned, makes the phyfical caufe of black- nefs i it is concluded that black bodies receive heat more freely than others. Black marble or tiles, expofed to the fun, become fenfiWy hotter than white ones. Black pa- per is kindled by a burning-glafs much fooner than white, and the difference is flrongly marked : a burning-glafs, too weak to have any vifible effedl at all upon white paper, ihall readily kindle the fame paper rubbed over with ink.. Hence black clothes, when wetted, are faid to dry fafter;: black habits, and rooms hung with black, to be warmer; black mould to be a hotter foil for vegetables ; and gar- den walls, painted black, to anfwer better for the ripening of wall fruit ; than thofe of lighter colours. 6. It is not however to be affirmed that the like dif- ferences obtain in the impreffions made by common fire. Black paper, held to the fire, does not feem to be affeded fooner, or in a greater degree, than fuch as is white; It' maybe proper to obferve alfo, that the combuflibility of the paper may be increafed,. by impregnating it with fub- ftances of themfelves not combuftible, and which give no^ colour to it. This is the foundation of one of the fympa- thetic inks, as they are called, made of a ftrong folution of fal ammoniac in water, which, though colourlefs when written with on paper, becomes very legible on expofing the paper to the fire -, that is, it occafions the parts moift- ened with it to fcorch or burn, before the reft of the paper is hurt, to a brown or black. All the falts I have tried. produced [ 3»9 ] produced this cffeft in a greater or lefs degree ; nitre, alum, tartar, very weakly ; fea fait moreftrongly ; fixed alcaline falts ftill more fo; fal ammoniac the moil ftrongly of all. Metallic folutions, made in acids, and diluted fo as not to corrode the paper, adled in the fame manner. 7. Befides the funple blacks, there are a multitude of compound ones, inclining more or lefs to other colours. Thus the painters have blue-blacks, brown-blacks, &c. which may be made by mixing pigments of the refpedtive colours with fimple black ones, in greater or lefs quantity, according to the fhade required. The dyers alfo have different blacks, and often darken other colours by {lightly palfmg them through the black dying liquor ; but the term brown-black is in this bufmefs un.known, brown and black being here looked upon as oppofite to one another. In effedt, the colour called brown-black is no other than that which ill dyed black clothes change to in wearing : no wonder then that it is excluded from the catalogue of the dyers colours. 8. The true or fimple blacks, mixed with white, form different fliades of grey, lighter or darker according as the white or black ingredient preyails in the mixt. The black pigments, fpread thin upon a white ground, have a like effed:. 9. Hence the painter, with one true black pigment, can produce on white paper, or on other white bodies, all the fhades of grey and black, from the ilighteft difcoloration of the paper, up to a full black : and the dyer produces the fame effedl on white wool, filk, or cloth, by continu- ing the fubjedts for a fhorter or longer time in the black bath, or making the bath itfelf weaker or ftronger. 10. Hence alfo the dilution of black pigments with white, or the fpreading of them thin upon a white ground, affords a ready method of judging of the quality or fpecies of [ 320 J of the colour; which, if it be a true black, will in this diluted ftate look of a pure or fimple grey, but if it has a tendency to any other colour, that colour will now betray itfelf. 1 1 . All the colours, in a very deep or concentrated ftate, approach to blacknefs. Thus the red liquor prepared by boiling or infufing madder root in water, and the yellow decoiflion or infufion of liquorice root, evaporated in a gentle heat till they become thick, look of a dark black colour, or of a colour approaching to blacknefs -, and thefe thick maffes, drawn out into flender ftrings, or diluted with water, or rubbed on paper, exhibit again the red and yel- low colours, which the liquors had at firft. Nature affords many black objedls, whofe blacknefs depends upon the fame principle, being truly a concentration of fome of the other colours. Thus in black cherries, currants, elderberries, &c. what feems to be black is no other than an opake deep red : their juce appears black when its furface is looked down upon in an opake veffel, but red when diluted or fpread thin. The black flint, as it is called, of the ifland of Afcenfion, held in thin pieces between the eye and the light, appears greenifh ; and one of the deep black ftones called black agate, viewed in the fame manner, difcovers its true colour to be a deep red. S E C T. II. Native black colours. *~\^ H E mineral kingdom affords abundance of bodies J^ uniformly tindlured or variegated with black, or with a deep colour approaching to blacknefs : Such are, the black flates, which make an ornamental covering for houfes: the black touchflone, on which pieces of metals being jubbed leave a mark of their own colour; which Ihews the colour [ 321 ] colour the more perfedlly by virtue of its blacknefs, and which thus enables us to judge and compare tlie colour and finenefs of metallic compofitions, with much more certainty than could be done by viewing them in the mafs : the black flint fo called, which performs the fame office with the touchftone, and being harder than the common touchftones, anfwers better for the hard metals: the com- mon black marbles, ufed for many kinds of ornamental works : the more rare black marble, called lapis ohfidianus or opjianus, which, in virtue of the very high polifh it re- ceives, was rriade into mirrours by the Greeks and Romans: the black gallinazo flone, defcribed by D'Ulloa, which anfwered the fame purpofes among the Indians of Peru before the conquell of that kingdom by the Spaniards: the black jet, and other fubllances of the fame clafs, which are formed into many elegant toys: the plain and variegated black agates, pebbles, cryftals, &c. which are cut by the jeweller. The ftone called tourmalin, remarkable for the fmgular phenomena it exhibits in fome eled^rical experi- ments, is in its rough ftate of a full black colour, though when polifhed it looks rather of a brown black: I have been informed by a fkilful jeweller that he had (&tn a black diamond, cut and fet in a ring; though perhaps the examination made of it was not fo rigorous as could be wirtiedfor determining its being truly of the diamond kind. However this may be, a black tinge in bodies of this clafs is looked upon as an imperfedtion or foulnefs, although, when the diamond is cut, a black foil placed under it im- proves its beauty: for the rofe diamonds, the collet, or focket in which the ftone is fet, is fpecked with ivory black in little dotsj and for brilliants it is all over blacked. Among vegetables and animals, blacknefs, though fre- quent, is lefs diffufed, or of more limited extent. The black colour of the ftalks and feeds of certain plants, that, of [ 322 ] of fhells, feathers, and hair, and that of the flcin of the human fpecies in certain climates, is only a fuperficial teint. The blacknefs of fruits, as already obferved, is generally rather a concentration of fome other colour than a true black. Some woods, particularly the ebony, are tinged throughout with a true blacknefs, on which great part of their value depends, and which art very happily imitates on more common woods. The bony matter which lines the mouth of certain whales has like wife frequently a pretty deep black tindture, joined to another quality, elafticity, on many occafions more important than its colour. Among the various fubftances wliich nature has im- pregnated with a deep and permanent black colour, there are few of which art can avail itfelf for communicating this colour to other bodies. There is not perhaps any in- Aance of a black colour being, as the reft of the colours frequently are, extracfled from one body by means of dif- folvents, and thence introduced into another. Nor can the generality of natural blacks be applied in their whole fubftance^ fome of thefe bodies being of fuch a texture, as does not admit of their being reduced into fufficiently iine powder, for being mixed with a proper cementitious matter, fo as to be fpread fmooth; and others having their colour deftroyed by the pulverization. Of thefe laft we fee an inftance in the common black flates, which may be fcraped into a white duft, in which the flighteft fcratches look white, and which, when drawn along any other black body, as hard as themfelves and not polilhed, leave a white mark; a property which, while it renders them ut- terly unfit for any purpofes in painting or ftaining, is that on which their ufe depends for occafional writing, or for making pencils for writing on other ftones. It is obvious^ that for this intention, black ftones are better adapted thaia thofc [ 323 ] thofe of any other colour; and that the ftone fhould be fomewhat harder than the pencil, that the marks may pro- ceed chiefly from the pencil, without fcratching the fub- ftancc of the ftone. The only native blacks I know of, that have been em- ployed as colouring materials, are the following. I. Black chalk. The black chalk or black marking-flone of the fhops, fo called from its ufe in drawing black lines on paper, is a light earthy fubflance, of a pretty deep black colour, moderately firm, in texture fomewhat flaky like flate, not of a rough harfh furface like common chalk, but rather foft and fmooth to the touch. It llains very freely, and, in virtue of its fmoothnefs, makes very neat marks. It is eafily reduced into an impalpable foft powder, without any apparent diminution of its blacknefs. In this flate, it mixes eafily with oil into a fmooth parte; and being dif- fufed through water, it flowly fettles, in a black flimy or muddy form; properties which make its ufe very con- venient to the painter both in oil and water colours. En- tire malTes of it, laid in water, are alfo by degrees pene- trated and difunited, though much more difficultly than thofe of white chalk. It appears, in effedt,- to be an earth of a quite difi'erent nature from common chalk, and feems to be rather of the flaty bituminous kind.- In the fire it becomes white with a reddifh cafl:, and very friable; retaining its flaky flrudlure, and looking much like the white flaky mafles which fome forts of pitcoal leave in burning. Acid liquors neither dif- folve, nor alter the colour of the black chalk itfelf; nor have they, fo far as I could obferve, any fenfible ad:ion upon the white alhes. U u Gur: [ 324 ] Our colour fhops are faid to be flipplied with this ufefid earth from Italy and Germany; though feme parts of England afford fubflances, nearly, if not entirely, of the fame quality, and which are found to be equally fervico- able, both for marking and as black paints. Such parti- cularly is the black earthy fubftance called Killovv; faid by Dr. Merrett, in his Pinax rerum Britannicarum, to be found in Lancafliire; and by Mr. Da Cofta, in his hiftory of fof- fils, to be plentiful on the fide, near the top, -of Cay- Avon, an high hill in Merionethfhire. The killow has fome- what of a bluifh or purplilh caft mixed with its blacknefs., as the black chalk likewife has: hence it is named by Merrett blue marking ftone, lapis caruleus killow diSlus ducendis lineis idoneus. There is a harder and fofter kind of it, killoia diiriiifcula et molliufcula of Woodward's me* thod of foifils. II. Pit coal. From the deep gloffy black colour of fome of the com- mon forts of pitcoal, I was induced to make trial of them as paints : their affinity to oils, in virtue of their bitumi- nous nature, promifed alfo fome advantages, in oil paint- ing, above the fubftances of a more earthy kind. Several of the finer pieces, levigated into an impalpable powder, were mixed both with oil and with gum water, and applied on paper and on wood. Both mixtures, when laid on thick, appeared of a pretty good black colour, though much in- ferior to that of the coal at firfl ; and the oily one feemed to dry fooner than oil paints generally do. Laid on thin, or in a dilute ftate, they looked brown, not of the grey colour which refill ts from the dilution of a pure black. Pitcoal therefore may be conlidered, not as a true black, but as a brown-black j a colour on many occafions wanted in painting, and which, as I have been informed by an in- genious [ 325 ] genious artift, is often in buiinefs produced with this ma- terial. As different forts of pitcoal, and different pieces from one pit, differ much from one another in degree and fpecies of colour, fome care (liould be taken in the choice of them, according to the purpofe they are intended for. All the forts, at leaft all which I have tried, require long grinding in order to their being reduced into a powder of fufficiont finenefs. III. Black fands. The black fands, one of the brighteil and moft beautiful ©f which is found in Virginia, lofe their colour on being ground into powder, and hence cannot be ufed as pig- ments. There are however cafes, in which they may con- tribute to the embellifliment of certain works, by being ftrewed upon oil paintings for a fparkling black, in the lame manner as fmalt is ftrewed for blue. In this inten- tion they are ufed on writings, preferably to the white fands, as they do not weaken the colour of the ink, but coincide with its blacknefs, and give an agreeable luftre. IV. Black-lead. This mineral is dug in our own country; and is here, as Dr. Woodward obferves, in the prefice to his method of foffils, more plentiful, and of a better kind, than in any other part of the world. According to Dr. Plott's account, in the Philofophical Tranladlions, No. 240, it is found only at Kefwych, in Cumberland, and is there called wadt or kellow, by which laft name, as we have already taken notice, an earth like die black chalk is diftinguiflied in other places. The colour of black-lead, rather a deep fhining bluifh frey than a black, may be feen, diluted a httle, in the black U u 2 melting [ 326 ] melting pots when broken or the furface fcraped off, and entire in the genuine fort of black pencils. It differs not a little in goodnefs, fome forts marking paper freely, and others very difficultly or fcarce at all. It is fmooth and as it were undtuous to the touch, and hence is fometimes ufed inftead of oil or foap, for giving flipperi- nefs to the rubbing parts of machines. Acids neither diffolve it, nor alter its colour or uniftuofity. Black-lead has not been found to contain any of the metal from which it receives its name, and its compofition appears to be of a very fingular kind. From its known refiftence to vehement degrees of fire, whether urged by itfelf in clofe veffels, or made with clay into melting-pots and placed among the burning fuel, it fhould feem that it could not partake largely of any volatile fubftance ; and it has been generally fuppofed to confift chiefly of a talky earth. But Mr. Quift relates, in a curious paper of ex- periments on black-lead, publilhed in the Swedifh tranf- aftions for i7i;4, that having expofed many different fpe- cimens of this mineral to a flrong heat, on a fcorifying difh under a muffle, they all yielded fulphureous fumes and flowers in great abundance; and that there remained be- hind, from one fort, only a fifth part of its weight, and from another no more than a twentieth part, of a yellow or brown calx,which being treated with inflammable fluxes^ yielded feven tenths its weight of a metallic mafs, which feemed to be a mixture of iron and tin. Agreeably to thefe experiments, in an effay for a new fyftem of mine- ralogy, publiihed lately in Sweden, afcribed to the cele- brated Mr. Cronftedt, and which bears ftrong marks of great knowledge and experience in the mineral kingdom, black-lead is claffed among the fulphureous minerals, and c^iWcdifuIphiir Jhtiated with iron and tin. I could [ 32? ] I could not perfuade mylelf tliat the minerals, on which the above experiments were made, could be fuch ;is arc called among us black-lead, till fome of the fineft black- lead of our pencil-makers, weighing one hundred and fixty-eight grains, in three pieces, having been kept of a moderately llrong red heat on a fcorifying di(h for three hours,with the common precaution of covering the veflel for a time, left: the matter (liould crackle, and fome par- ticles be thrown off" from it in fubftance ; I found it re- duced to about an hundred and twenty grains, and all the pieces changed on the outfide to a fparkling rufty brown calx, of which a confiderable part was attraded by a mag- netic bar, the internal parts continuing of the fame appear- ance as at firft. Being then broken into fmaller pieces, and expofed to a like heat for two hours, it fuffered the fame change as before, and was reduced to about lixty grains. Being further broken, and calcined with a mo- derate red heat for ten hours, it was diminifhed to thirty grains j and by a repetition of this operation, to twelve grains, jor a fourteenth part of its firft: weight. The remarkable diflipation, in thefe experiments, of a fubftance which in clofe velTels refifts intenfe fires, may he foniewhat illuftrated by the known property of charcoal, which when excluded from the aftion of the air, whether by being inclofcd in a vefl'el, or mixed with clay into a mafs, remains unconfumed and unaltered in the fire. Mafles of black-lead feem to calcine and fuffer a diftlpation only on the furface ; the internal part remaining long un- changed, unlefs the mafs be broken, or the calx rubbed off, fo as that i'rcih furfaces may be expofed to the air. The common black-lead melting-pots, made of clay and the coarfer kinds of black-lead powdered, like thofe made of clay and charcoal powder, lofe their external blacknefs with part of their weight, and thus have their ft:aining quality r 328 J ^ality deftroyed, by ftrong fire. Hence furnaces made of thefe pots, as defcribed at the beginning of this volume, after they have fufFered ftrong fire, ceafe to difcolour the hands. Black-lead in fine powder, ftirred into melted fulphur,, unites with it fo uniformly, and in fuch quantity, in virtue perhaps of its own abounding with fulphur, that though. the compound remains fluid enough to be poured into moulds, it looks nearly like the coarfer forts of black-lead itfelf. Probably the way which prince Rupert is faid to have had, mentioned in the third volume of Dr. Birch's Hiftory of the Royal Society, of making black-lead run like a metal in a mould, fo as to ferve for black-lead again,, confifted in mixing with it fulphur or fulphureous bodies. On this principle the German black-lead pencils arc fejd to be made ; and many of thofe which are hawkedi about by certain perfons among us, are prepared in the fame manner : their melting or foftening, when held in a candle, or applied to a red hot iron, and yielding a bluifli flame, with a ilrong fmell like that, of burning brimftone,, betrays their compolition ; for black-lead itfelf yields no fmell or fume, and fuffers no apparent alteration, in that heat. Pencils made with fuch additions are of a very bad; kind : they are hard, brittle, and do not caft or make a mark freely either on paper or wood, rather cutting or. fcratching them than leaving a coloured ftroke. The true Englifh pencils (which Vogel in his Mineral Syftem, and fome other foreign writers, imagine to be pre- pared alfo by melting the black-lead with fome additional fubftances„ and cafl:ing it into a mould) are formed of black-lead alone,, fawed into Hips, which are fitted into a groove made in a piece of wood, and another flip of wood glued over them : the fofteft wood, as cedar, is made choice of, that the pencil may be the ealier cut ; and a part [ 3*9 ] part at one end, too fliort to be conveniently ufcd afttt the reft has been worn and cut away, is left unfilled with the black-lead, that there may be no wafte of fo vaUiahle a commodity. Thefe pencils are greatly prefcraMe to the others, though feldom fo perfedl as could be wijhed, being accompanied with fome degree of the fame inconveiN- ences, and being very unequal in their quality, on account of different forts of the mineral being fraudulently joiaerf together in one pencil, the fore part being commonly pretty good, and the reft of an inferior kii\d. Some, to avoid thefe imperfedlions, take the finer pieces of blaek-lead itfelf, which they law into flips, and fix for ufe in port-' crayons : this is doubtlefs the fureft way of obtainii^ black-lead crayons, whofe goodnefs can be depended on. V. Black 'vegetable Juices. The excellent black varnifh of China and Japan, which has hitherto been but imperfedly imitated in Europe, and which was formerly thought to be an artificial compofition of refinous bodies coloured with black pigments, has beeft ■difcovered, by the later travellers in thofe countries, to be a native juice, exuding from incifions made in the trunks of certain trees. One of thefe trees, according to the ac- count given of it in Kaempfer's Amcsn'itates exoticee, is that whofe fruit is fometimes brought to Europe, as a medicinal drug, under the name of anacardium. The anacardium itfelf, as it comes to us, is remarkable f(7r a black-colouring juice. It is a kind of nut, with a double fiiell, containing, in the fpace between the outer and the inner (hell, « fungous fubftance filled with a dark- coloured vifcous fluid, which is eafily forced out, by out- ing the nut, and fqueezing it between the fingers : a Iittl. There are however trees, natives of our own American, colonies, which appear to contain juices of the fame nature with the valuable produdions of the Indian. Of this kind, are feveral, and perhaps the greater number, of the fpecies of toxicodendron or poifon-tree. Mr.Catelby,inhishiftory. of Carolifia, defcribes one, called there the poifon-afli, from whofe trunk flows a liquid, black as ink, and fuppofed to be poifonous : this reputed poifonous quality, as I have been informed by fome gentlemen of that country, has hitherto deterred the inhabitants from attempting to coi- led: or make any ufe of it. The abbe Mazeas, in the Philofophical Tranfadions, vol. 49, for the year. 1755,, gives an account of three forts of the toxicodendron, raifed in a botanic garden in France,. containing in their leaves a milky juice, w^hich in drying became of a deep black, and communicated the fame colour to the linen it was dropt< on : the linen, thus ftained, was boiled with foap, and came out without the leaft diminution of its colour; nor did ftrong ley of wood-afties make any change in it. Several of thefe trees have been raifed in the open ground in England: fome of them ftill, remain in the. biihoj), [ 331 ] bilhop of London's garden at Fulham, after having been long negleded, and fuffered many fevere winters : fee a catalogue of the exotic trees in this garden, by Dr. Watfon, in the Philofophical Tranfadions, vol. 47, for the years 175 1 and 1752. They appear alfo to perfedl their co- louring juices, in this, nearly as well as in their native cli- mate. The fpecies called by Mr. Miller the true lac tree, of wb'ch I was favoured with a branch by himfelf from Chelfea garden, was found to contain, in its bark, and in the pedicles and ribs of the leaves, a fomewhat milky juice, which foon clvanged in the air to a reddilh-brown, and in two or three hours to a deep blackiih or brownifli-black colour: wherever the bark was cut or wounded, the in^ cifion became blackifh; and on feveral parts of the leaves the juice had fpontaneoufly exuded, and ftained them of the fame colour. This juice, diopt on linen, gave at firft little or no colour, looking only like a fpot of oil; but by degrees the part moiftened with it darkened in the fame manner as the juice itfelf. On wafhing and boiling the linen with foap, the ftain not only was not difcharged, but feemed to have its blacknefs rather improved; as if a brown matter, with which the black was manifefHy de- bafed, had been in part waflied out, fo as to leave the black more pure. It were to be wifhed that fome attempts were made, for collefting the colouring juices of thefe trees, in fufficient plenty, for anfvvering the important purpofes to which they promife to be applicable. Perhaps alfo means might be found of introducing into fome parts of the extenfive dominions of Great-Britain, in which all varieties of foil and climate are now to be met with, the oriental trees themfelves, to which fome of the Indian manufadures are fuppofed to owe diftinguiflied advantages. This there ar£. now fome grounds to hope for, from the patronage of a ■X X fociety, , [ 332 ] fociety, whofe encouragement has already fo greatly pro- moted the culture of many valuable plants and trees. As the milky juices of fome of our common plants turn dark-coloured or blackifli in drying, I was induced to make trial of feveral of them on linen : The milks of wild pop- pies, garden poppies, dandelion, hawkweed, fowthiftle, gave brown or brownifh-red ftains, which were difcharged by wafhing with foap : the milks of the fig-tree, of let- tuces, and of different kinds of fpurges, gave no colour at all. The colourlefs juice which iilues from hop-flalks when cut, ftains linen of a pale reddilh or brownifh-red, extremely durable : I tried to deepen the colour by re- peated applications of the juice, but could never make any approach to blacknefs. The juice of floes gave likewife a pale brownifh ftain, which, by repeated wafhings with foap, and wetting with flrong folution of alcaline fait, was darkened to a deeper brown : on baking the floes, their juice turns red, ajid the red ftain which it then imparts to linen is, on wafliing with foap, changed to a pale bluifh, which alfo proves durable. The juices both of the raw and baked floes were applied repeatedly on the fame fpots, in order to deepen the refpeftive colours ; and the brown or reddifh-brown flain of the raw floe, and the blue of the baked, were applied on one another, on principles here- after explained. In all thefe ways a ftain was obtained, which when flightly wafhed with foap, looked of a pretty deep black ; but by longer wafliing, much of the colour was difcharged, and little more was left than a Angle ap- plication of the juice would have produced. The floes were tried in different ftates of maturity, from the begin- ing of feptember to the middle of december ; and the event was always nearly the fame. Though thefe experi- ments, with many others of the fame kind, proved unfuc- cefsful in regard to the produdion of the colour here inten- ded. I [ 333 J ded, they ferve to point out means, which may be convenient and ufeful on feme occafions, of marking linen with a colour, pale indeed, but fufficiently vilible, which foap does not difcharge. In the fifth volume of the celebrated Linnacus's Am [ 354 ] is difcharged by acids, and reftored again by the fulphure- ous vapour or Iblution. Calces of lead, melted with fulphur, form a black or blackifli mafs, which proves an ufeful matter for taking cafts from medals, being confiderably more tough than fulphur alone. For this purpofe, equal parts of minium and flowers of fulphur are put in an iron ladle over the fire, till they foften into the confidence of pap, and are then kindled with a piece of lighted paper, and flirred for fome time : the veflel being afterwards covered clofe and continued on the fire, the mixture becomes fluid in a few minutes, and is then poured upon the medal previoufly oHed and wiped pretty clean. This procefs, communi- cated to me by a friend, I have often tried with fatisfacflion. The cafts are very neat ; the colour, fometimes a pretty deep black, and fometimes a black grey, according to dif- ferent circumftances in the fufion ; they are very durable, and when foiled may be wafhed clean again with fpirit of wine. There are other metals alfo which produce a black co- lour with fulphureous bodies. When a folution of filver in aquafortis is added to a folution of fulphur made in alcaline ley, the filver and fulphur unite and precipitate together in the form of a black powder. Quickfilver and fulphur, by being barely rubbed together in a mortar, be- come black, and hence this mixture, commonly made for medicinal ufes, is called the mineral ethiops. But as thefe kinds of compolitions afi"ord nothing of importance for the art of colouring black, it would be needlefs in this place to confider them more particularly. IV. Black from the CQmhination of other colours. In the three foregoing articles we have feen blacknefs • generated from the a<3:ion of certain bodies on one an- other. [ 355 ] other, and, in the preceding fcftion, from alterations pro- duced in the bodies themfelves or their component parts. There is another general principle, on which Ibme of the mofl: common colours are obtained, the combination of two or more differently coloured bodies together, whence refults a new colour compounded of thofe of the ingre- dients : thus green is formed from a mixture of blue and yellow, and purple from blue and red. Thefe compound colours are found to fuccecd, by grinding together coloured earthy powders, by uniting coloured tiames or the funs beams which have palTed coloured through glafles, by mixing dyed wool, threads, &c. where there can be no fufpicion of any intrinfic change made in the fubjedls, or of any adtion of the ingredients on one another. Mr. le Blon, in his harmony of colours, forms black on the fame foundation, by mixing together the three colours called primitive, blue, red, and yellow; and Mr. Cartel, in his optique des couleurs, publiflied in 1740, fays that this compound black, has an advantage, in painting, above the fimple ones, of anfwering better for the dark- ening of other colours. Thus if blue, by the addition of black, is to be darkened into a blue-black, the fimple blacks, according to him, if ufed in fufficient quantity to produce the requilite deepnefs, conceal the blue, while the compound blacks leave it diffinguifhable. Le Blon does not mention the proportions of the three primitive colours neceffary for producing black. Cailel diredls fifteen parts of blue, five of red, and three of yel- low j but takes notice, that thefe proportions are rather fpeculatively than pradically juff, and that the eye only can be the true judge ; our colours being all very imper- fed:, and our pigments or other bodies, of one denomi- nation of colour, being very unequal in its degree or in- tenfity. He obferves that the colours ihould each be the A a a deepeft [ 3S6 ] deepefl and darkell in its kind ; and that, inftead of taking one pigment for each colour, it is better to take as many as can be got ; for the greater contrail there is of hetero- geneous and dilcordant drugs, the more true and beautiful, he fays, will the black be, and the more capable of uniting with all other colours, without fuppreffing them, and even without making them tawney. The trials I have made of mixing different blue, red, and yellow powders, have not fucceeded fo far as to afford a perfedl black ; but I have often obtained from them very dark colours, fuch as may be called brown-blacks and grey-blacks, fuch as we commonly fee in the dark parts of paintings, and fuch as the charcoal and foot blacks appear when diluted a little. The ingredients being each of a dark deep colour is a very neceffary condition ; for bright blues, bright reds, and bright yellows, mixed in fuch proportions that neither colour prevailed, produced only a grey. In effedV, all compolitionsof this kind, phy-^ fically confidered, can be no other than greys, or of fome of the intermediate teints between whitenefs and blacknefs j and thefe greys will be fo much the lighter or darker, as the component colours of themfelves are bright or dark. Some further experiments of producing a black by com- poiition, for the purpofes of dying and Gaining, will be mentioned in the fequel of this effay. SECT. V. BlacA paints, varniJJjes, Gfc. I. Black point with oil. LACK oil paint is prepared, by grinding, with a B proper quantity of oil, the charcoal or foot blacks, or the natural black earths, or pitcoal, till they are united into a fmooth, uniform, thick compound, which is occa- fionally [ 357 J fionally diluted with more oil, to a due confiilencc ioi being worked freely with the brufli or pencil. The fineft: black colour is made with ivory-Mack, ground, before the addition of the oil, into an impalpable powder. The material moft commonly made ufe of is lamp-black, whofe colour is for moft purpofcs of fuflicient deepnefs and beauty. The unduofity of lamp-black gives it an advantage above the other pigment;, of mixing more eafily and perfedly with tlie oil ; but from the lame qua- lity it receives a difadvantage, of being too flow in drying for the difpatch rcquifite in bufintfs. Some deprive it of this imperfeftion by burning it, that is, by heating it red hot in a clofe veflcl ; but being by this means reduced to the ilate of coal, it is deprived alfo of its eafy mifcibility with oil. It may, however, be made to dry as fpeedily as other oil paints generally do, by a due preparation of the oil i as particularly by fetting it on fire and boiling it, in the manner hereafter defcribcd, in the tenth article of this fedtion, for making printers ink. The oil, for all paints, requires fome preparation, to promote its own drying ; and the method here recom- mended appears for this purpofe both the mofi: expeditious and the moft effedlual : The dark colour, which it com- monly acquires in the procefs, and which renders it unfit for the brigliter coloured paints, is of no inconvenience to it for blacks. The oil is made confiderably thick by the boiling, and being in this ftate well mixed with the black matter, the mixture is diluted for ufe with unboiled oil, to which it communicates a fufiicient degree of the drying quality defired. II. Black paint with tvater. An opake deep black for water colours is made, by grinding ivory-black with gum water ; or with the liquid Aaa 2 which [ 358 ] which fettles from whites of eggs, after they have been beaten up and fiiffered to ftand a little. Some ufe gum water and the white of eggs together ; and report, that a fmall addition of the latter makes the mixture flow more freely from the pencil, and improves itsgloflinefs. It may be obferved, that though ivory-black makes the decpeft colour, in water as well as in oil painting, yet it is not always, on this account, to be preferred, in either kind, to the other black pigments. A deep jet black colour is feldom wanted in painting j and in the lighter {hades, whether obtained by diluting the black with white bodies, or by applying it thin on a white ground, the par- ticular beauty of ivory-black is in great meafure loft : the fame intentions may be anfwered by pigments of lefs price and more eafily procurable. A valuable black for water colours is brought from China and the Eaft-Indies, fometimes in large rolls, more commonly in fmall quadrangular cakes, generally marked with Chinefe charafters. By dipping the end of one of the cakes in a little water, and rubbing it about on the bottom or fides of the veflel, a part of its fubftance is taken up by the water, which may thus be readily tinged to any ihade of black or grey, from fuch as will juft colour paper, to a full black. The compofition of this Indian ink has not hitherto, fo far as I can learn, been revealed y and I therefore made fome experiments with a view to difcover it. Though the Indian ink is readily diffufed through water, it is not truly diffolved : when the liquid is fuf- fered to ftand for fome time, the black matter fettles to the bottom in a muddy form, leaving the water on the top colourlefs ; in the fame manner as the common black pigments fettle from diluted gum water. The ink, kept moift, in warm weather, becomes in a few days putrid, like [ 359 ] like the fluid or foft parts of animals ; as does likewife the clear water, after the black matter has fettled and been feparated from it. The Indian ink appears tlx:rc- forc to contain an animal fubllance foluble in water ; and to confift of a black powder mixed with fome animal glue. For the greater certainty in regard to this conglu- tinating ingredient, I boiled one of the China cakes in Icveral frelh portions of water, that all its foluble parts might be extracted, and having filtered the liquors through paper, fct them to evaporate in a flone bafon : they fmelt like glue, and left a very confiderable quantity of a tena- cious fubrtance, which could not be perceived to differ in any refpcdt from common glue: Being thus convinced of the compofition of the mafs, I tried to imitate it, by mixing fome of the lamp-black, which I had myfelf prepared from oil (fee page 342) with as much melted glue as gave it fufficicnt tenacity for be- ing formed into cakes. The cakes, when dry, anfwered fully as well as the genuine Indian ink, in regard both to the colour, and the freedom and fmoothnefs of working. Ivory-black and other charcoal blacks, levigated to a great degree of iinenefs, which requires no fmall pains, had the fame effeft with the lamp-black ; but in the flate in which ivory-black is commonly fold, it proved much too gritty, and feparated too haftily from the water. III. Cojnpofition for marking p^ee p. Great quantities of wool are annually made unfer- viceable by the pitch and tar, with which ilieep are marked, and which are commonly not laid on with a fpa- ring hand, as they confiderably increafe the weight of the fleece at a trifling expence. With a view to prevent, as much as poiTible, this great wafl:e of fo ufeful a commo- dity, the fociety inftituted in London for the encourage- ment [ 36o ] ment of arts, manufactures, and commerce, and who continue vigoroufly and judicioufly to profecute the im- portant ends of their inftitution, offered a confiderable premium for the dilcovery of any cheap compofation, that might fupply the place of thofe hurtful materials j whofe colour Ihould be ftrong and lafting, which fhould bear the weather a proper time, and not damage the wool. Several propofals for this purpofe were laid before the fo- cie'ty, but none of them have as yet been thought deferv- ing of the premium. The enquiry having been warmly recommended to nie by the late Dr. Hales, as an objedt of very great importance to the woollen manufacftury, I went through a fet of experiments with this view in the year 1759. It was hoped, that the ill qualities of tar and pitch might be corredled, by mixing with them fome foap or fize, which fhould prevent their too great adhefivenefs, and render them fo far diffoluble in water, as to be dif- chargeable from the wool by the means commonly prac- tifed for cleanling it ; or, in failure of tar and pitch, that fome compolition of refms, oils, or fats might be found, which iliould be rendred harmlefs to the wool by the fame correctors, and which fhould ferve as fufficient ce- ments for certain coloured powders, among which black appeared to be the bell:, as being the flrongefl: and moft confpicuous colour. On thefe principles many trials were made, but with little fuccefs : for the undtuous and re- finous materials, with the advantage which they received from the foap or fize, of being eaiily wafhed out from the wool, received alfo the difadvantage of being too foon difcharged by the weather. It was next confidered, that as wool has always a natu- ral greafinefs, which the workmen wafh out with ftale urine, foap, or ley, as defcribed in the fequel of this hif- tory; [ 36i ] tory; the common animal fats might probably be difchurged liom it by the fame means, fo as not to ftand in need of tliofe ingredients, from which the foregoing compofiti- ons had contradted the imperfcdtion of being too eafily diftliargeablc. Accordingly 1 melted fbme tallow ; and flirrcd into it fo much charcoal in fine powder, as made it of a full black colour, and of a thick conllftcncc. This mixture, eafily procurable and at fmall cxpence, being ap- plied warm with a marking iron on pieces of flannel, quickly fixed or hardened, bore moderate rubbing, refirted the fun and rain, and yet could be wafhed out freely with foap, or ley, or llalc urine. All the good qualities, that can be defircd in a compofition for marking (heep, appeared dierefore to be united in this fimple preparation. Though the mixture of tallow and charcoal powder was found fufiiciently durable when applied as above upon pieces of flannel ; it occurred, that it might neverthelefs, by the repeated attritions to which it is cxpofed on the body of the animal, be in danger of being rubbed off too fbon. If we could add to the compofition a little pitch or tar, we fliould efl'ediually fecure againfl: any inconveni- ency of this kind, and it was apprehended that thefe in- gredients might here be added with fafety ; for being perfedtly dillblved by the tallow, it might be prefumed that they would wafh out along with it from the wool. Thus we fee flains of tar got out from clothes by means of oil, which diliblving the tar, the whole compound is tlien difcharged by the fame detergents that the oil itfelf would be. I therefore melted fome tallow with an eighth, with a fixth, and with a fourth of its weight of tar, and having thickened the mixtures with charcoal powder, fpread them while hot upon pieces of flannel. None of the compofitions could be difcharged by any rubbing or ■wafhing with water. By foap they were all wafhed out completely ; [ 362 ] completely ; that which had the fmalleft proportion of tar, eafily enough ; that which had the largeft proportion, difficultly. If therefore it fhould be feared, that the tallow will fail in point of durability or adhefivenefs, which, however, I do not apprehend that it will j it is plain, that as much as can be defired of this quality may be commu- nicated, without damaging the wool, by a proper addition of the fubftances commonly made ufe of. I do not con- ceive that the nature of the thing can admit of any greater perfedlion. There is a material circumftance In this affair, which does not feem to have been fufficiently confidered by thofe who propofed the enquiry. If we could difcover, which fome have fruitlefsly endeavoured to do, a ftaining compo- fition in the nature of a dye, polTeffing all the good qualities that have been mentioned ; it would fcarcely be poffible, as matters fland at prefent, to induce the farmers to make ufe of it. They require a fubftance that will add weight : and I apprehend it will be no fmall recommendation to the above compofition, that in this reipedl, as in all others except its being innocent to the wool, it agrees as nearly as can be expefted, with the materials to which they have been long familiarized. IV. Compofition for preferving wood, &c. The great adhefivenefs, which renders tar unfit as a principal ingredient, and excellent as a fecondary one, for the purpofes of the foregoing article, adapts it to another ufe, on fome occafions not a little important ; the preferv- ing of fome kinds of wood on the outfides of certain build- ings, the covering of flieds, paling, &c. as alfo for coating common tiles, in imitation of the black glazed tiles, which are fold at a much higher price. Tar [ 363 J Tar and pitch of themfelves are too foft for thefe in- tentions ; even the pitch being Uable to be melted off by tlie heat of the fun in fummcr, however firm in the cold of winter. Different powdery fubftances, as afhes, ochres and other mineral pigments, have been mixed with them, but without remedying the imperfe<5tion fo effedually as could be wiihed. In the SwediHi tranfadtions for the years 1742 and 1740, two compofitions are recommended, which are faid to be firm, durable, and gloffy. One is prepared by melting the tar over a moderate fire, fo as to make it fluid but not to boil, and ftirring in as much coal dufl: as v\'ill render it thick : this mixture, the author fays, is to be laid on with wooden trowels, in a hot day, as thick or as thin as fhall be thought proper. The other is prepared by mixing the melted tar with a futficient quantity of lamp-black : a little of this mixture is fpread upon the upper fide of each tile with a fliff, fhort-haired, painting brufh : next day, when dry, the tiles are done over with tar alone, and two days after with tar again : this coating being well dried, which in fum- mer, according to the author, is generally in eight or ten days, fome powdered lead ore is ffrewed over it, and well rubbed in, hrit with a coarfe and afterwards with a fine linen cloth ; from this it receives a fparkling appearance. I tried both thefe compofitions, and found them of a good black colour : when the bodies coated with them are held before the fire till the furface begins to run, they become glofly. They are not however wholly exempt from the inconveniencies complained of in the others. For though the tar was made as thick, both with the coal dufl and lamp-black, as was confiffent with its being fpread fmooth even in a hot fun and while warm from the fire, it afterwards foftened in the fun confiderably j B b b though [ 364 ] though the parts, which the fiin did not immediately fhine upon, proved fufficiently firm in the hotteft weather. By coal duft, in the firft compofttion, is meant pow- dered charcoal. Sufpedling however that pitcoal, in vir- tue of its bituminous nature, might unite more perfeftly with the tar, and be in fome meafure diffolved by it, I made trial of this alfo, chufing the fineft coloured pieces, of thofe kinds which melt in the fire, and grinding them into impalpable powder. The mixture of this powder with the melted tar, made of fuch confidence as to be freely fpread while warm with a brufh, feemed to foften lefs in the funs heat than either of the other two. The durabi- lity of thefe compofitions I cannot yet determine : after having flood, without any apparent alteration, one fum- mer and v/inter, they continue expofed to the weather, for difcovering what effefts longer time and viciffitudes of feafons may have upon them. The mixture of tar and lamp-black is found the moft effedtual prefervative for the mafls and yards of fliips,. Such parts of the maft, as the Aiding up and down of the fails requires to be only greafed, and thofe which are covered with turpentine or refin mixed with tallow or oil,, generally contract large rents, while the parts coated v.ith tar and lamp-black remain perfeftly found. I have been favoured by a gentleman on board of a veffel in the Eafl- Indies, with an account of a violent thunder ftorm, by which the mainmaft was greatly damaged, and whofe effedts on the different parts of the mafl were pretty re- markable. All the parts which were greafed, or covered with turpentine, were burfl in pieces : thofe above, be- tween, and below the greafed parts, as alfo the yard arms, the round top or fcaffolding, &c. coated with tar and lamp-black, remained all unhurt. In [ 365 ] In this place it may be proper to obferve, that the coat- ing or painting of wood does not in all cales contribute to its prefervation : unlefs the wood be very thoroughly dry, efpecially thofe kinds of wood whofe juices are not oily or refinous, the painting, by confining the watery fap, haftens the corruption. Several prclfes for a paper manu- fadlure having been made of heart of oak feemingly very dry, fome of them, which with injudicious care had been well painted over, rotted and perilhed in a few years, while the unpainted ones continued for many years per- fec'tly found. V. Cotnpofitions for blacking leather. In the tanning of leather, it is fo much impregnated with the aftringent parts of oak bark, or with