# # # LIBRARY of'the University of Illinois CI.ASS. BOOK. VOLUME. # Accession Na. Digitized by tfie Internet Arcliive in 2015 littps://arcliive.org/details/treatiseonelectrOOmcmi ELEOTEO-METALLUEG-T I ELECTRICAL & METALLURGICAL PUBLICATIONS. ELECTRIC SMELTING AND REFINING: A Practical Manual of the Extraction and Treatment of Metals by Electrical Methods. Being the " Elektro-Metallurgie " of Dr W. BORCHERS. Translated from the Second German Edition bv WALTER G. MC.MILLAN, F.I.C., F.C.S., Secretary to the Institution of Electrical Engineers. "With numerous Illustrations and three Folding Plates. 21s. Contents : Part I.— Alkalies and Alkaline Earth Metals : Magnesium, Lithium^ Beryllium, Sodium, Potassium, Calcium, Strontium, Barium, the Carbides of the Alkaline Earth Metals. Part II. — The Earth Metals : Aluminium, Cerium, Lanthanum, Didymium. Part III.— The Heavy Metals : Copper, Silver, Gold, Zinc and Cadmium, Mercury, Tin, Lead, Bismuth, Antimony, Chromium, Molybdenum, Tungsten, Uranium, Manganese, Iron, Nickel and Cobalt, the Platinum Group. " Comprehensive and Authoritative." — Electrician. ELECTRICAL RULES AND TABLES (A Pocket-Book of). For the use of Elec- triciansand Engineers. By J. MUNRO, C.E., and Prof. A. JAMIBSON, F.R.S.E., M.Inst.E.E. With numerous Diagrams. Thirteenth Edition, Revised and En- larged. Leather, Ss. 6d. " Wonderfully perfect ; worthy of the highest commendation." — Electrician. THE ART OF THE GOLDSMITH AND JEWELLER. A Treatise on the Manipu- lation of Gold in the Various Processes of Goldsmith's Work, and the Manufacture of Personal Ornaments, etc. For Students and Practical Men. Bv THOS, B. WIGLEY. Assisted by J. H. STAXSBIE, B.Sc.(Lond.), F.LC. With numerous Illustrations. Large Crown 8vo, 8s. 6d. "An exhaustive fund of information which cannot fail to be of service."— Jeweller and Metal Worker. INTRODUCTION TO THE STUDY OF METALLURGY, By Sir W. ROBERTS- AUSTEN, K.C.B., D.C.L., F.R.S., Chemist and Assayer of the Royal Mint; Professor of Metallurgy in the Royal College of Science. Handsome Cloth. With additional Illustrations and Micro-Photographic Plates of Ditferent Varieties of Steel. Fourth Edition. ]5s. "No English text-book at all approaches this in completeness with which the most modern views on the subject are dealt with. Professor Austen's volume will be invalu- able." — Chemical Neics. THE METALLURGY OF GOLD. By T. KIRKE ROSE, D.Sc, Assoc.R.S.M., Assistant Assayer to the Royal Mint. Including the most recent improvements in the Cyanide Process. Third Edition, Revised and Enlarged. With numerous Illus- trations. 21s. "An addition to the literature of Metallurgy, of classical ya.-ljjv.:'— Nature. THE METALLURGY OF LEAD AND SILVER. By H. F. COLLINS, Assoc.R.S.M., M.Inst.M.M. In Two Volumes, each complete in itself. Part I.— Lead : A Treatise on the Manufacture of Lead, with Sections on Smeltinc^ and Desilverisation, and Chapters on the Assay and Analysis of the Materials involved. ° [ Volume II. Si-LYER— Ready shortly. ELEMENTS OF METALLURGY. A Practical Treatise on the Art of Extractino- Metals from their Ores. By J. ARTHUR PHILLIPS, C.E., F.C.S., F.G.S and H. BAUERMAN, F.G.S. Third Edition. In Royal Svo, with numerous Illustrations. Co7itents : Refractory Materials, Fire-Clays, Fuels, etc., Antimony, Ai^senic, Zinc, Iron, Cobalt, Nickel, Mercury, Bismuth, Lead, Aluminium, Copper, Tin, Gold, Silver, Platinum, etc. " The value of this work is almost inestimable."— il/mmf/ Journal. ASSAYING (A Text-Book of). For the use of Mine Managers, Assayers, etc. Bv J.iJ BERINGER, F.C.S., F.LC, Principal of the Camborne Mining School, and 'a BERINGER, F.C.S. Fifth Edition. 10s. 6d. " Really meritorious. May be safely depended w^on."— Nature. CHEMISTRY FOR ENGIN^^^^ AND MANUFACTURERS: A Practical Text- Book By BERTRAM BLOUNT, F.LC, F.C.S., Consulting Chemist to the Crown Agents for the Colonies, and A. G. BLOXAM, F.LC, F.C.S., Head of the Chemistry Department, Goldsmiths' Institute, New Cross. In Two Vols. Large Svo With numerous Illustrations. Vol. I.— The Che.aiistry of Engineering, Building, and Metallurgy 10s 6d ; Worthy of High Rank in the class of literature to which it helon^s."— Journal of Gas Lighting. ° Vol. IL— The Chemistry of Manufacturing Processes. 16s. " The authors have succeeded beyond all expectation."— T/ie Times. London : CHAS. GEIFEIN k CO., Ltd., Exeter St., Strand.. ft A TEEATISE ON ^ ■ ' . ELECTEO-METALLUKGY EMBEACING THE APPLICATION OF ELECTROLYSIS TO THE PLATING, DEPOSITING, SMELTING, AND REFINING OF VARIOUS METALS, AND TO THE REPRODUCTION OF PRINTING SURFACES AND ART-WORK, ETC. BY WALTEE G. MCMILLAN, F.I.C, M.Inst.M.M., LATE LECTURER ON METALLURGY IN MASON UNIVERSITY COLLEGE, BIRMINGHAM, AND FORMERLY CHEMIST AND METALLURGIST' TO THE COSSIPORE ORDNANCE FACTORIES. SECOND EDITION. EEVISED AND ENLARGED. WITH NUMEROUS ILLUSTRATIONS. LOISTDON : CHARLES GEIFFm & COMPANY, LIMITED, EXETER STREET, STRAND. 1899. [All Bights Reserved.] PREFACE TO THE SECOND EDITION. Since the First Edition of this Treatise was pubUshed the principal advances that have been made in the practice of Electro-MetaUurgy have been in the field of Smelting and Eefining ; but so great has been the progress in this direction that any intention to treat the subject adequately in the ^ space available had necessarily to be abandoned. Although, ^ therefore, the chapter dealing with this branch of the subject has been re-cast and extended, the reader who wishes to study the question of electric refining and electric smelting in detail is referred to one of the books dealing exclusively with these matters. At the date of the earlier edition the modern theories of ^electrolytic action based largely on the work of Van't Hoff Jj3n Osmotic Pressure, and that of Arrhenius on Electrolytic *^ Dissociation, both first published in 1887, were beginning rapidly to gain ground. These theories have now been developed, and are so widely adopted as an explanation of the facts of electrolysis that, although they are not by any means universally accepted, it has been thought necessary to give a short summary of their leading features. A separate chapter has therefore been devoted to the presentation of these theories in an elementary fashion. The remainder of the book has been re-read, and many vi PREFACE. additions and alterations have been made throughout in the direction of bringing it up to date. The Author desires particularly to record his thanks to his friend and former colleague, Mr. E. F. Herroun, for his kindness in reading over the proof of the chapter on the Modern Theories of Electrolysis. To the Electrolytic Plat- ing Apparatus Company he is indebted for the drawing reproduced in fig. 70, to Grore's Art of Electrolytic Separation of Metals (1890) for the numbers relating to the Pembrey installation of electrolytic copper refining incorporated in Table XXVII., to the Council of the Institution of Electrical Engineers for Table XXXVII.a, and to Borchers and McMillan's Electric- Smelting and Refining (1897) for several additional blocks used in the text. W. G. M. 28 Victoria Street, London, S.W. I2th August 1899. PREFACE TO THE FIRST EDITION. In the following pages I have endeavoured to systematise and explain the various processes of Electro-Metallurgy as far as possible. Believing fully that in teaching and writing upon such subjects a technological rather than a technical treatment is required, I have tried so to set the matter before the reader that, even if he be a novice, he may be led to take an intelligent interest in any practical work upon which he may be engaged; but I have avoided the accumulation of a mass of unnecessary descriptive detail, which would only tend towards confusion, and which would be dictated by common-sense to any who have grasped the principles involved. In many cases, however, success is in a large measure dependent upon strict attention to mechanical detail ; and here I have not hesitated to introduce such instructions as I believed needful to guide the worker in the special operations in hand, while indicating the reasons which should enable him to apply them to processes of kindred character. In short, I have aimed at a combination of theory and practice. The necessity for at least a fair knowledge of Chemistry and Electricity has led to the introduction of a chapter , dealing in - an elementary fashion with such laws as are required for an understanding of our subject ; but it is not VUl PEEFACE. pretended that this chapter shall in any degree supersede the text-books upon these sciences ; it is rather intended to lead up to them. In treating of the sources of current, especially the dynamo-electric machine, 1 have dwelt longer upon the general theory of construction and use as appli- cable to all than upon the special modifications adopted by different inventors and manufacturers. In addition to the journals, the following works among others have been consulted, and my general indebtedness to these authors must here be thankfully recorded: — Fontaine's Electrolyse, Gore's Art of Electro-Metallurgy, J aping's Elehtrolyse Galvano-plastik unci Eeinmetallgevmi' nung, Napier's Manual of Electro-Metallurgy, Eoseleur's Manipulations Hyclroplastiques, Schaschl's Gahanostegie, Thompson's Dynamo Electric Machinery, Urquhart's Electro- typing and Electro-plating, Volkmer's Betrieb der Galnano- plastik mit Dynamo-EWktrischen Maschinen zu Zivecken der Graphischen Kilnste, Wahl's Practical Guide for the Gold and Silver Electro-plater and the Galmno-plastic Operator, Watt's Electro-deposition, Weiss's Galvano-plastih, and Wilson's Stereotyping and Electrotyping. My thanks are also due to the Brush Electric Light Corporation, Messrs Hoe & Company, Messrs Siemens Brothers, and Messrs Townson & Mercer for diagrams of apparatus. I must further acknowledge my obligations to Professor Silvanus Thompson for his kind permission to describe and figure a special form of switch which he has in use. WALTEE G. MCMILLAN. CossipoRE, Calcutta, September 1890. CONTENTS. CHAPTER 1. Introductoky and Historical. .rjefinition of the term Electro-metallnrgy — Scope of the Art — The Germ of the Process in Primitive Times — The Growth of Electrical Knowledge — The Invention of the Thermopile Battery and Magneto-electric and Dynamo-electric Machines — First Attempts at the Electro-deposition of Metals — The Work of Nicholson and Carlisle, Cruickshanks, Wollaston, Brugnatelli, and Bessemer — BecquerePs Experiments in the Electrolytic Treatment of Ores — The Invention of the Daniell-cell the first step towards Electrotypy : De la Rue's Observation, and the Development of Electrotyping by Jacobi, Spencer, and Jordan — The question of prece- dence among the three Rival Inventors — Murray's discovery of Blacklead- ing Non-conductive Electrotype Moulds — Leeson's and Montgomery's first Elastic Moulds — The later Advances in the Art ; the value of the Dynamo and its eff"ect upon the Industry — Electrolytic Ore treatment — Electrolytic metal-refining — Electrolysis of Fused Substances — Electro- smelting and Refining — Recent Development of the Theory of Elec- trolysis, Pages 1-14 CHAPTER II. Theoretical and General. {See also Chapter XIX. ) Matter and Force — Conditions of Matter — Constitution of Matter— Molecules and Atoms — Elements and Compounds — Relative Weights of Atoms — Meaning of the Symbols and of Chemical Formulae — Laws of Definite Chemical Combination — List of Elementary Substances — Energy Dis- played by Chemical Combination — Eff'ect of Varying Heats of Combina- tion in Determining the Occurrence of Chemical Re-actions — The Elements placM in Electro-chemical Series — Transformations of Energy — The Conversion of Chemical into Electrical Energy, and its application X CONTENTS. in the Galvanic Battery — Re-conversion of Electrical Energy into Chemical Energy — The Theory of Electrolysis or Electro-deposition — Laws Governing Pressure or Electro -motive Force of Battery Currents — The Relation of Current-pressure to the Electro-deposition of Metals from Solutions under varying Conditions — Electric Conductance — Electrolytic Conduction — Electrolysis of mixed Solutions — The Deposi- tion of Alloys — Quantity and Potential of Currents — Units employed in Measurements, . . . . . . . . Pages 15-39 CHAPTER III. Sources of Current. The Galvanic Battery — The Use of Impure Zinc ; Local Action ; Amalgama- tion — The Earlier Forms of Battery — The Polarisation of Battery Cells, and its Remedies — Definition of Terms used in speaking of Batteries— The various Single- and Two-fluid Cells — Smee's, DanielPs, Grove's, Bunsen's, the Bichromate, and other Batteries — Practical Hints on the Use of Batteries — The Fittings and Connections of a Battery — Various manners of arranging several Cells ; the application of Ohm's Law — Thermo-electric Batteries — The Direct Conversion of Heat into Elec- tricity — The Thermo-electro-motive Force of varied Combinations of Metals at different Temperatures ; the choice of Metals for Thermo-elec- tric Couples — Clamond's and Noe's Thermopiles — The Conversion of Mechanical into Electrical Energy — The Elementary Theory and parts of the Dynamo- and Magneto-Electrical Machines — Various arrange- ments of Magnets and Armatures in Dynamo Construction — The Wilde, Gramme, Siemens, Weston, Brush, Victoria, and other forms of Dynamos — Conditions to be observed in using the Dynamo — Accumu- lators or Secondary Batteries — Use of Public Electricity Supply for Electro-metallurgical Work — Conversion of Alternating-currents into Continuous Currents, and of high-pressure Currents into low-pressure Currents, Pages 40-88 CHAPTER IV. General Conditions to be Observed in Electro-Plating. Necessity for Cleanliness — The Proportioning of Current to the Electrolytic Separation to be Effected— The Relation of Weight and Thickness of Deposit to Current-strength and Duration of Process — The effect of and means for altering the Electrical Resistance of the Circuit — The use of Measuring Instruments for Determining Current-strength — The Detector, Galvanometer, Ammeter, and Voltmeter — The Arrangement of Plating- Vats according to Current-strength, Volume and Pressure, and other Considerations ; Connection in Series and in Parallel — The Choice of CONTENTS. xi Anodes ; and the Regulation of the Distance between the Electrodes in the Yat— Necessity for Stirring the Plating Solutions — Motion to be imparted to the Solution, . . . . . . ■ Pages 89-104 CHAPTER V. Plating Adjuncts and Disposition of Plant. Necessity for Light and Air in Work-rooms — Apportionment of Rooms to Departments of Work — Requirements in the Disposition of Shops — Drainage and Ventilation — Arrangement of Plant for Electro-plating — The Material and Form of Vat, and Manner of Introducing the Connecting Wires — Relative Positions of Electrodes for different kinds of Work— The Method of supporting Anode Plates— The Systems of Stirring Plating Liquids, and of imparting Motion to the Suspended Objects — The Weighing of the Deposited Metal ; Plating-balances — Corrections to be made in using the Plating-balance — The Coating of Wires — Electro-plating large Surfaces— The Electro-plating of small Goods in Rotating Drums— Smith and Deakin's A})paratus— The Joints of Lead-lined Vats, Pages 105-122 CHAPTER VI. The Cleansing and Preparation of Work for the Depositing- Vat, AND Subsequent Polishing of Plated Goods. The Different Methods of effecting the removal of Grease and thick incrusta- tions of Dirt — The Chemical Action of hot Alkali on Grease — The Cleansing of Objects from Mineral Oils — The Construction and Use of the Potash Tank — Necessity for dipping in Acid before immersion in the Plating-Vat— The Cleansing of Copper, Brass, and German Silver, of Iron and Steel, Zinc, Lead, Tin, and Britannia Metal— The Quicking of Metallic Surfaces— The Mechanical Treatment of Metals to be Cleansed, and methods of ensuring a Smooth and Polished Surface — The Polishing Lathe — The manner of Bobbing and Mopping — The uses of Scratch- brushing by Hand and by Lathe — The Process of Burnishing — The Preliminary Treatment of Hard Steel — The Preparation of Objects for Nickeling, Pages 123-138 CHAPTER VII. The Electro-Deposition of Copper. Objects with which Electro-deposition of Copper is applied — Coating by Simple Immersion — Coppering of small Steel Articles — The Single-cell Process — The J^anner of Coating Flat and Spherical Surfaces by Single- cell Deposition — The Battery Process of Coppering — The Battery xii CONTENTS. employed— The various Acid and Alkaline Plating-baths— The Char- acter, Susi^ension, and use of the Anodes— The Character of the Copper Deposited ; the Strength, Hardness, and General Fitness of the Metal precipitated from various strengths of Solution by different Current- densities — The Process of Deposition — The Manner of Preparing the Articles for the Bath, Immersing them, noting the Progress of the Action, withdrawing and finally Cleansing them — The Coppering of Printing-rollers— Electrolytically formed -Tubes- Other Applications ot the Process, Pages 139-155 CHAPTER VIII. ELECTROTYriNG. First Principles of Electrotyping— Moulding Materials for Printers' and Alt Work in Solid and Elastic Compositions, applicable to Plain or Undercut Designs, and the manner of applying them— Gutta-percha, Bees'-wax, Plaster of Paris, Fusible Metal, Gelatin, and Sealing-wax ; and Mixtures in which these are employed — The manner of rendering the Mould Conductive— Plumbago, Silvered and Gilded Plumbago, Tin- and Copper-powders— Printers' Electrotyping— The Reproduction of Steel Plates ; the Arrangement of Baths and Distribution of Current, and General Conduct of the Process— Typographical Matter— The Preparation of the Type ; Moulding ; Black-leading of the Mould, and Deposition of Copper ; the final Backing and Treatment of the Electro- type—Treatment of Wood-blocks— Art Electrotyping— The Moulding , and Reproduction of Medals, of Busts and Statues, and of Natural Objects— The Special Treatment of Large Statues, in regard to Mould- ing, Disposition of Anodes and Application of Solution— The manner of ensuring Equalisation of Deposit upon the Moulds— Sundry Applica- tions of Electrotyping— Glyphography, Stylography, Galvanography, Electro-etching, and the like— Manufacture of Copper Reflectors by Electrotyping, Pages 156-191 CHAPTER IX. The Electko-Deposition of Silver. The Deposition of Silver by Simple Immersion ; the Composition and Use of various Plating Solutions and Pastes— The Single-cell Process— The Separate-current Process— The Battery— The Constitution and Pre- paration of various Silver-baths— The conditions to be avoided in Making Up and in Tending Plating-solutions— Bright Plating-liquids— The Anodes— The Character of the Metal Deposited under difl"erent Conditions— The Stripping of old Silver Coats from Copper, Brass, or German Silver, and from Zinc, Iron, Tin, Lead, and their Alloys— The CONTENTS. xiii Final Preparation of the Objects for the Bath — The Suspension of the Goods in the Vats, and manner of Depositing the Metal — Precautions to be adopted in certain cases — The use of the Striking-bath — Difficulties encountered in Plating — The Local Thickening of the Film to withstand Wear — The Thickness of Deposit^ — Silver Electrotyping — The Ornamentation of Silver Surfaces by Dead Lustre, and as Oxidised Silver, Antique Silver, or by Satin Finish — Niello Work, Pages 192-216 CHAPTER X. The Electro-Deposition of Gold. Advantages of Gold-plating — Solutions and Process for Deposition by Simple Immersion — The Single-cell Process — Battery Methods of Deposition — The Preparation and Characteristics of various Solutions — The Production of Coloured Gold — The Necessary Properties of the Anode — The Character of the Metal Deposited by different Current- strengths— The Stripping of old Gold-deposits — The Requirements and Conduct of the Depositing Process — The Gilding of Interior Surfaces — Plating with the ' Doctor '—The Thickness of the Films— Dead Gild- ing — The Treatment of the more Electro-positive Metals— The Gilding of Soldered Goods — Parcel-gilding — The Ornamentation and Treatment of Gilt Surfaces— The Contrasting of Coloured Gilding— The Colouring of Deposited Gold— The Gilding of Watch Mechanisms— The Mechanical Production of the Surface-grain, .... Pages 217-235 CHAPTER XL The Electro-Deposition of Nickel and Cobalt. The Applications and Advantages of Nickel-plating— Difficulties in Electro- nickeling— The Character of the Deposited Metal— The Battery Process for Depositing Nickel — the Nature and Preparation of the Various Nickeling Solutions— Cast and Rolled— Nickel and Carbon Anodes— The Necessity for Stripping old Nickel Coats— The Preparation of Objects for the Bath— The manner of effecting Electrolysis— Precautions to be Observed— The Finishing of the Plated Goods— Dead Nickel Surfaces— The Electro-deposition of Cobalt— The Solutions used, and the Method of Applying them— The Details of the Process, and the Nature of the Pre- cipitated Metal, Pages 236-250 CHAPTER XIL The Electeo-Deposition of Iron. The Advantages of Iron-faced Engraved Copper Plates— The Depositing Solutions, their Preparation, Maintenance, and Use— The Physical xiv CONTENTS. Properties of- Deposited Iron — Use of the Term Steel-facing — Strip- ping old Coats ; the Plating Process and Final Treatment of the Work, . . . . . . . . . Pages 251-258 CHAPTER XIII. The Electro- Deposition of Platinum, Zinc, Cadmium, Tin, Lead, Antimony, Bismuth, and Palladium ; Electeo-chromy. Platinising by Simple Immersion — Deposition by Single-cell Process — The Platinising of Silver Plates of the Smee's Battery — Platinating by a Separate Current — Removing Old Deposits — The Plating-solutions and manner of applying them — The Deposition of Zinc — Comparison of Electro-zinced Metal with so-called Galvanised Iron — The Solutions and their Use ; Use of Zinc Dust in the Solution ; Anomalies in the Applica- tion of the Current — The Character of the Zinc Deposit under various Conditions — The Electro-deposition of Cadmium — Electro-tinning ; Electrolytically-coated Goods contrasted with Tin-plate — Tinning of Brass Pins and small objects by Simple Immersion — Deposition of Tin by Single-cell and Separate-current Processes — The Electro-deposition of Lead — The Coating of Bodies with Antimony — Disqualifications of the Metal for Plating thin articles liable to be bent — Simple Immersion and Battery Methods of Deposition — The Plating-solutions and the Metals precipitated from them — The Preparation and Properties of Explosive Antimony — The Electro-deposition of Bismuth — The Electro-deposition of Palladium — Attempts to deposit Aluminium — The Colouring of Metallic Surfaces — Electro-chromy, . . . . Pages 259-279 CHAPTER XIY. The Electeo-Deposition of Alloys. The Electro-deposition of Brass — The Solution ; its Composition and Use — The Choice of Anodes — The Influence of the various Conditions upon the Character of the Deposited Brass — The Deposition of Bronze and of German Silver— The Precipitation of other Alloys, . Pages 280-290 CHAPTER XY. Electeo-Metalluegical Exteagtion and Refining Peoce^^ses. Conditions under which the Industry Exists — The Electro-refining of Copper — The Theory of the Process— The Behaviour of the various Impurities at the Anode, in the Solution, and at the Cathode— The Solution, Current-strength, and General Conduct of the Process — The Disposition of Vats according to the Current employed — Modern Systems of Refining — Practice in certain large Installations — The Electro- CONTENTS. XV extraction of Copper from Ores and Products— Classification of Extrac- tion Processes— Outlines of the Methods employed— The Electro-refining , of Lead— Treatment of Base Bullion— The Electrolytic Eefining of Silver —The Electro-extraction of Zinc— The Electro-reduction of Antimony— Electro-smelting- The Electric Furnaces of Siemens, Cowles, and others —The Electrolysis of Fused Compounds— The Electric Smelting of Aluminium, Magnesium, and Sodium— Electric Welding and Anneal- • • . • Pages 291-320 CHAPTER XVI. The Recovery of Certain Metals from their Solutions OR Waste Substances. Sketch of the Treatment of Residues of Cobalt, Copper, Gold, Lead. Mercury, Nickel, Platinum, and Silver, . . , . . Pages 321-326 CHAPTER XVIL ' The Determination of the Proportion of Metal in Certain Depositing Solutions. Outline of Methods for conducting Electrolytic Quantitative Analysis, and for determining Antimony, Cobalt, Copper, Gold, Lead, Nickel Plati- num, and Silver, . . . . . . . Pages 327-331 CHAPTER XVIII. Power Required for Electrolytic Work. Calculation of Power required for Electrolytic Work— Horse-Power required per Unit Area of Electrode Surface for different Solutions and different Metals— Board of Trade Units of Electrical Energy (Kilowatt-Hours) required for the Deposition of Various Metals from different Solutions- Cost of Electricity generated from Batteries and from Dynamos— Cost of Steam Power and of Water Power on the large Scale— Absorption of Power in Conductors— Loss of Electrical Energy by Transformation into Heat m Conductors of different Materials and Sizes, . Pages 332-340 CHAPTER XIX. Modern Theories of Electrolysis. Development of the Modern Theories of Electrolysis— Position of the Theory of Ionic Dissociation— The Theories of Solution Pressure and of Osmotic Pressure— Analogy between the Laws governing Gaseous Pressure and Osmotic Pressure in Dilute Solutions— Behaviour of Electrolytes in xvi CONTENTS. respect to Osmotic Pressure — The Theory of Dissociation of Electrolytes into Ions when in Solution — The Explanation of Electrolytic Conduction in the Light of the Theory of lonisation — The Meaning of Electrolytic Solution Pressure — The Application of Modern Theories to the Explana- tion of Simple Exchange of Metals, of Simple Galvanic Cells, of Two-fluid Cells, of the Current produced when one Piece of Metal is immersed in a non-homogeneous Solution, and of Electrolysis with Insoluble and Soluble Anodes — The Effect of Secondary Actions in Electrolysis — The Electrolysis of Mixed Solutions and Double Salts, and of the Salts of Complex Acids, Potassium Aurocyanide and Silver Cyanide, etc. — The Eff'ect of Unequal Rates of Migration of Ions in Solutions on the Relative Concentration of the Electrolytes at the two Electrodes — Conditions of Electrolysis, . . . Pages 341-368 CHAPTER XX. A Glossaey of Substances Commonly Employed in Electro-Metallurgy. Rules to be observed in dealing with Acids and other Chemical Reagents — Glossary of Common Substances, .... Pages 369-398 ADDENDA. Various useful Tables — The Bronzing of Copper and Brass Surfaces — Anti- dotes to Poisons, Pages 399-413 Index, Page 414 LIST OF TABLES IN THIS TREATISE. 4. 6. 1. The List of the Elements, 2. The Number of Heat- units Evolved in certain Com- positions, 3. The Arrangement of the Commoner Metals in Electro-chemical Series, The Electric Conductance of certain Metals for Electricity, The Thermo-electro-motive Force of various Metals in Relation to Lead, ...... Illustration of the Total E.M.F. produced by an Iron-and-Lead couple at different Temperatures, . The Average Current-values suitable to the Deposi- tion of certain Metals, . ... The Composition of Copper Baths, recommended by various Authorities, . . The effect of varying Current-strength on the Copper deposited from Neutral Copper Sulphate Solu- tions, The effect of varying Current-strength on the Copper deposited from Copper Sulphate Solutions con- taining two per cent, of Sulphuric Acid, The Maximum Current-density for Electrotyping with different Solutions, 12. The Composition of certain Fusible Alloys, 13. The Composition of Silver Simple-immersion Mix- tures, recommended by various Authorities, The Composition of Silver Baths for Separate- current Process, recommended by various Authorities, The Composition of Gold Simple-immersion Mix- tures, recommended by various Authorities, 16. The Composition of Gold Baths for Separate-current Process, recommended by various Authorities, Chap. 2 10 11 14 15. 10 10 h PAGE 21 23 24 34 65 69 145 148 148 150 1'62 193 1:97 218 221 xviii LIST OF TABLES IN THIS TEEATISE. 17. The Composition of Nickel Baths for Separate- current Process, recommended by various Authori- ties, Chap. 11 18. The Composition of Cobalt Baths for Separate-cur- rent Process, recommended by various Authorities, ,j 11 19. The Composition of Iron Baths for Separate-current Process, recommended by various Autliorities, . ,, 12 20. The Composition of Platinum Baths for Separate- current Process, recommended by various Authorities, ....... 13 21. The Composition of Zinc Baths for Separate-current Process, recommended by various Authorities, . 13 22. The Composition of Tinning Baths for Simple- immersion Process, recommended by various Authorities, ,, 13 23. The Composition of Tinning Baths for Separate-cur- rent Process, recommended by various Authorities, 13 24. The Composition of Antimony Baths for Separate- ^ current Process, recommended by various Authori- ties, J, 13 25. The Composition of Brassing Baths for Separate- current Process, recommended by various Authori- ties, ,,14 26. The Composition of Bronzing Baths for Separate- current Process, recommended by various Authori- ties, ,,14 27. Comparison of some of the Older and more Modern Systems of Copper-refining at certain Electro- metallurgical Works, ,,15 28. Data for Calculating Weight and Thickness of Deposit, produced by Current of known Volume in a given Time, for certain of the Commoner Metals, Addenda 29. Values of Equal Current-volumes, as expressed in Amperes per Square Decimetre, per Square Foot, and per Square Inch of Electrode Surface, . . 30. The Specific Gravities of Sulphuric, Nitric, and Hydrochloric Acids corresponding to varying percentages of H2SO4, HNO3, and HCl, . . ,, 31. The Specific Gravities of Solutions corresponding to the Degrees of the Baume Hydrometer, . . ,, 32. The Densities of Solutions of Crystallised Copper and Zinc Sulphates, ,, 33. The Specific Electrical Resistance of different Sul- phuric Acid Solutions at various Temperatures, . ,, PAGE 238 248 254 261 264 268 270 273 282 287 299 399 400 401 402 • 402 403 LIST OF TABLES IN THIS TREATISE. xix PAGE 34. The Specific Resistance of different Copper-sulphate Solutions at various Temperatures, . . . Addenda 403 35. The Electrical Resistance of pure Copper Wires of various Diameters, ...... 404 36. The Diameters corresponding to the Numbers of the old Birmingham Wire Gauge, . . . . 405 36a. The Diameters corresponding to the Numbers of the American (Brown & Sharpe's) Standard Wire Gauge, 405 37. The Diameters and Areas of Cross-section correspond- ing to the Numbers of the Standard (Imperial) Wire Gauge, ,, 406 37a. Maximum Currents Permissible for Copper Conductors Insulated and Laid in Casing or Tubing, . . 407 38. Comparison of Centigrade and Fahrenheit Ther- mometers, ,, 408 39. Avoirdupois Weight, ,, 409 40. Troy Weight, „ 409 41. Apothecaries' Weight, 409 42. Imperial Fluid Measure, 409 43. The Interconversion of certain Standard Weights and Measures, ........ ,, 410 A TEEATISE ON ELECTEO-METALLUEGY. CHAPTEE I. INTRODUCTORY AND HISTORICAL. The word metallurgy is understood to mean the art of working metals — extracting them from their ores and preparing them for apphcation to the varied uses of daily life. By analogy the term electro-metallurgy, originally suggested by Smee, might reasonably be expected to imply such extraction and preparation effected with the aid of electricity. This, however, is, strictly speaking, but one section of the subject, and, indeed, regarded from the standpoint of commercial practicability, it is one of the most recent developments of the art ; for the economical application of electricity to the recovery of metals from their ores by the separate-current process was scarcely possible until the invention of the dynamo-electric machine had placed a cheap source of electric energy at the disposal of the metallurgist. Just as the science of metallurgy also is but a branch of that of chemistry, and becomes elevated from an art to a science, in proportion as the laws of chemistry are made to regulate its processes, so the science of electro-metallurgy is dependent on the laws of chemistry and electricity, and will make more rapid progress as the accurate study and application of these laws are made to take the place of the ' rule of thumb ' methods, which are the inevitable outcome of the tentative experiments made in the early dawn of an art. Accepting, then, the broader use of the term, electro-metallurgy may be defined as the science which treats of the application of electrical methods to the separation or to the solution of metals from substances containing them, and (perhaps we may add) to the treatment of metals for certain specific purposes in the arts. A 2 IXTEODUCTORY AND HISTORICAL. Scope. — Thus the electro-metallurgist may be called upon to deposit metals with any of the following objects : — (1) To obtain a coherent and removable deposit on a mould, the form of which it is desired to reproduce with accuracy; this process is termed electrotyping : (2) To obtain a thin, but perfect and adhesive, film upon a metal of different character, in order to impart to it acid- or air-resisting, or aesthetic properties, in which it was naturally deficient ; this is known as ' eledro-platiiig : (3) To obtain the whole of a given metal from a substance containing it, either as a substitute for extraction by smelting, or for analytical or refining purposes. It will be evident that in the first two of these, the interest centres in securing the exact condition of deposit which is best suited to the work in hand ; whilst in the third, it is of paramount importance that the metal shall be completely separated, leaving no residue in the material from which it was to be extracted. Finally (-4), he may be required to dissolve metals, either to remove an existing coat of one metal from the surface of another, or to effect the complete or partial solution of a homogeneous body superficially, as in the case of electro-etching. Early History. — The history of the art is interesting, but per- haps too much involved to render anything more than the following brief sketch of value to the probable readers of this volume. The fact that certain metals become superficially coated with other metals when plunged into suitable solutions was known to the ancients, and such a covering of iron swords and shields with copper by immersion in copjoer solutions was described by the Greek historian Zosimus in the fifth century. Paracelsus, who lived in the beginning of the sixteenth century (1493-154:1), ascribed the apparent change of iron into copper, Avhen dipped into the blue waters of Schmollnitz in Hungary, to an actual transmutation of metals, a view which found favour even at a much later period. But although these may be considered as the beginnings of electro-metallurgy on the chemical side, it was not until the lapse of two centuries and a half from the latter date that the application of electricity to the deposition of metals became possible. Let us then glance at the gradual growth of electrical knowledge and its adaptation to the requirements of * electro-deposition.' Such a retrospect cannot embrace any long- term of years ; for, although the attractive force of rubbed amber was known to the ancients, it awoke only a wondering interest until 1647, when Otto von Guericke first constructed a machine Avhich exhibited the phenomenon in an intensified degree ; the un- INVENTION OF THE ELECTRIC BATTERY. 3 known force received the name electricity (from eleUron = amber), electrical machines were gradually improved, and in 1752 Franklin demonstrated the identity of the electric spark with the lightning flash. But, in spite of the marvellous disruptive effect of these ^frictionar machines, the actual quantity of electricity which €ould thus be generated was very minute, and could not avail for the deposition of metals from solutions. Its destructive power was derived from the enormous potential or ' pressure ' at which it acted, and no electrolytic effect could possibly have been observed except by a most careful experimenter actually searching for such a manifestation. In the year 1759 Galvani made his celebrated discovery that a metal wire at one end touching the lumbar nerves of a recently- killed frog, and at the other the muscles of its leg or thigh, caused a rapid muscular contraction. Finding the same pheno- menon producible with the aid of a frictional machine, he was led to connect the two incidents, and to ascribe the former to the action of electricity resident in the animal itself. Volta, on the contrary, finding—as, indeed, Galvani had done before him— that if two wires of different metals were used, the contractions became more vigorous, concluded that the electrical energy was due rather to the action of the wires than to any property inherent in the animal tissue. Led on by this assumed produc- tion of electricity by contact of dissimilar metals, he constructed the series of zinc and copper discs, separated by moist cloth, which bears the name of the Voltaic pile, and with which, for the first time, comparatively large currents, though of very low potential, were obtainable, such as might be applied to the purposes of electro-metallurgy. Meanwhile, Fabroni in Italy, and Wollaston, Davy, and others in England, showed that oxida- tion, or rusting of the zinc, invariably attended the production of electricity in this way, and ascribed the latter to chemical action. First Electrical Battery.— In 1800 Volta replaced the pile by his 'crown of cups,' in which each pair of copper and zinc plates was separated not by damp cloth, but by acidulated water placed m a series of vessels, the copper of each intermediate vessel being connected by a wire with the zinc of the next, leaving a free or unattached copper plate at one end of the series and a corresponding zinc plate at the other, these terminal plates being, of course, equivalent to those of the 'pile.' This, then was the original electric battery, the discovery of which has led to the invention, of the art of electro-metallurgy. Separation of Metals.— In the same year Nicholson and Carlisle 4 INTRODUCTORY AND HISTORICAL. succeeded in decomposing water, or, in other words, depositing- hydroqen, by means of the source of electricity thus placed at their "disposal; and in 1803, Cruickshanks, of Woolwich, con- structed a large battery of considerable power, with which he deposited, or 'revived,' as he termed it, many metals from their solutions, and e^en proposed the use of an electrolysing current in quantitative chemical analysis. Meanwhile, in 1801, Wollaston had obtained a coating of '^copper on silver, sufficiently adherent to allow of burnishing, by introducing the latter metal, in contact with one more oxidisable, into a solution of copper, thus forming a small electric battery in the depositing liquid itself. In the Philosophical Magazine, Brugnatelli, in 1805, described the gilding of two large silver medals by means of the Voltaic pile and a^olution of 'ammoniuret of gold,' and also the silver- ing of platinum surfaces, at the same time directing attention to the gradual solution of the plate through which the electric current entered the hquids. Then Davy, in 1807, made his grand discovery of the alkali-metals, potassium and sodium, by electro- lytic isolation. " Magneto- and Dynamo-Electric Machines.— The knowledge of the relation between electricity and magnetism gained^ in 1820, both by Oersted's researches on the action of the electric current upon a compass-needle, and by the success of Arago in magnetising a steel needle by means of the current, may perhaps be regarded as the primary step towards the invention of magneto-electric machines, the lirst of which was constmcted by Faraday in 1831 ; it consisted of a copper disc rotated between the poles of a horse-shoe magnet, with the necessary fittings for taking off the current thus generated. In the same year Faraday observed the mutual action of electric currents, and the conditions governing the formation of induced currents, and thus, as it were, paved the way for the subsequent invention of the dynamo-electric machine. Faraday's magneto-electric machine was not suffi- ciently powerful to have any practical value, but in the following year S'ixii produced a machine of this character; and this may perhaps be regarded as the prototype from which the subsequent generators of this class have been evolved. Thermopile. — The thermopile, another source of electrical energy which has been more or less largely used in electro- metallurgical work, especially abroad, owes its origin to Seebeck's observations in 1822, that a current is produced by heating a compound bar of bismuth and copper at the junction of the metals, provided that the free ends are connected by a metalUc wire. EARLY EXPERIMENTS IN ELECTROLYSIS. 5 Ohm's Law. — In 1827, Ohm enunciated his great fundamental ^ law, which governs all electrical work, formulating, as it does, the relation between strength or volume of current, electrical ^pressure,' and the resistance of bodies to the passage of the current. Seven years later, Faraday demonstrated the relation between the strength of the current and the amount of any metal electrolytically deposited by it, and proved that the quantity of electricity flowing in a given circuit could be measured by the amount of metal which it could deposit in a known period of time. It is by the systematic and intelligent application of these laws that the electro-metallurgist of to-day is able to arrange his plant with scientific accuracy, instead of by mere rule of thumb. Copper-coating by Bessemer. — In 1831, Bessemer had coated .articles composed of an alloy of lead, tin, iron, and antimony with a film of copper by simple immersion in a solution of •copper salt ; but finding, as he describes in a letter published by Watt {Electro-deposition of Gold, Silver, etc., p. 60), that the metal was not adherent, he tried for, and obtained, better results by placing the objects on a copper, iron, or, better still, zinc tray, and then sinking them in the liquid. In this way he formed a small battery in situ, as we have seen Wollaston had done in 1801. Becquerel's Electrolysis Works. — Becquerel, in 1836, was the first actually to apply the principles of electrolysis to the treat- ment of natural products for the recovery of the metal contained in them. He even planned out works upon a commercial scale for the treatment of complex minerals containing copper and silver, but they were never erected, owing to the prohibitive expenditure of battery zinc involved in the process. Dani ell-Battery. — In the same year a new era was started by Daniell's introduction of his two-fluid battery, which placed a Tery constant current at the disposal of the electro-metallurgist, .and almost immediately produced a ripe harvest of results. De la Rue at once took the first unconscious step in the direction of electrotyping, when he observed that the copper, which is de- posited in the cells of the Daniell battery whilst in use, exactly reproduces upon its surface evelry line or scratch upon the copper plate on which it forms. Intent, however, on other objects, he failed to follow up the Ime of research thus indicated. Elkington's Process. — In 1838, the Patent Office Records show that Elkington, who had two years previously protected a process of gilding fox copper or brass objects by simple immersion in a solution containing gold, produced a method of zinc plating, 6 INTRODUCTORY AND HISTORICAL. analogous in principle to that of Wollaston's, by which the copper, brass, or iron to be coated was immersed in contact with a more oxidisable metal in a solution of that which it was desired to deposit, thus forming a galvanic cell in the depositing bath itself ; and so for the first time the deposition of one metal upon another, through the galvanic action produced by the solu tion of a third, became the subject of a patent specification. Earliest Electrotypers— Their Eivar Claims. — De la Rue, as. we have just seen, had already, in 1836, indicated the possibility of copying uneven surfaces by electro-deposition, but had missed the practical application of his discovery. But three years later three individuals almost simultaneously, and it would seem quite independently, publicly described processes of electro- typing. These three were Jacobi of St Petersburg, and Spencer and Jordan in England. The tale of these rival inventors has. often been told ; it is briefly as follows : — Professor Jacobi published a method of converting into relief, by galvanic means,, even the finest lines engraved upon a copper plate, thus pro- ducing a printing surface suitable to the requirements of the printer. An account of this process found its way into the pages of the Athenceum on May 4, 1839. On the 8th of May following. Spencer gave notice to the Liverpool Polytechnie Institution of his intention to read a paper before that body on the ' electrotype process ' ; but this paper was not read until September of the same year. Meanwhile, however, the account of Jacobi's discovery had been copied into the London Mechanic's^ Magazine^ and had called forth a letter from Jordan, dated May 22nd, but not published until June 8th, in which he described his experiments in the same field, which were begun in the summer of 1838. He clearly set forth here the method which has since been known as the single-cell process of electro- typing, and claimed the possibihty of multiplying engraved plates, typographical matter or medals, by forming galvano-plastic matrices on the object, and using the 'negative' copy thus ob- tained to reproduce the original form ; and he even suggested making tubes by depositing copper around a wire or metallic core which could subsequently be removed. Strangely enough, neither of these accounts received public attention, and the matter remained unnoticed until the end of September, when Spencer's paper was read. This paper is especially interesting, because it shows how the process of electrotyping was gradually developed in his hands, mainly by an attentive and patient examination into the causes of a series, of apparently minor phenomena observed in September 1837, spencer's eeseakches in electkotyping. 