Chav. II.
COMPOSITION AN1) USE OF .MINERALS.
671
Copper
-
-
- 1
-
32
- 50
Sulphur
-
•
- 1
-
16
50
1
48
100
Carbonate of Copper (Malachite) is found native, but never regularly crystallised; it is ofa fine green colour, but sometimes of a beautiful blue.
Oxide of copper - 2 - 80 78*43
Carbonic acid - 1 - 22 21*57
1 102 100-00
Alloys of Copver. — Ormolu is an alloy of copper and zinc in equal quantities,melted at the lowest temperature at which copper will fuse; they are then stirredtogether, and when thoroughly mixed, a further quantity of zinc is added in small quan-tities, until the alloy has attained in the melting pot the desired colour. It should beobserved that the zinc will fly off in vapour if the temperature is too high, and the residuewill become spelter, or hard solder only ; but when the operation is properly managed thealloy will acquire first a brassy yellow, then, by adding a little more zinc, a purple or violethue, and afterwards become perfectly white, which is the proper hue for the compound in afused state.
Sterling or Standard Gold consists of 11 of gold and 1 of copper — specific gravity17*157.
Brass is an alloy of copper and zinc, usually in the proportions of from 12 to 18per cent, of zinc ; its specific gravity varies from 7*8 to 8*4. It may be also made by mixing50 parts of oxide of copper, 100 of calamine, 400 of black flux, and 30 of charcoal powder;these melted in a crucible till the blue flame is no longer seen, a button of brass of a goldencolour is found at the bottom, weighing about one-sixth more than the pure copper ob-tained from the same quantity of oxide.
Pinchbeck , or Dutch Gold , contains more copper than exists in brass ; a little tin is some-times added.
Speeidum Metal is an alloy of copper, tin, and arsenic.
Bell Metal and Bronze .* the former consists of 3 parts of copper and 1 of tin, the latter offrom 8 to 12 of tin with 100 of copper ; when a shrill sound is required zinc in a smallproportion is added, and sometimes lead. Bronze requires its texture to be softened whenheated; and then suddenly cooled.
Tinned Copper —Vessels intended to be tinned have their surface cleaned and washedwith sal-ammoniac; a bit of tin then rubbed over it unites and covers the copper.
Gun Metal • copper 663, tin 336 ; another variety, copper 764, tin 236.
Bronze for Statues usually contains 0*20 parts of tin. In an ancient Egyptian poniardwas found 85 copper, 14 tin, and 1 iron *. in a mirror 62 copper, 32 tin, 6 lead.
German Metal: copper 534, nickel 175, zinc 291; it resembles silver of 18 carats; it isemployed in the manufacture of all ornamental works where silver is used: 90 parts ofcopper, 5 of zinc, and 5 of antimony, is the best alloy to make plummer’s blocks for the ironor steel gudgeons of machinery to run in.
The Copper and Tin Mines in Devonshire and Cornwall may be considered to commenceat Dartmoor and terminate at the Land’s End. The surface of the country is gently undu-lating, the loftiest hills rarely exceeding 1000 feet above the level of the sea, and the planesat their bases are in general from 100 to 200 feet above high-water. The highest peaks aregranite, and the lower hills and the plains consist of various slates. The granite may be saidto occupy Dartmoor, the neighbourhood of Rough Tor and Brownwilly, the Ilengsbarrowdistrict, the Cairn Bren range, which is separated from that of Wondron by a narrow slip ofslate near Pendarvis, and the western tract, which extends from St. Ive’s to the Land’s End.Granite is also found at Kithill, Brenge, and St. Michael’s Mount, and in a few other localities.All the other parts of Cornwall (except the Lizard district, which is of serpentine,) may beconsidered to consist of slate of various kinds.
The granite, in general cross-grained and of porphyritic structure, contains felspar,quartz, and mica ; but in some places the mica is replaced by talc; the rock is then calledChina-stone, the felspathic portions of which, when decomposed, are washed out and pre-pared as porcelain earth for the manufacture of earthenware. In some of the granite schorlabounds. In the year 1838, 28,(XX) tons of this porcelain clay and China-stone were exportedfrom Cornwall to the Potteries.
The slates are in general felspathic, and near the granite their structure is often com-pact, whilst at greater distances they are lamellar and schistose in their structure, and stillfarther off they become fissile, and make excellent roofing slates. The lamina? of the slatesusually dip from the granite, round the flanks of which they are symmetrically arranged,