that matter which rtrikes a black colour with green vitriol, that rub- ing it over three or four times with a folution of the vitriol, or with a folution of iron made in vegetable acids, is fufficient for ftaining it black. Of this we may be convinced, by dropping a little of the folution on the unblacked fide of common flioe leather. This operation is performed by the currier, who, after the colouring, gives a glofs to the leather with a folution of gum-arabic and fize made in vinegar. Where the previous aftringent impregnation is infufficient to give a due colour, and for thofe forts of leather which have not been tanned, fome galls or other aftringents are added to the folution of iron; and in many cafes, particularly for the finer forts of leather, and for renewing the black nefs, ivory or lamp- black are ufed. A mixture of either of thefe with lin- feed oil makes the common oil blacking. For a ihining blacking, fmall beer or water are taken inftead of oil, in the quantity of about a pint to an ounce of the ivory- black, with the addition of half an ounce of brown fugar B b b 2 and [ 366 ] and as much gum-arabic. The white of an egg, fubfti- tuted to the gum, makes the black more Ihining, but is fuppofed to hurt the leather and make it apt to crack. It is obvious> that all thefe kinds of compofitions admit of many variations : it is fu£Ecient here to have given a general idea of them. VI. Spirit varni/Jj. Black varnifli, for japanning on wood or leather, is prepared by mixing lamp-black or ivory-black with a proper quantity of a llrong folution of gum lac in fpirlt of wine, fuch as that defcribed in the preceeding part of this work, page 224. The lamp-black is commonly pre- ferred to the ivory-black, on account of its uniting better with the fluid, and working fmoother. The thicker part of the varnilh, which fettles at the bottom, is ufed with the lamp-black for the firft coatings, and the mix- ture applied at different times, in a hot room, one layer after another is dry, till a full body of colour is obtained : after which, the piece is waflied over in the fame man- ner, feverd times, with the finer part of the varnifh, jufl tinged with the black, lb as to make a coating of fufhcioat thicknefs to bear polilhing with tripoli. VII. Amber varni/Iies for papier mache, &c. Papier mache is made of cuttings of white or brown' paper, boiled in water, and beaten in a mortar, till they are reduced into a kind of pafte, and then boiled with folution of gum-arabic or of fize, to give tenacity to the pafte, which is afterwards formed into different toys, &c. by preffing it into oiled moulds. When dry, it is done over with a mixture of lize and lamp-black, and after- wards varnillied. The black varnifh for thefe toys (of which the firft account I have feen is in a pamphlet on drawing. [ 367] drawing, &c. printed for Mr. Pcele in 1732, and laid to be taken chiefly from manulcripts left by Mr. Boyle) is prepared as follows. Some colophony, or turpentine boiled down till it be- comes black and friable, is melted in a glazed earthen vefTel, and thrice as much amber in fine powder fprinkled in by degrees, with the addition of a little fpirit or oil of turpentine now and then : when the amber is melted, I'prinkle in the fame quantity of farcocolla, continuing to Itir them, and to add more fpirit of turpentine, till the whole bedomes fluid : then ftrain out the clear through a coarfe hair bag, prefling it gently between hot boards. This varnifli, mixed with ivory-black in fine powder, is applied, in a hot room, on the dried paper paiici which is then fet in a gently heated oven, next day in a hotter oven, and the third day in a very hot one, and let ftand each time till the oven is grown cold. The parte thus varniihed is hard, durable, glofly, and bears liquors hot or cold. A more fimple amber varnilh, of great ufe for many purpofes, and faid to be the bafis of the fine varnifhes which we fee on coaches, 6cc. is prepared, by gently melt- ing the amber in a crucible till it becomes black, then re- ducing it into a powder, which looks brown, and boiling the powder in linfeed oil, or in a mixture of linfeed oil and oil of turpentine. Drying oil is commonly made choice of by the workmen; but it feems more eligible here to take the oil unprepared, that the boiling, requifite for giving it the drying quality, may be employed at the fame time in making it ad: upon the amber. By the previous melting of the amber, its nature is changed, and part of its oily and faline matter expelled, as happens in the common diftillation of it. When the diftillation is not far protradcd, the caput mortuum, or Ihining [ 368 ] ihining black mafs which remaims in the retort, anfwers as well as the iimber melted on purpofe. Hence fome of our chemifts, inftead of urging the diflillation to the ut- moft, by which the amber would be reduced to a mere coal, find it more advantageous to difcontinue the procefs when the thinner oil and greater part of the fait have arifen, that the remaining mafs may be in great meafure foluble in oils, fo as to fupply the common demand of the varnifh makers. It has generally been thought, that amber will not at all difTolve in oils, till it has thus fuffered a degree of de- compofition by fire. Hoffmann relates an experiment* in his obfervationes phyjico-chemicce, which difcovers the folubility of this concrete in its natural ftate. Pow- dered amber, with twice its quantity of oil olive, was put in a wide-mouthed glafs ; and a digeftor, or ftrong copper veflel, being filled about one-third with water, the glafs was placed in it, the cover of the digeftor fcrewed down tight, and a moderate fire continued an hour or more : when cold, the amber was found difiblved into a gelatinous tranfparent mafs. In Dr. Stockars veiy cuvions fpecimen inaugin-ale de fuc~ cino., printed at Leyden, in 1760, there are lundry more important experiments on this fubjedt, made by himfelf, conjointly with my worthy correfpondent Mr. Ziegler of Winterthur. They found that by continuing a fimmer- ing heat twelve hours, and confining the vapour as much as ftone-ware veflels would bear without burfting (the dan- ger of which was avoided by making a fmall notch in the cork floppers) powdered amber diffolved perfe6lly in ex- •preffed oils, in turpentine, and in balfam of copaiba : a ftrong copper veflel, with a cover fcrewed on it, feems moft eligible, and for the greater fecurity, a valve may be made in the cover, kept down by a fpring that fhall give way before [ 369 ] before the confined vapour is of fufficicnt force to be in any danger of burfling the vclfel. Though fuch a heat as converts part of the oil into ftrong elaitic vapours, and the forcible compreffurc of the vapour, are expedient for haftening the diflblution, they do not appear to be effen- tially neccflary ; for by digeftion for a week in clofe ftopt glafs vefTels, in wliich the coniprefTure could not be very great, folutions equally perfedt were obtained. The folution in rape-feed oil, and in oil of almonds, was of a fine yellowifli colour ; in linfeed-oil, gold coloured; in oil of poppy-feeds, yellowifh red; in oil olive, of a beautiful red ; in oil of nuts, deeper coloured ; and in oil of bays, of a purple red. It is obfervable that this lafl- oil, which of itfelf, in the greatert: common heat of the atmofphere, proves of a thick butyraceous confif- tence, continued fluid when the amber was diflblved in it. The folutions made with turpentine, and with balfani of copaiba, were of a deep red colour, and on cooling hardened into a brittle mafs of the fame colour. All the folutions mingled perfectly with fpirit of turpentine. Thofe made with the oils of linfeed, bays, poppy-feeds, and nuts, and with balfam of copaiba and turpentine, be- ing diluted with four times their quantity of Ipirit of tur- pentine, formed hard, tenacious, glolTy varniihes, which dried fufficiently quick, and appeared greatly preferable to thofe made in the common manner from melted amben. VIII. Varnipofor metals. Iron fnufF-boxes, mourning buckles, Sec. are coloured black, by making them confiderably hot, and applying on them in this flate a thick mixture of lamp-black, with a certain varnifh called gold-fize. There is a gold- fize, formerly mentioned, for gilding, or fixing gold-leaf on wood, &c. The fize here meant is a compofition of a- different [ 37° ] difFerent kind, confifting of drying oil, turpentine, and the pigment called Naples yellow ; which laft ingredient is ufed for giving a high gold colour to the mixture, to fit it for fome of the other purpofes for which it is em- ployed. In the prefent intention, the yellow might doubtlefs be omitted, and the varnilli formed at once by mixing lamp-black with a proper quantity of turpentine and drying oil. IX. Sealing-ivax, Black fealing wax is compofed of gum lac, melted with one half or one third of its weight of ivory-black in fine powder. The inferior fort of lac, called fliell- lac, anfwers as well for this ufe as the fineft. It is cuf- tomary to mix with it, for the ordinary kinds of fealing wax, a confiderable proportion, as two thirds its weight, of the cheaper refinous bodies, particularly Venice turpen- tine, by which the beauty of the mafs is here lefs injured than in the red wax, and of which a fmall addition is in all cafes expedient, to prevent the compound from being too brittle. The ingredients being melted and well flir- red together over a moderate fire, the mixture is poured out upon an oiled flone or iron plate, and rolled, while foft, into flicks, which afterwards receive their gloffinefs by being heated till the furface begins to fhine. The black figures on the dial-plates' of clocks and watches, which look like black enamel, are formed of the finer kind of black fealing wax, which is melted into ca- vities made in the plate, and afterwards poliflied. Black enamel or flones are fome times imitated in the fame manner in other works. X. Printing [ 371 ] X. Printing ink. Printing ink differs from the common oil paint, defcribed at the beginning of this fedion, only in the preparation of the oil, which mull; here have its confif- tence and tenacity greatly increafed, and its greafmefs di- miniflied, by means of fire. The fame way of prepa- ration, either not carried to fo great a length, or with a fubfequent addition of frefli oil to dilute the mixture, affords, as already obferved, one of the befl drying oils for the black paint. The oils of linfeed and nuts are made choice of for this ufe : the nut oil is fuppofed to be the beft, and is accord- ingly preferred for the black ink, though the darker colour which it acquires from the fire makes it lefs fit for the red. It is faid that the other expreffed oils cannot be fufficiently freed from their undtuous quality ; whencp the ink made with them dries exceeding flowly, is apt to come off and fmear the paper in the beating and prefTing which it undergoes in the book-binders hands, or finks into the fubflance of the paper, beyond the mark of the type, and ftains it yellow. Ten or twelve gallons of the oil are fet over the fire, in an iron pot, capable of holding at leall half as much more; for the oil fwells up greatly, and its boiling over into the fire would be very dangerous. When it boils, it is kept flirring with an iron ladle ; and if it does not itfelf take flume, it is kindled with a piece of lighted paper or burning wood ; for fimple boiling, without the actual accenfion of the oil, does not communicate a fufB- cient degree of the drying quality required : it feems to be in the more inflammable parts, which are fooneft con-^ fumed by the burjiing, that tlie injurious fatnefs or greafi* nefs confifls. The oil is fuffered to burn for half aij Ccc hour [ 372 ] hour or more, and the flame being then extlnguifhed by- covering the veffel clofe, the boiling is afterwards con- tinued, with a gentle heat, till the oil appears of a proper Gdnfiftence : in which ftate it is called varnifli. It is necelfary to have two kinds of this varnifh, a more and lefs boiled, or a thicker and a thinner, which are oCca- fionally mixed together as different purpofes may require'; that which is of a jufl confiftence in warm weather proves too thick in cold ; and that which anfwers well for large characters, proves in the fame feafon rather too thin for fmall ones. The thickeft varnifli is of fuch confidence when cold,. that it draws into threads betv/een the fingers nearly like. Weak glue : this is the mark by which the workmen judge ©f the due boiling, a little of it being from time to time taken but for this trial, and cooled by dropping it on a file or other cold body. It is very vifcous and tenacious,, like the foft refinous juices or thick turpentines. It is laot at all diffolvcd, any more than the oil at firrt:, by Water or fpirit of wine, but mingles readily enough with frelh oil, and unites with mucilages into a mals which: diffolves in water into a milky liquor : by boiling with ftrong alcaline ley it forms a foapy compound j whence the types, after an imprefiion, are cleaned from the ink,, by wafliing, and rubbing them with a brufli, in hot ley. The oil emits, during the whole time of the boiling, very offenfive penetrating fumes : when grown cold, it has an acrid difagreeable tafle, but little ill fmelL The oil is faid to lofe, in being boiled into thick varnifli, from a tenth te an eighth part of its weight, which proportions agree fufficiently with my trials : common linfeed oil, boiled dtown to a confiftence which appeared fomewhat too thick, loil about one fixth : being further boiled, till it became igpite firm when cold, the lofs was near one half. Diffe- rent [ 373 ] rent oBs, and perhaps the fame oil in different ftates, differ in this refpeft : firti oil, boiled to tliicknefs, loft: much more than that of linfeed, the thick matter amounting only to about one fourth of the original weight of the oil. The workmen are accuftomed to add, in the prepara- tion of ten or twelve gallons of oil, as foon as the burn- ing is over, a pound or two of dry crulls of bread, and a dozen or two of onions, by which they fuppofc the greafi- nefs to be more eftedually deftroyed. It may however be queftioned, whether additions of this kind are of much ufe ; for I have prepared the varnifli, feemingly of a very good quality, by fire alone. There is another fort of additions whofe eftedl is more apparent. To give a greater body to the varnilh, and in- creafe its drying quality, a proportion of turpentine is thought neceflary ; and with fome artifts, litharge has in this intention been ;i I'ecret. It is obferved, in the French encyclopedie, by Mr. le Breton, the printer of that work, that when very (jld oil is ufed, neither turpentine nor litharge are needful ; but that, when the oil is new, fome turpentine mull: neceflarily be employed, for without it, the linearing of the paper, by the fpreading or coming off of the Ink, caiuiot be avoided ; that it is much more eli- gible to ufe old oil than to have recourfe to this corredion of the new, both turpentine and litharge, particularly the laft, making the mixture adhere fo firmly to the types, that it is fcarce to be got entirely off" by the ley, whence the eye of the letter is loon clogged up. When turpentine is ufed, it is fird boiled by itfelf, untill, on dipping in a piece of paper, it is found to crumble and part from the paper when cold : the oil be- ing then taken from the fire, the turpentine, while ftill fluid, is poured into it, after which the boiling is re- peate4> and continued till they are fufficiently incorporated. C C C 2 It [ 374 ] It is here fomewhat more difficult to hit .the due point o£ boihng, than when the oil is prepared without addition ^ the mixture being more apt to grow too thick from con- tinuing the heat too long, and full of little hard grains from not continuing it fufficiently j which grains are pro- bably undiflblved particles of the turpentine. The ufe of boiling the turpentine firfh by itfelf is to diffipate its. moiflure or effeutial oil : by the boiling it becomes a re- finous matter, nearly the fame with common refm, which, poffibly would anfwer the fame end. For making the varnilh into ink, lamp-black is tiie- common material ; of which, according to Mr. le Breton,^ two ounces and a half are fufficient for fixteen ounces of the varnifh.' They are ground together on a flone with; a muller, in the fame manner as oil paints. The paper, for printing, is moillened with water; by which it is made more yielding and pliable, fo as not only to be lefs apt to be torn by the types in the prefs, but, likewife to be more clofely and evenly applied to them, , and confequently to take a neater and more perfed: im- preffion. The due moiflure of the paper, and the care and attention of the prefTmea in well working the in.k on the types with the balls, are very material points ; V/ithout which, how excellent foever the ink is, the im- preffion will not be beautiful. The adhefion of printers ink to wetted paper feems to fhew that it is not truly of an oily nature. All exprefled oils contain probably a gummy or mucilaginous matter; and perhaps the tenacity, confiftence, drying quality, and the property of adhering to bodies moiftened with water, . which the oil acquires in the procefs above defcribed, may be all owing to fome of the purer part of the oil being deftroyed, fo as to leave the remainder more gum- my. When the oil dries, it proves a tough flexible fub- ilance> [ 375 ] ftance, which has little difpolition to unite with frcfii oil any more than with water, as if the gummy and oily matter were in fuch proportions, tliat one defends the other from the menftruum that would otherwife diflblvc it: efieirtial oils on the contrary, being free from gum, harden into a nxerely refnious mafs, brittle like other* refins, and which dillblves, like the oil at firft, in frefli oil or in fpirit of wine. The differences obfervcd in different exprefled oils, in regard to the drying quality, may de- pend on the dificrcnt quantities of gummy matter; and the difference of old oil from new, on the gum being in the latter more intimately combined, fo as not to fepa- rate in the burning and boiling. When thefe oils are firll: preifed out from the fubjedts, they abound with muci- lage, great part of which is only luperficially mixed, fo as to give a turbidnefs and opacity to the fluid : in keeping, a part of this loof'e mucilage is thrown off, and the re- mainder may be prefumed to become at the fame time more intimately incorporated with the oil. The repug- nance which we obferve between oil and gum does not in the leaft invalidate thefe conje(5lures, any more than the repugnance between oil and water can be an argument againfl the exiflence of water in oils : indeed we have plain proofs of the coalition of oil with gum, in the ana- lyfis of the purefl gums, gum arabic, fenica, tragacanth, from which an adlual oil is obtained by diflillation. The diflillation of exprelfed oils themtclves feems to favour the opinion here propofed : from all of them there re- mains in the diftilling veffel a large quantity, though from fome more than from others, of a grofs coaly matter j and there arifes a fluid oil, which does not dry or grow thick in the air as the oils did at firfl ; and which is therefore found to anfwer for certain purpofes, as in the lapidaries bufmefs. [ 376] bufinefs, for which the thickening of the oils in their na- tural flate renders them unfit. XI. Rolling-prefs Ink. • Between the rolling-prefs printing.and that of the prin»- ters prefs ftridtly fo called, there is this elTential difference ; that in the former the impreffion is received from figures hollowed in a copper plate, but in the latter from promi- nent types. The damping of the paper is eerations, which wool has to undergo, render the pre- venting [ 4i6 ] venting of harflinefs of more importance here than in cloth. ^'^I. Black dye liiithout galls. O F the galls, direfted in the foregoing procefTes, a part is commonly omitted in bufmefs, and fupplied by cheaper aftringents, which, being weaker in virtue, are taken in quantity proportionably larger. From the pre- fent high price of galls I was induced to try whether this expenfive article could not be entirely fuperfeded. I pro- ceeded exactly according to the French procels with ver- degris, page 411, only inftead of the galls taking fix times as much oak bark, fuch as the tanners ufe : the cloth, well wafhed with foap after the dye, appeared of a black colour, not indeed quite fo beautiful as that dyed in the fame manner with galls, yet not a bad one. I tried fu- ;nach alfo, with the fame event. It appears therefore, that though no effedtual fubftitute to galls could be found for the purpofes of making ink, yet cheaper fubftances may often be made to fuffice in the dying bufinefs, where the great confumptlon of aftringent materials ren- ders the redu6lion of the price of more importance. In the Swedifli Tranfadlions for the year 1753, a fine black is faid to be dyed without galls or logwood ; the place of both which is fupplied by a plant common in Sweden, called there nijcelon or mjoslon-ris, which is ga- thered in autumn while the leaves continue green, and carefully dried that they may retain their green colour. A hundred pounds of woollen cloth are directed to be boiled with fixteen pounds of green vitriol and eight pounds of white tartar, for two hours j and the cloth next day to be rinfed out as after the common alum boiling. A hundred and fifty pounds of the dried mjaelon cut a little., or a fomewhat greater quantity if the plant has [417] has been long kept, are boiled in" water for two hours; and the 7njcelon being then taken out, a Httle madder is put into the Hquor. The cloth is put in along with the madder, boiled for an hour and a half or an hour and three quarters, and afterwards rinfed in water. This dye is faid to be ufed chiefly for fine cloth, and to give lefs harilinefs than the common black. What the mjcelon is, we learn from a paper by Linnaus in the fame Tranfadions for the year 1743- He obferves, that about a year before, a leaf cA\cd jackajhapiick was brought into England from North-America, and mixed with tobacco for fmoaking. Mr. Collinfon favoured him with large fpecimens of it, entitled " the plant Jackafha- " puck which is mixed with tobacco, gathered on *♦ Churchill river in Hudfons bay." This plant, he fays, was eafily known by a Swede, as it grows in Sweden in abundance, on uncultivated gravelly fandy hills. He gives its Swedilh names mjcelon, mjcelon-ris, mjxlbcers-ris y and likewife the latin names under which it is defcribed by different botanic writers, from which it is clear, that the mjcelon is the lame with the ir^\i luji that has lately come into efleem in Germany for medicinal ufe. Some quantity of the wca iirji has been brought from Germany, to be tried as a medicine in this country : the plant is raifed alfo in fome of our botanic gardens, and if the pro- pagation of it Ihould be found of any importance, it would doubtlefs thrive on many ot our now barren hills. I have been informed by a foreign correfpondent, that the uva urfi is faid to be ufed in England for dying black, and that it is imported for this purpofe from Hudfons bay. I cannot find that this plant, or any other from Hudfons bay, is known among our dyers or dry-falters ; but the two foregoing quotations account fufficiently for the report. I made [4i8J I made trial of the German uva urfi both on white and on blue cloth, exadtly according to the Swedifh di- redlions ; boiling the cloth firil with vitriol and tartar, and afterwards with a decoftion of the uva urji: on the ilue cloth I obtained a tolerably good black, but on the white cloth, as with other aftringents, the colour was only a dark brown. I repeated the experiment without the madder, and with a variation in the order of applying the other ingredients, boiling the cloth firfl in a decoftion of the uva urji, and then adding the vitriol and tartar : by this method I obtained, as before, a pretty good black on the blue cloth, but only a brown on the white. I afterwards omitted the tartar alfo, and did not obferve that the want of it occafioned any difference in the colour produced. All the famples dyed brown with uva urji and vitriol, became black on being paffed through log- wood liquor ; but without either logwood or a blue ground, no true blacknefs could be obtained. A dyer, whom I confulted on this head, made fome trials for me, on the uva urji, with the fame event ; this plant giving no black dye with vitriol alone, any more than the other affringents. On adding green viti'iol to a ftrong decodlion of uva urji, I took notice of a phenomenon which did not hap- pen at all with galls, and which I do not remember to have obferved, in fo remarkable a degree, with any of the other flrong aftringents. The liquor, inflead of the uniform appearance of the common black mixtures of this kind, looked like a black powder diffufed through water ; and being written with on paper, the ilrokes ap- peared everywhere unequal and fpecky, as if made with charcoal powder and water, though they were of a deep and durable black where the colouring matter lay thick. This hafty concretion of the black matter from the liquor, while [ 419 ] while It renders the uva urfi entirely unfit for the pur-« pofes of making ink, may pofTibly be of fome advantage to it for the black dye ; as the largenefs of the colouring particles, which concrete in the pores of the cloth, may render them more fixed, fo that lefs of the colouring matter is wafted in the liquor, and lefs of it can be dif- charged from the cloth. To this caufe may perhaps be afcribed a quality of the uva urfi dye mentioned by the Swedifli author, that the cloth is cleaner than after the other black dyes, or requires lefs walhing to free it from the loofe colour. Among many aflringents I have tried, oak wood came the neareft to the uva urfi in this concretion of the colouring matter. A piece of white flannel was boiled firft with oak faw-duft, and afterwards with an addition of vitriol as in the foregoing procefles. The liquor, as foon as the vitriol was put in, became bluilTi-black, though with much lefs bluenefs than the cold infufion of oak-duft and vitriol, page 383 : fome of it being poured off" into a glafs, it appeared full of powdery matter, which foon fettled to the bottom, leaving the liquor of a pale bluilh. From the blue colour of this mixture it was hoped, that a black dye might be obtained from it without logwood or a blue ground ; and in effe and laying bare the gold which it was luppofed to conceal. But the more it was examined, the more did this notion leein impro- bable ; and the more grounds were found for believing, that platina is a metal of a peculiar kind, diflindt in na- ture from gold, as well as from the other metals, though endowed with fuch properties, as had hitherto been lup- pofed to be characteriftic of gold, or to be pollelled by gold alone ; inibmuch that this new metal was reported to have been fometimes fraudulently mixed with gold, in conliderable quantity, without being either fcparable, or diftinguilhable, by any of the common methods in which gold is allayed or refined. A full examination of fuch a body appeared of the utmoft importance, as regarding not only the difcovery of the various properties of the platina itfelf, an objedl fuf- ficiently intercrting, but likewile, what is much more fo, the preventing the abules which it was liable to give* occailon to, and the lecuring the fineneis and value of the precious metal ; fo that if the platina fliould not be rendered an ufeful commodity, it might at leaft be no longer a dangerous one. I had begun this examination in the year 1749, but could not then procure enough of the platina for carrjing the experiments to luch a length as I aimed at; for a metal fo extraordinar}-, entirely new, at leail to tliis part of the world, of which only a few general properties were Mmm 2 known. [ 446 ] known, and thefe but partially and imperfectly, deferved to be ■ fubmitted to all the kinds of operations that are pradlifed on the other metals, and to all the agents by \yhich other metals are found to be affedlcd. In the be- ginning of the year 1754, his excellency general Wall, at that time ambaflador from Spain, enabled me to pro- ceed in the experiments, by fending me about an hundred ounces j and 1 was afterwards favoured with confiderable. quantities more by fome other gentlemen. The moil, ingenious and experienced chemifls in Europe, as foon as they could obtain any of the new metal, entered into the fame purfuits ; and feveral of thefe enquiries have from, time to time been made publick. The firil publication I have feen on this fubjeifl is that of Mr. Wood, in the 44th volume of the Philofophical Tranfadlions, for the years 1749 and 1750. To the hiftorical obfervations, of which an abftrad: has been given, above, Mr. Wood fubjoins a few experiments, made partly, as may be prefumed from their event, on the true platina in grains, and partly on the cafl metal; one- of which experiments, the cupellation of the caft metal, with lead, was afterwards repeated, more circumfpedtly,. by Dr. Brownrigg. In the 48th volume of the Tranfadions, part 2d, for the year 1754, is inferted an account of the principal' 'experiments which had been then made on the platina by me. They are divided into four papers, which are followed in the next volume by two papers more. On the publication of the firft four, I was informed that Mr. SchefFer alfo had given an examination of this metal in the Handl'mgar of the Swedilh academy of fciences for the year 1752. Thofe books being difficultly procurable in this country, and written in a language which I did not underftand, it was fome time before I could [ 447 1 could avail mylelf of his enquiries, which I found to be curious and interefling, and carried, tliough not fu far as could be wiflied, yet much further than could have been expeded, confidering that for his principal experiments he had only a hundred grains of the crude mineral, from which he could pick out but forty grains of the platina to work, upon, and that he had no previous notice of its polleirmg any remarkable properties, but looked upon it at firft as being only un iron mineral ; he afterwards in- deed obtained fome more, but it was only fuch another little quantity, Thefe experiments were made by the encouragement of Mr. aflellbr Rudenfchoeld, who has lately informed me^ in a letter from Stockholm, that he brought the platina from Spain in the year 1745,. nearly four years before it was known in England. In one of the following volumes of the Swedirti Handlingar, there is another paper by the fame gentleman, containing ob- fervationsr on fome parts of mine, concerning the fpecific gravities of mixtures of platina with other metallic bodies. A French tranflation of all the papers above-mentioned, except the 1j.11: of Mr Scheffers and the two laft of mine, which had not come to the tranflators knowledge, was publiflied at Paris in 1758, under the title of la platlne, I'or blanc, ou I'/juitietne metal : to this treatife is added an extra dl of a letter from Venice, relating to what may be called the alchemical hiftory of platina, not containing any new fadts, but fome reflections drav/n from mine. Profeflbr Marggraf, of the academy of fciences at Ber- lin, having obtained a quantity of platina from London, made a large fet of experiments upon it, repeating and further profecuting feveral of mine, and adding many new ones. Thefe appeared firft in a French tranflation, among the Memoires of the Berlin academy for the year *757» printed in 1759 ; they have fince been publiflied, more [ 448 ] more corredlly, in the original German, in the firfl: volume of a colledtion of his chemical writings, the continuation of which is earneftly wiflied for. In the Memoir es of the academy of Paris for 1758, printed in 1763, there is a paper on this metal by Mr. Macquer and Mr. Baume conjointly ; who, befides re- peating and varying fome of my experiments, and draw- ing from them fome new confequences, have expofed the platina to an agent which the other enquirers have not had opportunities of doing, a large burning concave. Their platina, in quantity a pound, was fent to them from Madrid. The foregoing are the only writers I know of, v^'ho have treated exprefsly and experimentally on platina. Some others have mentioned it occafionally, as particu- larly Mr. Cronlledt and Mr. Vogel, in their new mineral fyftems. The former has in general given a very juft account of it ; but the latter appears to me to be a little miftaken in fome points, which will be further taken notice of in their places. Since the publication of my experiments in the Tranf- adlions, I have at times been adding others, and en- deavouring to afcertain fome properties of platina which before had been too flightly examijied. Nothing now is fo much wanted, in regar4 to this extraordinary metal, as a regular hiftory of what has already been done, or a connefted view of the experiments that have been made upon it. Such a hiflory I fFiall here attempt, quoting every where the authors of fuch fadts as are not taken from my own diaries, and, where any doubts arile on comparing the different accounts, making new trials'. SECT. [ 449 ] SECT. I. Of the general properties of platina conjidcred by itfelf, or independent of its difpojition to unite or not unite ivith other bodies. I. Defer ipt ion of Platina. PLATINA in grains, as brought into England, is of a fhining whitifli colour, fomewhat approaching to that of filver, but lefs white : from this refemblance, which becomes much greater when the platina has palled through certain operations, it probably, as already taken notice, received its name.- Mr. Macquer refembles its colour to that of coarfe iron filings unrufted, but all I have feen was a good deal whiter than any iron filings : this difference from iron is mentioned alfo exprefsly by Mr. Scheffer, for whUe he had no fufpicion of the platina being a new diftindl metal, he fays it feemed to be iron which by fome accident had been made externally white. Mr. Marggraf calls tlie colour white inclining a little to that of lead. The colour of platina is not tarnifhed or altered, fo far as I have obferved, by air or moifture, or by any exha- lations that are commonly diftufed through the atmo- fphere : it refifts vapours which difcolour filver, and ap-- pears equally permanent with that of pure gold. The grains are of various fizes : fome few are as large as linfeed, but mofl of them a good deal fmaller. Their figure alfo is various and irregular : fome approach to a triangular, others rather to a circular form : mofl of them are flat, none globular, and few of any great convexity : the furface is fmooth, with the edges and angles generally rounded off. On viewing them with a microfcope, the furface [ 45° ] furface appeared in fome parts uneven : the prominencies looked bright and polifhed j the cavities dark coloured and roughifh, as if they w^ere fprinkled with a powdery- matter. A few of the grains were attracted, though very weakly, by a magnetic bar. II. Subftances mixed with the native platina. With the grains of platina, above defcribed, feverai heterogeneous matters are intermingled ; fome of which are in fmall particles or duft, feparable by a fine fieve ; others larger, fo as to be diftinguifhed by the eye and picked out. Thefe fubftances, in the different parcels of platina which I examined, were the following. I . A confiderable quantity of blackiOi duft, which ap- peared to confift of two diffimilar fubftances ; a part of it being attradled vigoroufly by a magnetic bar, and a part not attracted at all. The part attradled is of a deep fparkling black colour, much refembling the black fand from Virginia : the reft is of a brownilh hue, and has feverai bright particles intermixed, which appear to be fragments of the grains of platina itfelf. It is probable that the roughnefs and dark colour of the cavities of the grains of platina, and .the magnetic quality of fome of the grains, proceed from foxne portion of thefe extraneous powders adhering in them. .2. Among the larger grains of platina, feparated by means of a coarfe lieve, were obferved fundry irregular dark-coloured particles., fome blackifli, others with a caft of brownifli-red, in appearance refembling fragments of emery or loadftone. Some of thefe were attracted by the magnet, very weakly, and others not at all. The un- magnetic duft of the preceding paragraph fcems to be only fmaller fragments of this laft kind of matter. 3. There [45^ n. There were fome rough yellow particles, very mal- leable, which appeared to be gold, though not free from a mixture of platina. A further examination of thefe golden particles will be given hereafter. Their quantity differs in different parcels of the mineral : twelve ounces of the richeff that has come to my hands being diligently- picked, with the affiftance of a magnifying glafs, the grains partly or entirely yellow amounted to about two pennyweights, or one part on a hundred and twenty of the mixt. 4. A few globules of quickfilver containing gold, with fome particles of platina intermixed and pretty ftrongly adhering. Mr. Marggraf likewife obferved fame quick- filver among the platina which he examined, having been induced to look for it with attention, by finding, that when an ounce of platina had been urged with a ftrong fire in a glafs retort, a little true running mercury came over into the receiver. The yielding of quickfilver and containing fome magnetic parts, the former of which is particularly mentioned in the firfl of my papers in the Philofophical Tranfaftions, and the latter not only there, but by all thofe I know, of who have given any experimental account of platina, are ranked byVogel among the nev/ properties of thij) mineral difcovered by Marggraf. 5. Some fine colourlefs tranfparent particles,which were hard to bieak under the hammer, and were not fenfibly ?.(ll:ed upon by aquafortis. Thefe are probably fragments of the hard kind of ftone, which frequently inverts ores in mines, and in which native gold is oftencft found lodged, called by the Germans quartz, but which has not, tliat I know of, received any diilindtive Englifh name. 6. A very few irregular particles of a jet black colour. Thefe broke very eafily, and looked like the finer forts of N n n pitcoal : [ 452 ] pitcoal : laid on a red hot iron, they emitted a yellowifli ijBioke, and fmelt Hke burning coal. The foregoing obfervations afford fome room to fufpedl, that this mineral has not come to us in its native form, but has probably been ground in mills, and worked with quick- filver, in order to extract the particles of gold intermixed with it. But its mineral hiflory will be confidered more particularly after we have gone tlirough the hiftory of the experiments, as fome points cannot till then be fufficiently underftood. It is here only to be well attended to, that all thefe matters are entirely adventitious to the platina; that their quantities are very variable, and that one or more of them, in fome parcels, feem to be altogether wanting,, the magnetic or ferrugineous matter being always the moft confiderable, and poflibly the only conflant admixture. III,. Specific Gravity of Flatina. The mineral called platina being, as we have before- feen, a mixture of very diffimilar matters not uniformly blended, I weighed hydroflatically feveral different parcels, taking fometimes four or five ounces for one experiment, and in one twelve ounces. In moff of the trials, tlie gravity turned out, to that of water, very nearly as 1 7 to i : it was never lefs than 16,500, nor greater than 17,200. The gravity of platina was examined alio by Dr. Pembcr- ton and Mr. Ellicott,who both reported it to be about 1 7= The late Mr. Sparkes informed ine,. that a fpecimen which he made trial of turned out but 16 ; and Dr. Davies, that he weighed a parcel whofe gravity was found to be 17,233. , To come as near as might be to the fpecific weight of the pure platina, I feparated a quantity of the larger grains by a coarfe fieve, and endeavoured to cleanfe them from the duff that might adhere, by boiling them in aquafortis, .^xing them with fal ammoniac and forcing off" the fait [ 453 J by fire, and afterwards walliing them with water, Thfr gravity of thefe was found on many trials to be upwards of 1 8, though the microfcopc Hill difcovered a portion of blackifli matter in their cavities. Fahrenhcits thermo- meter (landing at the fortieth degree, a quantity of thefc grains which weiglied 642 in air, weighed in diftilled wa- ter 606], whence the fpecific gravity comes out 18,213. It was doubtlefs the larger and purer grains that Mr, Marg- graf examined, when he makes the gravity of platina to that of gold as i8y to 19. The remarkable weight of platina appears to have been the principal inducement for believing that it is rich in gold, and is flill infilled on by many as a proof of its be- ing fo, agreeably to the general axiom already taken notice of in the hiflory of gold, which, having long been univer- fally received, men cannot ealily think to be erroneous, that as mercury, among the bodies hitherto known, is the next in weight to gold, all bodies heavier than mercury, whofe gravity is about 14, muft therefore neceflarily par- take of gold. Accordingly it has been affirmed that a twentieth, a tenth, and fome have gone fo far as to pretend that a fourth part of platina is true gold, tiie reft being a ferrugineous matter enveloping the gold. But iiwc fuppofe platina to contain even this laft quan- tity of gold, I apprehend that the fame difficulty will ftill remain, and that the axiom will be as cffisdlually overturned as if we fuppofe it to contain none. If the matter mixed with the gold in platina is ferrugineous, its fpecific gravity cannot be admitted to be more than 8, for pure iron itfelf* does not come up to that weight. Now if 8 parts of this matter lofe i in water, 3,0000 parts will lofe ,3750; and 1,0000 parts of gold, the gravity of this metal being' about 19,300, will lofe ,0518; fo that 4,0000 parts of the compound will lofe ,4268 ; whence, dividing 4,0006' N n n 2 by [ 454 1 _ by ,4268, we have 9,372 for the gravity of the compounds The gravity of platina (hould be no more than this, if its compofition was fuch as is fuppofed ; fo that one part of gold, wrapt up in three of ferrugineous matter, fs very far from accounting for the great weight of the mineral. T6 make the gravity 17, the quantity of gold ought to be ro parts in 1 1 of the mafs. If it be fuppofed that the matter mixed with the gold is not iron, but fomething of a heavier kind, let us inveftigate what its weight muft be. If gold be blended with three times its weight of another matter, and the gravity of the mixt be 17; then 4^ parts of gold, and i2| of the other matter, will together lofe i in water : the /^j; or 4,25 of gold lofe ,22 in water, fo that the 12,75 of the other matter muft lofe ,78, whence the gravity of this laft comes out above 16. If platina therefore be fuppofed to contain, gold becaufe it approaches to gold in fpecific weight, we muft ftill admit that there is a fubftance which does. the fame though it contains no gold. To this way of reafoning the degraded gold of Mr. Boyle has been objed;ed j which however does not feem to me at all to affedl the argument. For in Boyles procefs, of which an account has been already given in the hiftory of. pold, page 2q6 of this volume, the gravity of the gold, by the mixture of an inconfiderable quantity of foreign mat- ter, was diminifhed betwe.en a fifth and a fixth part, proba- bly from accidental cavities in the mafs j whereas here, according to the fuppofition we have been fpeaking of, the gravity of the compound, inftead of being diminilhed, is increafed almoft to double of what it. ought to be. There may indeed be fome variation of gravity from ,th€ mixture of two bodies with one another, but of fuch an increafe as this I believe it will not be pretended that there is any inftance. The great weight of platina therefore, inftead, of; o [ 455 ] of being a proof of Its containing gold, affords rather a prefumption of its being a ponderous body diftin(fl from gold. IV. Malleability of Platina. Some of the purer grains of platina, by gentle ftrokes of a flat hammer upon a fmooth anvil, bore to be extended into thin plates, without breaking or cracking about the edges : fome cracked before they had been much flattened, and-difcovercd internally a clofe granulated texture : others were fo brittle as to be reduced, without much difficulty, into powder. Even the tougher ones foon broke from rude blows in. an iron mortar ; and they feemed all to be mofe brittle when red hot than when cold; Mr. Scheffer, in his little quantity of platina, did not take notice that the grains differed in toughncfs : the par- ticles he tried having been of the more malleable kind, he makes platina in general to be as malleable a metal as the beft iron. Mr.Macquer feems alfo to have tried only a fingle grain : he fays he took one of the largeft of the grains, and having beaten it with moderate ftrokes on a iteel anvil, he found that it fuffered itfelf to be flattened into a pretty thin plate, which however cracked upon con- tinuing the beating. But Mr. Marggraf examined feveral grains, and obferved.the fame difference in their malleabi- lity as I had done : fome ftretched confiderably ; others but a little, breaking from a few blows ; whilfl others bore to be extended into pretty thin plates : he takes notice that thefe laft were mollly the convex grains. Upon the whole,^ as many of the grains are apparently of confiderable malleability, and as the brittlenefs of the others proceeds doubtlefs from fome accidental caufe, we can by no means refufe platina the title of a malleable me- tal ; though little advantage can refult from this property.. 4anlefs means fliould be found of uniting the grains into, larger maffes. V. Platinc [ 456 ] V. Platina expofed to the fire in vejfels. 1 . An ounce of platina, containing its ufual admixture of magnetic duft, was kept for fome time of a moderate red heat in an iron ladle. The white grains became dark co- loured, and almoft loft their metallic brightnefs ; and the magnet feemed no longer to attradt any part of the mixt : in other refpedts no alteration was obferved. 2. Several ounces of platina, freed from the black duft, and in which no yellow particles could be feen, were heated to a ftrong red heat, and quenched in urine. The platina, as before, loft its brightnefs: many of the grains looked blackifli, others of a rufty or reddifli brown, and fome of a high yellow colour; which laft proved more malleable than platina, and appeared to be in great part gold. Sur- prifed at this event, and imagining at firft, agreeably to the common opinion, that the platina had fuffered a de- compofition, or been divefted of its coat, I repeated the ignition and extindlion upwards of thirty times, quenching the matter fometimes in urine, and fometimes in folution offal ammoniac and other faline liquors: the platina con- tinued ftill of the dark colour which it had contracted at firft-, and no xnore golden grains could be perceived. On examining the remainder of the packet of platina, the gold, which the firft- ignition had exhibited; was eafily ac- counted for: the particles of gold, naturally intermingled among the platina, were covered with quicklilver, which had doubtiefs been added with a view to extraft them; and the quickfilver, evaporating in the fire, had left the gold of its proper afpecft. It is poffible that others may have been impofed upon by the like appearances, and thought they had produced gold from the fubftance of the platina itfelf, wlien they had only colledted the golden grains, which ought to be looked upon as entirely adventitious. • 3. The [ 457 ] J. The platina, difcolourcd by the two forcgoln^i; cx- periments, was put into a crucible, which was covered, and kept for half an hour in a pretty (Iroiig fire, lufficient for the mehing of caft iron. The platina loft the ill co- lour which it hadcontrafted in a weaker heat, and became brighter and whiter than it had been at firft. The grains {luck together, fo as to come out of the crucible in one lump; but they readily fell afundcr again on a flight blow, and did not appear to have at all melted, or altered their Ihape. 