The Metallurgy of Gold ... Second edition

Front matter

THE METALLURGY OF GOLD. NEW METALLURGICAL SERIES EDITED EY W. C. ROBERTS-AUSTEN, C.B., F.R.S., Chemist and Assayer of the Royal Mint ; Professor of Metallurgy in the Royal College of Science. In Large Svo, Handsome Cloth. With Illustrations. 1. INTRODUCTION" to the STUDY of METALLURGY. By the Editor. Third Edition. 12s. 6d. "No English text-book at all approaches this in the completeness with which the most modern views on the subject are dealt with. Professor Austen's volume will be invaluable."— Chemical News. 2. GOLD (The Metallurgy of). By T. Kirke Rose, D.Sc, Assoc. R.S.M., F.C.S., Assist. -Assayer of the Royal Mint. Second Edition. " The Four chapters on Chlorination, written from the point of view alike of the practical man and the chemist, teem with considerations hitherto unrecognised, and constitute an addition to the literature of Metallurgy, which will prove to be of classical value." — Nature. 3.

COPPER (The Metallurgy of). By Thomas Gibb, Assoc. R.S.M. 4. IRON and STEEL (The Metallurgy of). By Thomas Turner, Assoc. R.S.M., F.I.C. Vol. I., Iron, 16s. "A most valuable summary of useful knowledge relating to every method and stage in the manufacture of cast and wrought iron down to the present moment . . . particularly rich in chemical details. ... An exhaustive and really needed compilation by a most capable and thoroughly up-to-date metallurgical authority." —Bulletin of the American Iron and Steel Association. 5. METALLURGICAL MACHINERY : the application of Engineering to Metallurgical Problems. By Henry Charles Jenkins, Wh.Sc, Assoc. R.S.M., Assoc. M.Inst.CE., of the Royal Mint. 6. ALLOYS. By the Editor. *»* Other Volumes in Preparation. LONDON: CHARLES GRIFFIN & CO., LIMITED, EXETER STREET, STRAJsD.

The 40-Stamp Battery of the New Heriot Company, showing the Stamps, Water-Supply, aud Copper Tables » THE METALLURGY OF GOLD BY T. KIEKE ROSE, D.Sc, ASSOCIATE OP THE BUT AL SCHOOL OP BOSES; FELLOW l.P THE CHEMICAL SOCIETY; ASSISTANT ASSAYER OF THE ROYAL MINT. BEING ONE OF A SERIES OF TREATISES ON METALLUHGI WRITTEN BV ASSOCIATES OF THE ROYAL SCHOOL OF MINES. EDITED BY prof. m. C. TKOberts^Husten, C.3B., jf.lR.S. WITH yUMEROUS lUMSTRATlOXS. SECOND EDITION. LONDON: CHARLES GRIFFIN AND COMPANY, LIMITED; EXETER STREET, STRAND. 1896. [AU Right* litxerval,] PBEFACE. 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.

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 -jold 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.

. 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

PREFACE.

. X 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 PAGE Magnetism, .... 3 Physical and chemical properties of gold, . . . 1 Conductivity and expansion, 4 Introduction, . . . 1 Atomic weight and volume, . 4 Colour, . . . . 1 Volatility, .... 4 Malleability and ductibility, 2 Crystallisation, ... 8 Hardness, .... 2 Solubility, .... 9 Tenacity 2 Preparation of pure gold, . 10 Specific gravity, ... 3 Allotropic forms of gold, . . 11 Cohesion, .... 3 Alloys of gold, .... 12 Specific heat, ... 3 Amalgams 14 Fusibility, .... 3 Gold and silver, ... 15 ,, copper, ... 16 Liquation 17 Spectrum, .... 3 Latent heat, .... 3 Chapter II. — Chemistry op the Compounds op Gold.

