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many cafes more than counterbalanced by theextreme brittlenefs, which even a moderate por-tion of tin imparts, and which is a Angular cir-cumftance confidering how very malleable bothmetals are before mixture, and the remarkablefbftnefs and dudlility of tin.
The fenfible qualities of the different mix-tures are the following. Copper alloyed withfrom I to about 5 per cent, of tin is muchharder than before, the colour yellow with acall; of red, and the fracture granular. It isftill confiderably malleable. This appears to bethe ufual compofition of many of the very an-tient copper tools and weapons before the com-mon ufe of iron; 1 whence it appears that theancients did not (as has often been fuppofed)poffefs any peculiar art of hardening pure cop-per, otherwife than by mixture. It is certainthat the quenching of red hot copper in waterwill not at all make it harder, or have any fucheffedt as it has upon iron. An alloy in whichthe tin is from ■— to ^ of the whole is hard,brittle, but ftill a little malleable, clofe-grained,and yellowifh-white. Where the tin is as muchas of the mafs, it is now entirely brittle, andcontinues fo in every higher proportion. Theyellownefs is not entirely loft till the tin isabove -/yds of the whole.
_ Copper, or fometimes copper with a littlezinc, alloyed with as much tin as will make fromabout T V to about of the whole, forms analloy which is the principal, and often the onlycompofition for bells, brafs cannon (fo called)bronze ftatues, and feveral fmaller purpofes,and hence it is called Bronze , or Bell-Metal (al-ways obferving that there is no perfect uni-formity in the different alloys under thefenames, either in the proportion or the actualnumber of ingredients) and it is excellentlyfitted for thefe purpofes, by its hardnefs, den-fity, fonoroufnefs, and fufibility, whereby theminute parts of hollow moulds may be readilyfilled before it fixes in cooling. For cannon, alower portion of tin feems to be ufed. Accordingto Dr. Watfon, * the metal ufed at Woolwich is100 parts of copper and 8 to 12 of tin. Henceit ftill retains fome little malleability, and ofcourfe is tougher than with more tin. Bronzecannon are much lefs liable to ruft than thofeof iron, but in large pieces of ordnance, by veryrapid firing the touch-hole is apt to melt downand fpoil the piece; of which there is a remark-able inftance at the Tower of London of amortar of the largeft calibre thus fpoilt at the«ege of Namur . On account of the fonorouf-
' Dize. Jour. Phyf.
nefs of bronze, thefe cannon give a much{harper report than thofe of iron, which for atime impairs the hearing of the people thatwork them. A common alloy for bell-metal isabout 80 of copper to 20 of tin ; or where cop-per, brafs, and tin are ufed, the copper is from70 to 80 per cent, including the portion con-tained in the brafs, and the remainder is tin,and zinc. The zinc certainly makes it morefonorous. Antimony is alfo often found infmall quantity in bell-metal. Some of the finerkinds ufed for fmall articles contain alfo a littlefilver, which much improves the found.
When the tin is nearly one-third of the alloyit is then moft beautifully white, with a luftrealnioft like that of mercury, extremely hard,very clofe-grained, and perfectly brittle. Inthis ftate it takes a moft beautiful polifh, andis admirably fitted for the reflexion of lightfor all optical purpofes. It is then called Jpe-culttm metal , which however, for the extremeperfection required in modern aftronomical in-ftruments, is better mixed with a very fmallproportion of other metals particularly arfenic,brafs, and filver. But the bafis of thefe com-pofitions is copper alloyed with nearly half itsweight of tin. The ufe of this alloy for thefame purpofe is of great antiquity, and certainlywas in frequent ufe in the time of Pliny. Klap-roth analyzed a portion of an antient lpeculumin the following manner. The fragment wascompadt, very hard, and brittle, the frelli fracturegreyilh white, which by polilhing acquired atbeautiful luftre. A hundred grains were heatedwith nitric acid, whereby a blue folution wasmade, and a part remained undiflblved. Thefolution being firft tried without cffedt withmuriat of foda for filver, was mixed with a fo-lution of fulphat of foda, which gave a whiteprecipitate of fulphat of lead, equivalent to 6grains of lead. The copper was then feparatedby iron, and amounted to 62 grains. The un-diflblved portion flowly digefted with muriaticacid, gave a ftraw-yellow folution, which, de-compofed by zinc, gave 32 grs. of metallic tin.This fpeculum therefore confifted of 62 partsof copper, 32 of tin and 8 of lead, which laftwas probably an adulteration of the tin and notadded defignedly.
When more tin is added than amounts tohalf the weight of the copper, the alloy beginsto lofe that fplendid whitenefs for which it isfo valuable as a mirror, and becomes more of ablue-grey. As the tin increafes, the texturebecomes rough-grained, and, as it were, rotten*1.36. • ElTays.