ALL 



was appropriated to tlie lowering the purity of gold or filver 

 by mixture with copper, previoufly to its being coined or 

 maiiufackured, it fecins gradually to hnve afTumcd the ortlio- 

 praphy and meaning of the Englifli verb to allay, /'. e. to 

 ab;'.te, to lowir. All the other known combinations of 

 metal; with eacli other were fimply called nii.\fil metals. But 

 as inercafcd attention has been paid to tiie accuracy of che- 

 mical Nomenclature, the term alloy has at length compre- 

 hended all the binary and more complicated metallic com- 

 pounds ; thofe of which mercuiy makes a part, being indeed 

 more ^encrallv known by the name Amalgam. 



Anailoy, therefore, may be defined, a combination of 

 anv two or more metals, into one homogeneous mafs ; to 

 tlie exprefs cxclufion of mere mechanical mixtures, which, 

 however, in fome cafes, are not eafily diftinguilhed from 

 ccnuine alloys. The moll valuable and ufeful of thefe have 

 acquired peculiar names, fuch as brafs, type-metal, tutenag, 

 bron/.e, fpeculum-metal, &c. all of which will be defcribed 

 in their proper places hereafter ; the objeA in this article 

 bein"' confmed to the ilatcment of fuch general fafts and 

 inferences as arc deducible from a comparifon with each other 

 of thofe experiments on the combinations of metals that 

 have been made with fuflicicnt accuracy, the number of which 

 is unfortunately very fmail. 



As no metal, except mercur)', is fluid at the common at- 

 mofpbcrical temperature, and as without the fluidity of one 

 at lead of the ingredients, no metallic combinations can take 

 place, it is ncceifar)' to expofe the materials in a crucible, or 

 other proner veflel, to a 1-eat fomewhat greater than is re- 

 quired for the liqutfaftion of their moft fufible part ; but as 

 this diminution of cohefion is equally favourable to intimate 

 mixture as to chemical combination, and fince all tendency 

 to reparation ceafcs, the very moment of the congelation of 

 the mafs, it no doubt often happens that thefe two circum- 

 ftances are confounded together, and thus caufe many of the 

 anomalies and peculiar difiiculties with which the fubjeft is 

 encumbered. 



The method that is given in moft chemical books for af- 

 certaining whether a mafs of metal is a real alloy, or only a 

 mechanical mixture, confifts in fufing it with as little heat as 

 poflible, and keeping it in this ftate till its component parts 

 feparate from each other, like oil from water, according to 

 their refpeftive fpecific gravities ; and perhaps there is upon 

 the whole no other way equally fimple and praflicable of 

 cffefting this ; at the fame time that it is liable to a number 

 of errors. In the finl place, when experimenting at high 

 temperatures, it is very difficult and indeed impoflible to 

 prefcrve an equal degree of heat through the procefs, and it 

 is highly probable that a metallic combination may take 

 place at a mere melting heat, which is decompofed by a 

 higher one, or -vice •uerfa. If this may happen in alloys that 

 confift of only two materials, it is ftill more likely to do fo 

 when three or a ftill greater number of metals are united into 

 one mafs. Thus, if an alloy made of one part zinc and two 

 of mercury be mixed with another of one part bifmuth and 

 one mercury, the whole may be fufed together by a heat 

 juft fufficient to make them flow, may be kept in fufion for 

 a confiderable time, and then poured into a melting cone 

 without any feparation of the conftituent parts ; but when 

 the altey is heated lo as to make the mercury boil, the 

 greater part of the zinc immediately rifes to the furface and 

 feparates, owing to the deftrurtion of the equilibrium between 

 the antagonift affinities, by the prefence of a certain quantity 

 of caloric. 



Again, fuppofing no change in the affinities to take place, 

 if the mixed -metals are nearly of the fame fufibility and 



ALL 



fpecific gravity, a fpontaneous feparation by fufion isfcarccly 

 to be expected : fo alfo, if they differ in thefe two particular;!, 

 and the metal of eaiieft fution is of the greatell fpecific 

 gravity, as in an alloy of copper and lead, where the two 

 parts are in equal proportions, the tirft efcft of the lieat 

 Avill be the feparation of part of the lead before the mais 

 enters into fufion, this will occupy the bottom of the cru- 

 cible, and the fufibihty of the alloy decreafir.g by the gradu.il 

 feparation of the lead, a temperature nearly equal to I'se 

 melting heat of copper will be required to bring the whole 

 to a fluid ftate ; when this is effected, the lead receiving ths 

 fiift imprefTion of the heat as it enters the crucible from 

 below, being alfo covered with melted copper, will be made 

 to boil, and in confequence will be continually thrown up 

 into the copper notwitiiftanding itsfuperior fpecific gravity. 



The only way, therefore, of determining with certainty 

 the difference between an alloy and a mere mixture, is by a 

 comparifon of the properties of the compound with thofe 

 of its elements, which if they are not intermediate, nor caiifed 

 by mere mechanical aftion on each other, may be received 

 as adequate evidence of a proper chemical union. Here, 

 however, a number of difficulties and doubts, as yet wholly 

 uninvelligated and incapable of being rcfolved by common 

 cafes of affinity, require examination. 



If two metals being fufed together produce a mafs, whofe 

 fpecific gravity is either greater orlefs than the mean fpecific 

 gravity of its elements, the refult is faid to be an alloy, or 

 proper chemical combination. How few however, if even 

 any experiments for the purpofe of aicertaining this have 

 been made with fufficient care ? It is not enough that the 

 fpecific gravity of each of the fimple metals fliould be taken 

 and compared with that of the alloy ; but they ought to 

 have been previoufly melted by themfelves, and cooled in the 

 fame circum ftances to which the alloy was afterwards to be 

 expofed. For example, fuppofe an alloy to be made of 

 copper and gold, equal parts ; the copper to have been cut 

 off from a piece of hard wire, and the gold to have been 

 laminated, the fpecific gravity of the firil ■ vs-ill be nearly 

 8.87 and of the latter 19.36; the two metals being thoroughly 

 mixed by fufion, and either left to cool in the crucible or 

 poured into a melting cone, are then weighed in the hydrof- 

 tatic balance, and the difference or agreement between the 

 fpecific gravity of the aUoy and the mean gravity of the 

 materials, is confidered as a fair ground of inference for the 

 reality of chemical combination, or the contrary. But tfs 

 fpecific gravity of copper cooled flowly and not wire-dravi-n 

 is only 7.7P, and that of gold in the fame circumftances is 

 19.25 ; now the alloy is precilely in this ftate, having been 

 merely melted and cooled gradually ; if therefore no che- 

 mical combination whatever had taken place, yet the fpecific 

 gravity of the alloy, inftead of being = 14. 1 1, as deduced 

 by calculation from that of the materials, would be = 1 3.51, 

 merely from the circumftance of flow cooling without com- 

 preffion. Ey cooling a malleable metal fuddenly, as by 

 pouring it into cold water, it becomes hard, and in fome 

 degree brittle, refembhng in this refpeft a piece of the fame 

 metal that has been laminated without fubfequcnt annealing ; 

 the fpecific gravity of the laminated metal is increafed, and 

 probably the fame effeft is produced by the fudden cooling ; 

 thus a great feeming change in fpecific gravity may exill 

 where there is none in faft. Befides, it is poffible that a real 

 alteration of fpecific gravity may appear in a mixture of two 

 metals, which, inftead of being an evidence of chemical com- 

 bination fliall be merely the effeft of the hardnefs and 

 tenacity of one of them. It is well known that all metals 

 expand by heat, and alter their dimenfions when pafTing from 



the 