Compounds of gold, . . 19 Haloid compounds, ... 20 Aurous cyanide, ... 27 Auro-cyanide of potassium, . 27 Chlorides of gold, ... 20 Auri-cyanide of potassium, . 28 Proto-chloride, ... 20 Oxides of gold, ... 28 Auro-aurichloride, . . 20 Aurous oxide, ... 28 Trichloride, .... 20 Auric oxide, ... 28 Chlor-auric acid, ... 26 Aurates, ..... 29 Fulminating gold, ... 29 Bromides of gold, ... 26 Proto-bromide, . . 2(5 Sulphites of gold, ... 29 Auro-auric bromide, . . 27 Hyposulphites of gold, . . 30 Tribromide 27 Silicates of gold, ... 32 Iodides of gold, ... 27 Sulphides of gold, ... 32 Cyanides of gold, ... 27 Purple of Cassius, ... CONTENTS. Chapter III.— Mode of Occurrence and Distribution of Gold. PAGE Forms in which gold occurs in nature, . . . .34 Vein gold, .... 34 Placer gold and nuggets, . 36 Calaverite, .... 37 Sylvanite, . . . .37 .

PAGE Petzite, .... 37 Nagyagite, .... 37 Composition of native gold, . 38 Geographical distribution of gold, ..... 38 Origin of gold ores, ... 41 Chapter IV.— Placer Mining [—Shallow Deposits. Placer deposits, ... 42 Methods of obtaining gravel from shallow placers, . 43 Appliances used in washing the gravel, .... 44 Pan, 44 Batea, 44 Prospecting trough, . . 45 Horn-spoons, ... 45 Cradle, .... 45 Long-tom, .... 46 Puddling-tub, ... 46 Siberian trough, 47 Sluice, 49 Sluicing, . . . . .50 Use of drops, . . .51 ,, undercurrents, . . 51 ,, mercury, ... 52 Cleaning-up, ... 52 Tail-race, .... 38 Ground-sluice, ... 53 Booming, .... 53 Dry diggings, .... 54 Cement gravels, ... 54 Tail sluices, .... 54 Fly catchers, .... 55 River mining, .... 55 1. Kiver mining proper, . 55 2. Dredging, ... 57 3. Deep bar mining, . . 57 Methods of working Siberian placers, .... 58 1.

Siberian sluice, . . 59 2. Trommel, ... 60 3. Pan, .... 60 Beach mining, . . . .61 New method of working shallow placer deposits, . . .61 Chapter V. — Deep Placer Deposits. Nature and mode of origin of deposits, .... 62 Distribution of gold in the gravels, .... 66 Origin of gold in the gravels, 67 Minerals occurring in the gravels, .... 69 Methods of working, . . 70 " Hydraulicking," . . 70 Commencement of operations 71 Supply of water, . . 71 Breaking down the bank, . 74 Sluicing gravel, . . 76 Cleaning-up, ... 79 Disposal of tailings, . . 80 Drift mining, . . .81 Shaft „ ... Si Hydraulic elevator, . . 83 Economic conditions, . . 85 Shallow placer deposits, . 85 Deep „ „ . CONTENTS. XIII Chapter VI. — Quartz Crushing in the Stamp Battery. page page Primitive methods of crushing and amalgamating, . . 88 Mortar, . . . .103 Splash-box, . . .105 Mortar, .... 89 Dies, .... 105 Maray, .... 89 Cam-shaft, . . .106 Chilian mill, ... 90 Cams, . . . .106 Arrastra, .... 90 Tappets, . . . .107 Iron prospecting arrastra, . 93 Stamp, . . . . 10S Stamp battery, ... 93 Guides, . . . .109 Rock -breakers, ... 96 Lever, . . . .109 Blake, . . 96 Screens, . . . .110 Dodge, .... 97 Order of fall of stamps, . 113 Gates 98 Automatic feeding, . .113 Schranz, .... 99 Stanford's feeder, . . 114 Material employed for crushing surfaces, . . 100 Challenge ,, . . 115 Water supply, . . .116 Amalgamated plates, . . 117 Position and use of rock-breakers, . . . 101 Mill site, 119 Battery proper, . . . 102 General arrangement of the stamp mill, .... 120 Foundations, . . . 103 Framework, . . . 103 Chapter VII.—. ■Amalgamation in the Stamp Battery.

