A Treatise on Metallurgy; comprising mining, and general and particular metallurgical operations, etc

Front matter

A TREATISE ON METALLURGY; COMPRISING MINING, AND GENERAL AND PARTICULAR METALLURGICAL OPERATIONS, WITH A DESCRIPTION OF CHARCOAL, COKE, AND ANTHRACITE FURNACES, BLAST MACHINES, HOT BLAST, FORGE HAMMERS, ROLLING MILLS, ETC. ETC. BY FREDERICK OVERMAN, MTNrNG- ENGINEER. AUTHOR OF A "TREATISE ON IRON," ETC, ETO. WITH THREE HUNDRED AND SEVENTY-SEVEN WOOD ENGRAVINGS. NEW-YOEK: D. APPLETON & COMPANY, 200 BROADWAY, AND 16 LITTLE BRITAIN, LONDON. M.DCCC.LII. Entered, according to act of Congress, in the year 1852, by D. APPLETON & COMPANY, In the Clerk's Office of the District Court of the United States for the Southern District of New-York. PUBLISHERS'

Preface

. This work is chiefly designed to present a practical view of the various and extensive operations of Metallurgy. It is not, however, merely a description of processes, and an enumeration of facts, which would be of comparatively limited importance to the active and inquiring mind of the pubhc ; but each division contains a philosophical in vestigation of the apparatus used, and the operations per formed, in all specific cases, as well as the principles upon which their advantages depend. Without entering so ex tensively into scientific considerations as to impart, in any degree, the character of a theoretical work to his book, the author has, nevertheless, amply enriched his pages with all such investigations as were necessary for the elu cidation of the principles involved in these extensive pursuits.

The progress of the work through the press was some what interrupted by the death of the author, when two thirds of it had been put in type. The manuscript, how ever, had been previously completed by him, with the ex ception of the last chapter, which has been added chiefly from Appletons1 Dictionary of Mechanical Science. In all other parts, it is entirely the work of Mr. Overman. There may, perhaps, be instances in which he is more or less indebted to the labors of other writers, in these pages. If such is the case, it would have, undoubtedly, been ac knowledged, had he lived to write this notice.

PREFACE.

. II The reader may discover imperfections, and even errors, in the work; and doubtless the author would have been fearful that such existed, if he had witnessed its completion. To this apprehension, he would, unques tionably, have alluded in a similar manner to which he has done in a former work, and which we take the liberty here to quote. — " It may even embody errors ; these, on the ground of human frailty, may be deemed by the kind hearted reader excusable. The expression of one fact will, we hope, disarm critics. "We make no claims as a writer. We make this statement not only because the language of the book is not our native tongue, but because, though it were, we doubt whether we should be able to exhibit a reasonable proficiency in its use." The following tribute to the memory of a worthy and accomplished man of science, is from the pen of a personal friend of Mr. Overman.

It is presumed that its insertion here will prove not otherwise than agreeable to the reader, while it will impart gratification to the author's friends. " Frederick Overman died on the 7th January, 1852, in Philadelphia, at the age of 49 years, from the effects of arsenited hydrogen, inhaled while engaged in a chemical analysis. He was a native of Germany, and born in Elberfeldt of humble parents, who could give him but the rudi ments of a common school education ; and when a boy, bound him to learn the mercantile business. This employment, however, little suited his taste and natural activity, and he bound himself afterwards to a cabinet maker, in whose business he acquired a proficiency. While thus engaged and acquiring habits of industry, he was sedulously employed to gain know. ledge and to enrich his mind.

Endowed by nature with a vast cerebral or ganization, indeed one of the very first order, and with a decided prepon derance of the higher faculties, the yearnings of such a mind could not be permanently kept under by poverty and opposing outward circumstances. He began to travel, as all mechanics do in Germany, and went to Berlin to work. There he succeeded in obtaining an admission as a pupil of the Royal Polytechnic Institute, the first institution of its kind in Europe, and then under the superintendence of the celebrated Mr. Beuth, who was not long in discovering the pre-eminent talents of his pupil, and consequently took an especial interest in the education of the young man. He remained

PREFACE.

. III in that Institute I believe four years, during which time he also attended lectures in the Royal University, and was also by his protector introduced to the unions of the first artists and scientific men of the £ige, as for instance Professor Rauch the celebrated sculptor, Schinkel the great architect, Alex ander von Humboldt, <fcc. It w.is under the spell of such happy and en nobling influences, that Frederick Overman acquired such a deep and abiding love of scientific pursuits, and cultivated his taste of the fine arts. Those who have been intimate with the deceased, appreciated his exquisite taste and mature judgment in all that is ennobling and beautiful. In his mind there was a rare union of the ideal with the useful, the practical and the rational. But the most prominent trait in his character was an exalted love of truth and moral freedom, those two features essential to form the basis of a really great man.