7 while experimenting with a single voltaic cell, consisting of a copper plate in copper sulphate solution connected by copper wire to a zinc plate immersed in a solution of common salt. The starting-point on the road to the new discovery was the observation that certain spots on the copper plate, which had accidentally been overlaid with molten sealmg-wax, received no metallic deposit when placed in the cell ; and he was thence led to attempt the formation of designs in relief, for use m the printing press, by coating a copper plate with an insulating varnish and then tracing the desired pattern by scratching the varnish completely away at the required points, and finally building up a deposit of copper upon the portions of the metallic plate thus exposed. While experimenting in this direction, he made the important observation that the nature of the deposited copper was dependent on the degree of 'intensity of the electro- chemical action,' dr, as we should say, on the strength of the current, strong currents giving rapidly deposited but highly crystalline and friable metal. He now met with difficulties, which however, were not insurmountable, and even led to further triumphs ; he found that the deposited copper would adhere perfectly only to an absolutely clean surface of the same metal Thus, when he required to obtain perfect adhesion between the metals, he first cleaned the copper surface with nitric acid, whereas if he wished afterwards to separate the deposited metal from the original plate he coated the latter with the thinnest possible film of bees' wax, previous to exposing it in the battery-cell. The subsequent apphcation of a gentle heat enabled the two plates to be separated with the greatest facility. Next, when using a penny-piece instead of a copper plate, he observed that the inner surface of the copper sheath with which it had been coated bore a perfectly sharp copy, in tntaglio, of the image and letters which were in relief on the coin itself ; in this way he obtained matrices from which the original could be faithfully reproduced. Now, ascertaining that copper would deposit as readily upon lead as upon itself, he secured exact copies of coins, of set-up type, or even of wood-blocks, by pressing them upon sheets of lead and depositing copper upon the indented lead matrix so prepared. And finally he found that clay plaster of Paris, wood, or other non-conducting materials could be covered electrolytically with copper, if they were first coated with a conductive film of bronze-powder or gold-leaf. It would appear hopeless to determine the question of real priority between the three inventors. Jacobi seems to have been the first to publish an account of his researches, and so 8 INTRODUCTORY AND HISTORICAL. far his claim is good; Spencer next declared his intention to describe his experiments, but was forestalled by Jordan; on the other hand, Spencer claims to be the earhest experimenter in the field, and his investigations appear to have been deeper and more fully developed than those of either Jacobi or Jordan. It is doubtless one of those frequently recurring instances, wherein the progress of knowledge has led several men to a simultaneous but independent development of the same line of thought ; and in such cases credit must be ascribed to each, but the palm awarded to the most thorough and painstaking. Murray's Blackleading Process. — The immediate result of Spencer's paper was the creation of a sudden mania for electro- typing, the simple and inexpensive character of the necessary apparatus enabling amateurs of all grades to vie with the fresh race of operatives which sprung up at the birth of a new industry. Evidence of this is to be seen in the rapidly increasing number of patents which were now apphed for in this branch of the Arts. With so many workers in the same field, it would indeed be strange if the scope of the work were not quickly and widely enlarged, and existing processes much improved ; the year 1840 was accordingly destined to see many improvements effected. The application of the art to the requirements of the printer was in this year made practi- cable by Murray's discovery, that moulds of non-conducting material could be made to take the deposit of copper by brushing them over with plumbago, so that metalHc moulds were no longer essential. In the same year the first-pubhshed news- paper print from an electrotype block is beheved by Smee to have appeared in the London Journal, Nevertheless, ^ Savage's Dictionary of Printing, which appeared in the following year, although it contained many good engravings from electrotypes, exhibited a page of 'diamond' tj^pe, also printed from an electro-deposited block, but this was so imperfect that, as Wilson has suggested, we may infer that the art of electrotyping formes of small type had not yet attained sufficient excellence to warrant its general application to this purpose. In 1840, Mason endeavoured to utihse the current generated by the single-cell electrotyping arrangement in a second depositing cell, and thus to carry on two operations simultaneously; and although this method was not practically adopted, it neverthe- less pointed to the possibility of applying a separate current from sources other than Daniell's battery. Cyanide Baths.— In this year Wright, after experimenting with many solutions, discovered the use of the cyanide bath for LATER PROGRESS IN THE ART. 9 the production of thick deposits of gold and silver, in place of the thin films obtainable by simple immersion. The invention Avas patented and at once put in operation by the Messrs Elkington, who were foremost in this field at the time. At the 'end of the same year, de Ruolz patented in France the use of similar solutions, not only for gold and silver, but for platinum, .copper, lead, tin, cobalt, nickel, and zinc. The following year witnessed the publication of a very complete work on electro- metallurgy by Smee, and this was followed by several others in rapid succession. Elastic Moulds. — Leeson, in 1842, greatly advanced the appli- cation of electrotyping to the reproduction of works of art by the use of elastic moulds made of glue and gum, which thus enabled objects of intricate or undercut design to be faithfully copied and indefinitely multiplied; and by the insertion of leading wires in the mould to distribute the current more uniformly, and hence, to facilitate a higher degree of simultaneity in the deposition. In the following year Montgomery proposed the application of gutta-percha as a moulding medium for slightly undercut objects. Bright Silver. — From that time the inventions for many years, although numerous enough, had not sufiicient novelty to render them worth recording in detail, excepting, perhaps, the important discovery by Mil ward, in 1847, that the addition of a small quantity of carbon bisulphide to the silver plating baths •caused the deposited silver to show no longer a dead or frosted surface, but to exhibit greater lustre and brilliancy. Cheap Sources of Electricity. — In 1842 and 1843 respectively, Woolrich patented the use of magneto-electric machines, and Poole, that of thermo-electric piles, for depositing metals; but neither of these seem to have been at the time successfully ^applied in practice. But the introduction of Pacinotti's dynamo- electric machine, first described by him in II Nuovo Cimento in 1864, of Wilde's magneto-electric machines in the following year, and of Siemens' and Wheatstone's more perfect dynamos, simultaneously invented in 1867, profoundly modified the scope of the art by affording a far cheaper source of electricity than had hitherto been possible. From this time, with the more careful study of the theory of the dynamo, and the consequent improvements in its mechanical and electrical efficiency, there have arisen a host of new machines, constructed especially to satisfy certain * specific objects, and approaching much nearer to perfection than did their original progenitors. Dynamo-electric machines are now made to suit the needs of the electro- 10 , INTEODUCTOEY AND HISTOEICAL. metallurgist, and thus new fields of labour have been opened, more particularly in the domain of metal refining and smeltmg; and the readiness with which mechanical energy may now be converted into electrical, renders the utilisation of natural waste water-power thoroughly applicable to these purposes. Ore-Treatment by Electricity.— The later history of our sub- iect is, in its more important branches, intimately associated with the application of dynamo-electric machinery, and ot powerful currents to the treatment of ores, furnace-products, or solutions. Becquerel, as we have seen, failed practically to apply his process for the treatment of ores on account ot the expense of the zinc; it therefore remained dormant until 18bb, when it was tried in San Francisco by Wolfe and Pioche, who seem, however, to have eff-ected but little. Marchese s method of treating copper mattes (impure fused copper sulphide), by using them as anodes, started a new epoch m 1882, and many modifications of this and analogous processes have since been carried into practice. How far this type of ore-treatment may be able to compete with the older smelting methods, can only be considered in connection with the particular circumstances ot each individual case, and will be more fully dealt with later m Up to the present time, the direct treatment of ores and mattes by electrolysis has not proved successful, chiefly on account ot the large proportion of sulphur and other insoluble material which has to be separated. The difficulties, however, are probably not in- superable, inasmuch as they are industrial rather than scientific. The case, indeed, forms an apt illustration of the necessity to make long and careful experiments on a fairly large scale before embark- ing in a costly electro-metallurgical enterprise, which must stand or fall by its financial possibilities. The electro ytic extraction of the copper from matte is quite possible, and may be efi-ected readily in the laboratory; and it may proceed satisfactorily even on the large scale for a time, but on prolonged use the processes which have been tried have usually been found to be too costly to allow of their competing with the improved methods introduced bv modern chemical or metallurgical science. It is theretore customary to prepare an impure copper by metallurgical means, and then to refine this copper electrolytically. . . Metal-Eefining.— The refining of copper by electi^lysis is now- one of the most important applications of electro-metallurgy, it is practically, however, a modification of the process of electro- typing, and the two arts, therefore, up to a certam pomt, have a common history. The earUest process of practical importance ELECTKOLYTIC REFINING. 11 was that of Elkington, patented in 1865. In principle it is the same as that used at the present tirne, and is specially interesting as being the first to employ the dynamo for the purpose. Among those who have applied this process on a large scale are Siemens and Halske and Borchers in Germany, and Thofehrn in America ; and the magnitude of the industry may be gauged by the fact that in one works alone, that of the Anaconda Company in the United States, the plant is capable of generating over 2500 horse- power, and producing 150 tons of refined copper per diem. The process is now being largely developed in the direction of increas- ing the rate of deposition. The electrolytic refining of other metals has been attempted with varying success, but in few cases has it succeeded in displacing the older methods. The Moebius process for treating impure silver in an electrolyte of silver nitrate has been largely employed in America and Germany. The commercial refining of nickel by electrolysis is being conducted on a relatively small scale, and the same may be said of zinc, but the inherent difficulties in the electrolysis of the last-named metal have prevented any very great development up to the present time. Gold has been deposited from solutions obtained by the action of dilute potassium cyanide liquors on gold ores, and this process, the electrolytic part of which is due to Siemens and Halske, is now a recognised method of gold recovery largely used for the treat- ment of poor ores and * tailings ' (ores which have already been treated by another process). But in no case has the electric refining of metals been so successful as in the case of copper. This, no doubt, is largely due to several contributory causes. In the first place, the solution required is simple, cheap, and easily managed; secondly, the precious metals contained in the copper are recovered practically completely as by-products ; thirdly, the rate of deposition is very rapid, so that the amount of capital lying idle is small ; fourthly, the copper may be deposited in a reguline condition and almost absolutely pure; and, lastly, there is a very great and ever- increasing demand for pure copper for conductors for electrical purposes. In addition to these a negative cause may perhaps be found in the relative difficulty and expense involved in obtaining equally pure copper by metallurgical means. The industrial questions in connection with these processes will be touched upon in Chapter XV. Electrolysis- of Fused Substances. — In another direction, electricity has been applied to the extraction of metals by the passage of a current through a fused salt ; it was in this way, in 12 INTEODUCTORY AND HISTORICAL. 1854, that. Bunsen and Deville reduced aluminium from its combination with chlorine, and recently many arrangements purporting to effect a similar result have appeared in the records of the Patent Office. Electrolytic processes have entirely replaced the older metallurgical methods of extracting aluminium, with the result that the price of the metal has fallen in ten years from £1 to Is. 6d. per pound. The processes 'of Hall (patented in 1886), Heroult (1887), and others, which are now operating so successfully in this direction, all consist in passing a current of electricity through a bath of a fused salt (usually a double fluoride of aluminium and an alkali metal), in which pure oxide of aluminium is dissolved. The bath is maintained at a red heat without external firing by the passage of the electric current through it, and the alumina is decomposed into aluminium, which is collected, and oxygen, which combines with the carbon block through which the current is passed into the bath, forming gaseous oxides of carbon which escape into the air. The alumina is replaced from time to time as it is decomposed, so that the process is strictly analogous to the electrolysis of copper sulphate dissolved in water. In the same way metallic sodium is now almost entirely prepared by electro- lysis, Castner's process (patented in 1890) for the electrolysis of fused caustic soda being largely employed in Great Britain, America and Germany, whilst other processes for the electrolysis of melted sodium chloride are also in use. A proposal by Pichou, in 1853, to reduce an ore admixed with a small percentage of charcoal in the electric arc passing between two large electrodes, found a later development in the electric furnace of the brothers Cowles, patented in 1885, for the treatment of ores containing aluminium and other metals. Latest Advances. — Among the more interesting modern developments in the field of electro-metallurgy are those in which electricity is employed merely as a heating agent, and not in electrolysis. One of the earliest experiments on a small scale in this direction was made by Depretz in 1849 ; Pichou used his electric furnace for smelting in 1853, and Siemens elaborated a furnace which was used on a comparatively large experimental scale in 1880. In all these the heat of the electric arc was used. Another type of furnace in which the heat was obtained by the incandescence of a rod of carbon, due to the resistance interposed by it in the circuit of a powerful electric current, was successfully introduced by Borchers in 1880. Since then, the electric furnace has been developed in two directions, first in pure scientific work, where, in the hands of Siemens, Moissan and others, it has been the means of melting metals hitherto regarded as infusible, and DEVELOPMENTS OF THEORY. 13 of studying the properties of such materials after fusion and solidification. In the other direction it has been extended in size and capacity, and has become a valuable industrial agent, as, for example, in the manufacture of calcium carbide for use in acety- lene generators and as a metallurgical reagent, and in the pro- duction of phosphorus. [N'early allied to this is the use of the current for electric welding and for the local annealing of hard steel plates and the like. Here the metal is raised to a welding temperature mainly by the use of the arc, although in the Lagrange-Hoho and Burton electric forges the heat is largely generated by the passage of the current through a very high resistance, aided in the last- named case by the combustion of the hydrogen set free by the electrolysis of the water in which the pieces are immersed. Several systems of electric welding are now in use, and although it is scarcely likely that at present they can compete with the' smith's forge for ordinary work, they have found many applications for special purposes. The applications of electrolysis to the manufacture of alkali, bleaching powder, chlorate of potash, and in the field of organic chemistry are now increasingly numerous, but these belong rather to the sphere of electro-chemistry than to that of electro-metallurgy, and need not be treated of in this work. Eecent Developments of the Theory of Electrolysis. — In the theory of the subject great changes have been wrought of late years, and the theory evolved by Grotthus, in 1805, and universally accepted until 1887, has since then rapidly lost ground, and is now replaced by explanations based on the investigations of modern electro- chemists. The work of Yan't HofF on the osmotic pressure of substances dissolved in liquid solvents was published in 1887, and showed that, at least in dilute solutions, the dissolved material was subject to laws similar to those governing the pressure and volume of gases. This led to Arrhenius, in the same year, advanc- ing the theory of the dissociation of electrolytes, when in solution, into separate ions, each carrying a charge either of positive or of negative electricity ; and this theory with the earlier discovery by Hittorf (in 1853) of the actual movement, or migration, of the ions from one electrode to the other, forms the basis of the modern explanations of which a short elementary account is attempted in Chapter XX. Conclusion. — The growth of the art on the whole has been rapidly progressive ; a few processes may have been superseded by furnace-methods, which have proved to be less costly, but the various branches have for the most part steadily gained ground as 14 INTRODUCTORY AND HISTORICAL. the work became more reliable and more economically conducted, until the electrotyper and the electroplater in gold, silver, and nickel occupy a quite important position among the manufac- turers of the world; whilst the electro-refiner and the electro- smelter, if one may use the terms, have, in certain branches of industry, competed successfully with the chemist and metallurgist, and in some few cases have even obtained a monopoly of the work. CHAPTER II. THEORETICAL AND GENERAL. {See also Chapter XX.) It has already been hinted that a right understanding of all the problems involved in the science of electro-metallurgy de- mands an acquaintance, not only with the manner in which certain forces act upon matter, but with the constitution of uiatter itself. A brief review, therefore, of a few of the fun- damental laws and theories of electrical and chemical science naturally finds a place at this point; although for a full ex- planation of these subjects reference must, of course, be made to the text-books devoted specially to them. Matter — Force. — It must be clearly understood, then, that matter (that is, anything which possesses masi) is variously con- stituted. It is within our common experience that different kinds of matter exhibit different properties and characteristics, or, as we say, are made of different materials ; and it is the object of chemical science to teach us what the materials are and how they behave when brought into contact with one another. Force has no material constitution, and therefore no weight, and is only made known to us by its action on matter. Physical (or mechanical) forces, as they are termed, may affect the relative position or the outward shape and appearance of material substances, but chemical forces affect the very in- gredients of which the substance is composed, and govern the more intimate mutual relationship between different bodies. Conditions of Matter. — We are conscious that various kinds of matter may exist at different times in certain distinct forms —solid, liquid, and gaseous — but these are solely physical, not chemical, differences (the constitution or component parts of the substance remaining unchanged throughout), and are broughj} about by physical means, such as alteration of heat or pressure, so that a return to the original conditions is accompanied by a reproduction of the body in its first form. For example — water 16 THEORETICAL AND GENERAL. at 15° C. is a liquid, but cooled to 0° C. it becomes solid ice, or heated to 100° C. it is converted into gaseous steam ; nevertheless, if the ice and the steam be respectively brought back to 15° C. they will again form a liquid quite undistinguishable from that originally experimented with. We may imagine, then, that water is made up of a vast number of almost infinitesimal particles, all of which are alike and are rapidly vibrating to and fro ; and that in ice they are so packed together with shorter paths of vibration that they will not readily separate, thus causing solidity ; but that when heat is applied to them, each particle vibrates through a- longer distance, and the difi'erent units are farther apart and more free to move among themselves, so that they present the character- istics of a liquid ; while above the boiling point the freedom is so- great that they are actually carried away as a gas. Constitution of Matter. — If now we imagine these minute similar particles to be so small that further sub-division by physical means is impossible, we are figuring to ourselves those penultimate particles which, in the language of the atomic theory, are termed molecules. It is with the molecule that the physicist has to deal, and it is on the molecule that physical forces act ; but the chemist is able to break up each molecule intO' a certain limited number of smaller particles, which are supposed by this theory to be indivisible, and are hence called atoms- (a = not, temno = l cut). Any molecule may contain two or a^ larger number of these atoms, and the atoms themselves may be similar or dissimilar; there are, indeed, two classes of bodies, the first of which includes those molecules in which all the atoms are alike, the number of substances in this class being, of course, equal to the number of difi'erent kinds of atoms in exist- ence, while the other group includes those whose molecules are made up of unlike atoms, and the number of these bodies is almost unlimited, because of the endless combinations possible between different varieties of atoms. In the first class both molecules and atoms are all ahke, so that such a substance can contain but one kind of matter, and is hence termed an element. In the other group, although the molecules of any substance are similar, the atoms are not ; but each atom being indivisible, and consisting of one body only, is an element, and, therefore, the substance is said to be a compound of such and such elements. Thus, if a molecule of water could be made to yield its atoms, it would be found to contain two of a gaseous element, hydrogen, and one of another gas, oxygen, each quite unlike water, and unlike the other; and if two molecules could be so treated at the same time, the two liberated oxygen atoms would, if kept apart^ CHEMICAL COMBINATION. 17 from the hydrogen, unite to form a molecule of oxygen, while the four hydrogen atoms would form two molecules of hydrogen. Atomic Weight. — Now, if it were possible to isolate and weigh a number of atoms, it would be found that all those of the same nature would possess equal weight, but that diverse atoms would have unlike weights ; and for all chemical and electrolytical calculations this must be thoroughly understood. But although the actual weighing of an atom is still an impossible feat, yet by studying the mutual relations of the elements the chemist is able to estimate the relative, though not the absolute, weights of different atoms. Chemical Symbols and Formulae. — Hydrogen, which is the lightest substance known, being regarded as unity, the atomic weight of each element is expressed as a multiple of that of hydrogen. Thus, if an atom of hydrogen be regarded as weigh- ing 1, it is found that an atom of oxygen will weigh 16. Now, as each molecule of water has been shown to consist of two atoms of hydrogen combined with one of oxygen, it is evident that it must contain 2 parts by weight of the former with 16 parts of the latter ; and as all molecules of water are alike in composition, it follows that in every 18 parts by weight of pure water there are 2 of hydrogen and 16 of oxygen. It should be clearly remembered that every true compound, no matter how it is produced, not only contains always the same elements, but contains them in the same proportion. Provided, then, that we are able to determine the nature of the different atoms present in a molecule of any substance, and the propor- tion in which they are there, we can calculate with precision the percentage of each element contained in it ; and, conversely, if we know the proportionate weights of the constituents of a given compound, we can at once determine the number of each kind of atom in the molecule. Hence it is possible to assign a definite chemical formula to every compoimd ; that of water might be written *2 hydrogen : 1 oxygen,' implying that there are two atoms of the former to one of the latter ; but in chemical work to write down the names of the elements in full would be both tiresome and clumsy, so that chemists are in the habit of using a system of shorthand notation, which is both simpler and more scientific. It consists in selecting a symbol, usually the first letter, or the first with some specially suggestive subsequent letter, of either the English or Latin name of the substance, to represent each element ; and this is understood to stand for, not an indefinite amount, but for 1 atom, and, therefore, so many known parts by weight of the element. Thus ' H ' implies 1 atom B 18 THEORETICAL AND GENERAL. or 1 part by weight of hydrogen; ' O,' 1 atom or 16 parts by weight of oxygen; Te' (Ferrum), 1 atom or 56 parts of iron; 'Sn' {Stannum), 1 atom or 118 parts of tin; and so on. By such a system of notation, then, we are able to express both the nature and the proportionate composition of any substance, and so, for example, to ascribe the formula to water. It should now be evident that elements can combine with others only in proportions which are multiples of their respective atomic weights, because no fraction of an atom can enter into the constitution of a molecule. But further than this, the propor- tionate numbers of each kind of atom in any molecule are no mere arbitrary or accidental figures, but are regulated by definite laws; and it is the fixedness of these laws which enables us to predict the exact weight of any metal Avhich should be deposited by a given current in a known time. Valency. — We have seen that water contains 1 atom of oxygen united with 2 of hydrogen ; but in hydrochloric acid (HCl) there is 1 of chlorine, not with 2, but with only 1 of hydrogen, while in ammonia (NH3) we find 1 of nitrogen with 3 of hydrogen, and in marsh gas (CH^) 1 of carbon requires 4 of hydrogen. Here we observe chlorine to be typical of a class of elements which combine with hydrogen in equal atomic proportion, oxygen typical of a class where the ratio is 1 : 2, nitrogen of one where it is 1 : 3, and carbon of one where it is 1 : 4. The terms monovalent, divalent, trivalent, and tetravalent are applied to the elements included in these classes respectively. The words monatomic, diatomic, etc., are sometimes used, but those indicating valency are preferable. All elements, however, are not capable of combining with hydrogen ; and to determine the valency of these, it is simply necessary to ascertain how many atoms of hydrogen are replaced in any compound by the element in question. For example, common salt is a compound of the metal sodium (Na) with the gas chlorine in the atomic propor- tion of 1 : 1, the formula being IS^aCl. But 1 atom of chlorine combines with 1 atom of hydrogen in hydrochloric acid, and, therefore, 1 atom of sodium is equivalent to 1 atom of hydrogen, inasmuch as each requires 1 atom of chlorine to combine with it— thus, sodium, like hydrogen and chlorine, is monovalent. This equivalency is clearly seen by the following reaction: — Hydrochloric acid , , j 1 sodium ) j 1 chlorine \ common salt and (1 hydrogen). or putting it in the form of an equation, HCl + Na = NaCl-rH. CHEMICAL COMBINATION. 19 Again, in oxide of sodium, l^a^O, 1 atom of oxygen combines with 2 of the metal, just as it does with 2 of hydrogen ; and by throwing metallic sodium into water, the latter is decomposed and part of the hydrogen is liberated as a gas, while an equivalent of sodium takes its place to form caustic soda. This exchange is thus represented : — Na + OH2 = NaOH + H 1 sodium added to 1 water yields 1 caustic soda and 1 hydrogen. In both these instances also, therefore, we find evidence that sodium replaces an equal number of hydrogen atoms, and is monovalent. The atom of the metal zinc (Zn) replaces 2 hydrogen atoms, as is seen in the following equations representing the action of hydrochloric acid and of water respectively upon metallic zinc : — Zn + H2O = ZnO + H2 1 zinc added to 1 water yields 1 zinc oxide and 2 hydrogen. Zn + 2HC1 = ZnCls + 1 zinc added to 2 hydrochloric acid yields 1 zinc chloride and 2 hydrogen. Zinc is therefore a divalent element. Similarly, aluminium may be shown to be trivalent, 1 of the metal replacing 3 of hydrogen ; or 2 of the metal replacing 6 of hydrogen as indicated — 6HCl + 2Al = Al2Cl6 + 3Ho 3H20 + 2Al = Al203 +3H2. Other elements are tetravalent, pentavalent or hexavalent, according as they replace 4, 5 or 6 atoms of hydrogen in com- pounds. To save the repetition of the full term monovalent or divalent elements, etc., it is often more convenient to classify the elements as monads^ dyads, triads, tetrads, pentads, or hexads. Now it sometimes happens that a single element may form two classes of compounds, in which the valency of the element is different ; but these classes are quite distinct from one another, for although they are interconvertible, yet they do not * In writing equations, a figure written before a group of symbols means that all the elements symbolised up to the first stop are to be multiplied by the number represented by that figure ; while a small figure below the line, to^ the right of a symbol, describes the number of those atoms to be taken into consideration. 2HC1 means 2 molecules of hydrochloric acid (2 atoms of hydrogen and 2 of chlorine). HgO means 2 atoms of H with 1 of 0 ; while SHjO means 3 molecules of water containing 6 atoms of H and 3 of 0. 20 THEOKETICAL AND GENERAL, arbitrarily pass over from the one to the other, but tend to remain separate, even, perhaps, through a long cycle of chemical changes; but one or other of these classes is generally more stable than the other — that is, less liable to change — and is, therefore, the one more commonly met with. Thus, copper (Gu = cu2)rwn) forms the more usual group of compounds in which it replaces 2 of hydrogen (CuO = cupric"^ oxide, CuCl2 = cupric chloride) and is generally recognised as a dyad, but it may, under certain circumstances, behave as a monad and form the group of compounds of which cuprous"^ oxide = Cu20, and cuprous chloride = CU2CI2, are typical members. To express, the valency of an element, it is therefore necessary to know with which class of compounds we are dealing. Elements. — In the following table will be found the names of the elements at present known, together with their symbols and valencies, their atomic weights, and their equivalent w^eights. By the latter term is meant the number of parts by weight of an element which are required to take the place of 1 part of hydrogen, or of 1 equivalent of any other element ; and these numbers are evidently found by dividing the atomic weight (i.e., the weight of an atom) of the element by the number of hydrogen atoms which are equivalent to it. Thus, when any element interchanges with another in a compound, it is always in the proportion of a multiple of their equivalent weights. In this table, the names of the more common metallic elements and of those metals which, as such or in compounds, are most largely used in electro-metallurgy, are printed in small capitals, while the non-metallic elements are distinguished by italics. Heat-evolution of Combinations. — We have used the term chemical forces in the earlier part of this chapter, and we must now devote a short time to the observation of the comparative effects produced by various chemical changes and combinations. It must first be understood that when any chemical combina- tion occurs, heat is usually evolved, owing to the conversion of chemical energy into its equivalent of heat-energy; and that just as given substances always combine in perfectly definite proportions, so each of these combinations is attended by the evolution of a perfectly constant amount of heat-energy. The * "When an element forms two classes of compounds, that class in which, the greatest proportion of oxygen or other equivalent substance is combined with it is distinguished by the suffix -ic attached to the name of the element, while the other class takes the termination -ous. When the name- alone is used, the more common group is usually understood — for example^, copper oxide would imply cupric oxide, CuO. CHEMICAL COMBINATION. oolpvpcp(^op?p(^^c<^^Hcx)oo o4wvO'ti6:)OOoocot^cJOi'^ oo5lOo:>r-HOiI-^ocoascooooo^^c^o<^^cccooo(^^o^— ii-Hu::)00'?t» w o S o HPS O 03 5^ OOOipO:pOOOcpoovpipvpi^OC0OOl;^OO 050ocoocoococooco!Oiob^a5'-^ooii?asi^:)-rti»OrHt^t>^ •^(M^OW COvOCOC> CO a aod acoT-i;Mr-<(MT*-ce OF CEETAix Metals. ame of MetaL i Aathorities. j Xame of Metal. 3C O < Oilv CAj . • • X V V \J R D L P W 1 Antimony, • • 4-2 lOO'O E D L 0 P W XJ. X>'. XJj -jx, vy, X J T T • Mercury P Af P Xj, -JX, X • S0"6 "R D L 0 P W jj . xy • lj . -jx , V/ J X , If. Bismuth. 1 '9 X ^ \r /A 1 II 111 1 11 1 Lllllj . Graphite 0*07 .JLL. OOUJ.U.III. t • 87 -4 0 1 .UX 1 1 Alloys, d'C ZjUH^j • . • Cu with. Si, AV 23*7 M Cu J J 12,, Si, AV. ' "Pn'f'aQsiiTTm X U LCaOOi.U.XLl J • 20*8 M. Cu ; » 9„ P, AV Platinum, 16-7 B,D,L,M,0,P,W.i 1 Cn J ; 10 „ Pb, 30-0 AV. Palladium. . 16'4 D. Cu J J 10„ Al, 12*6 AV. Iron, . . . 16-4 B,D,L,M,0,P,W. Cu t ) 10„ As, 9-1 AV. Tin, . . . 15-2 B,M,0,W. Cu > J 20 „ Sn, S'4 AV. Thallium, 9-2 M. Cu > > 35 „ Zn, 21-1 A\'. ] Lead, . . . 8-8 B,M.O.W. Cu } J 60 „ Ag, 86-6 A7. Nickel, . . 1 i 1 Au ; ? 50 „ Ag, 16-1 AV. ' Arsenic, . . ^ 4-8 M. ' ' Sn J J 12„ Xa, 46-9 AT. Physicists have obtained verT varied results in estimatins: the conductance of metals, probably owing to want of care in selecting perfectly pure specimens ; for, remembering that a mere trace of impurity is often sufficient to lower the conductance (or, in other words, to increase the resistance) of a metal by an alto- gether disproportionate amount, it is to be regretted that careful analyses of the samples operated upon were not made, and the nature and quantities of impurities published with the result of the physical observations. In this Table, whenever possible, the mean residts obtained by Becquerel, Davy, Lenz, Matthiessen, Ohm, Pouillet, and Weiller have been taken ; in many cases, how- ever, measurements have been made by only one or two of these, and in others a single result obtained by one of them is so divergent from the corresponding numbers quoted by the others ELECTROLYTIC CONDUCTION. 35 as to point to an error of observation, and to render its omission desirable in calculating the mean: for this reason we have in- dicated the sources from which the figures have been derived by the initial letters of the observers' names. The conductance of liquids is much lower, that of dilute sulphuric acid being 0-000133, and copper sulphate solution 0-00000542, that of silver being taken as 100; while pure water itself is a non-conductor. In passing through any substance, electric energy must always suffer loss, owing to partial trans- formation into heat in overcoming the resistance of the conductor, which is in its effect similar to that of friction on mechanical energy; hence, for conducting wires only that material which offers least resistance (or friction) to the path of the current should be used. Silver is, of course, placed out of the field on account of its costHness ; but copper, which is practically as good, is readily obtainable, and should, therefore, have the preference over all metals for this purpose. Electrolytic Conduction.— Up to this point we have been dealing with metallic conduction, which means that the electric energy is merely transmitted through a substance without pro- ducing any other effect than the conversion of a small proportion into heat, due to the resistance of the conductor. But there is another, or electrolytic conduction, which has only been observed to take place in compound liquids, and in this case the passage of the electric current from one particle to another is accompanied by a polarisation, or change in the condition of the liquid, which is only made apparent by the separation of the constituents of the electrolyte at the poles {i.e., the anode and cathode). This electrolytic conduction may occur in solutions of solid substances or in fused bodies, but it is essential that they shall be compounds (mercury is a Hquid conductor, but being an element it cannot be electrolysed and, therefore, conducts metallically), and that they shall be able to conduct the electricity. In electrolysing fused salts it may sometimes happen that the ions dissolve instantan- eously in the liquid and, diffusing through it, reunite as rapidly as they are separated; and thus to all appearances the fluid is conducting metallically rather than electrolytically. Fused alloys of metals appear actually to conduct metallically only, as up to the present no attempt to separate the constituents by electrolysis has proved successful, though it is of course possible that the re-diffusion of the separated metals just described may account for the failure of these attempts. The conditions, then, for electrolysis by the battery or dynamo are, that the solution, should be able to conduct electricity, and 36 THEOKETICAL AND GEXEKAL. that the electro-motive force of the battery used should be in excess of that set up between the separated ions ; and, further, that the ions, or at least the cations, should be able to exist as such in the electrolyte without bringing about secondary actions or decompositions. Deposition of Alloys. — It has elsewhere been stated that when a current is passed through a mixed solution, all the electro- positive ions appear to take part in conducting the current through the liquid, but that usually only the least electro-positive metal present is deposited, either because the others require a greater expenditure of energy, and are not precipitated at all, or because, if deposited, they exchange places with less electro- positive metals still in the electrolyte surjounding the cathode. Thus, in a solution of zinc and copper it is possible that both metals are deposited; but that the zinc instantaneously acts on the copper solution around and re-dissolves, whilst it precipi- tates at the same time an equivalent of copper in its place, so that only the latter metal is finally separated. Eut the deposi- tion of \he alloy may be understood in any of the following cases :— (1) If the current be so powerful that the secondary action of the more electro-positive metal on the solution of the other have not time to perfect itself. (2) If the particular solution used be of such a character that the more electro- positive metal, when it is separated, cannot chemically attack the solution of the more electro-negative, because the heats of formation of. the two salts are so nearly identical that the same electro-motive force is needed to deposit each. (3) If the proportion of the more electro-negative metal in the hquid be so small, compared with that of the other, that the solution around the cathode is exhausted of the former more rapidly than its replacement is possible through diffusion; so that the electro- positive metal, having no possibility of exchanging places with it, remains undissolved. Thus, from a mixed solution of copper and zinc, brass might be deposited if the current were so strong, and, therefore, the decomposition so rapid, that the separated zinc had not time to precipitate the copper around it, before it was protected by a further electro-deposit of copper ; or if such compounds were selected, that the copper salt could not be decomposed by metalhc zinc ; or if the quantity of zinc in the solution were so great as compared with the copper, that there was insufficient copper in the solution around the anode to take the place of the mass of zinc deposited. It should further be remarked that probably a small proportion of heat is generated in the alloying of certain metals, and that this would provide UNITS OF MEASUREMENT. 