The Metallurgy of Gold ... Third edition, revised and enlarged

Introduction

TO THE STUDY OF METALLURGY. By Prof. Roberts- Acsten, C.B., F.R.S. Foukth Edition 15s. 1. COLD. By. T. K. Rose, Assoc.R.S.M., D.Sc, Assistant- Assayer of the Royal Mint. Third Edition. : IRON. By Thos. Turner, Assoc.R.S.M., F.I. C. 16s. 4. STEEL. By F. W. Harbord, Assoc.R.S.M., F.I.C. 5. SILVER AND LEAD. By Henry F. Collins, Assoc.R.S.M., Assoc. Mem. Inst. CE. 6. METALLURGICAL MACHINERY. By H. Jenkins, Wh.Sc, Assoc.R.S.M., Assoc. Mem. Inst. C.E., of the Royal Mint. 7. ALLOYS. By the Editor. *** Other Volumes in Preparation. ELECTRIC SMELTING AND REFINING. A Practical Manual of the Extraction and Treatment of Metals by Electrical Methods. Being the " Elektro- Metallukoie" of Dr. W. Boi.chees. Translated from the Second German Edition hy Walter G. M'Millan, F.I.C, F.C.S., Secretary to the Institution of Electrical Engineers.

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Preface

. In the preparation of the Third Edition, the text has again been revised and considerable additions made to it, in order to keep pace with the progress of the metallurgy of gold. In particular, the chapters on the Cyanide Process will be found to have undergone great changes. The frontispiece is from a photograph kindly lent by Mr. Alfred James, late of Johannesburg, and several other new illustrations have been added. Royal Mist, April, 1S98.

PREFACE.

TO THE SECOND EDITION. In preparing this edition, the whole book has been care fully revised, due attention being paid to some useful suggestions contained in the various reviews of the first edition. Eight new illustrations have been added, in cluding the frontispiece, which is from a photograph kindly lent by Messrs. Fraser & Chalmers. The rapid progress made in improving the cyanide process has rendered it necessary to make considerable alterations in, and additions to, the chapters devoted to that subject, and a new chapter on Economic Considerations has also been added. The author desires to express his thanks to the old students of the Royal School of Mines and others, who have kindly sent information on work connected with the metallurgy of gold, now in progress in various parts of the world. Royal Mint, August, 1896.

INTRODUCTION.

TO THE FIRST EDITION. As this is the first of a series of treatises devoted to indi vidual metals, which is being prepared under my guidance it may be well to offer a few introductory remarks. Associates of the Royal School of Mines have taken their full share in conducting mining and metallurgical operations in all parts of the world, but, notwithstanding the wide experience they have gained, no treatise claiming to give a general account of the Metallurgy of Gold and adequately dealing with modern processes could hitherto have been attributed to a Student of the School. It may be claimed that in this country Dr. Percy founded the literature of Metallurgy, but his volume on "Silver and Gold," although unrivalled in accuracy of detail, is only a splendid fragment, and _rold is alone dealt with in the sections devoted to the refining of bullion and to assaying.

A large amount of valuable information concerning new methods and machinery used in the treatment of gold ores has appeared in various official publications during the last twenty years, and much is either scattered over the pages of the scientific press or incorporated in the proceedings of various learned societies. Any attempt, however, to study the subject as a whole from

INTRODUCTION.

. VIII these sources of information would be hopeless for those whose time is limited, or who have not good libraries at their disposal. Mr. Rose gained his practical experience of gold and silver extraction in the Western States of America, and has written a volume which should prove to be very useful, as its careful and conscientious preparation entitles it to confidence. W. C. ROBERTS-AUSTEN. Royal Mint, March, 1894.

PREFACE.

TO THE FIRST EDITION. In the present volume an effort has been made to supply a succinct summary of the existing condition of the metallurgy of gold, for the use of students and others who are interested in the industries connected with the precious metals. It has been said that mill-managers as a class have little to learn from books ; but, even if this be so, they will still need to keep themselves acquainted with the progress of their art in far distant countries. To them the bibliography appended will be useful. Although, in this book, brief accounts are given of some typical machines, attention has been directed rather to methods of procedure than to details of machinery, which will be dealt with in a separate volume in the series. The practice in particular extraction works has been described at some length, as such information, wherever it is available, is among the most valuable that can be given.