But few persons in this country have been on terms of intimacy with the late Mr. Overman, and by many of his super ficial acquaintances he has been either misunderstood or not at all com prehended. Such deep and great natures can never accommodate them selves to the superficialities of our unnatural social institutions, they can never be dragged down to the quagmire of competitive strife and hatred. In his scientific and technological researches he would pursue his object with indefatigaible industry only so long as the scientific interest, or the interest of the inventor was kept alive. This once satisfied, he would drop that pursuit, no matter how great the prospects of emolument and profit. This is the test of true genius. He will never exert himself without a noble ob ject in view ! The day may yet come, when human nature will be more appreciated, and when society will be so constituted, that the ordinary wants of nature's noblemen will be provided for, so that they may be at liberty to follow the bent of their genius, without being harassed by petty cares !

While in Europe, Mr. Overman moved in useful and extensive spheres. His improvements in the manufacture of iron have given him a reputation. In this branch of technology he has labored successfully a num ber of years ; he was well paid, and his engagements called him to all parts of Europe, where he introduced his improvements in the process of pud dling and other manufactures. He has been engaged in superintending the erection of various large manufacturing establishments ; he conducted at one time the extensive engineering establishment at Chemnitz, in Saxony ; and for a period was in the employment of the Austrian Government, working under the es pecial and personal charge of Prince Metternich, for the purpose of ascertaining the industrial resources of the empire, and collecting commercial data upon which to found the basis of a new commercial treaty with Great Britain.

All these engagements were but of a temporary duration ; his strong love of freedom and unfettered action would never permit him to accept of a permanent situation as a government official, neither in Aus-

PREFACE.

. tria nor Prussia. His political instincts were thoroughly democratic. Various disappointments and dissatisfaction induced him to emigrate from Germany to this country, in the year 1842, where he has been variously employed in introducing improvements in the manufacture of iron. In this he has met with success and failures. Had he enjoyed the good fortune of a James Watt, in finding another Bolton, who had appreciated his talents, and applied his inventive genius to mercantile use, he could have realized fortunes. For the last three or four years he was engaged principally in his technological writings. His work on the manufacture of iron is highly appreciated in Great Britain, and by the London Mining Journal is pronounced the best scientific treatise ever published on the subject. JOHN A. ROEBLING, Civil Engineer.

Trenton, N. J., 1th April, 1852. The Publishers, amid all the circumstances, have spared neither labor nor expense to render this work such as might be expected, from the extensive importance of the subject, and the ability of its author. They anticipate that its practical information will be found in advance of similar works. And although it might have been better, if it could have received the benefits of the author's re vision throughout, yet they are confident it will meet with that favor from the pubhc which is due to the last labor of one who has passed beyond the sphere of human criticism. D. APPLETON & CO. TABLE OF CONTENTS. »>t PART FIRST. MINING.

Chapter I. Paoe

Exploring . . . . . . . . .17 " by shafts . . . . . . . 22 Examination of the deposit . . . . . .27 Plan of a mine ....... 33 Irregular veins ........ 38 CHAPTER EL Boring for minerals ....... 39 " by steam ........ 43 Turning the spindle ....... 44 Accidents ........ 44 Cementing ........ 46 Saving of bore metal . . . . . . .47 chapter ra. Nature of a vein ....... 48 Theory of the formation of mineral deposit . . . .50 chapter iv. Working of a mine ....... 53 Subterranean workings . . . . . .54 Galleries ........ 55 Shafts ........ 56 Timbering ....... 57 Framing of timber . . . . . . .63 Extraction of ore ...... 64 Masses ........ 73 Pockets ........ 79 Practical remarks on mining . . . . .80 Blasting ........ VI CONTENTS. PAH

CHAPTER V.

Hauling and hoisting .....•• 83 Dog cart .-......• 83 The wheelbarrow . . . . . . .84 Horse wagons, or cars ...... 86 Inclined planes . . . . . . .88 Hoisting in shafts ...... 89 Ropes and chains . . . . . . . 95 Descent and ascent of the workmen .... 97

CHAPTER VI.

