COP 
a variety of yellow or gold-coloured alloys, 
known by the names of tombac, Manheim, 
or Dutch gold, tinsel, similor. Prince Ru- 
pert’s metal, Pinchbeck, &c. ; but tlie pre- 
cise composition varies according to the 
fancy or the experience of different manu- 
facturers. The Dutch gold may be beaten 
out into extremely fine leaves, which, when 
fresh, have nearly the brilliance of gold-leaf, 
and are used as a cheap imitation of it ; but 
they tarnish very soon. The mixture may 
be made, either by directly melting copper 
and zinc, or by mixing brass and copper. 
In either case the copper should be melted 
fimt, and the zinc added afterwards, the 
whole stirred together with wood, covering 
it with a little charcoal, and poured out im- 
mediately, to prevent the loss by the burn- 
ing off file zinc. A kind of tombac is 
the material of which a large proportion of 
the Roman coins was composed. Klaproth 
on analyzing several struck during the first 
century of the emperors, found them all to 
consist either of pure copper, or of copper 
and zinc, in which the latter metal made 
generally from a fifth to a sixth of the mass. 
A little tin and lead were found in some ; 
but in such small proportion as to appear 
only an accidental impurity. 
The alloys of copper and tin are ex- 
tremely important in the arts, and curious 
as chemical mixtures. They form, in differ- 
ent proportions, mixtures which have a dis- 
tinct and appropriate use. Tin added to 
copper makes it more fusible; much less 
liable to rust or corrosion by common sub- 
stances; harder, denser, and more sonorous. 
In these respects the alloy has a real advan- 
tage over unmixed copper; but this is in 
many cases more than counterbalanced by 
the extreme brittleness which even a mo- 
derate portion of tin imparts, and which is 
a singular circumstance, considering how 
very malleable both metals are before mix- 
ture, and the remarkable softness and duc- 
tility of tin. 
Copper, or sometimes copper with a little 
zinc, alloyed with as much tin as will make 
from about ^th to about rfii of the whole, 
forms an alloy which is the principal, and 
often the only composition for bells, brass 
cannon (so called), bronze statues, and se- 
veral smaller purposes, and hence it is call- 
ed bronze, or bell-metal ; and it is excel- 
lently fitted for these purposes, by its hard- 
ness, density, sonorousness, and fusibility, 
whereby the minute parts of hollow moulds 
may be readily filled before it fixes in cool- 
ing. For cannon a lower portion of tin 
COP 
seems to be used. Bronze cannon are 
much less liable to rust than those of iron ; 
but in large pieces of ordnance, by very 
rapid firing, the touch-hole is apt to melt 
down, and spoil the piece ; of which there 
is a remarkable instance at the Tower of 
London, of a mortar of the largest calibre 
thus spoiled at the siege of Namur. On ac- 
count of the sonorousness of bronze, fiiese 
cannon give a much sharper report than 
those of iron, which for a time impairs the 
hearing of the people that work them. A 
common alloy for bell-metal is about 80 of 
copper to 20 of tin ; or where copper, brass, 
and tin are used, the copper is fi'om 70 to 
80 per cent, including the portion contain- 
ed in the brass, and the remainder is tin 
and zinc. The zinc certainly makes it 
more sonorous. Antimony is also often 
found in small quantity in bell-metal. Some 
of the finer kinds used for small articles con- 
tain also a little silver, which much improves 
the sound. When the tin is nearly one- 
third of the alloy, it is then most beautifully 
white, with a lustre almost like that of 
mercury, extremely hard, very close-grain- 
ed, and perfectly brittle. In this state it 
takes a most beautiful polish, and is admi- 
rably fitted for the reflection of light for all 
optical purposes. It is then called specu- 
lum metal, which, however, for flie extreme 
perfection required in modern astionomical 
instraments, is better mixed with a very 
small proportion of other metals, particu- 
larly arsenic, brass, and silver. 
When more tin is added than amounts to 
half the weight of the copper, the alloy be- 
gins to lose that splendid whiteness for 
w'hich it is so valuable as a mirror, and be- 
comes more of a blue-grey. As the tin in- 
creases, the texture becomes rough-grained, 
and as it were rotten, and totally unfit for 
manufacture. The speculum metal is there- 
fore in the highest proportion of alloy of 
tin that copper will admit, for any useful 
purpose. See Aikin’s Dictionary of Che- 
mistry. 
COPPERAS is the sulphate of iron, 
and is commonly called green vitriol. If 
sulphuric acid be diluted with water, and be 
poured upon iron, much effervescence will 
be seen : the • metal will be dissolved, and 
the solution, when evaporated, will exhibit 
the sulphate of iron, or common copperas, 
which is a neutral salt in a very impure 
state. Copperas is the basis of many dyes: 
it gives a fine black, though it rafiier sub- 
jects the material to decay, unless used 
with extreme caution ; the least excess oc- 
