cubic centimetres of solution. I effected the estimation with the standard solution of iodide, of which 10 centimetres represent 10 of mercury. To saturate these 10 centimetres it required 115 divisions of the chloromercurial solution. These 115 divisions contain then 0.10 of mercury. Now. as all the mercury contained in the analysed cinnabar is spread through the 10,000 divisions of solution, we have the quantity of mercury found by the experiment by means of a simple proportion.' BLOWPIPE REACTIONS OF MERCURY. MERCURY.-The compounds of mercury are all volatile, and cannot, in consequence, be distinguished by their reaction with any of the fluxes. Substances containing mercury are assayed by being mixed with a little tin, iron filings, or oxide of lead, and heating the mixture to redness in the closed tube or matrass. In this operation the mercury is reduced, and collects in the coldest part of the tube as a greyish powder, which being brought together by the end of a feather, collects as metallic globules. When the quantity is very small, the globules may be distinguished by aid of the microscope. ORES OF MERCURY. CINNABAR, SULPHIDE OF MERCURY.-Alone, on charcoal, it volatilises without residue, giving off an odour of sulphurous acid. In the matrass it sublimes, giving a blackish subliIn the open tube, it gives, by roasting, mercury and sublimed cinnabar. In the matrass, with soda, globules of mercury are obtained. mate. CHLORIDE OF MERCURY, HORN MERCURY.-On charcoal, volatilises without residue. In the matrass, gives a white subliWith soda, in the matrass, gives much mercury in mate. globules. With microcosmic salt, fused on the brass wire, it communicates a fine azure blue colour to the flame, indicative of chlorine. 461 CHAPTER XV. ASSAY OF SILVER. ALL argentiferous substances may be divided into two classes, is follows: CLASS I. All minerals containing silver, Silver glance (AgS) containing 87 per cent. of Ag. Brittle silver ore (6AgS,SbS3) containing 70,4 per cent. of Ag. Argentiferous sulphide of copper (Cu,S,AgS) containing 53 per Polybasite (9(Cu,S,AgS) + (SbS,,AsS3)) containing 72-94 per cent. of Ag. Slags. Cupel bottoms. Litharge, etc. CLASS II.-Metallic silver and alloys, either native or otherwise. General Observations on the Assay of Ores and Substances of Class No. 1. In order to separate silver from this class of substances, an alloy of the precious metal with lead must be formed. The different methods by which this object can be obtained are the following: firstly, fusion with a reducing flux; secondly, fusion with oxidising reagents; thirdly, scorification. All substances containing lead in the state of oxide, such as carbonates, phosphates, &c., are fused directly with a reducing flux, as also are slags, old cupels, litharge, &c. All plumbiferous sulphides, &c., containing silver, are assayed as for lead by the processes already pointed out, taking care to follow the method which gives the largest proportion of lead. All argentiferous minerals containing copper may be assayed as copper ores; because an alloy of copper and silver can be cupelled by means of lead. In making assays of silver with lead or copper, it is sometimes necessary to commence the operation by roasting the ore; under other circumstances, also, argentiferous matters are roasted. There is nothing very particular to be observed in this roasting; the temperature alone requires attention by managing well at the commencement of the operation, in order to avoid softening, and especially to avoid a very rapid disengagement of arsenical vapours, because a very considerable amount of silver may be lost by that means. All substances which contain reducible oxides are fused with a reducing flux, as also those from which charcoal separates metals which alloy with lead, or metals which do not hinder the process of cupellation; but it is necessary to add to the reducing flux a certain proportion of litharge, in order to produce metallic lead, with which the silver may alloy. A mixture of metallic lead and any suitable flux may be substituted for that of litharge and a reducing flux; but the latter is preferable, because the lead produced is uniformly diffused throughout the whole mass of flux, &c., not allowing a particle of silver to escape its action. be, The reducing agent employed in nearly all assays is charcoal either in its ordinary state, or as it is found in black flux. Starch and other analogous substances may as before mentioned, substituted for it: crude argol is, however, the best reducing agent. The portion employed must be varied according to circumstances, so that the silver-lead produced be not too rich, or that too great