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of metals, are capable of penetrating very deeply into them, and tinging them with different colours. The fimple agates and jafpers, and other Jlones of the like uniform ftructure, are eafily coloured in an uniform manner; but tliofe which are variegated with veins, are compofed of feveral different forts of matter, and therefore are lefs eafily, and lefs evenly ftained. As the tinging matter does not penetrate the fe- veral beds, or veins, in the fame manner, therefore all that can be done to thefe, is to add fpots and veins to their na- tural ones, but they cannot be tinged throughout to one uniform colour, as the chalcedony, or white agate may. If a fmall quantity of a folution of filver, in fpirit of nitre, be poured upon one of thefe agates, and the Jlone expofed to the fun, it will in a few hours be tinged to a reddifh brown colour ; and if more of the fame folution be added, and it be again expofed, the colour will become ftronger, and will penetrate deeper into the body of the Jlone ; and if the Jlone be not too thick, and the folution be rubbed on both fides, it will tinge it throughout. Nor is this all the effect, for it will give it feveral veins and lineations, which were not difHnguifhable in it before; the reafon of which is, that in all thefe /tones there are fome parts harder than the reft, and confequently more difficultly- coloured, and thefe remaining, therefore, paler than the reft of the mafs, make the lines and veins in it.
If there be added to the folution of filver, ufed for this pur- pofe, a fourth part of its quantity of foot, and as much fait of tartar, the colour becomes grey ; and if, inftead of this foot and tartar, the fame quantity of plumofe alum be ufed, the Jlone will be tinged to a deep violet colour, tend- ing to black.
A folution of gold gives agate only a pale brown colour, and that penetrates but a little way into it; and a folution of bifmuth gives a colour, which appears white when the light falls directly upon it, but brown when it is held againft the funfhine, or a candle ; and all the metallic and mineral folutions, employed in the fame manner, affect the Jlone more or lefs in the fame way.
The expofing the Jlones to the fun is a very neceffary article in the procefs, fince without that the tinge is but very faint, and penetrates but to a very little depth. To trace in the chalcedony, or white agate, figures of any determinate kind, the ufuai method is this : before the agate is polifhed, mark out the intended figure with the point of a fine needle, and afterwards with a brufh, or a pen, follow thofe lines with a very ftrong folution of filver. One would imagine that the dendritae, or delineations of trees in Mocoa Jlones, might be imitated in this manner; but it is difficult to give the due blacknefs of colour, and to mark the figures with a like precifion and exaclnefs. If -any Jlone Ihould, however, be fufpected to be adulterated, or counterfeited in this man- ner, it is eafily brought to the trial ; for if it be thus made, a fmall heat over the fire will almoft entirely diveft it of its colour, and the rubbing a little fpirit of nitre, or aqua fortis over it, will have the fame effect. In both cafes the Jlone may, however, be reftored again to its beauty ; in the firft, by tinging it afrefh with the fame liquor, and in the laft, "by only expofing it for feveral days to the fun. It is well known, that by means of fire alone the amethyft, the fapphire, and the other gems, may alfo be wholly di- refted of their beautiful colours. The method of doing this, as, to put the gems into a crucible, furrounding them with fand, or with fteel filings ; then putting them into the fire, they lofe their colour as they become hot, and are taken out wholly colourlefs. If the white agate be calcined in this manner alone, it becomes of a cloudy or opake white; but if it has before been ftained with the folution of filver, thofe fpots or ftains become of a yellow colour, which aqua fortis afterwards has no power to take away. If the agate be calcined firft, and afterwards rubbed over with the folution of filver, it receives fome fpots and lines of brown. The cornelian lofes a great part of its rednefs by calcination, and becomes of a dufky flefh- coloured white ; and the Mocoa Jlone, treated in the fame manner, lofes all its colour, and the delineations of trees difappear.
There are many Jlones on which the folution of filver has no effect ; of this number are all the gems, rock cryftal, and the like. The dendrites of Catalonia is alfo of this kind ; and of this Jlone the artificers relate an odd pheno- menon, which is, that if it be fawn afunder there are very few delineations obferved in it, but if it be fplit by a blow, it is ufually found full of them. The reafon of this is only that thefe delineations are fo many flaws and cracks, and xhz Jlone breaks eafieft in thefe places. The effects of the folution of filver are different, as to degree of colour, on different Jlones. The Oriental agate receives from it a deeper and blacker tinge than the com- mon chalcedony. Some agates, naturally diftinguifhed by their yellow fpots, receive a purple colour from it. The jade Jlone, ufed by the Turks, takes only a faint tinge of brown. The common prime emerald, or root of the emerald, be- comes blackifh with it. The Oriental granile is tinged in many places with a violet colour by it ; this is principally effected in the white parts. The folution does not aft much Sufpl. Vol. II.
