COP

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arid totally unfit for manufacture. The fpecu-lum metal is therefore in the higheft proportionof alloy of tin that copper will admit for anylifeful purpofe.

A perfe£t fpeculum-metal lhould be quitewhite without (hewing any call of yellow whenpolilhed, not very liable to tarnifh, quite freefrom pores even when examined by a lens, of acertain coherence or toughnefs to bear the grind-er, and for the convenience of working, as foftas may be confident with the other requifites.

Mr. Mudge, whofe fpecula were celebratedfor their goodnefs, obferves, that the extreme ofwhitenefs is given by 32 parts of copper and 16of tin,' but this is exceffively hard and brittle;that 32 of copper with 14^ of tin is ftill quitewhite and as hard as can be wrought. He alfoobferved by many trials, that the metal to turnout free from pores fhould be twice fufed, thatis, the firlt time for the purpofe of mixture (inwhich the copper is to be firlt melted feparately)and then remelted with as little heat as poffible,for calling. As there is always fome lofs bythe calcination, chiefly of the tin, a little allow-ance in the proportion of this latter may bemade on account of the double fufion.

The mod elaborate mixture and accuratedirections on this important fubjedl are givenby Mr. Edwards, whofe fpecula are of extremeexcellence, and are publifhed in the nauticalalmanac for 1787,“ of which the following arethe leading particulars.

The quality of the copper fhould fird be triedby adding fucceffively from fo much fhort ofhalf its weight of tin that it proves a little yel-low, to the full half of tin, and by companionafcertaining the maximum of whitenefs, ob-ferving that beyond this point the alloy beginsto lofe part of the brilliance of fraCture and tobecome bluifli. When this is found, take 32 partsof the copper, melt it, add one part of brafs andas much filver, with a little black flux to coverthe furface ; when thefe are melted, dir with awooden rod, and pour in from 15 to 16 partsof tin (according as found neceffary by pre-vious experiment) fufed in a feparate cruciblewith a low heat, dir the mixture again, andimmediately pour it into cold water. Then inthe fecond melting (with as little heat as willfuffice) take for every 16 parts of the compo-fition 1 part of white arfenic, wrap it in paper,thrud it to the bottom of the fluid metal, anddir with a wooden rod till no more arl'enicalfumes efcape, immediately after which cad themetal in a fand-mould. Then while it is dill

red-hot lay it in a pot full of very hot embersand cool it very flowly: unlefs this precautionis particularly obferved, the metal will fly inpieces when cold, fometimes even long afterall fuch danger is thought to be pad; or it willfplit in the polilhing. For the particular man-ner of conltru Cling the mould, and the wholeof the nice and laborious operations of grindingand polilhing to a perfectly true figure, thereader is referred to the two papers above men-tioned.

Both the brafs, and the filver, and the arfenicappear to have their didindt ufe. The bralsmakes the mixture tougher and not fo excef-fively hard and brittle. The filver improvesboth the texture and colour, but is not an eflentia!though really an ufeful addition. The arfenicis found by a£tual comparifon to make themetal finer, and particularly clofer in texture,and therefore lefs liable to be porous. It fen-fibly increafes the fpecific gravity, which, be-fore the arfenic is added, is 8.78, but after-wards, 8.89. It is added in die fecond melt-ing, that as little as poffible may be diffipatedin vapour. A greater proportion would makethe metal liable to tarnilh. An alloy contain-ing 6 of copper, 2 of tin, and 1 of arfenic, isnearly the proportion of Sir I. Newton’s fpe-cula, which is very good, but poliflies fome-what yellow.

The feparation of copper from bell-metal andall the tin alloys in the large way, happened tobe an objeCl of confiderable importance inFrance in the midlt of the revolutionary war,when the importation of copper was nearly im-practicable, and a large quantity of it was re-quired for general purpofes, and particularlyfor the coinage. A number of very ingeniousexperiments were then made on the belt methodof freeing the metal of church bells from theirtin and other alloy, and obtaining thence a goodmalleable copper with the lealt poffible expenceor lofs.

The circumftances in which it is worth whileto melt down great bells for the fake of the purecopper do not often occur, but the various pro-cefles may be here jhortly noticed as importantto the hiftory of the chemical properties of cop-per alloys, and as a very happy example of theapplication of theoretic chemiltry to the pur-pofes of manufacture. Pelletier* and Fourcroy tfeem to have chiefly diftinguilhed themfelves inthis refearch, and each chemilt appears to havefollowed nearly the fame track, but with anacknowledged priority in the former.

• Phil. Tranf. vol. 67. » or Nicholfon’s Journal, 4to, vol. 3. x Memoire*, lorn. I & i. r An. Chita, tom. 9c,