In the accompanying sections, some of the properties of iron and steel, as employed in the construction of boilers, are given. It is, therefore, desirable that the meanings applied to the various terms used should be clearly understood. The definitions necessary are, then, briefly as follows:—

Tensile strength is equivalent to the amount of force which, steadily and slowly applied in a line with the axis of the test piece, just overcomes the cohesion of the particles, and pulls it into separate parts.

Contraction of area is the amount by which the area, at the point where the specimen has broken, is reduced below what it was before any strain or pulling force was applied.

Elongation is the amount to which the specimen stretches, between two fixed points, due to a steady and slowly applied force, which pulls and separates it into parts. Elongation is made up of two parts: one due to the general stretch, more or less, over the length; the other, due to contraction of area at about the point of fracture.

Shearing strength is equivalent to the force which, if steadily and slowly applied at right angles, or nearly so, to the line of axis of the rivet, causes it to separate into parts, which slide over each other, the planes of the surface at the point of separation being at right angles, or nearly so, to the axis of the rivet.

Elastic limit is the point where the addition to the permanent set produced by each equal increment of load or force, steadily and slowly applied, ceases to be fairly uniform, and is suddenly, after the point is reached, increased in amount. It is expressed as a percentage of the tensile strength.

Tough.—The material is said to be “tough” when it can be bent first in one direction, then in the other, without fracturing. The greater the angles it bends through (coupled with the number of times it bends), the tougher it is.

Ductile.—The material is “ductile” when it can be extended by a pulling or tensile force and remain extended after the force is removed. The greater the permanent extension, the more ductile the material.

Elasticity is that quality in a material by which, after being stretched or compressed by force, it apparently regains its original dimensions when the force is removed.

Fatigued is a term applied to the material when it has lost in some degree its power of resistance to fracture, due to the repeated application of forces, more particularly when the forces or strains have varied considerable in amount.

Malleable is a term applied to the material when it can be extended by hammering, rolling, or otherwise, without fracturing, and remains extended. The more it can be extended without being fractured, the more malleable it is.

Weldable is a term applied to the material if it can be united, when hot, by hammering or pressing together the heated parts. The nearer the properties of the material, after being welded, are to what they were before being heated and welded, the more weldable it is.

Cold-short is a name given to the material when it cannot be worked under the hammer or by rolling, or be bent when cold without cracking at the edges. Such a material may be worked or bent when at a great heat, but not at any temperature which is lower than about that assigned to dull red.

Hot-short is when the material cannot be easily worked under the hammer, or by rolling at a red-heat at any temperature which is higher than about that assigned to a red-heat, without fracturing or cracking. Such a material may be worked or bent at a less heat.

Homogeneous describes a material which is all of the same structure and nature.

A homogeneous material is the best for boilers, and it should be of suitable tensile strength with contraction of area and elongation best suited for the purpose, having an elastic limit that will insure the structure being reliable; it should be tough and ductile, and its elasticity fairly good, and be capable of enduring strains without becoming too quickly or easily fatigued. The material should be malleable and in some cases weldable; that which is of a decidedly cold-short or hot-short nature should be avoided.