among assayers, refiners, &c. little bars of gold, silver, and copper, combined together, in all the different proportions and degrees of mixture; the use of which is to discover the degree of purity of any piece of gold or silver, by comparing the mark it leaves on the touchstone with those of the bars.

The metals usually tried by the touchstone are gold, silver, and copper, either pure, or mixed with one another in different degrees and proportions, by fusion. In order to find out the purity or quantity of base metal in these various admixtures, when they are to be examined they are compared with these needles, which are mixed in a known proportion, and prepared for this use. The metals of these needles, both pure and mixed, are all made into laminae or plates, one-twelfth of an inch broad, and of a fourth part of their breadth in thickness, and an inch and half-long; these being thus prepared, you are to engrave on each a mark indicating its purity, or the nature and quantity of the admixture in it. The black rough marbles, the basaltes, or the softer kinds of black pebbles, are the most proper for touchstones.

The method of using the needles and stone is thus: The piece of metal to be tried ought first to be wiped well with a clean towel or piece of soft leather, that you may the better see its true colour; for from this alone an experienced person will in some degree, judge beforehand what the principal metal is, and how and with what debased.

Then choose a convenient, not over large, part of the surface of the metal, and rub it several times very hardly and strongly against the touchstone, that in case a deceitful coat or crust should have been laid upon it, it may be worn off by that friction: this, however, is more readily done by a grindstone or small file. Then wipe a flat and very clear part of the touchstone, and rub against it, over and over, the just-mentioned part of the surface of the piece of metal, till you have, on the flat surface of the stone, a thin metallic crust, an inch long, and about an eighth of an inch broad: this done, look out the needle that seems most like to the metal under trial, wipe the lower part of this needle very clean, and then rub it against the touchstone, as you did the metal, by the side of the other line, and in a direction parallel to it.

When this is done, if you find no difference between 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 same alloy, which is immediately known by the mark engraved on your needle. But if you find a difference between the colour of the mark given by the metal, and that by the needle you have tried, choose out another needle, either of a darker or lighter colour than the former, as the difference of the tinge on the touchstone directs; and by one or more trials of this kind you will be able to determine which of your needles the metal answers, and thence what alloy it is of, by the mark of the needle; or else you will find that the alloy is extraordinary, and not to be determined by the comparison of your needles.