37 an additional tendency to deposit the alloy, sufficient, perhaps, to determine its production in certain cases. The comparative electro-positiveness of two metals in a given solution may be ascertained by connecting the metals by wires to corresponding slips of copper immersed in copper sulphate solution, and noting on which strip a further copper deposit is produced ; the slip on which the copper is precipitated will, of course, be that which is joined to the more positive metal ; or it may be determined by connecting the plate with a^ wire, and finding, with the aid of a galvanometer, in which direction the current flows, remembering that outside the solution it must always pass from the electro-negative to the positive metal. Units of Measurement. — Before closing this chapter we will refer briefly to some of the units of measurement employed in practical work. In dealing with electric currents there are two distinct factors to be taken into account — quantity and potential. The potential, we have already seen, is the ' pressure ' under which the current commences to flow, the quantity or intensity refer to the actual amount or * volume ' which passes in a given time, irrespective of pressure. And these terms, pressure and volume, perhaps explain better than any others the sense of the two words in question, inasmuch as they efl'ect a comparison with hydraulic power, which is a more tangible force, so to speak, and, therefore, better understood. The power of overcoming resistance increases with the potential in electrical as in hydraulic work ; thousands of cubic feet (volume) of water may be flowing in a given time through a large pipe with but a slight fall (potential), and yet but little force is required to check the flow ; while a few cubic inches conveyed along a small pipe, if only from a sufficient height, may sweep away every obstacle placed in the path and will thus cause a current to flow, even through a great resistance. But in electro-depositing, provided that there is just sufficient electro-motive force to overcome the resistance, the amount of work done is exactly proportional to the flow of electricity in a given time ; and this follows naturally from the laws of current-production. A given amount of zinc dissolved in the battery produces a given amount of heat, which is converted into electricity, and can then, in the electrolyte, only yield, at most, its equivalent of chemical energy. In reality the amount deposited is somewhat less than that theoretically predictable by reason of loss of electrical energy as heat due to the resistance of the circuit. The unit of electro-motive force (pressure), then, is the volt, 38 THEOEETICAL AND GENEEAL. and is approximately equal to the E.M.F. of a single Daniell's cell. The unit of resistance is the ohm, which is equal to that of a column of mercury 106 centimetres long by 1 square millimetre in cross-sectional area. The unit of current (strength) is the ampere, and is that produced by a potential of 1 volt acting 'through 1 ohm. In scientific work the metrical system of weights and measures is adopted. The unit of weight is the gramme ( = 15*432 grains); ten, hundred, and a thousand grammes being termed dekagramme, hectogramme, and kilogramme ; and yV^h, y^th, and yoVo^^ being denominated decigramme, centigramme, and milligramme, respectively. The unit of length is the metre (= 39*37 inches), and the multiples and fractions have the same Greek and Latin prefixes respectively as those of the unit of weight. In cubic measurement a special term litre is used to denote the cubic decimetre, or 1000 cubic centimetres ( = 1*76 pints). The unit of heat is the calorie, and represents- the amount of heat-energy which must be expended in order to raise the temperature of 1 gramme of cold water 1° centigrade : some authorities use a kilogramme-degree unit, but the gramme-degree system is more general, and is the one adopted in this work, whenever it may be necessary to use it. Other units are used in different countries, as, for example, that of the pound- Fahrenheit-degree, intended to adapt itself to the English systems of measurement. The * pressure of heat ' or temperature used by preference in scientific work is measured by the Centigrade scale which denomin- ates the freezing point of water zero, and the boiling point 100°, the space between these points being divided into 100 equal degrees. In the Fahrenheit scale, largely used in England, the freezing point is 32°, the boiling point 212°, the space between them having 180 degrees. In the Reaumur system, adopted widely on the Continent, the freezing point is zero, and the boiling point 80°. The first two of these systems alone will be referred to in this book, and on page 408 will be found a Table of Comparison of Temperatures recorded by each. In addition to these units there are several of practical utility which may well be mentioned. The electrical unit of quantity is the coulomb ; it is the volume of current equal to that of 1 ampere passing through a circuit for one second of time. UNITS OF MEASUREMENT. 39 A current of 1 ampere at the pressure of 1 volt is termed a luatt ; it is useful for comparing different currents, and is really the product of volume into pressure. The English horse-jpower (H.P.) is taken at 550 foot-pounds per second, and is thus equivalent to raising 550 pounds through one foot, or one pound through 550 feet in a second. (The French H.P. is 542*48 foot-pounds per second.) The British H.P. is equivalent to 746 watts; or, in other words, 1 kilowatt is equal to about 1^ H.P. Finally to connect heat-energy with mechanical work. Joule found that 1 calorie (gramme) is equal to 0*424 kilogramme- metre or 3*066 foot-pounds — i.e., 1 calorie liberated per second is equal to 0*00557 H.P. The term current density is used in electrolysis to represent the number of units of electricity per unit surface of electrode. In England this is sometimes expressed in amperes per sq. ft., some- times in amperes per sq. in., of anode (or cathode) surface exposed to the current. On the Continent it is more often quoted in terms of amperes per sq. decimetre. In electrolytic analysis amperes per sq. centimetre are commonly quoted, and the symbol K.D.^qq is often used to express this: thus a current N.D.ioo = 0*5 would mean a current density of 0*5 amperes per sq. centimetre. The work expended in raising the potential of a coulomb of electricity one volt (i.e. a coulomb x a volt) is termed a joule. The watt, which is a very useful unit, represents activity, or the rate of doing electrical work, and is equivalent to one joule per second, or a current of 1 ampere at a pressure of 1 volt. This unit being rather small for practical purposes, a unit of 1000 watts, termed a kiloioatt, is commonly employed ; and a current of 1000 watts passing for an hour, known as a Jciloioatt-hour, is the unit adopted by the Board of Trade for Electricity Supply. A kilowatt-hour may obviously be a current of 1000 amperes at 1 volt passing for an hour, or one of 1 ampere at 1000 volts, or of 20 amperes at 50 volts (or of any other combination provided that the number of amperes multiphed by the number of volts is 1000). passing for a like period. CHAPTER III.' SOURCES OP CURRENT. Relative Efficiency and Cheapness.— From what has been said in the last chapter, it will be apparent that some other form of energy must be converted into electrical energy in order to yield the current required for electro-plating; and we can now understand why the only electrical current originally available, which was generated from mechanical energy through friction between unlike bodies, although capable of exhibiting wonderful disruptive effects, owing to its high electro-motive force, was useless for electro-metallurgical work by reason of its deficiency in intensity— that is volume. Then with the invention of the battery, which aimed at the conversion of chemical into electrical energy, currents of low electro-motive force but great volume were producible, and these soon found an apphcation in plating and electrotyping. But in nearly all batteries zinc is used up, and as a given weight of zinc can only produce a given (in many instances not very different) weight of deposited metal, it is too costly to be used in some branches of electro-metallurgy. The dynamo-electric machine, which is able to convert mechanical energy directly into a current of high intensity but low electro- motive force, is now largely used, not only in this direction, but also as a substitute for battery-power generally ; because the com- bustion of the coal in the boiler of the steam engine takes the place of the oxidation of the zinc in the cell of the battery, and is a far cheaper agent. Nevertheless, as the steam engine utilises only a very small fraction of the total heat produced by the combustion of the coal, some means of directly converting heat into electricity is even now a desideratum ; at present, the thermo-electric-pile does indeed effect this object, but it is not economical, and is not used for the generation of large currents, nor is it extensively employed in any way. The manner of producing currents by the battery, the dynamo, and the thermopile will be shortly discussed in this chapter. THE GALVANIC BATTERY. 41 The Voltaic or Galvanic Battery {named after Volta and Galvani), Principle. — The principle of the galvanic battery has already l^een explained, namely : — That when any two metals are placed in a fluid which can attack at least one of them, and are con- nected by any conductor or set of conductors, an electric current is at once set up, which flows within the cell from the more positive metal to the other, and in the opposite direction through the conductors; and that the greater the distance between the two metals on the electro-chemical scale, the higher will be the electro-motive force of the current produced. Zinc being a €ommon metal, comparatively inexpensive, and, at the same time, very fairly electro-positive, is almost universally adopted as the positive element. The electro-negative element and the exciting fluid are, however, frequently varied, such modifications consti- tuting the chief differences between the types of cell commonly 'employed. Local Action on Zinc. — Whenever commercial zinc is used as the positive plate in an acid solution, it must be well amalgamated with mercury. Impure zinc dipped alone into acid evolves clouds of hydrogen gas from its surface ; but, if a copper plate connected to the zinc be immersed in the same liquid, all this hydrogen evolution should be transferred to the copper strip {vide p. 31) ; yet, as a matter of fact, much still continues to be formed upon the zinc. This is to be explained by the presence of minute specks on the surface of the zinc of more electro-negative bodies {e.g., lead and iron), which, being impurities in the body of the metal and, therefore, in contact with the mass of the zinc, and also exposed to the acid liquid, act (each for itself) as minute independent batteries, with the zinc as the dissolving element, so that from each of them hydrogen is continuously evolved. This local action, as it is termed, is objectionable, because it entails a loss of zinc without any equivalent of work done outside the battery, all the energy being expended in heating the materials of each of these minute and local circuits within the cell ; this, of course, represents so much waste. Further, the phenomenon is not merely temporary, for the electro-negative impurities, although perhaps almost invisible, are not attacked by the liquid ; and the gradual removal of the zinc by solution only results in laying bare a greater number of them, thus increasing rather than diminishing the extent of the local action during the progress of the discharge. The most obvious remedy 42 SOURCES OF CURRENT. is to use only the purest zinc, but this is costly ; while the pro- tection afforded to the metal of commerce by a thin coat of mercury is found to impart equal efficiency and to be more economical. To amalgamate the zincs, clean them well ; then with a piece of flannel or sponge tied to the end of a stick, wash them with dilute sulphuric acid (1 of acid to 15 or 20 of water) ; then pouring a few drops of mercury upon the centre of each, rub it cautiously over the plate until the whole surface presents a silvery brightness. Mercury will adhere only to perfectly clean metallic surfaces, hence the necessity for the preliminary washing with acid ; if, however, the zinc be very dirty or at all greasy, this treatment will not suffice, and it should be first immersed in warm caustic soda or potash solution. This will remove all grease ; then, after washing thoroughly in water, the plate should be flooded with the acid and rubbed with mercury as before. An excess of mercury is to be avoided, as it soon penetrates the zinc and renders it brittle. The plates should be often examined, and re-amalgamated as soon as dark spots are seen upon the surface^ or w^hen gas is evolved from them during the working of the battery. The acid should not be allow^ed to touch the hands more than necessary; and if it fall upon the clothes it should be at once neutralised by a drop of strong ammonia, or it w^ill in course of time produce a red stain, and finally destroy the cloth. Dry Pile. — Originally copper and zinc were used with dilute sulphuric acid, the earliest form of battery being the dry pile, in which copper and zinc discs, separated by moistened circular pieces of flannel (in the order — copper, flannel, zinc, copper, flannel, zinc, &c.), w^ere clamped together in a wide glass tube until the whole was filled ; wires were then soldered to the last disc at either end, and caps were fastened on to maintain the whole arrangement in position ; on connecting the end wires a current could at once be produced. But the volume of current thus obtainable was still very small, althovigh the electro-motive force might be comparatively great owing to the arrangement of the discs in continuous series, as will be explained later in the chapter. WoUaston's Copper-zinc Cell. — The simple copper-zinc cell was a distinct advance on the pile. It was made by placing a strip of copper in dilute sulphuric acid opposite to one of zinc, and connecting the two strips with the wire which was to complete the circuit. It was, in fact, the cell shown in fig. 3. But this battery, too, is very weak, owing to the low electro-motive force produced by these two metals immersed in dilute sul THE GALVANIC BATTEKY. 43 phuric acid; it is true that by placing several cells in series the potential is raised, hut this is neither convenient nor eco- nomical, and the original Wollaston cell is rarely, if ever, seen now. Polarisation. — One of the chief objections to cells of this class is that the hydrogen gas evolved at the copper plate chngs to it and cannot readily escape. This formation of hydrogen films on the surface of the negative strip is termed polarisation, and pre- sents a double disadvantage : first, the mere formation of a film of any gas upon the metal prevents the contact of its whole surface with the liquid, and, by interposing a layer of an insulating medium, reduces the surface area of the plate as much as if a portion of it were coated with an acid-resisting varnish; and in the second place, a far more serious difficulty is found in the tendency of the separating (positive) hydrogen to set up a cell on its own account, in co-operation with the zinc which now forms the negative plate ; and this secondary current will, of course, start from the hydrogen to the zinc in the Hquid, and from the zinc to the copper (and thence to the hydrogen in contact with it) outside — that is, it will be in the reverse direction to the original current, and by partially opposing and neutralising it, must necessarily reduce the resultant current from the battery. Thus a copper-zinc cell may for the first few seconds of use give a fairly strong current ; but as soon as the action has proceeded sufficiently far to give rise to a pro- duction of hydrogen on the copper plate, it becomes polarised, and the opposing electro-motive force which is set up greatly impairs the efficiency of the cell. Parts of Battery. — It must here be explained that in describing a battery, the more oxidisable or electro-positive metal (that, in fact, which by dissolving produces the current) is termed the positive plate ; but as the current outside the solution starts from the other plate, this second one is called the positive pole. In the copper-zinc cell, the zinc is the positive plate, but the wire attached to it is the negative pole ; and the copper, while it is termed the negative plate, becomes the positive pole outside the liquid. This is not likely to be forgotten if the facts be clearly grasped : — (1) That the plate from which the current starts, whether within the solution or without, is invariably designated positive ; (2) that the current always starts from the electro-positive element to pass through the intervening Hquid of the cell, and always, therefore, starts from the electro-negative element to pass through the wires and outside-connections of the battery ; and (3) that the starting and finishing points within the liquid are termed the positive or negative plates, metals, or elements, and the starting and finishing 44 SOUECES OF CUREENT. points outside the solution are called the poles. The whole path of the current is called the circuit — that in the cell being the internal^ and that outside the external portion; and any con- ductive connection between the two plates is said to complete the circuit. When the two plates themselves come into contact within the cell, or they become united, either in the interior of the cell or close to the poles, by any thick metal substance which presents practically no resistance to the current, the battery is said to be short-circuited. When this short-circuiting takes place in any way while the current is flowing through the ordinary circuit, which probably presents a much higher resistance, nearly the whole of the current is diverted from the desired path, and passes through the easier passage, thereby causing a cessation of work and a great waste of battery-zinc ; for when there are two possible paths open to a current, it will divide itself between the two in inverse proportion to their resistances. Thus let us suppose a current of 200 amperes to arrive at a point in the circuit at which a junction is reached, so that it may continue to flow to another point in the circuit, either by one channel with a resist- ance of 1 ohm, or by a second with a resistance of 99 ohms; the result will be that the path having 1 ohm resistance will carry 99 99 4- 1 current, or 198 amperes, while the other takes the remaining y^oth, or 2 amperes. Reduction of Polarisation —A large number of voltaic cells have been invented to meet the different requirements of practice, but for the most part with the object of producing either a con- stant current (that is, a current of uniform strength for a long time), or a current of high potential. In any case, polarisation must be minimised, and this is effected either by mechanical or by chemical means. The mechanical method consists in so arranging the cell that the hydrogen shall escape from the surface of the negative plate immediately it forms ; the chemical method seeks to prevent the separation of the hydrogen altogether, by means of substances which combine with it to form harmless compounds, or which cause the deposition of another suitable metal in its place. The latter or chemical system of overcoming polarisation has the added advantage that it increases the chemical activity in the cell, and thus adds to the electro-motive force. The substances employed for converting the hydrogen into a neutral substance are usually of an oxidising character and thus effect their object by the formation of water ; and this operation may be conducted either by immersing both plates in a single oxidising solution, or by dipping the negative metal only in the oxidant, which must THE GALVANIC BATTERY. 45 then be contained in a porous vessel placed within that holding the dilute acid and the positive element. The former class are termed single-fluid cells, the latter twofluid cells. Other arrange- ments are possible but are not largely used, and will not be dealt with here— for example, those with two liquids (in different com- partments) and one metal, or the gas battery of Grove. Smee's Cell.— Of single-fluid cells in which polarisation is- mechanically remedied, the most noteworthy is that invented by S7nee, In this battery a plate of silver is placed in dilute sul- phuric acid between two plates of zinc, but the silver plate is coated superficially with a very thin layer of platinum, so that the hydrogen may escape with comparative readiness from the shghtly roughened surface which is thus produced. But even with this^ device a certain amount of hydrogen must exist on the surface of the silver so long as the cell remains in action. The separation and escape of the gas, however, occur with fair regularity, so that although the electro-motive force rapidly decreases during the first few seconds of use, it then remains constant at a point con- siderably higher than it would if the zinc were opposed to a pure^ silver plate, and yet higher than if it were opposed to copper, on account of the greater electro-chemical difference between the two former metals than betwen the latter. But the mechani- cal depolariser is palliative only, not remedial. The annexed sketch of the Smee-cell (fig. 4), shown in section, illustrates its construction. The zinc plate, Z, is doubled, so that it may sur- round the silver on two sides and thus utilise both surfaces of the latter. It may be formed of two zinc plates mounted with the platinised silver between them in a wooden frame, which being a very feeble conductor may carry away a minute fraction of the current, but serves to hold the metals in position, so that a quite thin sheet of silver may be employed without fear of its bend- ing out of shape and making a short circuit. The zinc alone dissolves, and requires, therefore, to be initially of fair thickness; the silver plate may theoretically be only sufficiently stout to carry the current without presenting undue resistance, for the E.M.F. of a cell is independent of the dimen- sions of its elements. The loss of energy by conduction through the wood may be obviated by steeping the frame in melted paraffin, or by coating it with shellac varnish. The acid, which is prepared by carefully {vide p. 372) adding 1 part of strong sul- Fig. 4. — Smee- cell. 46 SOURCES OF CUKREXT. phuricacid with constant stirring to 12 parts of water, is contained in a glass or stoneware vessel of suitable shape. These vessels are generally rectangular, and may be made to hold one pair or any greater number of the zinc silver-plates, which are supported by resting the wooden framework upon the tops of the side walls of the acid trough. The Smee-cells are very weak, the electro-motive force of one being only about 0"-i7 volt ; and they evolve much hydrogen, the escape of which into the air in a constant succession of minute bubbles, each mechanically carrying with it a trace of sulphuric acid, causes a slightly unpleasant choking sensation in breathing the atmosphere of the room in which they are working. But, on the other hand, they are compact and very simple in con- struction, and if kept clean are not liable to become disordered ; while the escape of the hydrogen produces a peculiar hissing sound, which, to the practised ear, may indicate by its varying intensity any accidental derangement either in the cells them- selves or in the baths which they are electrolysing. It will cease altogether if the circuit is broken and the current has ceased to flow, or it will grow louder if a short circuit has formed at any point, and so diminished the resistance and increased the activity in the battery and caused a needless waste of current. This cell is still largely used by electrotypers on account of its regularity and simplicity when properly supervised. Darnell-Cell. — The form of battery invented by Professor Daniell is one which, by reason of its constancy, is well suited to electro-depositing work. In it hj'drogen is not deposited at all, but is made to displace metallic copper from its solution, by immersing the negative plate in a saturated solution of copper sulphate. If both plates were placed in this liquid, then, as soon as the circuit was broken and they remained isolated, the zinc being more electro-positive than copper would at once commence to exchange with that element contained in the solution, a part of the zinc forming soluble zinc sulphate, and an equivalent amount of copper precipitating in a pulverulent or spongy form on the surface of the remaining zinc. This film, being pervious to liquid, would still allow the copper solution to attack the zinc core, and bv 4ocal action' would even facilitate the exchanc^e of the metals. The battery would be rendered wasteful of zinc, and, indeed, useless. To obviate this, the copper solution with its electro-negative plate of copper is separated from the simple acid liquid containing the zinc by a porous partition of some kind, generally of baked but unglazed earthenware, which will permit diffusion, but not free interchange, of liquids through its pores. This is usually effected by placing the zinc in a porous vessel THE GALVANIC BATTERY. 47 within the outer cell containing the copper; thus, a 'two-fluid cell' is produced. The action which takes place may be expressed as follows — number 1 representing the condition of the cell when it is standing inactive, number 2 that when the circuit is com- pleted and a current is being generated : — 1. (-Plate) Cu, Cu.SO4.Cu.SO4 : : : Hg. SO4 . H^. SO4, Zn (-f- Plate). 2. Cu, Cu. SO4.CU. SO4.H2 ::: SO4.H2.SO4.Zn. In outer cell. Within porous cell. These changes or reactions may be described in the form of equations thus, 1. In porous cell : Zn + H2S04 = ZnS04 + H2. 2. In outer cell : H2 + CUSO4 = H2SO4 + Cu, or the ultimate reaction of the battery may be summed up in one equation, Zn + CuS04==ZnS04 + Cu. Thus, the hydrogen finds its way, as an ion, through the liquid in the pores of the inner cell-wall, exchanges places with the copper, and produces a practical transference of sulphuric acid from the outer to the inner cell ; and as the acid on the one side of the porous division becomes neutralised by zinc, the strength of the copper solution on the other side is rapidly diminished by the deposition of its metallic constituent. The complete exhaustion of either side would set a limit to the utility of the cell ; but long before this point is reached the power of the battery is much lessened. As it is chiefly essential that copper alone, and no hydrogen, should be separated, care must be taken to keep the liquid in the inner cell saturated with copper sulphate, by suspending crystals of the solid copper salt {blue vitriol) just below the surface of the liquid, so that the solu- tion may be replenished as fast as it is impoverished. V The usual form of the Daniell-cell is indicated in fig. 5. It consists of a cylindrical copper vessel holding a saturated solution of copper sulphate, and acts both as a containing vessel and as Fig. 5. — Daniell-cell. 48 SOURCES OF CUERENT. the negative plate of the battery. Around the top is a perforated copper, ring-shaped trough, T, which is below the level of the liquid, and is always kept filled with crystals of the copper salt. Within this vessel is the porous clay-cell, P, closed at the bottom but open above, and in this is the amalgamated zinc rod or plate, Z, immersed in dilute sulphuric acid ( 1 acid : 1 0 water), or some- times in a solution of zinc sulphate. Instead of using a copper external vessel, a thin sheet of rolled copper may be bent into cylindrical shape and placed in a stoneware or glass jar; the spare crystals are then suspended in muslin bags within the outer cell. The prime cost of this latter arrangement is less, but it must be remembered that in the former the copper is the negative element, and the cylinders are, therefore, in no way injured, but on the contrary are thickened by the gradual deposition of metal while the current is passing, and are not appreciably attacked by the dilute acid contained in them. Modified Daniell-Cells. — The arrangements for maintaining^ the supply of copper sulphate crystals are very numerous ; for example, Breguet and others have adopted a globular receptacle of glass, which is charged with the salt, filled up with water, and, the neck being closed by a cork with two perforations, is inverted above the inner porous cell, so that the bottom of the cork lies beneath the fluid level within the latter : then as the battery liquid deposits copper it becomes lighter, and floating upwards becomes displaced by the heavy saturated solution in the flask, while the poorer solution, which has found its way into the receiver, again becomes enriched, and is in turn ready to take the place of a further quantity of im- Fig. 6.— Daniell-cell poverished liquor. In this cell the copper is (Breguet's form). placed in the inner compartment, as in fig. 6 : the globe, G, is supported by the neck of the porous pot, while between the two is inserted a copper tape, C, which serves for the negative element. Around the porous cell is a bent cylinder of zinc in sulphuric acid ; the current is led away by the wires marked -t- and - attached to the copper and zinc respectively. Somewhat similar to this is the Meidinger- cell, in which a flask containing the crystals dips into a small cup containing the copper plate, resting on the bottom of a glass vessel, the diameter of which is enlarged to contain the zinc cylinder at a point somewhat above the upper portion of the cup. In Kuhlo's cell a copper vessel is used, which carries the TR^ GALVANIC BATTERY. 49 perforated trough outside instead of inside, and the zinc is enclosed in parchment paper instead of in a porous cell. Other modifications in shape may be made. In the pattern adopted by the Post Office Authorities, a long teak-wood trough, coated inside with some water-resisting varnish, such as marine glue, is divided into a number of compartments by alternate divisions of slate and porous earthenware. It contains the zinc a.nd dilute acid in divisions 1, 3, 5, and the remaining odd numbers; and the copper as copper sulphate in the even divisions, 2, 4, 6, etc. It is simply a conveniently compact and portable arrangement of a battery of several cells. Large Daniell- cells may be made horizontal instead of upright, and without porous divisions, provided they are allowed to rest undisturbed in a position where they will not be subjected to any con- siderable amount of vibration, the difference in the specific gravities of the solutions employed sufficing to keep them in their required relative positions. A simple form of such a gravity I)attery is shown in fig. 7. A wooden trough of convenient size, say 14 inches long by 10 wide and 3 high, is lined with insulating varnish, and provided with a glass partition, G, near to one end, which must reach to the top of the trough, but only to within a quarter of an inch of the bottom. On the bottom rests a large sheet of thin copper plate with a narrow strip attached to it, which, passing under the glass partition, is bent up within the smaller compartment to form the positive pole, C. Above this plate, and resting on wooden brackets in the sides of the trough, is a stout grid or plate of cast zinc with perforations to allow of the escape of any gas which might be formed through incorrect working ; this should be well insulated from the copper and kept at a minimum distance of half an inch from it. A zinc strip attached to the grid serves for the negative pole, Z. A dilute solution of zinc sulphate, to which a few drops of sulphuric acid have been added, is poured into the trough until the zinc is covered ; the small compartment is then filled with copper sulphate crystals, and these, gradually dissolving, form a heavy solution which, finding its way beneath the glass partition, covers the floor of the vessel to the height of an eighth or a quarter of an inch. The zinc is, therefore, now in a dilute zinc sulphate, D Fig. 7. — Gravity battery. 50 SOUECES OF CURRENT. the copper in a strong copper sulphate sohitiou, and the battery is ready for nse. The crystals in the smaller diyision must, of course/ be renewed, as they gradually dissolye away. If well attended to, this form of the Daniell-cell will yield good results, but it is essential to ayoid all agitation which would tend to mix the solutions and bring copper-bearing liquids into contact with the zinc ; a parchment-paper tray to receiye the zinc plate will minimise this risk, but in any case the grid should be carefully remoyed from time to time and cleansed from any copper deposited upon it. Indeed, in all forms of the Daniell-cell the zinc plates should be constantly inspected, as not infrequently splashes of copper liquids find their way into the outer compartment; then, when the cell is at rest, a portion of the copper is at once deposited on the zinc by simple electro-chemical exchange, and local action being set up, the amount of copper separated rapidly increases. As soon, therefore, as the red-brown colour of metallic copper is- obseryed upon any portion of the zinc, it must be remoyed, and the surrounding liquid examined ; eyen the merest shade of blue colour in the solution indicates the presence of copper, and points to the necessity for its renewal. When copper is thus found in the outer yessel, the porous cell should be most carefully examined, as it may haye become cracked, and would thus permit a comparatiyely free interchange of liquids. The Daniell-cell has an electro-motiye force of about 1 yolt, varying with the strength of the acid, the nature of the solvent and the like, from 0-98 to 1 '08 yolts. The E.M.F. usually accepted is 1-079 yolts, and this is so near to unity that it is sometimes used as a standard-cell with which other batteries may be compared. It is well suited for electrolytic work, especially on account of the extreme constancy of the current, but requires greater care and attention than the Smee-cell, to which, however, it is in many respects preferable. G-rove-Cell. — In the Grove-cell a different depolarising system is adopted. Instead of substituting a more negative metal for the gaseous hydrogen. Grove uses a medium containing an excess of oxygen which oxidises the hydrogen into the innocuous compoimd, water, as rapidly as it is deposited upon the negative plate. This medium is nitric acid, the chemical action being* represented as follows : — 1. In outer cell: Zn-fHoS04 =ZnS04-rHo. 2. In inner cell: Ho + 2HN03 = N204 -f2H20. Or in one equation — Zn -f H,S04 2HXO3 = Z11SO4 + ^^204 + 2H,0. Zinc. Sulphuric acid. Nitric acid. Zinc sulphate. Nitric peroxide. Water, THE GALVANIC BATTERY. 51 The nitrogen peroxide (^"204) is a gas which at once dissolves in the nitric acid, imparting to it a red tint at first, and after- wards a deep green colour. It is this gas which, after the acid is saturated with it by the long-continued action of the battery, is seen to come off in the form of dense red-brown fumes possess- ing a suffocating odour. * The negative plate is not made of copper, which would be vigorously attacked by the nitric acid, but of platinum in the form of foil, as this metal entirely resists the action of the acid. Fig. 8 shows (in section) two Grove-cells set up in series to illustrate the method of connecting them. Within a glass or glazed earthenware outer vessel is the dilute sulphuric acid con- taining the zinc plate bent into the shape indicated, in order to Fig. 8. — Mode of connecting a Fig. 9. — Grove-cell (external pair of Grove-cells. view). give a larger surface by surrounding the negative plate, and for convenience in connecting with other cells. Enfolded by the zinc is the porous compartment containing the strip of platinum foil, P, in strong nitric acid. The platinum is not attacked when in use, and is only deteriorated by repeated mechanical manipulation in setting up or taking down the battery ; but its high value adds greatly to the prime cost of the cell, and for this reason the Bunsen-cell, with its block of carbon in lieu of platinum, is generally preferred for commercial purposes. Fig. 9 gives a general view of the external appearance of the Grove-cell, showing one element complete, with the attachments of the next platinum plate on the one side, and of the zinc with its porous compartment on the other. Bunsen-Cell. — The Biinsen-cell has the same reaction and, 52 SOUKCES OF CUEEENT. therefore, practically the same electro-motive force as Grove's, viz. — nearly twice that of Daniell's (1*8 to 1*9 volts). In a circular stoneware jar (fig. 10) is contained the porous cell with its rod of gas-carbon and strong nitric acid, while around the porous pot is a bent cylinder of zinc immersed in dilute sulphuric acid. The carbons are usually cut from the blocks of the deposit which gradually forms on the interior of gas-retorts, and is known as gas-carbon or retort- carbon. It is extremely dense, strong and dur- able ; but is sufficiently porous to allow the acid to pass upw^ard by capillary attraction, and so to attack the brass binding screios^ which form the connection between the battery plates and the wires through v/hich the current is conveyed to its place of application. To prevent this action, D'Arsonval recommends steeping the extreme upper end of the carbon for a short time in melted paraffin, then depositing a thin film of metallic copper on its surface by electro- lysis, and plunging the whole of this portion beneath melted type-metal ; the type-metal will thus form a covering which is in contact with the carbon at the edges of the joint, and must efi'ectually prevent the corrosion of the binding screw, while maintaining perfect electrical connection between the parts. In both Grove's and Bunsen's cells the exciting fluid may be prepared by adding 1 part of sulphuric acid to 19 of w^ater. The nitric acid must initially be highly concentrated, the specific gravity being not less than 1*3082 ( = 34° Baume). The tendency of the reaction is to produce nitric peroxide, which finally escapes as a gas, and water, as we have already seen ; thus, not only is the nitric acid constantly being w^eakened by decomposition, but the remainder is at the same time diluted by the water which is formed. In this manner the strength of the acid rapidly falls ofi", and its oxidising efficiency is lessened. By the time it has been reduced to a density of 30° Baume (sp. gr. =1-2624) the electro-motive force of the cell becomes quickly diminished, and when it has reached a strength of 28° Baume (sp. gr. = 1*2407) it is no longer serviceable, and should be at once renewed. These cells present many advantages ; they have a high electro-motive force, and are fairly constant for a few hours, but they require to be filled with fresh acids daily. The nitric acid, however, is a constant source of danger in inex- Fig. 10.— Bunsen-cell. THE GALVANIC BATTERY. 53 perienced hands on account of its extreme corrosiveness ; and the red nitric peroxide gas, which is constan% evolved, is not only disagreeable, but if inhaled for any time or in any quantity is positively injurious. Whenever either of these forms of cell is used it should be placed in a well-ventilated space outside the w^orkroom, or in a cupboard provided with the means of dis- charging the gases into a flue or directly into the outer air. In spite of these draw^backs they are largely used for nickel- plating and for some other classes of work, especially, perhaps, for those of an experimental nature. Bichromate-Cells.— Some operators have substituted chromic for nitric acid, because the products of decomposition are solid and remain dissolved in the Hquid instead of passing into and contaminating the atmosphere; a suitable solution, proposed by Higgins, consists of 1 part of potassium bichromate dissolved in a mixture of 3 parts of sulphuric acid with 9 parts of water. The disposition of such a cell is similar to those which we have just described. But there are single-fluid hiclirornate-celh in which no porous division is needed, and these have the advantage of a lower internal resistance. The best proportions for the exciting fluid are, perhaps, those which correspond to the equation representing its action as follows : — K2Cro07 + 7H0SO4 + 8Zn = 3ZiiS04 + KoSO, + Cr2(S04)3 + TH^O. Potassium Sulphuric Zinc. Zinc Potassium Chromium Water, bichromate, acid. sulphate, sulphate. sulphate. This would require about 3| ounces of potassium bichromate, and 8J ounces of sulphuric acid to be dissolved in a quart of water. As the zinc is here actually in the strongly-oxidising solution during*^ the action of the battery, and as a very notable amount of solvent action w^oiild take place even when the battery is at rest (though by no means comparable with that w^hich" would be set up in a single-fluid nitric acid cell), means must be devised for removing tlie zinc from the liquid directly the current is broken. This is generally accomphshed by drawling or winding them up out of the acid. Y\g. 11. — Bichro- In the bottle-form of this cell (fig. 11) two long mate of potash strips of carbon, united by a metallic connection cell (bottle-form), above, are fastened (parallel to one another) to ^ a vulcanite stopper, and are there connected with the bindnig screw -f ; these form the negative element and pass to the bottom 54 SOURCES OF QUERENT. of the bottle ; between them is a short thick strip of zinc attached to a brass rod passing stiffly through the centre of the ebonite cork and connected with the binding screw - . The zinc is entirely insulated from the carbon by the ebonite, and may be drawn out of the solution by means of the brass rod as soon as its services are no longer required. In the trough-form of cell (fig. 12) a series of carbon-and-zinc couples are immersed in the acid mixture contained in a long rectangular trough, from which they can be withdrawn when the battery is not in use. These batteries are simple, give a high electro-motive force (1*9 to 2 volts), and have a very low internal resistance ; but the chromic solution, although Fig. 12. — Bichromate of potash battery (trough-form). emitting no fumes, is highly corrosive, and, moreover, must be constantly watched, for as it becomes w^eakened by use the * current pressure ' rapidly declines. Leclanche-Cell. — The number of other batteries in use for various purposes is immensely large and is yearly increasing, but only a few are of practical use to the electro-metallurgist. Many are too weak, others are strong at first but speedily lose their power, while others again, which by their strength and constancy are well adapted to our purpose, are too expensive or too trouble- some for daily use under the conditions of the workshop. Thus, to take a single example, the Leclanche-cell, economical enough as to prime cost and in use, gives a current which, at first powerful, rapidly declines owing to polarisation, but almost as rapidly recovers itself when standing at rest; it is thus very suitable for intermittent or irregular work, such as the ringing of electric bells, but is useless for electro-plating. THE GALVANIC BATTERY. 55 Practical Hints on the Use of Batteries.