d is purplish when the metal contains cuprous oxide. Its
specific gravity varies between 8.91 and 8.95, according to the
treatment to which it may have been subjected; J. F. W. Hampe gives
8.945 (0 deg./4 deg.) for perfectly pure and compact copper. Ordinary
commercial copper is somewhat porous and has a specific gravity ranging
from 8.2 to 8.5. It takes a brilliant polish, is in a high degree
malleable and ductile, and in tenacity it only falls short of iron,
exceeding in that quality both silver and gold. By different authorities
its melting-point is stated at from 1000 deg. to 1200 deg. C.; C. T.
Heycock and F. H. Neville give 1080 deg..5; P. Dejean gives 1085 deg. as
the freezing-point. The molten metal is sea-green in colour, and at
higher temperatures (in the electric arc) it vaporizes and burns with a
green flame. G. W. A. Kahlbaum succeeded in subliming the metal in a
vacuum, and H. Moissan (_Compt. rend._, 1905, 141, p. 853) distilled it
in the electric furnace. Molten copper absorbs carbon monoxide, hydrogen
and sulphur dioxide; it also appears to decompose hydrocarbons (methane,
ethane), absorbing the hydrogen and the carbon separating out. These
occluded gases are all liberated when the copper cools, and so give rise
to porous castings, unless special precautions are taken. The gases are
also expelled from the molten metal by lead, carbon dioxide, or water
vapour. Its specific heat is 0.0899 at 0 deg. C. and 0.0942 at 100 deg.;
the coefficient of linear expansion per 1 deg. C. is 0.001869. In
electric conductivity it stands next to silver; the conducting power of
silver being equal to 100, that of perfectly pure copper is given by A.
Matthiessen as 96.4 at 13 deg. C.

Copper is not affected by exposure in dry air, but in a moist
atmosphere, containing carbonic acid, it becomes coated with a green
basic carbonate. When heated or rubbed it emits a peculiar disagreeable
odour. Sulphuric and hydrochloric acids have little or no action upon it
at ordinary temperatures, even when in a fine state of division; but on
heating, copper sulphate and sulphur dioxide are formed in the first
case, and cuprous chloride and hydrogen in the second. Concentrated
nitric acid has also very little action, but with the dilute acid a
vigorous action ensues. The first products of this reaction are copper
nitrate and nitric oxide, but, as the concentration of the copper
nitrate increases, nitrous oxide and, eventually, free nitrogen a