worker in sheet metals, since all sheets of tin, lead, copper, and zinc are sold by pounds weight to the foot. A few remarks by way of explanation of these several qualities, possessed in common by metals and alloys, therefore, preface the descriptions of the metals and alloys to follow.

Metallic lustre. This is, in fact, nothing more than the power of reflecting light rays. If a surface absorbs light rays largely, the reflection is broken, and the appearance of the surface will not be bright, but dull. A broken or rough surface absorbs and scatters the light rays, a smooth surface, in the sense of being polished, reflects them. A porous substance cannot be polished. For a surface to be capable of taking a polish and becoming lustrous it must be dense, close, or hard. Thus no amount of polishing would make the natural surface of wood lustrous like that of iron, and no amount of polishing would make the surface of iron as lustrous as that of the harder steel. Metals not hard enough in themselves to take a high polish can be rendered harder and more lustrous by the admixture of another metal. Tin and copper in various proportions form speculum metal and bell metal, each extremely hard and lustrous, and so of alloys and of other metals.

Tenacity is equivalent to strength, or the resistance. offered by a body to forces tending to pull its particles asunder. It is measured in pounds or tons per square inch. That is, if the ultimate tensile strength of a bar of iron is 40,000 pounds per square inch, that means that a load of 40,000 pounds suspended at the end of a bar 1 inch square, in cross section, would just suffice to tear the bar asunder. Tenacity, in this sense of

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breaking strength, is not of so much relative interest to the sheet-metal worker as it is to the engineer. Still, there are some matters cognate thereto which it is well to be aware of, such as the effect of the presence of impurities, the effect of temperature and the effect of drawing out. In brief, the presence of foreign matters varies, in some cases and in certain proportions tend- ing to increase, in others to diminution of strength. The effect of increase of temperature is to lessen the tenacity of metals, the effect of excessive drawing out is to lessen the tenacity by overcoming the cohesive strength, and replacing the fibrous condition by the crystalline, on the other hand, the tenacity is raised by moderate drawing out. Steel and iron possess the highest tenacity, while zinc, tin and lead possess the least.

Ductility. In proportion to the ductility of metals and alloys they are adapted for the purpose of wiredrawing, hence, steel, wrought iron, and copper, being highly ductile, are used for this purpose. Gold, silver, and platinum stand highest in the range of ductility, but their cost precludes their use for any but some special purposes. Tenacity is closely related to ductility, inasmuch as a weak metal will break before it can be reduced to a fine wire. Zinc, tin, and lead, though soft, will not stand drawing down, because their tenacity is so low. Ductile metals become hardened and crystallized during the process of wire-drawing, until they reach the limit of the coherence of their particles. Then annealing becomes necessary. This is effected by heating the metal, and allowing it to cool slowly, the effect of heat being to produce a natural rearrangement of the molecular particles.

Malleability is not identical with ductility, though in

some respects akin to it. The effect of hammering or rolling is to destroy the cohesion of the particles of metal, to restore which annealing is necessary. The softest metals are not the most malleable, neither are the most tenacious metals the most readily rolled and hammered. Lead and tin are soft, iron and steel are strong, or tenacious, but neither are malleable, as are gold, silver, and copper. Copper is the only really malleable substance used by sheet-metal workers, and that can be hammered into almost any form. Sheet-iron and steel can be bent and rolled, but cannot be raised under the hammer or in dies to anything like the same extent as copper. The malleability of thick metals is generally increased by heat, that of thin metals is not practically affected by it. The malleability of metal lies at the basis of the formation of work in sheet metal. There is an essential difference between the operations of the boilermaker and those of the sheetmetal worker. The materials are largely the samesteel, wrought iron, and copper-but the difference in thickness render the methods of working different. The first-named class of artisans do much of their work by the aid of heat, the second, in the cold. The difference is due to the relative thicknesses of the plates used by the first, and of the sheets used by the second. A thick plate cannot be bent to a quick curvature unless it is heated, a thin sheet can be bent, or hammered, or stamped, in the cold to almost any outline. The reason of this is readily apparent on a little consideration.

Take a plate of thick metal, a sheet of thin metal, and a sheet of rubber, and note the effect of bending

in each case. The thick plate can only be bent by the application of much force, assisted, if the curvature be quick, by heat, the thin steel can be bent most readily to the same curvature, the rubber also with extreme ease. In each case the effect of bending is to extend the outer layers, and compress the inner layers. The layers in the center of the plate, or sheet, are neither extended nor compressed, and this central plane of bending is called the neutral axis. The difference in bending thick and thin metal plates is due to the fact that in the first the layers which are in compression and extension are at a considerable distance from the neutral axis, while in thin plates these layers are practically coincident therewith, so that in a thin plate there is no appreciable amount of compression or extension, hence the ease with which they can be bent.

But if the metal in the plates were highly elastic and mobile, like rubber, then, even though thick, extension and compression would take place in thick plates as in thin. The effect of heating thick plates is to cause the molecules to move over one another, and to become rearranged permanently, and this not necessarily in a state of high extension or compression, such as would result if the plates had been bent cold, but in a safe and natural way, provided the amount of bending does not exceed the limit which the nature of the material will permit it to sustain. The same kind of thing occurs in thin sheet-metals which are subjected to severe rolling, hammering, or stamping. Some movement and rearrangement of the particles of metal takes place, and the greater the amount of curvature or distortion of form produced, the more severe will be the stresses produced in the substance of the material. If

a flat plate is raised by hammering, or if it is deeply beaded or dished, or set out, it will be brought into so high a state of tension that it will probably crack, unless heating is resorted to for the purpose of rearranging the particles of metal. It is therefore obvious that the result of hammering, rolling, and stamping is to cause the particles of metal to glide over one another, extending some parts and compressing others, with the frequent coincidence also of thinning down. some of the portions which have been subjected to the most severe treatment. If, therefore, the metals did not possess this property of malleability and of ductility, but were such that their particles could not be made to glide one over the other, no irregular metallic forms could be produced by hammering or stamping, but casting would be the only method available for obtaining these forms.

Conductivity of heat is a property which renders the metals so valuable for heating purposes. The conducting power of metals varies, but it so happens that copper, which is the best conductor among the metals in common use, is also the most malleable. Wrought iron is also an excellent conductor. The thinner the sheets, the more rapidly is heat transmitted through them. And, moreover, heat is transmitted so quickly through thin malleable sheets that there is no risk of fracture occurring, due to unequal contraction, as there is in many metallic substances.

Fusibility. The melting of steel, copper, and brass does not concern the worker in sheet metal, but the relative fusibilities of the numerous brass, lead, and tin solders are matters of much practical importance to him. These all melt at comparatively low tempera