— In all dealings with hatteries scrupulous cleanUness in every detail is necessary. The cells and the metals must be thoroughly cleansed before commencing work ; and it is of the highest importance^ that all metallic connections in the external circuit, such as the junctions of the plates with the binding screws or with one another (when placing them in series), be kept perfectly bright. A film of oxide or tarnish between two surfaces through which a current must pass increases the resistance of the circuit to the passage of the current, and so, w^hile retarding the action of the battery, adds to the amount of electrical energy converted into useless heat, and thus practically wasted. Porous cells must be kept clean; they are apt to become choked with insoluble deposits which add greatly to their resistance ; and in the Daniell-battery nodules of copper frequently form upon their surfaces,— these must be watched for and removed. When taking a battery to pieces after use, in order to replenish and re-fit it, it is well to soak the porous cells for some time in water, in order to extract the salts contained in them, and never to allow them to become dry until they are thus purified. A fairly pure zinc should always be used; it may be in the condition of rolled sheet, but is more usually cast into the desired shape ; in any case the whole surface must be thoroughly, but not excessively, amalgamated. As soon as bubbles of gas are observed to form upon the zinc, it should be carefully examined, as this is a certain sign that some more^ electro- negative substance is present, which may either be an impurity in the metal itself, when re-amalgamation will remedy the defect, or may be due, as in Daniell's cell, to a metal deposited from the battery solutions, in which case it must be removed by cutting, scraping, or fiUng. Old or worn-out zincs should be saved, as they contain much mercury which is recoverable (see p. 323). ^ ^ No metallic or semi-conductive connection between the two plates of a battery is permissible, because such a connection forms a short circuit, which diminishes or destroys the current flowing in the external circuit. Absolute insulation (or electrical separation) between the two poles, and between the wires at all intermediate points in the circuit, must be preserved ; and the higher the electro-motive force of the current, the greater is its power of overcoming resistance, and, therefore, the more care- fully must the insulation be ensured. Dry wood, which is a partial conductor for electricity of very high potential, is practically an insulator for ordinary battery currents ; but when moistened 56 SOUKCES OF CUEEEXT. with acid or with any of the battery solutions, it iDecomes capable of cansincr verv considerable current leakao:e. The battery solutions require careful watching. Any tendency to crystallise or deposit soHd matter upon the sides or bottom of the cell must be checked ; it may be due to the use of too concentrated solutions at the outset, which is curable bv the addition of water, or it may be that they have become saturated and worn out, when they, of coiu'se, require to be renewed. The quantities to be used in making up solutions should be weighed or measured ; this occupies but little more time, and in the result is more rehable than rule-of-thumb work or mere guess measurements. If the solutions made up from a crystalhne salt be tm'bid, they should be filtered through a blotting-paper cone inserted in a glass funnel (metallic ftmnels should not be used, as they are liable to be ^ corroded by the liuids passing through them). A circular filter-paper is readily made to fit the tunnel by folding it fii^st across one diameter, as shown at A B in divi- sion 1 of fig. 13 ; then on folding it again at right angles, as at C D in Xo. 2, it has the form of Xo. 3 : now on insert- ing the finger between the folds of the paper it may be opened out to the conical shape de- picted in ZSTo. 4, and is thus ready to place in the funnel. If. however, the paper should not fit well into the cone of the latter, it may be refolded along the line. EF, as in Xo. 5, or along any other suitable line, and may thus be adapted to suit a funnel constructed with any angle at its apex. Strondy- acid solutions, such as those used in the bichromate battery, cannot be thus filtered, as they destroy the paper ; but the solu- tion of the potassium bichromate may be passed through a filter before adding the acid to it. If it be necessary to clear any solution which attacks paper, a plug of spim glass or of asbestos may be hghtly rammed into the apex of the funnel, and will form an efiicient filtering-medium in lieu of paper. Fig. 13. — Mode of folding filter-paper. THE GALVANIC BATTERY. 57 It is a good plan to place the battery in a chamber apart from, but adjoining, the depositing room, remembering that the greater the length of copper wire to be traversed by the current between the battery and the vats, the greater will be the loss of energy through the resistance of the circuit. By keeping it outside, however, the operator is not annoyed by the fumes or acid spray produced by certain forms of cell ; and there is less danger of mistake or of accident in manipulating the battery and depositing solutions. If circumstances compel the double use of the same room, the battery should on no account be placed above or too near the electrolytic vats, but should be fitted up in a place easy of access, and preferably in a ventilated cupboard, as already explained. It will be found convenient to set up the battery in proximity to a sink provided with a good water supply, so that every facility may be afforded for cleansing the different parts when the work of the day is over. In charging two-flu.id cells, no drops of the depolarising fluid must be permitted to enter the zinc compartment. On the Fittings and Connections of a Battery. — It will be remembered that the electro-motive force of a voltaic cell is measured by the heat-energy produced by the combinations taking place within it ; hence the size and shape of a cell may greatly modify the volume of current generated by it, but are without influence on the pressure or E.M.F. Other things being equal, the volume of any current may be increased by raising the electro-motive force or by lowering the total resistance of the circuit, as will be auore fully explained in the next chapter {irtfra, p. 92) ; now, the internal resistance of a battery is in- creased, either by expanding the distance between the two plates, because the current has to traverse a longer distance through an inferior conductor ; or by using smaller battery plates, because the area of the liquid conductor is thus reduced, and the current has to flow through a narrower channel. By increasing the resistance by either of these methods, the total current must be diminished while the electro-motive force is constant. Conversely, the use of larger plates (that is, of larger cells) or the nearer approach of the metals in the cell, while leaving the pressure unchanged, increases the volume of current by diminishing the resistance to its path ; in other words, the output in amperes is increased but the E.M.F. in volts is constant. With any given type of cell, then, it is possible to alter the current strength but not the electro-motive force ; to effect the latter, changes in the chemical conditions are necessary. Modes of Arranging Cells. — By the use of more than one cell 58 SOUKCES OF CUERENT. the electro-motive force as well as the current-intensity may be altered at will. If two similar cells be joined, as shown in fig. 14, copper to copper and zinc to zinc, the electro-motive force is no more altered than would be the total fluid pressure produced by placing two pails of water side by side upon a level floor in place of one ; for both the cells are yielding the same pressure of elec- tricity, and the mere coupling them in j^^araZZe?, as it is termed, is only equivalent in eff'ect to increasing the size of a single cell, which as we have seen is without influence on the E.M.F. But if the cells be disposed as in flg. 15, with the copper of one joined to the zinc of the next, and the free elements con- nected to the main circuit, the current generated in the first cell has to flow through the second, and that of the second through the first in order to complete the whole circuit, with the result that the total electro-motive force is doubled. This arrangement, which is termed coupling in series, is exactly analogous to lifting the one pail of water above referred to and placing it upon Wi > - ; Fig. 14. — Two cells with like metals con- nected. ^^^^ i 5^ Fio;. 15. — Two cells with unlike metals connected. the other, when the pressure is of course doubled. In parallel all the chemical energy of cells is placed upon the same level, and can, as it were, pump the electricity only to the same height ; but in series the energy of one cell pumps the electricity to one level, and then that of the second cell starting from this point raises it as high again. And so in setting up any number of cells, if placed all parallel, the E.M.F. is only that of one cell, but the internal resistance is reduced, as it would be in one large cell of the same type ; while if all are arranged in series the E.M.F. will be raised in direct proportion to the number of cells in use. Intermediate dispositions to suit special requirements may be made by grouping several cells together in parallel, and then uniting this group as a whole in series with other similar groups. Let us with the aid of the diagram, fig. 16, consider the possible groupings of the 6 cells. A, B, C, D, E, F, each of which has, let us say, an electro-motive force of 1 volt and an internal resistance of 1 ohm. One such cell, short-circuited by a piece of metal having BATTERY CONNECTIONS. 59 practically no resistance, would give a current of 1 ampere ; for Ohm has enunciated the law that the volume of current m a .given circuit is proportional to its electro-motive force, and inversely proportional to the total resistance, which is expressed by saying that the current volume is equal to the electro-motive force divided by the resistance, or more shortly by formulating In other words. Ohm's law may be written for any volts ^jircuit as amperes = ohms When the 6 cells in fig. 16 are 3. 4. Fig. 16. — Modes of grouping six cells. placed parallel, as shown in position 1, all the coppers are united together, and as a group are opposed to all the zincs similarly united ; the E.M.F. is unaltered, and still stands for the whole group at only 1 volt ; but the resistance is divided by 6, because there are now 6 equal internal passages for the current instead of 1, and it is therefore ^ ohm instead of 1 ohm; so that now the volume of the current flowing through a short circuit is 1 (volt)^^ amperes. But if the 6 cells are joined in series, as f (ohm) in position 2, the E.M.F. is multiplied by 6 and is 6 x 1 = 6 volts, 60 SOURCES OF CURRENT. while the resistance is also multiplied by 6, and is now 6x1 = 6 ohms, because now the current has to flow through all the cells in series, and is, therefore, checked by the resistance of each ; the cur- rent yielded on short-circuiting is, therefore, ^""^j^^^^ ^ ampere, or the same as that of a single cell ! (Note, however, that this is only on short-circuiting — vide infra.) Now, by joining the pairs A and B, C and D, E and F, each respectively in parallel, and connecting the groups A B, C D, and E F in series, as in position 3, the electro-motive force of each parallel pair is only 1 volt ; but there are three pairs in series, so that the total electro-motive force is 3 volts ; while the resistance of each pair is of course half that of one cell, or ^ ohm, and the sum of the three resistances is # ohm, so that the ultimate current on short 3 circuit is — = 2 amperes. And, finally, by arranging the cells, as in position 4, with the three cells ABC parallel, and united tO' form a series with the group D E F also placed parallel, the electro-motive force of each group is 1 volt and the total pressure is 2 volts, while the resistance of each is ^ ohm and the total resistance § ohm, so that the resultant current is-^ = 3 amperes. The volume of current quoted in each case is, as we have stated, that produced when the external resistance is nil ; but this con- dition never obtains in practice, and the anomaly of six cells giving only the same current as would be afforded by one is not met with when the external resistance is high, for then the value of the increased electro-motive force is felt. Thus the current from one of these cells acting through an external resistance of 100 ohms (making with the internal resistance of the cell a total resistance of 100 + 1 = 101 ohms) gives an ampereage equal to 1 ^1 ^7^^^^^ ^ = ^-i- ampere, whereas the six cells in series would 101 (ohms) 101 give ^ (volt) ^ _6^ ampere : thus the current ratio in the two- ^ 106 (ohms) 106 ^ ' 1 6 cases is — : = 106 : 606 ; that is to say, there is now 5*72 101 106 ' times as much current produced from the six cells in series as there is from one alone. On the other hand, the arrangement of the six cells in parallel, so advantageous on short circuit, is little superior to a single cell when the current is to be passed through so high a resistance as 100 ohms. In this case the total resistance is 100 + ^ ohms; the total E.M.F. is 1 volt; the total current BATTERY CONNECTIONS. 61 ^ = _A ampere ; and the ratio of efficiency of 1 cell to 6 100 + 1 601 ^ ^ in parallel is only-l-:-A. = 601 : 606, or 1 : I'Ol instead of ^ 101 601 1 : 5*72 as when they are set up in series. From this it will be seen that to obtain the highest effect from a number of cells, they must be united in series when the external resistance is very high, or in parallel when it is very low. The highest efficiency of all is to be obtained when the cells are so grouped that the internal resistance of the battery is equal to the external resistance of the circuit. But although the coupling-up of cells in series may be desirable by affording the greatest possible volume of current, such a disposition is by no means the most economical. A certain weight of zinc, in dissolving, produces a certain E.M.F., and in one cell yields a certain volume of current capable of doing a certain quantum of work. And where one cell only is employed, a given weight of zinc, in dissolving, deposits electro- lytically a definite equivalent weight of any metal in the plating- baths. But when cells are multiplied in series, an equal amount of zinc dissolves in each, but only the one equivalent of deposited metal is yielded, the energy developed in the oxidation of the remaining zincs being used in raising the electro- motive force — in pumping the current to a higher level. For example, in the case of the six cells in series in short circuit, the same ultimate current is produced as would be yielded by one cell ; but in the former case six times the quantity of zinc is dissolved to produce the same effect. It is clear that economi- cally, the best result is obtained when the smallest number of cells is placed in series which is compatible with the production of the minimum electro-motive force required for the electrolysis ; and the disposition of the cells must be governed by the balance of the two considerations — time and economy. Connecting Screws. — In making electrical connections between the different parts of the circuit, the use of brass binding-screws is advisable ; these may be procured of any apparatus-maker and of any shape that may be required. Figs. 17, 18, 19, 20, 21, and 22 illustrate six of the more useful forms. Figs. 17 to 19 represent the commoner connectors for uniting plates or bars to wires. Fig. 17 is employed in the Bunsen battery to join the carbon block to the terminal wire ; but it is equally appUcable to the uniting of any thick substance to a wire, the former being held by the screw at the side of the large 62 SOURCES OF CUERENT. clamp, the latter by that in the spherical portion above. Fig. 18" may be screwed into a metal block, and is commonly used to joini the negative wire to the zinc of the Daniell-cell. Fig. 19 is em- ployed as the terminal binding-screw in a Grove-cell, and will serve to connect any thin sheet to a wire. Fig. 20 illustrates- the arrangement for uniting two wires end to end. Figs. 21 and. 1 m Fig. 17. Fi^. 18. Ficr. 19. Ficr. 20. Fior. 21, Fig. 22. Forms of binding-screws for batteries. 22 represent systems of coupling plates or blocks, the former as. applied to two thin sheets, the latter to a thick block with a thin, plate. Other forms may, of requirements, but these will which they may be put. Switch-Boards. — Often for mental work more especially. course, be devised to suit special answer most of the purposes to practical purposes, but for experi- it is desirable to have at hand a rapid method of altering the arrangement of a group of battery cells, so that any desired combination in series and parallel may be obtained. The constant alteration of battery connec- tions is tedious and clumsy ; while a switch-board used by Professor Silvanus Thompson is at once simple and efficacious. This instrument is made with a series of binding-screws, and of accurately-ground brass plugs fitting into cavities between ad- jacent brass blocks, such as those employed in the manufacture of resistance-coils for electrical measurement. Fig. 23 represents, a modification of Thompson's switch-board, which we have in constant use ; it is precisely the same as the other in principle, but is of cheaper construction, and may be made by any carpenter. A is a plan, and B a sectional elevation of the board. Two parallel strips of ^^brass plate, p and n, are let into the surface of a varnished board, and are connected each with a terminal bind- ing-screw at the end ( + and - ). At even distances along the surface of each brass rod is a series of upright split brass tubes, as at s, s in the section B ; the tubes on p being spaced midway between those on n, as shown. Along the lines, P and N, are similar rows of upright split tubes, s, s, each of which is connected- BATTERY CONNECTIONS. 63 by metal wire, with a corresponding horizontal split brass tube, s", s", as at a, b, c, a\ b', c, etc. The connection is made as shown in the figure, B ; every pair of split tubes, s and s\ must be perfectly insulated from every other pair, and from the series, p and n ; there is, therefore, no brass rod running along the lines, P and N. The up- right tubes on the row P correspond exactly to those in the row p ; those in N to the others in n; and the space between any corre- sponding tubes, s and s, and also the diagonal space be- tween s of row P and s of row N, must be equal in every case to those between the tubes on rows p and n. Twelve U-shaped pieces of brass rod, h, h, are prepared of such size that they fit w^ith each limb into one of any adjacent pair of vertical split tubes (the object of the splitting is to give sufficient spring to ensure a tight fit, and therefore good contact) ; a like number of short straight brass rods are prepared for the horizontal tubes, s\ s'. The necessity for even spacing of the tubes is evident, if the handles, h, h, are to be inter- changeable. Each cell of the battery is now connected by a wire to the apparatus, the positive (copper) pole of the first to the s" tube, marked a, the negative (zinc) pole to that marked a; the second is similarly attached to b and b', the third to c and c\ and so on, taking care that the positive poles are all connected to one side, the negative to the other, and that the two poles of every cell are attached to corresponding tubes (a and a ; b and b' ; etc.). The battery wires may be soldered, each to one of the above- mentioned short straight brass rods, to be inserted in horizontal split tubes. Supposing 6 cells to be thus connected up, no current can flow even if the terminal wires marked ^ + ' and ' - ' are joined, because each of the tubes, s, s, is isolated ; but by insert- ing the handles, h, h, in a suitable manner, the various con- nections are made in any required fashion, and the current passes through the circle from the terminal + to that denoted by-. By connecting every s in the P line with the corresponding s' ah c d e f - - - - mm a' h' c' d' e' f C D E F Fig. 23. — Thompson's Switch, modified. 64 SOUECES OF CUEEENT. in the p line, and similarly those in the X row with the others in rt, the positive pole of every cell is placed in direct contact with the brass strip, p, the negative with the strip, n, so that all the cells are in parallel, and can discharge thus through the circuit. This arrangement is shown diagrammatically in plan C. But if only the positive wire of cell a is placed in connection with jy, and only the negative wire of /' with n, and the inner lines of tubes are "^joined-up diagonally (a of line P with 5' of X ; Z> of P with c' of X : and so on),*^ the whole 6 cells are in series, as represented in diagram D ; because the copper pole of the first is connected to the outer circuit, while the zinc pole is joined to the copper pole of the next, and so on until the zinc pole of the last is free and is united to the wire of the outer circuit. Diagram E shows the cells, a, 6, and c, in series connected in parallel with d, e, f] also in series. Diagram F shows each pair, a and b, c and d, e and /; in series, but the three couples in parallel. Thus, diagrams C, D, E and F illustrate respectively the four possible methods of combining 6 cells, all parallel ; all in series ; two parallel groups of 3 cells in series ; and three parallel groups of 2 cells in series : and these connections may be altered at will in a few seconds of time. Thermo-Electric Batteries. The fact,- already alluded to in the introductory chapter, that a compound bar, made up by solder- -X- ing two unlike metal strips end to end, is capable of producing an electric current when the junction is alone heated, renders \ possible the direct conversion of heat into electrical energy. Instruments used to effect this change are termed thermopiles or tliernio-electric hatter ies. Thermo-Electrical Series.— Any two metal bars joined as in fig. 2-i will give a current always in the same direction when the point of union is heated, or in the opposite direction when it is cooled, more Yig, 24. Simple than the rest of the bars. In respect of Thermopile. their behaviour when thus treated the various metals behave very differently, and may be arranged in a thermo-electrical series similar to that indicative of their electro-chemical relations. Any single pair of THERMO-ELECTRIC COUPLES. 65 metals gives a constant electro-motive force so long as the differ- ence between the temperature of the junction and that of the remainder of the circuit is constant ; and, moreover, in many instances the E.M.F. produced is nearly proportional to this difference in temperature. The actual electro-motive force of any pair of metals is extremely minute compared with that of a galvanic battery, so that a large number of couples must be used to produce an equal effect. In the following table are grouped some of the principal metals with the electro-motive force produced by heating a single couple — made by heating them respectively at their juncture TABLE v.— Showing the Thermo-electro-motive Force of Various Elements in Relation to Lead, Expressed in Micro-Volts per Degree Centigeade. Metal. Observer. Micro-Volts. Bismuth, pressed wire. Bismuth, crystals. Bismuth, ordinary, Bismuth-Antimony Alloy (10 : 1), Cobalt Nickel, German Silver, .... Palladium, Mercury, ..... Lead, Tin, pure pressed wire, Tin, ordinary, .... Copper, commercial. Platinum, ..... Gold, ^ Cadmium, ..... Antimony, pure pressed wire, Antimony, commercial pressed wire. Antimony, ordinary, . Antimony, crystals. Silver, ...... Zinc, Copper, electrolytic. Arsenic, Iron, pianoforte wire, . Red phosphorus, .... Antimony-Zinc Alloy (2:1), Copper sulphide, .... Antimony -Cadmium Alloy (1 : 1), Tellurium, Selenium, B&M M B M B M M B M M B M M M M M M M B B B M B B B M B B M M B B - 6-0 - 17-1 - 0-1 - 0-4 - 0-1 - 0-9 - 1-2 - 2-4 - 2-8 + 89 to 97 + 65 to 45 + 40 + 64-5 + 22 + 15-5 + 11-6 + 6-8 + 3-2 99-0 196-7 231-9 502 807 3-0 3-7 3-8 13-6 22-6 29-7 22-6 to 26-4 Zero E 66 SOURCES OF CURRENT. with metallic lead — to a temperature 1° Centigrade higher than the rest of the circuit. The E.M.F. is given not m volts but in micro-volts (1 micro- volt is the one-millionth part of 1 volt). In harmony with the electro-chemical system of nomenclature, which designates that metal electro-positive from which the current starts to pass through the liquid in the voltaic cell, a substance is said to be thermo-electro-positive to another when the current passes from it to the second through the heated juncture. In the table lead is taken as the basis for comparing the others, so that all the metals standing above lead are marked with the -f sign, which indicates that the current would flow from them to the lead through the joint, while with the negative elements the current starts from the lead. The numbers cannot be accepted as absolutely accurate, or universally applicable, because the behaviour of metals in respect to these currents is greatly affected by the presence of impurities, and because the relations between them are dis- turbed, and in some cases reversed, at different temperatures. The letters M, B refer to Matthiessen and Becquerel, from whose results these numbers are taken. Thus, for example, a cobalt-lead junction might be expected to give a current of 22 micro-volts for each degree through which the junction was superheated; and similarly a lead-iron junction would give almost the same E.M.F., but the current would flow in the opposite direction. To determine from these figures the difference of potential between any other pair of metals, it is only necessary to deduct the stated E.M.F of the more negative metal from that of the other, for example: — (1) When both metals are positive : a nickel-palladium pair should give a current of 15*5 - 6'8 = 8-7 micro-volts per degree passing from nickel to palladium through the junction; (2) when one is positive and the other negative : a nickel-zinc junction should give 15-5 - ( - 3-7) = 15-5 3*7 = 19*2 micro-volts, flowing from nickel to zinc ; and (3) when both are negative : a platinum- silver couple should show - 0*9 - ( - 3-0) = 3-0 - 0*9 = 2-1 micro-volts, the current starting from the platinum. If the junction be superheated 100° instead of 1°, these numbers must of course be multiplied by 100 ; thus the nickel-zinc pair should yield a current of 1920 micro-volts or 0-001920 volt, so that it would require 521 of these couples heated to 100° C. to give the E.M.F. equal to that of a single Daniell-cell. Neutral Point.— A peculiarity of these couples has already been mentioned, namely, that the potentials often vary dispro- portionately to the rise of temperature. It may so happen that THERMO-ELECTRIC COUPLES. 67 a pair of metals giviDg a very low E.M.F. per degree at ordinary temperatures will give a comparatively high E.M.F. at high temperatures, while another couple which started well may gradually fall off as heat is applied, until at a certain temperature (varying with the metals, and known as the neutral point) there is no E.M.F., and, therefore, no current; yet, on continuing the application of heat, an E.M.F. is again set up, but it now tends to drive the current in the reverse direction. The behaviour of metals in this respect is best seen by reference to fig. 25. Lead Degrees Centigrade Fig. 25.— Thermal E.M.F. of metals at different temperatures. is here adopted as the standard of comparison, because, unlike most other metals, hot lead in contact with cold lead produces no difference of potential. To ascertain from this table the effect upon the electro-motive force of a rise in temperature of 1° C. at any given temperature, it is only necessary to find the vertical line corresponding to the required temperature, and then, noting the points at which the sloping lines representing the two metals cross it, to read off the difference in micro-volts between them with the aid of the scale on the left-hand margin. Thus, for example, an increase in temperature from 0° to 1° in the warmer junction of an iron-copper couple increases the electro-motive force by - l- (-15)=14 micro-volts, but a similar rise from 100° to 101° only gives - 2 - ( - 11) = 9 micro-volts, and from 200° to 201° only - 3 - ( - 6) = 3 micro- 68 SOURCES OF CURRENT. volts, in all these cases the current flowing from copper to iron through the juncture. But a rise of temperature from 260 to 261° produces no current, this being the neutral pomt of these two metals (their respective lines cross here), while if the two junctions of the copper-iron pair were at the temperatures^ ot 350° and 351°, there would again be an .E.M.F. of 0 - ( - 5) - D micro-volts, but the current would now be flowing from iron to copi)er With some pairs of metals there is practically no neutral point; palladium and iron give a constant rise m potential for each increment of temperature, while with others the neutral point is below the zero on the Centigrade scale, so that the higher the temperature to which the one joint is heated, the greater the efficiency of the pair— e.^., palladium and cadmium. ^ i . j Further, the total E.M.F. of a circuit may be readily calculated from this table. Of course if the rise in potential were strictly proportional to the increase in temperature, it would be only necessary to discover the difference in temperature between the two junctions of the metal and multiply it by the number of micro-volts stated per degree, but such a procedure would evidently give very misleading results. To estimate the total E M.F. from the table, find the diagonal lines representing the two metals, ascertain the mean temperature of the two junctions of the couple, and mark the points at which the two diagonal lines respectively cross the vertical line representing the mean temperature, then multiply the number of micro-volts between these two points by the number of degrees difference between the two temperatures, and the result is the required E.M.F. Thus it may be required to know the actual E.M.F. of an iron- copper couple of which one junction is at 200° C, the other at 100° C. The mean temperature is 150° C, and the difference of potential between the metals at this point is (per degree) - 2-5 - ( - 8-5) = 6 micro-volts. The required E.M.F. is, therefore, 6 x (200 - 100) = 600 micro-volts or 0*0006 volt. Another most important point, clearly brought out by this table, is that when the neutral point of any pair of metals is above zero Centigrade, not only does an increase of temperature yield no proportionate return in the value of the E.M.F., but as soon as the temperature of the hot end has exceeded that of the neutral point, there is an actual decrease of current, because the E.M.F.'s at the two junctions are tending in opposite directions. A single instance— that of lead and iron— will serve both to illustrate and explain this phenomenon. The neutral point of these two bodies is approximately 350° C. THERMO-ELECTRIC COUPLES. 69 TABLE VI.— Illustrating the Total E.M.F. Produced by an Iron- and-Lead Couple at Different Temperatures. Temperature. Total Micro- Volts produced under these conditions. Increase of Total E.M.F. per 100°. At Cool Junction. At Hot Junction. Centigrade. 0° Centigrade. 100° lO'Q \y inn 19Sn Micro- Volts. 1280 0° 200° lA'^ onn 01 Ad iU / X ZUU — Zl-iU 860 0° 300° 8-6 X 300 = 2580 ,440 0° 350° 7-5 X 350 = 2625 45 (per 50°) 0° 400° 6-4 X 400 = 2560 - 65 (per 50°) 0° 500° 4-3 X 500 = 2150 - 410 0° 600° 2-1 X 600 = 1260 - 890 0° 700° 0 X 700 = 0 -1260 The maximum current is thus produced at 350°, thence it decreases to zero at 700°, and would then begin to increase again, but in an inverse direction. The important bearing of this inversion upon the choice of couples for the production of thermo- electric currents and on the temperature to which they should be submitted is at once evident. Mode of Arranging Thermic Couples.— A variety of thermo- piles has been proposed, differing in the metals employed, in the construction, and in the manner of applying the heat. The essentials are that the two metals shall be as far apart in the thermo-electric series as possible, that there shall be no chemical or physical reason (too great fusibility or oxidisability) against the use of either, and that they shall be conveniently arranged so that the one point of juncture may be superheated as compared with the other. The simplest method of arranging a series of couples, so necessary where each pair produces but an infinitesi- mal electro-motive force, is that shown in fig. 26. The rules which apply to the fitting up of galvanic batteries in parallel or in series liold good equally with thermo-electric batteries. Clamond's Thermopile.— The thermopile most commonly used is that of Clamond, in which metallic iron is united to an alloy of 70 SOUECES OF CURRENT. antimony and zinc (combined preferably in the ratio of their atomic weights, viz. : — 122 : 65 or nearly 2 : 1). Fig. 27 shows the arrangement of ten couples in circular form : strips of tin- plate, P, are generally employed for the iron element, and good contact between the two metals is ensured by bending the strip into a narrow loop at one end, placing this portion in a mould, and pouring the melted alloy around 'it, so that it is actually em- bedded in the casting ; thin plates of mica are inserted externally between the metals to insulate them at all except the desired points of contact. Each block, N, may be about 2|- inches long by f inch thick, but the dimensions may, of course, be greatly Fig. 26.— Simple thermo- Fig. 27. —Ten thermo-electric couples electric couples. in circular form. (Clamond's pile.) varied ; they are arranged radially with a central space, H, between them through which the heat is applied to the junc- tions, J. The different pairs are arranged in series by bending the free end of each tin-plate strip and soldering it to the ex- ternal edge of the adjoining block next in regular succession, a break, B, being left at one point by which connection with the external circuit is made. By this arrangement the points to be heated are brought to a central focus, while the cool junctures are placed at the outside, where they are most free to radiate away all excess of heat conducted through the metal blocks. Generally there are ten pairs in the circle, and several tiers of similar circles are piled one upon another, but insulated from each other by a layer of cement composed of powdered asbestos moistened with a solution of potassium silicate. All the pairs THERMO-ELECTRIC BATTERIES. 71 in each row are in series, and each tier is placed in series with the preceding one, so that the resulting electro-motive force of a pile of ten tiers of ten blocks each would be 100 times that of a single pair. The source of heat is usually coal-gas, which is burned at jets from a perforated earthenware tube placed upright in the central cavity. In some forms of this pile the top of the €avity is closed, and a thin sheet-iron tube is inserted between the centre pipe and the metal couples, leaving a space above, so that the products of combustion are led down through the annular space outside the iron lining, and are passed away through a flue at the bottom, the object being the economy of a greater proportion of the heat from the burning gas. Charcoal or coke may be substituted for coal-gas when necessary. A battery of 6000 pairs, arranged in series and heated by coke, has been found to give an electro-motive force of 109 volts (equal to 100 Daniell cells) with a total internal resistance of 15| ohms. Noe's Pile.— In the Noe 'pile a cyhndrical bar of the zinc- antimony alloy is soldered to stout German-silver wires; the end to be heated is fitted with a brass cap, through which passes a stout copper-rod. The juncture of the metals is heated, not by direct contact with the ilame or with heated air, but by the conduction of heat along the cdpper rod, the free end of which is thrust into the flame. The arrangement of a single Noe pair is shown in fig. 28, to- gether with a sketch of the method of fitting several •couples in series around a central flame. The German- Noe pile. .silver combination is greatly preferable to that with iron, and the electro-motive force of a single pair of the Noe pile may m con- sequence amount, it is said, to even ^-V of a volt (by the apphca- tion of sufficient heat), with an internal resistance of ohm. . But even at best the thermopile is very wasteful, and it is questionable whether more than 1 per cent, of the heat-energy expended is recovered in the form of electricity ; indeed, Fischer found by experiments, conducted in 1882, that only 0*3 to 0*5 per cent, was utilised. Nevertheless, the study of the subject is very fascinating because there is a direct conversion of heat into electricity; and although at present an almost prohibitive loss 72 SOUECES OF CUEEEXT. is iiiTolved, increase of knowledge may be attended by the pro- duction of a higher efficiency : the simplicity and convenience of the arrangement would probably then ensm^e to it a wide apphca- tion as a source of electricity for electro-metallurgical iDui-iDoses. The Dynamo-Electric Machixe. In the generation of electric energy the choice at present practically lies between the oxidation of zinc in a battery, and the oxidation of coal, with a conversion of chemical energy first into mechanical energy, and thence by dynamo-electric machinery into electricity; but here also a large percentage of loss is incun-ed in the first stage of the conversion, owing to the waste which is inevitable even in the best constructed steam- or gas- engine. TThere, however, a steady and constant water-supply is available for actuating water-wheels or turbines, the dynamo may sometimes, but not always, be an economical machine, Vhich may convert a very large proportion of an otherwise waste form of energy into useful electricity. * The prime phenomenon on which the dynamo depends is the production of electric currents in closed circuits of metal which are caused to move in the vicinity, either of j^ermanent- or of electro-magnets, or of wires through which another current of electricity is flowing. Lines of Magnetic Force.— In considering the theory of the dynamo, one must not lose sight of the close^^connection^between electrical and magnetic phenomena. By coiling a piece of wire spirally arotmd a bar of iron, and then transmitting a current of electricity through the wire, the iron is converted into a magnet, and will continue to evince mas:- netic properties so long as the cuiTcnt passes, but no longer ; and a coil of wire through which a current is flowing will itself be magnetic, attracting light particles of iron to itself, and tending to set north and south, even though it have no iron core ; such a coil is termed a solenoid. If a series of li,crht compass-needles be suspended, as in tig. 29. above a bar-ma^-net or a solenoid, they will be found to place themselves at various angles as indicated; the needle marked 4, being central, would RELATION OF MAGNETISM TO ELECTEICITY. 73 have each pole equally magnetised by the equidistant poles of the magnetic bar, and would, therefore, occupy a position parallel to it; but numbers 3, 2, and 1, being each nearer than the last to the north pole, would fall more and more within its influence, and would be less and less affected by the increasingly distant south pole, and would thus tend to set themselves more nearly vertical, while the needles 5, 6, and 7 over the other half of the bar would behave similarly. Now, the position of each needle clearly indicates the direction in which the magnet is exerting its forces at that point ; and if a map w^ere made, showing the attitude of the needle towards the bar in every conceivable position, the various lines of force would be fully indicated. Such a map may be made by sprinkling iron filings on to a sheet of paper spread evenly above a magnet, and then very gently shaking the paper ; each filmg itself becomes a magnet by virtue of its proximity to the bar, and is indeed a miniature compass- needle. The lines of force thus indicated around a single bar- magnet are sketched in fig. 30. The existence of these lines of force may be thus readily de- monstrated, as well as the fact Fig. 30.— Bar-magnet lines that they are most numerous of force, in the regions of the poles. They are always considered to run from the north pole to the south. Conversion of Alternating into Constant-Direction Currents. — Whenever a coil of wire is caused to cut these lines of force, so that the number passing through the coil is altered by the move- ment, an electric current is produced in the coil ; and the greater the number of lines cut in this way the higher is the electro- motive force of the current ; the direction of current depends upon that of the lines of force, and upon that of rotation of the coil. A coil cutting an increasing number of lines of force running- through it from left to right produces a current in the same direction as it does when cutting a diminisliing number running from right to left; but opposite to that taken by the current produced when an increasing number passing from right to left, or a diminishing number from left to right, are entering through it. Supposing, for example, the rectangle of wire, A B C D (fig. 31), to be rotated on its axis, E F, between the poles of the magnets, N, S, from left to right (so that A B approaches N) ; 74 SOUKCES OF CURRENT. Fig. 31. — Production of currents in mao;iietic field. then as rotation proceeds the loop cuts a diminishing number of the lines of force running across the space from X to S until it is in a horizontal position, and the current produced during this period will pass along the coil in the direction D C B A ; now on continuino' the rotation, an increasing number of lines will be cut, but they will be entering the loop from the opposite face (from the face which was at first opposite S, but is now gradually becoming opposite to X) ; and the result of such increase, accompanied as it is by an in- version of the direction of the lines in regard to the loop (the lines still run from X to S, it is onlv the loop which has changed position), is to give a current still in the same direction, D C B A, but as soon as the coil has passed through an angle of ISO degrees and is again vertical, but now with A B underneath, a decreased cutting of the lines of force begins ; and as they are still threading the loop in the same direction, a sudden reversal in the direction of current occurs the vertical point is passed. At the next quarter turn, from vertical position, fewer lines of but once again their direction so that the current in the latter until when the vertical point is reversed again and the same cycle of Thus the rotation of such a loop of wire the magnet is accompanied by the produc- es the horizontal to the original force pass through the loop, relative to the coil is reversed, maintains its second direction, again reached, it is changes is repeated. between the poles of _ _ tion in it of a current of electricity, the direction of which is reversed each time the coil passes through the position parallel to the faces of the magnets. Thus if the loop were extended so as to include an external circuit, the current passing through the latter must change its direction twice as often as the loop rotates ; in order to avoid this constant alternation of the current, which would be fatal to electro-plating work, a commutator is added, which converts the alternate into a continiLOus current of uniform direction. The commutator for a single coil. C D E F, is arranged as shown in fig. 32, and consists of a metal tube split into two equal parts lengthways, the two halves being fastened around a small cylinder, but well insulated from one another ; one half is attached to one end of the loop wire, the second half to the other. Copper strips or brushes, B B', are fixed to the frame of the dvnamo, so that they press lightly upon the spht cylinder at RELATION OF MAGNETISM TO ELECTKICITY. 