T O U 



T O U 



■ The method of ufing your needles and the (tone is this : 

 the piece of gold to be examined, being weU cleaned in fome 

 convenient part of its furface, a ilroke is to be made with it 

 on the ilone ; and another, clofe by it, with fuch of the touch- 

 needles as appear to come the neareft to it in colour. If the 

 colour of- both, upon the ftone, is exadlly the fame, it is 

 judged that the given mafs is of the fame finenefs with the 

 needle : if different, another and another needle muft be 

 tried, till fuch a one is found as exattly correfponds to it. 

 j To do this readily, pradlice only can teach. 

 i In making the ilrokcs, both the given piece, andthe needle 

 of comparifon, are to be rubbed feveral times backwards and 

 forwards upon the ilone, that the marks may be ftrong and 

 i full, not lefs than a quarter of an inch long, and about a 

 ; tenth or an eighth of an inch broad : both marks are to be 

 i wetted before the examination of them, their colours being 

 thus rendered more diftinft. A Ilroke, which has been 

 drawn fome days, is never to be compared with a frefh 

 , one, as the colour may have fuffered an alteration from the 

 I air ; the fine atoms, left upon the touch-llone, being much 

 more fufceptible of fuel; alterations than the metal in the 

 mafs. If the piece is fuppofed to be fuperficially height- 

 ened by art in its colour, that part of it, which the ilroke 

 is defigned to be made with, ftiould be previoutly rubbed on 

 another part of the Ilone, or rather on a rougher kind of 

 ftone than the common touch-ftones, that a frelli furface of 

 the metal may be expofed. If it is fufpefted to be gilt with 

 a thick coat of metal finer than the internal part, it fliould 

 be raifed with a graver, to fome depth, that the exterior 

 coat may be broken through : cutting the piece in two is 

 a lefs certain way of difcovering this abufe ; the outer coat 

 being frequently drawn along by the flieers or chifel, fo as 

 to cover the divided parts. 



The metallic compofitions, made to refemble gold in 

 colour, are readily known by means of a drop or two of 

 aqua fortis, which has no effeft upon gold, but difTolves or 

 difcharges the marks made by all its known imitations. 



If gold is debafed by an admixture of any confiderable 

 quantity of thefe compoGtions, aqua fortis will, in this cafe, 

 alfo difcharge fo much of the mark as was made by the bafe 

 metal, and leave only that of the gold, which will now ap- 

 pear difcontinued or in fpecks. Silver and copper are in 

 like manner eaten out from gold on the touch-ftone, and 

 hence fome judgment may thus be formed of the finenefs of 

 the metal from the proportion of the remaining gold to the 

 vacuities. 



Ercker obferves, that hard gold appears on the touch- 

 ftone lefs fine than it really is. It may be prefumed 

 that this difference docs not proceed from the fimple 

 hardnefs ; but from the hardnefs being occafioned by an 

 admixture of fuch metalhc bodies, as debafe the colour in 

 a greater degree than an equal quantity of the common 

 alloy. Silver and copper are the only metals ufually found 

 mixed with gold, whether in buUion or m coins ; and the only 

 ones, whofe quantity is attempted to be judged of by this 

 method of trial. 



The Chinefe are faid to be extremely expert in the ufe of 

 the touch-llone, fo as to diftinguifh by it fo fmall a difference 

 in the finenefs as half a touch, or a two hundredth part of 

 the mixt. The touch-ilone, fays Dr. Lewis, is the only 

 teft by which they regulate the fale of their gold to the Eu- 

 ropean merchants ; and in thofe countries it is fubjeft to 

 fewer difficulties than among us, on account of the uniformity 

 of the alloy, where there is alnioil always filver ; the leaft 

 appearance of copper being ufed in the alloy gives a fufpicion 

 of fraud. As an aflay of the gold is rar-ely permitted in that 

 commerce, it behoves the European trader to be well prac- 

 VoL. XXXVI. 



