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regular edge of the hole, clamps the plates together and forms the rivet head.

Drop Hammers.- Heavy drop hammers are used for forging and also for welding, and are operated by hydraulic or steam power. They consist of an anvil upon a solid base of steel blocks laid over oak timbers to give elasticity to the machine. Above the anvil vertical housings capped by steel arches support the hammering ram and the platforms upon which the piston cylinder and other actuating machinery is placed. By the use of swages, fullers and flatters, hot metal is hammered into forms, often by the use of a progressive series of dies. The largest hammer of this kind was erected in 1891 at Bethlehem (Pa.) Steel Works. Its general dimensions and weight are as follows: Height, 90 feet; width, 42 feet; weight of anvil and foundation, 1,800 tons; weight of housings, steam chests, pressure cylinders and piston, about 1,000 tons. Ram 19 feet long, 10 feet wide and 4 feet thick,

weighing 100 tons. Lighter forms of drop hammers are actuated by compressed air and are called pneumatic hammers. They are used principally as cornice bending machines.

Bending Rolls and Benders are used generally in boiler and tank work; the metal plates being drawn by rotation between three rolls so arranged that their axes form the edges of a triangular prism, their relative adjustment determining the curve to which the plate is bent.

Rolling Machines are used to flatten out metal bars into plates and commercial shapes, such as I-beams, railroad rails, etc. As designed at present they accomplish in a short space of time a great variety of work which in times past was turned out by the more laborious and expensive processes of lathe turning and forging. One class is used to manufac

ture boiler and armor plate and the general run of heavy sheet metal; while another class produces the thin sheet metal down to the finest grades such as tinfoil.

Presses. By the use of presses sheet or plate metal is converted into utensils of any desired form. They are usually operated by hydraulic power; are provided with dies between which the metal is pressed into the required form, and are capable of being constructed to exert an unlimited amount of pressure. Forging presses are made in all sizes adapted for uses ranging from the pressing of watch-cases to the forging of steamship shafts and of armor plates weighing up to 14,000 pounds. Forgings thus made are superior to hammered forgings. They are designed for working metals either hot or cold.iesb albniq

Planers. Planing machines are used to obtain flat surfaces on metal. There are two types; those in which the motion of the table relative to the cutting tool is rectilinear and those in which that motion is rotary. In general construction a planer consists of a traversing table on which the work is fastened; a bed to receive the table and guide it in a right line; a cross slide to support the slide rest carrying the tool; standards bolted to the bed and supporting the cross slide, and the mechanical devices for feeding and regulating purposes. The power supplied from shafting by belts is transformed by gear-wheel attachments into the reciprocating motion of the table, causing it to slide back and forth between the vertical guides, thus bringing the work against the cutting tool which shaves off successively, side by side, narrow thin strips of metal until a perfectly flat surface is produced. Planers are built in various sizes, the larger machines being equipped with tables 7 to 8 feet in width and 20 feet long.

Saws.

In metal working, saws belong to the class commonly termed finishing machinery. For cutting plates and bars into shorter lengths they are in extensive use, affording a great economy of time. They are built in a great many sizes and forms, both stationary and portable, equipped with single cutters or cutters arranged in gangs. For cutting off large bars such as steel beams, rails and similar shapes, saws with single cutters are generally used, the object being fastened to a carriage and moved into contact with the edge of the circular cutter. Some machines used for lighter work are so arranged that the circular saws are moved

into contact with the bars. The cutters are of two kinds. The friction discs made of soft mild steel, without teeth, measuring about 44 inches in diameter and three-sixteenths of an inch in thickness, are used for cutting off either hot or cold metal. They are run at a high rate of speed, about 15,000 feet per minute, rim velocity. The toothed cutters vary greatly in diameter and thickness; are made of highlytempered steel, and are used for clean cutting in cold metal. They are usually run at a low rate of speed, the rim velocity varying from 130 to 150 feet per minute. Friction discs run at rim velocities between 20,000 and 25,000 feet per minute are called fusion discs, from the fact that the intense heat generated by the friction actually melts the metallic dust ground off by the cutter.

Special Machines.- This term includes an almost endless variety of metal-working machinery, capable of enumeration only in a very general way. They are used for special purposes such as making pins, nails, rivets and pens; the tapping of nuts and the threading of bolts. Although often of very complex construction they turn out work with great rapidity and precision. In its special field metal-working machinery has not only displaced hand labor in all countries, and especially in the United States, but by its use structural work such as the building of magnificent buildings and bridges, powerful engines and mammoth ships have been produced far beyond the capabilities of that which is known industrially as hand labor. Consult Adam, H. M., and Evans, J. H., 'Metal Work' (London 1914); Colvin, F. H., and Stanley, F. A., American Machinist Grinding Book (New York 1912); De Vries, D., 'Milling Machines and Milling Practice' (London 1916); Hasluck, P. N., Metal Working' (London 1904); Horner, J. G., Modern Milling Machines (London 1906); Pull, E., Modern Milling' (New York 1917).

