ALLOY' (in chemistry) is a mixture of two or more metals, either natural,or produced artificially by melting them together. The A., or mixture, has often different properties from the component metals, and bears a distinct name. Thus, bell-metal is an A. of copper and tin; tombak, of copper and zinc; brass of copper, with a larger proportion of zinc, etc. Alloys arc generally harder than the metals that compose them, and this is the motive for alloying the precious metals. Both gold and silver, when pure, are very soft, and easily worn away by use ; and therefore, a certain proportion of copper is added, to give these metals the requisite hardness. In this case the word "alloy" signifies the inferior metal added, and not the mixture. For coin, the proportion of copper to be added is fixed by law (see the following article), and differs in different states. It has been found by experiment that of A. gives the greatest durability. This is exactly the proportion in British gold coin, a pound troy of the metal containing 11 parts gold and 1 part copper. The A. in our silver coin is somewhat less, being 18 dwts. in the pound instead of 20 dwts. For convenience in reckoning, the standard of the coinage in France, and other countries that adopt its monetary system, as well as in the United States, is made rar pure. metal and A., usually stated 900 (in 1000) parts fine. Our gold and silver standards similarly stated would be 917 and 925 respectively. Gold is sometimes alloyed,with silver, or with a mixture of silver and copper. The color of gold and silver is affected by the nature and amount of the A. A strong A. of copper makes gold red; of silver, green; and a still stronger of silver, a bright yellow. A compound of mercury with another metal is an amalgam (q.v.).
Alloysseldom possess the density which theory or calculation fro& the specific gravity of their constituents would indicate. Thus, many alloys possess a greater density than the mean density of their constituents, whilst others have a less density. The increase in density of the A. indicates that the metals have contracted; in other words, that the
metallic molecules have approached each other more closely ; whilst the decrease in density denotes a separation of the molecules to greater distances from each other.
The strength or cohesion of an A. is generally greater than that of the mean cohesion of the metals contained therein, or even of that of the most cohesive of its constituents. Thus, the breaking weight of a bar of copper or tin (meaning the longitudinal strain it can bear) is very much lower than the breaking weight of a bar composed of an A. of tin and copper. The following tables represent the The power of conducting electrical currents is not so great in an A. as the mean con ducting power of its components.
The composition of the more commonly occurring and commercially important alloys is as follows : Plumber's solder, 1 tin and 2 lead ; soft solder, 2 tin and 1 lead ; common pewter, 4 tin and 1 lead ; gun metal, 9 copper and 1 tin ; bronze, 9 copper and 1 tin and zinc ; cymbals and Chinese gongs, 4 copper and 1 tin ; bell-metal, 3 copper and 1 tin ; speculum metal, 2 copper and 1 tin ; pot-metal or cock-metal, 2 copper and 1 lead; metal, 16 copper and 1 to 11 zinc ; Mannheim gold—pinchbeck or bath-metal, 16 copper and 4 zinc ; Bristol brass, for soldering, 16 copper and 6 zinc ; ordinary brass, for cast ing, '16 copper and 8 zinc : Muntz sheathing-metal, 16 copper and 19i zinc ; speltcr solder. for copper and iron, 10 copper and 12 zinc ;, speller solder, for work, 16 copper and 16 zinc ; Mosaic gold, 16 copper and zinc ; hardest silver solder, 4 silver and 1 copper ; hard silver solder, 3 silver and _ copper ; soft silver solder, 2 silver and 1 copper ; Ger man silver, 100 copper, 60 zinc, and 40 nickel ; type-metal, ordinary, 15 lead, 4 antimony, and 1 tin, or 14 lead, 5 antimony, and 1 tin—small types, 4 lead and 1 antimony—large types, 6 lead and 1 antimony ; stereotype metal, 48 lead, 6 antimony, and 1 tin ; Braun ilia metal, 50 tin, 4 antimony, 4 bismuth, and 1 copper.