Ventilation of mines . . . . . . .99 Natural means ....... 100 Artificial means . . . . . . .102 Distribution of air in the mine ..... 108 Quantity of air ....... 110 Form and dimensions of air passages .... 112 Safety lamp ........ 114 Trying the air ...... . 116 Tube of respiration . . . . . . .118 CHAPTER vii. Drainage ........ 119 By levels ........ 120 By pumps ........ 121 Principles of the pump ...... 121 Lifting pump ....... 125 Of pistons . . . . . . . 126 Force pump . ... . . . 128 Pipes • • • . . . . .130 The quantity of water furnished by a stroke of a pump . 133 Loss of power in pumps ...... 134 Length of stroke . . . . . . .134 Piston rods ........ 135 Setting of a pump ...... 139 Proposal of a new method for setting pumps . . . 140 Various forms of pumps ...... 143 Spring poles for pumps ..... I45 PART SECOND. GENERAL METALLURGICAL OPERATIONS.

CHAPTER I.

Assaying . . 151 The blow-pipe ..... 1 CONTENTS. VII PAGE Re-agents 153 Carbonate of soda 154 154 Cyanide of potassium Borax 154 Operation with borax 157 160 Operation with soda Roasting . 161 Color of oxides on the coal Selenium . 162 162 Tellurium Arsenic 162 162 163 Antimony Bismuth . Lead . 163 163 163 Cadmium Zinc . 163 163 Tin 163 Silver Sulphurets, chlorides Test on minerals . 164 164 164 Barytes, Heavy spar Lime . 164 Magnesia Alumina Silex 165 165 165 Manganese Zinc 166 166 166 Cobalt Nickel Iron . 167 167 Cadmium Lead . 168 168 Bismuth Uranium ■ Copper 168 168 168 Silver 169 Mercury Platinum Gold 169 170 170 170 Tin Antimony Titanium Chromium Tellurium Arsenic . 171 171 172 172 172 173 Selenium Crucible assay Instruments 173 CONTENTS, VIII PAGE 174 Mortar Sieves . Washing 175 175 176 Calcination Roasting . 176 177 Melting 178 Crucibles Furnace 180 181 Blast-furnace Cupel furnace 184 186 Cupel Fluxes 189 189 Silex 189 Potash Soda . 190 190 Potash and soda Baryta 190 Clay 190 190 Manganese Iron 191 191 Copper 191 Antimony 191 Lead and bismuth Borax 191 Fluor-spar Saltpetre 192 192 Chloride of sodium Black flux 192 193 Litharge 194 195 Sulphate of lead Oxide of copper 195 Reduction Hydrogen Carbon 195 195 196 Power of reduction Smelting 197 198 Gold . 198 Use of salt 201 Sulphurets and Tellurets Metals and gold . 201 205 Platinum, silver, and gold Moist analysis 206 207 Remarks 207 Platinum, and platinum metals Silver 208 209 Assay by metallic lead 210 Smelting with oxide of lead .

Cupellation . 212 CONTENTS. IX -a.e Amalgamation . . . . . . . . . 222 The globule in the cupel .... . . . 223 Separation of gold and silver . . . . . . . 223 Lead 225 Assay in crucibles ...... 227 Galena . .229 Lead and antimony . . . . . 230 Copper X X . X . . . .230 Refining .... ... 233 English process of assaying ...... 235 Crude copper ....... 235 Refining ........ 338 Iron . ...... . 239 Mercury ........ 243 Chromium . . . . . . 244 Zinc . 244 Tin ..... . . . 245 English assay ....... 247 Cobalt 248 Nickel . 249 Bismuth .... . . . . 249 Cadmium ........ 250 Antimony ....... 250 Other metals ........ 250 Assay of fuel . . . . . . . . 250 Particular value of fuel . . . . . . 251 Ashes . ■ . . . . . . .252

CHAPTER IL

Preparation of ore by hand ...... 252 Washing by machines ...... 258 Crushing . . . . . . . .259 Rollers . . . . . . . .260 Stamping . . . . . . . 262 Mills ........ 265 Wet stamping ....... 268 Number of wipers ...... 270 Form of trough ....... 274 The sieve . . . . . .274 Open bottom ........ 275 Variety of forms of stamping machines .... 276 Final washing ....... 281 In the sieve ....... 282 Sweep tables ....... 285 Percussion tables ...... 287 Roasting . . . . . . . .289 In heaps ........ 289 In mounds ........ CONTENTS. X .-.OB On furnaces .....-• 291 The reverberatory furnace ...••• Principle of roasting ....-" CHAPTER UL Smelting ......•• 294 Metals ...... • • 295 Slags 296 Oxidation 296 Reducing ....•••• 298 By metals 2S8 By oxides ....•••• 2"" Carbon 300 Alloys 304 Separation by weight ...... 306 Slags 306 Fusibility of slags ...... 307 General reflections on smelting ..... 310 Sublimation ....... 311 Refining 312 Liquefaction ....... 312 Crystallization ....... 313

CHAPTER IV.