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on the black ones, only that it takes fome of them Sway; The ferpentine marble receives an olive colour from it j but it is remarkable that the amianthufesi and the talcs, and other fofiaceous_y?(wj, are wholly unaffected by it. Marble being a fubftance greatly fofter than agate, receives the colours with much greater eafe, and the doing this in. an accurate manner has been tbe fubject of the attempts of many eminent men; Kircher has given fome directions for the ftaining marble, which have been tranfiated word for word into the Philofophicai Tranfaaions; but they are fo indeterminate and uncertain, that nothing can in reality be learned from them. Many others have written alfo on the fame fubjeft, but Mr. Du Fay Is the only one whofe expe- riments are plainly and clearly laid down, and may be fol-r lowed by any body.
This gentleman chofe the common white marble without veins for making his experiments, for the fame reafon that he chofe the plain agates, becaufe in the veined ones there are feveral different forts of matter, all of which are not to be penetrated with equal eafe. The folution of filver pene- trates into marble to the depth of an inch, or more, and gives a tinge, reddifh or purple, at firft, and afterwards brown, from which colour it never varies afterwards. It al- ways takes off the polifh of the marble, eating away a part of its furface. The folution of gold does not penetrate fo deep into marble as that of filver, but it gives a beautiful violet colour. Both thefe operations are much affifted by expofing the marble to the fun. The liquors ufually diffufe tbemfelves, and fpread every way in the marble, fo that it is notcafy to make any figure with them that fhall keep its outline tolerably regular:
The folution of copper gives marble a beautiful green tinge; but it does not penetrate deep, and on the application of boil- ing water becomes black ; when the furface is polifhed off afterwards, however, it becomes again of a beautiful green. Befide the powerful acid menftruums, there are many other liquors which have a power of penetrating deep into marble. Of this nature are all the oily fluids ; but the expreffed oils have this difadvantage, that they leave a fattinefs in the marble which will not fuffer it afterwards to take a good polifh. All fubftances which can penetrate marble, can carry colours into it; but fuch are moft eligible, which having lodged the colours evaporate, and leave them there, without injuring the Jlone, Spirit of wine is of this num- ber ; it is excellently qualified for the extracting beautiful tinctures, and finks them very deep. Oil of turpentine alfb has its value, but it does not take tinges fo well as the fpirit. Some have recommended lixiviums of the fixt alkaline falts, but they very rarely produce any beautiful colour; In the ufing thefe fluids the marble is to be gently heated, and the fpirit is by that means evaporated before it is cooled, leav- ing its colour always behind. White wax penetrates very deep into hot marble, and conveys colours into it in a very beautiful and determinate manner. There are however but a few bodies, which will impart their colours to wax, and therefore this valuable means is of a very limited ufe. Several of the gums alone are alfo able to tinge marble very ftrongly. Dragons blood, and gamboge, if rubbed on hot marble, penetrate it to the depth of about a twelfth of an inch : the gamboge requires the marble to be hotter than the other, and tinges it to a very beautiful yellow ; the dragons blood tinges to a red in different degrees, according to the heat of the marble.
If thefe gums have been ufed to polifhed marble, there is. no farther caution neceffary, than the cleaning them off from the furface with a little fpirit of wine : but the way to make them fink deeper into the Jlone, is to take off the polifh by rubbing the furface with pumice, or the like, and then the gums fink much farther, and the colours appear very beautiful when the marble is polifhed again. Though thefe gums act alone, yet they will fucceed much better if diffolved in fpirit of wine, and applied with a pen- cil ; for by this means they fink deeper in, and the figures traced out will keep their determinate form and outlines^ thefe folutions fixing immediately, without fpreading any way. It is alfo remarkable, that the folution of dragons blood hardens the marble, and renders it lefs foluble in acids than before; fo that if a piece, ftained in part with this folution, be afterwards rubbed over with an acid dtffolvent, and its furface eaten away to fome depth, the parts which are coloured will all ftand out above the reft. A tincture of Brafil wood in fpirit of wine tinges marble red, and if the heat given to the marble be greater, it be- comes purple ; but both thefe colours fade a little in keep- ing. A tincture of cochineal gives a purplifh red, and the more the marble is heated, the farther the colour pene- trates, and the deeper it is. In oil of turpentine the colour of cochineal penetrates much deeper into the marble, but it has a brownifh caft. AJkanet root,' by means 6f fpirit of wine, gives alfo a red colour, which, if the heat be too great, changes to brown ; and this, and moft other of the like matters, tinge the marble, in tincture with fpirit, to a flight depth ; and in oil of turpentine they fink much deeper, but then the oil leaves a greafy look upon the marble;
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