G OLD. —Assaying.

115

scale pans rest in stirrups of palladium, and can beremoved by means of forceps. The supports at thecentre and ends are shown on a large scale on thePlate. Each stirrup hangs from two steel pointsresting in agate cups, and the agate knife edge at thecentre doe's not come in contact with the agatepalates until after the supports have been removedfrom below the scale pans by means of the handle,moving in a vertical slot in front of the balance. Ashas already been described at page 96, two systemsare employed for expressing the composition of goldand silver ingots, and the weights used in “ weigh-ing in” and “ weighing out” the assays are usuallyso prepared that the results obtained directly repre-sent the composition, rendering calculations un-necessary. Thus when reports of gold assays arerequired on the trade system, the weight of metalemployed (varying from 5 to 16 grains), called the“ assay pound,” is divided into 24 parts, calledcarats, and each of these is subdivided in the mannershown by the table on page 95. If, however,assays are required on the millesimal system, theweight taken is called 1000, and in weighing thecornet after the completion of the assay subdivisionsof this 1000 are employed. Those employed inreporting silver assays on the trade system aresomewhat different.

The adjustment of the weight of the assay piece,so that the index of the balance should point tozero, would waste a considerable amount of time,and it is usual therefore to make a note of any slightdivergence, not exceeding about three scale divisions(each of which represents the P ar ^

assay piece) on each side of zero, and to apply thiscorrection in the final weighing. If the gold doesnot contain about times its weight of silver it isnecessary to add silver up to this amount, and thetwo metals are then wrapped up in a piece of sheet-lead, and placed on a tray divided into numberedcompartments, which correspond to the positions ofcupels in the muffle ; the number of the compartmentin which each assay is placed being noted with a viewto its subsequent identification. This description ofthe preparation of the assay piece applies only toalloys, such, for instance, as those used in thecoinage, and in which the next operation of fusionis not required, there being no earthy impuritiespresent.

Fusion with Oxide of Lead. —Assuming, as before,that the ore to be assayed is gold quartz, it mustfirst, as in all other cases, be reduced to a fine powder,by triturating in an iron mortar. This operation ismuch facilitated by heating the quartz to redness,and plunging it in cold water. Having pulverized afew thousand grains, it is usual to make at least twoassays of the sample, to test the correctness of theresult. Tor each of these weigh 500 or 1000 grains,according to the richness of the ore, and mix inti-mately on a clean surface of glass, or highly-glazedwriting paper, with about the same weight of lithargeor red lead, half the weight of dried carbonate ofsoda, and rather more than half of powdered char-coal. The precise quantities are not very important,

but it is better that the carbonate of soda should bein excess than that it should fall short of the aboveproportion. Phillips recommends the followingproportion for the assay of a rock which does notcontain an appreciable quantity of iron pyrites—600 grains of finely pulverized ore, 4000 grains oflitharge or red lead intimately mixed with it, andfrom 15 to 20 grains of flour, starch, or finelypowdered charcoal. The mixture is introduced intoa Cornish or black-lead crucible (Phillips, however,recommends ordinary French pots), which it shouldnot more than two-tliirds fill; it may then be coveredwith a thin layer of borax, and afterwards fixedsolidly in the furnace by surrounding it with fuel.

Furnaces suitable for assaying are of variousforms, and the required temperature may be ob-tained in them either by supplying an artificialblast of air by some blowing apparatus, or by con-necting the furnace with a chimney sufficiently highto establish a strong natural draught. When thematerials for its erection can be obtained the ordinarywind-furnace is to be preferred. This is represented

Fig. so.

'Cf

and fully described in connection with the assayingof copper ores, at page 570, vol. i., to which thereader is referred. For the travelling assaycr,however, a portable apparatus is required, and inthis respect Sefstrom’s blast-furnace offers peculiaradvantages. This consists of two cylinders of sheet-iron, placed one-within the other, as represented inthe annexed drawing, Fig. 30. A represents theouter cylinder, which answers the purpose perfectlywell when not larger than a man’s hat; and B theinner one, lined with a coating of fire-clay about 1inch in thickness. Both cylinders are provided witha bottom, and are fixed together at the top, air-tight,by a horizontal hoop or flat ring of metal, E E, in sucha way as to leave an equal space, c C, between theirsides and bottoms. The interior cylinder, B B, ispierced at about the middle of its depth with eightholes, D D, which pass through the lining of fire-clay,and all point to the centre of the furnace, where thecrucible, h, is placed on a piece of fire-brick, kept inits position by a little fire-clay, and surrounded withfuel. For a small furnace of this kind charcoal mustbe employed as fuel, and should be broken into