80

ASSAY.

thousandth of mercury the difference of color is not remarkable, and is perceived onlyin a very moderate light.

But when the quantity of mercury is so small that it cannot be detected by thedifference of color in the chloride of silver, it may be rendered quite evident by a verysimple process of concentration. Dissolve one gramme of the silver supposed to containjof a thousandth of mercury, and let only j of it be precipitated, by adding only | ofthe common salt necessary to precipitate it entirely. In thus operating, the J thou-sandth of mercury is concentrated in a quantity of chloride of silver four times smaller:it is as if the silver having been entirely precipitated, four times as much mercury,equal to two thousandths, had been precipitated with it.

In taking two grammes of silver, and precipitating only \ by common salt, theprecipitate would be, with respect to the chloride of silver, as if it amounted to fourthousandths. By this process, which occupies only five minutes, because exact weighingis not necessary, jL of a thousandth of mercury may be detected in silver.

It Is not useless to observe, that in making those experiments the most exact mannerof introducing small quantities of mercury into a solution of silver, is to weigh a minuteglobule of mercury, and to dissolve it in nitric acid, diluting the solution so that itmay contain as many cubic centimetres as the globule weighs of centigrammes. Eachcubic centimetre, taken by means of a pipette , will contain one milligramme ofmercury.

If the ingot of silver to be assayed is found to contain a greater quantity of mercury,one thousandth for example, the humid process ought either to be given up in th scase, or to be compared with cupellation.

When the silver contains mercury, the solution from which the mixed chlorides areprecipitated does not readily become - clear.

Silver containing mercury, put into a small crucible and mixed with lamp-black,to prevent the volatilization of the silver, was heated for three quarters of an hourin a muffle, but the silver increased sensibly in weight. This process for separating themercury, therefore, failed. It is to be observed, that mercury is the only metal whichhas thus the power of disturbing the analysis by the humid way.

Assaying of Gold. —In estimating or expressing the fineness of gold, the wholemass spoken of is supposed to weigh 24 carats of 12 grains each, either real, or merelyproportional, like the assayer’s weights; and the pure gold is called fine. Thus, ifgold be said to be 23 carats fine, it is to be understood, that in a mass, weighing24 carats, the quantity of pure gold amounts to 23 carats.

In such small work as cannot be assayed by scraping off a part and cupelling it,the assayers endeavor to ascertain its fineness or quality by the touch. This is amethod of comparing the color and other properties, of a minute portion of themetal, with those of small bars, the composition of which is known. These bars arecalled touch needles, and they are rubbed upon a smooth piece of black basaltes orpottery, which, for this reason, is called the touchstone. Black flint slate will servethe same purpose. Sets of gold needles may consist of pure gold j of pure gold, 23-Jcarats with \ carat of silver; 23 carats of gold with one carat of silver; 22 1 carats ofgold withTJ carat of silver; and so on, till the silver amounts to four carats; afterwhich the additions may proceed by whole carats. Other needles may be made in thesame manner, with copper instead of silver; and other sets -may have the addition,consisting either of equal parts of silver and copper, or of such proportions as theoccasions of business require. The examination by the touch may be advantageouslyemployed previous to quartation, to indicate the quantity of silver necessary to beadded.

In foreign countries* where trinkets and small work are required to be submitted tothe assay of the touch, a variety of needles is necessary ; but they are not much usedin England. They afford, however, a degree of information which is more considerablethan might at first be expected. The attentive assayer compares not only the color ofthe stroke made upon the touchstone by the metal under examination, with that producedby his needle, but will likewise attend to the sensation of roughness, dryness, smooth-ness, or greasiness, which the texture of the rubbed metal excites, when abraded by thestone. When two strokes perfectly alike in color are made upon the stone, he maythen wet them with aquafortis, which will affect them very differently, if they be notsimilar compositions; or the stone itself may be made red-hot by the fire, or by the blow-pipe, if thin black pottery be used; in which case the phenomena of oxydation will differaccording to the nature and quantity of the alloy. Six principal circumstances appearto affect the operation of parting; namely, the quantity of acid used in parting, or inthe first boiling; the concentration of this acid ; the time employed in its application ;the quantity of acid made use of in the reprise, or second operation ; its concentration;and the time during which it is applied. From experiment it has been shown, tftateach of these unfavorable circumstances might easily occasion a loss of from the half of