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they may be made much fhorter than thofe of filver, and afterwards foldered to plates of copper, that may be fuffici- ently long for ufe.
The ufe of thefe Needles is by means of the touchftone ; and arifes hence, that every metal when pure mull have its fpeci- fie colour, that diftinguifhes it from the reft : but metals be- ing the moft opake of all known bodies, the fpecific colour of every one appears moft diftinctly when you rub it againft a very black hard ftone ; and if the colours of two or more metals are expreffed by large lively fpots, made near each other on the fame plane, by rubbing them againft the furface of the ftone, you will by that means cafily dilcern their differ- ence, or their likenefs.
The ftone adapted to this ufe, and called from its office the "Touch-fane, muft have the following qualities. It muft be of the deepeft black, left the tincture of the metal fhould be altered by fpurious rays of light mining among it : it muft be capable of being pretty well polifhed, for when too rough, the colours of the metals rubbed againft it cannot be neatly or regularly diftinguifhed j and if it is too fmooth, the metals are but faintly, and too flowly abraded or fcraped by it, efpecially when gold is tried. It muft alfo be neither too hard, nor too foft. Tripoli, coal-duft, and tin afhes are ufed in rubbing off" the thin metalline crufts, and in a fhort time the ftone when very hard is apt to acquire too fmooth a fur- face ; and when it is too foft it eafily wears, throws off a duft, and contracts furrows.
The black rough marbles, and the foftcr black pebbles from the beds of rivers, are moft proper for this ufe, and are to be made into the form of a quadrangular prifm, about an inch thick, and two or three inches long.
The method of ufing your Needles and the ftone is this : When you meet with a piece of metal to be tried ; firft wipe it very well with a clean towel, or piece of foft leather, that you may the better fee its true colour; for from this alone an experienced perfon will in fome degree judge before-hand what the principal metal is, and how, and with what debafed. Then chufe a convenient not over large part of the furface of the metal, and rub it feveral times very hardly and ftrongly againft the Touch-flow ; that in cafe a [deceitful coat or cruft fhould have been laid upon it, it may be worn off by that friction. This however is done more readily by a grindftone or fmall file if you have them at hand. Then wipe a flat and very clean part of the Touch-Jlone, and rub againft it over and over, the juft mentioned part of the furface of the piece of metal, till you have on the flat furface of the ftone a thin metallic cruft, an inch long and about an eighth of an inch broad : this done look out the Needle that fecms moft like to the metal under trial, wipe the lower part of this Needle very clean, and then rub it againft the Touch-fane as you did the metal by the fide of the other line, and in a direction paral- lel to it. When this is done, if you find no difference be- tween the colours of the two marks, made by your Needle, and the metal under trial ; you may with great probability pronounce that metal, and your Needle, to be of the fame al- lay ; which is immediately known by the mark engraved on your Needle. But if you find a difference between the co- lour of the mark given by the metal, and that by the Needle you have tried ; choofe out another Needle, either of a darker or a lighter colour than the former, as the difference of the tinge on the Touch-flone directs ; and by one or more trials of this kind you will be able to determine which of your Needles the metal anfwers, and thence what allay it is of, by the mark of the Needle; or elfe you will find that the allay is extraor- dinary, and not to be determined by the comparifon of your Needles.
But if the metal under trial, has been altered by tin, arfenic, zink, or other fuch admixtures, the workmen may be deceived by the colour, fo as to take for pure gold, or filver, that which is not by any means fuch. Deceits of this kind how- ever are found out by the affiftance of acid menftrua. Aqua forth anfwers this purpofe, when the mafs is of the colour of gold, and aqua regia when it is of the colour of filver ; for the firft of thefe menftrua diflblves all metals except gold, and the latter all metals except filver. In this cafe then you are to pour upon your metallic ftreak on the ftone, one fmall drop of either of thefe liquors, and extend it gently over it with a feather. If it is neither gold nor filver the whole ftreak will be obliterated and confumed, but if there is any gold or filver in it, this remains undiffolved, and {hews ano- ther colour, becaufe the other parts have been feparated from it by the folution. When thefe menftrua are ufed, great care muft be taken that there is no oil in the way ; for that would fpoil and deftroy their effects.
Befide thefe the following particulars are to be obferved : Gold and filver when pure, whether fcparate or both mixed together, without the addition of any other matter, when made hot in the fire, not only preferve their colour, but if they were tar- nifhed before, they recover their fplendour there, not lofing the leaft part of their weight : and by this quality in thefe two metals, the caratura alba, or white allay, made by the mix- ture of gold and filver alone, is diftinguifhed from all the others. If you have not liberty to try the whole mafs in the fire, you may make this experiment on a fmall piece of it with a blow pipe.
