ALL

ALL

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die eafied fufible is alfo very inflammable, thefirfl method is had recourfe to ; as when copperand zinc or copper and tin are to be combined jbut the latter is praclifed in thofe cafes wherethe materials are either nearly of the fame de-gree of fufibility, or at lead where the tempera-ture required for melting the leaft fufible, isnot fufiicient to volatilize that which is the moltfo. In order to prevent oxydation, the crucibleought to be lined with charcoal, and a quantityof decrepitated fait fhould be flrewed over thetop of the ingredients, which melting at a verylow red heat, will float on the furface of themetal, and thus exclude the contaft of the air.By taking thefe precautions, a lower and longercontinued heat may be advantageoufly fublli-tuted for a fliorter and more violent one, theingredients will be mingled more perfectly, andthe lofs and confequent inaccuracy when ex-perimenting on the moll inflammable metals,will in a great meafure be avoided. As foon astbe mafs is liquefied, it will be of advantage toilir it repeatedly, to prevent the ingredientsfrom feparating according to their refpectivcfpecific gravity, before they have had an oppor-tunity of combining together ; for this purpofea charred flick, or rod of baked clay, is to bepreferred to one of iron, as this lali would inmany cafes be acled upon by the melted metal,and ipoil the procefs. The alloy fhould alfo bepoured alternately from one red hot crucibleinto another, and back again, to infure the inti-mate mixture of the ingredients. Thefe are byno means ufelefs precautions, for where one ofthe metals is of confulerably greater denfitythan the other, it requires particular care tomake the quality of the alloy equal throughoutthe whole of the mafs. Schlutter relates , 1that twenty pounds weight of filver, containingabout a 56 th part of gold, was melted in acrucible, and poured into cold water to be gran-ulated : famples from the top, middle, andbottom of the mafs were then alfayed, and werefound all to differ in their proportions of gold.In like manner Mr. Hatchett obferved, b thatgold made ftandard, with the ufual precautions,by filyer, copper, lead, antimony, &c. and thencall into vertical bars, was by no means didri-buted equally, but that the top of each bar,(being compofed of the portion of alloy whichoccupied the bottom of the crucible) was bothpurer and of greater fpecific gravity than thelower end of the bar.

It is a matter of confiderable importance toafeertain whether alloys are mere mixtures or* Ear. des Mines et des Metaux. p. 25 6 - b phiI - T c

proper chemical compounds: in mod inltancesthere feems to be little or no doubt of the latterbeing the cafe, and perhaps all the fuppofedexamples of the contrary may be looked uponas inltances of the fuperfaturation of one ofthe ingredients by the other.

The rule that is laid down in mod chemicalbooks for afeertaining whether a mafs of metalis an alloy or a Ample mixture, is to fufe itwith as little heat as poffible, and to keep it inthis date till its component parts feparate fromeach other, like oil from water, according to theirrefpeftive gravities. Thus if nearly equal partsof gold and filver are melted hadily together,and granulated by pouring into cold water,although the particles of gold and filver cannotbe didinguifhed from each other by their ex-ternal characters, yet they may be eafily madeto feparate by melting the mafs at a very gentleheat, keeping it in fufion for a quarter of anhour more or leis, and then allowing it to coolgradually; the crucible being now examined,will be found to contain the gold, alloyed witha fmall variable proportion of filver in a lumpat the bottom, and the red of the filver whollyfree from gold, lying upon it. c It feemshowever by no means to be a neceflary confe-quence of this fact, that the mafs in quedionwas not a proper alloy: the drong affinity ofcohefion that fubfids between the particles bothof gold and filver, the great difference betweentheir fpecific gravities, and the temperature atwhich they become liquid: all thefe forcesconcurring together in the mod favourable cir-cumdances, may well overcome a confiderabledegree of chemical affinity. Still lefs can it behence inferred that no proportions of gold andfilver can form a proper alloy.

Another objection to the method jud men-tioned of didinguilhing alloys from mixtures,is, that it is applicable only to a very few cafes.When, for example, a mixture of copper andlead is to be fubjecled to this trial, as foon asthe feparation begins to take place, part of thelead will occupy the bottom of the crucible,while the red dill mixed with the copper, willfloat above it: in proportion as the procefs goeson, and the copper becomes purer, a greaterheat will be required to keep it liquid; thelead will then begin to boil, and particles of thismetal in vapour will be condantly thrown upinto the copper, fo as to prevent the compleatparting of the two, even fuppofing that no che-mical affinity whatever fubfided between them.With dill more force will this obje£tion applyis, for 1803. p. 1 ifj. c Lewis. Comm. Arts. p. 86.