introduction of the hot air blast in forges and furnaces where bellows

or blowing apparatus was required. This was the invention of J. Beaumont Neilson, of Glasgow, and was covered by him in British patent No. 5,701 of 1828. This consisted in heating the air blast before admitting it to the furnace, and it so increased the reduction of refractory ores in the blast furnace as to permit three or four times the quantity of iron to be produced with an expenditure of little more than one-third of the fuel. [Illustration: FIG. 250.--MODERN HOT BLAST FURNACE.] An illustration of a modern blast furnace plant is given in Fig. 250. A is the furnace, in which the iron ore and fuel are arranged in alternate layers. The hot air blast comes in through pipes _t_ at the bottom, called tuyeres. As gas escapes through the opening _b_ at the top, it is first cleared of dust in the settler and washer B, and then passes through the pipe C to the regenerators D D D, where it is made to heat the incoming air. The gas mixed with some air burns in the regenerators, and, after heating a mass of brick within the regenerators red hot, escapes by the underground passageway to the chimney on the right. When the bricks are sufficiently hot in one of the regenerators, gas is turned off therefrom, and into another regenerator, and fresh air from pipe H is passed through the bricks of the heated regenerator, and being heated passes out pipe F at the top and thence to the pipe G and tuyeres _t_, to promote the chemical reactions in the blast furnace. In the earlier blast furnaces a vast amount of heat was allowed to escape and was wasted. The utilization of this heat engaged the attention of Aubertot in France, 1810-14; Teague in England (British patent No. 6,211, of 1832); Budd (British patent No. 10,475, of 1845), and others. To enable the escaping hot gases to be employed for heating the hot blast regenerators a charging device is now used, as seen at a in Fig. 250, in which the admission of ore and fuel is regulated by a large conical valve, and the gases are compelled to pass out at _b_ and be utilized. Among the world’s largest blast furnaces may be mentioned the Austrian Alpine Montan Gesellschaft, which concern owns thirty-two furnaces. This is said to be the largest number owned by any one concern in the world, but most of them are of small size and run on charcoal iron. The furnaces of the United States are, however, of the largest yield, and the leading ones of these are: No. Annual capacity Furnaces. in tons. Carnegie Steel Co. 17 2,200,000 Federal Steel Co. 19 1,900,000 Tennessee Coal and Iron Co. 20 1,307,000 National Steel Co. 12 1,205,000 The present annual output of pig iron in the United States is about ten million tons, of which these four companies make about one-half. [Illustration: FIG. 251.--PUDDLING FURNACE.] When the iron runs from the bottom of the blast furnace it is allowed to flow into trough-like moulds in the sand of the floor, and forms pig iron. Pig iron can be remelted and cast into various articles in moulds, but it cannot be wrought with the hammer, nor rolled into rails or plates, nor welded on the anvil, because it is still a compound of iron and carbon with other impurities, and is crystalline in character. To bring it into wrought iron, which is malleable and ductile, it is puddled and refined, which involves chiefly the burning out of the carbon and silicon. The pig iron is remelted (see Fig. 251) in the tray-shaped hearth _b_ from the heat of the fire in the reverberatory furnace _a_, the reverberatory furnace being one in which the materials treated are exposed to the heat of the flame, but not to contact with the fuel. The hot flame mixed with air beating down upon the melted iron on hearth _b_ for two hours or so, burns out the silicon and carbon, the process being facilitated by stirring and working the mass with tools. During the operation the oxygen of the air combines with the carbon and forms carbonic acid gas, which, in escaping from the metal, appears to make it boil. When the iron parts with its carbon it loses its fluidity and becomes plastic and coherent, and is formed into balls called _blooms_. These blooms consist of particles of nearly pure iron cohering, but retaining still a quantity of slag or vitreous material, and other impurities, which slag, etc., is worked out while still, hot by a squeezing, kneading, and hammering process to form wrought iron that may be worked into any shape between rolls or under the hammer. [Illustration: FIG. 252.