75 Fig. 32. — Commutator for a single coil. points diametrically opposite to one another; as the tube is divided equally, and as the brushes are parallel, it follows that whatever the position of the cylinder, both brushes are never in €ontact with the same section simultaneously. To the brushes are fastened the wires connecting with the external circuit, so that any current generated in the coil flows first to one section of the split tube, thence to the brush in con- tact with it, and so around the outer circuit to the other brush, then through the second section of the tube to the other end of the coil of wire, and so completes the circuit. Now, provided the rota- tion of the coil around its axis, A A, be always in the same direc- tion, and both the brushes and the breaks in the commutator tube be at the highest and lowest points in the circle, at the moment when the reversal of current takes place, the current in the main circuit flows con- tinually in the same direction ; because the brushes come in con- tact with difl'erent sections of the commutator directly the current is reversed, and the same brush is always connected with the descending side of the coil, the other brush always with the ascend- ing portion, no matter whether it be C D or E F. Supposing, for example, the coil is rotating from left to right as indicated, C D is now descending and brush B is touching section G, and the direction of current may be supposed to be (C-D-E-F-H)-B'-circuit-B-(G-C), etc. ; this direction continues until a half -re volution is completed and the side E F is uppermost, now as E F begins to descend, the current tends to flow in the coil in the reverse direction, thus, F E D C ; but at this instant the commutator also shifts, and section G is now in contact with the brush B' (instead of with B) and H with B ; now, then, the current flows through the circuit in the direction (F-E-D-C-G)-B'-circuit-B-(H-F), etc., and so on. It is very clear, then, that in spite of alterations in direction of the current in the coil, the current invariably flows from the brush B' to the outer circuit, and from the outer circuit through the brush B, to whichever section of the commutator happens to be touching it ; B' is, therefore, as much the positive pole of the system we have been describing as the binding-screw attached to the platinum plate is the positive pole of the Grove-battery. As 76 SOURCES OF CURRENT. a matter of practice, the brushes are usually given a Mead/ that is, they are slightly in advance of the true vertical diameter of the coil, for reasons, connected with the mutual reaction of the currents in the magnets and in the coils, into which it is un- necessary to enter here. Parts of Dynamo.— -In the actual dynamo the number of these coils is multiplied, and the methods of arranging them various, the whole system being known as the armature. Thus, in electric generators of this type, there are two main parts, the jield-rnagnet and the armature (including the commutator) ; and the different types of machine are made by varying the construction of one or both of these. It shoidd be noted here that the electro-motive force of a dynamo may be raised either by increasing the number of lines of force cut by the rotating rings {i.e., by using a stronger magnet), or by increasing the number of coils in the armature, or by increasing the speed of its rotation. Field-Magnets. First, as to the neld-magnets \ originally a coil of wires was rotated in front of the poles of a steel per- manent magnet of horse-shoe form : the lineal descendant of this machine is the small magneto-electric apparatus used for medical purposes. But the difficulty in obtaining any large store of magnetism in permanent magnets soon caused their abandonment in favour of the far stronger magnets made by circulating a ciurent of electricity arotmd bars of soft iron, which, being more powerful, have a greater number of lines of force, or, as it is termed, a stronger magnetic jiekl. All the dynamo-electric machines belong to this second class, but the manner in which the magnetisation, or exciting, of the soft iron core is conducted is variable. There are four principal methods of construction. 1. By Separately Exciting the Field-Magnets. — A current from a battery or from another dynamo circulates around the soft iron core and thus magnetises it : and the whole current generated in the armature is used in the outside or main circuit. Such a machine is equivalent to an impossibly strong magneto-machine (with permanent magnets), and has the additional advantage that the extent of magnetisation, and hence the strength of the magnetic field, is not only quite constant, but is imcler perfect control by altering the volume of the exciting current. 2. The Series-Wound Dynamo. — The whole cin-rent generated in the armature passes, not only through the main circuit, but also through the wire surrounding the field-magnets. By this aiTangement, if the resistance of the external circuit be increased, THE DYNAMO-ELECTEIC MACHINE. 77 the volume of current is, of course, diminished, and this reacts on the field-magnets by exciting them less, and thus producing a weaker magnetic field. The primary action of the machine is due to the small amount of residual magnetism, which remains even in the softest iron long after the exciting current has ceased. As soon as the armature acquires sufficient speed, the extremely weak magnetic field, resulting from the residual magnetism of the iron, causes a minute current to be set up in the coils ; and this current in passing around the exciting wires of the field-magnet adds slightly to the magnetism, and thus increases the magnetic field, so that a stronger current is produced in the armature, which again intensifies the magnetic field ; thus a constant action and reaction takes place, resulting in a continuous increase of current-strength, until the maximum capacity of the machine is reached. The whole action is, however, complete within a few seconds of starting the machine. A great disadvantage of this class of machine for use in electro-metallurgy is that, if by any cause the dynamo is stopped without first breaking the circuit, the opposing electro-motive force from the plating-vats (mVie p. 31) causes a current to pass through the wires surrounding the field-magnets of the dynamo, and produces a current in them which overpowers the residual magnetism of the iron-core and reverses the poles of the machine : the consequence of this is that when the dynamo is again started, the current passes through the vats in the wrong direction. Such a result is, of course, fatal to the work of plating, so that the greatest care is needed to guard against this catastrophe when series-dynamos are employed. It is usual to place in the circuit a cut-out, which automatically breaks the current as soon as it has diminished to a given volume. 3. The Shunt-Wound Dynamo, — In this a part of the current generated passes to the main circuit, while a smaller portion is passed or shunted around the field-magnets, which is accom- plished by making the wires around the magnets to have a high resistance as compared with the external circuit, and branching off the one circuit from the other, so that the current divides between the two in inverse proportion to the resistances (see p. 44). Hence any addition of extra resistance in the main circuit diverts a greater proportion of the current through the shunt wires of the magnet, intensifying the magnetic field and causing a larger amount of current to be generated, in spite of the higher resistance in the working portions, which is an advan- tage in dealing with irregular external work, as the machine becomes self-compensating. The starting of the machine is 78 SOURCES OF CUEEENT. due to residual magnetism in the same way as in the series- dynamo. 4. Combination or Compound-Wound Dynamos. — By combining any two of these methods in one machine, as, for example, making the magnetisation of the iron dependent partly on series-windingv partly on shunt-, the adaptability of the dynamo to divers purposes is practically unlimited. Armature. — The armature is also capable of modification; it may be a drum-armature, made up of a number of coils like that shown in figs. 31 and 32, wound on a rotating cylinder or drum; it may be a ring-armature (as in fig. 33), in which the wire is wound in smaller coils around a rotating ring; or it may be a pole-armattire, in which it is coiled around projecting pieces of iron, as shown in fig. 34 ; or, fourthly, it may be a disc-armature Eig. 33. — Ring-armature. Fig. 34. — Pole -armature. with the coils placed close against the faces of a rotating disc. The armature-coils are usually in series in order to increase the total electro-motive force ; the difi'erent loops might be attached separately, each to the ends of a copper plate, but the arrange- ment would then be like a number of voltaic cells in parallel, the total electro-motive force of which is equal only to that of one cell. But by connecting all the coils in a continuous series, as shown for the ring-armature of fig. 35, all the difi'erent electro- motive forces generated in each coil at any moment, according to the number of lines of force cut by it, are added together and make up the total electro-motive force between the brushes, B, B'. That the number of magnetic lines of force cut by a coil, varies at difi'erent periods of its rotation, and hence that the- E.M.F. produced by the passage of the same coil through equal angular distances in difi'erent parts of the field also fluctuates, will be clearly seen by the diagram (fig. 36). Here the coils A and C, when moved through (say) 5 degrees from their present position, are moving almost parallel to the lines of force, so that in this medial position between the magnets there is almost no current produced ; and it is at these points also that the reversal in the direction of the current is observed. But the coils B and D, when moved through an equal angle from their position on. THE DYNAMO-ELECTKIC MACHINE. 7& the magnetic axis, are meeting the lines of force nearly at right angles, and thus cut the greatest possible number in a given period. These points, B and D, in the magnetic axis are, there- fore, the centres of greatest current-production in a given coil, while A and C, perpendicular to the lines of force, are those of least energy, and all other points between them fall into a regularly graduated series ; so that, starting from the locality of no current-production. A, the electro-motive force in each succes- Fig. 35. — Ring- Fig. 36. — Yariation in lines of force armature. cut during rotation. sive coil increases until the maximum is reached at B, when it diminishes to zero at C, and rises to a second maximum at D, finally falling off to nothing again at A. But supposing the current to be flowing through the coils on the right-hand side of the armature-ring, from a brush placed at A to another at C, it will also flow downwards from A to C on the left-hand side, because, although the direction of current is reversed at C, so also is the position of each coil reversed in relation to the magnetic poles, so that now a reversed current flows practically through a reversed spiral. Thus the current flows in a parallel fashion from one brush to the other through the two halves of the ring, and the electro-motive forces generated in both halves are identical, if the armature is symmetrically wound, and are equal to the sums of those produced in the consecutive coils. Thus the points A and C, where the two parallel currents meet, are those of highest aggregate electro-motive force, so that they are in every way marked out as the positions for the brushes. As, however, it would be mechanically inconvenient to place them at the outside of the ring, it is usual to connect the circuit at intervals with the diff'erent sections of the commutator, as shown diagrammatically in fig. 37 ; the rings are thus divided into a convenient number of groups, and each group (not each 80 SOURCES OF CUKKENT. axis the brushes Fig. 37. — Modes of connecting groups of coils to commu- tator. ring) is joined to the collector, which consists simply of a series of copper bars insulated from one another, and fastened to the axis of the rino\ with one group of coils connected to each ; on this press lightly at the points of least action but highest E.M.F. The brushes only touch two diametrically opposite commutator-bars at the same time, and are thus insulated from all the others, so that the current generated cannot find its way to them except by passing through every coil upon the half-ring. This explana- tion has referred only to ring-armatures but the other forms are similarly dealt with. For electro - metallurgical purposes the essential recommendation of a dynamo is, that it shall be able to produce a perfectly steady current of low electro-motive force but large volume. In order to accomplish this, a small number of turns of stout wire or rod (because of its low resistance) is made to take the place of the many windings of wire, which are required to yield the high electro-motive force needed in electric lighting. The electro-motive force is reduced because there are fewer coils, the current volume is increased because the resistance is more than proportionately diminished ; but the product obtained by multiplying the number of amperes by that of the volts (or the number loatts, as this product is termed) may be the same in the machine before and after alteration. A few dynamos used in electro-metallurgical work may be briefly referred to ; after the general principles above set forth, no attempt will be made to give a detailed description of each machine. A few selected drawings are also given, but for the full description of the different instruments reference should be made to Thompson's i)^mmo ^/ecfn'c Machinery, ov other standard works. Wilde Dynamo. — The Wilde dynamo (fig. 38), which was one of the first practically used, was made with a small magneto- electric machine (with permanent steel magnets) as an exciter, the current thus produced circulating around the field-magnets of the dynamo beneath, and so generating the stronger current in the armature of the larger machine. Both exciter and generator were united on the same stand, and could be run from the same shafting by using suitable pullies and belts. Here, the horse- shoe form of magnet was adopted, which has since been retained in a few types of machines, but has been replaced in many by magnets with consequent poles. THE DYNAMO-ELECTRIC MACHINE. 81 Such a magnet is made by winding a soft iron coil in one direction (see p. 95), along one-half of its length, and revers- ing it in the other, so that the two extremities of the magnet are of like polarity, while the centre has the opposite polarity induced from each end of the bar. Fig. 39 illustrates diagram- matically the principal parts of a shunt-wound dynamo having field-magnets wdth consequent poles. FM are the field-magnets Fig. 38. — Wilde's first Fig. 39.— Magnet with consequent dynamo. poles. w^ound with wire in reversed directions as described ; A is a ring- armature with wires leading from each coil to the corresponding section of the collector, C, from which the current is withdrawn by the brushes, B. From the brushes the main circuit, MC, is taken together with a thin shunt-wire required for exciting the electro-magnets. The collector and armature are rotated by the spindle, Sp, which is attached to a pulley outside the machine not shown in the sketch. Gramme Dynamo. — A common form of the Gramme machine used for electro-plating is illustrated in diagrammatic form in fig. 40. It is series-wound, has consequent poles, and is not unlike the machine shown in fig. 39. The left-hand sketch shows a vertical section of the machine through the line of the brushes, the right-hand side gives an outline elevation. The field-magnets, are often wound with sheet-copper, instead of wire, to reduce the resistance, and with the same object the armature- coils are of copper rod ; they are, of course, more numerous than F 82 SOURCES OF CURRENT. those in the illustration, in which detail has been sacrificed to clearness ; the sections of the commutator must always be so close together that one comes into contact with the brush as Fig. 40. — Gramme machine. soon as the preceding one leaves it, otherwise the current becomes very intermittent, and sparks are produced at the points of contact. In Gramme electro-metallurgical dynamos Fig. 41. — Siemens machine. the'^armature is built up oil stout copper rods, insulated from onel^another by bitumenised paper, by laying them lengthwise upon the surface of a dram, then winding well-varnished iron DYNAMO-ELECTEIC MACHINES. 83 Avire around the central portion, and enclosing the latter by a •second series of copper bars attached by end-pieces to the front of one long bar, and to the back of the next, so that a con- tinuous spiral is formed, of which the front portions are joined up to the commutator. Siemens Djniamo. — The Siemens machine (fig. 41) is also made up with thick copper armature-plates instead of wire, and in one form the exciting of the field-magnets is efi'ected by a current passing through seven parallel turns of stout copper rod, in order to minimise the resistance. This machine also has conse- quent poles, but uses the drum-form of armature. When wire- wound armatures are employed, the resistance is frequently reduced by arranging the wires, not all in series, but grouped in four parallel circuits. Weston Dynamo. — In the Weston machine, a diagram of which is given in fig. 42, six electro-magnets with steel cores are fixed radially in the interior of a cast-iron drum, bolted to a bed-plate also of cast-iron. The wires in the alternate ■electro-magnets are wound in different directions, so that adjacent poles shall be of opposite denomination ; within the circle formed by the electro-mag- nets rotates a six-part pole-armature, the wdres from each pair of poles diametrically opposite being united, and coupled-up to one section of a three-part commutator. The alter- Fig. 42. — Weston nicxciiine. nate magnets presenting unlike poles renders this form of machine different from that which we have been considering, in so far as there were but two reversals in the ^ direction of the current in the twin-magnet dynamo, while here there are six, one between each pair of magnets, so that a somewhat different form of commutator is needed to rectify this alternating current and make it continuous. Weston obviates the danger of reversal of current by the back electro-motive force from the depositing vat on the occasion of sudden stoppages {vide p. 77), by the use of steel cores to the electro-magnets, which have a greater supply of residual magnetism, and by a special form of governor attached to the base-plate, which automatically regulates the current and prevents it falling to a dangerously low electro-motive force. Shlickert Dynamo. — The Shiichert dynamo is in principle similar to that of Gramme, but the armature is contracted in length until 84 SOURCES OF CURRENT. it is only a flat ring, and it is almost completely surrounded hy broad iron pole-pieces {i.e., continuations of the poles) attached to the magnets. Brush Dynamo." The Brush machine (fig. 43) was at a very Fig. 43. — Brush machine. early date adapted for electro-plating, and in its new form is. specially compound-wound, so that it shall give a current of constant potential through very wide ranges of intensity Victoria Dynamo. — Fig. 44 illustrates the ' Victoria dynamo^ Fig. 44. — Victoria dynamo. which is also adapted by the Brush Electrical Engineering Com- pany for electro-metallurgical work upon a large scale, such as for the refining of copper or other metals, or for electro-treatment of ores generally. DYNAMO-ELECTRIC MACHINES. 85 Krottlinger Dynamo. — The KrottUnger dynamo is used by many :firms abroad; it is shunt-wound with short wide field-magnets set upright on a base plate, with inwardly curved pole-pieces above, between which the armature rotates. The number of dynamos in the market at the present moment is very numerous, and most makers are prepared to wind machines upon their system, suitable for electro-metallurgical work; the .above list is, therefore, in no way complete, it has simply included a few of the best known and frequently used generators of this type. Instructions as to the Management of Dynamos. — In itself the "dynamo is a simple machine and should give but little trouble or difiiculty, provided that a due amount of attention is paid to its installation and superintendence. It should be firmly set on good level foundations, in a position where it is not likely to be exposed to contact with any of the liquids employed in the shops, nor to acid fumes or dust. It is best located in a dry place, enclosed in a box of wood or glass which may be easily removed for purposes of inspection, and which has openings cut in its side to provide for the passage of the engine belt or for the shaft of the pulley; it should be in a room adjoining the plating-room, but as near as possible to the vats, in order to minimise the loss of energy due to the resistance of the wires. If erected in the shop it must be in a place removed from splash- ings, and must be constantly examined. It must be kept scrupulously clean in every part ; the bearings must always be well lubricated with good oil, as the speed of shaft rotation is usually very high (500 to 1000 revolutions per minute), but excess of oil, and especially leakage of oil into the armature or on to the commutator and brushes, must be rigidly guarded -against. By the action of internal currents, as well as by friction, the machine is liable to become overheated, and may even be seriously damaged through the burning of the insulating material upon the wires ; the temperature of the fixed portions should, therefore, be ascertained while current is being generated by feeling them carefully with the hand. With fair usage the only part needing special attention is the commutator or collector. The brushes must be perfectly flat ; when they become ragged or turned up at the tips they should be clamped in a wooden holder and filed carefully ; they should rest with a gentle pres- sure upon the copper strips of the collecting shaft ; insufficient pressure combined with an uneven commutator will induce sparking at the brushes, which causes much wear and tear of the parts, whilst excessive friction will wear away the collecting 86 SOUECES OF CUREENT. rods unevenly and ruin the brushes, at the same time producing- more or less copper dust, which in course of time penetrates into and injures the machine. The brushes should also touch the- collector rods at opposite ends of a diameter, and the 'lead,' already referred to (p. 76) as necessary to the ]3rushes of nearly every dynamo, should be rightly adjusted or else sparking will ensue. Should the dynamo show sparks at the commutator, the , lead of the brushes may be altered backwards or forwards until a neutral point is found ; they are generally set on a frame by which they are maintained exactly opposite to one another, and by which they may be kept in any desired position. The com- mutator may be very slightly greased before commencing the day's work, preferably with a little grease or vaseline applied with the finger ; cloth or cotton should not be used, as fibres may be left behind, and it is essential that no foreign substance find its way to this portion of the machine. Oil must never be applied in any quantity, and the grease only in very minute proportions (they add to the resistance), while black lead or mercury, which some operators have applied to the surfaces, must be avoided altogether, as the latter gradually amalgamates, the copper and renders it very brittle, while the former gives a slightly conductive film to the insulating space between the bars, of the collecting ring, and so impairs the electrical efiiciency of the machines. The brushes must never be lifted out of contact while- the dynamo is running and the current passing ; the insulation maybe, and the commutator certainly will be, damaged by the sparking caused by such treatment. Irregularities in the com- mutator rods, or undue pressure, or sparking at the brushes will gradually wear the shaft to an oval or uneven shape ; this fault should be watched for and rectified at once by turning in the lathe to true circular section ; such a course should not, however, be necessary for several years. The dynamo may be driven by any regular source of mechani- cal energy, either the steam-, gas-, hot air-, or petroleum-engine (or, of course, water-power) may be used, the only requirements being sufficiency of power and uniformity of speed. Frequently spare power is obtainable from an engine used for other pur- poses, and may be applied with advantage, unless great irregu- larities in speed are caused by frequent variations in the amount of power absorbed by the machinery to which it was originally adapted. Accumulators or Secondary Batteries, which are so largely^ used in other branches of electrical engineering, may be also ELECTRICAL ACCUMULATORS. 87 employed in electro-metallurgical installations, but generally speaking little benefit will be derived from their use. Secondary Batteries. — The great advantage of the accumu- lator is that it converts the electrical energy of the dynamo, at times when it cannot be directly applied, into another form of energy (chemical) which may be, as it were, stored up until a more convenient moment has arrived for its re-con- version into electricity. Several kinds of secondary batteries have been brought before the public, but almost the only one in general requisition is that originated by Plante and after- wards improved by Faure and others. It consists of two lead plates opposed to one another in dilute sulphuric acid. On passing a current through this constantly in the same direction, the anode plate, by which the current enters the solution, be- comes superficially converted into lead peroxide (PbOg) by the oxygen liberated at its surface, and this, being insoluble in sulphuric acid, remains in situ, the action penetrating deeper and deeper into the plate the longer the current is continued. The cathode, at which hydrogen is deposited, remains, of course, unaltered. On the cessation of the current, two plates are opposed in the solution, one of metallic lead, the other practically of lead peroxide; on completing the circuit these act like a primary (or ordinary voltaic) battery, the clean lead plate (electro-positive) oxidising and receiving a deposit of the mon- oxide, which like the peroxide is insoluble, while the peroxide (electro-negative) gives up its excess oxygen and becomes also converted into monoxide. When both peroxide and metal have attained to the same condition the action ceases, but on re-charging by a dynamo, the original arrangement is restored, the lead oxide at the anode taking up oxygen and becoming peroxide, that at the cathode being reduced to the state of metal by the hydrogen liberated upon it. It is thus rehabili- tated and is ready for use again, and this cycle of changes may be repeated indefinitely. Now, instead of forming the cell from two clean plates, the positive and negative plates are generally coated with pastes of red lead and sulphuric acid and of litharge and sulphuric acid respectively ; cavities are frequently made on the surface of the lead with the object of affording a larger surface, and, more particularly, of maintaining the oxide pastes in position. Plates thus prepared behave at the outset like those which have been in use for some time, and do not require the fr.equent repetition and reversal of charging which is found necessary to ensure sufficient penetration of the oxide into the substance of the pure metaUic sheets. In use, these cells 88 SOUECES OF CUKRENT. are exactly analogous to ordinary batteries, and obey the same laws as to generation and distribution of current — indeed, they are practically nothing but an unusual form of galvanic battery ; but they should not be allowed to completely discharge themselves, nor should they be subjected to electrical shocks such as would occur if they became suddenly short-circuited. The electro- motive force of each cell is equal to about 2 volts. Use of Public Electricity Supply.— Most towns have now a pubHc electricity supply for Hghting purposes and for power; but this supply is usually at a pressure of 100 or more volts, which is far too high for direct use in electrolytic vats, unless many are coupled in series, which, for plating purposes, is practi- cally out of the question. Moreover, in many towns the current is alternating, and therefore useless to the electro-metallurgist. Such currents may, however, be readily utilised. For example, if the current from the supply mains be continuous (not alternating) and at a pressure of 100 volts, it may be passed through a number of accumulator cells (say 40) joined up in series. The cells, being thus charged together, may be separated and used individually, so that a pressure of 2, 4, 6, 8 or more volts may be obtained for any purpose by using 1, 2, 3, 4 or more cells. A simpler method than this may be used with advantage, and is applicable alike to continuous and alternate current systems ; it depends on the fact that a dynamo when reversed becomes a motor ; that is to say, if a current of electricity be sent through the brushes, and thus through the armature of a dynamo, the electrical energy will become re-converted into mechanical energy, and the armature will rotate and will act as a motor. Motors specially wound to suit any continuous or alternating current can be procured, and may be placed on the main town circuit, and used to drive an electro-plating dynamo by means of a belt ; or the two dynamos (motor and generator) may be mounted on the same shaft, in which case the couple are frequently called a inotor- generator ; or they may even be made, by special winding, to use the same field magnets, and are then practically one machine, often known as a rotatory converter. With an arrangement such as any of the above, the current from the town supply mains may be converted into a current of low E.M.F., suitable for electro- plating, and of correspondingly greater volume. Except for the loss in conversion, the total watts remain the same, so that what is lost in volts is gained in amperes. It should be unnecessary to point out that a plating dynamo must not be used as a motor on the town supply mains. CHAPTER lY. GENERAL CONDITIONS TO BE OBSERVED IN ELECTRO-PLATING. -Absolute Cleanliness Essential. — In electro-metallurgical work there is one prime necessity which cannot be too often or too jstrongly insisted upon, and that is, absolute cleanliness in every particular. Neglect of cleanliness in the electric generator, and in the connections throughout, causes the current to be deficient ;and the resistance of the circuit to be increased ; neglect of cleanliness in making up the baths introduces impurities into the solutions, and the character of the deposit suffers accord- ingly ; neglect of cleanliness in preparing the articles to be plated for their plunge into the depositing-vat ensures an irregular and non-adhesive coating; neglect of cleanliness after the plating is accomplished is likely to cause rapid tarnishing or rusting of the deposit. In short, want of cleanliness is the <}ause to which the largest proportion of electrotypers' and platers' troubles may be referred. And in respect of non- adhesiveness of a deposit, it should be noted that the articles to be treated must be chemically clean] a trace of grease, producible by mere handling, suffices to ruin the coating, be- cause the precipitated metal will adhere only to perfectly pure metallic surfaces, and the merest film of foreign matter, invisible perhaps to the eye, prevents this adhesion, or weakens it to so great an extent that very slight friction may cause separation to take place. As also Careful Adjustment of Currents. — It is further essential that the current shall be correctly proportioned to the work re- quired from it, and, if for this reason alone, it is to be regretted that old * rule-of-thumb ' methods find very general favour. Apparatus for measuring the current is comparatively inex- pensive, and the first outlay in instruments will soon be found to repay itself in the greater security insured against accidents due to careless work or imperfect knowledge, and in the im- mediate and certain indication of a failure in any part of the circuit, which may prove most detrimental to the work if 90 GENERAL CONDITIONS TO BE OBSERVED IN ELECTRO-PLATING. allowed to remain unremedied, but which, if attended to in time, might exert no evil influence. An ammeter ( = amperemeter) for measuring the volume of current to every bath, or to every series of baths placed in parallel, with a voltmeter for determining the fall in the pressure of the current between the electrodes, should be used in all large installations in which it is desired to produce good work of imiform character, or where varied work is undertaken. It is true that an experienced workman may know by inspection whether his baths are in good order; but the best workman is not infallible, and the use of measuring apparatus substitutes certainty for uncertainty, besides enabling the foreman to ascertain at a glance the conditions of w^ork at any moment. Where, too, a partial failure has occurred, the remedial measure to be adopted may often be indicated, and the current be at once restored to its original strength by adjusting the resistance of the circuit until the ammeter returns to its- first position. TABLE YIL— Showing the Average Current Values Suitable FOR Depositing Certain Metals. Amperes. Volts Metal between Per Sq. Anode and Decimetre Per Sq. Incb of Cathode. of Cathode Cathode Surface. Surface. Antimony, .... 0-4-0-5 0-020-0-030 10 -1-2 Brass, 0-5-0-8 0-030-0-050 3-0 -4-0 Copper, acid bath, 1-0-1-5 0-065-0-100 0-5 -1-5 ,, alkaline bath, 0-3-0-5 0-020 -0-030 3*0 -5-0 Gold, 0-1 0-006 0-5 -4-0 Iron, 0-5 0-030 1-0 Nickel, at first, .... 1-4-1-5 0-09 -0-10 5-0 ,, after, .... 0-2 -0-3 0-015-0-02 1-5 -2-0 ,, on zinc, .... 0-4 0-025 40 -5-0 Silver, ..... 0-2-0-5 0-015 -0-030 0-75-1-0 Zinc, ...... 0'3-0-6 0-02 -0-04 2-5 -3-0 A current which is either too strong or too weak gives un- satisfactory deposits, the most suitable strength in any instance depending upon the nature of the metal which is being deposited and that of the bath from which it is separated. A very weak current causes a slow and in some cases a bad deposit, while USE OF ELECTRO-CHEMICAL EQUIVALENTS. 91 a very intense current renders the metal non-adhesive, and may even reduce it to a pulverulent form. In the preceding table are given the quantity and electro-motive force of the currents which have been found by careful operators to be best suited for depositing the commoner metals ] but such general state- ments must be regarded only in the light of a guide ; every bath will be found to have its own most suitable current value, which will probably differ but little from those given above, but which may be readily determined, once and for all, by the use of the bath. Electro-Chemical Equivalents. — In dealing with electro-deposit- ing arrangements. Ampere's fundamental law that the current in amperes is proportional to the electro-motive force in volts divided by the resistance in ohms (C= -, see p. 59) must be borne in mind. The weight of metal deposited in a given time is dependent solely on the volume (amperes) of current passing through the solution. A coulomb of electricity 1 ampere passing for the space of one second of time), according to measurements made by Lord Rayleigh, deposits invariably 0*000010352 gramme of hydrogen; of any other metal it will deposit this fraction of a gramme multiplied by the equivalent weight of the metal, that is, by the atomic weight divided by the valency of the metal as it exists in the solution. Thus, a coulomb of electricity precipitates per second from silver cyanide 108 0-000010352 X -j- = 0-00118 gramme of silver, or 63*5 0-000010352 = 0*000328675 gramme of copper from a solution of copper sulphate ; while from a solution of a cujprous salt it would deposit twice the last-named weight of copper,, because in this class of compounds, copper is monovalent, and the atomic weight is therefore divided by 1 instead of by 2. The figures obtained by thus multiplying the coulomb-weight- value of hydrogen by the equivalent weights of the elements are termed their electro-chemical equivalents, and afford the data from which the electro-plater may determine the power which he will require to deposit a known weight of any metal, or to obtain a given thickness of coating upon a known area of surface. These numbers are collected together in tabular form on p. 399 ;, where also will be found the weights of the commoner metals, expressed both in grammes and in grains, which should be deposited in one hour by a current of 1 ampere, together with the thickness of the deposits produced in the same period of time •92 GENERAL CONDITIONS TO BE OBSERVED IN ELECTRO -PLATING. hj a current of 1 ampere per square decimetre, and per square inch of anode- and cathode-surface. The number of grammes precipitated per hour is calculated by multiplying the electro- chemical equivalent, or grammes per second, by 3600 (the number of seconds in the hour). The thickness of deposit is found from the weight deposited per hpur, taken in connection with its volume ; — a cubic centimetre of any metal weighing, in grammes, the number which represents its specific gravity (for example, the specific gravity of electrolytic copper being 8 '9 14, 1 cubic centimetre weighs 8*914 grammes). From this table, then, may be found approximately the time required to produce •either a given weight or a given thickness of deposit, provided that in the former case the strength of current, and in the second both current-strength and total area of cathode be known. Alteration of Resistance.— Since the current depends upon the E.M.F. and the resistance ; to multiply the strength of current, ^nd hence, also, the rapidity of deposition, either the electro- motive force must be increased or the resistance reduced. Of these alternatives, the latter is more readily modified than the former, and it is usual, for this reason, to introduce into the path •of the current an arrangement of wires forming an added resist- ance, which may be thrown in or out of circuit at pleasure, and will thus produce a commensurate effect on the current volume. Any alteration of the relative positions of the electrodes in the bath produces a change in the electrical resistance, and, therefore, •demands thoughtful attention, especially when measuring instru- ments are not available to indicate the extent of the derangement. Whenever an anode (or a cathode) is removed from the bath, the conducting surface through which the current enters (or leaves) the solution is diminished, and the resistance is conse- quently increased ; or when the anode is removed to a greater distance from the object being plated the current has to traverse a more extended length of the solution, which is a very inferior conductor, and the resistance is again increased. In each case the volume of current is correspondingly diminished, and the alteration is at once detected by the retrogression of the pointer upon the scale of the ammeter, or current-measurer. It sometimes happens that the electrodes touch beneath the liquid, or become connected by a fragment of metal, broken off perhaps from a faulty anode or a bad cathode-deposit ; the current then ceases to pass through the solution, and finds its way through the short circuit; electrolytic action is of course stopped, but •outwardly there may be nothing to indicate the casualty, €xcept in a few instances the behaviour of the battery, which USE OF KESISTANCE BOAKDS. may show signs of unwonted activity; here again the ammeter at once gives the alarm, because the diminution of resistance, owing to the substitution of a metallic for a liquid conductor,, causes a great increase of current ; it should be observed that this increase of current is accompanied by a greater consumption of battery-zinc, of which none is doing useful work ; not only is^ the whole energy wasted, but the deposit may even be seriously damaged. All this serves to emphasise the necessity for apply- ing measuring-apparatus at any time, but especially when dynamos are used, as they are more liable than batteries to- be injured by short-circuiting or unfair treatment. Again, since increase of resistance is attended by a decrease in current volume, the resistance of the circuit must be minimised in every possible way ; the generator must be placed as near to the vats as may be convenient, the copper connecting-rods or leads must be as thick and as pure as possible, and all surfaces of contact, through which the current has to flow, must be perfectly clean and bright, while the electrolyte-solution itself should be chosen with a view to high conductivity. Conversely, all wires must be prevented from mutual contact, except at the desired points of connection, otherwise a short circuit or leak may be set up which permits practically the whole, or at best a. large portion, of the current to return by the negative wire to the generator, not only without having done its appointed work, but with introduction of inconvenience and waste by its con- version into heat principally within the battery itself, which now imposes the principal resistance. Resistance Boards. — The arrangements which are employed to introduce additional resistance into the circuit are usually made of wire, which must not be too thin, or they will become over- heated by the passage of the current; they may be constructed of copper, brass, or German-silver wire, of electric-light carbons, or (for large currents) of moderately thin hoop iron. Of the three former, German-silver is the least conductive and therefore the best, brass standing second. A length of 20 inches of German- silver wire, or 10 yards of copper wire, either of No. 20 Birming- ham wire-gauge, or a foot length of a carbon -^^ of an inch in diameter, should each afford a resistance equal to about the quarter of an ohm. Fig. 45 illustrates a convenient method of arranging several coils of wire. The circuit is broken at one point, and the two ends of the conductor are connected to the system of resistance .wires at MC and MC ; the wire is mounted on a wooden frame by stretching it backwards and forwards alter- nately over the metal pins A, G, B, H, C, I, D, J, E, K, and F,, 94 GENERAL CONDITIONS TO BE OBSERVED IN ELECTRO-PLATING. of which A is attached to MC. The pins are so arranged that the handle H, which alone is attached to MC, may be moved upon its axis from contact with the MC rod at A, until it touches the buttons B, C, D, E, or F successively. Each section of the wire from A to B, from B to C, and so on, should have a resist- ance of (say) half an ohm. H consists of a flat brass rod with a wooden holder around the free end ; while it rests at A, the current flows through it directly from MC to MC without meeting with, any appreciable resistance, but as soon as it is shifted to B, the current has to pass through the section of wire passing from A over G to B, and so meets with a resistance of I ohm ; by moving the handle until it rests upon C, the resistance of the length of wire B H C is added to that of A G B, and thus a total of 1 ohm is no^v inserted ; similarly each succes- Fig. 45.— Mode of arranging Fig. 46.— Simplest resistance board. galvanoscope. sive move adds ah extra ^ ohm until, when H is resting upon F, the added resistance is 2 J ohms in all ; finally, by shifting the handle beyond the position E, the current is broken altogether. This instrument serves, therefore, as a switch to start or break the current, and as a regulator to control its strength. Galvanoscope. — Of the measuring and detecting instruments available, the galvanoscojoe or detector is one of the simplest : it is simply a magnetic needle surrounded \vith a coil of insulated wire through which, the current is made to pass ; it cannot well be employed for actual measurement, but may sometimes be useful in indicating the direction of the current, and thus enabling the operator to determine at a glance, and without reference to the battery or dynamo, which of two wires should be connected to DETERMINING DIRECTION OF CURRENT. 