and a little copper and nickel become oxidized and are absorbed by the flux; silver and gold and the greater part of copper and nickel remain with the lead (and bismuth, if present). When all volatile substances are driven off, the lead begins to become oxidized, and the button assumes a rotary motion; at this period the blast is discontinued, the assay is allowed to cool, and when perfectly cold the lead button is separated from the glass by some slight strokes with a hammer. It is now placed on a cupel of bone-ash and treated with the O Fl until it again assumes a rotatory motion. If much copper or nickel is present, the globule becomes covered with a thick infusible crust, which prevents the aimed-at oxidation; in this case another small piece of pure lead has to be added. The blast is kept up until the whole of the lead and other foreign metals, viz., copper and nickel, are oxidized; this is indicated by the cessation of the rotatory movement, if only little silver is present, or by the appearance of all the tints of the rainbow over the whole surface of the button, if the ore was very rich in silver; after a few moments it takes the look of pure silver. The oxides of lead, copper, &c., are absorbed by the bone-ash, and pure silver, or an alloy of silver with other noble metals, remains behind; the button may be tested for gold, &c., after the method given in § 79.

Sulphur.

§ 106. The presence of sulphur in sulphides may in many cases be detected by heating in a glass tube (v. §§ 11, 14), or on Ch with the O Fl.

§ 107. A very delicate test for the presence of sulphur, in whatever combination it may be contained in the substance, and which possesses moreover the advantage over all other methods of being very easily performed, is to mix the pulverized assay [No. 4] with some pure Sd or, better still, with a mixture of two parts of Sd and 1 of Bx, and to treat it on Ch with the R Fl. The fused mass is removed from the Ch, powdered, the powder placed on a silver foil or a bright silver coin, and a drop of water added. If the substance under examination contained any sulphur, a black spot will be formed on the silver foil, owing to the formation of sulphide of silver from the decomposition of the sulphide of sodium, which, in

The reactions of tin and its compwww Table in 2

§ 110. The presence of tin is indicated by its Ct whe stance [No. 13), alone or mixed with sil, is exposed to

on Ch.

When the substance under examination is an alloy, is conveniently added, which n

as it is formed, and allows

more volatile, ex. gr. antimo

their coatings. Arsenic is detected by its odor, and iron by the color which the Bx bead assumes when re-fused on platinum wire in the O Fl.

To detect copper in tin or its alloy, the assay [No. 52] is fused with a flux consisting of 100 parts of Sd, 50 of vitrified Bx, and 30 of silica. The flame is so directed that the metallic globule assumes a rotatory motion. When in this state the glass is kept covered, as much as possible, with the O Fl, care being taken that the globule is at one side in contact with the glass, and at the other with the Ch. The tin becomes oxidized and the oxide, in the measure as it is formed, absorbed by the flux; the remaining button is copper, pure or with a small quantity of tin, and may be readily tested with the usual fluxes.

Titanium.

§ 111. Titanic acid, when forming the principal constituent of any mineral substance, is easily detected by its behavior with the fluxes, v. Table II, 23; but when in combination with bases these reactions are not always clearly perceptible, being frequently sup pressed by the predominating reaction of the base. In such case we may subject the assay to the following treatment, by which even very small quantities of titanic acid will become apparent : the substance [No. 65] is reduced to a very fine powder, mixed with from 6 to 8 parts of bisulphate of potassa, and fused in a platinum spoon at a low red-heat; the fused mass is dissolved in a porcelain vessel in the smallest possible quantity of water, aided by heat. There remains an insoluble residue which is allowed to settle; the clear liquid is poured off into a larger vessel, mixed with a few drops of nitric acid and at least six volumes of water, and heated to ebullition. If the substance under examination contained any titanium, a white precipitate of titanic acid forms on boiling. The precipitate is collected on a filter, washed with water, acidulated with nitric acid, and tested with S Ph.

Uranium.

§ 112. The presence of this metal is easily recognized, in substances which contain no other coloring constituents, by the reactions given Table II, 25; the most characteristic test is that with

S Ph. In presence of much iron this reaction becomes indistinct; we may then operate in the following manner: the finely-pulverized substance [No. 70] is fused with bisulphate of potassa, the fused mass dissolved in water, mixed with carbonate of ammonia in excess, the liquid separated from the precipitate by filtration, and the filtrate heated to ebullition. If any uranium was present, a yellow precipitate is thrown down, which gives with the fluxes the pure reactions of uranium.

Zinc.

The reactions for zinc and its compounds, see §§ 12, 25, 45, and Table II, 27.

§ 113. A small amount of zinc, when associated with considerable quantities of lead, or bismuth, or antimony, or tin, cannot with certainty be ascertained by means of the Blp.

If the substance under examination contains the zinc as oxide [No. 36], or but a small quantity of sulphide, it is mixed with Sd and treated on Ch in R Fl. Substances consisting essentially of sulphide of zinc may be thus treated without the addition of Sd, and such as contain, beside oxide of zinc, other metallic oxides, are conveniently mixed with some Sd to which about one-half of its weight of Bx has been added. A ring of oxide of zinc is deposited on the Ch. When lead is present [No. 51] the Ct is frequently not pure, being mixed up with the Ct of lead. In this case it is moistened with some So Co and heated again with the O Fl. The oxide of lead is reduced by the red-hot Ch and volatilized, while the oxide of zinc remains behind with a green color (v. § 45).

6

FOURTH CHAPTER.

CHARACTERISTICS OF THE MOST IMPORTANT ORES; THEIR BEHAVIOR BEFORE THE BLOW-PIPE, AND TO SOLVENTS.

§ 114. Or the physical properties of the minerals which are treated of in this chapter, only those are enumerated which serve best to discriminate the different ores from each other. For a more detailed description I must refer to Dana's and other works on mineralogy. Among the distinguishing characters of minerals, their hardness and specific gravity stand foremost. The latter cannot be ascertained without a good balance, and will, for this reason, be of much less use to the practical man than the determination of hardness, an operation which may be performed in a few moments. A set of minerals, representing the scale of hardness, being not always at hand, it will be useful to give a series of substitutes for them, as arranged by Mr. Chapman:

1. Yields easily to the nail.

2. Yields with difficulty to the nail, or merely receives an impression from it. Does not scratch a copper coin.

3. Scratches a copper coin; but is also scratched by it, being of about the same degree of hardness.

4. Not scratched by a copper coin; does not scratch glass.

5. Scratches glass, though rather with difficulty, leaving its powder on it. Yields readily to the knife.

6. Scratches glass easily. Yields with difficulty to the knife. 7. Does not yield to the knife. Yields to the edge of a file, though with difficulty.

8. 9. 10. Harder than flint.

The scale of hardness, as introduced by Mohs, and enlarged by Breithaupt, is as follows:

1. Tale; common laminated light-green variety.

2. Gypsum; a crystalline variety.