The Metallurgy of Gold, etc

Introduction

to the STUDY of METALLURGY. By the Editor. Third Edition, " 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, B.Sc, Assoc. R. S.M., F.C.S., Assist. -Assayer of the Royal Mint. 3. COPPER (The Me^*ufgy>^f). By Thos. Gibb, Assoc. R.S.M. / '• M 4. IRON and STEEE$'/.THe14jietaUurgy of). By Thos. Turner, Assoc. B.S.M. 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. 8. ALLOYS. By the Editor. Other Volumes in Preparation, LONDON : CHARLES GRIFFIN & CO., LIMITED, EXETER STREET, STRAND. THE METALLURGY OF GOLD, liY T. KIliKE ROSE, B.Sc, ASSOCIATE OK TUB ROYAL SCHOOL OF MINES; FELLO*W*!**K THE CHEMICAL SOCIETY ; ASSISTANT ASSAYER OF THE ROYAL MINT.

BEING ONE OF A SERIES OF TREATISES ON METALLURGY, WRITTEN BY ASSOCIATES OF THE ROYAL SCHOOL OF MINE EDITED BY prof. TO. C. IRoberts-austcn, CJ6„ Jf.lR.5. WITH NUMEROUS ILLUSTRATIONS. LONDON: CHARLES GRIFFIN AND COMPANY, LIMITED: EXETER STREET, STRAND. 1894. [All Rights Reserved.]

INTRODUCTION.

. As this is the first ot' 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 adequate!}' dealing with modern processes could hitherto have been attributed to a Student of the School. It may be claimed that in this counts 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 gold 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 VI

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 Mini, March., 1894.

Preface

. 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 MaeArthur-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.

. VIII 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 too-ether form a comprehensive metallurgical series. T. K. ROSE. Royal Mint, March, 1894. CONTENTS. Chapter I.— The Properties op Gold and its Alloys. PAGE PAGE Physical and chemical properties of gold, . . . 1 Magnetism, .... 3 Conductivity and expansion, 4 Introduction, 1 Atomic weight and volume, . 4 Colour,. . . . . 1 Volatility, .... 4 Malleability and ductility, . 2 Crystallisation, ... 8 Hardness, .... 2 Solubility, .... 9 Tenacity, .... 2 Preparation of pure gold, . 10 Alloys of gold, . . .11 Amalgams, .... 12 Gold and silver, ... 12 Specific gravity, ... 3 Cohesion, .... 3 Specific heat, ... 3 Fusibility, .... 3 copper, . . .13 Spectrum, .... 3 Liquation, .... 14 Latent heat, .... 3 Chapter II. — Chemistry of the Compounds of Gold.

Compounds of gold, . . . 16 Aurous cyanide, ... 22 Haloid compounds, . . .17 Auro-cyanide of potassium, . 22 Chlorides of gold, ... 18 Auri-cyanide of potassium, . 23 Proto-chloride, . . .18 Oxides of gold, ... 23 Dichloride, . . . .18 Aurous oxide, ... 23 Trichloride, . . . .18 Auric oxide, .... 23 Chlor-auric acid, ... 21 Aurates, ..... 24 Bromides of gold, ... 21 Fulminating gold, ... 24 Sulphites of gold, ... 24 Proto-bromide, ... 21 Dibromide, .... 22 Hyposulphites of gold, . . 25 Tribromide, .... 22 Silicates of gold, ... 27 Iodides of gold, ... 22 Sulphides of gold, ... 27 Cyanides of gold, ... 22 Purple of Cassius, ... contknts. X and Distribution of Gold. Occurrence Chapter III. -Mode of page PARK Nagyagite, . . . .32 Forms in which gold occurs in nature, .... 29 Composition of native gold, . 33 Geographical distribution of gold, .... 33 Vein gold 29 Placer gold and nuggets, . 31 Calaverite, . . . .32 Annual production of gold, . 36 Cost of „ „ • 38 Sylvanite, . . .32 Petzite, .... 32 Chapter (V. — Placer MlNINI -Shallow Deposits.