Some recently devised methods of great importance in the metallurgy of gold, such as the MacArthur-Forrest cyanide process, the new barrel chlorination process, and the improved Gutzkow parting process, are given for the first time in a manual. Particular attention has been paid to the assay of gold bullion, the system in use at the Royal Mint and the precautions necessary to ensure X

PREFACE.

. the highest attainable accuracy being described. Little space is devoted to the geographical and geological distribution of gold ores, as this ground has been amply covered by other works. Moreover the processes of smelting, leaching, and pan-amalgamation used in the treatment of silver ores, whether they contain gold or not, have either been omitted or merely sketched in outline, as belonging to the metallurgy of silver. I am indebted to the President and Council of the Institute of Civil Engineers for leave to reproduce Figs. Nos. 14, 16, 17, 32, 33, 35, and 36, and to Mr. John Murray, for Figs. 58, 60, 61, 62, 63, 64, 66, and 67, which are copied from Dr. Percy's Metallurgy. I also take this opportunity of expressing my thanks to my colleague, Mr. F. W. Bayly, and to other friends for their kind assistance while certain sections were going through the press.

In conclusion, I desire gratefully to acknowledge the kindness of Prof. Roberts - Austen in giving valuable advice throughout the progress of the work. It was at his suggestion that the task was undertaken, and it is hoped that it will contribute to the realisation of his wish that the experience of School of Mines men should be collected in a number of works which will together form a comprehensive metallurgical series. T. K. ROSE. Royal Mint, March, 1894. CONTENTS. Chapter I. — The Properties of Gold and its Alloys. PAGE Introduction, . . . 1 Physical and chemical properties of gold, ... 2 Introduction, ... 2 Colour, .... 2 Malleability and ductility, . 3 Hardness 3 Tenacity, .... 3 Speci6c gravity, ... 4 Cohesion, .... 4 Specific heat, 4 Fusibility, .... 4 Spectrum, .... 4 Latent heat, . . 4 1*AQE Magnetism, .... 4 Conductivity and expansion, 5 Atomic weight and volume, . 5 Volatility, .... 5 Crystallisation, ... 9 Solubility, .... 10 Preparation of pure gold, . 11 Allotropic forms of gold, . . 12 Alloys of gold, .... 13 Amalgams, .... 15 Gold and silver, ... 16 ,, copper, ... 17 Liquation, . . 18 Chapter II. — Chemistry of the Compounds of Gold.

Compounds of gold, . . 20 Haloid compounds, ... 21 Chlorides of gold, . . .21 Protochloride, ... 21 Auro-aurichloride, . . 21 Trichloride, . . . .21 Chlor-auric acid, ... 27 Bromides of gold, ... 27 Protobromide, ... 27 Auro-auric bromide, . . 28 Tribromide, .... 28 Iodides of gold, ... 28 Cyanides of gold, ... 28 Aurous cyanide, ... 28 Auro-cyanide of potassium, . 28 Auri-cyanide of potassium, . 29 Oxides of gold, ... 29 Aurous oxide, ... 29 Auric oxide, ... 29 Aurates, 30 Fulminating gold, ... 30 Sulphites of gold, ... 30 Hyposulphites of gold, . . 31 Silicates of gold. . . . 33 Sulphides of gold, ... 33 Purple of Cassius, ... CONTENTS. Chapter III.— Mode op Occurrence and Distribution of Gold. PAGE Petzite 38 Nagyagite, .... 38 Composition of native gold, . 39 Geographical distribution of gold, 39 Origin of gold ores, ... 42 PAGE Forms in which gold occurs in nature, .... 35 Vein gold, .... 35 Placer gold and nuggets, . 37 Calaverite, . . . .38 Sylvanite, .... 38 Chapter IV.— Placer Mining \ — Shallow Deposits.