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If you find a Needle of the fame allay with the metal under trial, the ftreaks made by both upon the touch-ftone muft undergo exactly the fame changes when aquafortis is poured on them ; and this ought always to be made a part of the trial by the Touch-Needles, that no fraud may be at the bottom. All gold rendered brittle, when compared with thcToiich- Needles by the ftone, will appear lefs pure than it really is ; and on the contrary all filver rendered brittle, has the wbitenefs of filver in a higher degree : nor is there any wonder in this, when rightly confidered, lince the bodies which make gold and filver brittle are only a few metals, and femi-metals, all of a very bright white colour, and neceffarily adding to the whitcneis of filver, and taking from the ycllownefs of gold, fuch are tin, lead, regulus of antimony, bifmuth, zink, and arfenic. Thefe dilute the yellow colour of gold or copper, into a whitenefs, fo that the colour of copper mixed with the filver is hidden by admixtures of this kind, whereas gold on the contrary appears by them to have much more filver in it than it really has.
In a white allay, aquafortis docs not difcover the prefence of filver from twenty-three, to feven carracts, becaufe aquafortis does not feparatc filver from gold, unlefs the mafs contains three times more filver than gold.
Metallic ftreaks or crufts which have been left fome time upon the Touch-ftone, cannot be compared with frefh ones, with any degree of ufe, becaufe their remaining long on the ftone always alters their colour.
Silver when tempered with brafs appears whiter than it would do with a like quantity of copper, and as it may then be rendered fufficiently ductile by a proper operation, you will hardly be able to find out the fraud with the Touch- ftone t unlefs you make a fecond time the fame comparifon with the ftreak of a Needle of the fame colour, having previoufly poured aqua regia upon the metallic cruft laid by rubbing on the touch- ftone ; nor arc Touch-Needles tempered with brafs, of any great ufe on this occafion, fince this artificial metal is fome- times more and fometimes lefs yellow.
Laftly, if the metal laid upon the Toitch-fone by rubbing does" not appear neat or diftindt enough, Hck it over with fpittle that is not frothy; and the colours will be by that means more diftinctly and livelily reflected. Cramer's Art of Affaying 9 p. 116. Toucn-Stone (Cycl.) The trijb Touch - Stone, called Bafanus Hibernicas , by Molyneux and fome others , is a black marble found in the county of Antrim in that kingdom, in angular columns, forming that amazing pile cal- led by the vulgar, the giant's caufeway. This marble has the property of trying metals by the Touch beyond any other known ftone ; but it is not eafily wrought into form, being fo hard that it turns the edges of all the tools ufed to cut ftones. Were it not for this it is admirably calculated for building, and for ornamental works ; but no- body has attempted to ufe it in this manner, any where, ex- cept in the church of Ballywellan in the neighbourhood, and here the trouble of cutting is avoided, for the joints are taken as they found them, and the church is built of thefe in their natural fliape, piled one upon another. The outer furface of this ftone is of a whitifh colour like lime-ftone, but this is only the effect of the weather upon it ; for where-evcr it is broken it is found to be of a fine iron grey, and when polifhed appears of a true and deep jetty black. The accounts Pliny and others give of the bafaltes or bafanus found in ./Ethiopia and other places, agree extremely well in all refpects with this Irifh marble. They fay it was always found in form of columns, and was much harder than the common marble, and of an iron colour : Kcntman and other later writers, defcribe alfo a pillared ftone, found in Mifnia, the accounts of which agree perfectly well both with this and with the bafaltes of the anticnts : fo that there is no great room to doubt but that both the Mifnian and the Irifh colum- nar marble are the fame with the bafaltes of the Greeks and Romans ; and that this bafaltes is always found in the fame columnar form, in whatever part of the world it is met with ; and confequcntly that the giant's caufeway in Ireland, is no work of art as vulgarly thought, but only an immenfe conge- ries of this bafanus or bafaltes in its natural ftate and form. The columns of this marble are all regularly angular, but they confift of a great number of fides in fome, and of a fmall number in others. Some columns being octangular and others only triangular, and others of all the different numbers of angles between three and eight.
The Mifnian bafaltes is faid to have no columns of more than feven angles, or of lefs than four ; by this account it appears that the Irifh kind has two orders of columns which that wants, but this may be owing to want of due obfervation of the Mif- nian kind : for Dr. Molyneux in his firft account of this caufeway, fays, that its columns confift of five angles at leaft, or feven at moft, but future obfervations alone can fhew whether the Mifnian kind may not have other numbers of angles in fome of its columns, from thofe at firft taken notice of, as well as this. Some of ourEnglifh authors have been of opinion that the ftone of the giant's caufeway was of the entrochus or ajle- ria kind ; but this is Very ahfurd, for the fize of thofe ftones fcems to be limited, as they are the remains of animals, and
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