95 the anode of the depositing-vat. This it does in obedience to the law, discovered by Ampere, that a current flowing in a circuit, placed around a magnetic needle in a plane perpendicular to tha*t in which the latter is free to turn, causes the needle to set itself at right angles to the coil (fig. 46), the south pole being on that side of the coil from which the current appears to circulate in the direction of the hands of a watch. This arrangement will, perhaps, be better under- stood by the self-explanatory diagram (fig. 47). But for ordinary electro-plating currents even a detector is unnecessary ; a common compass-needle held in its case above or below the wire through which the current is passing suffices to indicate its direction by turning on its pivot with the north pole facing to the left or to the right. For this experiment the wire should extend from north to south so as to be parallel to the normal direction of the magnetic needle. The simple rule by which the •direction of current may be determined (and this rule applies equally to the last-considered example of a coil of wire) is, supposing a man Fig. 47. — Illustrating Ampere's rule. Fig. 48. Direction of electric currents. to be swimming with the electric current, inside the wire, head first, and with his face turned towards the magnetic needle, the north pole of the latter will set so as to be on his left hand. Prof. Jamieson's mnemonic rule is also simple and reliable here ; it will be readily understood in reference to figs. 48 and 49. ^ If a compass-needie be placed on one side of a wire through which ^6 GENERAL CONDITIONS TO BE OBSERVED IN ELECTRO-PLATING.. a current of unknown direction is flowing, and the right hand be placed on the other side of, and with the palm next to, the wire, so that the thumb points in the direction taken by the- north pole of the needle ; then the current will always be found flowing (from positive to negative) from the wrist towards the^ fingers. Thus, if the current in a wire run from north to souths the compass-needle will place itself with its north-seeking end pointing east when the compass is held beneath the wire, and pointing west when it is above it. Galvanometers. — For measurements, elaborations of the com- pass-needle detector are employed, and are termed galvano- meters. These are of many kinds, but all depend on the fact that a stronger current always causes a greater deflection of the needle than does a weak current. In its simplest form the galvano- meter is like the galvanoscope ; it consists of a number of coils of wire surrounding a delicately-poised compass-needle, which is^ attached to a thin vertical wire passing through a circular card, and carrying above the surface of the latter a light horizontal index-needle or pointer ; the card is firmly attached to the coils,, and is divided up into degrees, so that the angular motion of the pointer produced by a given current may be measured accurately. The angle traversed by the index is not, however, proportional to the strength of the current, and the instrument must be graduated by ascertaining the varying angles of deflection produced by currents of known strength; for the same volume of current always registers the same number of degrees upon the scale. The extent of the deflection is regulated by the resultant of the two forces — one the directive force of the earth, which tends to set the needle north and south, and the other that of the current, which tends to place it equatorially. In using this in- strument the index must first be allowed to come to rest under the action of the earth's magnetism alone, the coils (and the card with them) are then gradually shifted until the index points- to the zero ,of the scale, then the current is passed around the coils, and tlie angle through which the pointer has turned is measured as soon as equilibrium is restored. It is often incon- venient to be obliged to alter the position of the galvanometer, so that the nt^edle is initially north and south ; and an astatic pair is with advantage substituted for the single needle ; that is to say, two needles equally magnetised are fixed, one at a short distance above the other, but pointing in reverse directions, so^ that the north pole of the upper one lies above the south pole of the lower. The wire coil is wound around the lower needle only ; the object of this arrangement being to eliminate terres- CUKRENT-MEASURERS. 97 trial magnetism, by causing its opposite directive action on the two needles respectively to produce equilibrium, while it does not interfere with the relation of the needles to the current, because, although they are reversed in position, one lies above and the other beneath the upper portion of the coil, so that the effect of the current is also reversed. The method of fixing the astatic galvano- meter is shown in fig. 50 : the two needles are attached to a pointer which rotates above the fixed card- board scale, the whole system of moving parts being suspended by a single filament of raw (unspun) silk. This galvanometer must also be graduated. With a coil of considerable di- ameter, a magnetic field of uniform intensity is created at the centre, and a very small needle balanced at that Fig. 50. — Astatic galvano- point will set, under the influence of meter, different currents, at angles, of which the tangents are proportional to the volume of the directive currents. With such a tangent galvanometer^ when once the angle of deflection produced by any known current-strength is deter- mined, the value of any other current may be found without further graduation, by observing the angle through which it causes the index to turn ; the strength of the first current is to that of the second as the tangent of the former angle is to that of the latter. Thus with a table of natural tangents at hand, this galvanometer is practically a direct-reading instrument, and is of more general utility than the astatic form. Ammeter and Voltmeter. — Modified galvanometers are now made specially for determining the strength of current at a glance ; they are graduated on the scale in amperes, and are known as amperemeters^ or better as ammeters. By greatly increasing the number of coils in an ammeter, and with this its resistance, it becomes practically a voltmeter, and affords a direct reading in volts of the electro-motive force of the current employed. These two instruments are now so simple, compact, and inexpensive that they are within the reach of all users of electricity, and should certainly be included in the plant of every electro-metallurgical work. The ammeter has a very low resist- ance, and is placed in the main circuit, thus the whole strength G 98 GENERAL CONDITIONS TO BE OBSERVED IN ELECTRO-PLATING. of the current passes through it, and may be read off at any time ; where several baths are arranged parallel to one another, and the current is divided between them, an ammeter should be included in each section, in order that the operator may be sure that every vat is receiving its due proportion of current. The voltmeter will indicate the difference of potential between any two points of the circuit, but it is usually connected as a shunt, or across from one wire to the other ('parallel' to the vats) in close proximity to the electrodes in the vats; the resistance of the coils is so great, as compared with that of the baths, that only an infinitesimal portion of the current passes through them, and the distribution to the baths is unaffected. ^Nevertheless, it is customary to attach a small press button on a switch to the stand of the instrument, so that it is only thrown into circuit at the moment when the observation is made.^ A single volt- meter may be made to suffice for a small installation. Arrangement of Baths in Electro-plating.— In arranging the baths and selecting solutions the operator must be guided entirely by the class of work which he proposes to undertake. The necessity for pure solutions of high conductivity has already been insisted upon. The chemicals employed should be the purest obtainable, and the water should, if possible, be distilled, other- wise rain-water must be used. The solutions must be made up carefully to the required strength, and watched well, and occasionally tested while in use to ensure that they do not sensibly vary in composition, or become excessively alkaUne or acid. Moreover, they must be suited to the nature of the substance which is receiving the deposit; if this latter metal should of itself decompose the solution, unaided by any external current, the resulting deposit will probably be non-adhesive. Hence if a very electro-positive metal is to receive a coating of one which is highly electro-negative, it should first be covered with an intermediate metal from some solution which it cannot readily decompose, this metal, in turn, being of such nature that it will not break up the electrolyte of the ultimate metal to be precipitated. For example, in coating iron with silver, a pre- liminary film of copper may be given in the copper cyanide bath, and the coating of precious metal is readily deposited upon this. The vats should be thoroughly cleaned out from time to time to prevent the accumulation of slime, which always tends to collect, owing to dust and accidental impurities derived from the air, and to insoluble impurities contained in the anodes, that remain as a precipitate in the liquid after the rest of the metal has dissolved. The current should be switched on as soon as the ARRANGEMENT OF BATHS. 99 objects are introduced, or there will be a tendency for the basis metal to dissolve into and contaminate the bath before it can be covered with a protecting film ; this frequently renders the introduction of resistances necessary when first placing goods in the vat, in order to avoid using an excessive volume of current, as will be explained in dealing with silver (see p. 213). Arrangement of Baths in Series or Parallel. — When several baths are used on the same battery-circuit, they may be arranged either in series or parallel to one another ; but for miscellaneous plating the latter method is superior, although, in a few instances, when the work is quite uniform, as, for example, in electrotyping plates for printers, it may sometimes be advantageous to couple the vats in series, especially if a dynamo be used as a generator. The two systems may, of course, be combined, two or more series, with two or three baths in each, being arranged in parallel. When insoluble anodes are employed, it must be remembered that the electro-motive force required for the decomposition will be as many times higher than is required for one couple as there are baths in series. Two vats in series require twice the pressure needed for one, three vats demand thrice the pressure, and so on. When the anode is soluble and is made of the metal which is being deposited, the reduction of E.M.F. is not great, but the resistance is, of course, multiplied. Placed parallel, the resistance is diminished as the number of vats is increased, while the potential required is constant. Figs. 51 and 52 show dia- grammatically the arrangement of vats parallel and in series respectively. B is the battery; Y is the voltmeter, which may be thrown into circuit when required for use by means of the switch, S; A is the ammeter, of which there is one in each parallel circuit (with it may conveniently be a set of resistance wires) ; and E represents the electrolyte or plating- vat. It was shown on p. 61 that battery-cells coupled in series caused the solution of equal quantities of zinc in each cell, although altogether they could not deposit more than one equivalent of metal in the plating-bath, so that five times as much zinc was dissolved to precipitate one pound of silver when five cells were placed in series as when one cell alone was made to do the work. Conversely, in electro-deposition a given current with sufficient electro-motive force will deposit in each vat as much metal per unit of zinc dissolved in the battery as would be precipitated in a single vat. Just, therefore, as in the battery, coupling in series gives an increase of pressure and of power to do work rapidly, so a similar arrangement of plating-baths involves a greater absorption of pressure, and to a corresponding 100 GENERAL CONDITIONS TO BE OBSERVED IN ELECTRO-PLATING extent increases the time required to deposit a given thickness of metal. The economy of power, apparently promised by the precipitation of an increased weight of metal per unit of zmc dissolved, is thus discounted by the inconvenience of a slow deposit. A further objection to the series system as apphed to the plating-baths is that they become too mutually inter- dependent, for any increase in the resistance of one bath, as when a large article is removed from it, reduces the current-volume m the whole circuit. It is true that even when the parallel arrange- E 5 £ Fig. 51.— ^Parallel arrange- ment of vats. Fig. 52. — Vats arranged in series. Note —In these two figures one voltmeter is so arranged that, with the aid of the switch, S, it maybe used for either vat at will, and will thus show the difference of potential between either pair of electrodes. In fig. 52 two switches are necessary (as shown) unless the voltmeter used is capable of mpasuring deflections on either side of zero. One ammeter is used to each bath. ment is adopted, an alteration of resistance in one vat also influences the current-strength of the whole system, but the eff'ect is scarcely appreciable owing to the large aggregate surface presented in the difl'erent electrolytes ; the added resistance m one bath merely causes a greater volume of current to flow through the others, while on the other hand it tends to dimmish the total current. Where, then, the articles to be plated may be of every conceivable size, and the superficial areas are difficult to estimate exactly, the parallel system is preferable ; but when the articles present a fair uniformity of surface, and the current has sufficient potential, the alternative method may be substituted. For similar reasons it is best to hang the various articles m each THE ANODES. 101 iDath parallel to one another, as shown in figs. 53, 54, and not in series, so that the current entering the electrolyte by one anode passes to its corresponding cathode, and thence by a wire con- nection to the next anode, and so on, as depicted in figs, 55, 56. Fig. 53. — Plating-bath. Parallel arrangement of articles. Fig. 56. — Plating-bath. Arrangement of articles in series. Choice of Anodes. — In the matter of anodes stress has already been laid upon the necessity for their absolute purity and complete solubility in the electrolyte ; this latter condition is, however, a question to be more particularly regarded in the selection of the constituents of the bath. Unless under the action 102 GENEKAL CONDITIONS TO BE OBSERVED IN ELECTRO-PLATING. of the current the anodes dissolve freely in the hquid, the latter must become impoverished and change rapidly in composition; this is always a source of trouble and annoyance, and where the use of an insoluble or imperfectly soluble anode is unavoidable, small quantities of that salt of the metal which is undergoing electrolysis (or of metallic oxide) must be added from time to time to make good the loss due to deposition. Cast anodes will often be found more soluble than the rolled metal, as they are more porous and open in grain, and, therefore, more readily attacked by the hquid ; in some cases they may even be found to become spongy and friable, a condition which should of course be avoided, and which indicates the desirability of substituting the rolled sheet. When there is any difficulty in obtaining pure metal for anodes, the rolled material may generally be preferred, because the cast plates may contain a large percentage of foreign substances, which would escape detection on merely examining the exterior of the block, whereas any considerable addition of impurities would in many instances cause the metal to break up as it passed through the roUing machinery, so that the mere fact of a metal being in the form of rolled sheet is often a guarantee of at least a fair degree of purity. Cast-iron should on no account be used ; it always contains three or four per cent.^ of the insoluble substances, carbon and sihcon, with other bodies, such as manganese, phosphorus and sulphur; some at least of these are necessary to render it sufficiently fusible to melt in the cupola-furnace. Anodes should usually be of larger size than the cathodes to which they are opposed, so that the greater surface exposed to the solvent action of the electrolyte may compensate for the slower rate of solution as compared with that of deposition. Spacing of Electrodes— Polished Cathodes.— It has been seen that the resistance of a bath is higher when the electrodes are small, and that it increases as the distance between them is more extended; thus economy is effected by plating many objects simultaneously in parallel, and also by minimising the distance between them and their corresponding anodes. But there are objections to approximating them too closely— first, they are more hable to come in contact or to be united by a fragment of metal, and thus to produce short-circuiting; and, secondly, if the surface of the object is irregular the deposited metal is hable to be of unequal thickness, because a current passing between points at unequal distances tends to deposit most rapidly upon those portions of the cathode which most nearly approach the anode. By increasing the space separating AGITATION OF SOLUTIONS. 103 the two surfaces the irregularities of either have less influence on the deposit, because they are small as compared with the mean distance between them. This difficulty is chiefly experienced in electrotyping, where strong deposits of uniform thickness are required. In depositing copper when the solution is strong and at rest, small projections and striated markings may be observed, which increase in extent as the current is continued. Marks and scratches on the cathode do not become obhterated, but rather accentuated, as the metal is deposited over them; great care must, therefore, be taken that the surface to be coated is not only thoroughly cleansed, but that all tool- or file-marks are completely removed, as there is no remedy when once deposition has begun. Homogeneous Solutions necessary. — Another difficulty in- herent in the process is, that a current long continued with the solution at rest produces a gradual local alteration in the density of the latter. At the anode, metal is constantly dissolving into the surrounding liquid which thus becomes heavier, bulk for bulk, than the remainder of the bath, and sinks to the bottom ; while at the cathode the hquid is denuded of metal, and from its lower specific gravity rises to the surface. In this manner a very gentle but sure circulation occurs in the vat, producing an undue pro- portion of metal in solution at the bottom, and of acid at the top. The efi*ect of this is that thicker deposits form on the lower portions of goods immersed in the bath, owing largely to the higher conductivity of the strong solution and the greater pro- portion of current flowing through it ; moreover, a kind of local action may be set up in the deposited copper plate, which is rest- ing in two practically difi'erent solutions, with the result that the upper portion of the plate in the acid solution tends to dissolve, and to deposit a corresponding proportion of copper upon the lower half. It is probably the steady flow of liquid over the surface of the cathode which gives rise to the striated markings above re- ferred to. The only remedy is to keep the solutions thoroughly mixed by stirring or gently shaking them in any suitable way, and this precaution should never be omitted when thick deposits are required, which necessitate a comparatively long exposure m the bath. The solutions must also be kept free from suspended particles such as are apt to become detached in the form of shme from the anode, because the settling of these particles on the cathode is one of the principal causes of the rough nodular forma- tion on the surface of electro-deposited metals. Having seen the causes which operate to produce failure in electro-plating, it remains to be seen how they are avoided in practice. 104 GENEEAL CONDITIONS TO BE OBSERVED IN ELECTRO-PLATING. Motion of Solution. — The circulation of the electrolyte not only ensures the homogeneity of the solution, but enables a greater volume of current, and therefore a higher rate of deposition, to be used, by bringing a greater number of metal ions into contact with the cathode in a given time. By employing a very rapid flow of the solution around the cathode, the rate of deposition (for example, of copper in refining or in electrotypi'ng) may be enormously increased. Thus, in certain copper depositing processes, recently introduced, jets of the electrolyte are directed upon the surface during the whole period of electrolysis, with the result that a current density of over 100 amperes per sq. ft. may be used, «as against 10 amperes per sq. ft., which not long since was, with ordinary methods of depositing, considered a high density. By using strong solutions and a proportionately high current density, with rapid motion of the electrolyte, i\lr Swan has deposited good copper with current densities higher than 1000 amperes per sq. ft. CHAPTER V. PLATING ADJUNCTS AND DISPOSITION OF PLANT. Light and Pure Air. — In arranging an electro-plating establish- ment, due regard must be had for light and ventilation ; with in- sufficiency of the former bad work is almost sure lo result, as it is not easy to judge when the pieces are sufficiently stripped, polished, cleaned, or quicked, and the progress of the deposition cannot be watched with the requisite amount of care ; it is often necessary to stop a process immediately upon the appearance of certain signs, indicative of imperfect cleansing or the like, and it is of the highest importance that these characteristics should be noted at once, which cannot be done if the light be deficient. Badly- ventilated rooms are productive of ill-health and disease to the workmen ; this maxim, applicable, indeed, to all rooms in which men live or work, must be specially regarded in rooms where batteries or cyanide plating-solutions are in use ; the acid fume or spray given off by most batteries is most penetrating and injurious to health, even when considerably diluted with air, as it is likely to be found even in a large room, if it be insufficiently provided with means to carry off vitiated air and supply fresh in its place ; moreover, the cyanide solution becomes slowly decomposed by the carbonic-acid-laden air of towns, and evolves the deadly prussic acid gas — in minute proportion, it is true, but amply sufficient to become prejudicial to the well-being and comfort of the operator, when breathed continuously for any considerable period of time. Arrangement of Eooms. — At least three rooms should be available, if possible, for the purposes of the art. In the first of these the mechanical operations of cleansing and polishing are carried out ; these give rise to the production of more or less dust, and should not, therefore, be conducted in the same room with the plating-vats. Again, the engine driving the machinery should be in a separate chamber, apart from the dust of the polishing-room on the one hand, and from the fumes of the vat-room on the other, the power being communicated to the lathes and other machine tools by shafting running between the two rooms. When the 106 PLATIXG ADJUNCTS AND DISPOSITION OF PLANT. dynamo is the source of electric energy, it should be placed in the engine-room, but as close as possible to the baths in the adjoining chamber, so that there may be no greafc loss of energy owing to the resistance of the copper conducting Avires or leads to the passage of the electric current. But if batteries be employed, they should be carefully isolated from each of the three rooms above mentioned ; if a fourth be not available (a small one will suffice), a corner of the vat-room should be partitioned off, the chamber or cupboard thus formed being provided with a separate outlet into the open air, or, better still, into a chimney, so that all fumes may be at once and completely removed; a sink and a supply of fresh water may be fitted in this room with advantage. In the third or vat-room are the potash and all other baths used in chemically cleansing the pieces, together with those devoted to plating : an ample supply of water must be available in this department, and a steam-pipe should convey steam from the boiler — if one be used — to the potash-tank and steam-heated vessels. Here the various pieces of furniture should not be crowded together, but an ample margin of space should be left so that the operator may not be cramped for want of room. "When more than one plating process is employed, each should be kept to itself, and in large estabhshments a separate room may be set apart for each. It is, of course, impossible to lay down any general plan for the disposition of the plant of an electro-plating shop, because it must be arranged and modified, not only to suit the work to be per- formed, but also the floor-space, shape, and position of the rooms available. But, to sum up. it may be taken as a general rule that mechanical work shoidd be isolated from chemical, that the battery in the one case, or the dynamo and motor in the other, should be separated from both, that in each room the various classes of work should be kept distinct, but that where consecutive processes have to be followed, the arrangements for conducting them should be so placed that, when one stage of the work is completed, the articles may be conveniently transferred into position for the next operation. Drainage of Floors. — The floors should be of stone, asphalt, or concrete, or they may be covered with lead sheet, so that they may readily be kept clean, and be non-absorbent of the acids and chemical reagents splashed upon them : wood, besides being con- stantly wet, is liable to become rotten by the action of these sub- stances. Trapped gullies or sinks should be placed at suitable points flush with the floor ; they should not communicate with the house-drains directly, but should discharge into a pipe which ARRANGEMENT OF PLANT FOR ELECTRO-PLATING. 107 runs outside the house, and delivers into the open air above a second trapped gully communicating with the sewers or drains. In this way the floor may be kept free from accumulations of liquid, and may be readily and perfectly cleansed by flooding it with water, and then sweeping it into the gullies by means of india-rubber ' squeegees ' or brooms ; at the same time, there is no danger of sewer-air contaminating the atmosphere of the room (a fertile source of danger), because there is no direct communication with the drain. Pipes from sinks should discharge into the open air after a like manner. Ventilation. — To ensure the purity of the air, it is not well to trust simply to the ventilation of the room by doors and windows, but a systematic arrangement should be adopted, such as that of Tobin. Several ventilators should be made immediately below the ceiling of the room, by removing a brick, passing a tube through the wall, and bending it upwards in the open air (it may be shielded from rain by a cap placed a few inches above the exit) ; if possible, one of these ventilating-tubes should pass into a disused chimney-shaft. The vitiated air is thus carried away, and pure air must be admitted at the floor level to take its place ; this may be done by carrying two or three pipes through the wall close to the floor, and bending them up in the interior of the room to a height of five or six feet. Fresh air is delivered by them in a manner which does not give rise to draughts. Entering cold, it flows up these pipes and gradually distributes itself over the chamber, where, becoming heated, it rises to the roof and finds an exit through the upper row of ventilators. The guiding principles for the sanitary and safe conduct of an otherwise unhealthy occupation are expressed in a few words by saying — ensure abundance of light, water, and fresh air. Arrangement of Plant for Electro-Plating. The vats and apparatus used in cleansing are described in Chapter YI. Vats. — The vats employed to hold the solution for electro- plating should be considerably larger than the largest object to be coated in them, and must be made of, or at least lined with, some material which will resist the acid liquid that may be placed in them. Glass is by far the cleanest and best, but is rarely used, except for very small ^ work, on account of the initial expense and the risk of fracture. For very small objects glass vessels may be had in one piece as circular trays or jars ; but for large articles, the bath 108 PLATING ADJUNCTS AND DISPOSITION OF PLANT. should be made by joining five plates of sheet glass of the re- quisite sizes with marine glue, or white lead, or other cement, protected on the inside by a varnish made of asphalt dissolved in benzoline ; or of gutta-percha in benzene or in carbon bisulphide ; or, in fact, by any water- and acid-proof mixture. The glass should be supported by an outer frame of wood. Slate similarly arranged is a good substitute for glass; 'but though less brittle, it must still be used with care. For small work stone- or earthen- ware glazed troughs are readily obtainable and are very con- venient. Lead presents the advantage that it is readily formed into any shape. A tank made of this material should be supported beneath and around by a wooden case, so that it will not be subjected to the stress of a mass of liquid within. All the joints in the lead-lining should be made by autogenous soldering — that is, by melting together the two edges of the lead sheet, in- stead of uniting them with soft solder, which would set up galvanic action with the lead if it came into contact with the electrolytic solutions. But even when united into one continuous leaden trough, the metal should be completely protected in every part by a good layer of acid-resisting varnish, to prevent the de- composition of the liquid by the lead through simple exchange. Iron tanks also are very largely used, and, indeed, almost universally so for hot solutions. These also, being constructed of a highly electro-positive material, must be carefully preserved from attack by the solutions, either by varnish, or, better, by a good coating of enamel, which, since it is a fused complex silicate, forms practically a tank of glass, so long as it remains intact ; but as soon as the enamel is chipped and the surface of the iron is laid bare, its use must be discontinued until a new coating ^an be given. Wood is abundantly used, and, being a cheap material, easily worked, is especially well adapted for vats of unusual shape which may have to be constructed for one particular class of temporary work. These tanks are best secured at the ends by bolts and nuts, as shown in fig. 57, which serve to hold the sides firmly pressed against the end pieces. As wood alone is very absorbent, they should be lined with gutta-percha or any other water-proof material, and must be carefully watched so that they may be re-lined as soon as leakage into the wood is observed. Wooden vats are sometimes lined with thin lead sheet autogenously soldered, and this inner case may be var- nished, or may "be again lined with varnished wood. A *■ But see also page 122. THE PLATING-VAT. 109 mixture of 10 oz. of gutta-percha with 3 oz. of pitch and IJ oz. each of stearin and linseed oil, melted together and well incor- porated, has been found to afford a good protective covering to lead or wood. The tanks for hot solutions are best made of enamelled iron,^ and may be set over a small fire-grate in which charcoal is burned, or better — be- cause the heat is more under control — over a series of Bunsen burners of the ordinary upright form, or in the shape of horizontal rings ; or they may each be surrounded with an outer jacket, the intervening space being filled with waste steam, which is often avail- able in large works and may be economically applied. Iron tanks are often made of thin metal, and, if of large size, should be sup. ported by strong iron bear- ing bars beneath, to prevent them bulging when the weight of the liquid is applied. Vat-Connections. — Of whatever material the vat is constructed it should be provided with an insulated rim around the top, to carry the wires which conduct the current to the objects in the bath. This rim is best made of well-painted wood fitting on to the top of the bath, and the outer portion should be at a higher level than the inner, as indicated in fig. 58, which with fig. 59 illustrate the general ar- rangement as adapted to an iron vat. Around three sides of the raised portion Fig. 57. — Plating- vat. Fig. 58. — Rim of plating- vat. there runs a short brass or copper rod. A, ending in a binding-screw, B, attached to the positive pole of the battery. Around the corre- sponding three sides of the inner or lower level platform is a similar rod, C, insulated completely from A and from the bath by the 110 PLATING ADJUNCTS AND DISPOSITION OF PLANT. woodwork of the frame, and terminating in the binding-screw, D, attached to the negative (zinc) pole of the battery. Eesting on the two sides of the rod. A, may be placed any number of cross- rods of brass or copper, E, which can be held firmly in position by the screws, F F ; from these are suspended the anodes, which Fig. 59. — Rim of plating- vat. are thus placed in direct connection with the battery. Lying upon the lower rods, C, similar cross-bars serve to support the cathodes or objects to be coated. Any reasonable number of electrodes may be thus suspended in the same bath, the current flowing always from the anodes to the cathodes in parallel. When both sides of an object are to i> * be coated, the anodes and AAA A ^ ' cathodes must be placed alter- nately; such an arrangement has been shown in fig. 54, where A A represent anodes and C C cathodes ; here both sides of the anode are used and dissolved, as the current flows Fig. 60. — Arrangement of plates from them in either direction for electrotyping. to the cathode next adjoining. But where a deposit is re- quired on one side only of a plate, as in printers' electrotyping, one anode may be placed between each pair of cathodes set back to back (fig. 60), so that both sides of the former are used, but only that side of the latter which is nearest to its corresponding anode. Connection of Electrodes. — Other methods of connecting the electrodes with the battery are used, but that just described presents many advantages : thus the distance between an anode and cathode may be adjusted at will, and either may be removed SUSPENSION OF ANODE PLATES. Ill from the solution, examined and returned without disturbing the remainder, and without any manipulation of binding-screws ; the anode may be instantaneously transferred to a cathode rod at the beginning or end of an operation, if desired, to control the current (see p. 213); and the anode supporting rods are held firmly in position by the external screw^s — a similar arrangement, but of internal screws, may be applied to the cathode rods if required. Suspension of Anode Plates. — The anode plates are suspended from the cross-rods by suitable hooks. The anodes may con- veniently have a perforation in each of the upper corners, through which the suspending hooks are passed ; but as they are liable to dissolve irregularly, even when every precaution is used to ensure uniformity of liquid, the lower corners should also be perforated, so that the plates may be suspended alternately from opposite sides. They are sometimes hung so that they project above the surface of the liquid, and the supporting wires, not being immersed, are, therefore, not liable to corrosion and ultimate destruction by solution ; but the plates themselves (o\. will then have a shorter life, for they are most vigorously attacked at the line of uppermost contact with the liquid, and will be worn away at this point while the remainder of the plate is still sound ; moreover, the portion of metal above the water line is practically wasted. They are ^. ^^q^q frequently made, therefore, with projecting per- of suspending forated lugs, either at each corner, or only at two, anode plate, as in fig. 61 ; these alone project above the solution, and, while protecting the supporting wire from destruc- tion, minimise the amount of useless anode surface outside the liquid. To obviate the destruction of the suspending hook, without permitting any portion of the anode to remain above the bath, some operators use but one side of the anode to face the objects which are being coated. On the back reversed hooks are fastened by which the plate is suspended, as in fig. 62 ; the hooks are thus protected from dissolving by the anode which intervenes between them and the cathode, except in the space between the top of the plate and the surface of the liquid, which space should, therefore, be made as short as possible. Agitators. — The necessity for keeping the electrolyte in motion to ensure uniformity in composition when a thick deposit is re- quired has been dealt with already ; the manner of effecting it must depend upon the appliances at command. Stirring by hand is frequently relied upon, but it is liable to be accidentally omitted, and being necessarily intermittent allows time for partial separa- 112 PLATING ADJUNCTS AND DISPOSITION OF PLANT. tion to occur between two consecutive stirrings. Mechanical agitation, which is more certain in its effect, may be applied by such devices as working a small screw-propeller slowly at one end of the bath; or by blowing air into the solution constantly, through a tube passing to the bottom of the vat, by means of a fan-blower, or by a beating arrangement such as that used by V. Htibl in the electro-type baths of the Austrian Mihtary Geo- graphical Institute. In this system a glass rod, A (fig. 63), is fastened to a crank connected with an eccentric wheel or with any suitable device Fig. 62.— Mode of suspending anode. Fig. 63.— Yon Hiibl's agitator. for imparting a reciprocating, or to and fro, motion, so that at each reciprocation the rod is moved through the arc of a circle, from its original position at A to that represented by the dotted line A', and is then returned to its first place at A. Such a rod is placed between each pair of anodes and cathodes, all the rods or beaters being attached to the same crank-shaft which runs the whole length of the vat, so that they may all be actuated by the same mechanism. The motion of the beaters need not be very rapid — from 10 to 30 strokes a minute amply sufficing. The use of this or any similar device presupposes the existence of steam- er water-power and machinery in the shops ; where this is not available, manual power must be substituted in connection with any suitable mixing appHance, which must be set in motion at frequent intervals. Whatever motion is given must be sufficiently vigorous to ensure thorough mixture of the solution^ MOTION OF CATHODES. 113 Mode of attaching sliding-frame. but without disturbing the relative positions of anode and cathode, and the mechanism must be so applied that it in no way lessens the facilities for examining the progress of deposition. Motion of Cathodes. — Silver and some other metals require a gentle motion to be im- parted to the objects upon which they are being deposited. This may be done by enclos- ing the suspending rods of the objects within a wooden frame which is caused to move to and fro above the solution by means of an attachment to an eccentric. The frame may be suspended above the vat, or it may be caused to slide upon the edge of the bath. .1;.,^ Figs. 64, 65, and 66 illustrate a convenient method of fix- ing the sliding frame ; it is supported on wheels placed at the corners, each wheel rolling upon k/ an inclined plane, E (tig. 66), which may be set at any angle by means of a screw. The rod E, thus trough or vat imparts the necessary backwards and forwards motion to the system, so that, at every stroke, — czr a double action occurs, and the frame with the objects suspended from it moves in a horizontal direction by virtue of the pull of the rod R, and in a vertical direction on account of the inclined plane, the extent of the latter motion being con- trolled by the screw sliding-frame. which determines its angle of inclination. The caused to slide backwards three minutes. 00" 1? Figs. 65 and 66. — Mode of attaching through frame with or forwards a distance of 2 the objects should once in about two or 3 inches. H be or 114 PLATING ADJUNCTS AND DISPOSITION OF PLANT. Plating Balances.— In plating with precious metals it is fre- quently required to deposit only a given Aveight upon the various articles ; and although this may be approximately accomplished by calculating the time required to deposit a given weight of metal with a known current-strength and upon a known superficial area,* with the aid of the table given on p. 399, it is safer when absolute accuracy is required to use a plating-balance, by which the weight of metal deposited may be determined as the operation proceeds. In this instrument, a metal frame for carrying the cathode-rods is substituted for one scale-pan of a large balance of the ordinary description. The frame is supported from the beam of the balance by metallic connection, while the pillar of the scale is connected with the negative pole of the battery, so that electric communica- tion is made through the parts of the balance. Having attached the objects to the frame and immersed them in the solution, they are counterpoised by placing weights in the opposite pan of the balance until equilibrium is restored, and the frame and the objects are suspended freely, the former, of course, above and the latter in the solution ; an extra weight, equal to that of the metal which is to be deposited upon all the objects in the aggregate, is now added to those already in the scale-pan, and the current is switched on until the deposited metal, just over- balancing the added weights in the pan, turns the scale in the opposite direction — an action which may be indicated automati- cally by causing the pointer or beam of the balance to release the hammer of a small gong or to make contact with an electric bell. Eoselenr's Plating Balance. — Eoseleur has introduced a more elaborate balance by which the current is automatically cut off as soon as the beam of the scale is turned, so that the electrolytic action ceases directly the required amount of metal has been deposited; this was effected by making electrical contact, not with the pillar, but by attaching a platinum wire to the arm of the balance which supports the weights, and arranging underneath it a cup of mercury connected with the negative pole of the generator, into which cup it dips to such a distance that, as soon as the arm is raised by the reversal of the beam, the wire is lifted out of the mercury, and connection between cathode and battery is permanently broken. All the knife-edges of this balance work under mercury, in order to prevent overheating of * When it is only desired to deposit a total weight of metal, and not a certain weight per square inch, the super hcial area of the objects may be neglected ; all that is required to be known is the mean strength of current applied, in amperes. USE OF THE PLATING-BALANCE. 115 these parts by the current flowing through them, and at the same time to lessen friction and obviate the corrosive action of acid fumes. In the balance diagrammatically indicated in fig. 67 the current does not traverse the beam at all, but enters by a contact screw attached to the supporting rod of the cathode -frame. The objects are suspended in the bath from the flat cathode-rod, C, in the usual way ; then when these have been counterpoised by introducing weights into the pan, P, and the extra weights representing the total mass of metal to be deposited have also been added, the beam will turn, so that the arm. A', rests on the stop, K, which is rigidly attached to the pillar of the balance to prevent an excessive amount of play; at the same time the point of the screw, S, in the supporting rod of the cathode - frame, should just make good contact with the block, M, at the end of a spring, attached to a suitable fixed support, and connected with the negative pole of the battery ; the spring must, of course, be insulated from the pillar and all parts of the balance. Through this connection the current passes from the bath to the return- wire of the battery ; both S and M should, therefore, be tipped with platinum, which remains untarnished under all conditions, ^nd, therefore, secures good contact. If the extremity of S do not actually touch M, or if it press so hard upon it that the spring is bent, adjustment may readily be made by turning the milled head of the screw: the adjustment should be so made that when the balance is in exact equilibrium the points are just touching ; so that when the beam rests upon the stop, R', the spring becomes •slightly bent and ensures perfect contact ; but when it rests upon the corresponding stop, R, contact is entirely broken, and a space of at least the of an inch separates the two platinum surfaces. The current now flows through the circuit from the positive pole of the battery to the anodes, which rests as usual upon the sides of the vat, thence through the solution to the cathodes upon which it deposits the precious metal ; and from the cathodes it traverses Fig. 67.