OXIDES OP IRON.

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iron remains clean and bright for a long time, but veryrapidly rusts when exposed to the joint action of air,carbonic acid gas, and moisture, under -which circum-stances water is decomposed and oxide of iron formed.

205. The rust of iron is not a pure oxide, butcommonly a mixture of both oxides with a portion ofcarbonate or compound of the protoxide with carbonicacid.

206. Although iron cannot combine with the freeoxygen of the air at ordinary temperatures, yet whenstrongly heated it rapidly absorbs oxygen, and thenbecomes converted into a black scaly oxide ; whenevera piece of iron is heated in the fire, a quantity of abrittle, black oxide is formed on its surface, which easilyrubs off the iron—and hence this metal is graduallyworn away by exposure to fire ; it is from this reasonthat the iron pokers, fire-bars, and other things muchexposed to the fire, gradually get thinner and thinnerfrom the constant oxidation of the surface whilst hot,and removal by friction of the brittle coat of oxide thusformed.

207. When a piece of iron is very strongly heated, itat last begins to burn , that is to say, the combinationof the external part with oxygen goes on so rapidly,and evolves so much heat, that the whole mass of ironis kept sufficiently hot to continue this process ofcombination, and in consequence the iron glows brightly,gives off abundance of sparks, and runs down in dropsof the melted oxide for some time after it has been