--BESSEMER CONVERTER DURING THE “BLOW.”] _Bessemer Steel._--Steel is a compound of iron and carbon, standing between wrought iron and cast iron. Wrought iron has, when pure, practically no carbon in it, while cast iron has a considerable proportion in excess of steel. Steel making consists mainly in so treating cast iron as to get rid of a part of the carbon and other impurities. Of all methods of steel making, and in fact of all the steps of progress in the art of metal working, none has been so important and so far reaching in effect as the Bessemer process: It was invented by Henry Bessemer, of England, in 1855. About fifty British patents were taken by Mr. Bessemer relating to various improvements in the iron industry, but those representing the pioneer steps of the so-called Bessemer process are No. 2,321, of 1855; No. 2,768, of 1855, and No. 356, of 1856. The process is illustrated in Figs. 252, 253 and 254. The converter in which the process is carried out is a great bottle-shaped vessel 15 feet high and 9 feet wide, consisting of an iron shell with a heavy lining of refractory material, capable of holding eight or more tons of melted iron, and with an open neck at the top turned to one side. It is mounted on trunnions, and is provided with gear wheels by which it may be turned on its trunnions, so that it may be maintained erect, as in Fig. 252, or be turned down to pour out the contents into the casting ladle, as in Figs. 253 and 254. At the bottom of the converter there is an air chamber supplied by a pipe leading from one of the trunnions, which is hollow, and a number of upwardly discharging air openings or nozzles send streams of air into the molten mass of red hot cast iron. The red hot cast iron contains more or less carbon and silicon, and the air uniting with the carbon and silicon burns it out, and in doing so furnishes the heat for the continuance of the operation. When the pressure of air is turned into the mass of molten iron a tongue of flame increasing in brilliancy to an intense white, comes roaring out of the mouth of the converter, and a violent ebullition takes place within, and throws sparks and spatters of metal high in the air around, producing the impression and scenic effect of a volcano in eruption. In fifteen minutes the volume and brilliancy of the flame diminish, and this indicates the critical moment of conversion into tough steel, which must be adjusted to the greatest nicety. When the carbon is sufficiently burned out the blast is stopped and the converter turned down to receive a quantity of ferro-manganese or spiegeleisen (a compound of iron containing manganese), which unites with and removes the sulphur and oxide of iron, and then the lurid monster, with its breath of fire abated, and its energy exhausted, bows its head and vomits forth its charge of boiling steel, to be wrought or cast into ten thousand useful articles. [Illustration: FIG. 253.--POURING THE MOLTEN METAL.] [Illustration: FIG. 254.--SIDE VIEW, SHOWING TURNING GEARS.] Like most all valuable inventions, Mr. Bessemer’s claim to priority for the invention was contested. An American inventor, William Kelly, in an interference with Mr. Bessemer’s United States patent, successfully established a claim to the broad idea of forcing air into the red hot cast iron, and United States patent No. 17,628, June 23, 1857, was granted to Mr. Kelly. The honor of inventing and introducing a successful process and apparatus for making steel by this method, however, fairly belongs to Mr. Bessemer, to whose work was to be added the valuable contribution of Robert F. Mushet (British patent No. 2,219, of 1856) of adding spiegeleisen, a triple compound of iron, carbon and manganese, to the charge in the converter. This step served to regulate the supply of carbon and eliminate the oxygen, and completed the process of making steel. The Holly converter, covered by United States patents No. 86,303, and No. 86,304, January 26, 1869, represented one of the most important American developments of the Bessemer converter. The importance of Bessemer steel in its influence upon modern civilization is everywhere admitted. It has so cheapened steel that it now competes with iron in price. Practically all railroad rails, iron girders and beams for buildings, nails, etc., are made from it at a cost of between one and two cents per pound. In recognition of the great benefits conferred upon humanity by this process, Queen Victoria conferred the degree of knighthood upon the inventor, and his fortune resulting from his invention is estimated to have grown for some time at the rate of $500,000 a year. In a historical sketch of the development of his process, delivered by Sir Henry Bessemer in December, 1896, before the American Society of Mechanical Engineers at New York, Mr. Bessemer was reported as saying that the annual production of Bessemer steel in Europe and America amounted to 10,000,000 tons. The production of Bessemer steel in the United States for 1897 was for ingots and castings 5,475,315 