— Balance. 116 PLATING ADJUNCTS AND DISPOSITION OF PLANT. the supporting-rod, the screw, S, and the spring, M, and returns- to the negative pole by the wire, W. As soon as the weight ot metal precipitated is equal to that added to the pan, P, the balance comes to equilibrium and remains poised between the stops R and ; but the current is still flowing because b is^ not yet withdrawn from R' ; then, as soon as a slight additional amount of metal is deposited, the beam comes over and rests upon the stop, R, and, contact being broken, no further electrolytic? action can ensue. The cathode supporting-rod is made m two pieces joined by a ball-and-socket joint, B ; any disturbance of the knife-edge of the balance caused by the necessary slow reciprocat- ing motion imparted to the frame is thus obviated. The motion is imparted by an inverted fork, running between horizontal gmde- rollers, which spans the edge of the frame at the central point on one of its sides, the fork being, of course, attached to an eccentric rod ; the length of the fork may be so arranged that as soon as the balance rests upon the stop, R, the frame falls out of reach of its action. ^ Weight Corrections.— In using any of these balances, it must be remembered that the weight of a substance in water is less, than its weight in air, and that the plated article will appear to- weigh more after removal from the solution than it did when immersed; the actual weight of silver deposited by balance is, therefore, in excess of that indicated by the weights m the pan It is quite possible to rectify this error in making the needful calculation. The initial weight of the objects to be plated may be left out of account, because it is counterpoised while they are in the solution at the beginning of the operation, and they remain in the same liquid to the end. But the weight of silver (or gold> deposited upon them will be less while it is in the solution than it would be outside, by the weight of an equal volume of the liquid. For example, the specific gravity of silver may be taken, at 10-6; that is to say, 1 cubic inch of silver weighs 10-6 times as much as 1 cubic inch of water; thus 10-6 ounces of silver, weighed in the air in the usual manner, would show only 10'6 - l-oLg-e ounces if it were weighed while immersed in water. But the specific gravity of the solution is more than 1 as com- pared with water; regarding it as M, the weight of the lO'G oz of silver becomes 10-6 - 1*1, or only 9-5 ounces, if counterpoised while suspended in this liquid, and this is equivalent to a loss of over 10 per cent. Therefore, strictly speaking, to obtain an aggregate deposit of 10 ounces of silver upon a batch of articles, only 9 ounces need be placed upon the scale-pan. For gold, similar calculations may be made, but the loss is not so great USE OF THE PLATING-BALANCE. 117 owing to the higher specific gravity of gold ( = 19*3), so that the ratio of its weight in air to that in water is 19*3 : 18*3. In the same way, for other metals the weight in the pan should be ■divided by the fraction : — specific gravity of the metal ^ specific gravity of metal - specific gravity of solution We are not aware that these calculations are often made in practice, and the thickness of silver deposited must, therefore, always be perceptibly greater than that intended — certainly an ■error on the right side from the consumer's point of view. A •certain loss may be incurred in subsequent polishing processes, but this should not be greater than would be compensated for by the small excess of metal which is required to overcome the friction of the balance and bring the beam over to the opposite side. Should any articles or anodes accidentally fall into the plating- vat, they should be carefully picked out by means of a long wire, bent at one end into hook-shape, or by a pair of light tongs, which may be made of brass, previously coated with the metal which is being deposited, or with some more electro-negative metal; the bare arm should not be introduced, because of the risk of blood-poisoning which may be caused by the contact of recent wounds with the plating-liquid. For special classes of work special vats and special arrange- ments of all kinds may have to be made, and herein lies the scope for the inventive skill of the operator. But, with a know- ledge of the principles laid down in works on electro-metallurgy, there should be no serious difficulty in dealing with the various problems which may be presented. Roseleur's Wire-Gilding Process. — Among these special arrange- ments, the method adopted by Eoseleur for gilding iron by a single continuous process is especially interesting ; a diagram of the plant is given in fig. 68. The tin-coated and well-cleaned wire is slowly uncoiled from a reel, R, and passed to the drum, D, on which the finished wire is ultimately wound, and whose rotation causes the wire to travel through the various stages of the process. First it is passed into the hot gold-bath, E, heated by the furnace, F, and is maintained beneath the liquid by the glass rollers, G G ; here, by the current supplied through the platinum-wire' anodes, A A, and passed through the wire to the connection with the battery at B, the gold is deposited upon it. Passing thence, the gilt wire is guided by two wooden rollers, 118 PLATING ADJUNCTS AND DISPOSITION OF PLANT. W, to a cleansing-bath of potassium cyanide solution ; from thi& it is led into a vessel of clean wash- water, Y, and finally betweeik Fig. 68. — Roseleur's wire-gilding arrangement. the calico-covered draining rollers, C, to the drying and annealing-^ tube, T, which is maintained at a dull-red heat by a charcoal- furnace. Several parallel wires may, of course, be passed through the same process, simultaneously. Wire gauze, or fabrics of any kind, pro- vided that they conduct electricity, may be similarly coated by passing them beneath a roller in the depositing trough, and winding them finally upon a reel or drum. Wagener and Netto's Doctor. — A device invented by Wagener and Netto for coating surfaces which are too extensive to immerse in a solution may well be noted as an application of ingenuity to the solution of a difficult problem, although it is really only a modification of the apparatus long since known as a * doctor,' which is used for gilding the lips of ewers and the like (see p. 228). To a hollow wooden handle, H (fig. 69), is. attached a circular anode. A, of the required metal perforated in the centre, and connected by a wire, W, with the positive pole of the battery; in close contact with the lower surface of A is a flannel pad, E, held in place by the brass tube,. T, which passes through the length of the handle, and, being con- Fig. 69.— Wagener and Netto's ' doctor.' SPECIAL PLATING ARRANGEMENTS. 119 nected with the india-rubber tube, R, conveys the liquid electrolyte from any .convenient receptacle to the pad, E. This pad must be kept constantly wet, and the flow of solution is regulated by the clip, C, upon the india-rubber tube. The surface to be coated is connected with the negative pole of the battery; now, by brushing the apparatus over the surface, electrolytic connection is made between it and the anode. A, through the solution with which the pad, E* is wetted ; thus the electrolyte is decomposed and deposits its metal upon the required object, the thickness of film being regulated by the time during which the handle is held over each portion in succession. As the electrolyte is used up, fresh hquid is supplied through the central tube. For very irregular surfaces a long-haired brush, with short anode-wires admixed with the hairs, may be substituted for the sheet-anode and flannel-pad. Care must be taken, of course, as in all electro-depositing work, that the surfaces are perfectly clean before attempting to coat them. Smith and Deakin's Rotatory Plating Apparatus.— In the plating, especially in the electro-nickeling and brassing of small goods, much time is lost in attaching the wires required to suspend them in the bath, and in polishing the goods after deposition. By placing the objects to be plated in a revolving drum and at the same time connecting them Avith the negative conductor from the dynamo, the difficulty has been obviated by Smith and Deakin in their 1896 patent. Fig. 70, taken from a drawing supplied by the Electrolytic Plating Apparatus Company, shows the arrangement in use. V is an ordinary vat (part of the side being supposed cut away for the purpose of the illustration, to show the internal arrangements); within V is a hexagonal drum, D, mounted horizontally in the bath, and supported at each end from the longitudinal iron bar, B B. This bar is held at either end, in clamps which may be opened at will ; when they are open, the bar with all its attachments may be raised to any desired height above the vat by the cord, B B, passing over the pulleys, P P, attached to the roof immediately over the vat, and may be held in such position by bending the cord over the cleat, C. Passing transversely over the centre of the vat is the shaft, S, normally running at 100 revolutions per minute, carrying the cone pulley, C P, from which a belt is passed over the lower pulley, L P, attached to the rod, B ; this pulley, in turn, serves to rotate the drum, D, by means of a strap passing round the latter, as shown in the figure. The drum is made of wood, so perforated throughout that the area of the perforations is about equal to that of the solid wood between them, and the size of the holes used depends upon that 120 PLATING ADJUNCTS AND DISPOSITION OF PLANT. Yig, 70.— Smitli and Deakin's rotatory plating apparatus. of the goods to be plated, being, of course, larger when the goods are of larger size. One side of the drum is removable, so that the objects to be plated may be introduced and removed. The drum SPECIAL PLATING ARKANGEMENTS. 121 itself is mounted on a hollow spindle or sleeve with copper con- nections at intervals, which serve to make contact with the goods within, and which are connected by means of the sleeve and i^hence by conductors within the supporting brackets to the bar, B B, and so to the negative lead - . The anodes, A A, are sus- pended on either side of the drum and are connected with the positive lead + . In use the bar, B B, with the drum and attachments, is raised out of the vat, and supported just above the top rim; the drum is then half filled with the goods to be plated, which must, of <50urse, be thoroughly cleansed with potash, and dipped as usual. The removable side is then fastened in place, and the whole is lowered until the drum is completely immersed in the bath, and the bar, B B, is gripped in its clamps. The belt being started, rotation is commenced and kept up until the operation is finished. Eor ordinary small goods, without sharp corners, the drum should rotate at about 55 revolutions per minute, but with heavy goods, especially those with square or sharp corners, it is preferred to reduce the speed to from 3 to 6 revolutions. The current density and E.M.F. used, and hence the time required for plating, are the same as in the ordinary process. When the thus important that the object should be transferred with the- highest rapidity from the acid-bath to the wash-waters, and. from these to the electrolytic solution. Copper, Brass, German Silver, and other alloys, of which the chief constituent is copper, are, as already indicated, cleaned and brightened by dipping into acid after removal from the- potash solution. The common nitric acid of commerce, which emits red fumes and is known in the trade as nitrom acid, is very generally used for the acid dip; it consists of nitric acid in which lower oxides of nitrogen are dissolved, and the waste nitric acids from used Grove's or Bunsen's battery cells are not^ unsuitable for the work. It is used undiluted, the articles to- be dipped are slung on wires or in the baskets already described baskets of platinum-wire mesh are sometimes used for small articles, and as they are quite unattacked by the acid, and. therefore neither weaken nor contaminate the liquid, they are to be preferred to all others ; the prime cost alone is against them, but they soon repay this expenditure by their indestructi- bility and by the greater comfort and saving in their use. Since the acid exerts a powerful action upon the metal of which the articles are made, and the film of oxide is instantaneously cleared from the surface, the plunge into acid must be but momentary ; the objects are then rapidly transferred to a vessel, containing a large volume of water, and are afterwards washed again and again, the last time in quite clean water. It is false- economy in electro-plating to stint wash-waters at any stage. Another acid dip, sometimes used in preference to the above,, is made by mixing together, little by little and with the utmost care, equal volumes of strong sulphuric acid and water, and adding to each gallon of the mixture one pint of the common aqua fortis (nitric acid) of commerce. The subsequent manipu- lations are similar to those already described. A dead or dull surface may be given by plunging the article, previously dipped in nitric acid, into Roseleur's mixture of 200 parts of yellow nitric acid, 100 of oil of vitriol, and 1 part of common salt, together with from 1 to 5 parts of zinc sulphate (white vitriol), which should be made up the day before it is required for use. After remaining in this liquid for several minutes (from five to fifteen or even twenty) the articles are removed, plunged momentarily into a bright-dipping bath, to restore a certaia amount of the brilliancy which is generally too thoroughly removed in the dead dip, and are then rinsed as usual. SPECIAL CLEANSING PKOCESSES. 129 A bright lustre is given by placing for a few seconds in a mixture of the yellow nitric acid and oil of vitriol in equal parts, with 0*5 per cent, of common salt; like the last this mixture should be made up the day before it is required, or earlier, in order that it may be quite cold when required. Instead of this solution, a mixture of IJ to 2 parts of sulphuric acid with 1 part of an old nitric acid dip is frequently used; after mixing these acids, they should be put aside to cool, and decanted from any crystals of copper sulphate which may be formed by the action of the vitriol upon the copper contained in the old aqua fortis (due to the partial solution of the objects that have been immersed in it). Small proportions of hydro- .chloric acid or of sodium chloride, which by contact with the vitriol liberates hydrochloric acid, or of lamp-black are some- times added to this solution. Potassium cyanide, dissolved in ten times its weight of water, is often used instead of the acid dip for brass, especially when it is essential that the original polish upon the article should not be destroyed, as in the preparation of the objects for nickel- plating. A longer immersion in this Hquid is to be recom- mended, because the metalhc oxides are far less readily soluble in this than in the acid dips. In all cases the final cleansing in water must be observed. Iron and steel articles, which have been thoroughly polished, are dipped first into the boiling potash solution, and then, when thoroughly cleansed from grease, into a pickle consisting of water containing 10 per cent, of sulphuric or nitric acid or 25 per cent, of hydrochloric acid. Iron is vigorously attacked by acid, and by long immersion becomes unevenly dissolved and pitted. Cast- iron and steel also are liable to have a film of carbon and other insoluble impurities on the surface after pickling. Some operators therefore prefer, when possible, to substitute for pickling a mechanical process, such as that described on page 138. When the metal is covered with oxide or scale, as after forging or heating to redness, more vigorous measures are needed to effect their removal ; the objects are first suspended for two or three hours in a bath of dilute sulphuric acid (2 or 3 per cent, of acid for wrought-iron or steel, about 1 per cent, for cast-iron), which dissolves a little of the oxide and loosens much of the remainder, so that, after washing well with water, it may, for the most part' be detached by scouring with very fine sand. A second dip into the acid usually removes the last portions of scale; but, if necessary, the process must be repeated until the pieces are perfectly clean. I 130 PEEPAEATIOX FOR DEPOSITING-VAT — POLISHDsG. Zinc is first passed through the potash-bath, which exerts a distinct solvent action upon it, so that the process must be expedited, and is next dipped for a few minutes into water con- taining 10 per cent, of sulphuric acid : the mixture of acids used for bright-dipping brass (50 sulphuric acid, 50 nitric acid, and 0*5 salt) is sometimes used, but as it violently attacks the zinc, the operation of dipping must be rapidly effected, and the subsequent washing must be immediate and thorough. After treatment by either of these processes, scouring with fine sand and clean water must be resorted to, which has the effect, inter alia, of obhterating the black lines that indicate the position of solder after dipping in the acids. Lead, tin, Britannia metal, and the like are very rapidly passed through the potash-bath, or, as already described, are scoured with whiting and water subsequent to cleansing in benzene, if this be necessary by reason of extreme greasiness. They should be polished finally by rubbing with lime, even after the potash dip ; they then require only to be well rinsed in water to be ready for the electrolysic vat. All acid pickles used for different classes of work should be kept distinct from each other, so that one metal may not be dipped into a solution containing a more electro-negative metal, which would deposit upon it by chemical exchange. For example, zinc or iron must not be immersed in a pickle which is used for cleansing copper articles, because a certain amount of copper gradually dissolves into the liquid as successive objects are dipped, and this copper tends to deposit upon the more electro-positive metals afterwards brought into contact with it. ftuicking. — Many articles are ' quicked' before being subjected to the operation of depositing other metals, especially silver and gold, upon their surfaces. This simply consists in giving them a superficial amalgamation by the deposition of a thin film of mercury, in order that many metals, which alone would deposit the coating-metal from the plating-liquors by simple immersion, may be rendered practically incapable of so doing, the resulting deposit bein^ more adhesive and of better quality in consequence. But quicking is frequently resorted to in order to increase the adhesiveness of deposited metals on objects which would have no action on the bath; for the mercury, being but little liable to tarnish by oxidation, retains a bright surface when exposed to the air for a period which would suffice to produce a film of oxide upon an unquicked surface, and thus prevent adhesion. More- over, the solvent action of the mercury on both surfaces (especi- ally on gold and silver) tends to unite the two metals in the THE 'QUICKING' PEOCESS. 131 most intimate contact, and may even, by interamalgamation, form a superficial alloy which would thus make adhesion perfect. The principal metals so treated are copper, brass, German silver, and the like, previous to gold- and silver-plating, and zinc prior to nickeling. The quicking-solutions more commonly used are— the per-nitrate or proto-nitrate of mercury, the strength ranging from 1 to 2 ounces per gallon (Roseleur recommends 1 part of mercuric (per-) nitrate and 2 of sulphuric acid to 1000 parts of water) ; or the cyanide of mercury, which is made either by adding a sohition of potassium cyanide to one of a mercury salt (nitrate, chloride, or sulphate), until no further precipitate is produced, allowing the cyanide of mercury to subside, washing it two or three times with water by alternately stirring it up, allowing it to settle and pouring off the clear liquor from the precipitate, then dissolving it in a further quantity of potassium cyanide and diluting with water ; or by dissolving mercuric (per-) oxide directly in potassium cyanide solution. The objects are merely dipped into these solutions, when metal is superficially dissolved from them and mercury is deposited in its place by simple chemical exchange. The duration of the dip, never much more than momentary, is governed by ''the amount of mercury to be deposited, which in turn depends upon the thick- ness of the object to be plated and that of the coat to be applied. Usually a thick object and a thick coating demand, or at least permit, a heavier mercury deposit than thinner ones, which are * more liable to become brittle, and for which a mere momentary immersion will suffice. The character of the basis-metal ^ also influences the time required for mercury-deposition, inasmuch as the electro-positive metals have a more rapid action than those which are more electro-negative. Zinc, especially, requires careful quicking, as it not only deposits the mercury with rapidity, but is very readily penetrated by it, and is rendered brittle in consequence. The quicking-bath should be of such a strength that copper plunged into it becomes immediately covered with a silvery-white metallic film. The use of old or dilute solutions should be discontinued when they begin to yield a dark or almost black deposit of mercury, which is worse than useless, because the electro-deposited metal refuses to adhere to it. If the liquid be too strong, or contain too large an excess of free nitric acid. The word basis-metal is here applied to the metal which forms the object or base upon which an electro-deposit is ultimately to be given ; the term base-metal, though more euphonious, has a second signification which micrht prove to be misleading. ° 132 PREPAEATION FOR DEPOSITING-VAT — POLISHING. a similar result obtains ; it is, however, easy to decide to which cause failure is to be attributed. If the pieces have not been properly cleansed, the quicking- solution will give an irregular, patchy or discoloured film, instead of a clear silver-like uniform coat, owing to the presence of foreign* bodies such as grease or oxide. Mechanical Treatment. — Scouring with sand or pumice is best conducted on a wooden board placed above a tub containing the water or liquid to be used. The scouring brush should be made with moderately-hard bristles (hogs' hair is generally preferred), and is used by plunging it into the water, withdrawing it, shaking gently to remove the excess of the liquid, dipping in the powdered pumice-stone, and at once rubbing it over the whole surface of the object. To yield good results, the brush must be constantly charged with the powder, but while keeping it thoroughly moist,, excess of water is to be avoided ; the dipping into w^ater and powder may thus have to be frequently repeated if the object be at all large. When the scouring is not to be followed by a potash dip, the pieces should not be touched with the bare hands, and both brushes and powder must be examined to see that there is no trace of greasy matter attached to them. In preparing metal for the chemical treatment previous to^ actual electro-deposition, the pieces should be polished at least in. part ; as a rule, however, it will be found that the coating-metal adheres less satisfactorily to a surface which is perfectly polished. than to one which is in a slight degree, it may be almost imper- ceptibly, roughened. Nevertheless, the polishing must be so far- completed that all marks of the file or tool are obliterated, and the whole surface has only a regular, and hence almost invisible,, roughness ; for it is difficult to remove file-marks or scratches afterwards from the plated article without cutting through the- coating, or at least rendering it dangerously thin. Deep irregularities must first be removed, and this may necessi- tate the use of a file ; the marks of the latter 'must now be erased. by rubbing with some material such as emery ; and this in turn leaves finer markings, still too coarse to be left untouched, and which necessitate the use of polishing tools. Of these the most successful are those w^hich are caused to revolve rapidly in one plane by suitable mechanism, and which not only economise time, but have a perfect regularity of action and produce true parallelism. of the fine lines scratched by them. It is w^ell know^n that the best surface is always obtained when the polishing tool is passed, over it uniformly in the same direction, and that any motion which produces cross-lines, no matter how fine they may be, and POLISHING PROCESSES. 133 thus gives rise to cross-reflections of light, destroys the evenness of the appearance. Hand-poHshing is especially liable to produce these cross-lines, and thus entails a greater expenditure of time and care. Polishing-Lathe and Dolly.— The lathe-action is generally used for polishing ; discs or hohs of stout leather, usually hippopotamus- oi walrus-hide, about half-an-inch to one inch in thickness, and four or six inches in diameter, are rotated rapidly on the lathe- spindle by means of a treadle like that of a lathe or of a grind- stone, and while thus in motion the workman with one hand firmly presses the object to be polished against the lower side of the leather bob, while with the other he allows a gentle stream of fine sand to fall upon the top of the disc as it revolves towards him from above downwards. Trent sand is usually deemed most suitable for the work ; it may be used repeatedly. The pieces may be first treated with fresh rough sand to obliterate the deeper markings, and afterwards with worn sand, which has been used many times. For this purpose the bobs should make 1500 or 2000 revolutions per minute, but this is a high rate of rotation to be maintained by a treadle action, and is, there- fore, more satisfactorily com- municated by steam-power. B D A C D S B' Fig. 76 shows a bench power- spindle suitable for this pur- pose ; the base of the stand is bolted firmly to a strong table 1^ or bench ; the spindle, S, has J I a screw at either end, to which mfn , the bobs, B B', are firmly Fig. 76.— Bench power-spindle, attached; and between the forked arms of the stand, which carry the bearings, D D, of the rotating spindle, are fast-and-loose pulleys, one of which is keyed firmly to S at A, so that, when connected with a large pulley on the main shafting in the shop by means of a leather belt, it acts as a driving pulley and imparts the required motion to the bobs; the other is free to turn loosely upon S at C, so that when the belt is shifted to it from the driving pulley, it alone rotates, and the spindle remains at rest. A lever must be conveniently placed to shift the belt from the one pulley to the other at a moment's notice. Two workmen may use such a tool simultaneously ; one standing at B applies the coarse sand only, and when the object is thus sufficiently treated, hands it on to the second operator at B', who finishes it with the finer sand. Even now the metal is not absolutely bright, but requires a final 134 PEEPARATION FOR DEPOSITING-YAT — POLISHING. polish with a finer material, which may be given by bobbing the article with a little fresh, finely-crushed quicklime (Sheffield Hme is particularly well suited to the work) mixed with a little oil. The lime should be thoroughly caustic, and as it rapidly absorbs both carbonic acid and moisture from the air, it must be stored in air-tight closed boxes as soon as possible after burning, and should only be removed from these a little at a time as required for use. The last polish of all is given by a small quantity of lime applied without oil by the projecting edges of a series of calico rings clamped one upon another in a wooden holder with a central hole, by which it is screwed to the lathe-spindle in place of one of the bobs. This instrument is known as a dolly. Or a mop may be used for colouring the goods (see p. 137). Scratch-brushing consists in submitting the surfaces of articles to the polishing action of a number of fine wires set on end. The wire selected for this purpose must be harder than the metal to be treated by it, or it will have little or no action, and may even cover its surface with a thin film of metal wwn off from the wires by attrition ; when, for example, nickel is scratch- brushed with brass wire the surface becomes quite yellow in tint ; it must not only be relatively hard, but must also be actually and intrinsically rigid and stiff, so that the points shall not be readily bent over out of shape when in use. Hard-drawn thin brass wire, which may be made partially or w^hoUy soft if required, by suitable annealing, is the usual material for scratch-brushes, but occasion- ally steel, or even spun-glass, may be employed for treating extremely hard surfaces. Hand scratch-brushes are about 6 or 8 inches long, and are inch. One end is then dipped into soldering fluid (hydrochloric acid, in which as much zinc as possible has been dissolved) and then into a ladle of melted soft solder, firmly to unite the various wires and so form a solid brush ; the w^hole is then mounted on a wooden handle for convenience, with the free ends- of the wires extending beyond the handle, as in fig. 77. Occasion- ally a double length of the wires is taken, and they are simply bent over upon themselves and bound round the centre, leaving a loop at one end, and are afterw^ards mounted as before on a wooden handle (fig. 78) ; but it is less easy to produce regularity Fig. 77. — Short wire brush. made by firmly binding a large number of wires in the middle so that they form a compact bundle, with the ends free for the space of half-an-inch to an THE USE OF THE SCRATCH-BRUSH. 135 in the laying of the wires by this means. Either of these brushes may be used upon small surfaces; for large areas, the wires are mounted in handles in the form of a scrubbing-brush, as shown in fig. 79. When the motion is to be applied to the brashes by machine power — and this method is to be preferred to hand labour — a Yig, 78. — Long wire brush. Fig. 79. — Wire scmbbing-brush. number (4, 6, 8 or more) of hand-brushes may be mounted on the periphery of a wooden drum, as indicated in fig. 80 ; or a better form is made by setting the wires radially in a circular handle, so as to form a disc of from 5 to 6 inches in diameter (fig. 81). Both forms of circular brush are mounted on the spindle of the lathe, or on that driven by machine power, shown in fig. 76. For surfacing the interiors of vessels, a brush of the shape indicated in fig. 82. is useful, or on an emergency an old hand Fig. 80.— Machine brush. Fig. 81.— Rotary Fig. 82.— Wire brush machine brush. for inner surfaces. scratch-brush may sufiice, the wires of which have become turned over at the points. In using any of these brushes, a liquid lubricating medium is employed ; this is most generally stale beer, but many other liquids, such as crude tartar dissolved in water, diluted vinegar, or decoction of soap-wort, are supposed by some operators to produce a better effect, and are, accordingly, substituted for it. The brushes are useless when the ends of the wires have turned over upon themselves ; if they cannot then be straightened by 136 PREPARATION FOR DEPOSITING- YAT — POLISHING. means of a wooden mallet, the extreme tips must be cut off by a sharp metal chisel. The circular lathe-brush should be mounted upon the spindle, sometimes on one side, sometimes on the other, so that the direction of rotation is reversed, and the wires strike the object alternately with different sides of their surfaces; thus the latter are not unduly bent in one direction. It is essential that the wires be kept in good order, and an occasional dip into potash to remove grease, or into the acid dip to remove oxide, may have to be resorted to. The hand-bmsh is used by holding it in the same manner as, and imparting to it somewhat the motion of, a paint brush. The lathe-brush is mounted upon a spindle, and should be arranged w^ith a small reservoir above to contain the lubricating fluid, a small pipe with a tap serving to conduct the solution from this to a point immediately above the rotating brush, upon whicn the drops gradually (all; the piece is held firmly underneath the brush, but slightly on the side nearer the operator, so as to meet the wires as they descend. Around the brush is a metal screen to prevent splashings produced by the rapid rotation, and beneath it is a tray with an overflow pipe conducting to a receptacle placed below, to retain the waste solution for use again. The operation of scratch-brushing is had recourse to after deposition, in order to brighten the dull deposit ; sometimes even at intervals during the process to secure a good coating: some- times beforehand to brighten the object finally before immersion in the plating-vat. \Yhenever it is used prior to or during deposition it is obvious that every trace of the lubricating liquid must be washed away before placing, or replacing, the article in the bath. Burnishing. — This is a process applied to finally pohshing silver and some other deposited metals, and consists in rubbing the w^hole surface under considerable pressure by a very hard and, at the same time, highly-pohshed surface ; it may be effected after scratch-brushing the articles, or is often used as a substitute for this latter operation. The burnishing tools are usually made of steel for the first or grounding process, and of a very hard stone, such as agate or blood-stone, for finishing. These tools must be kept in the highest degree polished by rubbing them vigorously with very finely-crushed crocus- or rouge- powder on a strip of leather, fastened upon a piece of wood which is placed in a convenient position upon the working bench. The burnishers are of various shapes to suit the requirements of different kinds of work, the first rougli burnishing being often accomplished by instruments with comparatively sharp edges, POLISHING HARD METALS. 137 -while the finishing stages are accomplished with rounded ones. The annexed sketch (fig. 83) illustrates a few of the patterns commonly employed. Soap suds may be used to lubricate and moisten the burnishers. Silver-plated goods may be readily pohshed by submitting them to the action of the lathe-bobs, such as those already described, •or of wood covered with leather, or of brushes, upon which is maintained a small quantity of tripoli-powder mixed with a few drops of oil. The last poHsh is given, either by the application of rouge by constant rubbing with the fleshy portions of the hand, or by a dolly, termed a mop, in which swan's-down is substituted for the calico between the wooden clamps (see p. 134), using with it the finest possible paste of rouge-powder, entirely free from gritty matter, which would destroyrather than improve the existing polish. Fig. 83. — Burnishers. This mopping is commonly known as finishing or coloring, and igives the final perfect polish. The results of scratch-brushing and burnishing are quite dif- ferent, and each system has its own special advantage. Electro- deposited metal is always crystalline, however close the texture may be ; and being thus made up of an aggregation of minute crystals, the light falling upon it is not evenly reflected, but is more or less scattered by the varying facets of the crystalline surface ; and thus, although metallic, it has a dead lustre. Scratch-brushing followed by buffing, or bobbing, has the eff'ect ■of very slightly flattening the projecting portions downwards upon the surface, but mainly of grinding them off' until they are level with the lowest portions, and so a perfectly even and uniform surface being produced, light is reflected as it were from a mirror ; but no practical alteration of the physical condition of the coat- ing results. Burnishing, on the contrary, scarcely effects any grinding of the irregularities, but rather produces the level 138 PREPARATION FOR DEPOSITING-YAT — POLISHING. surface by flattening the raised portions into adjacent cavities, sa that the pressure exerted tends to fill up any pores or inter- crystalline spaces, and so to yield a more solid coat. Thus burnishing produces a denser, more durable, and more solid covering, but the colour and general appearance is somewhat less, satisfactory, possibly because the irregularities are merely rounded off and not entirely effaced, so that the surface is not so absolutely true as that yielded by good buffing and dollying. Steel, which is too hard to be polished by the methods given above, should be rough-polished with the emery-wheels, then glazed by the action of bobs of wood covered with leather, to which a mixture of the finest emery-powder with oil is applied. A strip of a soft alloy of lead and tin is sometimes substituted for the leather upon the bob. The finishing polish is administered with the best crocus-powder. For nickel deposits the object should be thoroughly polished so as to obliterate tool marks. At the same time, as above pointed out, the polish must not be too good, or the nickel will not adhere properly, and will be liable to strip. Iron or steel objects to be nickel-plated may with advantage be placed in boiling potash, re- moved, and allowed to cool without swilling. They may then be polished well with fine sand and water, which cleanses them me- chanically from soapy matter, and leaves very fine scratches inta which the preliminary coating of copper keys so that it adheres well. The objects may be coppered without an acid pickle, and after coppering they are well swilled, polished agaiii with sand and water, and nickeled immediately. CHAPTER VII. THE ELECTRO-DEPOSITION OF COPPER. It is frequently necessary to give a coating of copper to metals, chiefly to those, such as iron, which are more electro-positive, occasionally with the object of imparting to them the external characteristics of copper, but more often in order to enable them to receive a good deposit of a less electro-positive metal. But by far the most extensive application of electro-plating with copper is to be found in electrotyping, or obtaining facsimile copies of various objects for the use of the printer or sculptor. The (acid) copper solutions present fewer difficulties in management than perhaps those of any metal, permitting at once a wider range of current-strength and a greater variation of bath-composition. Coating by Simple Immersion. Iron. — Iron is practically the only metal that is coated with copper by simple immersion, and only small articles of this body are usually so treated. An acid solution of copper sulphate, made by dissolving about '2 ounces of the blue salt in a gallon of water, and adding about IJ to 2 ounces of sulphuric acid, may be employed with advantage, but considerable latitude is permissible in the proportions adopted. The deposition of the copper upon the surface of the iron is almost instantaneous, and, indeed, a long exposure in the solution produces a slimy precipitate which has almost no adhesion to the basis-metal ; such a deposit, Eoseleur recommends, should be mechanically consolidated and attached by rolling, if the metal be in the form of sheet, or by passing through the dies of a wiredrawer's plate, if it be in the condition of wire. Before dipping any iron or steel article into the copper solution, it must be thoroughly cleansed by plunging it consecutively into the caustic alkali liquids and the suitable acid dips, or by an alkali-dip followed by scouring, as described in the last Chapter ; then, when thoroughly cleansed, it is immersed in the copper-bath. 140 ELECTRO-DEPOSITION OF COPPER. Steel Pens. — Steel pens may be coppered superficially by treatment in the liquid already described, but are more satis- factorily coated by thoroughly stirring them, after cleansing, in sawdust moistened with a solution of half an ounce of copper sulphate with a like weight of sulphuric acid per gallon of water. The mixture is usually effected in a barrel or drum mounted upon a horizontal axis. The long hexagonal drum outlined in fig. 84 is a convenient arrangement for this purpose. It is mounted so that it may be turned on its horizontal axis, the pins at either end resting in bearings upon the upright supports ; one side is hinged, so that it may be opened to admit or discharge the damp saw- dust and pens, and when closed is held in position by a suitable catch. An improvement upon this form may be made by substituting short lengths of tube for the pins at the ends of the drum, and instead of causing them to rotate within bearings, passing a fixed rod completely through the drum, so that the tubes turn upon this rod which is held firmly by the uprights, and which carries fixed arms within the drum. Thus, on rotating the latter, the contents are turned over, and j====, 11 are more thoroughly mixed together by \^.\. the arms or beaters stationary within it. The fixed rod and beaters should be made Tig. 84.— Mixing-drum. of brass or of iron completely sheathed in copper. In using the apparatus, it is first half-filled with the moistened sawdust, then the pens are intro- duced ; the lid is closed and fastened in place, and the drum is rapidly rotated on its axis for a few minutes, until it is judged that every pen has been thoroughly coated, when it is stopped with the door at the lowest point, and this being opened allows the contents to fall upon the floor. The mixture is now placed on brass sieves, the mesh of which is of such size that it passes the sawdust through, but retains even the smallest articles that have been treated ; the sieves containing the latter are now plunged twice or thrice into fresh water, and the washed pens are transferred to a second rotating drum, in which they are dried by contact with hot, clean, and dry sawdust, which is subsequently separated from the finished nibs by means of sieves. Other solutions for coppering by simple immersion have been recommended, and notably those of Kopp, who coats iron in cupric chloride containing a little nitric or hydrochloric acids ; SINGLE-CELL PROCESS. 141 and Puscher, who treats brass by exposing it to a solution of copper sulphate and ammonium chloride. Obviously in all these processes the deposition of the copper is due to an exchange of a more electro-positive metal (e.