In 1877, Holley1 advanced the theory that irons weld in proportion to their mobility or flowing, and inversely as oxidation of the welding surfaces occurs. He thought that the more plastic or more nearly melting point the irons were, the more readily they would weld. But with every increase in heat was a corresponding readiness to oxidize - especially on the part of carbon and iron. This oxid interposed a mechanical difficulty to perfect welding.

This theory does not satisfy Campbell2 who insists that impurities tend to crystallization in the body of the iron. Carbon, which is the principal offender, and sulphur, phosphorus, and other ingredients, all form alloys or compounds with the pure ferrite. Ferrite itself is exceedingly malleable and mobile. But a mixture of ferrite and several of the carbon compounds, as cemen-tite, martensite, etc., is stiff above red heat in proportion to the carbon present. Campbell thinks that such a steel, which is really a mineral with a granitic structure, will not weld, because it refuses to flow. He claims that oxidation troubles are actually less, because the chemical combination of the iron oxid with the impurities and their oxids would give a self-fluxing surface.

According to Campbell, then, those impurities which caused decided crystallization with accompanying brittleness, interfered with the flow at high heat and prevented welding. Manganese, it is true, makes a more brittle iron, up to 1.20 per cent; but it prevents crystallization of sulphur, etc., and is an aid in welding.

For ordinary and commercial purposes the welding must be done in a few seconds' time, and the previous cleaning and heating must not take long. This at once limits to a very few the number of metals which can be welded; were it not for the recent remarkable advance due to the electric, oxy-hydrogen and acetylene processes of melting, welding would be confined to iron, platinum, nickel, and gold. Other metals would be joined by soldering and brazing, and even then the metal worker would have great difficulties with aluminum and many alloys.

1 Trans. American Institute of Mining Engineers, Vol. VI, p. 112.

2 "Metallurgy of Iron and Steel," p. 589.

I will first take up iron and steel welding. As much research work has been done on the metallurgy of iron as on all of the other metals combined. It is extremely probable that many of the difficulties and problems arising from proportions of impurities and methods of producing will apply equally to other metals. For this reason, and because of its overwhelming importance, I will treat of iron more thoroughly.