│ │ 0·340 │ 98·3 │ .. │ .. │ │Sulphur, │ 0·053 │ 100·0 │ 0·01 │ 98·5 │ │ │ 0·135 │ 99·0 │ .. │ .. │ │ │ 0·236 │ 98·9 │ .. │ .. │ │Tellurium, │ 0·065 │ 100·4 │ .. │ .. │ │ │ 0·181 │ 100·2 │ .. │ .. │ │ │ 0·405 │ 98·7 │ .. │ .. │ │Tin, │ 0·052 │ 97·6 │ 0·05 │ 100·5 │ │ │ 0·097 │ 92·7 │ .. │ .. │ │ │ 0·295 │ 79·8 │ .. │ .. │ │Zinc, │ 0·048 │ 98·3 │ 0·02 │ 98·5 │ │ │ 0·095 │ 96·3 │ .. │ .. │ └─────────────┴─────────┴─────────────┴─────────┴─────────────┘ PHYSICAL PROPERTIES.—The _colour_ of copper is familiar, being a fine salmon pink. The _appearance of the fractured surface_ is a useful guide in several respects as to the condition of the metal, and in the process of manufacture the refiner relies upon this appearance as an important criterion of the progress of the refining operation. Copper containing an excess of oxygen, for example, has a purplish-red colour and a coarse brick-like fracture; this is known as “_dry copper_,” and the metal is brittle and commercially useless when in that form. The ingot of dry copper is also characterised by a depression running along the surface. _Tough copper_ (“tough-pitch”) the mechanically useful variety resulting from the furnace-refining operation, possesses a bright salmon-coloured fracture, finely granular to silky in appearance, whilst “_overpoled copper_,” also brittle and industrially valueless whilst in that condition, has a very light salmon-coloured fracture, and is more coarsely fibrous. The _melting point_ of copper is 1,083° C., and is slightly lowered by the small quantities of impurity usually present in commercial metal. Molten copper is of a pale apple-green colour. The _boiling point_ under ordinary conditions is about 2,300° C. (1,700° C. _in vacuo_). The _electrical conductivity_ is of much importance. Copper ranks second only to silver as a conductor, the relative conductivity of the best copper being about 98 compared with silver as 100. The resistance of 12 inches of pure copper wire, 0·001 inch in diameter, is 9·612 ohms. The conductivity of the metal is decreased by mechanical working, and it follows the general straight-line law connecting conductivity and temperature. The effect of even small quantities of impurity on this property is very marked, so much so that only the purest varieties are suitable for electrical work, and for this reason electrolytic refining is often a necessary operation in the manufacture of copper intended for this purpose.