Placer deposits, ... 43 Methods of obtaining gravel from shallow placers, . 44 Appliances used in washing the gravel, . . . . 45 Pan, 45 Batea, ..... 45 Prospecting trough, . . 46 Hornspoons, ... 46 Cradle, . . . . 46 Long-tom, .... 47 Puddling-tub, ... 47 Siberian trough, ... 48 Sluice, ..... 50 Sluicing, . . . . . 51 Use of drops, ... 52 ,, undercurrents, . . 52 ,, mercury, ... 53 Cleaning-up, ... 53 Tail-race, .... 54 Ground-sluice, . . .54 Booming, .... 54 Dry diggings 55 Cement gravels, ... 55 Tail sluices, .... 55 Fly catchers, .... 56 River mining, .... 56 1. Kiver mining proper, . 56 2. Dredging, ... 58 3. Deep bar mining, . . 58 Methods of working Siberian placers, .... 59 1. Siberian sluice, . . 60 2. Trommel, . . .61 3. Pan 61 Beach mining, .... 62 New method of working shallow placer deposits, . . .62 Chapter V. — Deep Placer Deposits.

Nature and mode of origin of deposits, .... 63 Distribution of gold in the gravels, .... 67 Origin of gold in the gravels, 6S Minerals occurring in the gravels, . . .70 Methods of working, . . 71 " Hydraulicking, . . 71 Commencement of operations, .... 72 Supply of water, . 72 Breaking down the bank, . 75 Sluicing gravel, . . 77 Cleaning-up, ... 80 Disposal of tailings, . . 81 Drift mining, ... 82 Shaft „ . . .82 Hydraulic elevator, . . 84 Economic conditions, . . S6 Shallow placer deposits, . 86 Deep ,, ,, . CONTENTS. XIII Chapter VI. — Quartz Crushing in the Stamp Battery. PAGE Mortar, . . 108 Splash-box, . . .110 Dies, . . . .110 Cam-shaft, . . . Ill Cams Ill Tappets, . . . .112 Stamp, . . . .113 Guides, . . . .114 Lever, . . . .114 Height of drop, . .115 Screens, . . . .115 Order of fall of stamps, . 120 Automatic feeding, . . 120 Stanford's feeder, . .121 Challenge „ . .122 Water supply, . . .123 Amalgamated plates, . 124 Mill site 126 General arrangement of the stamp mill, . . . .127 PAGE Primitive methods of crushing and amalgamating, . . 89 Mortar, .... 91 Maray, . . . .91 Chilian mill, . . .92 Arrastra, . . . -92 Iron prospecting arrastra, . 95 Stamp battery, ... 95 Rock-breakers, . . . 101 Blake, . . . .101 Dodge, . . . .102 Gates, .... 103 Schranz, .... 104 Material employed for crushing surfaces, . .105 Position and use of rock-breakers, . . .106 Battery proper, . . . 107 Foundations, . . . 108 Framework, . . .108 Chapter VII. — Amalgamation in the Stamp Battery.

Galvanic action in amalgama Treatment of the amalgamated plates, 129 Discoloration of the copper plates, 130 Chemicals used to promote amalgamation, . . . 131 Use of mercury, . . . 132 Grade of plates, . . . 132 Muntz metal plates, . . 133 Shaking copper plates, . . 134 Corrugated plates, . . . 135 tion, 135 Designolle process, . . . 136 Cleaning-up, .... 137 The clean-up barrel, . . 138 „ ,, pan, . . 13s Retorting, . . . .140 Loss of mercury, . . . 142 ,, gold 145 Non-amalgamable gold, . .151 Gold in pyrites, . . . 152 134 Mercury wells, . . 135 Chapter VIII.— Other Forms of Crushing and Amalgamating Machinery. Schemnitz mill, . . . 165 Lazzlo amalgamator, . . 166 Pan-amalgamation, . . .167 The old system, . . . 16S The Boss continuous system, 171 Treatment of concentrates, . 172 Molloy's hydrogen amalgamator, 173 Jordan's amalgamator, . . 174 Special forms of stamps, . . 155 Husband, . - .155 Steam 156 Elephant 158 Morison's high-speed stamp, 1&8 Huntington mill, . . . 159 Crawford ball mill, . . . 1 63 Other ball mills, . . .165 Tyrolean mill CONTENTS, Chapter IX. — ( -Concentration in Stamp Mills.