tons, and for railroad rails 1,644,520 tons. The extent to which steel has displaced iron is shown by the fact that in the same year iron rails to the extent of 2,872 tons only were made, as compared with more than a million and a half tons of Bessemer steel. In the popular vote taken by the _Scientific American_, July 25, 1896, as to what invention introduced in the past fifty years had conferred the greatest benefit upon mankind, Bessemer steel was given the place of honor. A recent improvement in the handling of iron from the blast furnace is shown in Fig. 255. Heretofore, the iron was run in open sand moulds on the floor and allowed to cool in bars called “pigs,” which were united in a series to a main body of the flow, called a “sow.” To break the “pigs” from the “sow,” and handle the iron in transportation, was a very laborious and expensive work. The illustration shows two series of parallel trough moulds, each forming an endless belt, running on wheels. The molten cast iron is poured direct into these moulds, and as they travel along they pass beneath a body of water, which cools and solidifies the iron into pigs, and then carries them up an incline and dumps them directly into the cars. [Illustration: FIG. 255.--CASTING AND LOADING PIG IRON.] _Open Hearth Steel_ is not so cheap as Bessemer steel, but it is of a finer and more uniform quality. Bessemer steel is made in a few minutes by the most energetic, rapid and critical of processes, while the open hearth steel requires several hours, and its development being thus prolonged it may be watched and regulated to a greater nicety of result. For railroad rails and architectural construction Bessemer steel still finds a great field of usefulness, but for the finest quality of steel, such as is employed in making steam boilers, tools, armor plate for war vessels, etc., steel made by the open hearth process is preferred. It consists in the decarburization of cast iron by fusion with wrought iron, iron sponge, steel scrap, or iron oxide, in the hearth of a reverberatory furnace heated with gases, the flame of which assists the reaction, and the subsequent recarburization or deoxidation of the bath by the addition, at the close of the process, of spiegeleisen or ferro-manganese. The period of fusion lasts from four to eight hours. The advantages over the Bessemer process are, a less expensive plant and the greater duration of the operation, permitting, by means of sampling, more complete control of the quality of the product and greater uniformity of result. The British patents of Siemens, No. 2,861, of 1856; No. 167, of 1861, and No. 972, of 1863, for regenerative furnaces, and the British patents of Emile and Pierre Martin, No. 2,031, of 1864; No. 2,137, of 1865, and No. 859, of 1866, represent the so-called _Siemens-Martin_ process, which is the best known and generally used open hearth process. [Illustration: FIG. 256.--SIEMENS REGENERATIVE FURNACE.] _The Siemens Regenerative Furnace_, in which this process is carried out, is seen in Fig. 256. Four chambers, C, E, E′, C′, are filled with fire brick loosely stacked with spaces between, in checker-work style. Gas is forced in the bottom of chamber C, and air in bottom of chamber E, and they pass up separate flues, G, on the left, and being ignited in chamber D above, impinge in a flame on the metal in hearth H, the hot gases passing out flues F on the right, and percolating through and highly heating the checker-work bricks in chambers E′ and C′. As soon as these are hot, gas and air are shut off by valves from chambers C and E, and gas and air admitted to the bottoms of the now hot chambers C′ and E′. The gas and air now passing up through these chambers C′, E′, become highly heated, and when burned above the melted iron on hearth H produce an intense heat. The waste gases now pass down flues G, and impart their heat to the checker-work bricks in chambers C and E. When the bricks in E′ C′ become cooled by the passage of gas and air, the valves are again adjusted to reverse the currents of gas and air, sending them now through chambers C and E again. In this way the heat escaping to the smoke stack is stored up in the bricks and utilized to heat the incoming fuel gases before burning them, thus greatly increasing the effective energy of the furnace, saving fuel, and keeping the smoke stack relatively cool. _Armor Plate._--In these late days of struggle for supremacy between the power of the projectile and the resistance of the battleship, the production of armor plate has become an interesting and important industry. Three methods are employed. One is to roll the massive ingots directly into plates between tremendous rolls, a single pair of which, such as used in the Krupp works, are said to weigh in the rough as much as 100,000 pounds. Usually there are three great rollers arranged one above the other, and automatic tables are provided for raising and lowering the plates in their passage from one set of rolls to the other. The man in charge uses a whistle in giving the signals which direct these movements, and without the help of tongs and levers the glowing blocks move easily back and forth between the rollers. The men standing on both sides of the rollers have only to wipe off the plates with brooms and occasionally turn the plates. [Illustration: FIG. 257.