Fire-proof material ....... 314 Native fire-proof material ...... 314 Sandstone ....... 315 Clay and clay slate ....... 317 Slaty clay ....... 317 Clay 318 Talcose slate ....... 319 Mica slate ........ 319 Artificial stones, firebrick ..... 820

CHAPTER V.

Fuel ......... 326 Flaming fuel ....... 327 Composition of fuel ....... 328 The quantity of oxygen consumed .... 331 Intensity of heat ....... 331 Specific heat ....... 335 Wood ......... 335 Composition of the fibre ...... 336 Amount of water ....... 337 Ashes ........ 338 Flammability ....... 339 Drying of wood ...... . CONTENTS. XI PAGE Air-dried ........ 341 Kiln-dried ....... 343 Charcoal ......... 347 Absorption of water . . . . . . 348 Combustibility . . . . . . . 349 Quantity of heat in charcoal . . . . . 349 " coal . . . . . . . 350 Charring . . . . . . . . 352 " under movable covers ..... 352 " in ovens . . . ...... . 364 Cost of making coal . . . . . . .370 Peat or turf, and brown coal . . . . .373 Turf charcoal ....... 374 Mineral coal, pit coal or stone coal ..... 374 Bituminous coal . . . . . . . . 375 Quantity of heat . . . ... . 375 Quality of heat ....... 376 Coking of coal ....... 377 " in the open air ..... . 378 '- in ovens ....... 383 Anthracite ........ 389 Gaseous fuel ....... 391

CHAPTER VI.

Means which facilitate combustion ..... 393 Construction of furnaces ...... 394 Chimneys . ....... 395 Blast machines . . . . . . .397 Fans ......... 397 Cylinder, or piston blast machines .... 400 Parts of blast machines ...... 406 Regulators of blast ...... 409 Necessity of uniform density . . . . .410 Pipes and nozzels . . . . . .411 Tuyeres ........ 415 Manometer ....... 418 Hot blast . . . . . . . .418 Blast machines ....... 421 PART THIRD. PARTICULAR METALLURGICAL OPERATIONS.

CHAPTER I.

What are metals ? ....... 423 Classification of metals ...... CONTENTS, XII PAOE Affinity for oxygen ... Hydrated oxides . . . . . • Aifinity for chlorine . . . . • . . 428 429 Sulphurets . . • . ■ Phosphurets . . . • • .430 Carburets . .-.■.-. . • .431 Alloys . . . ... • • . • • 431 Fusibility of alloys . . . . ■ . • .433 Specific gravity . • . . . . . • - 435 The hardness of alloys . ... . • .435 Oxidation 436

Chapter IV

Zinc . . . * . . . . *. .453 Oxide of zinc . . • . . . . . 454 Blende . ... . . . . 454 Calumine . ... . . . . 455 Alloys of zinc ....... 455 Use of zinc . . . . . . . 456 Manufacture of zinc ....... 456 Silesian process ....... 457 Liege process . . ... . . 460 English process ....... 461 Refining •■-..... 462 Remarks ........ 463 Residue of distillation ...... 464 Rolling of zinc ....... 464 Cadmium ......... 465 Alloys ........ 465 Uses of cadmium . . . . . ... 455 Manufacture ... ... . . . CONTENTS. XIII PAGE

Chapter V

Manganese, properties 466 Ores 466 Alloys 467 Iron 467 Ores 468 Native iron 468 Magnetic iron ore Red oxide of iron 469 470 Brown hematite Sparry ore 471 472 Argillaceous ore Alloys of iron . 473 475 Phosphorus and iron . 477 Carburet of iron Silicon 477 480 Aluminum Arsenic 480 481 Chromium Titanium Zinc 483 483 483 Manganese 483 Nickel and cobalt Antimony 484 484 Lead Tin 484 484 Copper Mercury Sdver 485 485 485 Gold 485 Uses of iron 485 Manufacture of cast-iron . Roasting . Cleansing of ore Fluxes Artificial fluxes Amount of fluxes . Mixing of minerals Properties of cast-iron 486 486 491 493 495 496 496 499 Gray pig iron Mottled iron White iron 500 501 502 Smelting 505 Working the furnace Remarks . 512 515 Width of hearth Pressure of blast 518 519 Number of tuyeres CONTENTS. XIV PAGB 520 Size of boshes 521 Form and size of in-wall Size of mouth 523 525 Theory of smelting iron Hot blast . 527 Effect of hot blast Waste heat 530 532 534 Charcoal furnace Anthracite furnaces Coke furnaces 535 536 Manufacture of wrought-iron Qualities of wrought-iron Test of iron 536 536 538 Wrought-iron directly from the ore .

