ferrous sulphate, pyrogallic acid, hydroquinone, eikonogen, etc. When the plate is covered with the developing solution, an image appears; this is due to the precipitation of a film of silver, which produces variations of light and shade. Where the light acted strongly upon the plate, the deposit of silver is relatively heavy; and where there was little action, there is little metal deposited. Just what the action of light upon the silver bromide of a plate is, is not definitely known, but it certainly makes the reduction to silver take place more easily than is the case with ordinary silver bromide. The action of a developer may be illustrated by the following equation:

2 AgBr+C%H4(OH)2+2 KOH

hydroquinone

2 Ag+2 KBr+2 H2O+C6H402.

quinone

In this case the reduction of the silver bromide is due to the oxidation of hydroquinone to quinone.

Fixing. When the plate is sufficiently developed, it is rinsed and put into a bath of sodium thiosulphate ("hypo "; cf. § 268) to remove the silver salts not acted upon by light. This fixes the negative. The plate is called a "negative" because in it dark objects appear light, and light objects dark.

The action of sodium thiosulphate upon silver salts is to form the soluble "mixed " salt, NaAgS2O3.

A " print " is made by placing the film of a sensitized paper next to the negative and exposing both so that the light passes through the negative. The image may appear on the paper at once ("printing-out" papers) or may have to be developed ("developing" papers). In either case the prints are "fixed" by removing the unchanged silver salt.

Toning. Some papers are toned" in a bath of gold chloride,

METALLURGY OF GOLD.

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AuCl3, or platinum chloride, PtCl, before fixing. Toning replaces part of the silver by gold or platinum.

Blue prints are made on paper coated with a ferric salt (ferric ammonium citrate) and potassium ferricyanide, K,Fe(CN)6. After exposure, the picture is developed and fixed by washing it in water. The result is a blue print on a white ground. The process is used for copying plans, etc.

467. Gold. Gold is found both native and combined. Even native gold is not pure, however, but contains silver, and often iron, copper, etc. The metal is frequently found enclosed in quartz or quartzsand.

Gold is obtained chiefly from Colorado and other western states, from Alaska, and from Australia, Siberia, and South Africa. The gold produced in the United States during 1910 was 4,657,018 fine ounces, worth $96,269,100. This was about one-fourth of the world's yield that year.

468. Metallurgy of Gold. - Gold-mining is of two general kinds: (1) placer-mining, and (2) veinmining.

In placer-mining the clay and sand containing the gold are washed with water. The lighter particles are thus removed; while the gold and other heavy metals remain. Gold and silver are extracted from this mixture by mercury (cf. § 462; amalgamation).

Vein-mining consists in removing the gold-bearing rock from the earth and crushing it in stamp mills. The lighter materials are then washed away, and the gold is collected with mercury, as in placer-mining. Hydraulic-mining is a form of placermining done on a large scale with powerful streams of water.

Instead of mercury, chlorine and bromine are used to remove gold from the crushed ore. They form the soluble gold chloride, AuCls, or bromide, AuBr3. This is extracted with water, and the gold is precipitated by means of charcoal or ferrous sulphate (cf. § 469).

The Cyanide Process depends upon the fact that gold is converted into the soluble double cyanide, KCN. AuCN, by a solution of the alkali cyanides. The gold is separated from the cyanide solution by electrolysis or by means of zinc.

469. Purification of Gold. The gold obtained by the processes described above is not pure. It can be separated from silver by adding aqua regia, which reacts with the gold. The solution is evaporated to remove nitric acid, the residue is dissolved in water, and the gold is precipitated by ferrous sulphate or some other reducing agent.

3 FeSO4+AuCl3 →→→ Fe2(SO4)3 + FeCl3+Au.

In the treatment of silver and gold with sulphuric acid (cf. § 462, end) gold is left in the kettle as a brown, spongy mass. This is washed, dried, and melted in a crucible with charcoal and sodium carbonate. The resulting product, chemically pure gold, is poured into a mold, and leaves it as a gold brick.

470. Properties and Uses. Gold is the only common metal that is yellow. It is a good conductor, and the most ductile and malleable substance known. Its specific gravity is 19.3, and its melting temperature 1060° C.

Gold unites directly with chlorine and bromine, but not with oxygen. Aqua regia reacts with gold, form

ing auric chloride, AuCl, (cf. § 466; "toning "); but the common acids do not affect it.

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Gold is the standard of coinage of most nations. It is hardened by alloying it with copper (10% in the United States). For jewelry the proportion of gold varies from 40% to 75%; it is usually given as so many carats fine." Pure gold is 24 carats fine; hence 18-carat gold is 75% gold and 25% alloy. Because of its malleability and weak chemical action, gold is much used by dentists for filling teeth. Gold-leaf is used in ornamentation.

471. Exercises.

1. What are the atomic weights of copper, silver, and gold, respectively? Which forms the most stable oxide (cf. § 405)? The least? Which is affected most readily by the air?

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2. In the heating of copper matte in a converter," why are arsenic, lead, and iron oxidized before copper? Which of these form volatile oxides? What materials might "green wood yield in the "poling " process that could reduce copper oxide?

3. Cupric oxide "dissolves" in warm, concentrated hydrochloric acid. Write the equation. When the solution is heated with copper, a white solid separates out. What is it? Write the equation for its formation.

4. From the difference in the solubilities of silver chloride, bromide, and iodide (cf. Appendix viii), what would happen if the chloride and bromide were heated with a solution of potassium iodide? Give the equations.

5. How would you distinguish between the nitrates of barium, copper, cadmium, zinc, mercury (ic), and silver?

6. How would you separate the copper and silver of a silver coin so as to recover each metal in the free state? Write all the equations.

7. How would you separate brass into copper and zinc?

8. Give the uses of the alloys named in this chapter.

CHAPTER XXXIV.

ALUMINUM.

472. Occurrence of Aluminum. —

Although aluminum does not occur free, it is the most abundant and widely distributed metal. Only oxygen and silicon are more abundant. Some of its most important minerals are potash feldspar (KAlSi3O8), mica (muscovite), H2KAl(SiO4)3, and cryolite (cf. § 316). Granite is a mixture of quartz, feldspar, and mica. Clay results when granite and similar minerals are decomposed (cf. § 10).

All the other elements of the aluminum group are rare (cf. Periodic Table, § 383).

473. Preparation. — Aluminum was formerly produced from anhydrous aluminum chloride and sodium.

This method has been superseded by electrolytic processes, of which that of Hall (1887) is perhaps the most important. This is carried out on a large scale at Niagara.

Hall's Process. The furnace used in the Hall process is a box of boiler iron (Fig. 87), the bottom and sides of which are lined with a mixture of coke and tar, rammed hard. The bot