PAGE PAGE Concentration, . . .175 Settling boxes, . . .177 Classification according to size, 178 Screens, . . . .178 Pointed boxes, . . . 179 Early concentrating machinery, 180 Blanket strakes, . . .181 Riffled sluices, . . .182 Raising-gate concentrator, . 182 Round buddle, . . 162 Centrifugal concentrators, . 183 Hendy's, . . . 183 Duncan's, . . . .183 Percussion tables, . . . 1 84 Gilpin County concentrator, 184 Frue vanner, . . . .185 Method of working, . . 189 Riffle belted, . . .192 Embrey concentrator, . .192 Liihrig vanner, . . . 193 Hartz jigs, . . . .196 Pneumatic jig, . . . 197 Clarkson and Stanfieid 's concentrator, .... 197 Chapter X. — Stamp Battery Practice in Particular Localities. In California, . . . .199 In Colorado 204 On free-milling ores in Australia and New Zealand, . . 208 In the Thames Valley, New Zealand, . . . .212 In Dakota, . . . .215 In the Transvaal, . . . 216 Chapter XI. — Chlorination : The Preparation of Ore for Treatment.

The Plattner process, . . 224 Use of salt, . . . .242 Origin, .... 224 Losses of gold, . . 244 Method of working at Reichenstein, .... 225 Mechanical furnaces, . . 246 1. With mechanical stirrers, 247 Modern practice in chlorination, 226 (a) O'Hara, . . . 247 Crushing, . . . .227 (6) Spence, . . .248 Krom's rolls, . . . 228 (c) Pearce Turret, . 249 Rolls at Rapid City, Dakota. 231 (d) Brown horse shoe, . 250 Comparison between rolls and stamps, . . . 232 (e i Ropp straight line, . 252 2. With rotating beri, . 252 Drying the ore, . . . 233 3. Revolving cylinders, . 255 Roasting, . . . .234 (a) Bruckner, . . 255 Reverberatory furnace, . 235 (6) Hofmann, . . 256 Chemistry of oxidising roasting, 238 (c) White, . . .258 Decomposition of various minerals, .... 240 Elimination of arsenic and (d) White-Howell, . 259 Use of producer gas in roasting, 260 antimony, . . .241 Chapter XII. — Chlorination : The Vat Process.

Construction of vats, . . 261 Charging-in, .... 263 Generation of chlorine, . . 264 Impregnation, . . . 266 Reactions in vat, . . . 267 Amount of chlorine required, . 269 Leaching, .... 270 Precipitation of gold, . . 271 Cost of working, . . . CONTENTS. XV Chapter XIII.— Chlorination : The Barrel Process. PAGE History, 274 PACE Organic substances, . 287 Sulphuretted hydrogen, 287 Sulphurous acid, . 288 2. Solid precipitants, . . 289* Charcoal, . . . 289 Insoluble sulphides, . 290 Metals, . . .291 Modern patent processes of chlorination, . . . 292* Newbery-Vautin, . . 292 Pollok, .... 292 Swedish, .... 293 Cassel, 295 Julian, .... 295 Greenwood, . . . 295 Mears process, . . . 275 Thies process 277 Construction of barrel, . 2S0 Charging-in, . . . 280 Amounts of chemicals required, .... 282 Method of leaching, . . 283 Mechanical difficulties in leaching, 284 Leaching by pressure, . . 284 Riotte's method, . . . 285 Precipitation of gold , . . 285 1.