4. Some of tliis brightened platina, kept in a moderate red heat for an hour, contrafted a dark colour as before; and being afterwards urged haftily with a ftrong fire, it became again bright, almofl like filver, I tried the mal- leability of feveral of the grains, both when difcoloured and when brightened by fire, and found that in both ftates, as in the crude mineral, fome bore to be confider- ably extended, while others cracked or broke from a blow or two of the hammer. 5. I proceeded to try the effcQ. of greater degrees of heat, having fitted up for this purpofe a blaft-furnace or forge with two pair of large bellows. An ounce of pla- tina, in a black-lead crucible, was urged in this furnace with a fea-coal fire, for more than an hour. The heat was fo vehement, that the crucible in great part vitrefied; and the flip of Windfor brick which it was covered with, though defended by a thin coating of Sturbridge clay, as- alfo the internal parts of the furnace oppofite to the bellowfes, melted and run down. The grains of platina remained unmelted, being only fuperficially united into a himp of the figure of the bottom of the crucible: their colour was a good deal brighter and more filvery than at ferft; and they fcemed to cohere more firmly than thofe Twhich had undergone the weaker heat in No. 3. 6. The [ 4S8 ] 6. The foregoing experiment was many times repeated j in different kinds of crucibles, both German and Englifli; with fires of charcoal, of common fea-coal, and of fea-coal coaked or charred. In the moft intenfe fires I was able to excite, fuch as neither the befl of the crucibles, nor the furnace, could long fupport, the platina did not appear to melt, or foften, or alter its figure. I fometimes indeed obtained a few globular drops, of the fize of fmall fhot, of fmooth furfaces, which broke eafily on the anvil, and looked internally grey: thefe drops had evidently been melted, but it is probable that they were not pure platina, and that the fufion was owing to an admixture of the ferru- gineous part of the mineral, or of the golden grains : for when the purer picked grains of platina were employed, there was never any appearance of melted particles; and thofe parcels of the mineral which had once yielded fome melted drops, could never be made to afford niore, though urged with fires at leaft as vehement as the firft time. The cohefion of the grains of platina feemed to begin in a moderately firong red heat, and to become firmer and firmer as the fire was made more violent, though I never found them cohere fo much as to refift a fmall blow of a hammer. The colour, after flrong fire, was almofl: always bright and white, except on the furface of the mafs, which v/as often changed to a dark brownifh, with fometimes a faint yellowifh tinge : in one experiment, the metal, when violently heated, having been quenched in cold water, the grains which compofed the internal part of the lump ac- quired a violet or purple colour. 7. I picked out fome of the larger and brighter particles of platina, to the weight of about fifty grains, and fpread them on the bottom of a fmooth crucible: the veffel be- ing covered, and kept in a vehement fire, as in the above experiments, for about an hour, the platina cohered but nightly. [ 459 ] flightly, and being laid again in the balance, it rather out- weighed its former countcrpoife which had been left in the fcale. From this experiment, which was two or three times repeated with the fame event, I concluded, in the firft paper, publiflied in the Philofophical Tranfaftions, that platina does not lofe of its weight in the fire : Mr. Marggraf and Mr. Macquerhave fmce found, that it not only does not lofe, but really gains weight, and that when the fire is long continued, the gain is very confi- derable. 8. Mr. Marggraf put two ounces of crude platina in a fcorifying di(h under a mufBc, and kept up a ftfong fire for two hours, ftirring the platina at times with an iron rod. He obferved that no fumes arofe ; that when grown cold, the metal looked like fliavings of lead run together, but blacker and without metallic luftre ; and that its weight was not diminilhed but increafed, for it weighed two ounces and ten grains, or one part in ninety-fix more than it did at firll. 9. He repeated the experiment with one ounce of pla- tina, in a covered crucible, placed on a proper fupport, in a melting furnace,which, by means of a long pipe for con- veying in the air under the arti pit, and a long narrow chimney on the top, gives the ftrongefl; fire of all the fur- naces in his elaboratory. The fire being kept up in its greateft vehemence between three and four hours, the pla- tina was found fticking together but not melted, and weighed five or nearer fix grains, that is near one part in eighty, more than at firft. He takes notice that the grains were pretty eafily feparated by a blow of a hammer j that thofe in the internal part of the lump were whiter than at firft, but that they were ftill in tlxeir original form ; and that fome of them bore to be flattened on the anvil, Ooo 10. Mr. [ 46o ] 10. Mr.Macquer put an ounce of platlna into a German crucible, and expoled it to a ftrong fire for fifty hours, in- a furnace whofe heat,when continued for fuch a time, was capable of melting the mixtures which Mr. Pott fays, in his Uthogeognofia, yielded him glafles the moft hard and the leaft fufible. On examining the platina after this trial, he found that it had not melted, and that the grains only ftuck together fo as to form one mafs, which had exadlly the figure of the bottom of the crucible, and which had flirunk from the veffel fo as to come freely out -, that all the furface of the mafs was tarniflied and blackened, and changed^to a fiate colour, with a diminution of the metal- lic brilliancy; that the internal part of the crucible, where the platina had touched it, was tinged as if filings of iron had been calcined in it ; and that on weighing the platina after the operation, it was found increafed fourteen grains,, which amount (the French ounce confifting of five hun- dred and feventy-fix grains) to about one part in forty-one. The fame platina, fubmitted to another operation fimilar to the foregoing, received a further increafe of two grains, . the augmentation in all being fixteen grains, or one part in thirty-fix. There could be no fufpicion, he fays, of any coals or afhes falling in, becaufe the crucible was in a part of the furnace where fuch matters could have no accefs,. and becaufe it was alfo clofely covered, though not luted. . As the increafe was inconfiderable in the fecond operation, , he judges there would have been little or none on a third repetition. We may add, that fince after fifty hours ftrong fire, a further continuance of heat occafioned ftill a very fenfible augmentation of weight, the difference between j the refult of this experiment and Mr.Marggrafs, in regard ■ to the quantity of the augmentation, may be eafily account- - ed for, from the different lengths of time that the fires were continued. II. Iti [ 46i ] II. It is well known to the chcmifts, that the metals called imperfed:, or thofe which calcine in the fire, gain weight in their calcination ; a phenomenon not a little aftonilhing, and of which they have not been able to aliign any probable caufe, unlefs it be the ablbrption of air. As platina appears plainly, from many of its properties, not to be one of the imperfedt metals, Mr. Macquer very juitly fufpecfts, that the increafe of weight in the above experiments was owing to the calcination of fomc hetero- geneous fubllances mixed with the platina. The ferrugi- neous lining which it left in the crucible, and the obfcura- tion of the colour, feemed to confirm this conjedlure, and he further took notice, that after the fecond calcination there were fome grains of a friable matter like fcales of iron, and that the magnetic fand was no longer black and brilliant, but of the fame flate grey colour with the pla- tina. It may here be obferved, that if there was no miftake in Mr. Macquers weights, the quantity of this heteroge- neous calcinable matter muft be very confiderable. Of all the experiments I can recolledl of the calcination of bo- dies, there is no one in which the increafe was fo great as that which Mr. Schefler allows to iron, viz. one third of its weight, as we fliall fee hereafter in the fixth fedlion of this hiflory : admitting even this augmentation to the cal- cinable matter in platina, the quantity of this matter, to produce an augmentation of hxteen grains on the ounce, mufl: be forty-eight grains, or one eleventh part of the platina. 12. The obfervations in the foregoing paragraph account for the difference between my experiments No. 7, and thofe of M. Marggraf and Macquer in No, 8, 9, and 10 ; mine having been made with the purer grains, and theirs with the entire mineral containing its common mixtuj-e of calcinable parts. For further fatisfaciion in this point, I O o o 2 took [ 462 J took 360 grains of the larger and brighter particles picked out from platina, and the fame quantity of the blackifl^ duft feparatcd from it by a fieve : the two parcels, in two fmooth fcorifying dillies, were kept under a muffle, in a very flrong heat, for five hours ; and tliat both of then> might undergo an heat as equal as poffible, the places of the two dilhes were interchanged about the middle of the procefs. When cold, the picked platina, weighed with great exa£tnefs, was found to have gained two grains, or one part in a hundred and eighty ; while the duft was in- creafed near nine grains, or one part in forty. It was ob- ferved that the picked platina had. become darker coloured than it was at firft, but the duft paler ; and that the picked platina cohered very flightly, but the duft was agglutinated into a firm cake not eafily to be broken between the fin- gers. It muft be obferved that what is here, called duft contains a confiderable proportion of true platina, di\ ided into particles as fine as thofe of the impure matter i and confequently that the quantity of impure matter in the picked platina cannot be judged of from die proportional augmentations which the two parcels received in the fire. But we ftiall here drop an enquiry, which does not feem important enough to deferve the trouble of any further profecution, eipecially as we fhall hereafter find means of attacking thefe calcinable parts more effedtually than by fimple heat. 1-7. The experiments I. had made (Mo. 5 and 6 of this article) feem to prove, that platina cannot be brought in- to fufion in the common crucibles, by any heat that the veflels themfelves can fupport. Mr. Schefter concludes alfo from his trials, that to melt it in a crucible is impofll- ble, fince it refifts even a ftronger fire, than thatwhich vi- trefies the beft crucibles made of Waldenburg earth and quartz, which we may fuppofe, from this manner of fpeakr ing [463 ] ing of them, to be of a very good kind. Neverthclcfs, as the melting of phitina, if it could be effedted, would be a mofl: important acquifition in regard to its chemiual hillo- ry and mechanic ufes, Mr. Macquer made fome further trials with this view. He expofcd platina to the fire of a glaftihoufe furnace for five days and five nights, but with- out perceiving any other alterations than thofe already men- tioned : and indeed the glafshoufe fire could not be ex- pe(5led to fubdue this refradlory metal, which had already refifted fires much greater than the glafshoufe furnace can • produce, and greater than its materials or vcffels can lup- port. 14. For the laft effort, Mr. Macquer had rccourfe to a forge, inccealing the adlivity of the fire by an expedient fimilar to that which we have formerly mentioned in page 26 of the prefent work. The blaft from the bellows was divided into two pipes, which entered the furnace at two oppofite fides; and two other great bellowfes were fo dif- pofed, that their blafts entered oppofitely at the other two lides. Four ounces of platina, in a Hefiian crucible, being placed in the middle of the furnace, the fire was excited by the bellowfes to fuch a degree, that in lefs than a!n hour and a quarter, all the internal part of the furnace melted and run towards the bottom, forming in the lower part Biaffcs of glafs, which, flopping up the orifices of the blaft-pipes, made it necelfary to difcontinue the ex- periment: the crucible, which was all vitrefied, being, taken out fome time after, appeared ftill of fo dazzling a whitenefs that the eye could not fupport its luftre: yet notwithftanding this extreme fire which the platina had fuffered, it was no more melted than in the foregoing ex- periments; except that in the vitrifications, which fur- rounded the crucible, there were found fome grains, of a felver whitenefs, perfeftly round, which appeared to have had. [ 464] ■had a very good fufion, but which, from afmall blow of a hammer upon a fleel anvil, fell into powder. Mr. Macquer appears therefore, in this utmoft effort, to have produced no other effedls than thofe which I had obtained^ and his trials concur with the others in proving, that the beft of the common furnaces, and melting veffels, will themfelves melt fooner than the platina included in them. VI. Platina expofedto thejire in contaB with the burning fuel. As the power of fire, upon metallic as well as earthy bodies, is remarkably promoted by the immediate contadl of the burning fuel, and the impulfe of air upon the fub- jeft, platina was expofed to its ad:ion in thofe circumftauces. Mr. Scheffer feems to regret that he had not fome pounds of the metal for a trial of this kind, but the procefs may be managed in fuch a manner, that a very little quantity can be made to fufiice. A crucible, having a bed of charcoal in it, was laid on its fide among the fuel, in a good blaft-furnace, with its mouth towards the nofe of the bellows j and on the char- coal were fpread four ounces of platina. The fire was ve- hemently urged for above an hour; during which an in- tenfe white fiame pafl'ed through the crucible, and iflued Rt an aperture made for that purpofe in the end. Great part of the crucible was vitrefiedj but the grains of platina only fuperiicially cohered and became brighter, without feeming to have at all ibftened or altered their Ihape. The experiment Was feveral times repeated and varied: common fait, whofe fumes promote the vitrification of the •crucibles themfelves, was thrown on the fuel before the mouth of the vefTel, and its fumes ftrongly impelled upon the platina: the lumps of platina which had undergone the preceding operations, were dropt, before the nofe of the bellows, into violently-excited charcoal and fea-coal fire«. [465 ] firesy, (0 ftrong as almofl inftantly to melt off a piece of the end of the forged iron rod with which the fuel w.is at times ftirred down. The platina ftill came out unmelted, and unaltered in itS' form;, except thr.t there were fomc- times a few globular drops like thofe iiicntioned in the. preceding article. VII. P/atina expofed to a burning gli{/s. After all thefe fruitlefs attempts for the melting of platina, no other refource remains, for determining its tufibility or non-fufibility, than the action of large burning glafles or concaves; a trial which I have often regretted that I could not in this country find means of expoiing it to. What has earneftly been wiflied for by all thofe, whom profit, curiolity, or fcience, have interefted in thefe kinds of purfuits, Mr- Macquer and Mr, Baume have endeavoured to fupply. They ufed a concave of plate glafs, well filvered, twent)' two inches in diameter, and of twenty-eight inches focus. Before they proceeded to try its effedls on platina, they expofed to its aftion fe\'eral other bodies, that fome judge- ment might be formed of its force.. Black flint, powdered to prevent its crackling and flying about, and ftcured in a large piece of charcoal, bubbled up, and run into a tranfparent glafs in lefs than half a minute. Heflian crucibles, and glafshoufe pots, vitrefied completely in three or four feconds. Forged iron fmoked, melted, boiled, and changed into a vitrefcent fcoria, as foon as it was expofed to the focus. The gypfum of Montmartre, when the flat fides of the jilates or leaves, of which it is compofed, were prefented to the glafs, did not fhew the leafl difpofition to melt; but on prefenting a tranfverfe fedion of it, or the edges of the plates, it melted in an inflant, with a hiifing noife, into a brownifh-yellow matter. . [466] matter. Calcareous ftones did not completely melt ; but there was detached from them a circle, more compadt than the reft of the mafs, and of the fize of the focus ; the fe- paration of which feemed to be occafioned by the fhrink- ing of the matter which had begun to enter into fufion. The white calx of antintony, commonly called diaphoretic antimony, melted better than the calcareous ftones, and changed into an opake, pretty glofly fubftance, like white enamel. They obferve that the whitenefs of the calcareous ftones and the antimonial calx are of great difadvantage to their fulion, by refledling great part of the funs rays, fo that the fubjedt cannot undergo the full adtivity of the heat thrown upon it by the burning-glafs : that the cafe is the fame with metallic bodies, which melt fo much the more difficultly in the focus, as they are the more white and po- liflied : that this difference is fo remarkable, that in the focus of the concave whole effedls we have been fpeaking of, fo fufible a metal as filver, when its furface was po- liflied, did not melt at all : and that the whitenefs of pla- tina would doubtlefs in like manner have greatly weaken- ed the aftion of the concave on it. MelT. Macquer and Baume therefore took the platina which they had before kept five days' in a glafshoufe furnace, and which, while it had concreted into a lump large enough to be held in the focus, had at the fame time become tarniflied and browned on the furface, fo as to be in a ftate the moft favourable for the experiment. Their account of the experiment it- felf is as follows. " When the platina begun to feel the adlivity of the focus, it looked of a dazzling whitenefs : from time to time there ifiued from it fiery fparks, and there arofe a fume, very fenfible, and even pretty confiderable : in fine it entered into a true and good fufion, but it was not till the [ 467 ] the end of a minute and a half that this fufion took place. We melted it in this manner in five or fix parts : none of the melted parts however run to the ground, all of them remaining fixed to the piece of platina, prob.ibly becaufe they fet and hardened aflbon as they were no longer in the center of the focus. Thefe melted parts were diftinguifh- ed from the reft, by a filver brilliancy, and a rounded furface, fliining and poliHied. Wc flruck the largeft of thel'e melted malfes upon a ftecl anvil, to examine its duc- tility : it flattened eafily, and was reduced into a very thin plate, without breaking or cracking in the leafl ; info- much, that it appeared to us infinitely more malleable than the grains of platina are in their natural ftate, and that we believed it might be extended into as thin plates as gold -and filver. This platina grew hard and rigid un- der the ftrokcs of the hammer, as gold, filver, and other metals do : this rigidity was eafily deftroyed by the me- thod pradifed for gold and filver, that is, by heating it to a white heat and letting it cool." Mr. Baume, in his ma- nuel de chymie, printed in 1763, takes notice of another property of the platina thus melted; that it is found to be of a fpecific weight approaching (Jhnblable) to that of gold : on this, however, we can lay but little ftrefs, as he had laid before, in fpeaking of the crude grains of pla- tina, that their fpecific weight is equal (t'galej to that of gold. The above experiment, though not a little curious and interefting, is by no means entirely fatisfadlory ; and it were to be wifhed that fome further trials were made, with burning-glafles of greater force, for alcertaining with more precifion the real fufion of the platina, and for ob- taining fome quantity of the melted metal, that its duc- tility, gravity, hardnefs, and other properties, may be more fatisfadtorily examined. Thus much feems clear P p p from [ 468 ] from the experiment, that platina is a great deal more difficult of fufion than flint, and flint a great deal more (6 than gypfum j and as no means have been found of pufli- ing common Are to fuch a height, as to produce either in flint or gypfum the leaf!: appearance of fuflon, without the concurrence of the faline or earthy parts of the fuel, ■^vhich ferve as a llux for thofe bodies, though not for platina ; there appears no room to hope, as the author feems to do towards the end of his memoir, that we fhall ever be able to melt platina in great furnaces. It follows alfo, that the melted drops, which both Mr. Macquer and I obtained in our furnaces, could not be pure platina : for though it is not to be thought that our fires were of equal intenfity with that to which the platina was here expofed, our drops liad fullered a more perfedl; fulion, than thofe parts appear to have done that were melted in the focus of the burning-glafs : the drops like wife had nothing of the malleability, which platina melted by the burning- glafs is faid to poffefs in fo remarkable a degree, but on the contrary fell in powder under the hammer. If the fufion in one cafe was brought about by tlie mixture of fome foreign metallic matter with the platina, we cannot be certain but in the other alfo the fame caufe may have concurred in a lefs degree ; and confequently it is poflible that pure platina may require for its fufion a heat ilili more vehement. From the experiments related in this fedlion I think it may be concluded, that platina is a filver-coloured metal, of confiderable dudiility, not fufible by the ftrongefl: fires, that can be excited in the furnaces, or fuftained by the veflels, of the chemifl or the workman; that it approaches to gold in one of the reputedly moft difcriminating cha- radters of that metal, fpecific weight ; and that it agrees with gold and filver in being fixt and uncalcinable by fire, SECT. [ 469] SECT. II. Of the aEiion of Adds on Platina. I. Platina with the Vitriolic acid. SEVERAL parcels of the purer grains of platina were digefted for fome hours in a gentle heat, with the concentrated fpirit called oil of vitriol, and with the lame fpirit diluted with different proportions of water. No folution happened, nor any alteration either in the liquors or the metal. 2. Three ounces of ftrong oil of vitriol were boiled Avith one ounce of platina, in a tall narrow-necked glafs, for fome hours. The liquor remained nearly of the fame quantity as at firft, and no change could be perceived either in it or in the platina. 3. The glafs being cut off a little above the furface of the liquid, the fire was gradually increafed, till the liquor, which now begun to evaporate freely, had, in five or fix hours, wholly exhaled, and left the platina dry and red hot. The metal, when grown cold, being wafhed with water and afterwards dried, its weight was found to be the fame as at firft, and the furface of the grains fliewed no mark of corrofion. The only alteration obfervcd was, that many of the grains had become dull coloured and browniih; an effed: which, as we have already feen, fimple heat produces, and which therefore muff not be imputed to the adtion of the body fuperadded, when a heat fufficient to produce it is employed at the fame time. It appears therefore, that platina refifts the pure vitriolic acid, which, by on£ or other of the above methods of application, diffolves or corrodes every other known me- tallic body except gold. P p p 2 II. Platina [ 470 ] II. Plaiina with the Marine acid. 1 . We a k and ftrong fpirits of fait being digefted fe- parately with one third their weight of platina, in a gentle heat, for feveral hours, the liquors remained un- coloured, and the platina unaltered. The heat was after- wards increafed, and the liquors kept ftrongly boiling till they had totally exhaled, without making any fenfibk change in the platina. 2. When common fait is ftrongly heated, in mixture with certain vitriolic fubftances, its acid, forced out by the vitriolic acid, and refolved into fumes by the heat, corrodes fome metallic bodies, on which, in its liquid ftate, it has no adion. Two pajts of decrepitated or dried fea fait were therefore mixed with three parts of green vitriol calcined to rednefsj three ounces of the mixture prefled fmooth into a cementing pot; one ounce of platina fpread evenly upon the furface, and fome more of the mixture over it; the veflel clofely covered and luted, and kept in a moderate red heat for twelve hours. On examining it when grown cold, the faline mixture was found to have melted, and formed a fmooth uniform mafs. The platina, which had funk to the bottom, being fe- parated from the mixture by wafhing, appeared to have fuffered no change, though its weightwas a little diminifhed. •^. The experiment was repeated with a lefs fufible mixture, called the regal cement, compofed of one part of common fait, one of colcothar, or vitriol ftrongly calcined, and four of powdered red bricks. An ounce of platina,, furrounded as above with fix ounces of this compolition, and cemented in a clofe veflel, with a red heat,, for twenty hours, fuffered no material change, though there was, as. before, fome deficiency in the weight. Many of the grains were difcoloured; whereas, in the foregoing ex-- periment [ 47^ ] periment they were all nearly as briglit and white as at firft, on account, perhaps, of the mixture having melted, fo as to wafti and clcanlc tlieir kirfaces. 4. Of the other metallic bodies, gold is the only one which refifls the marine acid in the above way of appli- cation. As the platina in thefe experiments had no mark of difTolution, it was prefumed that this metal likewife had refilled it; and that the deficiency in weight was owing to fome of the fmaller grains having been wartied off along with the ponderous metallic matter of the vi- triol. The experiment was therefore varied, by fubfli- tuting, to the foregoing mixtures, mercury-fublimate, a combination of the concentrated marine acid with quick- lilver: when tliis compound is mixed with any one of the common metals, gold excepted, and the mixture cxpofed to a proper heat, tlae quickfilver feparates and exhales, while the acid unites witli the metal. An ounce of pla- tina was fpread upon three ounces of powdered lublimate in a glafs veflel, which being fet in a moderate fand heat, the fublimate totally arofe, leaving the platina of its ori- ginal weight, and uncorroded, though difcoloured a little. 5. As the adtion of fublimate on bodies depends not only on the acid being capable of corroding them, but on its having a flronger affinity to them than it has to the mercury, that is, a difpofition to unite with tliem in pre- ference to the mercury; it is poflible that there may be. bodies, really corrofible by the acid, but which, having lefs affinity to it than mercury has, will of confequence relift the adtion of fublimate. The regal cement was therefore again had recourfe to, but that none of the grains of platina might be in danger of being loft, twice their weight of gold was melted with them, and the mixture carefully hammered into a thin plate. A piece of the plate, weighing fifty grains, was lurrounded with. regal; [ 472 ] regal cement, the crucible covered and luted, and kept for twenty hours in a red heat. On examining the metal, it was found to retain the whitenefs and brittlenefs, which gold constantly receives from fo large a proportion of pla- tina, and to have loft in weight about half a grain, or one hundredth part. This lofs proceeded perhaps from alloy in the gold employed, which was above ftandard, but not perfedlly fine, or perhaps from the diffolution of fome of the heterogeneous parts of the platina, but by no means from the platina itfclf; for the fame plate, cemented again with frelli mixture for the fame length of time, fuffered no further diminution. If the marine acid was capable of corroding the platina, the corrofion would have continued in the fecond procefs, and inftead of a hundredth part, near a third part would have been eaten out. This ex- periment therefore determines with certainty the refiftence of platina to the marine fumes; and that the regal cement, fo called from its being fuppofed to purify gold from all heterogeneous metallic bodies, is incapable of feparating platina from it. 6. There are circumftancesin which gold itfelf isdiffolved by the pure marine acid ; as when it has been melted with tin, and the mixture beaten into powder and calcined ; or when it has been reduced into the form of a calx by preci- pitation from other menftrua, Platina calcined with tin, and fome of the precipitates of platina of wiiich an account will be given in the next feftion, were digefted in fpirit of fait; with a moderate heat, for feveral hours : the reddifli yellov/ colour which the menftruum acquired, fliewed that a part of the platina was diffolved, though it feemed to diilblve fomcv.-hat more difficultly, and more fparingly. than g-old treated in the fame manner. III. Platina [ 473 ] III. Platina ivhh the Nitrous acid. 1. Spirit of nitre diluted with water, proof aquafortis, and the ftrong fmoking nitrous fpirit, were digefled fepa- rately with one third their weight of platina, in a gentle heat, for feveral hours. During tlic digcflion, fome fmalj hubbies were obferved, as if a diifolution was beginning, but the liquors acquired no colour, and the fire being in- creafed, fo as to keep them boiling till they had to- tally evaporated, the platina remained unchanged, except only that many of the grains had become difcoloured. 2. Platina was treated likewife with nitrous cements, by proceHes fimilar to thofe in which it had been cxpofed to the marine fumes. An ounce of pure nitre, and an ounce aod a half of green vitriol calcined to rednefs,were ground together, part of the mixture prefled fmooth into a cru- cible, over which was fpread an ounce of platina, and the relt of the mixture above it. The crucible was covered and luted, and the fire gradually raifed, fo as to make the veffel of a full red heat, in which ftate it was continued for feven or eight hours. Red nitrous fumes iflued copi- oufly throHgh fome fmall cracks Avhich they had forced in the luting. The crucible being grown cold, the mixture was found not melted or baked together, but loofe and DOwder}^. The platina was of the fame weight and appear- ance as at firil, except that many of the grains had be- come, as in the foregoing experiments, dull coloured or brownifli. IV. Additional experiments with the foregoing acids, &c. on platina. Mr. Marggraf has given fome experiments on this head, which having been condufted in a fomewhat diffe- rent manner from mine, he took notice of fome pheno- mena [ 474 ] mena which did not occur to me. They were all per- formed in fniall glafs retorts, with receivers adapted to them ; and the fire gradually raifed, fo as to make the re- torts red hot. In this manner he treated platina with eight times its weight of each of the three foregoing acids; with twice its weight of mercury-fublimate ; with twice its weight of fal ammoniac ; and with thrice its weight of the mixture called fal alembrot, compofed of one part of mercury-fublimate and two of fal ammoniac. The quantity of platina in each experiment was fixty grains. With the nitrous and marine acids, he had a white cryftalline fublimate in the neck of the retort, which, viewed through a magnifying glafs, looked like cryftalline arfenic, but whofe quantity was too fmall to be fubmitted to any further examination. When the marine acid was ufed, there was alfo another fublimate of a reddifli colour ; and in all cafes, the remaining platina was changed, in part to a reddifh brown. Mercury-fublimate arofe uncoloured, and left the platina of a dark greyilh colour, here and there reddifh. The lal alembrot arofe alfo perfedly white, but was followed by a little yellowifli matter : the remaining platina was of a bright whitenefs, almoft like filver. With fal ammoniac there was a fine yellow fublimate (errone- oufly called blue in the Berlin memoirs) like that which rifes from a mixture of this fait with iron ; the remaining platina was rather whiter than at firft, and after fome time grew a little nioifl in the air. Mr. Marggraf exprefsly mentions his ufing in thefe ex- periments the crude unpicked mineral ; whereas in mine only the larger white grains were employed, from which all the heterogeneous parts and ill coloured grains, that could be diftinguill:ied by a good magnifier, had been care- fully picked out. It is pretty certain, that the fublimates did not proceed from the platina itfelf, but from its ad- mixtures, [ 475 ] mixtures, the white one polTibly from the mercurial glo- bules united with the acids, and the yellow from the fer- rugineous parts. The author himfelf concludes from the experiments, that the acids have no adion on the true pla- tina, but attack in fomc meafure its fcrrugineous matter ; and that the marine acid feems to have this efFedt in a greater degree than the other two. V. Plat ma ivith aqua regia. 1. Aqlta regia, the proper menilruum of gold, being poured upon platina, begun to adl upon it (lightly in the cold, and by the afTiftance of heat flowly and difficultly diflblved it; acquiring at firft a yellow colour, which deepened by degrees, as the menftruum became more fatu- rated, into a dark, almoft opake, brownilh red. 2. The experiment was feveral times repeated, with different forts of aquas regiae, made by diflolving fea fait and fal ammoniac, feparately, in four times their weight of aquafortis, and by abftradting the nitrous fpirit in a retort from the fame proportion of each of the falts. All thefe menftrua diflblved the platina ; and it did not appear to me that one dilTolved it more readily, or in greater quan- tity than another. Mr. Macquer tried alfo feveral aquae regire, compofed of different proportions of the nitrous and marine acids, and foujid that a mixture of equal parts of the two fpiiits was one of thofe which anfwered the befl. 3. In order to determine the quantity of menftruum ne- ceffary for the diirdution, I prepared an aqua regia by di- lutingtenounces and a halfof ftrong fmoking Ipiritof nitre with eight ounces of water, and abftradling the mixture from fix ounces of common fait. Five ounces of this aqua regia, which may be reckoned to contain three ounces of very ftrong acid fpirit, were poured upon one ounce of Q^q q platina. [ 476 ] platina, in a retort, to which was adapted a recipient. A moderate heat being applied, the menftruum a£ted pretty brifldy, and red fumes arofe in abundance. When about two thirds of the liquor had come over, the adion was fcarcely or not at all fenfible, though the fire was confider- ably raifed. The diftilled liquor, which appeared of a light reddiih colour, being poured back again into the re- tort, the diffolution begun afrefh : the vapour, which now came over, was much paler than the firft. The co- hobation being repeated four times, the diltilled liquor proved paler and paler every time : at length both the fumes and aftion ceafed, though the fire was augmented, and a confiderable part of the platina remained undiflblved. The folution was tlierefore poiu'ed off, fome more of the menftruum added, the diftiUation and cohobation renewed, and thefe procefies repeated, till all the platina appeared to be taken up, except a little blackilh matter which feemed to be its impurities. The laft portions of the men- ftruum feeming not to be fufficiently faturated, fome more platina was added ; and after the acid had ceafed to a(Sl, the remaining piatina was dried and weighed, to fee how much of it had been difTolved. It was found, that by this way of application, one ounce of platina was difTolved by eight ounces and a quarter of the m.enftruum ; which quantity of the menftruum, as appears from the manner of its preparation, confifted of about four ounces and a half of ftrong acid fpirit, diluted with three ounces and three quarters of water ; whereas, when the digeftion was performed in open veffels, and the fumes fuftered to efcape, about fourteen ounces of the above menftruum, contain- ing nearly eight ounces of ftrong acid fpirit, were neceffa- ry for diflblving one ounce of platina. Platina appears to require a much greater quantity than gold, and to difTolve far more difficultly. 4. Marggraf [ 477 ] 4- Marggraf ufed an aqua rcgia compolcd of one pari offal ammoniac and fixteen parts of aquafortis ; and found that twenty-four ounces of this mcnflruum were ncccflary for one ounce of platina. It may he fufpedtcd that the quantityof fal ammoniacwas not here fuffici:ntfor cnabhng all the aquafortis to adl on the platina, fo that the metal was diffolved only by a part of the nicnllnuim, the reft being fuperfluous aquafortis. The author obfcrves, that the folution depofited in the cold fmall reddifh cryftals : yet he diftilled off one half of it in a retort, and does not take notice of any cryftallization happening in the concen- trated rcfidiuim ; from whence it feems to follow, that one half of the liquor was ineffential to the diffolution. 5. Macqucr made an experiment of the fame kind, which agrees nearer with mine : of his aqua regia, compofed of equal parts of the nitrous and marine acids, fixteen ounces diflblved, by digellion, one ounce of platina ; and in my trials, about fourteen ounces were found to fufiice. Acid fpirits differ fo much in their ftrength, and the diffolution is fo much influenced hy the vapours being more or lefs confined during the proccfs, as appears from the expe- riments above related, that an exact agreement in this point is not to be expecTted. 6. In all the diffolutions of platina, a portion of black- ifli matter remained at the bottom, whether the platina had or had not been feparated from its black duft. In fome experiments, where the purer grains of the metal were ufed, the quantity of this indiflbluble matter amount- ed to about fix grains on the ounce, or an eightieth part : where the mineral was taken entire, without any fepara- tion of its heterogeneous mixtures, the reliduum was in one trial above a fortieth, and in another about a thir- tieth part. The proportion could not be determined v/ith much exaftnefs, the indilfoluble fubflance defending Q^q q 2 from [ 478 ] from the aftioii of the acid fome minute particles of the metal itfelf. 7, Great part of this refiduum, as Marggraf obferves, is attradted by the magnet ; its ferrugineous principle being probably bedded in fandy matter, fo that the acid could not reach it. As the finefl grains of the metal leave al- ways more or lefs of an indiflbluble fubftance, it fol- lows that platina is made fomewhat purer by the diffolu- tion. All the experiments related in this feftion concur in eftablifliing a ftrong agreement between platina and gold. There are fome other metals which diflblve indeed, and with much more facility, in aquaregia; but to refift either the pure vitriolic acid, or the marine acid, or the nitrous acid, in the circumftances wherein gold and pla- tina reiift them, are properties peculiar to thefe two. SECT. III. Experiments on Solution of Platina. I. Colour of the Solution, and trials of it for Jiaining. SOLUTIONS of platina in aqua regia, when fatu- rated with the metal, are of a dark, almofl opake, brown-red colour ; when only flightly impregnated, yel- low like thofe of gold. A few drops of the faturated li- quor tinge a large quantity of water of a fine golden hue. I know of no other metallic body whofe folutions in acids are fo rich or diifufive in colour, or tinge fuch large quan- tities of watery fluids. Notwithftanding this diffufivenefs of colour of the liquor itfelf, and its refemblance when diluted to folutions of gold, it is little difpofed to communicate any colour to other bodies, and in this refped it differs remarkably from gold. [ 479 ] gold. It corrodes the fkin, making it liarlli and rough, but I have not oblervcd that it gives any llain, not even the yellow one which the menftruum by itfelf communi- cates to the fkin. Ivory, feathers, filk, wood, and linen, Avere dipt in the diluted liquor and expofed to the fun, and the dipping and drying repeated three or four times : they all became brown, from the colouring matter of the folu- tion having dried upon the furface ; but water wadied it off readily, and left them colourlefs as at i\vi\, except that the filk retained a llight browniflinefs after the walhing. The folution dropt upon warm marble, immediately cor- roded it, but without giving any colour. Dropt into infu- fions of cochineal, it did not heighten but dcllroy the red or purplilh colour, and changed them to a brownirti or blackifli : fome of the mixtures applied on paper with a pencil, appeared nearly of the fame hue with Indian ink in its paler Ihades. II. Cryjlallization ofPlatina. Solutions of platina cryftallize much more eafilythan thofe of gold. As a confiderable heat is neceflary for making the aqua regia faturate itfdfwith the metal, the fatiated folution generally depofites, by the time it is grown cold, a brov/nilh red fediment, which is no other than a number of minute cryltals. A quantity of the folution having been fet by in an open glafs, in warm weather, the fuperfluous moifture gradually exhaling left moderately large cryftals, of a dark almoft opake red colour, of irre- gular figures, varioufly joined together, moll: of them in form of leaves, like flowers of benzoin but thicker. Their tafte was naufeous and fomewhat fharp, but not near fo corrofive as might be expedled from the great quantity of nitrous and marine acids combined with the metal. Waflied with proof fpirit, they became fomewhat paler, but ftill remained [ 48o ] remained of a high colour, refembling that of the deeper chives of faffron. In a moderate heat, they feemed to melt, though only imperfedly, and emitted white fumes fmelling of fpirit of fait ; at length they fell into a dufky afh coloured calx, ftaining the tobacco-pipe, in which they were expofed to the fire, of a pale dull reddifli colour. III. Volatilization of Platina. This metal, of itfelf as fixed in the fire as gold, appears to be equally volatilized by the hafty abftradlion of an aqua regia made with fal ammoniac. Marggraf put into a glafs retort fix ounces of a folution of platina made in a mix- ture of fixteen parts of aquafortis and one part of fal am- moniac : having fet the retort in fand, and fitted to it a receiver, he drew oft" the liquid by a gradual fire, which at lafl: was increafed, fo as to make the retort red hot and ready to melt. Tliere remained at the bottom a reddifh- brown powder,which being further calcined under a muf- fle, became more of a brilliant blackifli hue. In the neck of the retort was found a brown-red fublimate, which, on expofure for fome days to the air, run into a red liquor , refembling folution of 'platina. He poured fome of this liquor on a pcliilied copper plate, and found that the pla- tina, after fome time, precipitated upon the copper, as it does from its common folutions, covering the copper with a fliining blackifh powder. \Y . Solution of platina, 'with vitt'iolic acid. To a folution of platina, diluted with water, I added fome flrong fpirit, called oil, of vitriol : no precipitation or change of colour enfued, though a large quantity of the acid was at ditferent times dropt in, and the mixture fuf- fered to ftand for feveral days. But on adding the fame ftrong vitriolic fpirit to an undiluted folution of platina, the [48i ] the liquor immediately became turbid, and a dufky co- loured matter foon precipitated. The precipitate was not rediflblved on the aftiifion of watcrj nor was the preci- pitation prevented by adding water immediately after the acid had been dropt in. V. Solution of platbuif with volatile alctiU. The fpirits offal ammoniac, prepared both by quick- lime and by fixt alcaline falts, added to Iblutions of platina diluted with water, precipitated a dark redfparkling pow- der: but in whatever quantity the fpirits were added, the precipitation was not total, a conliderable part of the pla- tina remaining dilfolved fo as to communicate a high yellow colour to the liquor. The red precipitate, dried, and expofed to the fire in an iron ladle, became blackifli, without difcovering any thing of the fulminating power which precipitates of gold, prepared in the fame manner, have in a remarkable degree. On wafhing fome of the precipitate upon a filter, with repeated additions of water, greatell part of it difiblved, only a fmall quantity o^ blackilli matter remaining on the paper, and the liquor palling through of a deep bright golden colour: a very large quantity of water was tinged of this colour by a fmall one of the powder. VI. Solution of platina, ninth vegetable Jixt alcali. Salt of tartar, fait of wormwood, fixt nitre, and the lixivium faponarium of the London pharmacopoeia, had the fame eft'ed: on folution of platina as the volatile fpirits in the foregoing article, except that the precipitates were of a much duller reddilh colour, and lefs brilliant. The precipitation was equally imperfecfl ; the liquor ftill con- tinuing of a deep yellow colour, and greateft part of the precipitate being rediflblved on the addition of water. la [ 482 ] In the foregoing experiments, the precipitates of pla- tina by volatile alcalies were of a dark kind of red colour and confiderably fparkling, while thofe by the fixt were of a paler dull reddifh with little brilliancy. In the ac- counts which others have given of thefe precipitations, this difference, in itfelf of fmall importance, is not taken notice of. Scheffer calls the precipitates by both alcalies fimply red; and Marggraf calls them both orange yellow, a term applicable enough to the precipitates which I had obtained with the fixt alcalies, but not to thofe with the volatile. It lliould feem as if there had been fome real differences in the appearances of our refpedlive produdts, and I imagined that fuch differences might have arifen from differences in the folutions of platina made ufe of: fome late trials appeared to countenance this fufpicion, for while common folutions of platina yielded precipitates of the red kind, a folution of the cryflals of platina made in water gave only yellow ones. Macquer accounts for this difference of colour in another manner. He fays the precipitate proves red only when the quantity of alcali is no more than juft fufhcient to fatiate the acid ; and that the more of the alcaline liquor we add beyond this point, the precipitate