Placer deposits, ... 40 Tail-race, .... 51 Ground-sluice, ... 51 Methods of obtaining gravel from shallow placers, . 41 Appliances used in washing the gravel, .... 42 Pan, 42 Batea, 42 Prospecting trough, . . 43 Horn-spoons, ... 43 Booming, .... 51 Dry diggings, . . ... 52 Cement gravels, ... 52 Tail sluices, .... 52 Fly catchers, . . " . . 53 River mining, .... 53 1. River mining proper, . 53 Cradle,. . . . .43 2. Dredging, . . .55 3. Deep bar mining, . . 55 Methods of working Siberian placers, .... 56 Long-torn, .... 44 Puddling-tub, ... 44 Siberian trough, . . .45 Sluice, 47 1. Siberian sluice, . .57 2. Trommel, . . .58 Sluicing, . . . . .47 Use of drops, ... 49 3. Pan 58 undercurrents, . . 49 mercury, ... 50 Beach mining, .... 59 New method of working shallow placer deposits, . . 59 Cleaning-up, ... 50 Chapter V. — Deep Placer Deposits.

Nature and mode of origin of deposits, . . . .60 Distribution of gold in the gravels, . ... 64 Origin of gold in the gravels, 65 Minerals occurring in the gravels, . 67 Methods of working, . . 68 Supply of water, . . 69 Breaking down the bank, . 72 Sluicing gravel, . . 73 Cleaning-up, ... 76 Disposal of tailings, . . 77 Drift mining, . . .78 Shaft ,, ... 78 Hydraulic elevator, . . 80 Economic conditions, . . 82 Shallow placer deposits, . 82 Deep ,, ,, .84 Hydraulicking, ... 68 Commencement of opera- 69 tions, CONTENTS. XI Chapter VI. — Quartz Crushing in the Stamp Battery. PAGE; page Primitive methods of crushing and amalgamating, . . S5 Mortar, . ' . . .100 Splash-box, . . . 102 Dies, . . . .102 Mortar, .... 86 Maray 86 Cam-shaft, . . . 103 Chilian mill, ... 87 Cams, . . . .103 Tappets, . . . .103 Arrastra, .... 87 Stamp 105 Iron prospecting arrastra, . 90 Stamp battery, . . .90 Guides 106 Stone-breakers, . . .93 Lever, . . . .106 Screens, . . . .107 Blake, . . . . 93 Dodge, .... 94 Order of fall of stamps. . 110 Gates, .... 95 Automatic feeding, . .110 Stanford's feeder, .

Ill Challenge „ . .112 Water supply, . . .113 Schranz, . . 96 Material employed for crushing faces, . . 97 Position and use of stone- Amalgamated plates, . . 114 General arrangements of stamp mills 116 breakers, ... 98 Battery proper, ... 99 Foundations, . .100 Mill site 1 16 Disposition of the machinery, 1 1 7 Framework, . . .100 Chapter VII. — . Amalgamation in the Stamp Battery. Mercury wells, . . . 125 Treatment of the amalgamated plates, . . . .119 Discoloration of the copper plates, . . .120 Chemicals used to promote amalgamation, . . . 121 Use of mercury, . . 122 Galvanic action in amalgamation, . . . . .125 Cleaning-up, . . . . ,126 The clean-up barrel, . . 127 ,, pan, . . 128 Retorting, . . .129 Loss of mercury, . . . 131 Grade of plates, . . .122 Muntz metal plates, . 1 23 Shaking copper plates, . .124 ,, gold 134 Non-amalgamable gold, . 140 Gold in pyrites, . . .141 Corrugated plates, . . . 125 Chapter VIII. — Other Forms of Crushing and Amalgamating Machinery.

Jordan, . . . .155 Special forms of stamps, . .144 Husband, .... 144 Pan-amalgamation, . . .155 Steam, ..... 145 The old system, . . .156 The Boss continuous system, 159 Elephant, . . . .147 Huntington mill, . . . 148 Treatment of concentrates, . 160 Crawford ball mill, . . .152 Molloy's hydrogen amalgamator, 161 Jordan's amalgamator, . 162 Other ball mills, . . .155 Cyclops, .... contents. Chapter IX. — i Concentration in Stamp Mills. PAG1 PAGE Duncan's, . . . .171 Concentration, . .163 Settling boxes, . . .165 Percussion tables, . . .172 Classification according to size, 166 Gilpin County concentrator,. 172 Frue vanner, .... 173 Screens, .... 166 Pointed boxes, . . . 167 Method of working, . .177 Early concentrating machinery, 168 Riffle belted, . . .180 Embrey concentrator, . . 180 Blanket strakes, . . 169 Riffled sluices, . . .170 Hartz jigs, . . . .181 Raising-gate concentrator, . 170 Pneumatic jig, . . .182 Clarkson and Stanfield's concentrator, . . . .183 Round buddle, . . .170 Centrifugal concentrators, . 171 Hendy's 171 Chapter X. — Stamp Battery Practice in Particular Localities.

