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 or purity of any piece of gold or silver, by comparing the mark which 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 by fusion with one another, in different degrees and proportions. In order to discover the purity or quantity of baser metal in these various admixtures, when they are to be examined they are to be compared with these needles, which are mixed in a known proportion, and prepared for this use. The metals of the 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 a half long. These being thus prepared, a mark is to be engraved on each, indicating its purity, or the nature and quantity of the admixture. The black rough marbles, the basaltis, or the softer kinds of black pebbles, are the most proper for touchstones.