METHOD OF DISTINGUISHING IRON FROM STEEL. 123

appear in its metallic state though it is slightly oxidated; andpartly that it is capable of combining with different quantities ofcarbon.

Crude iron is not yet absolutely and perfectly de-oxidated orreduced to the complete metallic state: it also holds a consider-able portion of carbon in combination, the grey containing agreater quantity than the white crude iron.

Malleable iron is not only perfectly metallic, but likewise un-contaminated by carbon. It is a simple substance, and if per-fectly pure would contain nothing but iron. It however is,strictly speaking, never absolutely pure.

Steel is iron combined with carbon; it has for that reason beencalled carbonated iron. The proportion of carbon has not yetbeen ascertained with accuracy.

Cast iron, is iron combined with a much larger quantity ofcarbon than is necessary for forming steel.

METHOD OF DISTINGUISHING IRON FROM STEEL.

Rinman has long ago pointed out a ready method by whichsteel may be distinguished from iron. When a little diluted ni-tric acid is dropt upon a plate of steel, allowed to remain for afew minutes, and then washed off, it leaves behind it a blackspot; whereas the spot formed by nitric acid upon iron is whitishgrey.

Rationale. ..The colour of the black spot is owing to the car-bon of the iron, which is converted into charcoal by the acid, andin that state is distinguishable by the eye. Whereas upon ironwhich is nearly free from carbon this effect cannot take place.

Remark. ..This experiment shews that carbon is much morereadily oxidated when combined with iron, than when crystal-lized in the form of diamond.

INFERENCES.

From what has been stated above it is easy to comprehendwhy good grey crude iron is converted into steel by simple fu-