The alloys of copper and tin are ex tremely important in the arts, and cu rious as chemical mixtures. They form, in different proportions, mixtures, which have a distinct and appropriate use. Tin added to copper makes it more fusible ; much less liable to rust or corrosion by common substances ; harder, denser, and more sonorous. In these respects the alloy has a real advantage over unmixed copper ; but this is in many cases more than counterbalanced by the extreme brittleness which even a moderate por tion of tin imparts, and which is a singu lar circumstance, considering how very malleable both metals are before mix ture, and the remarkable softness and ductility of tin.
Copper, or sometimes copper with a little zinc, alloyed with as much tin as will make from about one-tenth to about one-fifth of the whole, forms an alloy, which is the principal, and often the only composition for bells, brass cannon (so called,) bronze statues, and several smaller purposes, and hence it is called bronze, or bell-metal; and it is excellent ly 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 cooling. For cannon, a lower portion of tin 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 account of the sonorousness of bronze, these cannon give a much sharper report than those of iron, which for a time impairs the hear ing 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 from 70 to 80 per cent. including the por tion contained in the brass, and the re mainder is tin and zinc. The zinc cer tainly makes it more sonorous. Antimo ny is also often found in small quantity in bell-metal. Some of the finer kinds used for small articles contain 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-grained, and perfectly brittle. In this state it takes a most beautiful polish, and is admirably fitted for the reflection of light for all optical purposes. It is then called specu lum metal, which, however, for the extreme perfection required in modern astronomical instruments, is better mix ed with a very small proportion of other metals, particularly arsenic, brass, and silver..
When more tin is added than amounts to half the weight of the copper, the alloy begins to lose that splendid white ness, for which it is so valuable as a mirror, and becomes more of a blue grey. As the tin increases, the texture becomes rough-grained, and as it were rotten, and totally unfit for manufacture. The speculum metal is therefore in the highest proportion of alloy of tin that copper will admit, for any useful purpose. See Aikin's Dictionary of Che mistry.