--14,000-TON HYDRAULIC PRESS FORGING AN ARMOR PLATE.] The second method utilizes great steam hammers weighing 125 tons, and striking Titanic blows upon the yielding metal. The most modern method, however, is by the hydraulic press forge, now used in the shops of the Bethlehem steel works in the production of Harveyized armor plate. In Fig. 257 is seen the great 14,000-ton hydraulic press-forge squeezing into shape a port armor plate for the battleship “Alabama.” After leaving the forge, the plate is trimmed to shape by the savage bite of a rotary saw and planer, seen in Figs. 258 and 259, whose insatiable appetites tear off the steel like famished fiends. The plate is then taken to be Harveyized by cementation, hardening, and tempering, as seen in Figs. 260, 261, and 262. The 125-ton mass of metal representing the plate in the rough, and weighing more than a locomotive, is thus handled and brought to shape with an ease and dispatch that inspires the observer with mixed emotions of admiration and awe. _Making Horse Shoes._--Anthony’s patent, April 8, 1831; Tolles’, of October 24, 1834, and H. Burden’s, of November 23, 1835, were pioneers in horse-shoe machines. Mr. Burden took many subsequent patents, and to him more than any other inventor belongs the credit of introducing machine-made horse shoes, which greatly cheapened the cost of this homely, but useful article. Nearly 400 United States patents have been granted for horse-shoe machines. [Illustration: FIG. 258.--ROTARY SAW, CUTTING HEAVY ARMOR PLATE.] [Illustration: FIG. 259.--ROTARY PLANER, TRIMMING HEAVY ARMOR PLATE.] [Illustration: FIG. 260.--THE CEMENTATION FURNACE.] [Illustration: FIG. 261.--HARDENING THE PLATE BY JETS OF WATER.] [Illustration: FIG. 262.--OIL TEMPERING.] _Making Screws, Bolts, Nuts, Etc._--Screw-making according to modern methods began between 1800-1810 with the operations of Maudsley. Sloan, in 1851, and Harvey, in 1864, made many improvements in machines, operating upon screw blanks. The gimlet-pointed screw, which allows the screw to be turned into wood without having a hole bored for it, was an important advance in the art. It was the invention of Thomas J. Sloan, patented August 20, 1846, No. 4,704, and was twice re-issued and extended. In later years the rolling of screws, instead of cutting the threads by a chasing tool, has attained considerable importance, and provides a simpler and cheaper method of manufacture. Knowles’ United States patent of April 1, 1831, re-issued March 1, 1833, described such a process, while Rogers, in patents No. 370,354, September 20, 1887; No. 408,529, August 6, 1889; No. 430,237, June 17, 1890, and No. 434,809, August 19, 1890, added such improvement in the process as to make it practical. In the great art of metal working the names of Bramah, Whitworth, Clements and Sellers appear conspicuously in the early part of the century as inventors of planing, boring and turning machinery for metals. Our present splendid machine shops, gun shops, locomotive works, typewriter and bicycle factories, are examples of the wonderful extensions of this art. In later years the field has been filled so full of improvements and special machines for special work, that only a brief citation of a few representative types is possible, and even then selection becomes a very difficult task. Many special tools, particularly those designed for _bicycle work_, have been devised, as exhibited by patent to Hillman, August 11, 1891, No. 457,718. In _turning car wheels_, an improvement consists in bringing the wheel to be dressed into close proximity to the edge of a rapidly revolving smooth metal disk, whereby the surface of the wheel is melted away without there being any actual contact between the wheel surface and the disk. This is shown in patent to Miltimore, August 24, 1886, No. 347,951. In _metal tube manufacture_ three processes are worthy of mention: (1) Passing a heated solid rod endwise between the working faces of two rapidly rotating tapered rolls, set with their axes at an angle to each other, as shown in Mannesmann’s patent, April 26, 1887, No. 361,954 and 361,955. (2) Forcing a tube into a rapidly rotating die, whereby the friction softens the tube, and the pressure and rotation