Metals. 229 



doses of oxygen; and when a metal has com- 

 bined with as much oxygen as possible, it is de- 

 noted by the term peroxide. 



The rusting of metals is a change to which 

 they are subject in consequence of exposure to 

 the air. We always find the rust to be an oxide, 

 such as that produced by calcination, or by . 

 acids ; and the reason is plain : a metal rusts by 

 merely attracting the oxygen from the atmo- 

 sphere, and therefore undergoes a gradual oxidize- 

 ment 



Metallic oxides are either a powder or a fria- 

 ble substance resembling the earths : they endure 

 violent heat without evaporation. Some of them 

 are fusible, especially when moderately calcined, 

 but always require more heat for fusion than the 

 metals from which they were obtained. They 

 form a mass which has no resemblance to any 

 metal, either when melted or congealed. Some 

 of them in this case form glass, like the fused 

 earthy substances. This melted matter or sco- 

 ria (as the litharge of lead,) will readily mix with 

 any of the earths in a melted state, and brings 

 many of them into fusion ; but it will not mix 

 with metals in their metallic state. 



The particular change which metals undergo 

 in consequence of oxidizement, is the means 

 which is commonly recurred to in separating 

 metals from one another. I have observed be- 

 fore, that some oxidize more quickly than others 

 when exposed to the same heat; and as no oxide 