Soluble precipitants, . 286 Ferrous sulphate, . 286 Chapter XIV. — Chlorination: Practice in Particular Mills. Cost of working, . . 303 Vat process 296 1. Butters' Mill,Kennel,Cali- 3. Modern barrel process at, the Golden Reward Works, Deadwood, Dakota, . 303 fornia, .... 296 2. Plymouth Mill, Amador Co., California, . . 297 3. Treadwell Mine, Alaska, 298 Cost of working, . . 310 4. Bromination Mill, Rapid City, Dakota, . .311 Barrel process — 1. Mears process at Deloro, Canada, . . .298 The Cassel- II inman bromine process, .... 314 2. Thies process in Carolina, 302 Future of chlorination, . . 316* Chapter XV.— The Cyanide Process. History, 319 Plant required, . . . 337 MacArthur-Forrest process, . 321 1. Preparation of ore, . . 321 2. Dissolving the gold, . 322 Treatment of ore slimes, . 337 ,, accumulated slimes, . 340 Vats, . . . .322 Testing of ores, . . . 341 Direct treatment of Rand ore, 342 Charging-in, . . . 323 Use of cyanide in stamp battery, .... 343 Separation of slimes, . 324 Leaching, . . . 324 Agitation, . . 325 Method of treatment — At the Robinson mine, . 344 Strength of cyanide solution, .... 328 ,, Sylvia mine, . . 344 T,10n, ....

"ASA, Disposal of tailings, . 330 ,, New Primrose mine, 346 3. Precipitation of gold, 331 Cleaning-up, . . . 332 Siemens-Halske process, . 347 Results of process — In South Africa, . . 351 4. Production of bullion from precipitate, . . . 333 ,, other parts of the world, CONTENTS, XVI Chapter XVI. —Chemistry of the Cyanide Process. PAGE PAGE Strength of solution required, 372 Consumption of cyanide, . 372 Methods of increasing the speed of action of potassium cyanide, . . • 373 The Hood process, . . . 374 ,, Sulman-Teed process, . 376 ,, Molloy process, . . 377 ,, Pelatan-Clerici process, . 377 ,, Johnstou process, . . 378 Wilde process, . . 378 Ores suitable to process, . . 378 Action of potassium cyanide on gold and other metals, . 354 Decomposition of potassium cyanide, .... 360 Reactions in the zinc boxes, . 361 Action of potassium cyanide — On metallic salts and minerals, 364 On oxidised pyrites, . . 366 The soda solution, . . . 368 Re-precipitation of gold and silver in leaching vats, 369 Testing strength of solution, . 369 Estimation of sulphides in potassium cyanide, . . 371 Chapter XVII. •Pyritic Smelting. 380 Chapter XVIII. — The Refining and Parting of Gold Bullion. •General considerations, . . 383 Refining, .... 384 Composition of bullion, . 384 Melting furnace, . . . 386 Crucibles 387 Melting, . . . .387 Refining, .... 388 Toughening, . . .390 Casting, . . . .391 Losses of bullion, . . . 393 liefining by sulphur, . . 395 Osmiridium in gold bars, . 395 farting processes, . . . 396 Cementation, . . . 397 Parting by sulphide of antimony, . . . .397 Parting by sulphur, . . 397 Nitric acid process, . . 398 1.

Granulation of alloys, . 398 2. Dissolving granulations, 399 3. Treatment of gold residues 400 4. Treatment of silver solution, . . . .401 5. Reduction of silver chloride, . . . 401 Parting by sulphuric acid, . 402 1. Mixing and granulating alloys, . . . 402 2. Dissolving silver, . . 403 3. Melting gold residue, . 406 4. Precipitation of silver, . 407 5. Crystallisation of sulphate of copper, . 407 Combined process, . . 408 Gutzkow process, . . 409 New Gutzkow process, . 41 1 Miller's chlorine process, . 414 Original method, . . 415 Later improvements, . 417 Modern practice at Melbourne, . . . 420 Modern practice at Sydney, 429 Refining brittle gold by chlorine, . . . 431 Parting by electrolysis, . 432 Parting of gold from platinum, iridium, &c. , 435 Chapter XIX.— The Assay of Gold Ores. General considerations, . . 437 Blowpipe assay, . . . 437 Sampling and crushing the ore, 438 Metallics, .... 439 •Crucible method of assay, . 440 Fusion, .... 440 Assay-ton weights, . . 441 General charges, . . 441 Fluxes, .... 442 Methods of operation, . 444 Roasting before fusion, . 447 Cleaning slag, . . . 443 Treatment of base ores, . 448 Cupellation, . . . 443 Influence of base metals, .