of the die spin it into a tube of reduced diameter, shown in patent to Bevington, January 13, 1891, No. 444,721. (3) Placing a hot ingot in a die and forcing a mandrel through the ingot, thereby causing it to assume the shape of the interior of the die, and greatly condensing the metal, shown in patents to Robertson, November 26, 1889, No. 416,014, and Ehrhardt, April 11, 1893, No. 495,245. In _welding_, the employment of electricity constitutes the most important departure. This was introduced by Elihu Thomson, and is covered in his patents Nos. 347,140 to 347,142, August 10, 1886, and No. 501,546, July 18, 1893. In _annealing_ and _tempering_, electricity has also been employed as a means of heating (see patent to Shaw, No. 211,938, February 4, 1879). It supplies an even heat and uniform temperature, and is much used in producing clock and watch springs. The making of iron castings malleable by a prolonged baking in a furnace in a bed of metallic oxide was an important, but early, step. It was the invention of Samuel Lucas, and is disclosed in his British patent No. 2,767, of 1804. The _Harvey process_ of making armor plate is an important recent development in _cementation_ and _case hardening_, and is covered by his United States patents No. 376,194, January 10, 1888, and No. 460,262, September 29, 1891. It consists, see Fig. 260, in embedding the face of the plate in carbon, protecting the back and sides with sand, heating to about the melting point of cast iron, and subsequently hardening the face. The Krupp armor plate, now rated as the best, is made under the patent to Schmitz and Ehrenzberger, No. 534,178, February 12, 1895. In _coating with metals_, the so-called “galvanizing” of iron is an important art. This was introduced by Craufurd (British patent No. 7,355, of April 29, 1837), and consisted in plunging the iron into a bath of melted zinc covered with sal ammoniac. In more recent years the tinning of iron has become an important industry, and machines have been made for automatically coating the plates and dispensing with hand labor, examples of which are found in patents No. 220,768, October 21, 1879, Morewood; No. 329,240, October 27, 1885, Taylor, _et al._, and No. 426,962, April 29, 1890, Rogers and Player. In _metal founding_ the employment of chill moulds is an important step. Where any portion of a casting is subjected to unusual wear, the mould is formed, opposite that part of the casting, out of metal, instead of sand, and this metal surface, by rapidly extracting the heat at that point by virtue of its own conductivity, hardens the metal of the casting at such point. The casting of car wheels by chill moulds, by which the tread portion of the wheel was hardened and increased in wearing qualities, is a good illustration. Important types are found in patents to Wilmington, No. 85,046, December 15, 1868; Barr, No. 207,794, September 10, 1878, and Whitney, re-issue patent, No. 10,804, February 1, 1887. In _wire-working_ great advances have been made in machines for making _barbed wire fences_. The French patent to Grassin & Baledans, in 1861, is the first disclosure of a barbed wire fence. This art began practically, however, with the United States patent to Glidden and Vaughan for a barbed wire machine, No. 157,508, December 8, 1874, re-issued March 20, 1877, No. 7,566, and has assumed great proportions. A machine for making wire net is shown in patent to Scarles, No. 380,664, April 3, 1888, and wire picket fence machines are shown in patents to Fultz, No. 298,368, May 13, 1884, and Kitselman, No. 356,322, January 18, 1887. Machines for making wire nails were invented at an early period, but the product found but little favor until about 1880, when they began to be extensively used, and have almost entirely supplanted cut nails for certain classes of work, since their round cross section and lack of taper give great holding power and avoid cutting the grain of the wood. In 1897 the wire nails produced in the United States amounted to 8,997,245 kegs of 100 pounds each, which nearly doubled the output of 1896. The output of cut nails for the same year was 2,106,799 kegs. The bending of wire to form chains without welding the links has long been done for watch chains, etc., but in late years the method has extended to many varieties of heavy chains. The patents to Breul, No. 359,054, March 8, 1887, and No. 467,331, January 19, 1892, are good examples. An interesting class of machines, but one impossible of illustration on account of their complication, are machines for making pins. In earlier times pins had their heads applied in a separate operation. Making pins from wire and forming the heads out of the cut sections began in the Nineteenth Century with Hunt’s British patent No. 4,129, of 