According to the Standard Dictionary, to weld is to "unite, as heated metal, in one piece or mass under the hammer or by pressure."

The Century Dictionary says, "To unite or consolidate, as pieces of metal or metallic powder, by hammering or compression, with or without previous softening by heat." * * * "term is more generally used when the junction of the pieces is effected without the actual fusing point of the metal having been reached." While the Standard adds, "Metals are weldable in proportion to the length of time they will stay under heat in a plastic condition without melting."

Welding is distinguished from soldering, which is, according to the Standard, "To unite, as two metallic substances, by solder." The Century, "To unite by a metallic cement." * * * "Every kind must be used as its own melting point, which must be always lower than that of the metals to be united."

I give these definitions because there is some confusion of the terms; and naturally so, as the two processes often are undistin-guishable. Thus two unlike metals, as iron and platinum, may be welded; while a fractured steel bar may be united by placing platinum foil between the pieces, pressing strongly together and heating moderately. This is strictly welding, yet the platinum foil is solder. In the recent processes of welding by fusion, the molten metal becomes a solder. Brazing is classed as soldering, but when brass is brazed the process is as nearly welding as the so-called autogenous weld.

The word autogenous is misapplied to welding. It means self-produced. The melted weld of the oxy-hydrogen or acetylene flame is a soldering process in which the metal produces its own solder. However, it makes a catchy trade-name.

Welding, under different names, is a property possessed by many substances, both elemental and compound. According to Roberts - Austen,1 who devotes considerable space to the flowing property of metals, "welding is the property possessed by metals, which on cooling from the molten state pass through a plastic stage before becoming perfectly solid, of being joined together by the cohesion of the molecules that is induced by the application of an extraneous force, such as hammering."

In general, welding occurs if cohesion between the molecules of the two pieces can be induced. This cohesion may amount to diffusion when the two pieces are of unlike substance, and the metal at the weld will be found to be an alloy of the two, Thus gold and lead, pressed together for several weeks, will weld at ordinary temperature. At 100 deg. C. they will weld in less time, and the weld will be an alloy of gold and lead.

Welding and diffusion are not inseparable, however. For the welding by diffusion of lead and gold is weaker than the first-named cold weld. While the diffusion of mercury through another metal invariably produces weakness of that metal, sometimes disintegration.

Regelation is the name given to the welding of two pieces of ice. Faraday is credited with this discovery. He found that two pieces of ice slightly below freezing point, if pressed together will weld. Wrightson2 states that both iron and ice suffer a drop in temperature when pressed together. He heated two irons to the plastic state in an electric welder and pressed them together. The recording pyrometer showed a sudden fall of from 19 deg. to 57 deg. C. He further states that iron increased almost 7 per cent, in volume on becoming plastic, and tries to trace an analogy between the behavior of iron and ice. It has since been found that revelation is a property possessed in some degree by most crystalline substances. Pure crystalline salts will regelate under pressure at moderate temperature. Even such a substance as bismuth will regelate.

Evidently welding depends upon two things:

1. The flow.

Most of the so-called solids are fluid to some extent. Highly crystalline, refractory rocks will flow under great pressure. The walls of some deep mines have flowed together in the course of time. A rod of glass or of sealing-wax will bend or flow if it supports a weight for several days. Lead, sodium, etc., flow readily under pressure.

1 "Introduction to Metallurgy," p. 47.

2 Journal of the Iron and Steel Institute, 1895, Vol. I, p. 499.

Flow is almost synonymous with malleability, the difference being a matter of time. Many substances which flow slowly will not withstand the shock of the hammer.

Most metals flow at all temperatures from normal to melting point, but they are the most easily weldable within the range of greatest plasticity. But their welding also depends upon:

2. The wetting or cohesion of the two substances.

Two pieces of the same or different substance will not weld if their surfaces do not cohere, no matter how malleable or fluid they may be. Aluminum is a notable instance. The metal is quite malleable at most temperatures, but a microscopic film of oxid prevents the two surfaces from wetting one another. Iron in a lesser degree is troubled with a coating of oxid at welding temperature. Any flux which will clean off both surfaces will allow a weld to be made. In proportion to the ease with which one can have and hold a clean surface of the metals in the range of plasticity, in that proportion will welding be feasible. The welding of malleable metals is dependent on the behavior of the oxids which form on their surface. In proof of this is the remarkable experiment of Chernoff1 in 1877. He showed that a partial weld of two pieces of iron could be made at the low temperature of 650 deg. C, which is at least 700 deg. below common welding heat. The two surfaces were planed and highly polished. Pressure was applied for several days, when it was found that there was a partial weld. This is similar to the well-known experiment in physics where two plane and highly polished surfaces of glass are pressed together. The surfaces will cohere to some extent.

These two experiments seem to show that whatever assists cohesion, assists the welding. There are numerous instances of welding among non-metallic substances which do not oxidize at the welding heat. Glass is too well-known to need explanation. Pieces of horn can be joined under pressure of hot plates if the horn be kept moist.

1 Revue Universalle des Mines, Vol. I, 1877, p. 411.

Metals newly nascent, in a fine powder, can be welded into a solid piece by a stroke of the hammer. Apparently for the reason that the grains of powder have bright, clean faces. Most of the malleable metals are so weldable.