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METALLOID (Gr. "metal-like"), chemistry, any non-metallic element. These are 13, namely, sulphur, phosphorous, fluorin, chlorin, iodine, bromine, silicon, boron, carbon, nitrogen, hydrogen, oxygen and selenium. The distinction between the metalloids and the metals is slight. The former, excepting selenium and phosphorus, do not have a "metallic" lustre; they are poor conductors of heat and electricity, are generally not reflectors of light and not electropositive; that is, no metalloid fails of all these tests. The term seems to have been introduced into modern usage instead of nonmetals for the very reason that there is no hard and fast line between metals and non-metals, so that "metal-like" or "resembling metals" is a better description of the class than the purely negative "non-metals." Originally it was applied to the non-metals which are solid at ordinary temperatures.

METALLURGY, the extraction of metals from their ores and so refining and fashioning them as to fit them for use in the metal industries, is the most ancient of arts. The annals of history show that the degree of civilization attained by a race was directly proportional to

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the extent to which they made use of metallurgical processes. In fact, a prominent metallurgist of the 19th century when delivering an introductory address to students of metallurgy was accustomed to charge them that "in proportion to the success with which the metallurgic art is practised in this country, will the interests of the whole population directly or indirectly in no inconsiderable degree be promoted."

As an industry, it requires a wide knowledge of the sciences, embracing chemistry, physics and mineralogy, as well as the professions of mechanical and electrical engineering. As some of the oldest historical works make reference to metallurgical processes, the actual practice of the art must go much farther back into prehistoric ages. It had its inception at the time when men, ever advancing, replaced their stone implements by others of greater usefulness made from metals. Their methods of extracting and refining were in many respects comparable with those used to-day except in the introduction of mechanical improvements. The early development of chemistry was very closely allied with metallurgy. Many of the older treatises on theories of chemistry are attempts to explain metallurgical reactions. Prominent among these was the separation of lead from the precious metals, gold and silver, by its combination with oxygen, a process practised and much speculated upon by the ancients. Gold was probably the first metal known as it occurs naturally in the metallic state, is bright, heavy and easily worked. Iron, copper, silver, tin and lead were other metals known to prehistoric men. Quicksilver is mentioned in the times of the Greeks and Romans. In the 15th and 16th centuries we hear of antimony, bismuth and zinc, with arsenic added at the close of the 17th century. Nickel, cobalt, manganese and platinum came in the 18th century, the remainder belonging to the 19th and 20th.

There are four great periods in the annals of metallurgy. The first extended from prehistoric times to the 1st century A.D., when Pliny, the Elder, in his work, Naturalis Historia,' collected all existing knowledge concerning metals to his day. The second period ending with the 15th century had as its chronicler Agricola, in 'De Re Metallica. Next came a period of comparative stagnation with relatively few important advances until the 19th century ushered in the fourth, our present period, one filled with development.

Properties of Metals.-All metals are distinguished by numerous characteristic properties which fit them for their various industrial applications. A brief outline of these follows: Density. All common metals except aluminum are relatively heavy, specific gravities ranging from 6 to 222. The density of metals varies, within moderate limits, due to methods of casting, rates of cooling and subsequent mechanical treatments. All metals except bismuth are lighter when liquid than when solid; i.e., they contract on solidification. Color and Lustre.The metals vary considerably in color and lustre and many industrial uses depend upon these differences, particularly in reference to the use of metals for ornamentation. Opacity. -This is a property of all metals under normal conditions. Crystallization. All metals crystallize in definite geometrical forms on free

