SALTS OF THH METALS. 123 



when exposed to the air : the reason of this is that 

 they have a strong aflBnity for oxygen, and under 

 these circumstances they gradually become covered 

 with a film of an oxide, or compound of the metal with 

 oxygen. Many of these metallic oxides are bases, 

 and form with acids a very numerous series of salts. 



270. Modern discoveries have shown that both the 

 earths and alkalies, are, in fact, the oxides of peculiar 

 and very oxidizable metals (202). Hence the earths, 

 alkalies, and ordinary metallic oxides, are all classed 

 together under the general term, base : they combine 

 with acids to form salts ; thus gypsum, or sulphate of 

 lime, is a compound of lime, which is the oxide of a 

 peculiar metal, and sulphuric acid. Green vitriol, or 

 sulphate of iron, is a compound of oxide of iron and 

 sulphuric acid ; and Cheltenham salts, or sulphate of 

 soda, consists of soda (the oxide of sodium) and sul- 

 phuric acid. 



271. To speak correctly, green vitriol should be 

 called sulphate of oxide of iron, but such a system 

 would be very inconvenient ; it is therefore customary, 

 when speaking of the salts formed by the oxide of a 

 metal, merely to call them by the name of the metal. 

 Hence, when chemists speak of sulphate of iron, and 

 carbonate of lead, they always mean salts of the 

 oxides of those metals ; the metals themselves, not 

 being bases, could not combine with the acids to form 

 salts. 



272. The salts formed by the combination of the 

 different metallic oxides are called just as if they were 