In California 184 In Colorado, . . . .189 ■On free-milling ores in Australia and New Zealand, . . 193 In the Thames Valley, New Zealand, . . . .197 In Dakota, . . . .200 Chapter XL — Chlorination: The Preparation of Ore for Treatment. The Plattner process, . . 202 Origin, . . . .202 Method of working at Reichenstein, .... 203 Modern practice in chlorination, 204 Crushing, .... 205 Krom's rolls, . . . 206 Rolls at Rapid City, Dakota, 209 Comparison between rolls and stamps, .... 210 Drying the ore, . . .211 Roasting, . . . .212 Reverberatory furnace, . 213 Chemistry of oxidising roasting, 216 Decomposition of various minerals, .... 218 Elimination of arsenic and antimony, . . . 219 Use of salt, . . . .220 Losses of gold, . . . 222 Mechanical furnaces, . . 225 1. With mechanical stirrers, 225 (a) O'Hara, . . .225 (b) Spence, . . .226 2.

With rotating bed, . 227 3. Revolving cylinders, . 230 (a) Briickner, . . 230 (6) Hofmann, . . 231 (c) White, . . .233 {d) White-Howell, . 234 Use of producer gas in roasting, 235 Chapter XII. — Chlorination: The Vat Process. Construction of vats, . . 230 Charging-in, .... 238 Generation of chlorine, . . 239 Impregnation, . . .241 Reactions in vat, . . . 242 Amount of chlorine required, . '244 Leaching, .... 245 Precipitation of gold, . . 246 Cost of working, . . . CONTENTS. Chapter XIII.— Chlorination : The Barrel Process. PAGE PAGE History, 249 ■»» OJ-.fl Mears process, . • • —J" Thies process, .... 252 Construction of barrel, . . 255 Charging-in, . . • 255 Amounts of chemicals required, .... 256 Method of leaching, . . 258 Mechanical difficulties in leaching, 259 Leaching by pressure, . . 259 Riotte's method, . . . 260 Precipitation of gold, . . 260 1.

Soluble precipitants, . 261 Ferrous sulphate, . . 261 Organic substances, . 262 Sulphuretted hydrogen, 262 Sulphurous acid, . . 263 2. Solid precipitants, . . 264 Charcoal, . - .264 Insoluble sulphides, . 265 Metals, . . .266 Modern patent processes of chlorination, . . .26/ Newbery-Vautin, . . 267 Pollok 267 Swedish 268 Cassel, 270 Julian, 270 Greenwood 270 Chapter XIV.— Chlorination : Practice in Particular Mills, Cost of working, . . 278 Vat process, .... 271 1 Butters' Mill, Kennel, Cali- 3. Modern barrel process at the Golden Reward Works, Deadwood, Dakota, . 278 Cost of working, . . 285 fornia, .... 271 2. Plymouth Mill, Amador Co., California, . . 272 4. Bromination Mill, Rapid City, Dakota, . . 286 Cost of working, . . 289 Future of chlorination, . . 291 3. Treadwell Mine, Alaska, . 273 Barrel process — 1.

Mears process at Deloro, Canada, . . • 274 2. Thies process in Carolina, 277 Chapter XV.— The Cyanide Process. History, 292 MaeArthur-Forrest process, . 294 1. Preparation of ore, . . 294 2. Dissolving the gold, . 295 Vats, .... 295 Charging-in, . . • 296 Separation of slimes, . 297 Leaching, . . • 297 Agitation, . . . 298 Strength of cyanide solution, . . . .300 Disposal of tailings, . . 301 3. Precipitation of gold, . 302 Cleaning-up, . . • 303 4. Production of bullion from precipitate, . . . 304 Molloy's method of precipitation, . . . 306 Plant required, . . . 307 Treatment of ore slimes, . 308 Testing of ores, . . . 308 Use of cyanide in stamp battery, .... 309 Method of treatment — At the Robinson miue, . 309 ,, Sylvia mine, . . 310 Results of process — In Utah, . . . .312 „ New Zealand, . . 312 312 „ South Africa, . . 313 Hannay's cyanide process, .

