Effect of Heat upon the Colours of Metals. 73 



(1.) Pure tin becomes covered with a greyish -black oxide, 

 increases much in bulk, then exhibits the appearance of coin- 

 l.ustion, and at hist leaves a red-coloured oxide, which, on cool- 

 ing, becomes first yellow, and at last white. 



(2.) Antimony becomes first black, then melts, resuming its 

 metallic splendour, and allowing a vapour to fly off. The whole 

 metal is volatilized in a white smoke, leaving yellowish and red- 

 dish spots on the cupel. 



(3.) Zinc melts, blackens on the surface, takes fire all of a 

 sudden, and burns with a very brilliant greenish-white flame, 

 giving out a white thick smoke. The oxide is gradually elevated 

 into a cone. When removed from the fire, it is at first greenish, 

 but, on cooling, becomes snnw-v.'hite. 



(4.) Bismuth soon melts, and is covered with a coat of oxide 

 which melts likewise. A small portion of the oxide sublimes ; 

 the rest sinks into the pores of the cupel, leaving it of a fine 

 orange-yellow colour with some spots of green. 



(5.) Lead exhibits exactly the same phaenomena, and differs 

 from bismuth merely in the colour which it leaves on the cupel, 

 which, when lead is used, is always lemon-yellow, becoming pale 

 and dirtv by exposure to the air. 



(6.) Copper assumes on its surface different iridescent shade:, 

 which succeed each other with rapidity, leaving at last a coating 

 of black oxide, which is detached as tiie metal cools. If the 

 furnace be hot enough, the metal melts, and is soon covered with 

 a coating of black oxide. 



When tin is contaminated by any iron, the presence of this 

 last metal becomes manifest by the spots of rust with which the 

 white oxide is tarnished after the metal has been e:{posed on the 

 «upel. 



The presence of a quarter per cent, of antimony in tin may be 

 recognised by the greyish-black spots with which the white oxide 

 of the metal is mixed after exposure on the cupel. 



When a small quantity of zinc is alloyed with tin, this last 

 metal loses the property of burning by covering itself with incan- 

 descent points, as happens when the tin is pure. The oxide, 

 when cold, has a shade of greenish-grey even when the zinc does 

 not exceed one per cent. 



Bismuth alloyed with tin, even when the proportion does not 

 exceed five per cent., gives to the oxide a greyish colour mixed 

 with yellow, or, if it does not exceed one per cent., merely a greyish 

 colour. 



Less than five per cent, of lead may be detected in tin by the 

 colour of rust which it communicates to the oxide of this last 

 metal. 



Less than one percent, of tin can be detected in lead, because 

 Vol. 56. No. 2U7. July 1820. K the 