CONTENTS. XVII Chapter XIX.— Continued. PAGE PAGE Inquartation and parting, . 452 2. Amalgamation, . . 459 3. Chlorination, . . . 459 Examination of assay materials, . . . 454 4. Whitehead's method, . 461 Examination of cupel, . . 454 ... Assay of pyrites, . . 461 Assay by scorification, . 455 (a) Swartz's method, . 461 (6) Stapff's ,, .461 6. Assay of purple of Cassius, 462 Detection of gold in minerals, . 457 Estimation of gold in dilute solution, .... 458 Special methods of assay, . 458 7. „ a Mint sweep, . 462 1. Mixed wet and dry method, 45S Chapter XX. — The Assay of Gold Bullion. Assay of alloys of gold, silver, and copper, . . . 483 Effects of other metals, . . 484 Assay of various gold alloys, . 484 A. Alloys requiring scorification, . . .484 Arsenic and antimony, . 484 Iron and manganese, . 485 Cobalt and nickel, . . 485 Zinc, . . . .485 Tin, 485 Aluminium, . . . 485 B. Amalgams, . . . 486 C. Platinum group, . . 486 (1) Platinum, . . .486 (2) Palladium, . . .487 (3) Rhodium and iridium, 487 D. Tellurium compounds, . 488 Wet methods of assay, . . 48S Other methods, . . .489 1.

Touchstone, . . .489 2. Colour and hardness, . 490 3. Density, . . . .490 4. Spectroscope, . . 490 5. Electrolysis, . . . 491 6. Induction balance, . . 491 Auxiliary balance for weighing cornets, .... 492 Parting assay, .... 462 1. Selection of sample, . 463 2. Preparation of assay-piece for cupellation, . . 464 3. Cupellation, . . .467 Assay furnace, . . . 467 Cupels, . . . .469 Method of operation, . 469 Flashing, . . .470 Temperature of muffle, . 471 4. Preparation of cupelled buttons for parting, . 474 5. Parting, .... 475 (a) In parting flasks, . 475 (6) In platinum trays, . 476 Relative advantages of these, . . . 477 6. Weighing the cornets, 478 Losses of gold in bullion assaying, ..... 478 Silver retained in cornets, . 480 Occluded gases, . . . 481 Checks or proofs, . . .481 Limits of accuracy in assay, . 482 Parting by sulphuric acid, . 482 Preliminary assay, . . . 483 Assay by cadmium,. . . 483 Chapter XXI.— Economic Considerations.

Table of production in different countries, . . .498 Amount produced by different processes, • ■ • aOO Consumption of gold, . . 501 Management of gold mills, . 493 Cost of production of gold, . 493 Annual production of gold, past and present, . . -495 CONTENTS. BIBLIOGRAPHY PAGE Periodicals 503 PAGE Roasting, .... 511 General, ..... 504 Chlorination, .... 511 Properties of gold and its alloys, 505 Distribution of gold, . . 507 Placer mining, . . . 508 Cyanide process, . . .512 Pyritic smelting, . . . 512 _ j _,, . . . .,__. Refining and parting of bullion, 512 Assaying, . . . .513 Crushing and amalgamation, . 509 Concentration, . . . 510 !ni>ex THE METALLURGY OF GOLD.

Chapter I. The Properties Of Gold And Its Alloys

PHYSICAL AND CHEMICAL PROPERTIES OF GOLD. Introduction. — From very early times the ancients were attracted by the beautiful colour, the brilliant lustre, and the indestructibility of gold, aud spared no pains in the endeavour to acquire it. In the code of Menes, who reigned in Egypt in 3600 B.C. or about 2000 years before Moses, the ratio of value between gold and silver is mentioned, one part of gold being declared equal in value to two and a half parts of silver, and it is, therefore, clear that the extraction of both metals from the deposits containing them must have been carried on before that time. It is, indeed, probable that gold was the first metal observed and collected, since it occurs in fragments of all sizes in loose sand, and the operations of collecting the larger pieces and melting them together are so simple.

