ALLOY 



heated, does not expose a clean surface, like lead, but is covered at times with oxide 

 of tin. Tin 75 and copper 25 gave a black oxide : if the heat be much elevated, the 

 underpart of the oxide is white, which is oxide of tin ; the upper part is black, being 

 the oxide of copper, and the cupel becomes of a rose colour. If the tin be impure 

 from iron, the oxide produced by it is marked with spots of a rust colour. 



The degree of affinity between metals may be in some measure estimated by the 

 greater or less facility with which, when of different degrees of fusibility or volatility, 

 they unite, or with which they can, after union, be separated by heat. The greater 

 or less tendency to separate into differently proportioned alloys, by long-continued 

 fusion, may also give some information upon this subject. Mr. Hatchett remarked, 

 in his elaborate researches on metallic alloys, that gold made standard with the usual 

 precautions, by silver, copper, lead, antimony, &c., and then cast, after long fusion, 

 into vertical bars, was by no means an uniform compound ; but that the top of the 

 bar, corresponding to the metal at the bottom of the crucible, contained the larger 

 proportion of gold. Hence, for a more thorough combination, two red-hot crucibles 

 should be employed, and the liquefied metals should be alternately poured from the 

 one into the other. To prevent unnecessary oxidisation from tho air, the crucibles 

 should contain, besides the metal, a mixture of common salt and pounded charcoal. 

 The metallic alloy should also be occasionally stirred up with a rod of earthenware. 



