These, then, are some of the methods which are now being used to combat our eternal enemy, the rust that doth corrupt.  All of them are useful in their several ways.  No one of them is best for all purposes.  The claim of “rust-proof” is no more to be taken seriously than “fire-proof.”  We should rather, if we were finical, have to speak of “rust-resisting” coatings as we do of “slow-burning” buildings.  Nature is insidious and unceasing in her efforts to bring to ruin the achievements of mankind and we need all the weapons we can find to frustrate her destructive determination.

But it is not enough for us to make iron superficially resistant to rust from the atmosphere.  We should like also to make it so that it would withstand corrosion by acids, then it could be used in place of the large and expensive platinum or porcelain evaporating pans and similar utensils employed in chemical works.  This requirement also has been met in the non-corrosive forms of iron, which have come into use within the last five years.  One of these, “tantiron,” invented by a British metallurgist, Robert N. Lennox, in 1912, contains 15 per cent. of silicon.  Similar products are known as “duriron” and “Buflokast” in America, “metilure” in France, “ileanite” in Italy and “neutraleisen” in Germany.  It is a silvery-white close-grained iron, very hard and rather brittle, somewhat like cast iron but with silicon as the main additional ingredient in place of carbon.  It is difficult to cut or drill but may be ground into shape by the new abrasives.  It is rustproof and is not attacked by sulfuric, nitric or acetic acid, hot or cold, diluted or concentrated.  It does not resist so well hydrochloric acid or sulfur dioxide or alkalies.

The value of iron lies in its versatility.  It is a dozen metals in one.  It can be made hard or soft, brittle or malleable, tough or weak, resistant or flexible, elastic or pliant, magnetic or non-magnetic, more or less conductive to electricity, by slight changes of composition or mere differences of treatment.  No wonder that the medieval mind ascribed these mysterious transformations to witchcraft.  But the modern micrometallurgist, by etching the surface of steel and photographing it, shows it up as composite as a block of granite.  He is then able to pick out its component minerals, ferrite, austenite, martensite, pearlite, graphite, cementite, and to show how their abundance, shape and arrangement contribute to the strength or weakness of the specimen.  The last of these constituents, cementite, is a definite chemical compound, an iron carbide, Fe_{3}C, containing 6.6 per cent. of carbon, so hard as to scratch glass, very brittle, and imparting these properties to hardened steel and cast iron.