German forge 541 545 552 Steel Refining of crude iron Puddling 553 561 Single furnaces Double furn.ice Puddling . 561 564 567 Boiling 570 General remarks Steel, hardening Annealing 574 582 585 Nature of steel Natural steel . . " 587 588 Steel of cementation, blistered steel Cement Furnaces 589 591 592 Shear steel Cast steel 595 596 Hammers Squeezers Rollers 598 602 604 General arrangement of rollers Sheet rollers 607 609 Shears 613 Reheating furnaces Stoves . • 614 618 Cost of making iron Nickel . . • 621 623 Ores of nickel Copper nickel Cobalt speise Alloys of nickel Argentan . 624 624 624 625 625 Uses of nickel alloys . CONTENTS. XV PAOB Manufacture of nickel ...... 626 Cobalt 630 Ores of cobalt . . . . • • .630 Alloys of cobalt . . . . . . .631 Smalt, azure, and zaffre . . . . . .631 Manufacture of smalt ...... 631

CHAPTER VL

Tin 636 Ores ......... 636 Alloys . . . • • • .637 Pewter . . . . . • • .637 Britannia metal . • • • • ■ .637 Speculum metal . . . • • • .638 Tin plate ........ 638 Uses of tin . . . . • • • • 639 Manufacture of tin . . . . . ■ 639 Antimony ....•••• 646 Ores ........ 646 Alloys 647 Uses of antimony ...... 647 Manufacture ........ 647 Bismuth ........ 650 Ores ......... 650 Alloys 650 Uses ......... 651 Manufacture ....... 651 Lead ......... 652 Ores 652 Galena 653 Alloys of lead ....... 654 Uses of lead ........ 655 Manufacture of lead ...... 656 Theory of smelting lead ore ...... 668 Lead smoke ....... 675 Sheet lead 676 CHAPTER VH. Copper ........ 678 Ores ......... 678 Native copper ....... 678 Sulphuret of copper ....... 679 Copper pyrites ....... 679 Gray copper ........ 680 Oxide of copper ....... 680 Silicate of copper ....... 680 Carbonate of copper ......

CONTENTS. PAGE Alloys of copper ....... 681 Uses ........ 684 Manufacture ........ 684 Smelting in reverberatory furnaces .... 684 The blast furnace ....... 691 Theory of smelting copper ..... 695 CHAPTER vm. Gold 701 The extraction of gold ...... 704 Silver ......... 710 Mercury . . . . . . . 717 Platinum ........ METALLURGY PART FIRST. MINING

Chapter I

Exploring. — In order to obtain those minerals which form valuable metals in the course of metallurgical operations, we must first find the ores. These, however plentifully distributed over the globe, must be sought by intelligent inquiry, if their discovery is not made accidentally. Any heavy material on the surface of the globe is a metallic ore, an oxide or compound of some metal ; we may with propriety range potash, lime and clay, magnesia and silex among the metallic ores. At the present time we use only some few metals in the arts, but it does not follow from this that sodium and others are excluded for ever from being useful. The time may come when calcium will be as valuable as any other metal. Those ores which are useful in our operations of manufacturing metals, are seldom found on the surface of the earth; they are in most instances buried beneath the soil, and penetrate the solid rock often to considerable depth.

Some may be found on the surface, at least those which are insoluble in water, as all the oxides of iron ; but not so the carbonates, and the sulphurets. As iron is distributed in every part of the earth's surface, as it has affinity for all other matter, and as it is insoluble in water, we may expect to find it in every locality where we explore for ore. Iron ore appears in so great a variety of forms, it is often so far blended with other matter, that we are justified in asserting that it represents almost all the known varieties of other ores in appearance ; it is frequently so dis guised as to make an assay necessary in order to recognize its presence. In most instances, when we find a heavy mineral, our first ■i METALLURGY. thought should be that it is iron ; and if, after a rigid examina tion, we find it is not iron, we are at liberty to infer another kind of ore.