CONTENTS. XIV Chapter XVI. -Chemistry of the Cyanide Process. PAGE On oxidised pyrites, . . 327 The soda solution, . ■ • 328 Re-precipitatiou of gold and silver in leaching vats, . 329 Testing strength of solution, . 329 Strength of solution required, . 331 Consumption of cyanide, . . 331 Ores suitable to process, . . 331 PAGE Action of potassic cyanide on gold and other metals, . 317 Decomposition of potassic cyanide, .... 322 Decomposition in the zinc boxes, . . . • 322 Action of potassic cyanide — Onmetallicsaltsand minerals, 324 Chapter XVII. Pyritic smelting, 333 Chapter XVIII.— The Refining and Parting of Gold Bullion. 4. Treatment of silver solution, .... 354 5. Reduction of silver chloride, . . . 354 Parting by sulphuric acid, . 355 1. Mixing and granulating alloys, . . 355 2. Dissolving silver, . . 356 3.

Melting gold residue, . 358 4. Precipitation of silver, . 359 5. Crystallisation of sulphate of copper, . 359 Combined process, . . 360 Gutzkow ,, . .361 New Gutzkow process, . 363 Miller's chlorine process, . 366 Original method, . . 367 Later improvements, . 369 Refining brittle gold by chlorine, . . . 373 I'arting by electrolysis, . 375 General considerations, . . 336 Refining, . . . . • 337 Composition of bullion, . 337 Melting furnace, . . . 339 Crucibles, . . . .340 Melting, . . - .340 Refining, .... 341 Toughening, . . . 343 Casting, .... 344 Losses of bullion, . . . 346 Refining by sulphur, . . 34S Osmiridium in gold bars, . 34S Parting processes, . . • 349 Cementation, . . . 350 Parting by sulphide of antimony, .... 350 Parting by sulphur, . . 350 Nitric acid process, . . 351 1. Granulation of alloys, . 351 2.

Dissolving granulations, 352 3. Treatment of gold residues, .... 353 Chapter XIX. — The Assay of Gold Ores. Fluxes, . . . .382 General considerations, . . 377 Sampling and crushing the ore, 378 Methods of operation, . 3S4 Metallics, . . . - 379 Roasting before fusion, . 386 Crucible method of assay, . 3S0 Cleaning slag, . . . 3S7 Fusion, .... 380 Treatment of base ores, . 387 Assay-ton weights, . .381 General charges, . . 381 Cupellation, . . . 387 Influence of base metals, . CONTENTS. XV Chapter XIX Continued. page pack lnquartation and parting, . 391 Examination of assay materials, . . 393 Examination of cupel, . . 393 Assay by seorification, . . 394 Detection of gold in minerals, . 390 Estimation of gold in dilute solution, .... 397 Special methods of assay, . 397 1. Mixed wet and dry method, 397 2.

Amalgamation, . . 39S 3. Chlorination, . . . 398 4. Whittliead's method, . 400 5. Assay of pyrites, . . 400 (a) Swartz's method, . 4(10 (6) Staptf's ,, . 400 6. Assay of purple of Cassius, 401 7. ,, a Mint sweep, . 401 Chapter XX. — The Assay of Gold Bullion. Parting assay, . . . .401 1 . Selection of sample, . 402 2. Preparation of assay-piece for cupellation, . . 403 3. Cupellation, . . . 406 Assav furnace, . . . 406 Cupels, . . .408 Method of operation, . 408 Flashing, . . . .409 Temperature of muffle, . 410 4. Preparation of cupelled buttons for parting, . 413 5. Parting 414 (a) In parting flasks, . 414 (b) In platinum trays, 415 Relative advantages of these, . . .416 6. Weighing the cornets, . 417 Losses of gold in bullion assaying 417 Silver retained in cornets, . 419 Occluded gases, . . . 420 Checks or proofs, . . . 420 Limits of accuracy in assay, . 421 Parting by sulphuric acid, . 421 Preliminary assay, . . . 422 Assay by cadmium, . . . 422 Assay of alloys of gold, silver, and copper, . . . 422 Effects of other metals, . . 423 Assay of various gold alloys, . 423 A. Alloys requiring scorification, .... 423 Arsenic and antimony, . 423 Iron and Manganese, . 424 Cobalt and nickel, . . 424 Zinc 424 Tin, 424 Aluminium, . . . 424 B. Amalgams, . . . 425 C Platinum group, . . 425 (1) Platinum, . . .425 (2) Palladium, . . .426 (3) Rhodium and iridium, 426 D. Tellurium compounds, . 427 Wet methods of assay, . . 427 Other methods, . . .428 1.

