ALLOY, either a natural or artificial com pound of two or more metals, except when mercury is one of them, in which case the mixture is called an amalgam. Native or natural alloys, considered as such, are not useful bodies, the only exception, if indeed it may be so reckoned,is the alloy of iron and nickel, constituting meteoric iron, of which the knives of the Esquimaux appear to be made; but artificial alloys are of the highest importance, since by uniting different metals compounds are formed which possess a combination of qualities not occurring in any one metal. Gold, silver, tin, antimony, and bismuth are generally alloyed; the first three on account of their softness, and the two latter because they are extremely brittle. Gold and silver are hardened by alloying with copper; copper is hardened by zinc, ct-c.
The formation of alloys appears to depend upon the chemical affinity of the metals for each other; and in some instances it seems to be wanting, for no combination occurs. Various facts may be assigned for supposing the combination to be the result of chemical affinity. M. Boussingault analysed six dif ferent native alloys of gold and silver, and found in all cases that the metals were com bined in definite proportions. The change of properties which metals undergo by com bining, furnishes strong evidence of its arising from chemical affinity and action ; thus, with respect to colour, copper, a reddish metal, by union with zinc, which is a white one, gives the well-known yellow alloy, brass. The fusing point of a mixed metal is never the mean of the temperature at which its consti tuents melt; and it is generally lower than that of the most fusible metal of the alloy.
All alloys formed of brittle metals are brittle; those made with ductile Metals are in some cases ductile, in others brittle ; When the proportions are nearly equal, there are as many alloys which are brittle as ductile ; but when one of the metals is in excess, they are most commonly ductile. In combining ductile and brittle metals, the compounds are brittle, if the brittle metal exceed, or nearly equal, the proportion Of the ductile one ; but when the ductile metal greatly exceeds the brittle one, the alloys are usually ductile. The deli sity of alloys sometimes exceeds, and in Other cases is less than, that which would result from calculadoti.
Not only are the properties of Metals altered by combination, but different proportions of the same metals produce very ilifferent alloys.
Thus, by combining ninety parts of copper with ten parts of tin, an alloy is obtained of greater density than the mean of the Metals, and it is also harder and more fusible than the copper ; it is slightly malleable when slowly cooled, but on the contrary, ivheii heated to redness, and plunged into cold water, it is very malleable : this bompbund IS known by the name of bronze. If eighty parts of copper be combined with twenty parts bf tin, the compound is the extremely sonorous one called bell-metal ; an alloy consisting of, {we thirds copper and one third tin, is susceptible of fine polish, and is used as speculum metal.
It is curious to observe in these alloys, that in bronze the density and hardness of the denser and harder metal are increased by corbbining with a lighter and softer one ; while, as might be expected, the fusibility of the more refractory metal is increased by uniting with a more fusible one. In bell metal the copper becomes more sonorous by combination with a metal which is less so : these changes are clear indications of chemi cal action.
The principal alloys employed in manufac tures are the following :—Arsenic and copper make packfong ; tin and lead make solder and common pewter ; tin, antimony, copper, and bismuth make best pewter and Britannia metal ; tin, lead, and bismuth make/as/Lk metal; tin and copper, in various proportions, make bronze, speculum metal, bellmetal, and gun metal; copper and zinc, in different propor tions, make brass and Dutch gold; copper, ,zinc, nickel, and iron make German. silver; sil ver and copper make standard silver; gold, silver, and copper make standard gold. In many of these alloys, such as the last two, the quantity of one of the metals is extremely small, but still it is sufficient to give a distinct property to the alloy.
Dr. Weiger, of Vienna, proposed in 1845 many mode's of inaking fine alloys, chiefly for the use of dentists. Gold, silver, platifia, and Palladium are the metals employed ; and these are taken—tivd bi three together—in such proportions its to fermi eight different alloys : platintmi being in every case one of the metals. In making alloys for such deli cate uses as thOse of the dentist, the piirity of the single metals IS as important as the com bining of the whole ; and Dr. Weiger details processes in respect to both theSe qualities.