SAYING.
The quantity of alloy is never consi dered as part of the value of metals which contain either gold or silver. In estimat ing or expressing the fineness of gold, the whole mass spoken of is supposed to weigh 24 carats of 12 grains each, either real or merely proportional, like the assayer's weights ; and the pure gold is called fine. Thus, if gold be said to be 23 carats fine, it is to be understood that, in a mass weighing 24 carats, the quantity of pure gold amounts to 23, carats.
In such small works as cannot be as sayed by scraping off a part, and cupel ling it, the assayers endeavour to aster.. taro its quality or fineness by the touch. This is a method of comparing the colour and other properties of a minute portion of the metal with those of certain small bars whose composition is known. These bars are called touch-needles ; and they are rubbed upon the black basaltes,which, for that reason, is called the touch-stone. Black flint, or pottery, will serve the same purpose. Sets of golden needles may consist of pure gold ; pure gold twenty-three and a half carats with half a carat silver ; twenty.three carats gold with one carat silver ; twenty-two and a, half carats gold with one and a half carat silver, and so forth, till the silver amount to four carats, after which the additions may proceed by whole carats. Other needles may be made in the same man ner, with copper instead of silver ; and other sets may have the addition, consist ing either of equal parts of silver and copper, or such proportions as the occa sions of business require.
In foreign countries, where trinkets and small works are required to be sub mitted to the assay of the touch, a variety of needles are necessary ; but they are not much used in England. They affbrd,
however, a degree of information, which is more considerable than might at first be expected. The attentive assayer not only compares the colour of the stroke made upon the touchstone by the metal under examination with that produced by his needle, hut will likewise attend to the sensation of roughness, dryness, smooth ness, or greasiness, which the texture of the rubbed metal excites when abraded by the stone: When two strokes, per fectly alike in colour, are made upon the stone, he may then wet them with aqua fortis, which will affect them very differ ently if they be not similar compositions ; or the stone itself may be made red hot by the fire, or by the blowpipe, if thin black pottery be used, in which case the phenomena of oxydation will differ according to the nature and quantity of the alloy.
Gold ores may be assayed in the moist way by pounding them very fine, weigh ing a determinate portion, and attempting their solution in nitric acid, which will dis solve the matrix if it consist of calcareous earth ; or if it be sulphatq of lime, the powder may be digested in aqua-regia as long as any metallic substance is taken up ; after which the gold may be precipi tated by an addition of sulphate of iron, which will cause it to fall down in the metallic state.
The principal use of gold is as the me dium of exchange in coin, fbr which it has been chosen to occupy the first place, on account of its scarcity, its great weight, and its not being subject to tarnish. The gold coins of Great Britain contain eleven parts of gold and one of copper. See Cozy.
Gold is likewise used in gilding. See