proves lefs and lefs red ; agreeably to which his coadjutor Baume fays afterwards more determinately, in his maJiuel de chyniie, that with a due quantity of fixt alcali the precipitate is orange yellow, and with an over quantity pale yellow. Mr. Macquer, judging from hence, that the rednefs was owing to a large quantity of acid retained by the platina, digefled fome of the red precipitate in a folution of fixt alcaline fait : the alcaline liquor, ablbrbing the acid, deflroyed the red co- lour of the powder, and made it white. It has long been known, that precipitates carry down with them a portion both of the diflblvent and of the body they were precipi- tated [ 483 ] tated by : the author oblerves that this appears more fenfible in our precipitate of platina, at Icaft with regard to the diflblvent, than in moft others ; and that this oh- fervation dilcovers the caufe ol' fundry fingular phenomena, which I had remarked in the precipitation of platina, and of which 1 had not given the theory', as of the red preci- pitate being folublc in water, and of part of the platina remaining fufpended whatever quantity of alcali we add in the cold: a detail and explication of thefe phenomena, with others of the fame nature, he referves for another memoir. Some experiments I have made do not very well agree with this theory, but I forbear any further obfcrva- tions till the authors memoir appears. • VII. Solution of platina, with mineral fixt alcali. As the two foregoing kinds of alcaline fait precipitate platina only in part, there is a third which has not even that effed:. The mineral alcali or bafis of fea fait, the method of preparing which will be defcribed in the fol- lowing part of this hiftory, produces no precipitation at all. This remarkable experiment, which we owe to Mr. Marggraf, will be further confidered hereafter. VIII. Solution of platina, ivith PmJJian alcali. Mr. Marggraf oblerves, that when folution of pla- tina is mixed and laturated with a lixivium of fixt alcali that has been calcined with blood, it yields a fine blue precipitate, which in certain circumflances proves as beau- tiful as the bell Pruflian blue, though there falls alfo at the fame time a little orange coloured matter. On re- peating this experiment, the liquors when firft mixed ap- peared, of a pretty deep blue, but when the precipitate had fettled, greateft part of it looked yellow, on account, probably, of the platina I made ufe of containing lefs fer- ^ R r r rugineous [ 484 ] _ _ rugineous matter, or the alcaline lixivium being lefs fatw- rated with the fubftance which tinges diflblved iron blue, than thofe which Mr. Marggraf employed. To obtain a foturated folution of this tinging fubftance, which cannot be expecSed to be done by calcining alcaline falts with blood or other like matters, I digefted fome common Pruffian blue both in folution of fixt alcaline fait, and in volatile fpirit of lal ammoniac prepared with quick- lime. Both menftrua foon became yellow; and the irony •bafis of the Pruffian blue, thus freed from its colouring matter, remained in a rufly form. To both tindlures I added fome more Pruffian blue, till they ceafed to have any ad:ion on "it. The fixt alcali, along with the tinging fubftance, appeared to have taken up fom.e of the iron;; for it flruck a blue colour with good aquafortis, with the acid of lulphur, and with difbilled vinegar, in which there were no grounds to fufpedt any iron to be previoufly con- tained. The volatile tindlure appeared free from iron, for •in the fame acid fpirits it produced no change, though it inflantly turned them blue when a little iron was firll dif- Iblved in them. This faturated folution of the tinging fubftance was added by degrees to folution of platina. The liquor turned at firfl to a deep blue, but on further additions, to a greenilh yellow. The precipitate was of two kinds, yellowifli at the bottom, and blue on the top. The whole being fhaken together and fet by till next day, a white matter appeared at the bottom, above this a yellow, and on the top a more copious brownifli grey. The liquor was of a deep gold colour. IX. Solution of platina i with compound falts. Solutions of alum, offal mirabile, of vitriolated tar- tar, of the fufible fait of urine, made feparately in water, and [ 48? ] and folutlon of chalk in aquafortis, were found by Marg- graf to produce no precipitation or apparent change in di- luted folution of platina. Sal ammoniac, one of the ingredients to which the men- ftruum owed its power of dilfolving the platina at firft, precipitated great part of it in form of a reddiHi or yellow- iHi powder, nearly fnnilar to tliat thrown down by alcalies. It is obfervable, that though neither fal ammoniac nor al- calies, feparately, occafioned a complete precipitation, the liquor llill remaining of a high colour ; yet when one was added to the folution remaining after the a(5tion of the other, a new precipitate fell, which left the liquor colourlefs. X. Solution of platina, ivith 'vinous fpirits. As gold is revived from its folutions by vinous fpirits, and made to rife in yellow films to the furface; I mixed a folution of platina with a large proportion of highly redlified fpirit of wine, and expofed the mixture for many days to the fun, in a wide-mouthed glafs ilightly covered with paper to keep out duft: there was no appearance of any yellow llcin, nor was any other alteration perceived, than that the platina had begun to cryllallize from the evaporation of the fluid. Sufpeding that though the liquor lliould really contain gold, yet the platina might ftrongly retain the gold and prevent its being feparated by the fpirit, I mixed three or four drops of folution of gold with two hundred drops of folution of platina, and after fliaking tliem well together, added fome rectified fpirit of wine: the whole being ex- pofed as above to the fun, a golden film was in a few days obferved upon the furface. R r r 2 XL Solution [ 486 ] XI. Solution of platina, with ejfential oils. A COLOURLESS efletitial oil of rofemary was poured into about half its quantity of folution of platina, the mixture well fliaken, and fuffered to reft: the oil quickly arofe to the furface without receiving any colour, and the acid underneath remained coloured as at firft. A compofition of platina and gold, which had been melted together, being diflblved in aqua regia, and the folution treated in the fame manner, the gold was imbibed by the oil, and the platina remained diflblved by the acid: the oil, loaded with the gold, appeared of a fine yellow colour, and on {landing for a few hours threw off great part of the gold to the fides of the glafs, in bright yellow films, which appeared to have no mixture of platina. Some other diflilled oils v/ere made trial of, with the fame event. XII. Solution of platina, with tether. The vitriolic xther or aethereal fpirit of wine, the pre- paration of which has been defcribed at the end of the eiglith fedtion of the hiftory of gold, was poured into a- Iblution of platina, and into a folution of a compofition of platina and gold. The two vials being immediately ftopt, to prevent the exhalation of the volatile fluid, and lightly fhaken; the sther received no colour from the folution. of platina, but became inftantly yellow from that of the platina and gold. XIII. Solution of platina, with tin. Asa minute proportion of gold contained in acid fo- lutions is difcoverable by their ftriking a purple colouf with tin, fome bright plates of pure tin were put into a folution of platina diluted with water. The plates in a little [ 48? ] little time looked of a dark olive colour, and fdon after were covered over with a rtddiih brown matter. The liquor became at firft darker coloured, and afterwards, by degrees, as the precipitate fell, nearly colourlefs, without exhibiting the leafl appearance of a purplifli or reddifli hue. Some platina was digeflcd in a quantity of aqua regia lufficient to diflblve only about half of it, and the remain- der was diifolved in a frefli portion of the mcnftruum. The two folutions, treated as above, yielded fomewhat different phenomena, but no tendency to a purplifh call: could be perceived in either. The latter folution, which looked yellow from its not being fully faturated with the platina, was, when diluted with water, almort colourlefs: neverthelefs, on the addition of tin, it became yellow again, then of a dull red, and at lafl of a dark brownifh red, confiderably deeper than the other more faturated fo- lution : on (landing for fome time it grew perfectly clear, depofiting a paler yellowifli precipitate. To determine whether platina was capable of pre- venting a fmall proportion of gold from difcovering itfelf in this way of trial, one drop of a folution of gold was let fall into fome ounces of a diluted folution of platina. On adding fome plates of tin, the liquor became quiekly purple. The foregoing experiments were made with a folution of the picked grains of platina. I fubmitted alfo to the four lafl trials, with tin, aether, effential oils, and vinous fpirits, a folution made by digefling in aqua regia the en- tire mineral, with its mixture of yellow particles, as it comes to us; which folution, in all thcfe trials, gave ex- adlly the fame appearances, as the other folution did after it had been firfl mixed with a very little quantity of fo- lution of gold, flriking a purple colour with tin, com- municating a yellow tinge to aether and effential oils, and yielding a yellow film with rediified fpirit of wine. XIV. Precipitaie: [488 ] XIV. Precipitate of platina expofed to a burning concave. Mr. Macquer and Mr. Baume, after examining the adion of a burning concave on crude platina, as already mentioned in page 466, expofed t\\c red precipitate of pla- tina made by alcalies, to the focus of the fame burning mirrour. " It immediately begun to boil, and diminifhed confiderably in volume : there arofe at the fame time a very abundant and very thick fume, fmelling ftrongly of aqua regia, and which appeared fo luminous and fo white in the neighbourhood of the focus, that we could not decide whether it was not a true flame: the precipitate at the fame time loft its red colour, to refume that which is natural to platina, and it now had the appearance of metalline lace. Being continued in the focus, the white fume fmelling of aqua regia was fucceeded by another fume or flame lefs copious, whofe colour inclined to violet. A little time after, there was formed, in the hotteft part of the focus, a button of fmooth brilliant matter perfedly melted, and then the vapours ceafed. Examining this button after it was grown cold, we found it to be a vitrefcent opake mat- ter, of a hyacinth colour on the furface, internally blackifh and pretty compacl. We dare not affirm that this was a true ^ itriiication of the platina in virtue of the faline mat- ters which were joined to it in the precipitate : the experi- ment ought to be repeated with pure platina, and with a burning glafs or concave flronger than that we uied." In- deed as the platina reflimed its metallic afpe<£t, it fliould feem to have been difengaged from the falts, before the time that the vitrification begun ; and perhaps a button of blackifli vitreous matter, formed in the middle of the focus, may be eafily enough accounted for from the ferrugineous calx, which the precipitate cannot be fuppofed to have been free from : fee the experiments of the relation of pla- tina [ 489 ] tina to vitreous bodies at the end of the following fe>rtion. The matter on which the powdery precipitate was cxpofcd to the focus of the burning concave, might alfo have con- tributed to the vitrification ; what this matter was, the au- thor does not mention. The experiments of this feftion point out fome ftriking differences between platina and goldj not only in the power of producing, when diflblved, a purple colour with tin, and communicating a like ilain to different kinds of colourlefs fubjedts, a power for wrhich gold is remarkable, and which platina wants j but likeviife in properties more importantly charadferillic, as they afford means of dif- tiuguifliing and parting the two metals when blended together. They have fliewn platina feparated in part from its folutions, by a fubflance which does not at all feparate gold, to wit, fal ammoniac; and gold feparated completely by fubffances which do not at all feparate pla- tina, viz. the mineral alcali, vinous fpirits, elfential oils and xther. It appears likewife from thefe experiments, that befides the black dufl which remains behind in the dilfolution of platina, the part dilfolved is not pure pla- tina; for the blue colour produced by tiie Pruffian alcali amounts to a proof that the folution contains iron. SECT. IV. Platina expofed to Jirong Jires, with faline, injiammable, fulphureous, vitreous and earthy bodies. HAVING feen the effefts of the purer acids on pla- tina, and the general properties of its folutions, we fhall proceed to apply to it what are commonly called Jinxes and dry menftrua, that is, fubftances which either barely promote the fufion of metals without corroding them, or which corrode and unite with them, when pro- perly [ 49° ] ^ ^ pcrly heated, nearly on the fame principle as humid menr- flrua dilTolve them. I. Platina with Borax. Half an ounce of platina was dropt into an ounce of melted borax, and urged with an intenfe fire for an hour. The platina appeared to have fuffered no alteration, but the borax was changed to a dark blackifli colour, probably from its having diffolved and vitrified fome of the ferrugineous duft. The whole was returned to the fire, which was kept up ftrong for a confiderable time longer, till the borax had funk through the crucible : it left the grains of platina of a bright white colour, flightly cohering but unaltered in form. II. Platina with Alcali. I TREATED platina in the fame manner both with the common fixt alcaline falts well purified, and with the cauftic alcali prepared by evaporating foapboilers ley, but could not perceive that either of them had any other effedt than contributing to brighten its colour. Mr. Marggraf mixed a dram of platina with half an ounce of fait of tartar, and gave them a vehement fire, in a clofe luted crucible, for tv/o hours. When cold, he found a hard mixt, of ayellowilh green colour, in which the platina was difperfed. The whole being feparated, as much as poffi- ble, from pieces of the crucible, by fcraping and wafliing, the water above the matter was next day found like gelly : the platina was whiter than ufijal, almoft of the whitenefs of filver, but of its wonted figure. The gelatinous con- fiftence which the water acquired in this and fome of the following experiments, is probably not owing to the pla- tina, but to fome of the earth of the crucible diffolved by the faline matter. III. Flatina [ 491 ] III. Platina with Nitre. Nitre, which reduces all the known metallic bodies, except gold, filver and quicklilver, into a calx, was mixed with equal its weight of platina, the mixture tlirown into a red hot crucible, and the fire kept up for about a quarter of an hour. No deflagration happened; and the platina, freed from the fait by repeated wafliing with water, appeared to have fuffered no other change than having its colour darkened, an efted: which the fnnple heat would have produced in it. The nitre was neverthelefs in great part alcalized. Four ounces of platina and eight ounces of the purefl nitre were put into a crucible, the crucible covered with a larger one inverted over it, and kept in flrong cementation, in a wind furnace, for three days and three nights without intermifhon. The matter being now boiled in water to feparate the fait, the platina looked rufty coloured, and had loft almoft half its weight: the faline liquor, on being filtered, left a brownifli powder fomewhat more than equivalent to this diminution, and being afterwards eva- porated to drynefs, yielded a fmall quantity of a greenifh cauftic Mcali. The fame platina was cemented thrice more with the fame quantities of frcfli nitre, and the fire continued for three days and three nights every time. In the two firft repetitions, a fmaller quantity of a paler powder feparated, and the remaining metal in good mea- fure loft the rufty hue which it had contracted before. After the laft cementation, the little quantity of metal which remained had much the fame appearance as the pla- tina at firft: on wadiing it, there was fcarcely any further feparation of powdery matter, but the nitre was ftill alca- lized. The platina was then mixed with fal ammoniac, and the fait fublimed in a Florence flalk : the fait arofc S f f uncoloured. [ 492 ] uncoloured, and left the metal white and bright. The powders feparated in the cementation were treated in the fame manner, and the fublimation repeated thirty times with frefli quantities of the fait: in the firft fublimations, ferrugineoLis yellow flo«.vers arofe, but at laft the fait re- ceived no tinge, and the powder remained of a greyifli colour. Mr. Marggraf gives an account of an experiment of the fame kind, in which he takes notice of fome phenomena, which either did not occur, or were not attended to, in mine. He threv/ into a red hot crucible four ounces of nitre, and one ounce, or four hundred and eighty grains, of platina : no detonation happened, but a confiderable fume arofe. The fire being continued, with care to prevent the falling in of any piece of coal, the matter, after feme time,, begun to fwell up, and a portion of it being taken out looked greeniili : it afterwards turned to a deep olive green> and grew confiderably tough and thick : after fome hours of flrong fire, it proved as thick as pap. The thick mat- ter was taken out while hot with an iron fpatula : it was of a deep olive-green colour. As much as pofiible of \vhat adhered to the crucible was colied:ed, and fet to digefl with the other in diflilled water : next day the whole was as thick as gelly. Being then diluted with more water, lliiTed about, and fuffered to fettle, the liquid was poured off, and this repeated till all the lighter parts were waflied over : this light matter, feparated from the faline liquor by filtration, well wafl^ied on the filter with hot water, and dried, weighed two hundred and twenty-five grains : it was of a dark grey colour, and by flrong calcination under a muftle became black like pitch. The more ponderous part was ground in a glafs mortar, by which fome more became fine enough to be wafhed over : this was of a clear brown colour, and amounted to thirty grains. The pla- tina [ 493 ] tina weighed three hundred and ten grains, and confe- quently had lolt above a third : it ftill rclenibled crude pla- tina, and retained its lullrc; the brownilli ruily coat, with which mine appeared covered after the firll cementations, having probably been here rubbed off in the grinding. The nitre was totally dccompofed, and had acquired all the charadlers of alcalicit)\ The crucible and its fupport were tinged almod throughout of an amcthyll colour, as ufually happens in the calcination of mangancfe witii nitre. The platina was treated in the fame manner with three ounces of frcfli nitre. The crucible and its fupport v/ere rtill tinged of a fine amethyft colour, the nitre was to- tally alcalized, and all other circumftances happened as in the firft operation, except that the lighter parts, firll wafhed off, weighed only lixty grains: by calcination they became, as before, of a pitchy blacknefs: the remaining powdery matter was of a clear grey, and weighed forty five grains : the platina, ftill bright, weighed two hundred and fifteen grains, or lefs than half of its firft weight. The operation was repeated with three ounces more of nitre. The crucible and its fupport were now lefs ftrongly tinged. The firft wafliings gave two grains of a light powder, in appearance much rcfembling the Eckertfl)erg blue earth; and by rubbing the rcit of the platina in water, there were obtained forty grains of a light pov/der of a grey-brown colour. The platina loll in this operation but five grains; and fo inconfidcrable a diminution giving little hopes of any further effect from a repetition of the procefii, the experiment was here dropt. It had been affirmed, that platina is a compound of gold and fome other matter, fo intimately combined together, as not to be feparable without other methods of procedure than are commonly pradtifed or known. An adept, in the pretended art of this higher metallurgic analyfis, Sff2 boalled [ 494 ] boafted of having deftroyed the heterogene matter, fo as to leave the gold pure, by long continued and repeated cenientations with nitre. To remove all fcruples on this head, I permitted him to make the experiment of which I have above given a fhort account, and of which I have ventured to infert only fuch particulars as came under my ov/n obfervation. The experiment, Vi^ith which that of Mr. Marggraf, fo far as it goes, fufHciently correfponds, was decifive. It fliewed much the greatell part of the platina changed to a powder, and the remaining platina as remote from the nature of gold as it was at fiift. I tried it both by acids,- and by cupellation with lead, a procefs of which an account will be given hereafter, and found it to preferve its own difcriminating charafters, without any marks oi gold, though it appeared to be purer than pla- tina in its common flate. I tried alfo, by the lame me- thods, the powders which had been feparated in the ce- mentations, after the reiterated fublimations of fal ammo- niac from them; and found thefe likewife to be no other than platina, not reduced to a calx, but barely divided. It may be prefumed that the adtion of the nitre was not direftly upon the platina itfelf, but on the irony matter,, adhering to the furfaces of the grains, or more intimately blended in their fubftancej which irony particles being changed to a calx, the platina intermixed became divided- along with them into a powdery ftate. This Ibppofition, accounts fatisfadlorily for the principal phenomena of the procefs; as the feparation of the powder being plentiful- in the firft cementation, and more and more fparing in the following ones; the firll powder bemg of a deep colour, and the others paler, as if the iron prevailed in the firft, and the platina in the others; the powders yielding yellow, ferrugineous flowers with fal ammoniac, while the platina. that remained entire gave no colour to the fait. lu [ 495 ] In regard to the pretenfions to the obtaining of goid by this procefs, it is not perhaps unreafonable to luppofe, that the remarkable reparation of powdery matter in the cementation, and the appearance of fome golden grains which had been naturally intermingled among thofe of the platina, led n>en of warm imaginations to anticipate the further effedl of the procefs, and to make the aflertion which the above experiments overturn. IV. Platina with common fait ^ An ounce of common fait, dried, was kept in fufion with a dram of platina, in a clofe crucible, for an hour and a half. The fait appeared yellowilh, and on breaking the mafs, there were found in the middle of it fome red cryftalline grains. The platina was all at the bottom of the crucible, and preferved its figure, having fuffered no change, except being made very white. The experiment was repeated with what is called regenerated common fait, and the phenomena were exadlly the fame. Both thefe experiments are from Marggraf. V. Platina with vitriolic fait s. Marggraf mixed a dram of platina with fome pure Glaubers fal mirabile, and kept the mixture in a ftrong fire for two hours : the fait funk through the cru- cible, and left the platina of a dark grey colour: on wartiing the platina with water, and rubbing it in a glafs- mortar, there feparated a little light matter of a bright blackilli colour, and what remained was platina un- changed. A dram of platina, and an ounce of vitriolated^ tartar, were treated in the fame manner: the fait melted' and became reddirti; the platina fuffered no change, ex- cept that it looked fome what more grey. VI. Platina [ 496 ] VI. P 1(7 fin a with the eJJ'ential falts of urine. From putrefied urine infpiiTated to the confiflence of a fyrup, is obtained by cryftallization a Angular faline con- crete, called the fulible or eflential fait of urine, or micro- cofmic fait, containing the acid ofphofphorus united with a volatile alcali. This fait, expofed to the fire, parts with its alcali, and aflumes a glafly appearance; in which ftate, all the common metals, gold not excepted, are faid to be corroded by it in fufion. A hundred and eighty grains of this fait were mixed with thirty grains of platina, and urged in a crucible with a ftrong fire for two hours: the platina was found unchanged at the bottom, covered with the fait, which likewife appeared little altered. Sixty grains of this fait, the fame quantity of calcined borax, and thirty grains of platina, were treated in the fame manner: there was a vitreous fcoria, fomewhat opake, of a yellowifh green colour: the falts and lighter parts being feparated by wafhing, the dried platina appeared of its natural form, but whiter than at firfi:. After the cryftallization of the foregoing fait from urine, there cryftallizes another, not containing the phofphorine acid, and whofe compofition is as yet unknown. Three drams of this fait and half a dram of platina being urged with a ftrong fire in a clofe crucible, the fait run all through the crucible, and the platina, after rubbing in a mortar and wafhing with water, appeared in its original form, being only fomewhat whiter than before. A dram of the fait, a dram of calcined borax, and a dram of pla- tina, treated in the fame manner, gave a green-yellowiih, dark-chryfolite-coloured vitreous mafs, under which lay the platina unaltered, except that as in the former cafe it was whiter than at firft. All thefe experiments are from Marggraf. VII. Platina [ 497 J VII. Platina ivith pbojphorinc acid. When the phofphorus of urine is fct on fire under a glafs bell, nearly in the fame manner in which fulphur has been ufually burnt for obtaining its acid, it yields fpongy flowers, in appearance much like thofe of zinc; and both the flowers, and what matter remains on the glafs dilh which the phofporus was placed upon, imbibe moif- ture from the air and run into a thick acid liquor, which expofcd to the fire leaves a dry matter that melts into a glalfy form. Mr. Marggraf mixed fixty grains of platina with twice as much of this acid liquor, and put them into a retort, whole juncture with the receiver was only clofed with paper. The watery moilUirc being drawn off by a gradual fire, the retort was fet, while hot, upon burning coals, till it begun to melt; after which, being taken from the fire, a flafli like lightening filled both the retort and receiver, and a violent explofion followed. The author very ingenioully, and with great probability, attributes this effeft to a regenerated phofphorus, to which the iron mixed with the platina had contributed the inflammable principle ; the adtion of which phofphorus could then only take place, when the abatement of the heat fuffered air to pafs in through the ill-clofed jundture. The pieces of glafs being colledted, the bottom of the retort appeared covered with a white faline matter, which being fcraped off, the platina was found under it unchanged. It is evi- dent that the platina itfelf neither was, nor was fuppofed by the author to be, anywife concerned in producing the fulguration ; though Vogel makes this fulguration one of the new properties of platina difcovered by Marggraf. VIII. Flatina [498] VIII. Platina with black jlux, ^c. ' The black flux commonly employed by the chemifts for the fufion of metallic minerals and calces, compofed of one part of nitre and two of tartar mixed together and burnt in a covered veflel to an alcaline coal, was kept in fufion above an hour, in a clofe crucible, with one fourth its weight of platina. Compofitions of wood-foot, char- coal powder, common fait, and wood afhes, direfled by M. de Reaumur for changing forged iron into fteel, were mixed with platina, and cemented feveral hours, in clofe crucibles, both with moderate degrees of heat, and with fires fVrongly excited. In all thefe trials I could not ob- ferve that the metal fuffered any change, except that its colour was darkened, IX, Platina with fulphur. An ounce of platina was fpread upon twice its weight of fulphur, with which fome powdered charcoal had been previeully mixed, to prevent its becoming fluid in the fire, fo as to fufi'er the platina to fubfide. The crucible, having another crucible, with a hole in the bottom, inverted into its mouth, \^•as kept in a cementing furnace for fome hours : being then taken out, it was found that the fulphur had entirely exhaled ; and that the platina, fepai-ated by wafh- ing from the charcoal powder, had the fame weight and appearance as at firft, except that its colour was changed to a blackilli : by rubbing it in a glafs mortar with a little alcaline fait and water, the blacknefs was deflroyed, and its original brightnefs reftored. I varied the experiment, by flirongly heating the platina in a crucible by itfelf, and repeatedly throvv ing upon it pieces of fulphur : it fl:ill re- mained unaltei-ed, the fulphur feeming to have no more a like coarfe gold. To the naked eye it appeared uniform j but a good mag- nifying glafs difcovered, in this as well as in the other, fome inequality of mixture, notwithilanding the fufion was two or three times repeated,, witli as great a degree of heat as we could eafily excite by the bellows. 3. Twenty carats of gold and four of platina were kept in ftrong fufion above an hour and a half. Thefe united into an equal mafs, in which no granule of platina or difii- milarlty of parts could be diftinguifhed. The colour was ftill fo dull and pale, that the compound could fcarcely be judged by the eye to contain any gold. It hammered well into a pretty thin plate, but we could not draw it into wire of any confiderable finenefs. 4. Twenty-two carats of gold were melted in the fame manner with two carats of platina, the fame proportion that ftandard gold contains of alloy. The mixture was uniform, and had a good deal of a golden colour, but with a particular dull dark hue, by which the eye could at once diftinguhh it, not only, from fine gold, but from all the common forts of alloyed gold. It worked well, was forged into a thin plate without cracking, and drawn into mode- rately fine wire, 5. Twenty-two carats and a half of gold and one and a half of platina, or fifteen parts of the former to one of the latter, melted into an uniform mafs, which after the ufual nealing [ 52? ] Healing and boiling, proved foinewhat tougher than the preceding, and of a better colour. 6. Twenty-three carats of gold were melted with one of platina, which is nearly half the proportion that flan- dard gold contains of alloy. The compound worked ex- tremely well ; but was diftinguiftiable from fine or ftandard gold by fome degree of the ill colour of the two forego- ing, which it retained after repeated forgings, fufions and boilings. 7. Twenty-three carats and one fourth of gold, and three fourths of a carat of platina, or thirty-one parts of the former to one of the latter, formed an equal mixture, very malleable, duftile like the three foregoing while hot as well as cold, but not altogether free from their parti- cular ill colour, 8. A mixture of twenty-three carats and a half of gold with half a carat, or one forty-feventh its weight of pla- tina, was very foft and flexible, of a good colour, without any thing of the difagreeable call by which all the forego- ing were readily diftinguifhable by the eye from any kind of alloyed gold I havefeen., 9. A mixture of twenty-three carats and three fourths of gold with one fourth of a carat, or one ninety-fifth its weight of platina, could not be diftinguiflied, by the eye.. or the hammer, from the fine gold itfelf. 10. In all the above procefles, even where the quantity of platina was very fmall, the fufion was performed with a vehement fire, that the platina might be the more per- fedlly diflbWed, and equally diffufed through the gold. This appeared to be a very necelTary precaution. Having once melted gold with one fourth its weight of platina, the button appeared not much paler than ftandard gold with filver alloy, but on a fecond fufion it loft its yellow- nefs, and looked not much unlike bell metal. The gold colour [ 528 ] colour appeared to have been only fuperficial, from an im- perfedl mixture ; moft of the platina having been concealed in the internal part of the mafs, and covered as it were ■with a golden coat. II. In fome circumftances I have feen the gold, after it had been thoroughly mixed with the platina, fpued out again in part to the furface. The foregoing bell-metal coloured mixture, after repeated fufions with and without additions, and in different degrees of heat, became once yellow on the furface. On cupelling mixtures of platina and gold with lead, I have oftener than once feen the re- maining button covered with a golden fkin, and all the internal part grey. J 2. In melting the platina and gold together, a little borax was always ufed as a flux ; with an addition of nitre, which fomewhat heightens the colour of gold, or at leaft prevents the borax from making it pale. Pieces d( fome of the mixtures were remelted, with borax alone, with ni- tre alone, with common fait, with fixt alcaline fait, and with powdered charcoal : thofe with borax feemed to be the paleft, and thofe with charcoal powder the beft co- loured, though the differences were very inconfiderable. 13. As a fmall portion of copper fomewhat heightens the colour of pale gold, I melted platina with eight times its weight of flandard gold made with copper alloy ; that is, three parts of platina with twenty-two parts of line gold and two of copper. The fufion was performed, as in the preceding experiments, with a ftrong fire, in a clofe cruci- ble, but without any flux, and continued about an hour. ■ The metal appeared covered with a black fcurf, and had loft about a two-hundredth part of its weight. It was much duller coloured, much harder under the hammer, and cracked fooner about the edges, than mixtures of fine gold with confiderably larger quantities of platina. By repeated [ 529 ] repeated fufion and frequent nealing.it became a little fofter and tougher, lb as to be drawn into pretty fine wire; but the colour was ftill exceeding dull, more refembling that of very bad copper than of gold. It appears from thefe experiments, that platina dimi- niflies the malleability of gold much Ids than it does that of the other malleable metals ; and infinitely lefs than lead, tin, iron, and the brittle metals do that of gold: that in confiderable proportions it debafes the colour of gold far more than the ufual alloy, communicating a peculiar and remarkable ill colour; and that it both hardens, and de^ bafes the colour of ftandard gold, with copper alloy, much more than fine gold: that in fmall proportions, as one forty-feventh and downwards, it does not fenhbly injure either the colour or malleability of gold; and confequently, that large proportions of platina mixed with gold are dif- coverable at fight, but that fmall proportions, if perfedly united with the gold, will not betray tliemfelves either to the eye or in the workmans hands.. X. Platina with Copper. 1. Equal parts of platina and copper, expofed, with- out addition, to a ftrong fire halHly excited by bellows, foon became fluid, but not thin; and loft about one fixty- fourth. The metal proved extremely hard to the file, broke difficultly on the anvil, flew afunder upon endea- vouring to cut it with a chifel, and appeared internally of a coarfe grained texture and white colour. 2. One ounce of platina and two of copper, urged with a quick fire in a blaft furnace, without addition, flowed fufficiently thin, and fcarcely fuffered any lofs. The me- tal was ftill very hard, and ftretched but little under the hammer. It looked darker coloured than the foregoing, with a flight reddifli cafl. 3. OncL [ 530 ] 3- One ounce of platina and four of copper, treated in the fame manner, united without lofs into a pretty tough compound, which bore to be confiderably flattened, cut with a chifel, and bent almoft double before it cracked. Internally it looked of a fine texture, and of a very pale copper colour. 4. A mixture of one ounce of platina and five of copper ilretched fomewhat more eafily under the hammer than the preceding, and appeared of a redder colour. 5. Upon increafing the copper to eight times the quan- tity of the platina, the compound proved fufiiciently tough, broke difficultly, and hammered well. It was much harder than copper, and of a paler colour. 6. A mixture of one part of platina and twelve of cop- per was fomewhat more eafily extended under the hammer than the preceding, and proved fofter to the file. It fi:uck a little in the teeth of the file, which the compofitions with a greater proportion of platina did not. 7. A mixture of one part of platina and twenty-five of •copper was flill fomewhat paler coloured than pure copper, and confiderably harder and fl:iffer, though very malleable. On increafing the copper a little further, the mixture con- tinued fomewhat harder than tlae copper by itfelf, and ap- peared of a fine rofe colour. 8. In the foregoing fufions, though in general no flux was made ufe of, there was fcarcely any lofs of weight, ex- cept in No. I, where the large proportion of platina re- quired the fire to be raifed to a violent degree. This feems owing in great meafure to the platina preventing the fcori- fication of the copper: for on melting pure copper, a great number of times, both with and without fluxes, there was conftantly fome lofs. 9. The mixtures with large proportions of platina are difficultly extended under the hammer when cold, and when [53M when red hot they fly in pieces : they bear a good poH/h, and do not fcem at all tarniihed in keeping ten years ; of the mixture of equal quantities in particular, the polifhed part continues very brilliant. No. 7. has tarnilhed a little, but fecmingly not fo much as pure copper. Platina appears therefore from thefe experiments to harden copper, to dilute its colour, and diminifh its dif- pofition to tarnifh; in fmall proportions, to improve its hardnefs, w^ithout much injuring either the colour or mal- leability; and in larger proportions, to injure the mallea- bility lefs than it does that of any of the other dudtile me- tals, except gold and perhaps filver. Platina and copper feem to form valuable compofitions, of which I doubt not but the workman may avail himfelf. In a letter from Spain to a perfon in London, a tranf- lation of which has been communicated to me, there is an account of an experiment on platina and copper, which, though imperfecfbly related, may deferve to be mentioned here. The author hrll tried platina with an equal weight of filver, and found them to melt together .... he after- wards melted it with copper, which united perfedlly well; but whether it was the platina itfelf, or the mixture with fdver, that was melted with the copper, is not clear from the words, though it feems to have been the former. The mixture with copper," on trying to hammer it, flew about like glafs; but having melted it over again with a flronger fire for fomc time, and thrown in a little faltpetre, mer- cury-fublimate, and other corrofives, it became malleable, and was then made into rings, which were worn for a good while without foiling the fingers, and preferved always the fame colour and lufl:re as thofe called in Spanifh tombagos, which confifl: of two parts of copper and one of gold." Z 2 z A mixture [ 532 ] A mixture of equal parts of platina and copper (No.i. of the above experiments) was tried by Mr. SchefFer, who reports, that they melted as eafily as copper by itfelf; and that the mixture proved tolerably malleable, as mixtures of gold with a like quantity of copper: in both thefe points, the little quantity he could allow for the experiment may be fuppofed to have occalioned fome deception. He adds, that wlien this compound is urged by a ftrong tlait impelled upon the lurface, as in the purification of copper before the bellows, it throws out fparkles like iron in welding, and that thefe fparkles are found in form, of round grains, which partake of both the metals; a phenomenon which gold does not exhibit with copper. After this operation, he found the mixture lefs malleable than before, like copper over-refined. XI. Platina with Copper and Zinc. 1. Equal parts of platina and brafe, covered with borax and expofed to a quick fire in a blaft furnace, melted perfedtly together, and fuffered very little lofs. The mixt was of a greyiih white colour, filed hard like bell-metal.» broke from a blow of the hammer without ftretching or receiving any impreffion, and flew afunder upon endeavour- ing to cut it with a chifel. Internally, it appeared of an, uniform fine grain, a clofe texture, and a darker colour- than on the outfide. It bore a very fine polifli, which in. ten years does not appear to have at all tarniflied. 2. One part of platina and two of brafs, melted together, in a flow fire, loft about one thirty-fixth. The ingot was of a duller colour than the foregoing, with a faint yellow- ish caft. It filed fofter, and broke lefs readily from the chifel, but cracked and fell in pieces under the hammer*. ]tt received a good golifh, and continues untarnifhed. 3. One [ 533 ] 3- One part of platina and four of brafs, covered as be- fore with borax, and expofed to a quick fire, melted toge- ther without lofs. This compound proved yellower than the preceding, and fofter to the file; it bore to be cut fome depth with a chifel before it broke, and received fomc impreffion from the hammer, ftretching a little, but foon cracking in various diredtions. 4. Upon increafing the brafs to fix times the weight of the platina, the compound appeared yellower, though ftill very pale. It proved fofter to the file; and ftretched more under the hammer, and received a deeper impreflion from the chifel, before it broke. 5. A mixture of one part of platina and twelve of brafs was confiderably paler, and much harder, than l)rafs. It broke under the chilel ; and cracked, before it had ex- tended much, under the hammer. Both this and the two preceding compofitions bore a tolerably good polifh, and have not tarnifhed fo much as brafs by itfelf; though in both refpeiSts they fall fhort of No. i and 2. XII. Platina tvith Copper and Tin, 1. Fifty parts of platina, icventeen of copper, and fix of tin, covered with borax, became tiuid in a llrong fire, and fuffered very little lofs. The ingot proved confider- ably hard, lb as fcarce to be touched by the file; and very brittle, breaking from a moderate blow, of a rough fur- fiace, and dull bell-metal colour. It bore a good polifh, and continues untarniflied. 2. Platina and copper of each one ounce, and four ounces of tin, melted perfedly together, with little or no lofs. This compound filed freely and eafily, and bore to be cut with a knife, but broke readily on the anvil ; the fradure was of an irregular furface, and a dull whitifli co- lour. Polifhed, it looked like poliflied iron: the fradlure Z z z 2 fcorj [ 534 ] Coon tarnilhed to a yellowifli hue; the poliflied part grew dull but retained its colour. 3. A mixture of platina and copper of each one part, and eight of tin, proved fofter than the foregoing, and flattened a little under the hammer. Broken, it fhewed a very irregular fur£\ce, compofed of a great number of bright white pktes. It did not polifh well. The fradure foon tarniflied; the poliflied part retained its colour. XIII. Platina with Iron. 1. Half an ounce of platina and an ounce of iron wire were placed on a bed of gypfum in a HefTian crucible, and covered and furrounded with more gypfum: after being urged in a blaft furnace with two pair of bellows for abouj an hour, the crucible was in great part vitrefied, and a large hole made in its fide, by which moft of the metal had run out. The experiment v/as four or five times repeated,. but. a perfedl union of the platina and iron could not be obtained, the crucible being corroded and vitrefied by the gypfum before the iron flowed thin enough to diflblve the. platina. It was cbfervable that the iron, thus melted, proved very malleable; though fome have thought that forged iron, brought into fufion, is of the fame nature with common cail iron. 2. Caft iron and platina, of each three ounces, expofed without addition to a flrong fire, united into a thick fluid, which, on adding an ounce more of the iron, flowed pretty thin. The black lead crucible having become too foft from the. great heat, to admit of being lifted with the tongs, the metal was fuffered to cool in it. On breaking it, the metal was found in one lump, not convex, but of a very concave furface : its weight was about one fixteenth lefs than that of the platina and iron employed. It proved txcelTively hard, fo as not to be touched by the file, and yet [ 535 ] yet (o tough, as not to be broken by repeated blows of a fledge hammer, from which it received fome impreflion. Heated red, it broke eafily, and looked internally of an uniform texture, not compofed of bright plates as the iron was at firft, but of very dark coloured grains which had no metallic luftre. 3. One ounce of platina being thrown upon four ounces of cafl iron beginning to melt, and the lire kept up ftrong, the whole came quickly into fufion. The compound, like the foregoing, was extremely hard, and feemed to ftr^tch a little under the fledge hanuner, without breaking. The texture \uas grained, as before, but the colour fomewhat lefs dark. 4. One part of platina and twelve of iron melted with- out difficulty, and with little or no lofs. This mixture alfo was much harder than the iron at firft, and received fome impreflion from the hammer. Like the others, it * could not be broken while cold without extreme violence, but proved very brittle when heated red. 5. All the foregoing compolitions received a good po- li(h. The firft, in keeping ten years, has fuffered no fen- fible change ; the fecond has fome finall fpecks of tarnifli, and the third, is tarnifhed fomewhat more, but not fo much- as a piece of the iron itfelf. 