Iron is very plentiful in our country ; it is hardly worth the pains and time taken to hunt for the ore. Its value is so small, it does not pay more than the expenses of digging and hauling in the majority of instances. We may expect to find iron every where — in ploughing, in making roads, and in digging canals. It is not so with other ores. Gold, if in its metallic state, may be found in the soil and in the ravines where a gentle current has accumulated alluvial deposit. Is the soil or sand composed of clay and other sub stances, and light, so as to form readily soft mud, we always find the gold lying deep, on the face of the rock ; particularly if the gold is coarse. It is in vain to look for fine gold in the bed of a river : its place is in the soil, and ditches made by the gen tle currents of rain-water.

Metallic gold cannot be found in the coal regions ; it belongs to an older period of creation. At the time when coal was deposited, it was too wet and cold for its formation, and that which may possibly have been liberated by abrasion of the old rocks, was rubbed to fine invisible dust, which probably may exist in all rocks of the younger series. Gold is soluble in sulphurets, wherein is an excess of sulphur : we find gold, therefore, in all pyriteous ores, particularly in those of iron, copper, and lead; but the quantities are often so small, that its value would not pay the expenses of extraction. We may find gold in the pyrites of all the Atlantic States, but, to all appearances, the near approach of coal is the cause of its being in small quantities in those States. In Virginia and North Carolina we find coal near the gold ore deposits, and even in the belt of the gold-bearing rocks ; but that coal is a secondary deposit, the gold rock was formed before the coal was deposited — it under lays the coal.

In the western bituminous coal-fields, it is of no use to look for gold. Along the lakes there may be gold, but we are not informed of its having been found in sufficient quantity for extraction. Both slopes of the Eocky Mountains are the legitimate localities for gold deposits. Gold is often found in ter tiary formations, in lignite and the rocks around it, but is of no practical use. If we suspect gold, either in alluvial soil or in rocks, the surest way to discover it is to wash the matter that is, remove the rocky matter by water in a manner described hereafter, and if the particles of gold are so heavy as to remain MINING. after the debris of rock is removed, the ore may be useful. If the particles of gold are so small as to float away with the rocky matter, the ore is of no use. Pyrites are tested by means of the assay.

Near limestone rock, no gold is found. Copper ore is in most instances recognized by its green color : if the body of the ore, in the interior, is blue, or red, or black, that on the surface contains always more or less of the green kind ; and we are generally correct in supposing copper where green silicious matter is found. Lead is not often discovered on the surface of the ground, or imbedded in the soil : freshets may expose a vein of galena and carry particles away from the general deposit, but this is no rule. Galena is easily decomposed by the atmosphere, and as the result ing sulphate, or carbonate, or oxide, is very light, and in the form of dust, it is easily carried away by currents of water. Deposits of galena must be looked for in steep hills and rapid currents, in limestone rock and the old slate, in graywacke, gneiss and por phyry; but in all stratified rocks of the coal series, and the younger rocks, it is in vain to look for it.

There may be lead ores — such as carbonates, sulphates, and others in young formations — but we have such an abundance of good galena, which will keep the price of lead down, that the manufacture of that metal from any other ore is unprofitable. If, therefore, other ore is found, it is of little value for smelting lead ; but it may be valuable for silver or gold, provided it contains so much of the pre cious metals as to pay for their extraction without depending on the amount of lead. Galena is easily recognized by its metallic lustre and cubical crystals ; that of the slate formations contains always more precious metals than that from the limestone rocks. Silver is generally a companion of lead, and it is not often found in any other form than combined with galena. In native copper, particularly that found at Lake Superior, there is in many instances a large quantity of metallic silver.

Such ores, however, cannot be considered silver ores, for we cannot extract the silver without incurring heavy expenses. Silver occurs also in antimony, but always combined with lead. If we find an ore accidentally, and it is not in a compact vein in the old or igneous rocks, we may doubt its being a useful silver ore ; if it comes from a limestone region, it is generally very poor in silver. We are there fore not justified in expecting to find silver ore in alluvial soil, or young rocks. METALLURGY. All other ores form compounds, or are found in a form which is easily destroyed. Neither zinc in all its forms, nor antimony, tin, or arsenic can be expected to appear at the surface ; we must look for them either in crevices or upon the rock below the soil. The presence of an ore may be presumed where a specimen is found, or where fine particles of it occur.