Among the rock carvings of Upper Egypt there are several illustrative of the art of washing auriferous sands by stirring and working them up by the hand in hollowed-out stone basins, and subsequently melting the gold in simple furnaces with the aid of mouth blow-pipes. The earliest of these carvings is supposed to date back to about 2500 B.C. However, in ancient times gold appears to have been mainly derived from India, and that country continued to supply most of the gold used in Europe until the discovery of America by Columbus. In order to collect alluvial gold, the sands were washed down over smooth sloping rocks by means of running water, and the particles of gold, sinking to the bottom of the stream by reason of their high density, were entangled and caught in the hair of raw hides spread on the rocks.

Among the hides used were THE METALLUROT OP GOLD. sheepskins, and hence originated the form of the legend of the Golden Fleece. Stripped of its heroic dress, this legend merely describes a successful piratical expedition about 1200 B.C. to win gold, which was being laboriously obtained from streams with the help of sheepskins by the inhabitants of what is now Armenia. Similar expeditions have not been unknown in much later times, and the method of obtaining gold by washing river sands is still practised, with improvements in matters of detail, in many parts of the world. Metallurgists are almost pro verbially conservative in their methods. Hides are even now occasionally employed to catch the gold, but sheepswool, when used, is generally in the form of blankets. At the present day, however, when auriferous sands are washed, the aid also is invoked of what Baron Born called in 1786 the "elective affinity" of mercury for gold when mixed with impurities.

The ease with which gold-amalgam can be collected, in spite of its being less dense than gold itself, is due to the fact that it is miscible in all proportions with mercury, so that, under proper conditions, large globules of liquid alloy are formed by the running-together of smaller particles, and the former are readily caught in suitable crevices. In the history of gold, it is also of interest to the metallurgist to remember that the earliest dawn of the science of chemistry was heralded by the study of the properties of gold, and by the efforts which were made to invest other matter with these properties. From the fourth to the fifteenth century, chemistry, which was first called " chemia " (yrit/jstia,), and then "alchemy," was defined as the art of transmuting base metals into gold and silver, almost all the labours of philosophers being intended to aid directly or indirectly in solving this problem.

At the end of this period, while Paracelsus was giving to chemistry a new aim — that of investigating the composition of drugs, and their effect on the human body — Agricola was reducing to order the numerous empirical facts which together made up the art of metallurgy, and although alchemy died hard, its era of usefulness may be said to have ended here. Gold has doubtless been the cause of many of the wars and marauding expeditions from which the world has suffered, but on the other hand it has been instrumental, in a far greater degree than most other commodities, in promoting the growth of civilisation, the efforts of the alchemists having laid the foundations ofthe science of chemistry, and those of the gold-seekers having resulted in the discovery of new countries, and in the spread of knowledge of all kinds.

Colour. — The lustre and fine colour of gold have given rise to most of the words which are used to denote it in different languages. The word "gold" is probably connected with the Sanscrit word "jvalita," which is derived from the verb "jval," to shine. It is the only metal which has a yellow colour when THE PROPERTIES OF GOLD. in mass and in a state of purity. Impurities greatly modify this colour, small quantities of silver lowering the tint, while copper raises it. In a finely divided state, when prepared by volatilisa tion or precipitation, gold assumes various colours, such as deep violet, ruby and reddish-purple, the tint varying to brownish purple and thence to dark brown and black. This purple colour has been supposed by some experimenters (viz., Guyton de Morveau, Biichner, Desmarest, Creuzbourg and Berzelius) to be due to the formation of a coloured oxide of gold of unknown com position, but Buisson, Proust, Figuier and, more recently, Kriiss have shown that no oxygen can be obtained from this coloured material, and that it probably consists of metallic gold.