6. About an ounce of a compofition of one part of pla- tina and four of iron was furrounded, in a crucible, with: Reaumurs fteel-making mixture, compofed of eight parts of wood foot, four parts of wood aflies, four of charcoal powder, and three of common fait: the crucible was covered: and clofely luted, and kept in a ftrong red heat for twelve hours. The metal gained an increafe of about one thirty- ninth of its weight, yielded to the file more eafily than at firft, feemed to receive no additional hardnefs on being ig- nited and (quenched in water, and did not appear to have. acquired^ [ 536 ] acquired any of the qualities by which fteel is diftinguifli- cd from iron. 7. A piece broken off from the fame ingot, weighing about three quarters of an ounce, was treated in the fame manner with the powder for foftening caft iron, viz. bone afh with a fmall mixture of charcoal powder. The me- tal was found increafed in weight about one thirty-fourth : it was lefs hard to the file than at firft, but harder than the part which had been cemented with the fteel-making mix- ture. It may be proper to obferve, that caft iron is by no means a pure or fimple metallic body, like thofe whofe re- lations to platina have been examined in the foregoing ar- ticles. It feems generally to contain mineral fulphur, to which perhaps its brittlenefs is chiefly owing, and which is feparated in the procefs by which the iron is made mal- leable. As platina appears incapable of contracting any union with pure fulphur, I have fufpedled, that while the platina and cafi: iron unite together, fome of the fulphure- ous matter is thrown out and confumed, and that the de- gree of toughnefs, obferved in the compounds, may proceed in part from this caufe; but experiments have not yet been carried to a fufficient length to enable us to enter fatis- fadtorily into dilquilitions of this kind. If however the caft iron fhould be as effectually purified by the platina, as it is even at the finery in the iron works, yet the toxtghnefs of the mixtures would ftill be pretty re- markable, confidering how much platina, when its propor- tion is large, is difpofed to diminifli this quality in all the other metals. Perhaps platina, for certain purpofes, may prove a valuable addition to this moft ufeful metal; a metal to which the workmen cannot communicate the hardnefs which is often required, without communicating at the fame time brittlenefs and intradtability. XIV. Platina i [ 537 ] XIV. Platina ivith metallic ghtjes. Mr. Marggraf, after having f^itisfied hinirdf tliat platina perfedtly rclifls the common unmetalhc fluxes of the vitreous and faline kind, as related in the fourth fediion of this hiftory, proceeded to try if the more active glafs of lead would ferve as a Hux for it. A glafs of lead, prepared from four parts of the fined minium and one part of pure flint, was reduced into pow- der andpafled through a fine fieve, to feparate any metallic grains that might remain in it. Eight ounces of the pow- der were mixed with one ounce and a half, or 720 grains, of platina, and the mixture urged with a ftrong fire, in a clofe luted crucible, for two hours : a white or grejr- iih brittle regulus was obtained, covered with a yellowifli fcoria. The regulus was remelted with more of the fame glafs of lead, and kept again two hours in fufion : it look- ed as before, had a like yellow fcoria, and weighed 606 grains, or about a fixtli part lefs than the platina employ- ed. Kept in fufion two hours in a clofe crucible, it loft fix grains, or about a hundredth part. It was then beaten in pieces in an iron mortar, and mixed with an ounce of common green glafs in fine powder : the mixture being, kept melted for three hours in a covered crucible, the fcoria proved turbid, inclining to greenifli, and in fome parts to bluifh ; the metal had loft thirty grains, or about a twentieth ; it filed well, looked very white in the marks of the file, had fome toughnefs, and did not very eafily break under the hammer. It was again expofed to a ftrong fire for two hours in a clofe crucible, with half an ounce of calcined borax : the borax run through the crucible,, but the metal did not perfedlly melt, only baking into a mafs^of an unequal rough furface, porous, eafy to break, in tte frafture of a grey and white colour intermixed, im weight: [ 538 ] weight 540 grains, fo that it had loft above a twentieth part more. It was further treated with half an ounce of calcined borax, the fame quantity of powdered white flints, and an ounce of fait of tartar: the mixture beipg urged for two hours in a clofe crucible, with a vehement fire, the fcoria^ were of a topaz inclining fomewhat to a chryfolith colour; the metal of a fine white colour, fpongy, rough on the furface, in weight 450 grains, fo that it had loft in this fufion one fixth part, and weighed now three eighths lefs than the platina at firft. It may be prefumed, that the metal obtained in this ex- periment was no other than a mixture of part of the pla- tina with fome lead revived from the glafs. Though the author took care, by covering and luting the crucible, to guard againft the falling in of any inflammable matter that might revive the lead, yet fuch a matter might have hap- pened to be introduced in the pounding and lifting of the glafs; and independently of any accident of this kind, there was, perhaps, in the platina itfelf, a power fufficient for producing the cfFedl. Common platina, fuch as Marggraf employed, plainly contains iron 3 and on barely ftirring glafs of lead in fufion with an iron rod, part of the lead is revived, I mixed fojme of the purer grains of platina both with glafs of lead and with glafs of antimony, and expofed both mixtures to a fire as ftrong as I could excite; the pa- tina lliewed no difpofition to melt, the grains remaining of their ufual appearance. Vogel feems therefore to have ill underftood Marggrafs experiments, when he con- cludes from them that platina yields a white regulus with glafs of lead. Mr. Marggraf gives alfo another experiment of the fufion of platina, v/ith an arfenicated glafs of lead. A glafs was prepared by melting together eight ounces of minium, two of flints, and one of white arfenic. Six ounces [ 539 ] ounces of this glafs in fine powder were mixed with onci ounce of platina, and the mixture melted in a clofe cru- cible for two hours. A brilhant regulus was obtained, greyifli on the fradlure, but when filed pretty white, weighing twenty-eight grains, or about one leventeenth, more than the platina : the fcoria was of a dark browji colour. Here the increafe of weight is a full proof that the fufion of the platina was owing to its having imbibed cither lead or arfenic from the glafs: in the brilliancy of the furface, and the grey colour of the internal part, Mr. Marggrafs metal agreed with our maffes of arfenicated platina already defcribed j and probably the ufing of arfe- nic in a llate of vitrification with fubftances which ferve to detain it in the fire, may be the mofl effecflual means of combining this volatile metallic body with platina. XV. Getter a! ohfervations on the mixtures of platina njoith other metals. I. It appears from the foregoing experiments, that pla- tina, unfutible by itfelf in the Itrongeft fires of our fur- naces, and proof againft the moft: aftive unmetallic fluxes, melts witli, or is diilolved by, every one of the common metallic bodies : That the different metals dillblve it with different degrees of force, and this not in proportion to the degree of their own fufibility : That there are remarkable differences in its relation to different metals, in regard to the change which it produces in the quality of the metal j that it hardens, and diminilhes the malleability of, all the malleable metals, but feems to communicate fome degree of toughnefs to one which of itfelf has none, viz. call iron; that it diminiflies the malleability of tin more, and of gold lefs, than of the other metals; that in certain quantities, it debafes the colour of all the metals, communicating to A a a a fome [ 540 ] fome its own whitenefs, as to copper, and producing with others a new colour, as with bifmuth, lead, and gold ; that it preferves copper and iron from tarnifhing or railing in the air, but occalions lead and bifmvith to tarnifli in a remarkable manner. 2. Though platina, when its quantity is not very large, becomes fluid with moft of the metals in a moderate fire, a ftrong one feems to be always requiiite for its perfedt and total folution. Compofitions of copper, of filver, and of lead, with one third their weight of platina, which had flowed thin enough to run freely into the mould, and ap- peared to the eye perfedtly mixed ; on being digefled in aquafortis till the menfl-ruum ceafed to aft, left feveral fmall grains of platina in their original form. Upon viewing thefe with a microfcope, fome appeared to have fufi-ered no alteration: others {hewed a multitude of fmall,. bright, globular protuberances, as if they had jull begun to melt. 3. Mixtures of copper, filver, and lead with fmaller proportions of platina, which had been kept in flrong fufion for fome hours that the platina might be wholly in- corporated, were digefted and boiled in frefli portions of aquafortis, till the platina was left by itfelf in fine powder, free from any thing that aquafortis could extract. Thefe powders were expofed to very vehement fires, Mdthout ad- dition, with the addition of borax, with alcaline falts, and with flint glafs: they proved as unfufible as the platina at firfl^, neither melting, nor communicating any colour to the falts or glafs. It appears therefore that the platina is only fimply dilfolved by the metals in fufion, and does not by their means become truly fufible itfelf. 4. As platina unites with feveral metals into compounds^ of new qualities, fuch as the ingredients neither pofiTefs feparately, nor can be conceived, on any known mecha- nical; [ 541 ] nical principles, to produce by their fimple jumflion j aB<| as fuch new properties leem to be in no metallic mixture more confpicuous than in thofe which pktina aftbrds ; it follows, that the diflblution of platina by metals is by no means a fuperficial mixture, but as pcrfedt and intimate a coalition as we have grounds to believe that of any one metal to be with any other. SECT. VI, Of the Jpecific gravities of mixtures of platina with different metals. AMONG the experiments communicated to the X~\. Royal Society by Mr. Wood, there is a remarkable one of the fpecific gravity of a mixture of equal parts of platina and gold. The gravity of the heaviefl: platina he examined was to that of water as 15 to i ; and the gravity of gold, as we have feen in the hiflory of that metal, is about 19-.-0. If 15,0 parts of platina lofe one on being immerfed in water, and 19,3 parts of gold lofe i ; then, if the two metals be mixed in equal quantities, 34,3 parts of the compound fliould lofe 2; whence, dividing 34,3 by 2, we have j 7, i 50 for the gravity of the compound. Such ought the gravity to be, if the two metals were joined fu- perficially, and each preferved its own proper volume ; but when melted together, the fpecific weight of the mafs is faid to have been confiderably greater, amounting to no lefs than 19. If this be the cafe, 19 parts of the melted mafs mulT: occupy no more fpace than 1 7 .--'0 did before the fufion ; fo that near a fourth part of one metal is re- ceived into the pores of the other, without increafing the bulk of the mafs. It may be fufpefted, that the fubftance which Mr. Wood weighed by itfelf under the name of platina was the lighter caft metal mentioned at the begin- A a a a 2 inp [ 542 ] ing of this hiftory, and that what he melted with gold was the true platina ; in which cafe, the gravity of pla- tina being fuppofed 1 7, the increafe of gravity on mixture comes out about a twentieth part, fo that about a tenth part of the platina has its bulk lofl in the mafs. To fatisfy myfelf in this point, I weighed hydroftatically the mixture, already mentioned, of equal parts of platina and gold. The fpecific gravity of the gold was 19,285 : the platina was the larger grains, whofe gravity, as we have feen in the firft fcdion, was at leaft 17. The compound weighed in air 13605, and loll in water 750, whence its gravity was 18,140: the gravity by calculation comes out 18,071 ; fo that though the platinas gravity had been no more than 17, the increafe of gravity from the mixture was not very coniiderable. As a little lofs had happened in the fulion of this mixture, and as the fpecific weight of the platina employed was not certainly known, I made two frefh ones, with pieces cut off from the fame mafs of gold, and fome of the largeft grains of platina, whofe gravity was nearly 18. One of thefe mixtures, weighing 5129, loft in water 276 ; and the other, weighing 6415, loft 345; whence the fpecific gravity of the former turns out 1 8,583, and that of the latter 18,594, which come as near to one another as can well be expeiled in experiments of this kind : the gravity by calculation is 18,622 ; fo that both mixtures were a little fpecifically lighter, or expanded in- to a larger volume, than if the metals had been weighed feparately, or joined by fimple appofition of parts. As thefe experiments were made with a good deal of care, it may be prefumed that in thofe, where there feemed to be a great increafe of gravity, or contraction of volume, either fome error happened in the weighing, or the platina had not been all taken up by the gold in the fufion. I weighed [ 543 ] I weighed alfo hydroftatically the other mixtures of pla- tina and gold, and fundry mixtures of it with different proportions of the other metals. Such mafles as could bear the hammer, were gently hammered a little, with care not to make them crack ; for the pure metals them- felves, after fulion, are feldom found to come up to their true fpecific weight, till brought to greater folidity under the hammer. The furface was filed fmooth, where any cavities or irregularities were likely to retain air; and moft of them were kept immerfed in water for an hour or more, that the air might be more effeftually extricated, and the water more clofely applied to them. The effedl of this precaution was manifeft in fome trials purpofely made ; when the metal, hanging in water from the end of the beam, had been cleared from all vifible air bubbles, and exadlly counterpoifed ; on (landing for an hour or two, it fenfibly, and fometimes very confiderably preponderated. The water was in fome of the trials melted liiow, and in others diftilled water, which were both found to be of the fame fpecific weight. The temperature of the air was from the 50th to the 60th degree of Fahrcnheits thermo- meter. The balance, made ufe of in thefe experirnents, was of great fenfibility, but not exadlly equibrachial : and here it may be proper to obferve, that though the writers on ba- lances require, and are very felicitous about procuring, a perfedl equality of the arms ; yet as this equality is ex- ceeding difficult, if not impoHible, to be obtained, fo nei- ther does it appear anywife neceffary to the accuracy of the inflrument. If ten equal fmall weights, put into one fcale, are counterpoifed by a weight in the other ; and if the ten weights be then removed, and a bit of filver or brafs plate put in their room ; it is evident, that when this plate Ihall be made equiponderant to the counterpoife, it wiU [ 544- ] will be exactly equal In weight to the ten, how unequal foever the arms of the balance may be ; and confequently, that any unequal- armed balance may, on this principle, have a fet of weights adjufted to it, which being ufed al- ways in one fcale, the inftrument (hall be of the fame ac- curacy as if the arms were moft exadly equal. The beft way of procuring equal fmall weights is, by cutting off equal lengths of the finefl filver wire : the filver thread, kept equally ftretched by a heavy body at the end, may be coiled clofe round a thicker piece of brafs wire, and all the coils cut through at once by a fharp inftrument applied lengthwife. Silver wire is drawn to fuch finenefs, and of fo uniform thicknefs, that weights, thus made by menfu- ration, are of greater nicety than it is polTible for any ba- lance to weigh. A piece of the v/ire,whofe length is very fenfible and much further divifible, ihall not have weight enough to give any fenfible motion to the tendered balance. Thefe fmall pieces, or fuch as will but jufi: move the ba- lance when empty, and which confequently will not move it at all when loaded, I have found to be a very ufeful ap- pendage to the adjufted feries of weights. Though a balance appears exadlly in equilibrio, yet one fide may really preponderate, by any quantity of force, lefs than that which is fufficient to overcome the fridtion on the center: as lefs additional force will ferve to deprefs this fide than the other, one of the fmall weights, tried firft in one -and then in the other fcale, will enable us to judge whether the equipoife is exad:, or on which fide the preponderation lies. The refults of thefe experiments were publifhed in the Philofophical Tranfadtions, together with the gravities of the feveral mixtures deduced from calculation ; from which it appeared that the experimental gravities were al- moft always Icls than the computed. But an error in thofe [ 545 ] thofc calculations has made the computed gravities in ge- neral too great: for though the ingredients in each mixture were proportioned to one another by weiglit, the calcu- lations were inadvertently made as if they had been taken by volume. The difcovery of this miflake I owe to Mr. Scheffer, who gives a paper on this fubjed in the Swedilh tranlailions for the year 1757. The computed gravities being rediified, there appears to obtain, in feveral of the mixtures, fome degree of what the above experiments fliew not to obtain in thofe with an equal quantity ot gold ; the compounds being of greater gravity, or more contracted in volume, than the two me- tals confidered feparately. This excels of the experimental gravities above the computed is attributed by Mr. Scheffer to the gravity of the platina being greater than that which I had afligncd to it. He imagines, that particles of air, adhering in the cavities of the rugged grains, had occafioned them, when weighed in water, to occupy a larger fpace than that of their own proper bulk ; and that, when the platina was melted into a mafs with other metals, it then difcovered its true gravity. On this foundation he endeavours to de- duce, from the fpecific weights of the mixtures, that of the platina itfelf j one of the moil important points, as he obferves, in its philofophic hiftory, that I had left undifco- vered. Though I failed, on account of the above inad- vertence, of attaining to its true weight, my experiments^ he thinks, lead to it; and from thofe experiments he con- cludes, that it is certainly more ponderous than pure gold. This point feems to require fome further examination :. for fuch a conclufion is not to be received without the ftrongeft proofs ; and if the principle of indudlion is not perfedtly iuft, it may give rife to fallacies of worfe confe- qjience than an error in the gravity of platina.. I have [546] I have therefore computed the gravities anew^, together* with the gravity w^hich each mixture gives for the platina. The firft column, in each of the following tables, contains the proportions of the two metals in the feveral mixtures, the iofs fuftained in fufion, where there was any, being dedudled: as platina itfelf fuffers no diminution in the fire, it is from the quantity of deflrudtible metal mixed with it that this deduction is made. The fecond column contains the fpecific gravities of the mixtures as found by experi- ment, and the third their gravities by calculation fuppofing the platinas gravity to be 17: in the fourth is fhewn the difference between the experimental and computed gravi- ties, with the marks -{- or — according as the former is greater or lefs than the latter. The laft column gives the gravity of platina deduced on Mr. Scheffers principle from each of the mixtures. Specific Gravity Platinas | By Exper jBy Calcul. DifiFerence Gravity GOLD 19,285 refulting Platina i. Gold 2 - - 18,37818,458 >o8o~ 16,797 Platina i. Gold 3 - - 18,613 18,658 ^''ZS-- 16,852 Platina i. Gold 5 - - 18,812 18,862 ,050— 16,759 Platina i. Gold 1 1 - - 18,835 19,071 ,236 — 14,988 Platina i, Gold 15 - - 18,918 19,124 ,206 — H.723 Platina i. Gold 23 - - 19,089 ^9^^!! ,188- 15,481 Platina i, Gold 31 - ,- 19,128 19,204 ,076 — 15.273 Platina i. Gold 47 - - 19,262 19,231 ,031-- 18,711 Platina i. Gold 95 - - 19,273119,258 ,0154. 18,214 As the experiments with gold had not come to Mr. Scheffers hands when he wrote his paper, he was in hopes, that when fuch experiments fhould be made, they would give platinas gravity with certainty j gold, being free r547 ] free from fome of the caufes of error which attend tlic other metals. It appears however from the foregoing ac- count, that of twelve mixtures of platina anil gold, there was not one fo lieavy as the gold itfelf, whereas oA Mr. ScheiFers principle they ought all to have been heavier. It is plain therefore that either platina is not fo heavy as gold, or tliat the principle of induction does not obtain in the mixtures of gold and platina. From the two lalt mixtures, the gravity of platina comes out between 1 8 and 1 9 ; bat on thefe no dependence can be had, the difference between the experimental and com- puted gravities being fo inconfiderable, that it may be at- tributed to the unavoidable imperfections of the inftru- ments ufed for the weighing; for an error of lefs than a thirty-thoufandth part of the weight makes a difference of ,012 in the fpecific gravity of the mixture, and of 1,000 in that of the platina deduced from it. The cafe is the fame in the mixtures with other metals where the platina is in fmall proportion. The other compofitions give the platinas gravity lefs than 17; and as the platina is found by itfelf to be 17 or more, it feems to follow, that there mull ncceflarily be a diminution of gravity produced by the union of the two metals with one another. A phenomenon obferved in the fuhon appeared to confirm this. Moft metallic bodies, made fluid by fire, flirink and aflutne a concave llirface in their return to folidity : pure gold (lirinks perhaps rather more than any of the others. But mixtures of gold and platina, where the platina was in conlkicrable proportion, were obferved to flirink little; fome of them even ex- panded and became convex. Of this expanfion or dila- tation of volume, a decreafe of fpecific gravity is the ne* ceiTary confequence. B b b b As [ 54S ] As the pvireft grains of crude platlna ha^e fomfe ad- mixture of heterogeneous matter, it is poffible that this matter may prevent the intimate union of the platina and gold, and thus occafion the two metals, when blended to- gether, to occupy a larger volume than naturally belongs to them. I therefore melted gold with platina that had palled through fome of the operations hereafter defcribed, and which may be prefumed to have been thereby purified from moft of its heterogeneous parts. One of the neateft beads of platina cupelled with lead, (article vi. No. 5, of the foUov/ing fedlion) was melted with equal its weight of gold in a ftrong fire, and continued, in fufion about an hour : the mafs proved fpongy and very light. I remelted it feveral times, with the moft vehement fires I could excite ; and in order to feparate as much as poffible of the lead, to which its fponginefs feemed owing,, I beat it in pieces, and boiled it in aquafortis, and repeat-- edly injedled corrofive fublimate upon it during the fufion. The mafs neverthelefs ftill turned out cavernulous, and brittle, and fpecifically lighter than either the gold or the bead of platina were by themfelves. Platina difiTolved in aqua regia was precipitated by mer- cury, and the precipitate boiled in aquafortis and well, wafhed with hot water. Twenty-fix grains of this pre- paration were melted with four times as much gold: the platina feeming to be imperfedlly mixed, the fufion was three or four times repeated, and the quantity of gold in-- creafed to about eight times that of the platina. This . mixture proved as ponderous as the gold itfelf, or rather more fo : it weighed in air 16802, and in water 15934,. whence its gravity was 19.357. It was examined by fome. ether gentlemen as well as myfelf, who all agreed in its being remarkably heavy : Dr. Pemberton, with a very exadt balance, found the weight in air 229.735 grains, and; [ 549 ] in water 2 17.S85, from which numbers the fpccific gravity comes out 19.387. SpecificGraviiy Pljlinas By Expcr By Calcul Difference Gravity LEAD 11,386 rcfulling. Platina i. Lead 0,97 14,029 13,679 .3504- 18,105 Platina i. Lead 1,92 12,92.5 12,838 ,087+ ^7'459 Platina i. Lead 3,97 12,404 12,196 ,308 + 19.242 Platina i. Lead 8 - ii»947 11,819 ,128 + i9'732 platina i. Lead 12 - "'774 11,682 ,092 + 19,923 platina i. Lead 24 - ^''S75 11,538 '"^37+ 19,238 From this table it appears that the gravity of lead is af- fefted by crude platina in a different manner from that of gold; the mixtures with gold being fuch, as if the crude grains were of lefs gravity than 1 7, but thofe with lead as if they were of greater; fo that in one or the other cafe, or in both, an alteration of volume muft neceffarily be pro- duced by the a53S 12,452 11.790 10,867 cGravity By Calcul. 13.342 I 2,449 12,046 11,488 Difference 'I93 + ,003 + ,256 — ,621 — Here we fee the effects of the ebullition and difperfioh of the filver taken notice of in the hiftory of the fufion of platina with this metal. The laft mixture is lighter even than filver itielf, a proof that the metal is rarefied or made cavernulous by the adtlon of the platina : the greater gra- vity of the two firft mixtures was probably owing to a part of the filver having been thrown off in the fufion, and the B b b b 2 platina pktina not perfeiftly diflblved by the reft. I took all poC- lible precautions for preparing a fet of mixtures of thefe two metals on purpofe for this examination, but they al- ways fputtered up fo much about the crucible, that no de- pendence could be had upon the proportions of the two m the remaining n\afs.. Specific Gravity Platiras Bv Exper. By Calcul. Difference Gravity COPPER 8,830 refulting Platinai, Copper 0,969 1 1,400 I 1,869 ,469 - Flatina i. Copper 2 - - 10,410 10,514 ,104 — Platina i. Copper 4 - - 9,908 9,768 ,140-1- 19,364 Platinai, Copper 5 - - 9.693 9,598 ,095 + 18,970 Platina i. Copper 8 - -. 9,300 9,328 ,028-. Platinai, Copperia - -■ 9,2-5'i 9,168 ,083-1- 21,607 Platinai, Copper25 - - 8,970 8,996 ,026 — Mr. SchefFer remarks that copper of itfelf can never be cafl clofej that when melted with a weak heat, it proves fp incompad; as not to bear the hammer; and that when melted in a ftrong heat, with the addition of inflammable matter, in order ta render it malleable, it proves cavernulous . on the outfidc. The irregularity in the above let of ex- periments feems to iliew that fomething of the fame kind happens in the mixtures of copper and platina; fmce four. mixtures out of the. {even v/.ere lighter than they ought to have been, and this not from any uniform adtion of the two metals on one another, but apparently from accidental porofity. 1 melted fome of the mixtures a fecond time, i\nd found their gravities confiderably altered .• that of 11,400 was incrca,le.d to 11,693; ^""^ that of 9,2.51 was njiminiflied to 8,985. Little therefore can be concluded, from thefe mixtures, in regard either to the gravity of the pUtinci, cr its effeift in varying the gravity of copper. IRON. t S5^ ] 1 - I RO N Specific By Exp. 7,100 CJrjvily ByUlc. DifTcrencc Platina* Grsvit . re fulling Platina I, Iron 1,295 Platina i. Iron 3,333 Platina r. Iron 5,150 Platina i. Iron 10 - - 9.9^7 8,700 8,202 7,862 9.511 8,202 7>«42 7'49^ ,406-1- >49B + ,-60+ /^66+ 20,403 34.963 40,951 Platina i, Iron 12 - - 7,800 7'4.>- ,368+. The compofitlons with filver have afforded a proof of the diminution of gravity from mixture, or of the mafs being dilated, from the atflion of the ingredients upon one another, into a larger volume than they occupied feparately. The above compolitions with iron Icem to be as ftriking inflances of a contrary effedt : the gravity of the two lail of them is luch, as no fubflance, however ponderous, could polTibly produce by the fimple appofition of its own parts to thole of the iron; for it appears in the calculation, that the platina and iron together occupy lefs volume than even the iron by itfelf, ^^ Mr. Scheffer very ingenioufly accounts for' this remark- able phenomenon from a fingular property of iron. When metals are deprived by calcination of their phlogiflon or in- flammable principle, their abfolute weight is increafed: iron, by complete calcination, receives an augmentation of one third of its weight. Caft iron has this particularity, that it can bear a coniiderable difTipation of its phlogirton, without calcining, or without lofmg its metallic form; and in proportion to this diflipation its abfolute weight is in- creafed. Now, as the above ponderous mixtures were melted without any inflammable addition, he thinks a part. of the phlogiflon of the iron muft neceilarily have been, burnt out in the fufion, and the metal of confequence ac- quired an additionaL weight; but that, as no increafe was- obfervedl ■obferved on weighing it, a part of the Iron, equal to the acquired weight, mull: have been fcorified and loft, and confequently the volume of the metal diminiihedj fo that there remained with the platina as great a weight of iron as at firft under a lefs volume. To fatisfy myfelf whether the increafe of fpeclfic gra- vity, or diminution of volume, was owing wholly to this caufe, I made another mixture. But as caft iron Is a very impure metal, I took a piece of a bar of the beft forged iron, and cemented It with a mixture of wood foot and .powdered charcoal, till it had imbibed fo much of the In- flammable matter as to become fteel ; repeating the ce- mentation, with a freih mixture, till the fteel melted. The metal in this ftate was very brittle, fo as without much difficulty to be reduced into powder. A portion of this powder was mixed with charcoal powder, and melted again: 7000 grains of the fteel powder, and 1000 grains of platina, were like wife mixed with charcoal powder and melted in a clofe crucible. The fpecific gravity of the forged iron was 7,795 ; which by the Introdudiion of phlogifton in tlie lirft cementation was dimlnllhed to j',618. By the repeated cementation and fufion, the gra- vity was diminilhed to very little more than 7. Of the powdered fteel melted with the charcoal powder, the gra- vity was 7,032, very nearly the fame as before this laft fufion. Of the powdered fteel and platina melted with charcoal powder, the gravity was 7,760, which ftill ex- ceeds the computed gravity, though not in fo great a degree as that of the mixtures with as large proportions of caft iron. The melted mixture weighed 30 grains lefs than the two ingredients before the fufion, on account, perhaps, of fome fmall grains of the metal remaining difperfed among the charcoal powder. Though this lofs be fup- pofed to have been of the fteel only, yet, as there will re- main [ 553 1 main 697 parts of flccl with 100 of platina, and as 7,76 parts of the niixt lofc i in water, the gravity of the pla- tina conies out on calculation no Icis than 27,813. It appears therefore that iron is very variable in its fpc- eific weight, in the different circumflances of being melted or forged, and impregnated more or lefs with phlogiftonj but that probably fome other caule alfo concurs in varying the gravity of mixtures of it with platina. This caufe tnay perhaps be found in a remarkable property of iron, which the experiments related in the former part of this work, (page 261) feem to have eftablillicd. Melted iron, in the inltant of its becoming folid, is dilated into a larger volume, and one of the marks of this dilatation is the con- vexity of its furface in circumftances wherein that of other metals is depreffed. Platina feems to deftroy this power in iron.. In the firfl mixture I made of caft iron and pla- tina, the furface was as much hollowed as that of any metallic mafs I remember to have feen, nor was this phe- nomenon omitted in the account of the experiments printed in the Tranfad^ions. If then fluid iron expands in fixing,, and the admixture of platina occafions it to contra6t, or to expand lefs, we need not wonder at the increafe of gravity in the hydroftatical experiments. Specific Gravity Platinas By Expcr. ByCalcul. Difference Gravity TIN 7,180 rel'ulting Platina i, Tin 0,984 10,827 10,129 ,698 + 21,649 Platina i. Tin 1,966 8,972 8,920 ,032 + 17,619. Platina i. Tin 4 - - 7'794^ 8,117 ^Z^2>" Platina i. Tin 8 - - 7>'7°S 7,672 '033-f 18,61^ Platina 1, Tin 12 — jMz 7'5i3 ,ioo-j- 26,745 Platina i. Tin 24 - - 7A7^ 7'349 ,I22-j- 27,36s 1h& [554] The firft of thefe mixtures with tin is that from which Mr. Scheffer endeavours to obtain the true gravity of pla- tina, and from this it comes out 21,649. He obferves that tin is not variable, as iron is, in its gravity, or quan- tity of phlogilton, fo long as it preferves its metallic formj and hence concludes, that when platina and tin are melted together, the excefs of the fpecific weight of the mixture above that of the tin, mufi: give the true fpecific weight of the platina. As the experiment on equal parts of tin and platina, makes the gravity of platina, on this principle, above 21, he feems to think that all the mixtures, whofe gravity was found fuch as to make platinas gravity lefs than this, muft have been porous, and are therefore to be difregarded in the prefent enquiry. He remarks, from the whole, that though the fpecific weights of fluids may be determined accurately enough by hydroftatical experi- ments, we cannot be fo certain about that of folids, on account of cavities, incompafetty remarkable manner ; the gold to become diflolubler: yi the pure marine acid, which gold by itfelf relifts ; andi the tin to become eaiily vitrefcible, though otherwife it- can fcarce be vitrefied at all ;. I treated platina and tin in-- the fame manner- Two parts of the picked grains of platina and three parts of tin were melted together, the mixture reduced in- to powder in a clean iron mortar, and a. hundred and lixty. grains of the powder fet in a cupel, under a muffle, ia fuch a heat as is employed for the cupellation of fdver«. The cupel being taken out, the matter appeared of a dark- purplilli colour, and part of it fluck together into a lump.. It was then put into an unglazed porcelain faucer, fet again under the muffle, and flirred evesy now and theii for two hours : here and there fome grains appeared glow- ing, like bits of burning coal ; a phenomenon which tin. ufually exhibits in its calcination. The powder, when cold, looked of a mixed, greyifh-reddiili colour, the red- prevailing : it weighed thirteen grains more than, at firft^ fo that it had gained an iacreafe of about one twelfth, ex- clufive of part of it which had ftuck both to the cupel and to the roughifh furface of the unglazed faucer.. A part of the calx was urged in a covered crucible, with. 3 ftrqng fire in. a blall furnace, above an hour. It did not in the lealt melt, and baked together but very, ilightly : its. colour was darkened almofl to a black. Both the red and. the black calces, dig«i1:ed in fpirit of fait, gave pretty deep yellow tinctures, like diluted folutions of platina in aqu;:^ regiaj whereas neither, the grains of platina, nor the tin calcined by itfelf, give any colour to the acid. ■ I-I. Separation of Mircury-from -platina^ Some quickfilver, which by long trituration with pla- ti;aa had diirolve.d a, part of the metal, was put into an iron ladle,. kdle, and expofcd to a moderate tire. The mercury eva- porated freely, and left the platina behind, in form of a dark, coloured powder intermingled with fume fmall briglit iJiining particles. It may be prefumed that the platina, by this dilTolution in quicklilver, is purified from great part of its iron, a metal which quickfilver has little difpofitioii to unite with. III. Separation of Arfcnic from platina. Pieces of platina, which had been melted with arfenic, were urged with a very ftrong fire in an open crucible.. Arfenical fumes, diftinguilhable by their garlic fmell, arofe in abundance for fome time : at length the fumes entirely ceafed, and the platina remained in a fpongy mafs. Oa tliis mafs I injedted a frefli quantity of arfcnic, fo as. to bring it into fufion, and having then hallily excited the fire till the fumes ceafed, found the matter again ipongy,. and nearly of the fame weight as after the firfl: operation. This was repeated three or four times, with the fame event. It did not appear that the arfenic carried off with it any part of the platina, as it does of all the other metals, gold itfelf not excepted : but a portion of die arfenic feemcd to be retained by the platina even in ftrong fires. Though the maf& was pretty compadl when fo far fatiated with the arfenic a& to be in fome meafure fufible, it always became- fpongy when fo much of the arfenic had been dilhpated as to leave the platina unfufible. All thefe mafies were fpe- cifically lighter than the platina at firft, the gravity of the heaviell of them being only about 16,800. IV. Separation of Regulus of antimony from, platina.. A MIXTURE of platina and regulus of antimony was- melted in a ftrong fire, in a fliallow wide crucible, and the. aafe of a. bellows diredicd obliquely upon the furface of the? fluid.. [ 560 ] fluid. The matter continued to flow, and to fume copi- cully, for fome hours : at length it became confiflent in an intenfe white heat, and fcarcely emitted any more fumes though flrongly blown on. The mafs, when grown cold, broke eafily, appeared very porous, blebby, of a dull grey- colour, and weighed confiderably more than the quantity ■of platina employed. Its fpecific gravity was only about This experiment was feveral times repeated, and the ^vent was always the fame j the platina not only refifling, as gold does, the volatilizing power of the antimonial regu- lus, but likewife defending a part of it from the adlion of the fire and air, and refufuig to melt after a certain quantity had been difiipated. I likewife treated platina with crude antimony. Four ounces of antimony and two ounces of platina, kept for fome time in a fire pretty flrongly excited by bellows, ap- peared melted only in part : four ounces more of antimo- ny being added, and the fire renewed, a reguline matter was found partly at the bottom and fides of the crucible, and partly intermingled among black fpongy fcoriie : the whole was returned to the fire with black flux and com- mon fait : it now melted fufticiently thin, and the regulus perfedlly fcparated. This regulus did not difter in ap- pearance from mixtures of regulus of antimony and platina melted togetlier, and exhibited the fame phenomena alio on trying to blow off the antimonial part. Mr. Schefter likewife tried platina with antimony, and the refult of his experiments was the fame as of mine. He obferves diat as platina refifls fulphur equally with gold, it camiot be fcorified by the fulphureous part of an- timony, and therefore remains, as gold does, in the regu- lus ; but that the regulus cannot be blown entirely off from it, as it is from gold, on account of the platina not continuing fluid. [5^1 ] V. St'/)aralio/i of Zinc from plat ina. A MIXTURE of platina and zinc, expoled haftily to a flrong fire, deflagrated and appeared in violent agitation. This continued but a little time : the matter quickly be- came Iblid, and could no longer be m.ide to tiow, or the zinc, of which a confiderablc quantity Hill remained in it» to flame. The mals was very brittle, dull coloured, fpongy, and, like the two foregoing, fpecilically lighter than the crude platina, VI. Cupellation of platina luitb head. 1. A MIXTURE of platina and lead was cupelled under a muffle 'u\ an allay furnace. For fome time the procefs. went on well ; the lead fmoking moderately, and changing ijito fcoria-, which were thrown of? to the fides and ab- Ibrbed by the cupel.. In proportion as the lead worked off, the matter required a llronger fire to keep it fluid ; and at length, colletfling itfelf into a dull flat lump, it could no longer be made to flow in the greatefl degree of heat which the furnace was capable of giving. The lump broke eafily under the hammer, appeared of a dull grey colour both internally and externally, and of a porous texture. It weighed near one fifth part more than the quantity of pla- tina employed. 2. This experiment was many times repeated and va- ried. I endeavoured to fcorify the lead in aflay crucibles, by intenfe fires in a blaft furnace; to work it oiFon bone— afli prefled into the bottoms of crucibles; and to blow it off on tefts before the nofe of a bellows.. The event was iiill the fame; the platina not only refilling the power of lead, which in thefe operations deflroys or fcorifies every other known metallic body except gold and filver, but likewife retaining, and preventing the fcorification.ot* ai ^act of the lead itfelf. [ 562 ] 3- In the hiftory of die fufion of platlna with lead it has been obfervcd, that lead depofites, in a gentle heat, great ipart of the platina which had been united with it in a itrong one. As the part which remains fufpended in the •lead, might be fufpedted to differ from that which fubfides, a quantity of lead was decanted from frefh parcels of pla- .tina in a heat below ignition, and both the decanted metal and the refiduums fubmitted to cupellation feparately. The event was the fame in allj the metal becoming con- fiftent after the lead had been worked off to a certain point, and refufmg further fcorification. 4. Mixtures of platina and lead, which had been cupelled in an affay furnace as long as they could be kept fluid, were expofed to ftronger fires in a blaft furnace, by them- felves, with powdered charcoal, with black flux, with bo- rax, with nitre, and with common fait. None of them .perfedly melted, or fufFered any confiderable alteration j they only became fomewhat more porous, probably from the exudation of fomeof the lead and a partial liquefadlion or foftening of the mafs. The immediate contact of burn- ing fuel, agitated by bellows, made fome of thefe mixtures flow after they had refufed to melt in crucibles aded on by intenfe fires : the beads by this means became fome- '.vhat neater and more compaft, but very little of the lead was feparated. 5. The cupelled beads were in general brittle, breaking -eafily under the hammer, without iftr^tching in any con- liderable degree. They were of a grey colour both on the upper furface and in the fradlure, but pretty bright and white on the lower furface, and when ground or filed; they had nothing of tlac purplifli hue, which the mixtures -gf platina and lead (page 515) had in fo remarkable a der ■gree; nor does their colour appear anywife altered after liceeping for .ten years in the fame circumilances in which thofc [ $(>! ] thofe mixtures were kept. On weighing them hydroflu- ticaHy, t]ie more fpongy ones were found nearly as ponde- rous as the crude platina. Among the more compadl, tlic gravity of one turned out 19,083, that of another 19,136, and of a third 19,240. It is probable liiat thcfe remark- able gravities proceeded partly from the plutina having been purified in the procefs from its lighter heterogeneous admixtures, and partly from an increafe of gravity occa- fioned by the coalition of the platina with the lead. The laft of thefe mixtures, whufe gravity was 19,240, is that which was melted with equal its weight of gold, as men- tioned in page 548. 6. A mixture of one part of platina and three of gold was cupelled Avith lead in an allay furnace. The matter worked well for a conliderable time: at length it colledtcd itfelf into a height hemifpherical lump, which by degrees became flatter, dull coloured, and rough. The button, orr being weighed, was found to retain about a twelfth part of lead. 7. The experiment being repeated with a mixture of one part of platina and fix of gold, forne part of the lead appeared ftill to be retained. The bead proved rounder and brighter than the foregoing, and of a good golden co- lour on the outfide; but it broke cafily under the hammer, and appeared internally grcyilli : fome of the fragments hung together by the outward golden coat. 8. Mixtures of platina and filver, fubmitted to the com- mon procefs of cupellation, retained likewife a little of the lead. Thefe, in becoming confident, formed not he- mifpherical beads, but flat m-illes, very rough and brittle,, and of a dull grey colour both extern nlly and internally. 9. The cupellation of platina with lead was one of the experiments made by Mr. Wood, and communicated to the Royal Society in the year 1750; but the platina being D d d d thca [ 5^4 1 then very imperfedtly known, fome deception happened in this point. Mr. Wood relates, that platina having been melted in an affay furnace on a tefl with lead, and there- with expofed to a great fire for three hours, till all the lead was wrought off, the platina was afterwards found remain- ing at the bottom of the ted, without having fufFered any alteration or diminution by this operation. Dr.Brownrigg, furprized at this reliilence of platina to lead, repeated the experiment. He melted twenty-fix grains of platina upon a cupel, with fixteen times its weight of pure lead, which he had himfelf revived from litharge : the lead being fcorified, there remained in the cupel a button of platina weighing twenty-one grains, fo that the platina loft in this- operation near a fifth part of its weight. From this ex- periment he conjecflured, and not without probability con- fidering the little that was then known of the properties of this new metal, that a part of tlie platina was fcorified by the lead; that the whole might have been fcorified by repetitions of the procefsj and that confequently gold and filver may be purified from platina, by cupellation with larger quantities of lead than are commonly employed. What the author has modeftly propofed only as a con- jedlure, to be confirmed or refuted by further trials, has by fome been taken for a certainty : in a letter prefented to the Royal Society foon after, the procefs is fpoken of as a method difcovered by Dr. Brownrigg, for feparating pla- tina from gold and filver. It is plain that this experiment muft- have been made, and the author has lately informed me that it was, with the caft metal mentioned at the be- ginning of this hiftory, which was then fuppofed to be true platina, and does lofe of its weight in the common procefs of cupellation. ID. Mr. Scheffer tried the cupellation of the grains oi platina with lead, and the event was exaftly the fame as in my [ S(>5] my experiments. The bead was dark coloured and rugged at top, white uinlcrncath, and rctiined a portion of the lead amounting to two or three parts in a hundred. He obferves that the lead cannot, by common lire, he worked off clean from this metal, as it is from gold and filver, on account of the platina not continuing fluid after the lead has been fcparated to a certain point; and judges that a fufficient heat for the complete fcparation of the two metals is not to be obtained by any other means than by large burning glaflcs. 1 1. I have already obferved, page 494, that platina di- vided by cementation with nitre, and afterwards purified by repeated fublimations of fal ammoniac, appeared no- wife different in cupellation from the common grains. Mr. Marggraf made trial of platina attenuated by folution and precipitation. The orange coloured precipitate thrown down by fixt alcali from folution of platina in aqua regia, being well waflied with hot water and ignited under a mufHc, became brownifli : nine parts of this matter were melted with an ounce of pure granulated lead, and the mixture expofed to the fire in a fcorifying difli till a con- fiderable part of the lead was fcorified : the remainder worked in a cupel, left a rough bead, of a whitifh grey colour, very brittle, perfcdlly like that obtained in the cu- pellation of crude platina : its weight was one grain. The experiment was repeated with a precipitate made with vo- latile alcali, and the event was the fame. He tried alfo the powder which remained on diflilling a folution of pla- tina to drynefs : this powder, calcined under a muflle, ac- quired a fliining blackifli colour, in which ftatc thirty grains ■of it were mixed with twenty times as mucli granulated lead, and the mixture worked as above, firll: on a fcorify- ing difli and afterwards in a cupel : the fcoriae were of a black-brown colour : the cupelled bead was brittle and of D d d d 2 a grey- [ 566 ] a grey-white colour like the others, and weighed forty- two grains, or two fifths more than the platina employed. This was treated in the fame manner with the fame quan- tity of frefli lead : the fcorias were of the fame colour, and the bead ftill weighed jufl forty-two grains. 