Treatment of the amalgamated plates, 122 Discoloration of the copper plates, 123 Chemicals used to promote amalgamation, . . . 124 Use of mercury, . . .125 Grade of plates, . . . 125 Muntz metal plates, . . 126 Shaking copper plates, . . 127 Corrugated plates, . . . 128 Mercury wells, . . . 128 Galvanic action in amalgamation, 128 Designolle process, . . . 129 Cleaning-up, .... 130 The clean-up barrel, . . 131 „ ,, pan, . . 131 Retorting, . . . .133 Loss of mercury, . . . 135 „ gold, . . . .138 Non-amalgamable gold, . . 144 Gold in pyrites, . . . 145 Chapter VIII. — Other Forms op Crushing and Amalgamating Machinery. Special forms of stamps, . . 148 Pan-amalgamation, . . . 158 Husband, . . . .148 The old system, . . . 159 Steam, 149 The Boss continuous system, 159 Elephant, . . . .151 Treatment of concentrates, . 163 Huntington mill, . . .152 Molloy's hydrogen amalgamator, 164 Crawford ball mill, . . .156 Jordan's amalgamator, . . 165 Other ball mills, . . .158 CONTENTS, Chapter IX. — ( -Concentration in Stamp Mills.

PAGE Concentration, . . . 166 Settling boxes, . . . 168 Classification according to size, 169 Screens, .... 169 Pointed boxes, . . .170 Early concentrating machinery, 17'1 Blauket strakes, . . .172 Riffled sluices, . . .173 Raising-gate concentrator, . 173 Round buddle, . . .173 Centrifugal concentrators, . 174 Hendy s, . . - . 174 PAGB Duncan's, . . . .174 Percussion tables, . . . 175 Gilpin County concentrator, 175 Frue vanner, .... 176 Method of working, . . 180 Riffle belted, . . .183 Embrey concentrator, . . 183 Liihrig vanner, . . . 184 Hartz jigs, . . . .187 Pneumatic jig, . . . 188 Clarkson and Standfield's concentrator, .... 188 Chapter X. — Stamp Battery Practice in Particular Localities. In California 190 In Colorado, . . . .195 On free-milling ores in Australia I and New Zealand, " . . 199 In the Thames Valley, New Zealand, . . . .203 In Dakota, .... 206 In the Transvaal, . . . 207 Chapter XI. — Chlorination : The Preparation of Ore for Treatment.

The Plattner process, . . 215 Origin, . . . .215 Method of working at Reichenstein, .... 216 Modern practice in chlorination, 217 Crushing, . . . .218 Krom's rolls, . . .219 Rolls at Rapid City, Dakota, 222 Comparison between rolls aud stamps, . . . .223 Drying the ore, . . . 224 Roasting, .... 225 Reverberatory furnace, . 226 Chemistry of oxidising roasting, 229 Decomposition of various minerals, .... 231 Elimination of arsenic and antimony, . . . 232 Use of salt 233 Losses of gold, . . . 235 Mechanical furnaces, . . 237 1. With mechanical stirrers, 238 (a) O'Hara, . . .238 («) Spence, . . . 239 (c) Pearce Turret, . 240 2. With rotating bed, . 240 3. Revolving cylinders, . 243 (a) BrUckner, . . 243 (6) Hofmann, . . 244 (c) White, ... 246 {d) White-Howell, . 247 Use of producer gas in roasting, 248 Chapter XII. — Chlorination: The Vat Process.

Construction of vats, . . 249 Charging-in, .... 251 Generation of chlorine, . . 252 Impregnation, .... 254 Reactions in vat, . . . 255 Amount of chlorine required, . 257 Leaching, .... 258 Precipitation of gold, . . 259 Cost of working, . . . CONTENTS. XV Chapter XIII. — I •Chlorination The Barrel Process. PAGE PAGE History, 262 Organic substances, . 275 Mears process, . . . 263 Sulphuretted hydrogen, 275 Thies process, .... 265 Sulphurous acid, . 276 Construction of barrel, . 268 2. Solid precipitants, . . 277 Charging-in, . . .268 Charcoal, . . . 277 Amounts of chemicals re- Insoluble sulphides, . 278 quired, .... 270 Method of leaching, . . 271 Mechanical difficulties in leach- Metals, . . .279 Modern patent processes of chlorination, . . . 280 ing, 272 Newbery-Vautin, . . 280 Pollok 280 Swedish 281 Leaching by pressure, . . 272 Riotte 's method, . . . 273 Precipitation of gold, . . 273 Cassel 283 1.

