MINERAL TESTING.

Otago Witness, Issue 1825, 12 November 1886, Page 11

MINERAL TESTING.

By f, A. Joseph. No. VIII. GROUP II.— GOLD. Gold, like platinum, ia soluble in aqua' regia, and is not affected by either nitric or hydrochloric acid ; therefore treat the finely powdered stone exactly as in the case of pjatinura. A hurried test might be made by bulling in the acid for 15 or 20 minutes, evaporating to dryness, and diluting with water; but the better way is to allow the sample to digest in the cold acid for a couple of days. Gold is much easier of solution in aqua regia than platinum, colour* iug the solution yellowish, while platjnunj gives a reddish colour to the solution; but as the acid is slightly yellow, the tinge imparted to the solution could oply be detected on evaporating to near dryness and adding water. The group reagent—sulphuretted hydrogen— throws down a black precipitate which! re-dissolves on the addition of ammonium sulphide. This latter liquid is the special-, reagent of the next group of metals, and when that group is dealt with instructions will lie.* given how to make the solution. The best test for gold in solution is iron sulphate (copperas), as a solution of the latter throws down metallic gold as a brown powder—brown if looked at from tho light, but; greenibh if held between you and tho light. Gold may be detected in minute ! quan" tities by applying the following tests s— A small quantity of the finely powdered stone is shaken up with tincture of iodine (prepared by,d!Sßolv« iug crystals of iodine in spirit of wine)'}, Allow the insoluble part to settle, when, it' gold be present it will be in the solution. Dip a piece of filter or fine white blotting paper into the solution, and after drying burn the paper in a porcelain dish. If gold is present the ash will be coloured purple. Ability to test a sample of quartz and decide with certainty whether it carries gold or not should' be, at least, one of the accomplishments of every mine manager, and there are no difficulties in'the way to debar the average miner from performing the tests for himself. Of course pounding a piece of quartz and washing it in a tin di&h in the great majority of cases gives a very fair idea of tho quality of the stone ; but the history of mining in the colonies shows that rich gold-bearing stone may be staring the miner in the face-for years and yet passed by unheeded. In the celebrated Mount Morgan mine, in Queensland, a .whole mountain of auriferous stone has been found, in which the gold is invisible to the eye, and also eludes the search by the ordinary methods. yet some of this very stone yields when* assayed at the rate of 18oz of gold to the ton. The stono is a kind of spongy, porous iron stone, \througb. which quartz is intimately impregnated.- In many mines where good payable stone is found the gold is quite invisible, and unless the chemical test is applied the more costly one of crushing with stampers and amalgamating must be resorted to. This is seldom done unless the stone shows gold, and for that simple reason payable reefs innumerable may' be passed over unnoticed. Since the chemical test can be so easily applied no likely looking reef should be passed over until a sufficient test is made, and oncehavingdeterminedthepresen.ee of gold, yon would be justified in sending a sample for, atisay. If miners could test for themselves, all needless expense in the way of having essays of barren specimens made need not be incurred. It will not be necessary to say much -about the methods of gold saving, as they have been touched upon in dealing with mercury] It will be sufficient to say that in almost all of our alluvial gold workings only a part of the gold is saved, a considerable and important part of the finer precious metal being carried away by the rush of water to enrich the tailiagsf or the future benefit of the industrious Mongolians. The history of the Tailings Company at.Gabriel's Gully (Tuapeka) is sufficient evidence that the statement is correct, and I can back it with the opinion of an expert. M. Schostak, mining engineer to the Czar of all the Russias, informed me that in the matter ot gold saving the Russians were far ahead of us, aud he stated pretty freely .'-that we we were wasting gold in a shameful' manner. The rich tailings lying broadcast all over the country amply bear this out. As a. rule sluices are too short and too steep, and. the 'rush of water too great, the common idea appearing to prevail that returns depend almost • entirely upon the quautity of stuff rußhed through, while in many cases at any rate better -returns would be obtained from half the stuff if all or nearly all the gold were saved.. But. this a matter for the practical miner to solye, and certainly as the precious metal becomes, scarcer better methods will be sought for. With this, genei - ally considered the most precious of. all metals, we finish Group 11, but before taking up the more important .metals of the next group it will be' /as . well to give a short resume. The group' reagent is made by treating pounded iron sulphide with cold dilute sulphuric acid; a part of the 'acid, which is itself a compound, combines With the metallic iron of the stone (more, familiarly known as mundic) forming iron sulpnate (copperas), while auother part takes up the sulphur of the faulphide aud forms sulpuretted hydrogen (more properly hydrogen sulphide). ' This gas delivered into water is dissolved ; toi a large extent, inakiug the solution strong enough for all practical purposes. A fresh solution must be made at iutervals of a few weeks, or as soon as tests show that the solution is too weak. This can alwa ys be determined by trying the Teagent on a solution known to contain a metal of the group. The reagent always gives the 1 best results with acidulated solutions ; therefore before using it always add alittle hydrochloric acid to the sample to be tested. This serves the. double put pose of acidifying the solution, and at. the same time of showing whether a metal of group I. be present or not. A glance at the precipitates given with tho various metals dealt with shows how a kind of classification may be made. On the addition of sulphuretted hydrogen, mercury goes through a gradation: of colour — white, yellow, grey, and finally' b'lack,~which cuts it off from all the other, metals of the group. Copper and gold both give black • precipitates. The gold black is dissolved by ammonium sul» phide, while that of copper is insoluble Ammonia added to the copper solution containing the black precipitate at once changes the colour to blue, while in the case of gold no change takes place. Bismuth and tin both give dark-brown precipitates. The addition of water whitens the bismuth precipitate, while it does not affect that of tin. Arsenic and cadmium both, giv? yellow praeipitates; arsenic slowly, and cadmium at ouca. The arsenic yellow is soluble in ammonium sulphide, while that of cadmium is not affected » Ammonia clonrs up fefac arsevrio precipitate and

whitens that of cadmium, fche white re*dlssolving £ more ammonia is added.

Antimony, with its characteristic orange precipitate, also stands alone. It is therefore appaient that the group splits up into handy Bubttivisior.s, containing respectively two metals th it jield black precipitates with hulphuretted hydrogen— two brown, two yeilow, oue orange, aud one a gradation of colours.