solidification. Structure. This is revealed in fractures examined by the eye and in polished and etched sections examined under the allpowerful eye of the microscope. Unmistakable evidence of the thermal and mechanical treatment which the metal has undergone is furnished the expert by such examination. Such a study often serves to distinguish one metal from another, making unnecessary the resort to chemical analysis. Hardness.-There are several methods of determining this property: scratch hardness, or resistance to abrasion; indentation hardness, or the resistance offered to penetration by a body of greater hardness, as in the familiar Brinell method where a hardened steel ball is used for this purpose; rebound hardness, measured by the height to which a hard body will rebound when allowed to fall upon the metal being tested as typified by the Shore sclerescope method; cutting hardness, or the resistance offered to cutting by a hard tool. This is usually measured by determining the number of revolutions of a standard drill to produce a given penetration. Strength. -This property is measured by resistance to rupture; by slow direct pull, bend, compression or twist; by sudden blow; by repeated bending or blows or a combination of both. Brittleness and Toughness. These two, opposing properties, are indicated by the way in which the metal behaves when subjected to strength tests. The same metal may appear as brittle or tough, depending on the conditions of test. Plasticity. -This property accounting for malleability and ductility, is the ability of the metal to flow under pressure in the hot and in the cold state. It is all important in the working of metals in the industries. If metals did not possess this property, the operations of hammering, rolling, drawing, spinning, etc., through which all metal articles must pass, during fabrication, would be impossible. Weldability. This is the capacity of pieces of metal to unite into one continuous metal when heated and brought into intimate contact. Fusibility. All metals except mercury are solid at ordinary temperatures, yet they can be melted-brought into a state of fusion — by application of sufficient heat. Volatility.—All metals may be vaporized at high temperatures. Diffusion.- This is the property of molecules of one metal to migrate into another when brought into intimate contact, either in the liquid or solid states. Occlusion. The rapidity and ease with which metals dissolve gases is augmented by high temperatures. Expansion and contraction with changes in temperature; thermal and electrical conductivity; magnetic permeability, or ability to conduct lines of magnetic force, are all properties which find great application in the industrial use of metals.

Although metallurgy is concerned with the extraction and purification of metals, few of them are used in the pure state. Practically all of the metal produced finds its way into alloys before it comes to the consumer's hands. Alloys, from the Latin, alligo, "to bind to," were defined by Birminguccio as "amicable associations of metals with each other." Alloys may be formed by fusion, compression or electro-deposition. By far the most of them are formed by fusion. By the alloying of metals new properties result which increase their suitability for commercial uses; for in

stance, an alloy is produced with a coefficient of expansion of zero to make possible the manufacture of clocks which will be accurate at all temperatures. Wire is produced with the same coefficient of expansion as glass for use in electric light bulbs. Before the metallurgist gave us this alloy we used the rare metal platinum for this purpose. Alloy steels of unheard-of strength and anti-fatigue properties are produced for use in automobiles and aeroplanes. Innumerable articles of common and special use show that most metallic objects are alloys possessing special properties to meet special needs. Frequently the change of a few tenths of a per cent of the amount of a constituent present in an alloy will cause the most startling changes in the properties of the resultant metal. Of all the innumerable operations of metallurgy there is hardly one which does not require the application of heat for its successful promotion. Heat is usually supplied by the combustion of a fuel and must be applied in many different ways and under various conditions to suit the metallurgical process to be carried out. Therefore, knowing the metallurgical requirements we must know the characteristics of the available fuels in order to determine which are to be used and the methods of using them to produce desired results.

Fuels. Although fuels are complex chemical combinations, the main heat-producing elements of all are carbon and hydrogen. When a fuel is once kindled the combustion will continue as long as the heat evolved is sufficient to keep the temperature above the ignition point. Essential conditions for perfect combustion are: first, sufficient air brought into intimate contact with the fuel; second, sufficient time in which it may act; third, temperature suitable for combustion must be maintained throughout. From these considerations it is evident that less excess air need be used with gaseous than solid fuels to get an intimate mixture. Mechanical methods of producing mixtures are frequently employed. The form of solid fuel affects combustion, porous fuels taking more air, caking fuels causing imperfect combustion, etc. With perfect combustion, the products are carbon dioxide and water. The atmosphere in the furnace resulting from the products of combustion may be either neutral, oxidizing or reducing. A neutral atmosphere which results from perfect combustion is very difficult to maintain. Under ordinary conditions we either supply an excess of air which results in an oxidizing atmosphere, or we do not furnish enough air for perfect combustion and this results in a reducing atmosphere. There are long and short flame fuels depending upon the amount of combustible gases evolved and the facilities existing for mixing air and gas. All these are important considerations in the use of fuel and design of combustion chambers. Fuels are rated by their calorific values. There are three classes: solid, liquid and gaseous. The solid are classified as natural (wood, peat, lignite, coal) and prepared (charcoal, coke, begasse, etc.). The liquid fuels are natural, as petroleum; and artificial, as distilled oils, tars, etc. The gaseous fuels may be divided as natural gas, or artificial (manufactured gas, oil, water, coal, blast furnaces, producer gas, etc., etc.). Each of these different classes of fuels presents its own problem in