Soluble precipitants, . 274 Julian 2S3 Ferrous sulphate, . 274 Greenwood, . . .283 Chapter XIV. — Chlorination: Practice in Particular Mills. Vat process, .... 284 Cost of working, . . 291 1. Butters' Mill,Kennel,California, .... 284 3. Modern barrel process at the Golden Reward Works, Deadwood, Dakota, . 291 2. Plymouth Mill, Amador Co., California, . . 285 Cost of working, . . 298 3. Treadwell Mine, Alaska, 286 4. Bromination Mill, Rapid City, Dakota, . . 299 Barrel process — 1. Mears process at Deloro, Canada, . . . 2S7 2. Thies process in Carolina, 290 Cost of working, . . 302 Future of chlorination, . . 304 Cyanide Process. Chapter XV.— The Molloy's method of precipitation, . . .321 Plant required, . . .322 History, 306 MacArthur-Forrest process, . 308 1. Preparation of ore, . . 308 2.

Dissolving the gold, . 309 Treatment of ore slimes, . 323 Testing of ores, . . .323 Vats, . . . .309 Charging-in, . . . 310 Direct treatment of Rand ore 324 Separation of slimes, . 311 Leaching, . . . 311 Use of cyanide in stamp battery 325 Method of treatment — Agitation, . . . 312 Strength of cyanide solution, .... 315 Disposal of tailings, . 316 At the Robinson mine, . 326 „ Sylvia mine, . . 326 3. Precipitation of gold, . 317 ,, New Primrose mine, 328 Cleaning-up, . . . 318 Siemens-Halske process, . 329 4. Production of bullion from Results of process — In South Africa, . . .331 precipitate, . . . 319 In other parts of the world, CONTENTS. Chapter XVI. — Chemistry of the Cyanide Process. page PAGE Action of potassium cyanide on gold and other metals, . 333 Re-precipitation of gold and silver in leaching vats, . 345 Testing strength of solution, . 345 Decomposition of potassium cyanide, .... 338 Strength of solution required, 347 Decomposition in the zinc boxes, .... 339 Consumption of cyanide, . 347 Methods of increasing the speed of action of potassium cyanide, .... 348 Action of potassium cyanide — Onmetallicsaltsandminerals, 340 On oxidised pyrites, . . 343 The Hood process, . . . 349 The soda solution, . . . 344 The Sulman-Teed process, . 351 Ores suitable to process, . . 352 Chapter XVII.

Pyritic smelting, , 353 Chapter XVIII. — The Refining and Parting of Gold Bullion. General considerations, . . 356 5. Reduction of silver chloride, . . . 374 Refining, .... 357 Composition of bullion, . 357 Parting by sulphuric acid, . 375 Melting furnace, . . . 359 1. Mixing and granulating alloys, . . . 375 Crucibles 360 Melting, . . . .360 2. Dissolving silver, . . 376 Refining, . . . .361 3. Melting gold residue, . 378 Toughening, . . . 363 4. Precipitation of silver, . 379 Casting, . . . .364 5. Crystallisation of sulphate of copper, . 379 Losses of bullion, . . . 366 Refining by sulphur, . . 368 Combined process, . . 380 Gutzkow process, . . 381 New Gutzkow process, . 3S3 Miller's chlorine process, . 386 Osmiridium in gold bars, . 36S Parting processes, . . . 369 Cementation, . . . 370 Parting by sulphide of anti- Original method, . . 387 370 Later improvements, . 389 mony 370 Parting by sulphur, . . 370 Modern practice at Melbourne, . . . 392 Modern practice at Sydney, 401 Nitric acid process, . . 371 1.

