In the open tube, much sulphurous acid is given off. With Soda, reduced iron and globules of copper are obtained, provided the ore has been sufficiently roasted.

SULPHIDE OF TIN AND COPPER, TIN PYRITES.-Before the blowpipe it becomes, by roasting, covered with a snow-white powder, which is oxide of tin. The white powder also

encircles the globule to the extent of about two lines.

In the open tube, sulphurous acid is given off.

NEEDLE-ORE, AIKENITE.-Alone, it fuses, giving off vapour, which coats the charcoal snow-white, slightly yellowish on the interior edge, finally giving a metallic bead resembling bismuth.

In the open tube it gives off a white smoke, one part of which is fusible, and the other volatile. The first part is converted by fusion into limpid drops, which become white by cooling; there is also an odour of sulphurous acid. Treated by fluxes, the resulting bead of bismuth gives the reaction of copper. After a long blast, a grain of copper may be obtained, which by cupellation with lead gives traces of silver. A fusible white smoke, at the commencement of the operation, indicates the presence of tellurium.

CHLORIDE OF COPPER.-Alone, colours the flame blue, with greenish edges. A red pulverulent deposit forms on the charcoal around the assay; the fused matter reduces, giving a grain of copper, surrounded by slag.

With fluxes, the chloride behaves as the oxide.

CARBONATE OF COPPER.-Alone, in the matrass, gives water, and blackens.

On charcoal it fuses, and behaves like oxide of copper. ARSENIATE OF COPPER behaves with fluxes in the same manner as the oxide of copper, but exhales a strong odour of arsenic, and gives, when reduced with soda, a white and brittle bead.

OXIDE OF COPPER.-Alone, in the oxidising flame, it is fused into a black bead, which is reduced on charcoal. In the reducing flame, at a temperature which does not suffice to fuse copper, the oxide is reduced, and shines with the lustre characteristic of metallic copper; but as soon as the blast ceases the metal re-oxidises, and becomes black or brown.

Exposed to a stronger heat, it gives a bead of metallic on fusion.

copper

With borax, oxide of copper readily fuses in the oxidising flame, forming a beautiful green glass, which loses its colour in the reducing flame, but which on cooling becomes cinnabar-red and opaque. If the oxide of copper be impure, the glass is generally deep brown, and only becomes opaque in an intermittent flame.

With microcosmic salt it fuses, attended with the same phenomena as with borax. If the quantity of copper be small, the glass occasionally becomes transparent and rubycoloured in the reducing flame; this change takes place at the instant of solidification. Commonly the glass becomes red and opaque, similar in appearance to an enamel.

When the quantity of the copper is so small that the character of the red oxide cannot be made evident in the reducing flame, a small quantity of tin must be added, and the flame kept up only for an instant. The glass, previously colourless, becomes red and opaque by cooling. If the blast be kept up too long, the colour is destroyed, owing to the reduction of the copper.

With soda, on the platinum wire, a beautiful green glass is formed, which becomes opaque and colourless on cooling. On charcoal it is absorbed, and the oxide reduced. The blowpipe is capable of detecting a smaller quantity of copper than any other test; especially when it is not in combination with other metals, which by their reduction would disguise its presence. In the latter case we must use borax and tin. When copper and iron are associated together, a single assay separates them into distinct particles; the one may be told by colour, and the other by being attracted by the magnet.

It is difficult, by borax or microcosmic salt, to determine this copper in slags as protoxide or suboxide, on account of the small quantity generally present; and, moreover, the other ingredients, which are chiefly silicates of different earths and difficultly reducible metallic oxides, destroy the reaction of oxide of copper. For this reason, instead of employing the reduction process, the slags must

be treated with soda on charcoal. If by this method, also, copper should not be detected, a greater quantity, about 100 milligrammes, must be reduced with its own quantity of soda, half of borax, and 30 to 50 milligrammes proof lead, and the lead, united to a globule, treated with boracic acid till all is dissolved, or the copper, is concentrated. If the slag contains a trace of copper, this becomes reduced, and combines with the lead, and, in the first case, colours the boracic acid red, green, or blue. If the copper present is very minute, the tinge is seen on those parts only where the latter part of the lead containing copper was dissolved.

A small quantity of copper contained in a substance can often be detected, if not in combination with sulphuric acid, by one or two drops of hydrochloric acid. It is only necessary to moisten the substance with this acid, and heat it in the forceps, in the azure of the blue flame, when, by this means, the outer flame is coloured greenish-blue, and often reddish-blue, by the chloride of copper formed.

373

CHAPTER X*

ASSAY OF LEAD.

ALL minerals and substances containing lead may, for the purposes of the asssayer by the dry way, be divided into four classes:

Class I. comprises sulphides, antimonial or otherwise (galena, &c.).

Class II. includes all plumbiferous substances containing neither sulphur nor arsenic, or mere traces only of these elements (litharge, minium, carbonate of lead, native and artificial, lead fume, cupel bottoms, furnace hearths, lead slag, &c.).

Class III. comprises all substances into whose composition either sulphuric, arsenic, chromic, or phosphoric acid, or a mixture of either, enters (pyromorphite, wolframite, &c.).

Class IV. Alloys of lead.

CLASS I.

Before describing the different modes of assaying substances of this class, it will be as well to pass in review the action of various reagents on sulphides of lead, in order that the rationale of the assay of those ores may be better appreciated.

Action of Oxygen.-If galena be roasted at a very gentle temperature, care being taken to avoid fusion, it will be converted into a mixture of oxide of lead and sulphate of lead, with evolution of sulphurous acid, thus:

2(PbS)+70=PbO+PbO,SO2+ SO2.

Action of Metallic Iron.-This metal completely and readily

decomposes sulphide of lead, giving metallic lead in a pure state, thus:

PbS+Fe-Pb+ FeS.

On the one side we have sulphide of lead and metallic iron, on the other metallic lead and sulphide of iron.

The Alkalies and Alkaline Carbonates decompose sulphide of lead, but only partially; pure lead is separated, and at the same time a very fusible grey slag is formed, which contains an alkaline sulphate and a compound of sulphide of lead and an alkaline sulphide. A certain proportion of the alkali is reduced by the sulphur, which is converted into sulphuric acid, so that no oxide of lead is produced. reaction may be thus expressed :

:—

7 (PbS)+4(KO) = 4Pb+KO,SO,+3(PbS,KS).

This

Nitrate of Potash completely decomposes sulphide of lead, with the reduction of metallic lead and formation of sulphate of potash and sulphurous acid, thus :—

2(PbS) + KO,NO,=2Pb+KO,SO3+SO2+N.

If the nitre be in excess, the lead will be oxidised in proportion to the excess present, and if there be a sufficiency added, no metallic lead at all will be produced.

Argol. The presence of carbonaceous matter much favours the decomposition of galena, by determining the reduction of a larger quantity of potassium to the metallic state, and thereby the formation of a larger quantity of alkaline sulphide. With 4 parts of argol to 1 part of sulphide, 80 parts of lead are reduced. If the reaction were complete, the decomposition would be as follows:

PbS+ KO+ C = Pb+ KS + CO.

For the reactions of oxide of lead (litharge) and the sulphate of lead on sulphide of lead, see pages 179 and 181.

From the reactions above given, it will be seen that there are many substances capable of completely reducing the lead from its sulphide, and yet few can be used safely with any advantage, as so to use them would imply a knowledge of how much sulphur and lead were in the ore to be assayed,