ALLOY 



97 



Dentist's Alloy. For the ordinary purposes of mounting artificial toeth a peculiar 

 metal is required. It must bo sufficiently hard and tough, and it must not bo liable 

 to corrosion by either acid or acrid fluids. Experience has shown that an alloy of 

 gold, silver, and copper most nearly meets all the required conditions. Dentists use 

 16-carat gold, which is fine gold and ^ alloy, the alloy being al\vays nearly equal 

 portions of silver and copper, which is not, for these purposes, in the .slightest degree 

 injurious. See AMALGAM. 



COPPER ALLOYS. Copper alloyed with zinc forms BRASS, and with tin wo have 

 BRONZE. (See those articles.) The best Kingston's Metal is an alloy of copper, tin, 

 and mercury ; it is much used for bearings. (See KINGSTON'S METAL.) The alloys 

 of the ancients were usually either brasses or bronzes. The following analyses of 

 ancient coins, &c., by Mr. John Arthur Phillips, are of great value;: . 



Copper, when united with half its weight of lead, forms an inferior alloy, resem- 

 bling gun-metal in colour, but it is softer and cheaper. This alloy is called pot-metal 

 and cock-metal, because it is used for large measures and in the manufacture of taps 

 and cocks of all descriptions. 



Sometimes a small quantity of zinc is added to pot-metal ; but when this is consi- 

 derable the copper seizes the zinc to form brass, and leaves the lead at liberty, a largo 

 portion of which separates on cooling. Zinc and lead are not disposed to unite ; but 

 a little arsenic occasions them to combine. 



It is not a little curious to find that some of the coins of high antiquity contain 

 zinc: this must have been introduced by the use of calamino, since it does not appear 

 that zinc was known as a metal before 1280 A.D., when Albertus Magnus speaks of it 

 as a semi-metal, and calls the alloy of copper and zinc golden mnrcasite ; or rather, 

 perhaps, ho means to apply that name to zinc, from its power of imparting a golden 

 colour to copper. The probability is that calamine was known from tho earliest times 

 as a peculiar earth, although it was not thought to bo an ore of zinc or of any other 

 metal. Sec ' Watson's Chemical Essays.' 



VOL. L H 