tifed in this way of examination : by carefully attending to 

 the above direftions, and by accuftoming himfelf to com- 

 pare the colours of a good fet of touch-needles, it is pre- 

 fumed he will be able to avoid being impofed on, either iu 

 the touch itfelf, or by the abufes faid to be fometimes com- 

 mitted, of covering the bar or ingot with a thick coat of finer 

 metal than the interior part, or of including niaffos of bafe 

 metal within it. A fet of needles may be prepared, for this 

 ufe, with filver alloy, in the feries of the Chinefe touches ; 

 or the needles of the European account may be eafily ac- 

 commodated to the Chinefe, by means of a table formed for 

 that purpofe on the principles already explained. It may 

 be obferved, that the gold flioes of China have a depreffion 

 in the middle, from the fhrinking of the metal in its coohncr, 

 with a number of circular rings, like thofe on the balls of the 

 fingers, but larger : Dr. Lewis fays, that when any other 

 metalhc mafs is included within, the fraud is difcoverable at 

 fight, by the middle being elevated inflead of deprefied, and 

 the fides being uneven and knobby ; but that the fame kind 

 of fraud is fometimes praftifed in the gold bars, where it is 

 not difcoverable by any external mark. See Cramer's Art of 

 Affaying, p. ii6, &c. Lewis's Com. of Arts, p. 122, &c. 

 See alfo Assaying. 



Toucii-Stone, a black, fmooth, gloffy ftone, ufed to try 

 metals upon. 



The ancients called it lapis Lyd'ms, the Lydian ftone, from 

 Lydia, a country of Afia Minor, whence it was brought. 

 See Tovcu-Needlcs. 



TovcH- Stone, Irijh, called bafamts Hibernicus by Moly- 

 neux and fome others, is a black ftone, found in the county 

 of Antrim, in that kingdom, in angular columns, fonning 

 that amazing pile called by the vulgar the Giant's Caufeway. 

 See Basaltes and Marmaiio-prosera. 



The bafaltes being an excellent touch-ftone, authors have 

 confounded it vWth the touch-ftone of the ancient Greeks, 

 called by them bafauos, bafanites, lapis Lydius or Hcraclitis, &c. 

 which were, probably, only black jafpers ; and they have 

 derived bafanos and bafaltes from the fame Greek word, 

 Sxj-y.ni^ii, to try or prove : but the bafaltes was not known to 

 the Greeks, and Pliny exprefsly tells us, that it is an Ethiopia 

 or Egyptian name : whence it is probable that bafaltes is a 

 corruption of the Hebrew word '^f^^^, barzel, which figni- 

 fies iron, a very proper name for a (lone, which they defcribe 

 to be of the colour and hardnefs of iron. Many authors 

 affirm, that the touch-ftone is a kind of black marble, or that 

 moft black marbles may ferve for touch-ftones : black mar- 

 bles, it is true, take the colours of metals, and may be ufed 

 as touch-ftones in the ancient manner mentioned by Theo- 

 phraftus and Pliny ; /. e. by touching them with needles of 

 different alloys ; which manner is ftiU praftifed in Germany 

 and other countries : but in the manner of trying them 

 with aqua fortis, it is abfurd to ufe any calcareous fub- 

 flances, as the acid will immediately deftroy the fubftance of 

 the ftone, and confequently affedl and fomewhat deface the 

 ftrokes of the metals. For a good touch-ftone, choice fliould 



be made of a black ftone of tlie genus of; 



lo-profera, or 



of the jafper genus, on which the aqua fortis will only diffolve 

 tlie copper, filver, &c. without touching the gold, or affect- 

 ing the ftone. The firfl genus fhould be preferred, as a nice 

 touch-ftone (hould only have a certain degree of polifh, and 

 (hould be of a compail fine texture, and neither too hard nor 

 too foft ; whereas the jafpers are too hard, and do not fhew 

 with fufHcient exaftnefs the different degrees of alloy ; for 

 the touches of the metal give them a higher iuftre than their 

 natural polifh, and that Iuftre injures the diftinftnefs of the 

 ftrokes made upon it. Cofla's Hift. Foff. p. 364. See 

 T(»ucii-A''i?f(//i-.f. 



K Tcrucii- 