12. The fame author gives an account of another ope- ration, in which platina and filver were combined together, the mixture melted with lead, the lead fcorified, the filver feparated by aquafortis, and the remaining platina again cupelled. He took thirty grains of crude platina, and .thrice as much of the combination of filver witli marine acid, called liina cornea. The mixture being expofed to as great a heat as a glafs retort would bear, no liquid pafled into the receiver, but a little white matter fublimed into the lower part of tlie neck of the retort, as commonly hap- pens when juna cornea is expofed to fuch a heat by itfelf. The mixt run clean together into a dark yellow hyacinth coloured mafs, and appeai-ed v/ell united : the glafs was flained of a dark yellow. The mixt was pounded, along with pieces of the glafs, which could not eafily be fepa- rated, in a clean iron mortar, the pov/der mixed with two ounces and a half of granulated lead, and melted in a cru- cible with a flrong fire : the fcoria was greeniiL. The" metal, w^orked on a cupel, drove well, as in the common filver afiay, till towards the end of the procefs, at which time it came afunder, grew flat and rough, and looked like filver fprung on the cupel by being too hafi:ily cooled, but without the lead metallic brightnefs on the furface : it was very brittle under the hammer, but bore to be filed, and the m.ark of the file looked white : it weighed a hun- dred and ten grains. It was cupelled with an ounce more of lead, and the produdl was the fame as before, with the lofs of feven grains in weight. This lafl: bead was beaten in pieces, mixed with fix drams of pure nitre, and melted with [ 5(>7 ] with a ftrong fire: the metal was of a lilver whitenefs, and weighed feventy grains : the fcoria was cauflic, liver coloured, and when liquefied in the air looked greenilh. The regulus was melted again, with half an ounce of the purefl nitre and a dram of borax : the fcoria proved clou- dy, inclining to yellow underneath and to greenifli above : the regulus was of a fine white, and weighed flill feventy grains ; it had fomething particular in its appearance on the furface and about the fides, refeinbling the radiated cobalt ; it llretched pretty well under the liammer, and bore to be flatted into a thin plate, but was harder than fine filver. A part of this plate was digefted in purified nquafortis : the mcnflruum became firll of a high grafs- green colour; afterwards, in a boiling heat, the plate grew black, and the folution brownilla. The filver being at length difiblvcd, there remained at the bottom a black ponderous matter like calx of gold. This was thoroughly wafi^ed with hot diftilled water, then flried, and made red hot, but it received no gold colour. It was mixed with granulated lead, and the mixture worked firft on a fcorify- ing dilh, and then on a cupel : there remained a convex bead, without metallic luftre, which fprung under the hammer, and refembled the other beads obtained by cu- pelling platina with lead. 13. It appears upon the whole, that Marggrafs trials for working ofi-" lead clean from platina, fucceeded no better than Schefiers and mine, fo much of the lead being always retained as to make the metal very brittle, whereas platina by itfelf, whether in its crude flate of grains or when melted by a burning-glafs, is of confiderable malle- ability. Macquer and Baume made another effort: they were " defirous of feeing, whether a heat of a good deal longer continuance would not produce that, which one couj) defeu, perhaps more flrong but of fliortcr duration, had. [ 568] h;id been unable to produce. They put upon a cupel of a proper fize one ounce of platina and two ounces of lead, and having placed the cupel in a furnace like that of Mr. Pott for the vitrification of earthy bodies, they raifed the fire by degrees, and kept it up without intermiffion for fifty hours, in fuch manner, that during the laft twenty- four hours it continued in its full violence. The cupel being then taken out, they found that the platina, inftead of being in a round brilliant button as gold and filver are after cupellation, was extended and flattened on the cupel: its upper furface was tarnifhed, dark coloured and wrinkled, from whence it was judged at firft that the operation had fucceeded no better than thofe we have been fpeaking of: the platina parted eafily from the cupel, which was become very hard, of a yellowifh white colour, femitranfparent, and ftruck fire freely with fteeh But upon exaftly weigh- ing the platina, they found, that inftead of receiving an augmentation of weight from fome of the lead remaining undeflroyed, it had loft one fixteenth of its weight: its lower furface was white and filvery : finally it was not eager, but bore very well to be extended under the ham- mer: they diiTolved a part of this cupelled platina in aqua regia, and this diflblution did not fhew any veftige of lead." As Mr. Macquer appears to have employed in this ex- periment the platina luch as he received it, containing a large admixture of ferrugineous and other foreign matters undoubtedly deftruftible in the procefs ; it is obvious that it might have retained a very confiderable proportion of the lead, notwithftanding the diminution in weight. Nor can aqua regia be looked upon as an infallible teft of its having been pure from lead; for this menftruum, in certain cir- cumllances, will diffolve lead as well as platina. But whatever might be in this, the event of the experiment, in regard to the malleability of the cupelled mafs^ appeare4 too [ 5^9 J too intcrcAing to be pafled over, in this hiilory, witiiout being verified by further trials. 14. Having at hand a wind furnace, formed of a mixture of Sturbridge clay and powdered glafs-houfe pots, fecurcd by iron hoops on the outfide, about two feet high from tiie grate to the top of the dome, fourteen inches wide in the middle and ten inches at the grate, with a chimney of nearly half the diameter of the grate and fourteen feet high ; I firft made trial of this furnace, and found its eifedt to- be fuch, that there was no occafion to have recourfe to any other. I fitted into it a mufllc, in the manner dc- fcribed by the ingenious author in a memoir on the vitrc- fication of clay with chalk, formed of the fame compofi- tion with the furnace, two inches high, three inches wide, of fuch length as to reach acrofs the furnace, fupported at the height of five inches above the grate by a brick of fire-ftanding clay, whicii was cut Hoping downwards fo as to cover as little as poffible of the grate. 15. A large cupel having been kept red hot in the muf- fle about an hour; two ounces of lead were put in, and one ounce of the picked grains of platina dropt into the melted lead. The fire being raifed with coaked pitcoal to its greateft vehemence, the whole internal part of the muffle appeared of a dazzling brightnefs, and the cupel could not be diftinguifhed, till cold air was fuffercd to pafs through by keeping the door open for fome time, which was done frequently, to promote the fcorification or dilli- pation of the lead. In this ftate the heat was continued, untill, in five or fix hours, penetrated by the vitrefcent cinder of the coal, the muffle begun to fail : all its back part, and fome of the internal part of the furnace, melted,, forming partly irregular vitreous lumps, and partly run- ing down through the grate in large drops of black hard glafs. The cupel was hard, yellowifli white, and femi- tranfparent,. [ 5/0 ] trafifparcnt, like Macquers. The platlna was in a flat cake, coated with the femivitrified matter of the cupel and glafly drops from the muffle, fo that nothing could be judged from the weight : it broke pretty eafily under the hammer, and did not feem to differ from that of former cupellations. 1 6. What was here wanting in the continuance, I en- deavoured to fupply by a repetition of the fire. The pla- tlna, pounded and wafhed, was placed under a frefh muf- fle, on a fcorifying difh ; and the heat kept up in its full violence, chiefly with wood and charcoal, for fourteen hours. Greateft part of the platina ftuck fo firmly to the dilh, in virtue of part of the lead which had exuded and vitrefied, as not to be got off without pounding the veflel. Where the platina on the difli was fl:ruck with a hammer, or rubbed with a fl:eel burniflier, it flretched, and acquired a continuous furface like filver or tin leaf. After the powder had been pafl"ed through a fine fieve, and wafla- :ed, on beating it again fome broad flat grains appeared, which flretched eafily under the hammer, and on being fqueezed with a pair of plyers, bent almoft double : one of thefe bore to be opened, and bent again in feveral di- reftions, without cracking. This powder, whofe particles appeared fo duflile and flexible, I tried to reunite into a mafs, by urging it with a vehement fire, in a covered cru- cible, for four hours : it cohered into a button, of the form of the crucible, not at all fl;icking to the veflel, and free from difcolourment : the button broke from a blow or two of a hammer, but not very eafily, filed tolerably fmooth, and burniflied like fine filver. 17. I cupelled four parcels of platina, with thrice their quantity of lead, till they would no longer continue fluid in a good aflTay furnace ; and repeated the cupellation orj irefli cupels, with the fame quantity of lead;,, a fecond and a third a third time. The firft cupels were tinged of a deep rufly colou'-, probably from the irony matter in the pla- tina; the others only yellowilh, as from lead alone. The plates of metal, after the firfl: cupellation, were dull co- loured and fluck to the cupels j after the others, they were brighter and did not rtick. The four plates, weighing 3031 grains, kept for twelve hours on a fcorifyifig dilli, in as llrong a fire as could be excited in the allay furnace, be- came whiter, and loft 2 1 8 grains : the white difli was co- vered all over with a yellow glazing. The plates, which had fuftered no appearance of fufion, and which ftill proved brittle though much lefs fo than before, were broken into fmaller pieces, and let on four cupels, under a mufBe, in the wind-furnace above defcribed : during eight hours ve- hement fire, the two cupels in the fore part of the muffle, which was lefs hot than the back part, were obferved, as often as the door was kept open for a little while, to fiioke confiderably; but all the air that could pafs into the muffle, did not fo far diminilh the dazzling heat, as that any fumes could be dirtinguiflied in the back part. The arch and further end of the muffle were, found all over glazed by the fumes; the cupels friable and unftained; the metal of a lilver whitenefs, and diminillied 105 grains. The pieces in the front cupels were ftill brittle; thofe in the further ones bore to be flattened confiderably under the hammer, and feemed nearly as foft as alloyed filver. 18. I made many other cupcllations of the lame kind; of which, as no other remarkable phenomena occurred than have been already mentioned, it would be unneceflary to give a particular detail. They agree in eflablilhing an important fa6t, that though in the common pro- ccfs of cupellation, even when performed with ftronger fires than the cupelling furnace can give, and continued fome hours beyond the time in which the fixing of the E e e e metal [ 57^ ] metal feems to {hew that the fire has produced Its full cffedl, platina has been always found to retain fo much of the lead as to break under the hammer^ yet by continuing thefe vehement fires for twenty hours or more, fo much of this retained lead is feparated, as to leave the platina mal- leable. Much of the lead was forced out after the metal had become folid, as appears in the experiment. No. 17, in which the quantity expelled from the cupelled plates, without tlieir having anywife foftened or altered their figure, amounted to above a tenth part of their weight. The thinner the metalline plates, the fooner and more effedlually were they freed from the lead, and rendered malleable: in one cupellation, a part of the metal having run into the form of a fine wire, this wire, after fix hours llrong heat, proved io flexible, as to bear bending back- wards and forwards feveral times without breaking, while a thick piece of tJie fime mafs, after eighteen hours longer continuance cf the fire, was fi:ill brittle : when a fmall quantity of platina, worked in a cupel of a proportionate lize, had, from the ^ape of the vefTel, formed a pretty thick mafs, which was the cafe in moft of the firfi: cupel- lations (No. i"to 8 of this article) a vehement fire, of much longer continuance than that of Macquers experi- ment, v/as infufficient for rendering the mafs malleable; but when beaten into powder and fpread thin, a fire not ex- tremely vehement, continued ten or twelve hours, made the particles of the powder fo dudile, that they ftretched under the peflle into fine plates like fragments of filver leaf: the powder thus flatted was remarkably foft or undluous to the touch, like talk ; and being rubbed on paper, fliuck to it, fo as not to be eafily brufhed off, making it look like what is called filver paper. It was therefore a happy circumftance in Macquers experiment, and indeed efiential to its fuccefs, that he ufed a coniiderable quantity [ 57], ] of platlna fo as to form a thin plate on the bottom of .i large cupel. Much of the lead exudes at Hrfl in a vitreous form, and glares or llains the difh or cupel on which the plate is cxpofed to the lire; but towards the end it feems to be forced out only in fume, no vifible mark remaining on the vefl'el : in one experiment, the metal loll about a twenty-fifth part of its weight, after it had ccafed to give any tinge to the cupel. 19. It may be proper to obfervc, that in mofl of the cupellations of platina with lead, efpecially where the quantity of the mixt was conliderablc, and a pretty llrong fire made ufe of, the cupelled plates appeared of a regular and fingular figure on the furface, fuch as no other metal or metallic mixture I know of aflumes in fixing. In tiie middle was a broad flat-bottomed depreflion, w^ith a lip or margin round it, like a common table plate ; and the lip was frofted as it were with regular tranfverfe rows of pro- minent dots. The finooth parts were in general foft or flippery to the touch. VII. Cupcllatlon of platina ivitb Bifnmth. Mixtures of platina with bifmuth were fubmltted to the common procelFes of cupellation under a muffle, fco- rification in afi'iiy crucibles, and tefling before the nofe of a bellows. The general event was nearly the fame as when platina and lead were treated in the fame manner: the mixtures, which at firft flowed eafily, became lefs and lefs fufible in proportion as the bifmuth was driven off, and at length could not be kept fluid in an intcnfe fire, though they appeared, on weighing, to retain a confiderable quan- tity of the bifmuth. Nor could bifmuth, any more than lead, be worked off clean, by the common procefs of cu- pellation, from mixtures of platina with lix times its weight of either gold or filver. E e e e 2 When [ 574 ] When one parcel of platina was cupelled with three or four frefh quantities of bifmuth, the firft cupels were al- ways tinged of a blackidi rufty hue, the next paler, and the third for the moft part only of the orange yellow colour which bifmuth itlelf communicates, and which is confi- derably deeper than the flain imparted by lead. In many of the cupellations, the furface of the metal was found covered with a leafy fubflance like deep co- loured litharge; and fometimes, under the cupelled plate, there was a large quantity of rough fpongy greenifli matter, adhering in many parts flrongly to the platina, running into cavities in its bottom, and in fome places lying as it were between plates or flakes of the metal. It appeared that bifmuth, in cupellation with platina, does not diffufe or fpread itfelf fo thin, or fink fo deep into the cupel, as lead does; but loads the parts which it touches, in fuch a man- ner, as to be prevented from extending further, and to be colledled there in its femivitrified ftate; fometimes lying in large quantity on the cupel, though a confiderable part of the cupel at bottom was not tinged with it. This does not feem to happen when bifmuth is worked ofF alone, and therefore probably proceeds from this metal being a lefs powerful menftruum than lead for the ferrugineous and other foreign matters blended with platina. Many of the cupellations however went on well, without any appear- ances of this kind, and yielded brittle plates, fometimes dull coloured and fometimes bright, according as lefs or more of the bifmuth was worked off, of uneven furfaces, with large protuberances irregularly and fometimes ele- gantly difpofed. Some of the minutes of thefe experi- ments having been loft, I cannot recolledt whether it was with large, or with fmall proportions of bifmuth, that the procefs fucceeded beft. From [ 575 ] From the cfFctll of a long continuance of flrong fire on mixtures of platina and lead in the preceding article, I uas induced to fubniit to the fame treatment mixtures of it with bifmuth, a metal whicli promifcd to be eafier lepa- ratcd tlian lead, as being itfeif much more ealily diflipatcd by fire. Some of the cupelled plates of the foregoing operations were kept for fix hours, on four cupels under a muffle, in as ftrong a heat as could be raifed in a good allay furnace. Of one of the plates a part had melted, and fprcad in fine filver-like leaves over the edge of the cupel : of all of them the thin edges bore to be hammered pretty well, and bent confiderably before they cracked : the cupels wc-e flained of a pale orange yellow. The plates being then urged for fix hours longer on frefii cupels, in the wind furnace be- fore mentioned, they all proved of a bright filver colour, and hammered well in the thinner parts, but Hill conti- nued brittle in the thick ones : the cupels were very flightly tinged. From the experiments related in this fedion it appear?, that platina perfedly relifls the deflruftive power of lead and bifmuth, which, with the concurrent aftion of fire and air, reduce all the other known metallic bodies, except gold and filver, into a calx or fcoria : that it refifls anti- mony, by which filver as well as the bale metals are fcori- fied, and which has always been efieemed the feverefi: tefl of gold : that it is not fenfibly volatilized by arfenic, which in flrong hafty fires carries off a portion of gold itfeif: that in degrees of heat confiderably flronger and of longer con- tinuance than have hitherto been employed for thefe kinds of operations, the platina preferves a part of thofe de- flrudtible metalUc bodies themfelves, retaining fo much of them as to be rendered brittle ; but that by further continuance of vehement fire, thofe bodies, at leafl lead and [57^] and bifmuth, may be wholly or almoft wholly diffipated, fo as to leave the platina in a mafs, as malleable as the finefl grains were feparately, and perhaps more fo, in vir- tue of their being purified in the operation from their irony or other foreign matters, as gold and filver are by the hke means from all the bafe metals. How far this diflipation of lead or bifmuth may be practicable in the large way, or on maffes of confiderable thicJcnefs, cannot be abfolutely determined from the ex- periments hitherto made, for as' yet, with me at leaft, the procefs has fucceeded only on thin pieces of the metal. Mr. Macquer feems to make no doubt that platina may on this foundation be rendered manageable by the work- man in large, fo as " to furnifh us with burning concaves, fpecula for telefcopes, an infinity of vefTels and utenfils for ■chemical and culinary ufes, and almoft all the works of the lockfmith." He obferves that platina would for thefe purpofes be an excellent material, " as its vivid and brilli- ant polifli is never tarnifhed by any kind of ruft, and as it not only refifts the aftion of air, water, fire, acids, and the voracious metals, as well as the purefl gold does, but joins to thefe admirable qualities a property ftill more precious, which is wanting to gold, viz. the force and hardnefs of iron." In my experiments, the cupelled plates, both with lead and bifmuth, were confiderably harder than fine gold or filver, but fofter than iron : the hardnefs here attributed to them appears to be inferred from the experiment on precipitate of platina related in page 505. SECT. [ S17 J SECT. VIII. Of the Affiituics ofpLitina. IN this fedion it is propofed to give an account of futlx experiments as have been made relative to the com- parative afiinities of platina and other metals, to one ano- ther, and to faline diirdvents ; the feparation of platina from one metal by the intervention of another, or of one metal from another by tlic intervention of platina ; the fe- paration of platina by other metals, or of other metals by platina, from their folutions in acids. For the greater difliiidlnefs, the refults of the feveral experiments are cx- preffcd in the rcfpedtive titles, after the fame manner as in the common tables of affinity- The body placed upper- mofl is always to be underftood to have a greater affinity to that which is immediately under it, than to the third or lowermofl: ; infomuch that if the firft and third be com- bined together, the middle one, properly applied as men- tioned in the experiment, will break their union, and throwing out the third body, join itfelf to the firft, though the feparation is not always complete. Where no fuch. affinity, or feparation, appears in the experiment, the feveral bodies are placed in a continued line.. I. Mercury: Platina: Lead. One part of platina and about four of lead were melted perfedlly together, and after the heat had fomewhat abated,, the fluid was poured gently, in a fmall ftream, into three times its quantity of quickfilver heated fo as to fume. On ftirring them with an iron rod, a blackifli powder was im- mediately thrown to the furface, which appeared to be chiefly [ 578 ] chiefly platina. On grinding them together in an iron mortar, a frefli powder gradually feparated, which, being occafionally waflied off, in appearance greatly refembled the foregoing, but was found, on- proper trials, to partici- pate more laigely of mercury and lead than of platina. The amalgam was of a very dull colour, and on being ex- pofed to the fire in an iron ladle, fwelled and leaped about, though the heat was fcarcely fufficient to make any of the quickfilver evaporate. I therefore had the grinding con- tinued, in a kind of mill, compofed of a thin iron plate, cut into the form of a crofs, and made to turn in an iron mortar : the plate was bent up nearly to the fliape of the bottom of the mortar, and between two of the ends was fixed a piece of wood, the other two ftanding loofe, and accommodating themfelves to the mortar in virtue of their elafticity : the piece of wood received the end of an up- right fpindle, which being fecured by crofs pieces to keep it in the middle of the mortar, and a fmall weight, fome- times greater and fbmetimes lefs, placed on the top, a wheel and pulley procured a rapid motion with little la- bour. After conftant agitation in this machine, with wa- ter occafionally renewed, for feven or eight days, the amalgam looked bright and uniform, and fuffered the quickfilver to exhale freely. The mercury being all eva- porated, there remained a dark grey powder, v/hich proved upon examination to be platina with a very little lead. For a part of the powder being digefled in aquafortis, a fmall portion of it difiblved, and the folution appeared to be no other than a folution of lead : the undillolved part, now of a dark purplifii colour, was moftly taken up by aqua regia, to which it communicated, not indeed the common hue of folutions of platina, but a kind of dull olive colour : plates of tin, however, quickly difcovered tliat the matter diffolved was platina, by occafioning a pre- cipitate [ 579 ] cipitate of the fame appearance with that which tin throws down from common fohitions of phitina. The reft of tlic powder was cupelled with lead : it left a rough, flatted, bright mafs, which would no longer melt, and which ex- adlly refembled thofe obtained in cupelling crude platina with lead. Mercury is fuppofed to have a greater affinity to lead than to any other metallic body, gold and filver excepted. In this experiment it fliewed a greater affinity to platina than to lead, fmce it retained much of the platina, after the lead, which was at firft in much larger proportion, had been almoft all thrown out. II. Mercury: Gold: Platina^ A MIXTURE of one part of platina and two of gold, which proved very white and brittle, was well nealed, and cautiouffy flatted into thin plates, which were thrown red hot into boiling quickfdver. On grinding and waffling with water, a powder feparated, copiouffy at firft, and by degrees more fparingly. After the procefs had been con- tinued about twenty-four hours, there was no further fe- paration, except of a little blackifh matter into which a part of the mercury itfelf is always changed in thefe kinds of operations. The amalgam, which looked bright, was put into a crucible, and the quickfilver being evaporated by moderate heat, there remained a fpongy mafs, of a high colour, which being melted and caft into an ingot, proved very foft and malleable, and not diffiinguiffiable by the eye from the pure gold made ufe of. How far this procefs is applicable to the feparation of platina from gold in the way of bufinefs, will be confidered in the following fed:ion. It is fufficient here to have eftabliflied the greater affinity F f f f of [ 58o] of mercury, to gold than platina, and to platina thaa lead. III. Platina: Lead: Iron. One ounce of a mixture of iron and platina, and two- ounces of lead, were covered with black flux, and urged with a pretty flrong fire, but which did not prove fufficient for the fufion of the platina and iron : the lead being^ poured off into a cylindrical mould, the lower part of the cylinder looked of a duller hue than the lead was at firft, and proved fpecifically heavier, in the proportion of 1 1,598 to 11,386. The lumps of iron and platina were mixed with the lead a fecond time, and expofed to a ftrong fire till the whole came into perfect fufion: on cooling the crucible too haftily in w'ater, the fluid matter exploded and threw off the cover, and the lead was found reduced into, fmall filaments, filling the crucible, which before was not one fourth part full. The iron regulus at the bottom was in a round, fmooth, very hard lump, and feemed to retain a confiderable portion of the platina. The lead, melted into a mafs with a little refin, appeared, from its fpecific gravity, and more manifeftly on cupellatiOn, to have im- bibed more of the platina than the iron retained. Call iron being dropt into a melted mixture of platina and lead covered with black flux, and the fire kept up flrong till the iron melted, moft of the platina appeared to be retained by the lead, and very little if any of it to be- taken up by the iron. It was judged at firfl: that this did not happen from the platina having lefs affinity to iron than to lead, but from its not having come fufliclently in contadl with the iron; for v/e have elfewhere feen that great part of the platina finks to the bottom even of the. lead, and the iron floats on, the furface of the lead. • A. mixture [ 5Si ] A mixture of pktina and iron was melted vvitli thrice its weight of lead upon a cupel, and a ftrong fire kept up till greatefl; part of the lead was worked olY. The remain- ing mafs was rugged and cavernulous: in its cavities, and at the bottom, was a very confidcrable quantity of a dark blackifh powder with fomewhat of a purpliih cafl:, which was attraded, though not vigoroufly, by a magnetic bar. This experiment feems dccifivc of the greater affinity of platina to lead than to iron; as it fhews iron, which had been previoufly well combined with platina, thrown out again in its metallic form by lead. It may therefore be prefumed, that the abforption of part of the platina from iroij by load in the firll experiment, proceeded from this fuperior affinity of the platina to the lead, and not, as was at firfl fufpeded, from its having an equal affinity to them botli, IV. Aqua rcgia: Zinc: Platina. Platina, digefled in a faturated folution of zinc made in aqua regia, did not appear in the leaft corroded; but zinc, put into a faturated folution of platina, foon begim to diliblvc, and to precipitate the platina. The precipitate was of a brownilli black colour: the liquor, after the zinc ceafed to be a6tcd on, continued j'ellow, a mark that the precipitation by zinc was not total, any more than by the unmetallic precipitant? in fedlion iii. Marggraf found, that when folution of zinc in aquafortis was rnixed with folution of platina, an orange-red or brick-coloured preci- pitate fell, tlie liquor continuing yellow as in the other <:afe. Ffff 2 V. Aquu [ 582 ] V. Aqua regia: Iro7i: Plat inn. A SATURATED folution of iron in aqua regIa did not jtenfibly ad on platina: a faturated folution of platina rea- dily corroded iron, the platina precipitating. A good quantity of yellow ochery powder fettled at the bottom, and the undifToIveii part of the iron appeared incruftated: with a dark coloured matter: it could not be judged from the colour whether the precipitation was complete, the fo-. lutions of platina arid of iron having a great refemblance. in colour. "^ VI. Platina:. Aqua regia ajtd folution of iron-vitriol:. Gold. Solution of iron in the vitriolic add, or a folution of the common green vitriol of iron made in water, which, totally precipitate gold from aqua regia, made no change in folution of platina. A mixture of platina and gold, which had been melted together and kept in fulion for,- feme hours, being diffolved in aqua regia, and the vitriolic, folution added, the gold was precipitated and the platina, remained diffolved. Solutions of iron in the nitrous and. marine acids did not precipitate either, platina cr goldo. VII. Aqua regia:-. Copper: Platina. . Platina, put into a folution of copper in aqua regia,. was not fenfibly ailed on: plates of copper, put into folu-. tion of platina, begun quickly to diffolve, and to precipi- tate the platina. The precipitate, was of a dark greyifl: cplour, , [583 ] colour, and was found on trial to have a coiifidcrablc quan- tity of tlie copper blended vvirh it : the liquor was of a more duiky green than folutions of pure copper, probably from its retaining fome of the platina. Solutions of cop- per in the vegetable, nitrous, marine and vitriolic acids, mixed feparately with Ibhition of phuina, produced no pre- cipitation or turbidnefs: Marggraf indeed found, that with tlie folution in the nitrous acid, a reddilli orange coloured powder was depofited after long ftanding, but in tliis pre- cipitation the copper folution probably had no Hiare, for the folution of platina by itfelf, as he obfervcs, yields, in- time, a like precipitate.. VIII. yljua regia: Tin: Platina. We have feen in the third fedlion that plates of pure tin precipitate platina, and that they do not produce with it the red or purple colour which they do with folutions of gold, but a dark brownilh or olive: it muit here be added,, for ellablilliing the atlinity more fully, that when platina- is digelled in a folution of tin made in aqua regia, no pre- cipitation of the tin, or corrolion of the platina enfues. The precipitation by tin is not total, any more than by the metals hitherto mentioned, but it may perhaps be queftion- ed whether the matter which remains dilfolved, and which gives colour to the liquor, be true platina, or the ferrugi- neous fubftance that was blended with it," fince in a former experiment, page 487, after the more foluble parts of the mineral had been extradled by. aqua regia, the remainder,, dilfolved in freHi aqua regia, appeared to be completely precipitated by tin, the liquor proving perfectly colourlcfs.. Solution of tin, mixcil with common folution of platina,, ieemed to. have. nearly the fame cffc(5l as tin in fubftance;. a. dark; [ 584 ] a dark rsddilh onnge coloured powder precipitated, a part of the platina or its iron remaining diflblved, fo as to give a high colour to the menflruum. IX. Aqua regia: Mercury: Platina. Mercury, which is faid to precipitate from aqua regia ■no one of the common metalHc bodies except gold, being put into a diluted folution of platina, feemed to be in a little time corroded, and did not run freely : foon after, it appeared covered with a greyifli powdery matter, which at firfl was apprehended to be a precipitate of the platina, but was found afterw^irds to be only a part of the mercury corroded.: upon applying a moderate heat, the whole of the quickfilver, the quantity of which was very confider- able, was diflblved, without any precipitation of the pla- tina. This folution of the two metals, being evaporated -a little fo as to difpofe it to fhoot, yielded cryftals not at all like thofe of platina, but in form of needles, externally of a yellowifh hue : the cryflals, flightly waflied with proof Spirit, became colourlefs: expofed to the fire, they emitted copious white fumes, with a hilling or crackling noife, and left a very fmall quantity of a reddith powder, giving a dull red ftain to the tobacco pipe which ferved for the vcilel; the cryilals laid on marble, and heated almoft, if not quite, to a red heat, fcarcely gaie it any tinge or injured its po- .lini. It appears from this experiment, that aqua regia, faturated M'ith platina, is capable of dillolving a conlider- able quantity of mercury, and that in cryftallization great part of the mercury fhoots before the platina. To another quantity of folution of platina I added more quickfilver than it was capable of taking up. The platina .now gradually fell down among the undiflblved mercury, in in form of a dark browjiilh powder, leaving tlie liquor very little coloured. Platina therefore agrees with gokl in having Icfs affinity to aqua regia than mercury has, though it differs in its affinity to the mercury, gold in this preci- pitation uniting with the mercury into an amalgam, while platina remains in a dillindl powder. This obfcrvation accounts for a phenomenon obfcrved by Marggraf in the following experiment. Half an ounce of quickfilver, and an ounce of folution of platina being Hiakcn together, the mercury run fluggifh,, and foon after a quantity of yellowiOi white powder fettled, at the bottom. The folution being fet to digeil, it ap- peared next day fomewhat green illi. The digellion was continued a day longer, and the mixture diluted with waterj the clear liquor being decanted off, the matter at the bottom was thoroughly edulcorated, and the yellowi/li white powder walhed off from the mercury and dried. The uncorroded mercury was not of the nature of an amalgam, but run pretty freely: being diilillcd in a retort, it left a metalline grain behind, fo fmall, that its appear- ance could not well be diftinguilhed without a micro- fcope, which flaewed it yellow. The white powder^ fet to fublime in anotlicr little retort, yielded a fublimate of a reddifh yellow colour in the lower part, and whiter above: there remained a little grey matter, which being preffed looked like an amalgam. It is remarkable that the mercury had here borne a very ftrong fire, by which tlie whole belly of the retort had been melted, tiiough without, any hole being made in it. It is probable that the little yellow grain, left upon, diftilling the uncorroded mercury, was a particle of gold which the platina had contained ; and that, agreeably to- the foregoing remark, platina and gold, diffolved together in aqua. regia, may be parted on this principle, the gold. being [ 586 ] being Imbibed by the mercury, while the platlna is preci- pitated in powder, which may be feparated from the amal- gam by wafliing. Solution of mercury in aquafortis rendered folution of platina inflantly turbid, and precipitated a greyifli brown powder. Solution of mercury-fublimate in water, poured into folution of platina, precipitated a red matter, with numerous bright fparkling particles, the liquor continuing yellow : the precipitate boi-e wafhing with water, without lofmg its red colour, X. Aqiiaregia: Nickel : Platina. Marggraf relates, that a piece of pure regulus of co- balt, or cobald-fpeife, from the fmalt works at Schneeberg in Saxony, after being repeatedly melted with glafs till all its blue-colouring matter was extradted, was readily at- tacked by folution of platina : the regulus lofl its bright- nefs, and became black, a yellowiih powder precipitated, and the liquor looked greenifh. The fubftance by which the platina was here precipi- tated, and which communicated a green colour to the li- quor, I apprehend to have been the metallic body called ■nickel, difcovered and defcribed by Mr. Cronftedt, in the -Swedifh tranfaflions for the years 1751 and 1754, one of whofe charadlers is to diflblve green in aqua regia, whereas the regulus of cobalt, ftriftly fo called, gives a reddilli fo- lution. Mr. Cronfledt obferves, that cobalt generally con- tains, befides its proper regulus, or the metal which gives a blue glafs, a quantity both of nickel and of bifmuth : that the fpeifc, or metal which feparates to the bottom of the melting-pot in making the blue glafs, generally con- fifts of all the three metals ; the cobalt-regulus and bif- ;tnuth, which of thenifeives are averfe ta any union with one [ 5S7 ] one anotlicr, being rendered millible by the intervention of nickel : that when this mixture is again melted with glafs, the cobalt-regulus vitrelies firrt ; the nickel, more 'difficultly calcinable or vitrefcible, preferving its metallic form to the lafl:. It may be prefumed therefore that the operations, which Marggrafs inetal paded through, fepa- rated the true cobalt-regulus, and left only the nickel. XI. Platina, Gold, and Aqua regia. Into a faturated folution of platina made in aqua regia, Mr. Marggraf put a plate of fine gold, and digefted tiie ■whole in a moderate warmth for fome days : the gold was not in the leaft aded upon, and there was no precipitation of the platina, except that a little dark orange coloured cryftalline powder fettled to the bottom, which the folu- tion of platina would have depofited by itfelf. The purer grains of platina were treated in the flime manner with a faturated folution of gold, and with the f^me event, the acid Ibewing no difpolition to quit either of thcfe metals in order to attack the other, fo that its affinity feems to be equal to both. I melted the two metals together, and di- gelled the compound in aqua regia : the menftruum dif- folved them both, but the gold moft readily; forthefirft portion of the liquor having been infufficient to diffolve the whole of the mafs, and the relT: being digefted in frefli aqua regia, the firfl: folution was found to have the greateft proportion of gold, and the other of platina. When the quantity of gold was fuch, as to give any thing of a gold colour to the mixture, the acid foon made the plates white, by eating out the gold firfl:. I likewife mixed together folutions of the two metals, and did not obferve any tur- bidnefs or precipitation to enfue, though Mr. Marggraf ■found, in his repetition of this experiment, a reddifh orange coloured precipitate : in this refpedt variations may happen G g g g from r 588 ] from the nature of the aquae regime made ufe of, as from an over-proportion of fal ammoniac in the aqua regia in which the gold is diffolved, for fal ammoniac, as we have formerly feen, is of itfelf fufficient to precipitate a part laf the platina. Though I could not perceive any feparatioa on mixing the two folutions, yet on diluting the mixture with water, and fuffering it to Hand for fome days, a bright gold coloured pellicle was thrown up to the furface : that this, however, was owing to the adtion of the platina, I will not affirm ; for I have feen a like feparation from di- luted folutions of gold alone. Another mixture of folu^ tions of gold and platina was evaporated a little, fo as to tlifpofe it to flioot : it yielded firft fine red cryfbls, which feemed to contain chiefly gold, with very little platina j Mfd afterwards deep faifron coloured cryflals, in which the platina apparently prevailed. XII. Platina, Silver, and Acids. Platina, digelled in a folution of filver made in aqua- fortis, was not at all afted upon, as indeed might have been expefted, the platina not being foluble in the acid itfelf by this treatment. A plate of filver, digefled in fo- lution of platina,, was ftrongly attacked : a white calx fet- tled upon the filver, and incrufted it all over, and the plate was (o corroded as to become friable between the fingers, the liquor fiiill continuing of a gold yellow colour. This experiment is from Marggraf : it feems to Ihew, that filver abforbs the marine acid from folution of platina, and that the platina remains diflblved in the nitrous acid, for if any of the platina had precipitated we mayprefume that the calx would not have been white. He found, however, that when the filver was previoufly diflblved, either in the nitrous or, vitriolic acids, it then occafioned a precipitation of the pla- tina, for on mixing thefe folutions with folution of platina, a yellow precipitate fell. I 5^9 ] XIII. Platina, Lead, and Acids. Thin plates of lead put into folution of platina arc fooit corroded, and white cryftals form at the hottom with a blackifli matter intermixed, the liquor continuing yellow; the cryftals dillblve in water, Iciving the blackilli powder, which appears to be platina. Marggraf, from whom this experiment is taken, tried alfo folutions of lead, made botli in aquafortis and in diftilled wine vinegar, and- relates that on mixing thefe folutions with folution of platina, no pre- cipitation cnfucd; a phenomenon not a little remarkable, as folutions of lead, made in either of the above mcnftrua, are in general precipitated by aqua regia or liquors contain- ing the marine acid. If there was no error or deception in thefe experiments, it might be concluded from them, that the marine acid has a greater affinity to platina than it has to lead; but with me the event was otherwife. A folution of lead in aquafortis, and a folution in diftilled water of cryftallized faccharum faturni which I had pre^ pared myfelf, being dropt into feparate portions of folution of platina, the firft drops produced no apparent change, but on continuing to add more of the lead folutions, both mixtures grew turbid and milky, and depofited quickly very copious white precipitates, the liquors continuing yellow like diluted folutions of platina. I repeated the experiment three or four times with different folutions of platina, and the appearances were always the fame. XIV. Platina, Regulus of antimony, and Aqua regia. Marggraf found, that a piece of pure regulus of an- timony, digefted in folution of platina, v/as attacked by the acid. A good deal of white powder fettled at the bottom, which was doubtlefs for the moft part fome of the regulus corroded: the reft of the regulus was reduced into fmall G g g g 2 brilliant [ S90 ] brilliant parts, and appeared to be intermixed with preci- pitated platina: the liquor continued yellow, XV. Platina, Bifmuth, and Acids. The author above-mentioned relates, that on digefting bifmuth in folution of platina, the efFedl was nearly the fame as with regulus of antimony, the bifmuth appearing corroded, a wliite powder fettling at the bottom, and the liquor continuing yellow: and that folution of bifmuth in aquafortis being mixed with folution of platina, no preci- pitation happened. The experiments in the five lafi: articles of this fedlioa are too obfcure and ambiguous for points of fuch impor- tance as the affinities of bodies to be eftablifhed on them; but thofe of the firft ten feem to be fufficiently clear and decifive. It may be obferved, that in fome of thefe affini- ties platina agrees with gold, as in being precipitated from aqua regia by zinc, iron, copper, tin, and quickfilver ; but that in others, it differs fo effentially from gold, that when the two metals are intimately combined together by long fulion, they may ftill be parted from one another in virtue of this contrariety in tlieir affinities to particular bodies, platina being rejefted by quickfilver while gold is retained, and gold being rejefled by aqua regis, when vitriolic fo- lutions of iron are added, while platina is retained. S E C T. IX. Of diJl'mgutJJmig and purifying Gold mixed with Platina. W'^E have now finiflied a laborious examination of the properties of this new metal and its relations to other bodies. One of the moft important advantages, that were expedled to refult from this enquiry, confidered in a commercial view, was the preferving of the finenefs and value [ 591 ] value of gold i or preventing it from being fraudulently dcbafed, by the admixture of a body, endowed with (o many of what have been univerfally reckoned the mod peculiar and inimitable charadters of the ])recious metal. This advantage has been obtained in the moft ample man- ner that could be wiflied for; the experiments having pointed out different means, by which finall proportions of platina mixed with gold, or fmall proportions of gold mixed with platina, may be eafily diilinguiilied, and by which the two metals, however blended together, may bs eafily parted from one another, either in the way of affay, or of bulinefs in large. The principal of thefe means it will here be proper to colledl together from the different parts of the hiflory, and to confider them more particularly in regard to their ufc and application in pradlice. I. Amalgamation with ^lickfilver. In an experiment related in the lafl fedtion, page 579, a mixture of platina and gold being united with mercury, and the amalgam ground with water for a confiderable time, the platina was gradually tlirown out, and the gold retained by the quickfilver. This procefs, fimple and convenient in the execution,, is accompanied with fome uncertainties in regard to its effedt, which render it of lefs general ufe than it may at firfl promife to be. Repetitions of the experiment have fliewn, that though the feparation fucceeds in fome cafes, it does not perfectly in all: that if there is any particle of the platina not fully diflblved by the gold, which will crenerally happen unlefs the quantity of gold is three or four times greater than that of the platina and the mixture is melted with an intenfe fire, this part will be retained in the amalgam, not diflblved by the mercury, not commi- nuted by the peflle, and too ponderous to be waflied off ia its [ 592 ] its grofs fonn. Various mixtures of platina and gold were treated in the manner above defcribed; and the gold, recovered from the amalgams, was fubmitted to further examinations. Where the proportion of platina was large at firft, the microfcope almoft always difcovered feme grains of it remaining with the fpongy mafs of gold after the evaporation of the mercury j and even when the gold had been melted, and made fluid enough to be poured into a mould, I have fometimes feen diftindt grains of platina on the fradlure of the