. Metals. 227 



remain unchanged by heat, except by that of the 

 voltaic flame. These have therefore been called 

 perfect metals. Some become oxidized at the 

 ordinary temperature of the atmosphere, as 

 sodium, potassium, &c. 



Metals are also calcined, or more properly, 

 according to the new chemistry, oxidized, by 

 being immersed in the acids, which part with 

 their oxygen to the metals, and reduce them to 

 a calx or oxide. The perfect metals, however, 

 require a peculiar acid to oxidize them, which is 

 called aqua rcgia, and is in fact a super-oxygen- 

 ated acid. But of this I shall treat more in 

 speaking of the action of acids on them. 



All the metals are capable of uniting with 

 chlorine, and even in preference to oxygen. 

 Iodine also acts upon many of the metals. 



The calcination of volatile metals, such as 

 zinc, Sec. is similar to the burning of combusti- 

 bles. Their vapour, when it mixes with the air, 

 is oxidized and has the appearance of white 

 smoke, like that of phosphorus. It attaches it- 

 self to the surface of other bodies, and, when 

 collected, proves to be an oxide of the metal. In 

 some cases their dissipation is manifest, as in 

 zinc, where the heat and light are evident ; but 

 in most others there is no light, though perhaps 

 if inspected in a very dark room, by a person 

 who had been long in the dark, light might be 

 perceived in many of them. 



Copper oxidizes in a heat below that neces- 