Granulation of alloys, . 371 2. Dissolving granulations, 372 Refining brittle gold by chlorine, . . . 403 Parting by electrolysis, . 404 3. Treatment of gold residues, .... 373 4. Treatment of silversolution, . . . .374 Chapter XIX.— The Assay of Gold Ores. General considerations, . . 407 Blowpipe assay, . . . 407 Sampling and crushing the ore, 408 Metallics, .... 409 Crucible method of assay, . 410 Fusion, .... 410 Assay-ton weights, . .411 General charges, . .411 Fluxes 412 Methods of operation, . 414 Roasting before fusion, . 416 Cleaning slag, . . . 417 Treatment of base ores, . 417 Cupellation, . . . 417 Influence of base metals, . CONTENTS. XVII Chapter XIX. '.. — Continued. PAGE Inquartation and parting, . 421 Examination of assay materials, . . . 423 Examination of cupel, . . 423 Assay by scorification, . . 424 Detection of gold in minerals, . 426 Estimation of gold in dilute solution, .... 427 Special methods of assay, . 427 1.

Mixed wet and dry method, 427 PAGE 2. Amalgamation, . . 428 3. Chlorination, . . . 428 4. Whitehead's method, . 430 5. Assay of pyrites, . . 430 (a) Swartz's method, . 430 (b) Stapff's „ . 430 6. Assay of purple of Cassius, 431 7. „ a Mint sweep, . 431 Chapter XX. — The Assay of Gold Bullion. Parting assay, .... 431 1. Selection of sample, . 432 2. Preparation of assay-piece for cupellation, . . 433 3. Cupellation, . . . 436 Assay furnace, . . . 436 Cupels, . . . .438 Method of operation, . 438 Flashing, . . . .439 Temperature of muffle, . 440 4. Preparation of cupelled buttons for parting, . 443 5. Parting, .... 444 (a) In parting flasks, . 444 (b) In platinum trays, . 445 Relative advantages of these, . . . 446 6. Weighing the cornets, . 447 Losses of gold in bullion assaying 447 Silver retained in cornets, . 449 Occluded gases, . . . 450 Cheeks or proofs, . . . 450 Limits of accuracy in assay, . 451 Parting by sulphuric acid, . 451 Preliminary assay, . . . 452 Assay by cadmium, . . . 452 Assay of alloys of gold, silver, and copper, . . . 452 Effects of other metals, . . 453 Assay of various gold alloys, . 453 A. Alloys requiring scorification, . . . 453 Arsenic and antimony, . 453 Iron and manganese, . 454 Cobalt and nickel, . . 454 Zinc 454 Tin, 454 Aluminium, . . . 454 B. Amalgams, . . . 455 C. Platinum group, . . 455 (1) Platinum, . . .455 (2) Palladium, . . .456 (3) Rhodium and iridium, 456 D. Tellurium compounds, . 457 Wet methods of assay, . . 457 Other methods, . . . 458 1.

Touchstone, . . . 458 2. Colour and hardness, . 459 3. Density 459 4. Spectroscope, . . . 459 5. Electrolysis, . . . 460 6. Induction balance, . . 460- Chapter XXI. — Economic Considerations. Management of gold mills, . 462 Cost of production of gold, . 462 Annual production of gold, past and present, . . . 464 Table of production in different countries, . . . 467 Amount produced by different processes, . . . 469 Consumption of gold, . . CONTENTS. BIBLIOGRAPHY. page Periodicals, . . . .471 General, 472 Properties of gold and its alloys, 473 Distribution of gold, . . 475 Placer mining, . . . 476 Crushing and amalgamation, . 477 Concentration, . . . 478 P.4GE Roasting, .... 478 Chlorination, .... 479 Cyanide process, . . . 480 Pyritic smelting, . . . 480 Refining and parting of bullion, 480 Assaying, .... 480 PAGE Index, , THE METALLURGY OF GOLD CHAPTER T. THE PROPERTIES OF GOLD AND ITS ALLOYS.

PHYSICAL AND CHEMICAL PROPERTIES OF GOLD. Introduction. — Some of the more obvious physical properties of gold were already well known and taken advantage of at a very early period in the history of man, long before any exact science could be said to exist, and it is 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," 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 been accused, with some justice, of having been the cause of many of the wars and marauding expeditions from which the world has suffered, but it may be asserted with equal truth that 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 of the 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 TnE metallurgy 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. Similar colours are seen in purple of Cassius, and in Roberts-Austen's purple alloy of aluminium and gold, the colour in each case being probably due to a particular form of finely divided gold.