ingot. Where the proportion of platina had been ftnall, the recovered gold was frequently, but not conftantly, found to be pure. It appears therefore, that though mercury has a greater affinity to gold than to platina, and though platina, on this principle, is capable of being feparated from gold; yet the procefs is too vague and precarious to be applicable in the way of affay, as we can have no mark of the precife time for difcontinuing it, and as we can never be certain, with- out making another affay, whether the whole of the platina is feparated or not. As a preparatory operation, where the quantities of platina and gold to be parted are large, it is neverthelefs of good ufe ; as greateft part of the platina may by this means be wafhed over with little trouble, and the gold brought into a lefs compafs, fo as to be commo- dioully fubmitted to a further purification by the means hereafter pointed out. This procefs may be confidered as anfwering the fame purpofe, in regard to mixtures of gold and platina, as that of ftamping and wafliing does in metallic ores, which could not be reduced to pure metal in the furnace to advantage, without the previous feparation of great part of their earthy or ftony matter by water. To enfure fuccefs, the mixt, if brittle enough to be pulverable, fhould be reduced into very fine powder, in ftamping mills, or in an iron mortar: the [ 593 ] the pulverifation may be facilitated by mtajis of hc:itt both the grains of platina itfclf, iuid mixtures of them witli other metals, being conliderably more brittle when hot than when cold. Or what is flill bett<;r ajid tiUitr, tlie mixt may be melted with a fuitablc quantity of Iwad, and this compound fubmitted to the trituration with mer- cury and water. If tliere is any truth in the report, that certain gold mines are negledted, on account of their in- tradlablenefs from platina contained in them, this lafl pro- ccfs may turn out a very important and advantageous one. II. Pixcipitation by vegetable Jixt Alcalics. As gold is totally precipitated by fixt alcaline falts, but platina only in part, and as a minute portion of platina remaining dillblved tinges a furprizingly large quantity of tlie fluid of a yellow colour; it was prefumed that a Imall admixture of platina with gold might by this means be readily difcovered. A few drops of a Iblution of platina were therefore mixed with above a hundred times the quan- tity of a folution of gold, and a pure fixt alcaline fait gra- dually added fo long as it occafioned any effervcfcence or precipitation. The remaining liquor was ilill fo yellow, that it was judged the platina would have difcovered itfelf, though its proportion had been lefs than one thoufandth part of that of the gold. It may be obferved, that though it is cuflomary to dilute metallic folutions pretty largely with water in order to their precipitation, yet here, as we •want only to fee whether any colour remains in the liquor after the precipitate has fettled, the lefs dilute the liquor is, the lefs quantity of colouring matter we Ihall be able to diftinguifh. It has been objedted to the above experiment, tliat though tlie platina is difcoverable when thus mingled iii- perficially [594] perficklly with the gold, it may neverthelefs, when com- bined more intimately by fulion, elude this method of trial. Mixtures of gold with fmall proportions of platina were therefore kept in fufion for feveral hours, with a very ftrong fire, and afterwards diffolved in aqua regia. The folutions were diluted confiderably with water, and a fo- lution of pure fixt alcaline fait gradually added, fo long as any effervefcence or turbidnefs enfued. The liquors proved paler than when the two metals had been diffolved fepa- rately, but retained colour enough to betray the platina. As the degree of colour was not here fo great, as might have been expefted from the quantity of platina which there was realcn to believe they contained, I tried to dif- cover the platina in them by fome chara<3:er more con- fpicuous. I put into the filtered liquors fome plates of pure tin : the tin prefently contradled an olive hue, and threw down a large quantity of brownifh precipitate, as it does from the common folutions of platina: it was obferva- ble, that the tin plates were often fenfibly acfled upon even ■while the liquor was overcharged with alcali. It has been further fuggefted, that fince a part of platina is precipitated as well as gold by fixt alcaline falts, if only 'this part be mixed with gold, it will elude this trial, and be thrown down by alcalies again, along with the gold, i'rom the Iblution of the compound. To determine this point, I meited with gold a precipitate of platina made by fixt alcali, and kept them in ftrong fufion for an hour and a half: they feemed to unite more eafily than gold does with the crude platina, and formed a fmooth neat •bead, which hammered pretty well into a thin plate before it cracked, and appeared internally uniform and equal. This compound being diffolved in aqua regia, the folution diluted with a little water, and a folution of fixt alcaline fnh added by degrees till the acid was more than faturated, the [ 595 ] the liquor became, not indeed colomlefs, but lo pjc, that it could hardly be judged to contain anyplatina : ncvcr- thelefs, on putting into it fomc tin plates, they quickly fhewed, as in the foregoing experiment, that it held a very confiderable quantity of platina. It appears there- fore that in all thefe circuniftances tiic platina remains partially dillblved in the neutralized liquor ; and tbat on this foundation, fmall proportions of it, mixed with gold, may be difcovered, either by the colour of the liquor after precipitation with alcali, or, in a more fenfible manner, by further precipitation witli tin. In all the above expe- riments the folutions were diluted with water, not as be- ing a circumftance advifable where gold is to be thus exa- mined, but that the ufcfulnefs of this way of trial might be eftablidied with greater certainty. Volatile alcaline falts or fpirits have the fame effedls as the fixt alcalies on folutions of platina, but their effeifls on folutions of gold are in fome circumflances different. After the acid has been fatiated, and all the gold precipitated, an addition of the volatile alcali beyond this point redif- folves fome part of the gold, fo that the liquor becomes yellow again though there be no platina in it. For this trial therefore, only the pure fixt alcalies are to be ufed, which, in whatever quantity they are added, have not been found to rediffolve any of the gold. III. Precipitation hy mineral Jixt Alcali. The vegetable fixt alcalies ferve only for dirtinguilhing whether gold is mixed with platina or not : theyare infuf- ficient for the purification of the precious metal, as they always precipitate a part of the platina along with the gold. With the mineral alcali, or the alcaline bafis of fea fait, the cafe is other\vile. Though tliis alcali, as appears from Marggrafs experiments, precipitates, equally with H h h h the [ 596 ] the veo-etable, all the common metallic bodies, gold, lil- ver, copper, iron, tin, lead, zinc, bifmuth, regulus of an- timony, cobalt, &ZC. yet in iblution of platinj; it produces no precipitation or turbidnefs ; fo that when this alcali is mixed with a folution of gold containing platina, the gold alone is precipitated, and all the platina remains diUblved^ The manner of obtaining this alcali from the acid with. which it is united in fea ialt, as it would in this place too much interrupt the hiftory, is referred to the appendix. The mineral alcali is in many places, particularly in the eaflern countries, found native, either in a pretty pure ftate,, or blended chietiy with earthy fubfcances, from which it is eafily feparated by folution in water. I have been fa- voured by Dr. Heberden with a quantity of this native fait fent to him from Tcneriff, and find that it anfwers the. prefent intention as eifedtually as the alcali extradled from fea fait. The folution of platina efFervefce.d with it, but in whatever proportions the folution of the alcali and of the platina. were mixed together, I could not obferve the: leafl precipitation or cloudinefs. A fait of the feme nature, though generally perhaps mingled with fome foreign faline matters, is obtained from the afhcs of certain plants, called Kali, which growing chiefly in fait marflies or on the fea fliore, the marine fait is fuppofed to be imbibed by them, and to be decom- pounded, or to have its acid feparated, partly by the power of vegetation in the plant itfelf, and partly by the burnings The befl fort of thefe afhes is faid to be prepared at Ali- cant in Spa,in, from an annual procumbent kali with fliort leaves like thofe of houfeleek. The ajQies, which are one of the common kinds of potaft in France, and there called fo2(de or Jb^a, are brought to us, under the name of Spanifi , aJJies or bar'iglia, in hard fpongy maffes, partly whitifli or grey, and partly blackifli. From thefe. maffes the faline part [ 597 ] part Is extrafted pure by powdering and digcfting them in water. Though it might be fiiipefted thit this lalt, in virtue of its containing not only the mineral but a portion of vegetable alcali, would precipitate part of the platina as well as gold, I could not fini that folution of platina fuf- fered the leaft alteration from it, any more than from th6 native or marine alcalies. How far thefe falts may fuffice, for tlie pcrfcft fcparation of platina and gold that have been intimately combined with one another by fufion, I have not yet had diredt experience. But it may be proper to obferve, that though both the native alcali and bariglia are fuppofed ge- nerally to contain fome fea fait in its whole fubftance, which for fomc purpofes renders them unfit, yet this fait does not appear to be here of any difad vantage, for pure fea fait occafioned no precipitation or turbidnefs in a folu- tion of platina, any more than in folution of gold. The platina employed in thefe experiments was fuch as had been cupelled with lead and urged afterwards with repeated ftrong fires, page 570. IV, Precipitation by fal amvioniac. ■ The alcaline falts in the two foregoing articles precipitate the gold, and leave the platina wholly or partially diflblved in the liquor. Sal ammoniac has a contrary effee form in which it is brought to us. The obfervations on the appearance of the grains, and the matters mixed with them, mentioned at the beginning of this eflay, induced me, on the firfl examination of it, to think that it had been ground in mills with quickfilver. Marggraf, whofe pla- tina came from London, and probably from the fame parcel with that in which I had obferved the drops of quickfilver, feems to have entertained a fufpicion of ths fame kind ; for he doubts whether the platina is a native mineral, or a metallic recrement from which the Spaniards have extradted the perfedt metal it contained. I have fince been informed that the quickfilver we obferved among it, ■which doubtlefs inliuenced Marggraf as well as me, did not come with it from the well Indies, but was added here by the proprietor vvitla a view to get out the golden particles.. Some accounts however feem to countenance the above conjefture, that platina is found in large malfes, and is re- duced into Imooth grains by ftamping and grinding. A fubllance in fmall grains, like the platina as brought to us, one fhould think could hardly be called a ftone, as platina. is by don Antonio: d'Ulloa. D'Ulloa is the firfl; writer Lhave met with who mentions- platina by name. In his voyage to South America, in- 1735 and the follov.ing years, fpeaking, of the gold and, filyer mines of Quito, he relates, that in the territory of. Choco there are mines, in which the gold is fo enveloped in other mineral fubflances,. bitumens, and flones, that they are obliged to ufe quickfilver for its feparation; that fome- times they find mineral fubflances which, from their being mixed with platina, they chufe to negle(3:; that this pla- tina is a Hone (piedra) of fuch refiflence, that it is not eafily broken by a blow upon an anvil j that it is not fub- dued by calcination j and that it is very difficult to extradt the metal it contains even with much labour and expence. /^/g J [ 6o7 ] Some have furpedcd that the picdras dd luga or ht>:(i> defcribed by the fame author as being untranlparcnt and of a leaden colour, and which were made into mirrours by the ancient Indians, confill of platina mixed with a ll;ony matter. This mineral cannot be the fame with that to which he gives the name of platina in tlic foregoing para- graph, for he cxprefsly mentions that tjic picdra del Ingo is foft, and liable to be broken by a flight blow. The Inca ftone is now pretty common, and as the French tranflator of the papers on platina (fee page 447) obferves, appears to be no other than a fcrrugineous mineral of the pyrites or rather mundick kind. Alonlb Barba mentions a fubftancc, under the name of chumpi, which feems to have more refemblancc to the pla- tina of d'Ulloa. He defcribes tlie cliumpi as a hard flone, of the emery kind, participating of iron, of a grey colour fhining a little, very hard to work bccaufe it refills the fire much, found in Potofi, Chocaya, and other places, along with blackilh and reddifli ores that yield gold. If platina is really found in large maflcs, either generally or only now and then, one might reafonably expedt thofe maffcs to be fuch as are here defcribed. Of the fame kind perhaps alfo is the mineral mentioned by feveral authors under the name of Spanifli emery, Smirii' Hijpanica, which fliould feem, from the accounts given of it, to be no other than platina or its matrix. T\\t fmin's is faid to be found in the gold mines, and its exportation- prohibited; to contain films or veins of native gold; to be in great requefl: among the alchemlfts ; to have been fomc- times ufed for the adulteration of gold ; to ftand, equally with the noble metal, cupellation, quartation, antimony, and the regal cement; and to be fcparable from it by amalgamation with mercury, which throws out the /w>/> and retains the gold; properties ftrongly charadlerillic of platinaj [ 6o8 1 platina, and which do not belong to any known fubftance befides. This debafement of go\di per extraBum fmi7-idis Hifpanici is mentioned by Becher in his minera arenaria, and feveral times hinted at in his phyfica fubterranea. Both Becher and Stahl indeed call the fubftance, which the gold receives from the emery, an earth, whereas platina is undoubtedly a metal; but this does not at all invalidate our fuppofitlon, for they give the name of earth alfo to the fubftance which copper receives from calamine in being made into brafs, which is now known to be metallic. From thefe obfervations 1 have been led to fufpeft that the European emerys likewife might poffibly participate of platina. If this was certain, it would account fatis- fadlorily for the ufe which fome of the alchemifts are faid to have made of emerys and other ferrugineous ores ; and we fhould no longer doubt, or wonder, that by treating gold with thefe kinds of minerals, they obtained a perma- nent augmentation ; that this augmentation, though it refifted lead, antimony, aquafortis, and the regal cement, was feparable, as Becher owns it was, by quickfilver ; and that, when it exceeded certain limits, it rendered the gold pale and brittle. If emery contains platina, I imagined it might be dif- coverable by boiling the powdered mineral in melted lead, and afterwards working off the lead upon a teft or cupel. The experiment was made with eight ounces of the finefl powder of common emery, and the fame quantity of lead, wliich were covered with black flux to prevent the fcori- fiCation of the lead, and urged with a ftrong fire for two or three hours. The lead became hard, rigid, of a dark colour, and a granulated texture> as if it had really im- bibed fome platina from the emery ; but in cupellation it worked almoit entirely off, leaving only a bead about the- fize of a fmail pius head, which was probably no- other, than filver contained in the lead.' [609] I repeated the experiment, with lome variation, tliink- ing to obtain a more perfe being mixed with ten parts of the fait, and melted in a crucible with a ftrong fire; moll of the fait rofe over the crucible, Iciving a brownilh glafs, and the gold revived into grains. One part of a calx of gold made with quickfdvcr, and ten of the fait, melted in a ftrong fire, gave likewife a yellow- brown glafs, and the gold was found revived. One part of gold, two parts offal ammoniac, and eight parts of the microcofmic l^ilt, being melted together, the fait run all through the crucible, the gold remaining in grains. Solution of gold in aqua regis is precipitated by folution of the microcofmic fait, provided the aqua regis is fully faturated with the gold, and the microcofmic folution added in fufficient quantity. If the mixture of the two folutions be poured upon compofitions for glafs, the whole boiled down to drynefs, then well ground together, and brought into fufion, the gold either wholly difappears, or hardly a footftep of it is to be fecn. A compofition of three parts of powdered flint, two parts of faltpetre, a:id one part of calcined borax, being treated in this manner with the mixt folution";; a blue fandivcr, almofl like tur- cois, was found on the top of the melted matter, and a pure blue glaf^ underneath. With regard to the blue cOf lour, which may feem pretty extraordinary,, the author- obferves that fome variation may happen from tJiQ d?grcc. of fire, and that the common ruby glafs, when in giving LIU it [ 620 ] It colour by the flame of wood it Is kept too tong ia k rtrong heat, changes to an amethyft blue. Page 114. Gold plates for enamelling. That a certain mixture of alloy is necelTary in the gold plates defigned for being enamelled on, I have related on the autliority of a writer in the French Encyclopedk'i who fays the gold muil be of the finenefs of twenty-two carats at moft, that if finer it will not have ftrength enough, and that if coarfer it will melt. I am told by an experienced artifh that this is a miilake; that ducat gold is generally ufcd, whofe finenefs appears to be from 23 1 to- 23 1 carats; and that the fin eft gold is for this ufe the befl, unlefs where fome parts of the gold are left bare and after- wards poHfhed;, as is often done in watch cafes, fnuff boxes^ &c. for which purpofes a mixture of alloy is neceffaryj, that filver is preferred as an alloy to copper, the latter dif- pofing the plates to tarnifh and turn green ; and that the pilates are flrengthened, by covering them on the back, fides with enamel. Thus much is certain, that the finer tlie gold, the more foft and flexible ; and the coarfer, the . more fufible it proves. Page 121, 123. "Tciichjlone — not of the 7narble kind. Mr. Pott alfo has taken notice, that the touchflone is not of the marble kind.; and that black marbles, how well . foever they, may anfwer for receiving a coloured flroke from metals, are unfit for the ufe of touchflones, on ac- count of their being difljolved by aquafortis. He makes the touchfl:one a clayey flate, partaking of iron; and finds, that in a ilrong fire, like many other ferrugineous clayey '.biaerAls, it melts perfedlly into a blackiih brown flag, and thatv [ 621 ] that a fmall quantity of it, mixed with vitreous com- pofitions, gives them a notable green tinge. He obfcrvcs that the impcrfedlion of Hints, &c. for the ufc of touch- ftones, confifts in^their hardiwfs, which occafions them to give too great brightnefs to the metalline ftroke, fo that its colour, and confequently the proportion of alloy, cannot be exadtly judged of- Page 172, &c. CiiJJitiss precipitate— Ruby glafs. Cassius does not appear, from his treatife de auro, to have been the difcoverer either of the precipitation by tin, or of the tinging of glafs by the precipitate. He defcribc5 the preparation of the precipitate, and (lightly mentions its ufe in this intention ; but gives no account of the manner of employing it, nor any pradiical hint in regard to the operation, except that in fpeaking of the fmallncfs of the parts of gold, he fiys that one dram of gold duly prepared will tinge ten pounds of glafs. A procefs for making the ruby glafs has been communi- cated to me by an artift, who fiys he was aflured it came from Kunkel, and that he had found it a good one for enamelled colour, but had never tried it for glafs. The gold is directed to be dillolved in a mi.xture of one part of fpirit of fait and three- of aquafortis; and the tin, in a mixture of one part of the former of thefe acids with two of the latter. The folution of gold being properly diluted with water, the due proportion of which is to be found by trials made on a fmall quantity as mentioned in page 1 76, the folution of tin is added, and the mixture fuflcred to ftand till the purple matter has fettled to the bottom. The colourfefs liquor is then poured off; and the purple fedi- ment, while moifl: and not very thick, is thoroughly mixed \vith powdered flint or find: this mixture is well ground L, 1 1 1 2 with [ 6-22 ] v/ith powdered nitre, tartar, borax, and arfenlc, and the compound melted with a fuitable fire. The proportions of the ingredients are, 2-560 parts of fand, 384 of nitre, 240 of tartar, 240 of borax, 28 of arfenic, 5 of tin, and 5 of gold. I have not yet had an opportunity of trying this proeefe, but am convinced, that the mixing of the precipitate with the fand, &c. in a moift ftate is a very material circum- llance, if not the principal one upon which the fuccefs of the operation depends. Perhaps the mofb certain- way would be, not to wait for any precipitation at all, but to moiften the powders with the purple liquor, grinding them well together, in a moderate warmth, till dry, and if ne- cefTary, repeating the humedlation. Mr. Potts experi- ments with the ml-crocofmic fait, mentioned in page 619, confirm the irtility of this method ef mixture* Page 185. Sluantity of gold colleSied in rivers. In a paper drawn up by Mr.Guettard, from the obler* vations of Mr. Pailhes, and publiflied in the volume of the French memoirs for the year 1761, the gold found in- rivers is reckoned an objedl of more importance than it has been ufually reprefented. It is faid that the mint of To- loufe received commonly every year two hundred marcs, ox one hundred pounds weight, of gold colledted from the Ariege, Garonne, and Salat; and that fince the year 1750, t^velve pounds have been carried, into the bureau of Pa- niiers, though this bureau comprehends at moft an extent- of only two leagues round, and though the whole of the gold is not fent thither, ftrangers and hawkers buying it up <;very davi. Ea^e [ 623 ] Page 186. Source of the gold found in rivers. It has been generally thought that the particles of goKI, found among the lands of rivers, have been torn off by the violence of the ftream, in palfing over fome rich beds or veins. The obfervations of Mr. Pailhes, in the memoir beiore mentioned, fcem to prove, that the gold is not con- fined to any particular fpot, but dilfcnunated, though very fparingly, tlirough all the adjacent earths ; and that the particles found in the rivers proceed from part of the banks vvalhed down by floods and rains, the lighter earth being carried away by the current, while the gold particles, with the ponderous black lands and flints, fettle to the bottom. Tlie author relates, that thofe who employ themfelves ia colled:ing the gold, Ibmetimes anticipate the effe6l of ths floods, by privately cutting down or undermining the banks,, that the gold particles may be feparated, which occafions frequent law-fuits between them and the proprietors of the grounds : That in the town of Pamiers, lituated on one of the celebrated auriferous rivers, Arlege, on digging for wells or foundations of buildings, the earth thrown up is always found to contain particles of gold : That he has dif- covered abundance of auriferous trafts in other parts of the territory of Foix, infomuch that he imagines it would even be more difficult to procure water for the wafliing than to find the gold : and that befides the gold met with in detached particles, the flints that accompany them con- tain alio gold, which might be fcparated to advantage by ftamping and wafliing. A quantity of thefc flints was lent by Mr. Pailhes to the academy, but in the allays made of them they appeared to be merely ferrugineous, yielding Bear half their weight of iron, without any mark of gold. Fage [ 624 ] Page 221. Gold coloured metal. The celebrated Mr. Pott, in a German letter to Von ■Jufti printed in 1760, affirms that tombac, or a gold coloured metal, may be made from a mixture of copper and tin ; and in a further difcuffion of Von Juftis objecti- ons, printed in 1762, he gives the particular compofition of this metal. " Take one half-ounce of tin afhes, and four half-ounces of copper : melt them well together, in a clofe luted crucible, with a ftrong fire. Or take one half- ounce of the pureft tin cut in pieces, and fixteen half- ounces of pure copper beaten into thin plates : lay the tin between the copper plates, lute the crucible clofe, and ,melt with a ftrons; fire." Page 224. Gold coloured "jarnijh. The compofition of a gold coloured varnifh, ufcd by the Englifli artifts for brafs and filver, was communicated to fome of the French academicians, in 1720 by Mr.Scar- let, and in 1738 by Mr. Graham, and has lately been pub- liHied in the volume of the French memoirs for 1761. Though I do not apprehend that this varnifli is anywife fuperiour to that defcribed in the page above referred to, I iliall here infert it for the fatisfadlion of the reader. ** Take two ounces of gum lac, two ounces of yellow am- ber, forty grains of dragons blood in tears, half a dram of fafiron, and forty ounces of good fpirit of wine: infufe and digeft in the ufual manner, and then flrain through a linen cloth. The piece to be varniflied muft be heated before the liquid is applied : it receives from the varniili a gold colour, little different from that of the mercurial gilding, and may be cleaned, when fullied, with warm wa- iter." It may be prefumed that the amber is of no great .ufe [ 625 ] ufe in the compoiition, this concrete being very fparingly difTolvcd by the fpirit. As the fpirit for varniflies ought to be freed as much as poffible from its phlegm or watery part, and as this is mofl conveniently cffedlcd by means of fixt akaline falts, I ac- cordingly dire(5led the fpirit to be fliakLn v.ith fo much- alcali as fhould be fufficient to imbibe the phlegm. In Meyers very ingenious German trcatifeon quicklime, pu'b- lilhed in 1764, it is obferved, that fpirit rcdlified with thefe falts is unfit for varnilhes, particiiKirly for fuch as are to be applied on gilt works, the fpirit taking up a part oi the fait, which darkens the colour, and prevents the fpeedy drying of the varnifli. This obfervation feems to relate to fpirits that have been dcphlegifjated, not fimply by fuffer- ing them to ftand fome hours on the alcali in the cold, and occafionally fliaking the veflel, but by the application of a conliderable heat ; in which laft cafe the fpirit is known to take up a fmall portion of «he alcaline filt, but that it dilTolves any in the firlT; is not fo clear. If, however, the fpirit Ihould in either way have received an alcaline im- pregnation, it may eafily be purified by means of a little alum well dried and powdered, the alcali being abforbcd by the acid of the alum, and forming therewith a com- pound not combinable with vinous fpirits. Some pcrfons, when a perfedlly pure fpirit is required, firft dephlcgmate it with alcaline falts, and afterwards purify it from fuch part of the alcali as remains in it, by a frcfh diftillation from a quantity of alum : perhaps fimple infufion and agi- tation with the alum would be as eftlftual, at leaft for the purpofe of making varniflies, as the more troublefome pro- aefs of diftillation. Go/d [ 626 ] Gold coloured Glafs, with metallic fiibjlances. Precipitates of filver, baked on glafs, ftain it yellow, and likewife give a yellow colour on being mixed and melted with forty or fifty times their weight of vitreous compofitions : the precipitate from aquafortis by fixt al- cali feems to anfwer beft. I have likewife obtained yel- low glaffes with certain preparations of iron, particularly with Prufllan blue. But neither with filver nor with iron does the colour fuccced conftantly, or approach to the high yellow of gold. The neareft imitations I have obtained of the colour of gold in glafs, were produced with antimony and lead. A quantity of crude antimony in fine powder was calcined by a little at a time in a flat iron pan, with care to prevent as aiiuch as poflible its running into lumps, by ufing a very gradual fire, and keeping it conftantly ftirring, till at length, when brought to a full red heat, it neither foftened nor emitted any fumes. The afti coloured calx, weighing little more than half of the crude antimony, was put into a crucible, and urged with, a ftrong fire in a blaft furnace: it melted into a glafs, dark coloured and opake in pieces of any confiderable thicknefs, and of a tranfparent yellow when drawn cut thin. Some of this glafs, reduced to pov/der, was mixed and melted with four times, three times, and twice its weight of powdered flint glafs : the glafs refulting from the firft mixture was of a tranfparent pale yellow, from the fecond deeper, and from the third of a pretty deep yellow, with- out any mixture of greenifh or brown. Equal parts of the glafs of antimony, of flint calcined and powdered, and of minium, formed a glafs of a hiah yellow, and with two parts of glafs of antimony, two of minium, and three of powdered flint, the colour approached ftill more to that of gold. I <>- ] gold. All thelc conipofitions were brigiit and tranfpa- rent, without any Icum on tJic furface or rcgulus at ihc bottom. The luft exhibited a inuhitude of fmall fpurkles intcrfperfed througli its whole fiibAance, which guve it a beautiful appearance in the mafs, though in tlie lapidaries hands they were found to be hnpcrfedtions, ariling I'rom air bubbles. It is pretty remarkable, that in feveral repe- titions of this experiment, in a glafshoufe furnace as well as in my own elaboratory, the produd: was always full of thefe brilliant fpecks^ Glafs of lead and glafs of antimony make likcwife a gold coloured glazing for porcelain and earthen ware. The fineft gold glazing is faid to be made with an addition of filver. A glafs of lead is prepared by melting minium or litharge with a third or a fourth part of its weight of pow- dered riint. This yellow glafs, reduced into Hne powd-cr, is either fprinkled on the porcelain made red hot, or mixed with beer or other glutinous liquids to a due confiftence, and applied with a pencil : the ware is then placed in the furnace, under a muffle, till the glafs begins to melt,which is known by its gliftening ; after which, while warm from the fire, it is moillened with a diluted folution of filver, and baked again. Or the powdered glafs of lead is moift- ened with the filver folution, then melted, and the glazing of the ware finifiied in one procefs, by applying on it this compound glafs. After the baking, the glazed vefiels, while ftill red hot, are held over the fmoke of burning ftraw, &c. Gold coloured glaffes loithout 7nctaUic fubjlimces. There are fundry earthy bodies, as chalk and gypfum, which make a yellow colour in glafs, efpecially when the vitrification is procured with borax or alcaline falts. Thefe ^lafles however have generally more or lefs of a M m m m green [ 628 ] green tinge, and never, fo far as I have obferved, a goM yellow. Neri diredls, for a gold yellow colour, one part of red tartar, and the fame quantity of manganefe, to be mixed with a hundred parts of fritt or the compofition for glafs. Kunkel, in his experimental remarks on Neri, fays that this procefs gave him more trouble than any other in the book ; that the proportions are quite faulty; that the quan- tity of manganefe is too much for the tartar, and the tar- tar too little for the fritt ; that one part, or one and a quar- ter of manganefe, is fufficient for a hundred of fritt, but that fix parts of tartar are hardly enough, efpecially unlefs. the tartar is of a dark red colour almolt blackifh ; and that he found it expedient to add to the tartar about a fourth of its weight of powdered charcoal. He takes notice that the compofition fwells up greatly in melting; that if the- glafs be much ftirred with the iron, as is cuftomary for, other kinds of glafs, it will rife up fo as to run over the pot, though at firft no more than half full ; and that there- fore it muft be left unftirred, and worked as it ftands in fu-. fion. Speaking afterwards of a yellow enamel tinged with the fame materials, he adds, that the colour muft be- carefully watched, too long a continuance of fire defi:roy-. ing it. My ingenious friend Mr. Samuel More, in repeating and varying this procefs with a view to render the colour more perfect, found that the manganefe is entirely ineffen- tial to the gold colour, and that the tartar is no otherwife of ufe, than in virtue of the coaly matter to which it is in part reduced by the fire. Different kinds of coals, as that of tartar, common charcoal, foot, dried blood, &c. on be- ing melted with colourlefs fritts or glafles, gave always pale yellow, dark yellow, reddifli, brownifh, or blackifli Colours, according as the inflammable matter was in fmaller or [629 ] or larger proportion ; the plilogillon, or infl.iniinalilc part of the coal, fecming to be the diredl tinging luhlliancc. When the phlogUlic matter was thus dit^ulcd through glafs, he did not find it to be affected by continued ilrong fire, any more than charcoal is when excluded from the air : though fome pots of the coloured glafs flood for a fortnight in the glalshoufc furnace, they ftill retained their colour ; nor did the mofl intenfe fire of a lamp alter it in the leafl. How fixt the colour when once united with the glafs is, we may judge from the indeflrudlibility, by very ftrong fire in open veflels, even of the fuperficial browns and blacks which charcoal and foot communicate to glafs in its converfion into porcelain : fee page 242. Mr. Pott, in his neue •wichtige ■phyficalifch-chymifcbe ma~ terien,(Sc, printed in 1762, obfervcs aUb that common coals give a yellow colour to glafs ; and that even by ce- mentation, provided the heat is not fulhcicnt to change it towards porcelain, they flain it of the fame colour. He fays that different coaly matters differ in their tinging power J that caput mortuum of foot, and lamp black, an- fwer better than common charcoal, and that he has known fome pcrfons employ the coal of indigo : That the fpark- ling coal, which remains in the retort after the redfifica- tion of the thick empyreumatic animal oils, is one of the moft adlive of thefe kinds of preparations, being as it were the heavier part of the inflammable fubflance of the oil, and very rich in colour : That this preparation, powdered and then again burnt a little in a' clofe veflel, is excellent for tinging glafs, and gives yellow, brown, reddilh or black- \(h colours according to its quantity : That the fritt mufh not be very hard of fufion, for if it is, the flrong fire will deflroy the colouring fubflance before the glafs melts ; and that he has found the following compolitions to be nearly the befl, viz. fand 2 parts, alcali 3 parts j or fand 2, al- M m m m. 2 cali [ 630 ] call 3, calcined borax i i or fand 2, alcali 2, calcined bo- rax I : That though faltpetre is hardly, or very fpa- ringly, yfed for yellow glafles, as it too much volatilizes tjie colouring fubilance ; yet here for the moft part a cer- tain proportion of it, which proportion will be eafily found by trial, is very neceflary, for without it the concentrated colouring matter is apt to make the glafs too dark, and- even of an opake pitchy blacknefs. That there is any material divcrfity in the effefts of dif- ferent coals, may be juftly queftioned ; for Mr. More ob-. tained, with common charcoal, the fame colours, as with the coals which are here fuppofed to be of greater excel- lence and adivity : it is probable, that the only difference- confilts in their containing different quantities of the in- flammable matter, fo that a little more fliall be required of one kind than of another, fop producing the fame de- gree of colour in the glafs. Nor does the faftnefs or fuli- bility of the- fritt appear to be any wife neceffary, for my. friend informs me that he has tinged, with coal, glaffes which were fo hard of fufion, that the glafsmen could not ■work them, whereas the above compoiitionsare all rather too foft to be fervice.able in the large way. . Pi7ge 233. Converfiqn of green glafs into fo7-ceIain. Some have complained, that on repeating thefe experi-^ ments, the change did not fucceed; and further enquiry has fhewn, that fome fort-s of green glafs are. unfit for thiS: operation. Green glafs has been chiefly made of vegetable aflies and fand, brought into fufion together by a fi:rong. fire: with this kind, v/hich is the green glafs common about London, the experiraents fucceed in the manner de- fcribed. In fome parts of this^kingdom, inflead of vege- table afhes, the vitrification of the. fand has been procured princi- [631 ] principally by means of another ingredient, the flags of the iron furnaces; and glafs of this compofition is found not to be convertible into porcelain. Tlie failure of this kind of glafs fervcs to confirm the general refults drawn from the former experiments ; that earthy and metallic glafles made without fuline matter, are not fufccptible of this change ; and that the change depends on the faline fub- llancc contained in the vcsretable afhcs. Pi7ge 314. Machines for blowing air by a fall of water. I HAVE received an account, from a worthy corre- fpondent in Swiflerland, of a machine which he has con- ftrudled for a fmelting furnace according to the foregoing diredlions : he fays, it has fo much the advantage of all other kinds of bellows, that it- deferves to be introduced univerfdly wherever the fituation of the place will permit. The only inconvenience he finds in it is, that the cullender and gratings are liable to be ftopt up by leaves, fee. With regard to the cullender, the obflrudlion m^y be obviated by enlarging the holes. The gratings ought to be of a large furface : the wire grating in the ciftern on the top > may be a cylinder nearly as large as the cifl;crn will re- ■ ceive, for if it is no more than fufficicnt to cover the mouth of the pipe, it will doubtlefs be loon choaked up: when fo much 01 the cylinder becomes flopt, that the water has no longer a free paiTage through, it may be lifted up and cleaned, another being placed in the room of it, without the trouble of turning off the water, or inter- rupting the going of the machine. The gratings here can be liable to no other inconveniences, than thofe which . are common in other water machines, mills, aqaxduds, 6cc. Some further improvements have occurred in the con- ilrudlion of thefe machines, by which they may be made efftdlual i [ 632] efFedual in cafes where the quantity or fall of water would otherwife be infufficient. Of conJlruBing blowing machines with falls of •water' of great height. Where the height of the fall is great, the quantity of water is ufually fmall; and in all the ways of application that have hitherto been contrived, the height will by no means make amends for the deficiency in quantity. In the common conftrudlion of thefe machines, where the upper pipe or funnel is no more than three, four, or five feet high ; though the fall fhould be fuch as to admit of the lower pipe being thirty or forty feet or more, it does not appear that any material advantage could refult from fuch a height. For, as the air is admitted into the water ■only at the top of this long pipe, it cannot, I think, be fup- pofed, that the quantity admitted will be the greater for the length of the paffage under the place of its admiflion. Water indeed has been found by Mariotte to run fafter, through an upright long pipe, than through a fhort one : a quantity of water which was forty-five feconds in running through a pipe three feet long, was difcharged in thirty- feven feconds, or near a fixth part lefs time, through a pipe of the fame bore and a double length ; fo that as more water pafles fuccefiively through the long pipe than through a Ihort one, in equal times, more air alfo muft be carried down by it. But in the cafe which we are here confidering, no benefit can be exped:ed on this princlplej for as the fupply of water is fuppofed to be limited, the bore of the pipe muft neceflarily be made lefs, in pro- portion to the increafe which its length may produce in the velocity. If the lower pipe is of fuch height, that the watery column It contains may fufficlently refift the force of the comprelTed air in the air-veflel, it fhould feem that any [633] any further addition to its height could be of no manner ©f ufe. We have feen, in the foregoing part of this efiay, that it would be more advifable, in fuch cafes, to (horten the lower pipe, and to lengthen the upper one: by this means tJie water, acquiring greater velocity at the place of its difcharge from ihc upper pipe into the lower, is enabled to divide or fpread more, and thus to receive more air into its interftices. The advantage, thus obtained, does not how- ever increafe in fo great a proportion as the height does. From an experiment related in page 310 it appears, that by increaling the height four-fold, the effedt was not in- creafed three-fold; and this even in fmall heights, where, the effedt is much more influenced by a variation of the height than in great ones. The obfervations already mentioned point out means of availing ourfelves more advantageoufly of high falls; fo as to produce always with certainty, from a fall of a double or treble height, a double or treble effedl if the quantity of water be the iame; or an equal effedt, with one half or one third the quantity cf water. Experiments have convinced, me, that a fall of fourteen, feet is more than fufficient for compreffing the air to fuch a degree, as to.be able to fuflain the gage at the height of four feet; or to raife, on an opening of a fquare inch, a weight of about a pound and three-quarters averdupois, oi". above two pounds troy; a comprcfllire, which .is appre- hended to be as great as there will in general be occalion for. Where we have plenty of water, with fuch a fall, we can drive in air, with this force, in any quantity: for if one machine, with a certain portion of the ilream, pro- duces a continued blaft of this {Irength through a pipe of a certain bore, as an inch or three-quarters of an inch; it is evident, that .the quantity of air may be doubled, trebled, &c. At at pleaiure, without diminifhing the compref^lire or forc« of the blaft, by adding another and another machine, till all the ftream is employed. It is plain, in like manner, that the fame advantage may be received from high falls, by placing one machine over another ; that after the wa- ter has performed its office in falling through one machine, it is ftill capable of exerting the fame adion in another and another machine, fo long as equal fpaces remain for it to fall through ; fo that the total effed: muft be the fame, as if -a quantity of water, fufficient for working all the ma- chines, came at firll in one .ftream. A fall thus divided into two machines is reprefented in the middle of the annexed plate. In the lower machine, whofe air-veffel is funk to a confiderable depth in a pit made in the ground, the water is forced up in the pit, on ihe outfide of the vefTel, four feet higher than the furface of the water within the veflel, or of the flone on which the water daflies, called by the workmen the dafli-board, (fee page 287). The air-veffel of the upper machine having an additional part at one fide, which performs the fame office as the pit, the water is in like manner forced v.p to the fame height in this outer part ; which outer veffel ferving as a refervoir for the machine under it, the water begins to ad: in this lower machine four feet higher up than the dalh-board of the firft. Whatever number of machines the fall will admit of, the cafe is the fame in them all : though in each of them the water falls eighteen feet, yet as it is prelTed up again four feet for the fucceed- ing machine, one machine takes up but fourteen feet of the real fall. The outer veffel, and its communication with the air- veffel, may be conveniently formed by an upright partition in the air-veffel itfelf, not reaching quite to the bottom. The outer divifion may be open at top, and needs not be fo [ (>3S ] lb high as the clofe air-veUll ; it is luilicicnt if it rcai-hc' a little more than four feet above the level of the dafli- board, the water, which it is defigned to receive, not rifing higher than this. In other rcfpeds, tlie flrudiirc of thefc jnachincs agrees entirely with that of the finglc ones already defcribed. It mull be oblcrved only that the cullenders of the lower machines fliould be, as nearly as polTiblc, of the fame dimenfions with thole of the upper ones. For if they are of fmaller bores, they will not admit of all the water whicii pall'es through the upper ones, fo that part of it mull run to walte : if they are larger, the water will pafs off too fall, without producing its due effedt. The regulators, defcribed in page 312, are here particularly ufeful, affording ready means of incrcafing or diminilhing the apertures occafionally while the machines are at work. Of Mowing macJj'mes 'with low falls of 'water. The dimenfions hitherto given are fuch as appear the moll advantageous. Much lower falls, however, than thofe which the foregoing machines are calculated for, as ten, eight, or perhaps feven feet, may be made to afford a flrong blaft. To produce fuch a comprellure of the air in the air-veffel, as to raife the gage four feet, a fall of about fix feet is neceflary for the lower pipe. l( the upper pipe is only about a foot and a half or two feet, the water, when divided by means of the cullender, will carry dovyn a certain quantity of air; and though the quantity, from an equal ftream of water, will not be fo great as when the fall is higher, yet, as there are in many parts of the king- dom, large bodies of water running with fuch a defcent, the deficiency may be compenfated, as already taken notice, by enlarging or multiplying' the tnachines. N n n n For [ 636 ] For many purpofes ftill lefs falls will fuffice. The fmiths bellows, as we have formerly feen, raifes the gage only about fourteen inches; and fuch a compreffure, it is prefumed, may be gained from a fall of five feet or lefs. Small flills may be applied alfo to another purpofe, of no little importance, the ventilation of mines and coal-pits, or the driving in of frefh air, in the room of that, which the mineral vapours have rendered unwholefome or pernicious. In all thefe machines it muft be obferved, that the height of the colum^n of water, falling through the pipe,^ determines, not' the adlual force of the blaft, but the greateft force which can be given it in that machine; that the height of the gage is always the meahire of the aftual force; that this force depends on the width of the pipe through which the air is difcharged from the air-veffel, and may be diminifhed, or increafed in any degree up to the greatefl that the column of water can refifi:, by widen- ing or narrowing the aperture of the pipe; that different r^achines will give blafts of equal force through pipes of greater or lefs width, according to the greater or lefs quan- tity of air which the water carries down with it; and that therefore the fize of the blafl-pipe rauft be adjufted by trial for 'each particular machine. The diftance of the dafli-board under the pipe may likewife admit of fome variation, and require to be re- gulated according to the fize of the pipe. In fome of the common machines, this diftance is three or four feet or more ; but fo large a fpace is apparently a difadvantage ; for fo much of it, as is more than fufiicient for the free pafling off of the water, is entirely ufelefs, being, in effedl, fo much taken off from the height of the fall. The diftance of fix inches, laid down in the foregoing machines, is defigned for a circular pipe of twelve inches diameter; in which cafe, the area, by which the water is difcharged all [ 6.37 ] , all round, is juH: double to the area of the pipf, and confc- quently more than large enough for letting the water off without impediment. Explanation of the Plate. The two blowing machines, rcpreilntcd ou the plate, are both drawn to one ftale, that the eye may judge more readily of their comparative heights and dimenfions. The fupports of the refervoirs, ficc. are not cxprcffcd, that the eflcntial parts may be the more dilUn<5t. The machine on the left hand is that of Dauphiny, de- fcribed in page 274, with a fall of about thirty feet. The other is a natural fall of twenty-eight feet, formed into two artificial ones of eighteen feet each; fee page 310 and 634. This double machine, though fomewhat lower than the other, may be prefumed to have twice its effcd, in virtue of the divifionj befides the advantage of the more free ad- miflion of air, and the fpreading of the ftream through a pipe of a much larger bore, by which it is enabled to carry down in its interfaces a much greater quantity of air. The dotted^ lines, in the upper rcfervoir, rcprefent a cy- lindrical grating of iron wire, to keep back weeds, &c. Thedivifion of the air-veflel,and thecourfeof the water from the upper machine to the lower, are apparent from the figure. On the rigl\-t hand is a perfpedtive view of the cullender, fcrewed to the upper pipe, drawn to a larger fcale, to fliew the difpofition of the holes. The holes may be made wider than formerly propofed, as an inch each fide, to pre- vent any danger of their being choaked up. Page 321. Black Diamond. I HAVE been favoured with a fight of this ftone, and am affured it is a true diamond. At a dillance, it looks uni- N n n n 2 formly [ 638 ] I'ormly black; but on clofer examination, it appears in feme parts tranfparent, and in others charged with foul- nefs, on which the black hue depends. Pnge 359. Indian ink from lamp-black and glue. Since the experiments on the compolition of Indian ink were made, I have met with an account in Du Haldes hillory of China, which feems to confirm them. He gives three receipts for the preparation of this commodity,. two taken from Chinefe books, and the third com- municated hy a native to one of the miffionaries. The colouring material in all of them is lamp-black, to which is added, in one, a quantity of horfe-chefnut burnt till the fmoke ceafes : he does not determine whether the wood- or the fruit of the horfe-chefnut tree is meant, but adds, from the Chinefe author, that if ufed in over-proportion, it inclines the black colour to a violet. The congluti- nating matter, in one of the prefcriptions, is a thin fize of neats leather; in another, a folution of gum tragacanth; and in the other, a mixture of fize with a decodion of certain vegetables to us unknown. The firft, viz. lamp- black and fize, which is that from verbal communication, is the very compofition which our experiments pointed out. As to gum tragacanth, it certainly is' not the con- glutinating ingredient in any of the famples of Indian ink. that have come under my examination, the vegetable gum not putrefying with water as the Indian ink does: if gum was really made ufe of, gum tragacanth fhould feem the mofl unfit for the purpofe, on account of its difficult and imperfcdl folution in water. The vegetable decodtions or infufions I cannot apprehend to be of any manner of ufe where fizc is employed, unlefs it fliould be to give a fcent to the compofition, in which intention, mulk and other perfumes [ 639 ] perfumes are fald to be frequently mixed witli it. The author obfcrvcs that the Chincfe have inks of diftcreiit goodnefs and price; that the moll cfTential diftlrence pro- ceeds from the quality of the lamp-black ; and that the befl lamp-black is the foot of oil, which is burnt ii\ lamps, in apartments fitted up for this purpofc. The Chinefe, according to his account, imagine the differences in the foot of different oils, &c. to be much greater than the experiments related in page 342 give room to believe they are. Page 369. Black varniJ/}J'or met ah. The workmen frequently employ for this purpofe, as I am informed, a mixture of lamp-black with the fcum- mings, &c. of different oil paints: the inixture is applied with a pencil, and the piece afterwards baked in an oven, with a heat fomewhat greater than is ufed for the papier mache. Naples yellow, a fuperfluous ingredient in thtr black varnifh, is the bafis of the dark brown which we fee on fome iron fnuff-boxes, this pigment changing to a brown in baking with the varnifli. Page 596. Si'paratiort of the alcali of fea fait . I . Purif cation of fea fait. Pure marine fait is a combination of the mineral alca- line fait with marine acid : but all the common forts of this fait have a mixture of one or more faline matters of a different compofition, their bafis, inffead of an alcaline fait, being an earth; which earth is generally the fame with that called magnefia, though fometimes, perhaps, it may be of the calcareous kind. I . Thcfe [ 640 ] 1 . Thefc falts with an earthy bafis are difcovered, by diflblving the marine fait in water, and dropping in a fo- lution of any alcaUne fait. The earth, of whatever kind it be, precipitates ; the acid, which held it diflblved, quitting it, to unite with the fuperadded alcali; fo that by continuing to drop in more of the alcaline folution, till it ceafes to occafion any precipitation or cloudinefs, we pro- duce in the liquor, inftead of the fait with an earthy bafis, a true neutral fait with an alcaline bafis. 