1817. This art received its greatest impetus, however, under Wright’s British patent No. 4,955, of 1824. A paper of pins containing a pin for every day in the year, and costing but a few cents, gives no idea to the purchaser of the time, thought and capital expended in machines for making them, and yet were it not for such machines, rapidly cutting coils of wire into lengths, pointing and heading the pins, and sticking them into papers, the world would be deprived of one of its most ubiquitous and useful articles. Many tons of pins are made in the United States weekly, and it is said that 20,000,000 pins a day are required to meet the demand. In the metal working art the making of firearms and projectiles has grown to wonderful proportions. Cutlery and builders’ hardware is an enormous branch; wire-drawing, sheet metal-making, forging, and the making of tools, springs, tin cans, needles, hooks and eyes, nails and tacks, and a thousand minor articles, have grown to such proportions that only a bird’s-eye view of the art is possible. In the _making_ of _shot_, the old method was to pour the melted metal through a sieve, and allow it to drop from a tower 180 feet or more in height. David Smith’s patent, No. 6,460, May 22, 1849, provided an ascending current of air through which the metal dropped, and which, by cooling the shot by retarding its fall and bringing a greater number of air particles in contact with them, avoided the necessity of such high towers. In 1868, Glasgow and Wood patented a process of dropping the shot through a column of glycerine or oil. Still another method is to allow the melted metal to fall on a revolving disk, which divides it into drops by centrifugal action. _Alloys._--Over 300 United States patents have been granted for various alloys of metals. The so-called _babbit metal_ was patented in the United States by Isaac Babbit, July 17, 1839, and in England, May 15, 1843, No. 9,724. This consists of an antifriction compound of tin, 10 parts, copper, 1 part, and antimony, 1 part, and is specially adapted for the lubricated bearings of machinery. _Phosphor bronze_, introduced in 1871, combines 80 to 92 parts copper, 7 of tin, and 1 of phosphorus (see United States patents to Lavroff, No. 118,372, August 22, 1871, and Levi and Kunzel, No. 115,220, May 23, 1871). The addition of phosphorus promotes the fluidity of the metal and makes very clean, fine and strong castings. In alloys of iron, _chromium steel_ is covered by patents to Baur, No. 49,495, August 22, 1865; No. 99,624, February 8, 1870, and No. 123,443, February 6, 1872; _manganese steel_, by Hadfield’s patent, No. 303,150, August 5, 1884; _aluminum steel_, by Wittenström’s patent, No. 333,373, December 29, 1885, and _phosphorus steel_, by Kunkel’s patent, No. 182,371, September 19, 1876. The most recent and perhaps most important, however, is _nickel steel_, used in making armor for battleships. This is covered by Schneider’s patents, Nos. 415,655, and 415,657, November 19, 1889. In 1878 England led the world in the iron industry with a production of 6,381,051 tons of pig iron, as compared with 2,301,215 tons by the United States. In 1897 the United States leads the world in the following ratios: Tons Pig Iron. Tons Steel. United States 9,652,680 7,156,957 Great Britain 8,789,455 4,585,961 Germany 6,879,541 4,796,226 France 2,472,143 1,312,000 The United States made in that year 29.30 per cent. of the world’s production of pig iron, and 34.58 per cent. of its steel. The total output of the whole world in that year was 32,937,490 tons pig iron, and 20,696,787 tons of steel. _Metallurgy of Rarer Metals._--Although less in evidence than iron, this has engaged the attention of the scientist from the earliest years of the century. The full list of metals discovered since 1800 may be found under “Chemistry.” The more important only are here given. Palladium and rhodium were reduced by Wollaston in 1804. Potassium and sodium were first separated in metallic form by Sir Humphrey Davy in 1807, through the agency of the voltaic arc; barium, strontium, calcium and boron by the same scientist in 1808; iodine by Courtois in 1811; selenium by Berzelius in 1817; cadmium by Stromeyer in 1817; silicon by Berzelius in 1823, and bromium by Balard in 1826. Magnesium was first prepared by Bussey in 1829. Aluminum was first separated in 1828 by Wohler, by decomposing the chloride by means of potassium. Oersted, in 1827, preceded him with important preliminary steps, and Deville, in 1854, followed in the first commercial applications. In late years the metallurgy of aluminum has made great advances. The Cowles process heats to incandescence by the electric current a mixture of alumina, carbon and copper, the reduced aluminum alloying with the copper. This process is covered by United States patents to Cowles and Cowles, No. 319,795, June 9. 