The ruby coloured gold, suspended in a liquid in which it has been precipitated by ether and phosphorus, is present in the most finely divided state obtainable, and perhaps it then approaches the condition of separate atoms. Such gold shows no tendency to settle by gravity even after being kept undisturbed for several years. Somewhat less finely divided gold gives a faint blue tinge to light transmitted through the liquid in which it is suspended. Gold precipitated from its solution as bromide is in a different molecular condition from that formed from chloride, giving out 3-2 calories in passing into the latter state.* The surface colour of small particles of native gold is often apparently reddened by being coated with translucent films of oxides of iron. Very thin plates of gold are translucent, and appear green by transmitted light, while remaining yellow by reflected light.

On heating, the green colour changes to ruby red, but is restored by the pressure of a hard substance by which the state of aggregation is again altered (Faraday). Molten gold is green, and its vapour is also probably greenish. Malleability and Ductility. — Malleability and ductility are possessed by gold at all temperatures to a far higher degree than by any other metal. A single grain of gold can be drawn out into a wire over 500 feet long, and leaves of not more than g <■, oo o t) °^ an incn in thickness can be obtained by beating. Faraday has shown that the thickness of these leaves may be still further reduced bjr floating them in a dilute solution of potassic cyanide by which they are partly dissolved. Hardness. — The hardness of gold lies between that of alu minium and that of silver, corresponding to the number 979 in Bottone's scale, in which the diamond is 3,010.

Tenacity. — The purest gold obtainable has a tenacity of 7 tons * Thomsen, Thermochemische Untersuchungen, vol. iii., p. 412. per square inch and an elongation of 30 '8 per cent., but the presence of minute traces— i.e., -zjy_-_- °f other elements, especially those having high atomic volumes — e.g., bismuth, tellurium, lead, &c, greatly lowers these constants, as well as the malleability and ductility of the metal, while its hardness is increased.* Gold containing of bismuth can almost be crumbled in the fingers. Specific Gravity. — The specific gravity of gold when precipi tated from solution by oxalic acid is 19*49 (G. Rose);f when cast it varies from 19*29 to 19*37, but this can be raised by compression to over 19*48. J Henry Louis has shown § that the specific gravity of unannealed "parted" gold (i.e., the residue left after boiling silver-gold alloys in nitric acid) is 20*3, its density being lowered by the process of annealing, a fact which Louis considers is indicative of the existence of an allotropic modification of the metal.

Cohesion. — On heating, gold can be welded like iron below the point of fusion, and finely divided gold agglomerates on heating without being subjected to pressure. Pressure alone is also sufficient to make gold dust cohere, while a true flow of the particles of gold can be induced in the case of the pure metal and some of its alloys. Specific Heat. — The specific heat of gold is -0324- (Regnault) or -0316 (Violle). Fusibility. — Gold fuses, after passing through a pasty stage, at a clear cherry-red heat, just below the fusing point of copper and much above that of silver. The metal expands considerably on fusing and contracts again on solidifying. Carnelley gives the temperature of fusion as 1,037°, Violle as 1,045°, whilst recent determinations have given it as 1,061*7° (Heycock and Neville), 1,050° to 1,060° (Le Chatelier), and 1,072° (Holborn and Wien).

Spectrum. — In the gold spectrum Huggins saw 23 lines, the wave lengths of the most important ones being 523*1, 583*5, and 627*6 respectively.|| Latent Heat. — The latent heat of fusion of gold is 16*3, and the normal lowering of the freezing point for 1 atom of impurity in 100 atoms of gold is 10° *6, but the presence of from 0*1 to 4*0 per cent, silver does not cause any alteration in the freezing point. Magnetism. — Gold is diamagnetic, its specific magnetism being 3*47 (Becquerel), if that of iron is taken as 100. * Roberts-Austen, Phil. Trans. Boyal Soc., vol. clxxix. (1888), p. 339. i Pogg. Ann., vol. lxxiii. (1848), p. 1, and vol. lxxv. (1848), p. 403. X Eighth Beport of the Boyal Mint, 1877, p. 43. § Trans. Am. Inst, of Mng. Eng., Chicago Meeting, 1893. || For full information on the spectroscopic characteristics of gold, see Lockyer and Roberts, Phil.