2. In fome forts of marine fait, the acid united with the earth is the vitriolic. This may be known, by dropping, into a folution of the fait, a folution of chalk, or other calcareous earth, made in the nitrous, marine, or vegetable acids. The vitriolic acid quits the earth which it was be- fore combined with, and joins itfelf to the calcareous earth, forming thereviMth a felenitic concrete, not diflbluble or exceeding fparingly, and which therefore fettles to the bottom in a pov/dery form; fo that by continuing to drop in a due quantity of the calcareous folution, all the vitriolic acid may be feparated with the calcareous earth, while the magnefia, now combined with the acid in which the cal- careous earth was before diflblved, remains in the liquor along with the marine fait. 3. There is another method in which we can feparate the vitriolic acid, and this without communicating any foreign impregnation lo the liquor. Add to the folution of the marine fait, fome ftrong lime-\vater : the vitriolic acid unites and precipitates with the linne; and the mag- nefia, thus deprived of its acid folvent, precipitates alfo. Though this fimple procefs effedlually purifies the fait from the combination of vitriolic acid and rnagnefia (com- monly called bitter fait, or bittern) it does not anfwer fo well, for merely diilinguilliing that acid, as the foregoing method; lime-water producing a turbidnefs and precipi- tation in many liquors which contain no vitriolic acid. [ 641 ] 4- In many kinds of marine l^If, the Jictcrojcncous earth is united with tlic true marine acid: wc may always judge that this is the cafe, when the method of trial No. i difcovers that the fait contains an earth, and when the cal- careous folution No. 2, by producing no cloudinefs, flicws that the acid is not the vitriolic. The combination of either magncfia or calcareous earth, ^\ ith the marine acid, or with the nitrous acid if fuch an acid Ihould ever exilt in marine fait, I know of no other means of feparating, than decompounding it by alcalies as in No. i, or careful cryllallization. The combination of earth with marine acid I have found to be by much the moft frequent and moft conlider- able admixture in the common marine falts ufed among us at table. This compound liquefies e.ifily in the air, a well known imperfedion in the common forts of marine fdt; and on this difpofition to liquefy depends its being in great meafure feparable by cryftallization. The bay falts, cryftallized by the llow evaporation produced by the funs heat, have much lefs of this deliquiable fait, and hence are much lefs fubjedt to grow moill in the air, than thofe prepared by the hafty boiling down of the brine j though they generally have a pretty large admixture of the bitter fait, which cryftallizes as perfedly, though not fo foon, as the marine lalt itfelf. On this bitter lalt probably depends a property of the common marine filts, which has given occafion to fome milVakes in regard to their compofition. When common fait has been melted in the fire, it afterwards deliquiates very fpeedily in the air, though before it was of fuch a kind as to be little difpofed to grow moill. This does not feem to proceed from the fait being rendered alcaline, or lofing any of its acid, but from fuch a tranfpofition of its acids as we find to happen when artificial mixtures of the fame [ 642 ] fame ingredients are treated in the fame manner : the vi- triolic acid of the bitter fait, loofened from its earth by the heat, unites with fo much as it can faturate of the alcali of the marine fait ; and the marine acid, difengaged by the other from this part of the alcali, unites with the magne- fia which the vitrioHc acid has forfaken, forming there- with, inflead of the cryftallizable bitter fait, the very deli- quiable compound above mentioned. It has been found indeed, that common fait gives out a portion of marine acid, when folutions of it are haftily boiled down, or when the dry fait is expofed to ftrong fire : but the compound of earth and marine acid parts with fome of its acid in the fame circumftances, and Mr. Baume has fliewn, in his manuel de chymie, that marine fait, purified from that com- pound, does not. The purification of marine fait from its earth, by the addition of alcaline falts. No, i, how ufeful foever it may be to the faltboiler, muft never be had recourfe to in the prefent intention, unlefs we have an alcali exadlly the fame with the marine alcali itfelf ; for by whatever means we can disjoin the marine alcali from its acid,we ihall disjoin along with it this extraneous alcali. Nor indeed is fuch a purification anywife wanted herej for in feparating the acid from the alcali, we feparate it from the earth alfo, and the alcali is afterwards purified from this earth, along with the other earthy matter which it has contradled in the operation, by folution in water. For the two firft proceifes of the following article, it is fufiicient if the fait is well purified from vitriolic acid ; and for the third, even that purification is unneceflary. 2. Preparation of cubic nitre. The acid of common fait can neither be expelled from its alcali by fire, nor transferred from it, fo far as is known, to [ 643 ] to any other body. But though \vc cannot transfer the marine acid from the alcali ; wc can transfer the alcali,; from the marine acid, to the nitrous acid ; and from this laft acid we can feparate tlie alcali pure. The combi-: nation of this alcali with tlie nitrous acid is called, from the iigurc which it aflumcs in cryltallization, cubio nitre. I. Cubic nitre may be prepared, by putting into a glafs- retort fome common fait, pure from vitriolic acid, tho- roughly dried over the fire, and reduced to powder ; fetting the retort, on as much f^nd as will keep it fteady, in an- iron pot placed in a proper furnace ; pouring in thrice the* weight of the fait, of ftrong fmoking fpirit of nitre, witlii' care to avoid the fumes ; immediately luting on a large receiver, with fome water in it to promote the condenfa-f tion of the fumes ; and proceeding to diftillation, with ar very gradual fire, increafcd at laft fo as to make the bot-^ tom of the retort red hot. The marine acid, with part o£ the nitrous, comes over into the receiver: the marine al-; cali, combined with the reft of the nitrous acid, remains irv the retort. The mafs of fait is to be dilfolved and walhedt out of the retort with diftilled \vater or pure rain water,' the folution filtered, evaporated with a moderate heat till a pellicle begins to appear on the furface, and then fet iru the cold : the fait ilioots into cubical or rather rhomboidul cryftals,, generally cluftered together. Mr. Marggraf, in a differtation on the beft method of feparating the alcaline fubftance of common fait, found, that two parts of fmoking fpirit of nitre, of Uith llircngth as inftantly to fire pure oil of cloves, were fufficient for> one part of purified common fait ; but of the weaker ni-' trous fpirit, called aquafortis, he reconimends eight times' the weight of the fait. He fays the cryftals obtained withi tlie fmoking fpirit (for he does not feem to have actuallyj O o o o tried [ 644 ] ^ ^ tried the weaker one) were pure cubic nitre, wtlch defla- grated on a burning coal without crackling, and had not the leaft mixture of common fait. Some have reported, that though a pretty flrong fpirit of nitre was ufed in more than double the weight of the fait, the refiduum after the diicillation confifled chiefly of marine fait unchanged, with only a fmall proportion of cubic nitre intermixed. On what caufe the failure depended, the few experiments I have made on this head do not enable me to judge: per- haps it may be necellary that the nitrous fpirit fhould be very ftrong, for a concentrated acid may produce decom- pofitions, as well as diflblutions, which the fam^e acid, di- luted, is incapable of efted:ing. II. Cubic nitre niay be obtained alfo in the procefs of making filver into luna cornea, which is the mofl eftedual way of purifying filver. Solution of common fait in water being dropt by degrees into a folution of filver made in aquafortis, fo long as any cloudinefs enfues, the marine acid precipitates with the filver, as the vitriolic did with chalk in No. 2 of the foregoing article, and the re- maining liquor is a folution of cubic nitre, blended with, the copper which the filver contained. How far this copper may be injurious in the intention for which cubic nitre is here wanted, has not been fully examined. III. The ftrong affinity of the vitriolic acid to calca- reous earth afix)rds a method of obtaining cubic nitre, more eligible than either of the foregoing.. Spirit of fait Is commonly prepared by difl:illation with the vitriolic acids and in this cafe, what remains in the retort is a combination of that acid with the alcali of the.marine fait. This compound is common in the fliops, under the name ■gf Glaid't'rs fait or Jal mirabUe. If a faturated folution of fill mirabile be made in water, and a folution of chalk in aquafortis added by degrees fo long as it occafions any. cloudinefs j , [ 645 ] cloudlnefsi the vitriolic acid and the chalii precipitate tO-» gethcr, and the nitrous acid and mineral alcali remain in tlic liquor, which accordingly, on cryllallization, yields a true cubic nitre. The Iblutions ought to be well fatu- rated, that the milkinels, which grows fainter and fainter as we continue to add more of the calcareous folution, may be the better diftinguifhcd; and after the cloudinefs feetns to have entirely ceafcd, a little more of this l.ill fo- lution may be dropt in, for a fmall cxccfs in its quantity will be of no inconvenience, but a fmall deficiency, by leaving part of the fil mirabilc undccompoundcd, will oc- cafioa the mineral alcali, for which this proccfs is only preparatory, to be impure, as will appear in the following operation, 3. Separatiofi of ihc mineral alcali from cubic nitre. The marine alcali being by the above methods com- bined with the nitrous acid, the acid is to be feparated from it by deflagration with inflammable fubftances. Mix the cubic nitre with one fiftli or one fixth of its weight of powdered charcoal, grinding them thoroughly together : the coal of animal fubftances is preferable to that of vege- tables, as the latter will leave, after burning, fome fmall portion of an alcaline fait, of a diiferent nature from that which is here required. Throw the mixture, by a very little at a time, into a large crucible made juft red hot, covering the crucible, as fpeedily and as clofe as may be, after each injecStion, to prevent the matter from being dif- fipated by the llrong deflagration which enfues. When the mixture has been all thrown in, and the detonation has ceafed, the fire may be augmented, and a pretty ftvong red heat kept up for half an hour or more, the crucible during: this time beins; left uncovered. The nitrous acid being thus burnt out, there remains In the crucible a blu- O o o o 2 iili-greenilh. [646] iih-greeniili alcaline mafs> which is to be purified by folu* ition in diltilled water. It diflblve's more difficuhly than, .the vegetable alcalies, and on duly evaporating the folutioa •flioots into fine white cryftals, which do not liquefy in the •air. This laft property of the marine alcali tends to con- jinn the obfervation already mentioned, that the deliquia- lion of marine fait after fufion does not proceed from a •part of the alcali having been, divcfted of its acid. If the marine falt^ ufed for the preparation of cubic ni- tre by the firft and fecond proceffes, contained any fait with •an earthy bafis, or if the folution of chalk in the third way of preparation was employed in too great quantity, the ;cryflallization of the cubic nitre will in great meafure fe^ parate thofe deliquiable compounds ; and indeed, without cryftallization, as the nitrous acid is diffipated or deflroyed in the fire, it will leave with the alcali only the earth, which will be feparated, as well as the aibes of the coal^ ■by the diflblution in water. If the cubic nitre contained any marine or vitriolic fait, the marine fait will continue -after the deflagration unchanged, and the vitriolic fait will, produce with the inflammable matter a fulphureous comit- ■pound. I: N D E X u\^ . INDEX. A. A C I D, m.iririf, container] in moft ^/^ ip'i'ig waters — — — — 133 Mc.iriiof dillinguii'hing in liquidsijS PurilicAtioirof liquids from it 133,641 Gold difljlvcd in it — — — — g^ PiatinadilFjIved in it — — 472,558 Fornjs with earths x di-Iiquiible fait 641 Add, nitrons, Gold di/ToIved in it — 91 I'orm ■ with eirtiis a deliquiablc fait 646 Add.vilnolii-, diftingui(bed in liquids 640 Purification of liquids from it — 640 Forms with earths a crylhllizable Cilt 641 ■JBolipile, defcribed 20 In foiiie circumllances animates fire 2 1 . In others extinguiihss fire 21 \,T.therei>lffirU, Preparation of 143 Imbibes gold from aqua regia — 101 Does not imbibe platir.i -486 Its ufe in allay ing gold 142 ^j<»/c. Staining of it black — — 437 Diftinguidiing of natural from artificial Ila'.us — — — — — 43g Drawing of regular figures on it — 439 Producing vegetations on it like mocho ftone 440 ^y<>, influences folution and precipitation 93 Its power in changing or d^Mlroying colours — — — 378 Probably abforbcd by metals in calcina- tion 461 Air, different means of fupplyir.g to fmelting furnaces 267 See Bello'jn and Bloivitig tnacbi/ies Experiments of its pa'iing down through pipes svitb fUiing water — — 201 General principles of its pafling dowik 2y6, 29g Gage fnr Ihcwing its degree vi com- picfTureiii blowing tn.ichincs — jSj Quantity required b/ large finc'tii^ furnaces 282,286 Q^untity required for fmithslorgc 286 Dilfcrcnt force of the blall for d^llcrent fo,:ls 236 Vapour from the lolipilc whether ca- pable of fupplying its o(li--e for lur- naces ' — -^ 21 Means of renewing in mines, Jtc. 27} Its comparativegrivity to water — 43 Apparent weights ot bodies how influ- enced by variations of its gravity 4"} AJt.ili, minern!, native — — 596 Method of extrafting from fea fait 639 Obtained from certain vegetables — ^96 Ciylhll^zcd : 646 • Precipitates moll metals, like vegetable alcalics — 59^ . Does not precipitate platina — 483, 596 Ahbertfj, hirtory of — 199 Its principles — — 200 Its prctenfions inconfillent with its principles 201, 202 ■ Deceptions in regard to the tranfmutabi- lity ol b.ife metals iniogold93, 96, 203 Frauds herein zoa Alloy of goM 114 How far ufcful 114, 1 1 5,620 Its proportion in dllFerent coins — 1 16 Amalg/imation of gold 7^ Of platina 508 Of iron — 50S Platina and gold fcparaled by— — 591 AJiil for grinding amalgams — 578, 194 Ami:rt. I N D Jmiir, mei.]\oi ofdiflblving inexprefled o'ls 367 More effcdlual method of diiTr.Iving ia oils 368,606 Advantages of this folution — 369,61 J Varnilhes 366, 614 ^n,tcardiim. Staining of linen with its juice 3^8 jinnotlo. Its preparation — 224 Its ufe in gold coloured varnidies 225 Jntimi)nt, Refining of gold by it — i ;6 Does notmakegoldcompletelypureijg Platina treated with it ;6o Platina witli its regulus 521, 559 Jjuaforlls, Preparation of 13; Purification 138 Adjullment of its (Irength for the part- ing aflay 139,140 Recovered after parting 152 Recovered from folution of gold in aqua regia 99 yip.i regia, different ways of preparing 96 DiiFerences in the volatilizing power of different aqus regi.-e lOO ^/■.T'W, gives durable blackilh ilaia to mar- ble 43^ Jiffefiic, with gold 87 with platina S'S'ISQ Method of uniting with ditSouitly fuli- ble bodies 539 4fj)'''g of gfi'd '27 Cupellati jn 129 Parting vjkh squafortis 135 Further affay, lor diicove'ingpLitinai 42 Quantity allowed for an zK.y 127 Alfayers Report of the line.iefs — 141 Jjirir.ient m«ter of vegetables, its general properties — 345 Phenomena on raixturi with grijen vi- fiol ■ ■ 3+^ Difurenccs in the phero^vcna produced by ditlerent altringents — 382, 418 Aurumfdminnns, Prcpuation of 104, 10; Its explofive power compaicd with that of ganpowuer 106 Conjcdure on the ciufe of its llrong aftijn on the body which fupporis it 107 E X. The gold recovered — — — 107, 108 Atirum mujtvum. Preparation of — — 222 B. Balances, obfervations on — — — 543 Equality of the arms not necefTary for their exaftnefs J43 Adjuflment of the weights — — 544 BarigUa, of the faoie nature with mineral alcali — — — — — — 595 Bas relitfi of gold, made on works of gold or filver — . — 80 Baths, themical, their ufe 31 Sand bath -— S Water bath — — — — 9 Imperfections of fand and water — 31 Mercurial bath •"■3* Compoand metalline bath — — — 32 Bellows, for large furnaces — — — 268 for froall fuinaces — — — — 18 Wooden bellows — -^ 268 their inconveniences — — _ 269 Water bellows 269 See Blowing machines Quantity of air afforded by bellows of iron furnaces — — — 282 by bellows of the iron finery 282 Quantity of air retained in them whea clofed 283 Made to deliver nearly all their air 283, 284. Made to give an equal blaft — — 18 Made to move with lefs force 20 Blj'muth, melted with platina — — 509 Worked on a cupel — rya. Agreement with lead 515 Bijlre, Preparation of — 340 Black bodies receive heat from the fun more freely than others — — 313 not from common fire — 5455 BlackncfsprodueeJhy ab brption of lights 1 7 Qy Itrong reflexion of light 318 By the concentration of other colours 320, 441 By the combination of other coloprs 420, 441, 442. 354 By the mutual aftion of bodies on mixttire — — — — 344 I N D By fire , 33 J By air and fun 329, 350, 438 Sliiii, diluted, becomes grey 31a Means of judging of (he fpcLics oi the colour 319, 3:0 Black nutiira! fiihflancet — — 320 Minci.ils 320, 323 Vcgetahle juices — . 322, 329 Animal juice 331 B'tuk Comf^Jitiotis 3-5 Enjmcl ^^2 Glals 4m Inks — 3-1 Paints 356 dealing wax ^-q Varnifhcs 366, 639 For marking flieep 3:9 For preftrviiig wood 362 For blacking Icalhtr 365 3'ack Dye ^c-i on Cotton J.1* en Hats ^..g on Linen 433. 4^9 on lii'k 422 on Thread 4.3 3, 429 on Woollen ' — 401 Jl!aik Sljins 45. on Agate ^]s on Boiic ^-^r on Hair '43 5 on Ivory 435 on Marble — 456 on Wood 434 Si'jmify Vearths for running down iron ore 266 Biixiiig inacbiitcs by a f.i!l of water with- out comp:eI!!ble bellows — 269 at Tivoii — 270 in JJaupliiny 27 1,2-4 on the Tibur:inc mountain — 272 a: Lead hills in Scotl.ind 3-3 in the tenitcry ot Foix 276 in Li'iguedoc 277 Of ihequantiry of water tliey rcquirc279' Qiiantiiy of air afforded by ihcni — 2S0 Means of determining the turce of the air 2S5 Of the iirudlure of the air vclfel — 2S6 Ifielhad of drying the air — — — 290 E X- Expsrlmer.li l,t iniproving the mi- chinci 2-,^ for afcrrraining ihcir principles of ai'Hon — — — _ — — JO, (or regulating the flrudjuie of (he funnc! and pipe j^g for increaling the effeft by diviJiig the llreain — 303 Dcfcriptioii if a machine deduced Irjni the ixprri.nenis 310 Machines lur la Is o( great height — 652 /or low (alis 635 Blm-fipt for concentrating flame 19 \U ufej .0 Blue ^!/ifi, with zjfTrc 343.441 vviihgold C19 B'.ut Upon, from oalc duU and vitriji 352 1 roni logwood and virdtgris 407 Irom logwood and blue vitriol — 4:8 From an clfeniial oil and volatile (piiic ^ 436 Bme fljinccl purple 97 lUincd black 4:5 Br'afi melted wiih (latina — 532 Made harder and lofb difjiol'cd to tar- „i(h 532.533 Breath human, its f^-rcc 73i Broc.idt!, method of fpinning gilt wire for them 61 Slips of gilt p^iper ufed inliead of gilt thread 62 Inconveniences of the p.ipcr — — 62 ■ The;^old extended by palling the llnfF between rolls 226 Method of cleaning when fullicd — 39 Burnirg-glifi, flin; melted by it — 46; Gypium melted by it 465 , I'latina melt.'d by it 467 tfTetfls faid to be produced bv it on Examination thereof — 71 Whaenefs of bodies impedes its aflioiri on thciii 31-^, 466 Does not niclt poHQicd filvcr — 46b C. Ca/jKire, in ore of zinc 5'9 ■ Means ol cxtrafting the zinc pu'c •; 19 . Metliod.i I N D .Method of combining its zinc with dinicultly fufib'e metals 519 with pktina 520 CahinaticTt, increafe of weight produced by it 465 in iron — — 55* in niinera! plaiina 459 in .1 mixture of platina and tin 558 ki a mixture of gold and tin — 618 Conjeftuie on thecaufe of the increafe — 461 Cai-tJt weights explained — 1 1 J Cri/ls frjm rnedals, a matter for tai— 440 Dillblved by vitriol of iron — — 390 Precipitation of gold by it 109 Glafs coloured with it, on being baked with white earth, gave a red tinge to the earth — — 247 Cctton, Rained black 433 purple 97*434 red 434 Crane, air imbibed by water paffing throngli it 291 Quantity of the water influenced by the fucking pipe being of en or clofe zQjf Crafcnr, Charcoal ; — jjj Differences of coals for crayons - — 337 Excellence of that of willow — 33^ Black lead J2g Different forts, how made — 32S Beft way of obtaining good ones 329 Cruiibki, nrethcd of preparing fo as tb Hand (ulphureous mixtures — 156 Black-lead, their advantages •67 whether hurtful to gold 616 made into portable furnaces — — 2 Cupels, method of preparing — — 129 CupcUotiots, furnace for — r \Z Prii cipic of the operation — ga Method of cor.diiiling the procefs 131 Proportion of lead — — -^ — ^\%% I'Jarks for regulating the lire 132. 133 Gold not made quite pure by cupelia- tion ,14 EtBorefcence of filver in the cupel— 260 Cuttle bone, its ufe for taking iinpreiU'jns, &c. 33} Cuttle ftp ink 334 D. Diamond, its foil — 3,21 OBS faid to be black — — 321, ^37 DiJIillation, I N D DiJiiUaticn, apparatus for 9 of aquafortis in iron po'.i — 136 of aquafortis from metallic folutions, in ftone ware veffels — 153 of acid fpirits in giafs retort* 64.3 of Kther 143 DjedClotbs, method of comparing famples of 406 Dying, apparatus for experiments in — 10 Dying black, on woollen 401 on filic • — 422 on hats — 428 on linen and thread — 429 Differences io woollen, filk and linen, in regard to their receiving this dye 426, 427, 429 EiVential differences of the d)ing liquor from writing ink 401, 407 Neceflily of a blue ground, or of log- wood, for woollen 404, 407 Means of difcovering whether the cloth has been blued 404 Whether maddering ufeful 40J Improvement of the dye by verdegris 411, 429 Effeft of the verdegris on the other in- gredients — — — 407 Whether cloth weakened by this dye 402 The dye made to take uniformly — 409 Subflitutes to galls 416 Black dye from combination of colours 420 E. Enrtbi, feparately unfufible, brought into fulion when mixed 245, 618 EteHricity, colours imprefTed by it on glafs, from metallic bodies — — — 175 Emery, Spanifb, probably platina 607 Common, its effeds on lead 608 En.mcl, bafis for — 442 Tinged black by concentration of other colours 441 by combination of colours — 442 Black imitated on dial plates of watches, &c. 370 Hillory of tinging it purple or red 170 Preparations of gold lor colouring it 176 E X. Proccfj of colouring it wiih gold 621 Enamelling furnace — ,|i Gold plates for enamelling 620 EiLir, fee ^tbir E.xp/infuin, does not al>va)s proceed uni- foimly by equil au^meniaiions of lunt 256,260 Some fluids cxpind in congealing 757 wa'.cr, in freezing — —^j-j gyplum diluted with water, in fciiing — 259 cafl iron in beconiinj folid 26a Exttnfibilitj of metals, compaiaiive, how influenced by their gravities 61 F. Featbiri flained pOrple — — — — —97 Fire common, not rcflcfled from elliptic fur- naces into a focui — — 26 Not animated by the elaflic fleam cf water 21 Concentrated bv ftreams of air 26 A furnace for this concentration of fire 27 Its differences from folar heat in pro- ducing colours — — 350 in the burning of black b::dies 3 iS Fijbes, method of gilding on them — 64 f/ijw# concentrated into a kind of focus 19, 26 Flalting-miH for gold wire "~S7 Flint melted by a burning-glafs — • — 465 Frankfort black — — — 376 a vegetable coal — 377 Freezing expands watery fluids 258 Contrails oily ones — — 25S Fuel; eta ke. I lilt coiil, its advantiges — 246 its inconveniences 569 Furnaces, defcription of a fct of portable ones for experiments '> f"3 For open lire 7 For a find pot — 3 For a flill 9 For enamelling — 12 For cupellation — ~ — i2 P p p p F.r I N D For making fmoke and fumes pafs down lluough the grate 79 For llrongfire «6 For fmeltiiigores, &c. among the fuelzi Wind furnace i i, 24, 569 Blali furnace — — 18 .AifSion of wind and biaft furnaces com- pared 25 Alijy turnace — — 12 Rei/erberatory 13 Lamp furnace 2S Fufibilily of metals increafed by mixture G. 6ti!h, their pr-rduiflicn on oak trees — 344 Difference in llrength of the bluifh and whili/h forts 344 Phenomena on mixture with green vi- tiiol 346 S'ubftitutes fur in dj irg — 345, 416 Gilding on paper, wood, &c. 62 on books — — — 64, 61; on glafs 65, 100, 614 on live fiihes 64 on copper, with amalgam of guld — 77 mejns ofcoikiling the mercury, and preferving the workmen from :ts fumes — —r- y-g of heightening the colour of the gild- ing - ',8 on iron, w i.h the amalgam 81 on iron, uith folution of gold — 108 on copper, earthen -ware, &c. by regulus o! antimony and gold — r- 88 Cold gilding — . . 93 Gilding imitated by .a varnifh — 225 Alcans of difcovering this cou-nterfeir gilding r — 225 Cildijig wax 78 QJaf:, its general compofition — — 245 From fiinple earths — 24;, 246, 618 F,'om metallic bodies -^ 245, 247 From. earths with fahs . 246, 248 EfFefts of cementation on the different forts of glafs ,. — 231, 246 earthy and metallic, ories not altered — 246, 630 green glal5,from earths with fait, changed v^v pprceUm . — -— — 230 EX. fucceflion of the changes — — — zjj of the caufe of the change 2^1 efFefts of different cementing materials — — — — 241 of the qualities of the porcelain — 236 Gltfi, a very fufible one ^42 Tinged blue, by gold 619 Black, by manganefe — 442 Black, by iron 4^1 Brown, by iron 441 Green, by iron ^ 621 Gold coloured.by antimony and lead 626 With gold coloured fpanglcs, by yellow- talk — — 223 Yellow, by earths 627 Yellow, by iron 626 Yellow, reddifh, &c. by phlogiilon 628 Purple, by manganefe — 173. Purple or-ruby coloured, by gold 171 liiliory of the ruby-glafs — - — i 70 prepaiitioivs of gold for it 176 expciimenis on a ■ glafs tinned-, by gold ^. 1 7. 9 method ofpreparing the ruby giars62i GLtfi Ifained \ ellow without fufion 626.629 Klethod of gilding on — 65, icc,6i4 Glazing, gold coloured 627 Glue, common and fifh-glue, difference in the mifcibiliiy of foktions of them with certain liquids 387 Gold, its mineral hillory iSi Where chiefly found — 1S2 Sometimes in large malTes 182 In the fands of European rivets — 1 84 Dilftminatexl through various earths — —r 'T : 1S5, 623 Thence wafiied out by,rivers ^ 623 Marks of the lic-hefl fpots in rivers 1 S6 Found in Britain — — — 187 Intimately mixed in moll fands — . 1 g-g Gold, Separation from fands, &c. by wafh- Jng 1 90 Separation from earths, &c. by quick- filver 193 Extradlion from fands by fufion — 19 j Extraflion from the ores of other me- tals 1 99 Extraction from large proportions of fil- ver .— .161 . of copper 165 E-2i I K D Extraflion from large proportion!! of iron i68 Means of readily difcovcring it when dilTolvcd in acids ic2.i to Dillinguifljed from other yellow me- tals — 123, 134. Very minute quantities in minerals, in lome circumllanccs extraflcd to ad- vantage 164. Not fcparable from copper, us filver ii, by eliquation with lead 1 67 Scpjraiion from gilt works 168 Co'i!, ufe of mixing alloy wiih it — 11 + Metals fit for its ailoy 1 1 5 Engli(h (landard gold 116 Crnvn gold — 117 Way of accounting its finenefs in diffe- rent countries 115 Finenefs of different coins — 1 16, 117 Latitude of finenefs in the Englifh coin- age — — J16 Its propottional value to filver in diffe- rent iiations u3 Go/J, of judging of its finenefs from its weight 125 from its colour on the touch ftonei 19 AHavingnf gold 127 Refining of gold on the tell 144 by aquafortis — 147 hy cementation 154 by antimony — i ;6 by vitriolic acid - — — — —-9; by vitriol — — — \^g Purification by nitre 63 by fublimate 70, 94 Dirterent means of purifying it from pla- tina 590 Separated from bafe metals by calcination and amalgamation — — 89 Means of avoiding impofition in the Chinefe gold trade— • 124,126 Purification from emery 147 GoliJ, Recovery of its ludre when fu]iied3R Its colour heightened — 83,68 Made pale 68, 78, 1 1 2 Obfervations on melting it 67 Mails of its finenefs and toughnefs in fufion — — 6q Of preferving or refloring its toughnefs 67 Whether black-lead crucibles injurious 616 Solden for if 435 Hals, method of dying black 428 He.jf, means of communicating an equal one to different fubjefls — 31, 32 Produced by the mixture of two cold liquors — 137 Horn flained black 435 I. Jee, lighter than water — • — — _ 257 E X. • Becomes ftill lighter from a continuance of cold 25S Arrangement of its parts 258 Imprejfi'jns from medals, &c. matters for taking 333, 354, 442 Matter for making feals from imprefiions in wax 7^ Ink, Indian, its analyfis 358 Imitation of it 3 59 How faid to be made in China — 638 Ink, priming, its preparation 371 Ink for the rolling-prefs, its preparotion376 Differences from printing ink 376 Inks, fympathttic, made vifible bv fi/e 3 1 8 by paffing a liquor over the writing 31; 3 by expofure to fumes 353 Ink, hlatk writing, with vitriol and aftrin- gents 361 Phenomena of the produflion of the co- lour 346. Properties of the colouring matter 347 Analogy with Pruffian blue 34S Experiments for determining its bell preparation 362 Coropolition of ink deduced from the experiments 392 Cauies of its fading or changing colour 380,38.;.. Its colour difcharged 349 Improved by keeping 386 Injured by fome metalline veffels — 390 Made to write black at once 392 Duration of writings prolonged by pre- paration of the paper — 393 Ink from more duiable materials — 396 ■ Ink of the ancients 399.334 - Iron, brought into fufion by gypfum joz, 534- Malleable after fufion jj^ Melted with gold 82, 8; Melted with platira 534, 552 Improved by platina — 536 Soldered by gold — ^ gz Amalgamation of it with mercury 508 Experiments of its expanfion in fixing — 261 Increafe of weight in calcination — 552 Variations in its gravity from its having jriore or lefs phlogifton — — 552 Colouis I N D Colours it gives toghfj 441, 6zo.( zd iiubflinces whirl) tinge diirolvcd iron bljck 345 General properlics of the bhtk matter 347, 348 Colour docs not fuccecd unle(s liic iron is difTulvcd in acids 349 Subflancc which tinges dillblvcd iron blue — — 348 Means of obtaining a faluratcd lolution ol this fubllance — — 484 Method of gilding iron with amalgam of gold 81 with lolution of gold io3 Jutia bianco 4^3 li'ory Aained bhck 435 purple 97 K. Kellou) 324, 32 J L. Lac 225 Solution of it in fpirit for varnill)es224, . — 624 Laces, gold, method of cleaning when ful- lied 59 Separation of the go!d from them by fufion 165 Lamp, improved 28 Lamp furnace — — z3 Lamp hl-iick, method of preparing — J41 A liner fort 342,639 From pitcoal 342 DifFerfnces from other kinds of foot 341 Difference from charcoal as a pigment 3^7,376 Lead, melted with different proportions cf platina — 512 Changed blue, purple, &c. — 516, 517 Agreement with blfmuth -~ 5 ' 5 Cupelled with platina "J 61 Melted with emery 6o3 Lead, b.'ack, where found — — — 32 j Its ufes 2, 326, 328 Difference in goodnefs — — — 326 Its analyfis — — — — 3*6 E X. Its llaining quality dcflroyedby fiie 327 Greatcll p.^rl kI it volatile in the f)iti2f> How made into pencils 328 Genuine penci's dilUnguidicd fiom counterfeit — 328 Lt/ifgoU, the procefs of making it — 4 j Its fizes — ^o Calcuhti'jn ol iis ihinncfs ~~ ^19 Green gjld leaf ^o Party leaf 51 Method > of filing it on other bodies 62 Linen embelliHicd uith tljivcrs of it 226 Leather, (laincd black ■>^C<, puiple .■'97 i/gi/, blacknefs produced by itsabforption 3,7 by its ilrong rcfljdioa 318 Lightening, wood prefcrved from, by a coaling of tar and lamp black — 564. Linen, cmbelliflied with gold leaf — 226 Stained black by vegetable juices 329, 530, 531 Dyed blaci; 429,433 Marked of durablecolour by fliet — 332 Stained of bufF colours 433 red 434 purple 97, 434 Lcgamcd, i's ufe in ih? bhck dye — 406 in writing ink 382 Produces blue and black with verde^^ris 407 Colour of its decoflion improved by keeping 411 M. MaJJer, produces a black dye on blue cloth —420 Produces a black dye on yellow iron fia'n . 433 Produces a purple on dilute iron flain 433 A boiling heat injurious for the red and purple, ulcful for the black 421, 434 Manganef;, colours it imparts to glafs 174, 44 1 Increases the fufibility of glafs — 44* T N D In calcination with nitre, tinges tV.e cru- (ible and its I'upport aincthvft co- loured 493 MaiUc, ftained black 436 violet or purple 98 Med/)h, a matter for taking cjfts fiom 354 Mtniltuum fine Jlrej'iiu 97 Ulerairf, amalgamated with gold 7; Means of preferving the workmen from its fumes in gilding 78 Amalgamated with platina 508 •Has greater affinity to platina than to lead 577 Has greater affinity to gold than to pla- tina 579 Precipitation of gold by it no Extraclion of gold and filver by it from earths '93 from bale metals 89,167 Minei, of renewing the air in them 273, 636 Modojlone imitated ■_ 44° Mortars, ^Ak, hardened into a fubftance like porcelain 240 iW»jJ7^ defci ibed ; — 130 Supplied by a crucible laid onits fide 12 N. llilre, its ufe in purifying gold 69 AV/'f, cidi.', preparation of 642 Separation ol marine alcali fiom it 645 O. Oik jnzO'iujI produces a blue with vitriol 382.383 SublUtuted to galls in dying 419 to oak bark for tanning 382 Oih, made mifcible v.ith water 397 0»/<, ejfintialy feparategold from acids 102 Ot thyme, changed blue by volatile fpirit 436 ■Olli, exprefet'.f'cmAQ to dryfoon fcr paint- ing 3i7. 37« jDifFerences in their dr^^ing quality 371 E x: Conjeflure onthe CTJife of their diffe- rent difpofition to dry 374 Differences in their diflblving amber369 Deprived of their property of growing thick in the air 375 Alterations which they undergo from fire— 372, 374 Reduced to a thick varnith 371 Ores, method of melting among the fuel 21 P. Paint, bliid, with oil — 356 with ivater — 337 Paper, method of preparing fo as to pro- long the duration of writings — 393 Papier macVi, how made 366 varnilh for it 367 Parchment, prepared fo as to prolong the duration of writings — — — 395 Parting ofgold from filver 135 Bell proportion of the filver to the gold in the way of aiTay 1 2S in the refining bufinefs — 148 Method of conducing the procefs by the afTa-yer 13^ by the refiner — 148 of fecuring the glafTes from breaking of pieventing any gold from being dilfolved 03 Does not make gold compleatly pure 96, 143 Pencils, black lead — ^28 Different forls, how made 32S True dillinguillied from counterfeit 328 Phofpbo'iis, with gold 1 1 1 with platina .qj Separation of its acid Any PiiJra del inca, what — 607 Pitcoal, as a pigment 324 Plajler of farts, fee Gypfum Platina, a metal of the fame gsnu? vsith gold and filver 610 Stands all the teds by which gold is allayed 44J Goli I N D Gold aduIteraleJ wiili it — 445, 6c ; Its mineral hillory 605 IJilloiyofits iiitroduclion into Europe — 442 Suppofcd to be contained in fonie ILuro- pein minerals 6o3 hence the ufe of fuch minerals (or the augmentation of gold 6o3 Hillory of experiments mad; on it by different pcrfons ^ J5 Platinn, Defcription of — ■ 449 SubUanccs mixed with it 453 Quantity of gold feparated from it 596 Malleability ^jjj Gravity, nearly equal to that of gold452 Kftcfls ot fire on it 456 C«ini-weight in the fire 4;9 the increafe oa jng to its 3dniixturcJ46 l Not fufible in fires that vitrcfy all com- mon vefTeis and furnaces 458, 462, 463, 468 Melted by burning glafs — — — 466 Refills the fimple aci'ls, equally with gold \i9fil- DifTolvcd, like g''ld, in certain circum- ftr.nccs, b.' marine acij — — 472 D:(r3lved, like gold, by aqua re^ia 47; Quantity of rr.enllruum necelTary 476 Cololir of the lolution, and trials for llaining 478 Volatilization of the plalina 4S0 Cryftallization 479 Precipiiation by different alcalics 481 Not precipitated by mineral alcali 4S3 Precipitated in part by falamm' niac 48J Not feparared from its foluticn by vi- nous fpirits — — 4SJ Not imbibed by eflential oils or xther 485 Produces no puiple with tin ^ij Precipitates expo'ed to burning gIafs4S3 Gl Is melted with the precipitates in firing fires — JC3 Its particles agglutinated by file into a metalline lump 4-,7, 505, 570 Expofed to llrong fires with various fluxes 489 Divided, and an irony matter feparated, bv long cementation with nitre 491 Difl^lvcd, fparingly, by fulphurated al- cali 50i Unites intimately with all the common metals ^- 54' E X. Strong fire necenary for its fcifcfl f^ lution by metal; 540 Pioduccs with them conipoundsof new quilities 539, i^^o Fxamination of the fpccific gravitic! of the coir.p- unJj ^^\ The gravityol plalina not deducible front that of any mixture of it with other mcta's 55; Amalgamation of it with mercury jcS Full n wiih arfcnic — 51 ; iiifmulh — — — — 509 Urals 53Z Coppi r 521; G.IJ 525 Ifon— — — — — — — 534. Lead 511 Ri'gulus of aniimnny — 521 'iin jio Silver — — 522 Zinc 519 Calcination wiih tin — — 557 Expofed to firorg fire with rcgulus of aniimnny 559 wiih .-.rfenic — — • 559 wiih zinc 561 Cupellati n uiih lead 561 N^i^h bifmuth — 573 Retains in cupdbii n f. me leid and bilmutli, fo as to prove brittle j6 1 freed roni them and made malleabit by comir.ucJ Itroiig fire — jt^ liiiiitations herein — 571 Experiments of its greater afiinity to q.e n,7f, method of miking 145 From mineral Hones-— — — — 1 • « Tejlii.g of gold and filler, procefs of — 1 46 Ibernoneten, made with difTorent fluidt, not coniparable from twofixt poinui 57 A mcrcuiial and (piiit one ir.adc to cor- relpond — 256 Not influenced by difference in theglafi 256 Thread, common method of J ;:-g black 429 Propofal for a more durable black 453 Tiles, painted black in imitation of black glazed ones 362 Tin, Its efieiflj on gold 8; Gold coated with it by boiling in tin ve/lel with alcali 39 Precipitation of gold by its folution 1 ic, 176 Produces a gold colour with copper 624 Melted with platina in different pro- portions — 510 Calcined, &c. with gold — — 89, 617 with plaiina — 557 Toucb-needlet, manner of preparing them — "9 Touch Jlotie, for metals — 121 Diic(fiions for its ufe — — 122 J ts requifile qualities 121, |2J Of what genus of Hones 620 ToxiccJendra, juices from them which flain linen of durable black 35c>33' Trees, conjeflure on the caufe of foine re- filling, and others being burll, by ftronK frolls 258 V. Varnijbei, Amber 367, 614 Black, for japanning, &:e. 366 hard, for papier machc 36b Q.q q q t'arniifbii, I N D nr/iijh/i.'^ck, for metals — 369, 639 of C^l^a and J.ipan 329 a ii;nTlaV one from plants railed here 1 330 Erown for njetals 639 Gold coloured 223, 624 Printers, ink for • 372 Fi'getalio/is of metih in the cupel — 260 from metallic loluiiofl.s on Hones 440 Veriiegris, improves the black dye 407, 41 1, 429 Debafes black ink 3S9 Produces blue and black with logwood Precipitation of gold by it 109 Kerditer, \h preparation 151 V(Jfeh, glafs, made harder and more ca- pable of bearing fire, by baking — 240 Vitriol, gre&n, how prepared in England — -,- 3+5 V/hether Englifh or foreign prefeiable in' dying — .•^- 408 Mirks of its gooSnefs ■ . — . — 409 Phenomena on mixing it with allrin- gents ^346 Calcination of — — 135 Its ufe in purifying gold — ■.- 1 09 VitriolU acta, hrow didinguillied in liquids Purification of liquids from it — 640 Uva urfi, asanallringcnt indying4i6, 418 Differences from other aflringents 418 W, Wadt : 325 Jfattr, its bulk increafed in freezing 257, ^ 258 Mark of fmall degrees of alcalefcence or putrefjclion in it — — ' 346 ■ Its vapour does not animate fire — 21 Weight of a cubic incVof it 43 its expcnce through different apertures at different depths -. ^ 279 Runs filter through an upright long than fJiort pipe ^ 632 Carries air down with it in falling tljrough pipes ■ — 2g\,feg. Account of blowing machines on this principle — 270 Experiments for their ianprov^roent 279 F / JV E X. iVix, gilding — .— — ——._- yg^ Black fealrng -^ — 370 Weights, fmall ones how obtained exact ,544 U'i/low, its excellence for charcoal crayons whence — , — 33^ U'ire, gold or gilt — 51 Procels of gilding and drawing the wiie — 53 Method of flatting it 57 , Method of fpinning the flatted wire on thread ~ 71 Obfervations on the firienefs of the gold ufe4 for it -^i — — " 51 Proportion of the gold to the filvcr 53 Calculation of the thinnef; of the golden coat : — — —^ 59 The filver extrafied, fo as to leave a tube of gold •. — 60 Wire gilt only on one fide ■■ 61 Colour of the gilding altered by heat 56 Method of nealing the wire — 55, 5!i wire from platinaand gold 326 Wire-drawing plates, of fteel 53 Of Lyons metal, their qualities — 54 Iron the bafisof the Lyons metal — . 54 Wtii)^, ilained black — 434 Stained purple— — — 97 Cbmpofuion for preferving — — 36^ In fome cafes damaged by painting 36; Wool, method of cleanling from its natural greafe —'■ — — ■ — 415 of dying black 415 Woollen dotl, procefs of dying black 410 of dying grey — 413 flained purple — i 97 Writing black with colourlefs liquors 39; Sympathetic, fee ink £ilt letters — — — — 64 Z. Zaffre, colours it gives to glafs — 441 Mark for diftiftguilhing the minerals fit for making it — — 343 Z/w, way of extrafting pure from its ore, and combining with metals 519 Melted with platina 5"^y. a^^ ^Mmm:^