74

SALTS OF TIIE METALS.

and very oxidisable metals (130). Hence the earths,alkalies, and ordinary metallic oxides, are all classedtogether under the general term base, they combine•with acids to form salts; thus gypsum or sulphate oflime is a compound of lime, which is the oxide of apeculiar metal and sulphuric acid. Green vitriol orsulphate of iron, is a compound of oxide of iron andsulphuric acid; and Cheltenham salts, or sulphate ofsoda, consists of soda, (the oxide of sodium), and sul-phuric acid.

219. To speak correctly, green vitriol should be calledsulphate of oxide of iron, but such a system would bevery inconvenient; it is therefore customary, whenspeaking of the salts formed by the oxide of a metal,merely to call them by the name of the metal. Hencewhen chemists speak of sulphate of iron, and carbonateof lead, they always mean salts of the oxides of thosemetals ; the metals themselves not being bases, couldnot combine with the acids to form salts.