Touchstone, . . .428 2. Colour and hardness, . 429 3. Density, . . . .429 4. Spectroscope, . . . 429 5. Electrolysis, . . . 430 6. Induction balance, . . 430

Appendix

A. The Liihrig vanner 433

APPENDIX.

B. Stamp battery practice in the Transvaal, ...... CONTENTS. BIBLIOGRAPHY PAGE PAGE Periodicals, .... 439 Roasting, .... 446 General, ..... 440 Chlorination, .... 447 Properties of gold and its alloys, 441 Distribution of gold, . . 443 Placer mining, . . . 444 Cyanide process, . . . 448 Pyritic smelting, . . . 448 Refining and parting of bullion, 448 Assaying, .... 448 Crushing and amalgamation, . 445 Concentration, . . . 446 Index,

Chapter I. The Properties Of Gold And Its Alloys

PHYSICAL AND CHEMICAL PROPERTIES OF GOLD. Introduction. — In beginning the study of the properties of gold, it is impossible to ignore two striking facts. Oil the one hand, 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. On the other hand, it is still more important 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 denned 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. — Gold is the only metal which has a yellow colour when 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. Tbis 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 to that formed from chloride, giving out 1'6 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.! 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 °f an incn in thickness can be obtained by beating. Faraday has shown that the thickness of these leaves may be still further reduced by 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 * Ditte, Lecons sur les Metaux, vol. ii., p. 561. t Phil. Trans. Royal Soc, 1857, p. 145. THE PROPERTIES OP GOLD. 3 per square inch and an elongation of 30-8 per cent., but the presence of minute traces — i.e., -^ijin) °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 rr-^j 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. + 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.

Cobesion. — 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 tbe fusing point of copper and much above that of silver. The metal expands considerably on fusing and contracts again on solidifying. Carnelley gives the tempeiature of fusion as 1,037°; while, according to Violle's determination, it is 1,045°, and this result has been generally accepted.

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. Royal Soc, vol. clxxix. (1888), p. 339. fPogg. Ann., vol. lxxiii. (1848), p. 1, and vol. Ixxv. (1848), p. 403. X Eighth Report of the Royal Mint, 1877, p. 43. § Trans. Am. Inst, of Mng. Eng., Chicago Meeting, 1893. i| For full information on the spectroscopic characteristics of gold, see Lockyer and Roberts, Phil.

Trans. Royal Soc, vol. clxiv. (1874), part ii., p. 495; and Fremy, Ency. Chim., vol. iii. (1888), L'or, p. 40. Conductivity and Expansion. — Its electrical conductivity is 76-7, and its thermal conductivity 53-2, those of silver being 100 in each case. Its coefficient of linear expansion is 0-0000144 between 0° and 100°.* Atomic Weigbt and Volume. — Its atomic weight, formerly believed to be about 196-2, has been more recently given by Kriiss as 196-64, by Thorpe and Laurie as 196-85, and by Mallet as 196-79. It is probable that 196-8 is not far from the truth. The atomic volume of gold is 10-2. Volatilisation of Gold. — The boiling point of pure gold has not been determined ; calculated according to Wiebe's formula it would be about 2,240°, or nearly 500° above the melting point of platinum. , However, contrary to the belief of the older experi menters (Gasto Claveus, and others), it is sensibly volatile in air at far lower temperatures.

Robert Boyle was unaware of this fact, but Homburg gilded a silver plate in 1709 by holding it over gold strongly heated in the focus of a burning mirror (Encyclo pædia Britannica, 1778, and Gmelin's Handbuch), and St. Claire Deville volatilised and again condensed gold when melting it with platinum. The rapid volatilisation of gold, when heated by an ordinary blowpipe, was first proved in 1802 by Dr. Robert Hare, of Philadelphia (Tilloch's Magazine), a purple stain being thus produced on bone-ash in a few seconds. Lastly, a discharge of high-tension electricity from gold points causes its volatilisa tion, and if the discharge is sent through a fine gold wire stretched on paper, it converts it into a purple streak of finely divided condensed particles of the metal. Tbe rapid distillation of gold caused by heating it in a current of air of considerable velocity, such as that furnished by a blow-pipe, by which the liquid is thrown into waves, may be shown at any time by heating a fragment of the precious metal of the size of a pin's head on a bone-ash cupel in the oxidising flame of a good mouth blowpipe.