a proportion of lead be reduced. If the silver-lead be too rich, much of the precious metal may be lost in the slag, and if too great a quantity of lead be produced, silver is again lost, owing to the long exposure to the fire during cupellation; and indeed this is the most fruitful cause of loss, for more is lost in this manner than by having too little lead produced. In order to know the right proportions, the following data will serve as a guide:-1 part of charcoal reduces about 30 parts of lead from litharge, and 1 part of black flux reduces about 1 part of lead. The fluxes employed in this kind of assay are litharge, black flux, carbonate of potash or soda, and borax. Litharge is an exceedingly convenient flux, because it occupies very little room, and fuses without bubbling, producing very liquid scoria with nearly every substance. Experiment has shown that nearly all argillaceous, stony, and ferruginous. substances fuse very well with from 8 to 12 or more parts of litharge. If from to 1 part of black flux, orth to ath of charcoal, be added to 1 of ore, from a part to 1 part of silver-lead will be produced. Black flux is employed in the fusion of all substances containing a large proportion of alumina, or in which lime is the predominant substance: from 2 to 3 parts of this flux generally suffice: 1 part of litharge is added to the assay, which is wholly reduced, producing nothing but lead. The carbonates of potash or soda produce exactly the same effects as the alkali of the black flux. A certain quantity of charcoal must, in this case, be added to the assay. Schlutter fuses the poor refuse of goldsmith's workshops, mixtures of fragments of crucibles, glass, &c., with 2 parts of carbonate of potash, when they are very earthy, and with 1 part only, when they contain much glass, adding, at the same time, to the mixture, a little litharge and granulated lead. Borax has, like litharge, the advantage of being an universal flux; it is useful especially for the fusion of substances containing much lime; but it is necessary to take great care in the assay, in order to avoid the loss which its boiling up might occasion. This only applies, however, to its use in its ordinary state; if previously fused, that is, used as glass of borax, no particular care need be taken. FUSION WITH OXIDISING REAGENTS. Litharge. The oxidising agents employed in the assay of argentiferous substances are litharge and nitre. Litharge attacks all the sulphides, arsenio-sulphides, &c., and oxidises nearly all the elements, excepting silver, when employed in sufficient quantity, and a quantity of lead equivalent to the oxidisable matters present is reduced, so that there results from the assay a slag containing an excess of oxide of lead, and an alloy of lead and silver, very little contaminated with foreign metals, if no copper be present, and which can be submitted directly to cupellation. This method of assay is exceedingly convenient and quick. The pulverised mineral is well mixed with litharge, and the mixture placed in a crucible, which may be very nearly filled, as there is scarcely any boiling up when the pot and its contents are submitted to the fire. A thin layer of pure litharge is placed above the mixture, the whole is then heated rapidly, and as soon as the litharge, &c., is completely fused, the crucible is taken from the fire. It is inconvenient to heat it for any length of time, on account of the corrosive action litharge has on the substance of the crucible, which it rapidly destroys. The proportion of litharge which must be employed depends upon the nature and quantity of oxidisable matters present in the ore. It ought in general to be very great. because it is absolutely necessary that no sulphurous matters be present, so that the slag may not contain the least trace of silver. But it is known how much litharge is required to decompose the metallic sulphides. Pyrites requires about 50 parts; mispickel, blende, sulphide of antimony, copper pyrites, grey cobalt, and grey copper, require from about twenty-five to about forty times their weight. For sulphide of bismuth 10 are sufficient, and for galena or sulphide of silver but 4 or 5 parts need be employed. The proportion of litharge will not be so great for a mineral containing much stony gangue as for one entirely metallic. Experiment has proved, that the assay of rough schlichs, such as those treated in the large way by amalgamation, can be made very exactly with from 10 to 12 parts of litharge. Alloys of silver with the very oxidisable metals can be assayed by means of litharge, such as those of iron, antimony, tin, zinc, &c.; but in order to have a successful result the |