1885, and Nos. 324,658 and 324,659, August 18, 1885. It has, however, for the most parts been superseded by the process patented by Hall, April 2, 1889, No. 400,766, in which alumina dissolved in fused cryolite is electrically decomposed. In the metallurgy of the precious metals probably the most important step has been the _cyanide process_ of obtaining gold and silver. In 1806 it was known that gold was soluble in a solution of cyanide of potassium. In 1844 L. Elsner published investigations along this line, and demonstrated that the solution took place only in the presence of oxygen. McArthur and Forrest perfected the process for commercial application, and it is now extensively used in the Transvaal and elsewhere. It is covered by their British patent, No. 14,174, of 1887, and United States patents No. 403,202, May 14, 1889, and No. 418,137, December 24, 1889, which describe the application of dilute solutions of cyanide of potassium, not exceeding 8 parts cyanogen to 1,000 parts of water: the use of zinc in a fine state of division to precipitate the gold out of solution, and the preparatory treatment of the partially oxidized ores with an alkali or salts of an alkali. By this solution-process gold, in the finest state of subdivision, which could not be extracted by other processes from the earthy matters, may be recovered and saved in a simple, practical and cheap way. [Illustration: FIG. 263.--EDISON MAGNETIC CONCENTRATING WORKS. THE GIANT CRUSHING ROLLS.] [Illustration: FIG. 264.--EDISON MAGNETIC CONCENTRATOR.] In the working of ores of gold and silver the old method of comminution of the rock and the separation of the gold and silver by amalgamation with mercury has given birth to thousands of inventions in stamp mills, amalgamators, ore washers, concentrators and separators. In the treatment of iron ores, and especially those of low grade, the magnetic concentrator is an interesting and striking departure. This method goes back to the first half of the Nineteenth Century, an example being found in the patent to Cook, No. 6,121, February 20, 1849. Edison’s patent, No. 228,329, June 1, 1880, is however, the basis of the first practical operations in which magnets, operating by attraction upon falling particles of iron ore, are made to separate the particles rich in iron from the sand. In Fig. 263 is shown the Edison magnetic concentrating apparatus. The ore, in masses of all sizes up to boulders of five or six tons weight, is dumped between the giant rolls, and these enormous masses are crunched and comminuted more easily than a dog crunches a bone. These gigantic rolls are six feet in diameter, six feet long, and their surfaces are covered with crushing knobs. They weigh with the moving machinery seventy tons, and when revolved at a circumferential speed of 3,500 feet in a minute, their insatiable and irresistible bite soon chews the rock into fragments that pass into similar crushing rolls set closer together until reduced to the desired fineness. The sand is then raised to the top of the concentrating devices, shown in Fig. 264, and is allowed to fall in sheets from inclined boards in front of a series of magnets, of which four sets are shown in the figure. These magnets deflect the fall of the particles rich in iron (which are attracted), while the non-magnetic particles of sand drop straight down. A thin knife-edge partition board, arranged below the falling sheets of sand, separates the deflected magnetic particles from the straight-falling sand. These magnetic particles are then collected and pressed into little bricks, which are now so rich in iron, by virtue of concentration, as to make the final reduction of the iron in the blast furnace easy and profitable. More recent developments in this art are shown in patents to Wetherill, No. 555,792, March 3, 1896, and Payne, No. 641,148, January 9, 1900. In the production of copper the well-known Bessemer process for refining iron is now largely used, as shown in patent to Manhes, No. 456,516, July 21, 1891, in which blasts of air are forced through the melted copper to remove sulphur and other impurities. Electrolytic processes of refining copper are also largely used, as described in Farmer’s patent, No. 322,170, July 14, 1885. The production of metals, other than iron, in the United States for the year 1897, was as follows: Gold, 2,774,935 ounces; worth $57,363,000. Silver, 53,860,000 ounces; worth $32,316,000. Copper, 220,571 long tons. Lead, 212,000 short tons. Zinc, 99,980 short tons. Aluminum, 4,000,000 lbs.; worth (37½ cents lb.) $1,500,000. (This was three times the product of 1896.) Mercury, 26,691 flasks; worth $993,445. Nickel, 23,707 pounds; worth (33 cents pound) $7,823.