^., iron) for the copper contained in the solution ; thus the latter gradually accumulates a large quantity of iron, while it loses a corresponding amount of copper (56 of iron being equivalent to 63 '5 of copper) ; and for this reason the bath must be watched to ensure that it is maintained at approximately the right strength. The simple immersion-process is not strictly electrolytic, but merges into a single-cell process when, as by WeiFs method, a piece of zinc is placed in contact with the metal to be coated, to facilitate the deposition of the required metal from a solution which is tardy or inactive. Single-Cell Process. Weirs Process. — Weil's process which he has used for cop- pering cast-iron pieces, even of large size, consists in dissolving 5 J ounces of copper sulphate, 13 ozs. of soda-lime (containing 50 per cent, of caustic soda), and 24 ozs. of potassium-sodium tartrate in each gallon of water, and in submitting each piece of iron, with a fragment of zinc attached to it, to the action of this solution. The zinc, being in metallic connection with the iron, sets up a current as it dissolves in the liquid, and deposits the copper, therefore, not on itself but upon the iron, to which it is electro- positive ; the duration of the immersion may range from a few hours to several days, as the deposition proceeds very slowly. The single-cell process was actually the source of all the others, for by its aid the art of electrotyping was first accomplished. In its simplest form it is well represented by the arrangement of Weil's which we have just described, but a porous cell is almost everywhere used to contain the zinc, so that it shall not be im- mersed in the copper liquid. Fig. 85 illustrates a depositing apparatus of this type; the outer jar, a, which may be made of glass or earthenware, is filled to about two-thirds of its height with a nearly saturated solution of copper sulphate, and contains an inner cell of porous earthenware, closed at the bottom, within which is a plate of zinc, ^, standing in a moderately strong solution of common salt or sal-ammoniac, or, preferably, of dilute sulphuric acid. In the latter case the zinc must be amalgamated; the liquids in the two cells should stand at the same level. The zinc plate should project above the porous pot, and have soldered to it a piece of copper wire, which serves to connect it with the object 142 ELECTKO-DEPOSmOX OF COPPER. to be electrotvped, c. Thus a species of Daniell-cell is formed, in which the zinc, dissolving in the acid liquid of the porous cell, deposits copper upon the conductor in the outer jar : and crystals of copper sulphate should be suspended in the liquid at the upper portion of the outer cell, to replace the metal deposited from the solution upon the negative pUte, just as they are in the ordinary battery -cell. Fig. 85. — Single-cell depositing Fig. 86. — Single-cell depositing apparatus for one object. apparatus for two objects. Several objects may be coated simultaneously without detri- ment to the working of the cell ; all must of course be attached to the zinc, and they may be suspended around the central zinc rod, as indicated in lig. S6 : the arrangement here figured on a small scale may be made of any required size by substituting wooden vats for the glass containing-vessel, and using porous cells and zinc plates of corresponding dimensions. In all the methods of deposition which we have been considering, one face ouly of the object is turned towards the zinc, and that face alone will receive a deposit of copper ; this is suitable enough when it is only required to produce an electrotype fi'om a coin, the two sides of which are separately treated : but when an object is to be completely covered with copper, prior to receiving a coating of a different metal, some such arrangemeut as that sketched in plan in fig. 87 is to be recommended. The object is suspended in the centre of the tub containing the copper solution from two cross-rods which rest on a circular wire connecting all the zincs in their separate porous cells, these being arranged around the circumference of the containine-vessel. Fig. 87. — AiTange- ment for electro - typing all sur- faces at once. DEPOSITION BY BATTERY. 143 In this manner the object to be coppered is completely surrounded with zincs, and the deposition proceeds with equal regularity on all portions. Porous diaphragms may be made of parchment- paper or of plaster of Paris, but are less satisfactory in use, and should only be adopted as a temporary substitute for the unglazed earthenware cells upon emergency. The single cell would seldom be used in practice when a separate battery-plant could be obtained, because it is more clumsy in its arrangements, the process is less under control, and the solution gradually becomes exhausted of copper unless well tended. Deposition by Battery ; or Separate Current Process. The principle of this process has already been fully explained ; a current of electricity is passed from a copper plate (anode) to the object which is to be coated (cathode), both being immersed in a solution containing copper ; a quantity of copper, depending entirely on the strength of the current, is thus dissolved from the anode, and an equal amount is deposited upon the cathode. Such details as strength of current, duration of process, com- position of bath, and disposition of plant must be determined by the character of the work under treatment. In the remainder of the chapter it is proposed to treat first of the electro-deposition of copper generally, then as applied to the covering of iron, brass, or other metals for protective, ornamental, or other purposes; while the electrotyping of printers' plates and art- electrotypy will be dealt with in a separate chapter. The Battery. — The battery employed is very frequently that of Smee, which is a favourite with printers' electrotypers ; the Daniell and bichromate, or modifications of them, are, however, also largely used. For the acid copper-baths a comparatively weak current of low electro-motive force is required, and any attempt to hasten the deposit by increasing the battery-power will result in defeat, owing to the production of brittle and erystalline or spongy copper. The alkaline bath requires a higher electro-motive force, such as would be provided by two, or even three, Bunsen- or bichromate-cells in series; but the volume of current must not be excessive, on account of the lower solubility of the anodes in the solution, which would lead to a portion of the oxygen deposited at the anode escaping without combining with copper, and this in turn would result in a lower rate of solution than of deposition, and so to a gradual impoverishing of the liquid. The number of cells to be used 144 ELECTRO-DEPOSITION OF COPPER. must depend upon the quantity of the work ; with the acid copper-solution they will all be arranged in parallel and should. be increased in number as the area of cathode surface is multi- plied. A dynamo may, with great advantage, be substituted for the battery, but it must have a very low electro-motive force, and must, of course, be selected to suit this class of work. The Solutions. — For coating metals which are less electro- positive than copper, and for the production of electrotype-plates, a simple solution of 1 J pound of copper sulphate and ^ pound of concentrated sulphuric acid in each gallon of water, will be found to give excellent results with a current of about 1 ampere per square decimetre ( = 0*064 ampere per square inch, or 9*3 amperes per square foot) of cathode surface. The bath should be made up by placing the weighed quantity of crystallised copper salt in a suitable vessel, and pouring upon it about four or five pints of boiling distilled or rain water, and stirring until the crystals have quite dissolved. If the solution be not now perfectly clear, owing to the presence of insoluble impurities in^ the copper sulphate, it must be filtered by passing it through a cone of blotting-paper fitted into a glass funnel (see p. 56), which will remove all mechanical impurities. The remainder of the water, necessary to make up the solution to the volume of one gallon, is now added cold ; and when the mixture is thoroughly cool, the sulphuric acid is cautiously added in a gentle stream, while the liquid is briskly stirred with a glass, rod, or if glass be not at hand, with a clean wooden stick or a length of copper rod. Iron must on no account be used, nor may iron or zinc containing- vessels be employed to hold copper solutions, because these metals deposit a portion of the copper- and contaminate the liquid by passing into solution themselves. Iron vessels, protected internally by a sound coating of enamel, may, of course, be used, but glass, glazed stoneware, or even wood are preferable, unless the enamel is frequently examined, to ensure that the iron is nowhere exposed to the solution. For treating metals such as zinc and iron which, being more electro-positive than copper, would take a non-adhesive deposit in the acid solution we have just described, recourse must be had to a special bath. In the following table are given the percentage compositions of a number of different copper solutions which have been advocated by various authorities. It will be seen that the majority of these take advantage of the solubility of copper cyanide in the solution of potassium cyanide, while the- remainder, for the most part, use copper oxide dissolved in. 02 to O ifl t t pq CO O _ aT W 00 H P5 Ph CO ifs /-t.i-; lo lo us CCS T5« fl-, o ^ O (M PQ -rs o • o a> ^ o ^ o 02 p m 02 ^ w -♦^ o o > s c . o ^ I— I ti §«^«> JO O U-l "~' _ (M ^-13 ^ =^ ^ 'CS a fe; .S ^ -2 > '^^> M ^ O 03 i-H !/: ft P) ^ o o .ca . o 4J -^7:3 w _, ^ — o s 2 00 r^H oi-'^.S a o 1> .^2 I "1 o eS^fl ^•^^ II II o O O w H t-H o W Pure Water, | 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 Sulphuric Acid. o • • • • • • • • o • • • • Sodium-Potassium Tartrate. . o • • I kO tH Potassium Bi- tartrate. o . . . . : o . . . . Potassium Car- bonate. a; • . la . . • • . 45 (M • • • Sodium Carbonate. O O O O • • • . lO • • . . • • • (>4 CO (M • • • • G. o • coco Copper Cyanide. • • • 6i Copper Carbonate. ( .opper Acetate. . .,h • '>i r-< >^ r-t r-t • -(M Wr* O o w to .s :3 o O CO 1^; o g ce O 02 o o S.2 o -shaped toggle-joints. 176 ELECTROTYPING. T T, which are pivoted at their centre and held by the framework beneath at F F, and attached to the plate at D D, the result of the pressure being that the >-shaped jointed bars tend to straighten, and being constrained from moving at their lower end, thrust the plate carrying the mould with great force upwards, against the cover, and in so doing press the face of the type mta the wax. The pressure must be gradually, steadily, and evenly apphed, until, for large surfaces, it may amount in the aggregate to several tons. Excessive pressure renders it almost impossible Fig. 94.— Hoe's toggle-press. to separate the forme from the wax without damaging the latter,, whilst an insufficient pressure does not give the depth necessary for printing to prevent the inking of the paper in the spaces between the letters ; the mean between the two pressures is soon learnt by experience, and may be judged with tolerable accuracy by the force applied on the hand-wheel or pump of the press. ^ When the pressure is released and the press opened,* the surfaces of mould and type must be separated by inserting bent screw-drivers gently between them at either end, and applying TRIMMING THE WAX IMPRESSION. 177 slight leverage until they are almost disengaged ; after similar assistance, applied if necessary at the sides also, and when the forme is quite free of the matrix, the latter is removed by lifting it vertically upwards ; it must be inspected to ensure that it is Trimming the Wax Impression. — All the wax which has been forced up around the sides or into deep spaces must now be carefully pared away with a sharp knife, and other spaces, the levels of which are so dangerously near to that of the type face that there is danger of their typed, are filled up by means ^ ■ ^ of a heated knife (fig. 95) ^^S- 95.— Building-knife. or a building-tool (fig. 96). These tools are heated to a temperature a little above the boil- ing point of water, so that a fragment of wax placed against them melts, and passing down to the point of the tool, may be run on to any desired point. This is a critical operation, which requires much care and no little skill to accomplish satisfactorily. Black-leading the Impres- sion. — The surface must now be most completely black- leaded. This is done by sprinkling a quantity of the Fig. 96. — Building-tools, finest plumbago over the whole, and then gently stippling and beating it into the wax by means of goats'-hair brushes. The great volume of black dust produced is very unpleasant, and many operators use black-leading machines, which not only prevent the scattering of dust, and consequent loss of valuable material, but effect a more certain covering of the matrix, which will be recognised as a matter of vital importance when it is remembered that a small area of surface insufficiently coated will give rise to a flaw in the electrotype plate, while even a speck will produce a pin-hole. The black- leading machine (fig. 97) is a large rectangular frame with a box beneath, a cover over the whole, and a trellis-table to support the wax matrix ; a reciprocating motion is imparted to the table by a hand-wheel placed outside the case, but connected with the necessary mechanism within ; and this serves also to produce a vertical reciprocating or dabbing motion to a brush which extends perfectly sound in every part. printing off with the type when they have been electro- M 178 ELECTROTYPING. across the whole table at its centre. On sprinkling the mould with a fair supply of plumbago, placing it face upwards on the table, and actuating the hand-wheel, the wax cast will be drawn to and fro beneath the moving brush, which will force the plumbago into every interstice ; the excess of black-lead falls into the box, while the cover prevents the escape of dust into the air. The cover may be made of glass, so that the progress of the operation may be watched, as far as the dust will permit ; occasionally even the hand-brushing is conducted under a shade, Fig. 97. — Hoe's black-leading machine. but this is not altogether satisfactory, as the operator is some- what cramped in his position, and, therefore, less able to ensure thorough work. The silvered or gilt plumbago, the tin-powder or the precipi- tated copper process for metallising the mould, may be used here if desired. Any system which necessitates the use of phosphorus is to be avoided, because this element may render the copper superficially brittle, and this defect is fatal to work that has to withstand the wear of the printing-press. When the mould is thus rendered superficially a conductor of electricity, every portion which is not required to receive a coating of copper — for example, the back of the frame and the edges of the wax — is painted over with melted wax by means of a soft brush. MAKING ELECTRICAL CONTACT. 179 Making Electrical Contact. — It is now ready for the electro- depositing process. Electrical connection must first be arranged for by embedding a frame-work of warm copper wire around the wax edge of the mould, then black-leading the surface of the wire to ensure good contact with the plumbago coating upon the wax ; and attaching the end of the wire itself to the cathode-rod of the bath. Sometimes the end of the wire only is embedded, but then the depositing action of the current has to spread itself over the whole of the plumbago surface from one point; whereas, by using the frame, it starts simultaneously from the whole circum- ference and gradually covers the surface towards the centre. When the electric contact gripper is used no further trouble need be taken. The smaller cavities in the wax would remain filled with air if plunged at once into the copper-vat, especially as the plumbago has a somewhat repellent action upon water until it is once whetted ; and as any air space of this kind prevents deposition locally by destroying contact between the wax and the solution, the mould is finally prepared for the bath by placing it in a tray and flowing an ounce or two of spirits of wine over it, to facilitate the wetting of the plumbago, then filling the tray to a depth of about 3 inches with water, and directing a high- pressure jet of water upon the surface from a rose held at a height of a few inches above the surface. After washing in this way for a minute or two, the surface should be inspected in various lights while still under water; any air-bells yet adhering to the surface of the wax are thus at once seen, and the washing must be continued until they have disappeared. Depositing the Copper. — The tray of wax is at once trans- ferred to the acid copper-bath, in which it may be suspended after the manner recommended for steel-engravings. It is advisable to increase the electro-motive force of the current employed beyond the normal at first, in order to force the copper deposit over the comparatively weak-conducting surface afi'orded by the plumbago. Copper should be deposited immedi- ately on the exposed metallic surfaces of the conducting wire, and should gradually spread from this until the whole surface is covered, w^hen the potential of the current should again be reduced ; the metal should then continue to precipitate evenly over the entire plate until it has attained to a thickness of the lio to the 3^2" of inch, w^hich is generally adequate. Progress is tested as in depositing upon metallic plates, by gently lifting one corner with a penknife; from four to fifteen hours usually suffice. When sufficient copper has been deposited. 180 ELECTEOTYPING. the frame is removed, rinsed with water, rested upon a level or slightly-sloping board, and suddenly flooded with hot water on the back of the newly deposited metal ; this immediately releases- the latter from the wax so that it may be detached at once (every precaution being taken against bending it) and examined by holding it up to the light. If many holes be visible the plate- should be discarded, if only a few, and these small ones, they may be made good in the next process. The wax matrix can rarely, if ever, be safely used a second time. These electrotype- plates will be subsequently strengthened by ' hacking metal,^ hence they need not be so strong intrinsically as those which have to bear the strain of the press unsupported ; and, therefore,, a more intense current may be used than is permissible — for example, in the reproduction of engraved plates ; but on no account must the volume be so intense that hydrogen is deposited with the copper, for not only is the metal itself weak (even if it be coherent at all) under these circumstances, but the clinging of the bubbles of gas to the work is certain to produce pin-holes- in the plate. With a 20 per cent, solution of copper sulphate^ slightly acidified and constantly agitated, a current of 0*2 to 0*225 ampere per square inch (3 or 3 '5 amperes per square deci- metre) is quite the maximum that should be adopted. For work which will be carefully used a thin deposit may suffice, such as might be deposited by 0*13 to 0T6 ampere per square inch (2 or 2*5 amperes per square decimetre) in three or four hours; but for plates which may have to withstand rough treatment or long wear, fifteen or twenty hours may have to be given. Backing the Copper-Sheet. — After examination, the trace of wax which adheres to the copper is removed by a rinsing with caustic potash solution, followed by a thorough washing with water. The next operation is to protect the thin shell with a- strengthening metal. The back of the copper sheet is painted over with a solution of zinc chloride containing a little sal- ammoniac (ammonium chloride), or borax; it is then rested on an iron tray which is suspended in contact with the surface of melted backing-metal. Granulated tin-lead alloy or foil con- taining 50 per cent, of each of these metals is then placed upon- the copper sheet, and the heat is continued until the white metal, has just melted — not higher, lest the copper become oxidised.. The tray is now removed to a level place and a small ladle full of backing-metal, which has been well skimmed, is slowly poured over the surface, commencing at one corner, until a depth of about one-eighth of an inch is attained. It is then allowed to cool. The backing-metal is an alloy of 91 per cent, of lead, 5 of BACKING THE COPPER-SHEET. 181 antimony, and 4 of tin (by weight) ; it should not be overheated — a temperature of about 600° F. is suitable. A rough practical test is to dip a scrap of white paper into the molten bath, when it should become only just discoloured, any stronger signs of scorching showing that the temperature is too high. The object of the preliminary coating with tin and lead is to ensure a sound union between the copper and the backing-metal, such as could not otherwise be guaranteed. After subdividing with a circular saw, if necessary by reason of the treatment of separate blocks or pages upon the same plate, the copper is examined with a steel straight-edge, and if not truly level, the positions of defective portions are marked on the back : it is then straightened by gentle blows with a polished hammer, taking every care that the face be not damaged. After obtain- ing a plane surface, the excess of backing-metal is shaved off in a specially constructed lathe or hand shaving-machine ; it is then trimmed and again tested with the straight-edge; irregularities are again rectified, and it is finally reduced to exactly the required thickness (usually that of a small pica) by a hand planing-machine ; after finally bevelling at the edges it is mounted on wood, type high. Any backing-metal which has found its way to the surface through pin-holes in the copper may generally be removed, unless the soldering-fiuid has also penetrated, and so caused the two clean metallic surfaces to unite. Unevennesses and defects of this character must be set right by a competent workman with a knowledge of engraving, to whom the final examination of the finished plate should be entrusted. Gutta-percha composition-moulds from type-formes are treated in the same manner as wax-impressions. Wood-Blocks. Wood-blocks, like typographical matter, may be copied by wax; but since they are liable to be damaged by extreme pressure, it is safer to mould them in gutta-percha rendered plastic by heat, or by pouring the melted gutta-percha and lard mixture over them as described on pp. 158, 159 ; then, after ren- dering them conductive, they are electrotyped, trimmed, backed, planed, and mounted in the same way as those produced from wax-matrices. 182 ELECTEOTYPING. AeT ELECTROTYPrNG. In this group may be arranged the reproduction of medals, medallions, busts, and statues, or objects of vegetable and animal origin and the like. Among the principal points to be observed are the choice of a suitable moulding-material, and the carrying out of the casting on the one hand, and the arrangement of the anodes in the bath on the other. Moulding. Medals. — Medals, coins (or medallions if metallic) may be coated directly with copper after the manner of copying engraved steel-plates, the copper-matrix then being used to electrotype upon, provided that they are not in any degree undercut. Only one side can be treated at a time, and the back must be protected by a stopping-off varnish, while the face is brushed over with the solution of wax in turpentine, already described, to prevent adhesion. If possible, the connecting wire should be soldered lightly to the rim of the medal ; but as this is rarely allowable, the object may be slung in a wire loop, or it may be placed in a copper tray as described by Urquhart, and depicted in tig. 98 ; these trays are made of thin sheet copper painted on the outside with Japan black to prevent local deposi- tion, but left bright inside to make connection with the medal ; they are supported in the bath by the hook shown above, the medal being merely fitted into the bottom of the tray. They may be made of various sizes to take any coin or medal of which a copy may be required. When the object is slung from a wire loop, its position must be shifted from time to time to prevent the formation of wire-marks. Medallions made of a non-conducting material must be made conductive by plumbago or thin metal *leaf,' a process which is rarely admissible. It is usually safer, in any case, to prepare a mould from the medal in preference to taking an electrotype-matrix. To this end the medal is rubbed lightly over with plumbago by means of a brush to which a circular movement is given ; it is then' placed upon a flat surface, preferably protected on the under side by a disc of chamois leather, if there be designs on both Fig. 98.— Copper trays. THE MOULDING OF STATUARY. 183 sides and the ^relief of the lower one is nearly as high as its surrounding rim ; it is then moulded with plastic gutta-percha or with the fluid composition, as explained in the section deahng with moulding materials. Either of these methods can be recommended, but any of the other materials described m the beginning of this chapter may be employed as there directed The mould is then rendered conductive with plumbago or metal and is ready to be electrotyped, the methods of doing which have been already dealt with in full. rv> ^ j • Busts and Statues.— The moulding may be first effected m the elastic composition (see p. 163) by placing the object, shghtly oiled on the surface, if permissible, within a box with tapering sides slightly greased, and gently pouring in the warm liquefied mixture, until the object is covered and the box completely filled, taking care that no air-bells form upon the surface of the former during the operation. The whole is now allowed to stand in a cool place until solidification is complete, when the box is removed, and the composition is cut through to the statue or bust from top to bottom, with the aid of a sharp knife, along a line previously determined by the shape of the object to be the most convenient. Being elastic the mould may now be opened out and withdrawn from the object, even if it be some- what sharply undercut ; once removed, it returns to its ongmal shape by virtue of its elasticity. The interior cavity, which ot course has taken the form of the moulded object, may be hardened and waterproofed as above described, and then coated with a conductive film and subjected to electrolysis; but owing to the difficulty in rendering it completely water-resisting and to the fact that wherever liquid may penetrate the mould will swell out of shape, it is not advisable to adopt this plan. It is better to prepare a special wax-composition by melting together a mixture of bees'-wax, rosin, and Russian tallow, m the propor- tion of 50 : 40 : 10 respectively, and pouring this, just at the moment before it sets, into the hollow space within the elastic mould, which should be re-closed for the time m its original containing-box ; the wax-mixture must not be too hot, or the two compositions will unite and the whole operation will be ruined. When the wax has solidified throughout, the mould is stripped from it, and an exact wax-reproduction of the original bust should result. This in turn is placed in a suitable box, and the space around is filled with a cream of plaster of Pans which must be allowed to harden ; it is then removed from the box, dried and heated over a trough, in an oven or stove, to a temperature sufficient to melt the composition from the interior ; 184 ELECTKOTYPING. the side of the plaster block, which had been in contact with the bottom of the box, and upon the surface of which the base of the wax-object is visible, is, of course, placed downwards, so that as the wax melts it runs into the tray prepared for its reception. A small proportion of the wax is absorbed by the plaster, and thus renders it non-absorbent. The interior of the cavity in the plaster is now rendered conductive by black-leading or metallisa- tion, and is ready for the electrolytic process. When, however, the whole plaster-mould is to be immersed in the vat, the outside must also be waterproofed by painting it with wax and subse- quently applying heat; and it is desirable also in this case to cut a small aperture through the plaster at the highest part of the object (the top of the head in the case of a bust) so that a constant circulation of the electrolyte may be effected during the time of deposition; this channel also must be made impervious to water. A similar but shortened process is applicable to the copying of models moulded in wax, if the original may be destroyed— the operation is taken up at the second stage of the above cycle, the plaster being poured around the object at once, leaving only an opening at a convenient point, through which the composition may be melted out, and the copper solution and anode intro- duced. When the original wax-model may not be sacrificed, the longer process of taking a first impression in elastic com- position must be resorted to, but the fracture of projecting portions of the brittle wax must be carefully guarded against. Other systems of moulding are also in vogue. Lenoir's method for reproducing statues in a manner approaches in principle to that of the foundry. He moulds the figure in gutta-percha in a sufficient number of different parts, the sections being so disposed and marked that when united together thev form a complete mould of the object; the different internal surfaces are black-leaded and then fitted around a skeleton-anode of wire ; a convenient number of apertures are made above and below to afford communication between the exterior and interior of the mould, for connecting the anodes with the battery and for the circulation of the solution ; and the arrangement is ready for electrolysis. A knowledge of the moulder's art is very valuable, if not indispensable, in determining the most suitable method of dividing up the surface of the statue into sections. Large statues moulded in plaster have been copied by render- ing them impermeable by liquid, coating the whole exterior with plumbago, and then immersing them as cathodes in an acid copper sulphate bath, until a thickness of about one-sixteenth THE TKEATMENT OF STATUARY. 185 of an inch of copper has been deposited. They are then cut through at suitable points, where the marks of the joins will be least conspicuous on the finished reproduction, and the plaster being completely removed, the outside is joined to connecting wires and covered with stopping-out varnish, and the inside is rendered dirty by the turpentine solution of wax, or by painting with dilute ammonium sulphide, which gives a superficial tarnish of copper sulphide ; the excess of the ammonium sulphide must be washed away, and copper is then deposited upon the diff*erent sections of the copper matrix individually ; when a thickness of copper of at least one-sixth of an inch, but preferably a quarter or even a third of an inch, has been acquired, the thin copper mould is stripped away, and the separate portions of the electrotyped statue are mechanically finished off and fitted together to form the complete figure. The mould having been prepared and rendered conductive by any of these processes, it is finally arranged to receive the de- posit of copper ; the main point now to be observed is that the anode shall be as nearly as possible equidistant from every part of the cathode-surface. If this be not attended to — for example, in electrotyping statue-moulds — the chief recesses in the mould will receive the thinnest deposit of metal, whereas they will afterwards be subjected to the greatest wear, being the most prominent portions of the finished surface, and should, therefore, bv preference have increased rather than diminished thickness. This matter needs careful consideration, and the ingenuity of the workmen may often be taxed to find the best possible arrange- ment. For shallow-cut metals or plane surfaces with no design in high relief, a flat anode placed at some little distance may sufiice; for reasons fully given on p. 102, the electrodes must not be allowed to approximate too closely. But for surfaces which are raised at any point, and which, therefore, produce deeply-cut moulds, the anode-surface should be dished out into an approxi- mate representation of the mean lines of the original object. It may then be placed nearer to the cathode, and thus impose less resistance in the circuit. But in dealing with statue-moulds, the problem is more difficult. Lenoir meets it by using an anode of thin platinum wire, bent backwards and forwards into a frame- work or skeleton of the figure, of course of smaller size, so that there shall be no danger of contact with the plumbagoed mould. The mould is then built up around this {vide supra), and the whole of the cavity is filled with the copper-solution ; the metal is deposited, and the wire-skeleton is finally removed by withdrawal through one of the cavities in the plaster. Plante 186 ELECTROTYPING. used a similar skeleton composed of perforated lead sheet, fashioned roughly into the required shape, and this, being comparatively inexpensive, was left within the statue when the process was finished. When the statue is moulded in sections, there is, of course, less difficulty in adapting a suitable anode. The lead and platinum anodes do not dissolve in the solution ; the strength of the latter must be kept up by adding crystals of copper sulphate from time to time, as the copper which it contains initially becomes exhausted. It is mainly for this reason that apertures must be provided in the mould-walls for the circulation of the liquid, which may be maintained by plac- ing a muslin bag or copper- wire box, containing crystals of copper sulphate, above the head-aperture, as this produces a gradual downward flow of heavy hquid containing fresh supplies of copper salt. Another result of using insoluble anodes is that a current with higher electro-motive force is necessary to effect the deposition (see p. 32) ; and, again, when platinum is used, oxygen gas is evolved, and for this reason the head-aperture must be at the very highest point to allow the gas to escape, otherwise an accumulation of gas forms at the summit of the figure so that the mould will not be in contact with the solution, and from that time can there receive no further deposit of metal. The lead anode, especially at first, combines with and thus absorbs a large pro- portion of the oxygen to form lead peroxide ; and in proportion as this is formed less electro-motive force is required because the heat of the lead undergoing oxidation is a substitute for that of the copper dissolving at the anode in ordinary electrotypy. When copper anodes are used it is more than ever of importance that they should be of the purest electrotype-copper, to prevent the formation of insoluble mud, which would deposit upon the interior surfaces of statue-moulds, and give rise to much inconvenience. Care is needed to ensure that the anodes at no time short- circuit the current, and stop the process by coming in contact with the cathodes. This is especially liable to happen in electro- typing statues or busts, because the slightest movement may alter the relative positions of the surfaces inside the moulds, and it is impossible to watch the progress of the deposition. Lenoir has suggested that the outside wires of his platinum skeleton should be encircled by an extended spiral of india-rubber filament, which is an insulator; but although for a time this would be successful, it is probable that by the gradual growth of the deposit the precipitated copper might creep up to the anode and effect contact at some point, at which it happened THE USE OF GUIDING-WIRES. 187 originally to approach the cathode too nearly. Such short-cir- cuiting would, of course, be fatal to the deposition upon all the moulds which might happen to be in the same circuit; Lenoir, therefore, introduced into the circuit of each individual mould a short length of thin iron wire sufficient to carry the com- paratively small current required for the electrolysis, but which would heat, and almost immediately fuse, by reason of its high electrical resistance if subjected to the much stronger current entailed by a short circuit. In this way the iron acts as a safety- valve, automatically breaking the circuit in the branch in which the accident has "^occurred, and restoring it to the remaining electrotypes in the bath. It is, in fact, what is known as a fusible cut-out, such as are now usually made of lead foil or wire for electrio-lighting circuits. The more modern form would answer the same purpose, being made to melt and break the circuit as soon as it is subjected to an undue intensity of current, the thickness of the lead being determined by the strength of the current which is to call it into use. For this class of work the ammeter should always be em- ployed ; it would not only indicate short-circuiting as soon as it occurred by registering the greatly-increased current flowing in the circuit, but it would also show whether the process was taking its normal course. Any undue approach of the electrodes would diminish the resistance and give rise to an increased current-volume, while any break or defect in the wires would be shown by the diminished ampereage recorded by the instru- ment. It, alone, affords an opportunity of judging of the pro- gress of the work within a closed mould. Having decided upon the best arrangement of anodes, the method of ensuring the most rapid covering of the mould demands attention. When the matrix or mould is of metal, no difficulty arises, because of its high conductivity; it is only necessary to connect any part of the mould with the generator to ensure an immediate deposition over the whole surface exposed. But when connection is made between the battery and cathode at only one point in a large non-conductive mould, a long time must elapse before the deposit will spread to the more distant portions of the plumbagoed surfaces ; but the deposit is more uniform, and the result, therefore, more satisfactory, in proportion to the rapidity with which the mould is initially covered with copper. In order to convey the current to several parts of the mould at once, light guiding-wires may sometimes be temporarily arranged so that their points rest lightly on tlie plumbagoed surface ; these act as so many nuclei or starting-points for the deposit, and, as 188 ELECTKOTYPING. soon as the copper has spread from them and covered the inter- vening surfaces, they are no longer required and may be removed. These guiding-wires are specially useful in carrying the deposit into the deeper or under-cut portions of the mould, into which it is often difficult to drive it at the outset, but which continue to receive a deposit when once they have been covered by a better conducting surface than the plumbago. The wires cannot be well arranged in the internal cavities required for reproducing busts and statues, as they are liable to make contact with, and to disturb the position of, the anodes. They may, however, be some- times passed permanently through the walls of the mould itself by adjusting them within the casting-box, so that their points rest very lightly upon raised portions of the wax-model (preferably at such points that they may not mar a flat surface on the finished figure by any mark indicative of their position). The several wires are collected into a bundle together outside the mould, and are then connected with the negative wire from the battery. The tip of the wire should be flush with the internal surface of the plaster or composition in the finished matrix, and, being black-leaded, will not adhere to the deposited metal. The deposit may often be coaxed into a refractory corner by using a supple- mentary anode of stout copper wire or thin sheet, which, being connected with the battery (positive pole), is held temporarily with its surface very close to, but not touching, the part to be covered ; thus the local resistance of the solution is much diminished and the deposit is readily started, and, when once formed, will continue to increase without difficulty. In all art-w^ork of the description to which we have been latterly referring, the conductive film must be of the finest quality in order to transmit the current with the utmost rapidity from the points of original contact to the remainder of the surface. The silvered plumbago off'ers great advantages in this connection ; and if ordinary plumbago be employed, only the best description is permissible. The finished electrotype generally presents a dirty appearance, owing to the black-leaded surface with which it has been in con- tact ; it may, however, be cleaned by rubbing with turpentine or benzene, sometimes after a preliminary plunge into boiling oil. Reproducing Natural Objects.— Animal or vegetable objects are often simply coated with a thin film of copper, and used in this condition for ornamental purposes, an additional deposit of gold or silver being added to that of the copper. To effect this, a conductive surface is first formed upon the object to be coated ; warm spirits of wine are shaken with crystals of silver nitrate GLYPHOGKAPHY AND STYLOGRAPHY. 189' until no more of the solid is dissolved. The object is then painted superficially with this solution, and placed under a glass bell-jar, or clock-shade, together with a saucer containing a few drops of a solution, made by dissolving a small fragment of vitreous phos- phorus in an ounce of carbon bisulphide. The vapour of phos- phorus evolved reduces the silver nitrate to the metallic state, and thus covers the whole surface of the object with a thin but continuous conductive film of silver, and enables it to receive an electro-deposit of copper of any desired thickness by merely sus- pending it as the cathode in an acid copper-bath. The greatest care is required in using this solution of phosphorus ; it is liable to produce painful sores if it fall upon the skin of the operator and be not immediately washed off, and to cause spontaneous ignition, even after the lapse of a considerable time, if it evaporate in con- tact with organic fabrics. Instead of merely covering the object with a film of copper, it may be reproduced by taking a mould in suitable material, black- leading its internal surface, and electrotyping it in the manner of statues or busts. The copying of any insect or leaf becomes thus- a question of moulding. Many other appUcations of electro-metallurgy in connection with copper are used, especially in connection with the multi- plication of drawings and designs : among processes of this kind that have been proposed may be mentioned — Ghjphography, which requires a flat copper plate to be either coated with two layers of composition, one black, the other white, or, preferably, to be itself rendered black by exposure to ammonium sulphide solution ; it is then coated with a white material. The required design is scratched through the wax until the black surface of the copper is visible ; when the drawing is complete— which is readily seen, because the lines appear black upon a white ground— the whole surface is coated with plumbago and electro- typed to a thickness of about the of an inch. This is supported with backing-material, like an ordinary electrotype-block, and is ready for printing in the typographical printing-press, the lines of the drawing being, of course, in relief upon a flat surface in the finished plate. Stylograjpliij is somewhat similar as to the mechanical part of the process. The copper plate being covered with a mixture of 67 per cent, of shellac and 33 of stearine, with sufficient lanip- black to render it black, is varnished and sprinkled hghtly with silver dust. The latter is then removed along the lines of the intended design until the black composition is seen beneath : the whole is then plumbagoed and electrolytically coated with 190 ELECTEOTYPING. copper. The lines are not sufficiently raised for ordinary type- printing ; a second plate must, therefore, be taken from the first, reproducing the etched lines of the original, and this is used for printing as from an engraved surface. Galvanograpliy consists in building up a picture in coloured varnish, the gradation of light and shade being given by varying the thickness of this film. After black-leading, the surface is coppered, and, being cleaned with oil of turpentine, is used like the last as an engraved plate. It is obvious that any of the photo-mechanical printing processes of the present day, in which the printing surfaces in relief are obtained from photographic reproductions of any drawing or suit- able object, may also be aided by the sister art of electro typy : the electrolytic part of the process is practically the same in all, and differs not from those already instanced, so that it is unnecessary to describe any of them in further detail. Electrolytic Etching. — By the reverse of these processes the same result is attained. The design is traced on the waxed surface of a copper plate, taking care that the etching-tools lay bare the metal in all the lines. The plate is now introduced into the copper-vat in connection with the an£)de wire instead of the cathode, and the copper dissolves at all places where it is exposed to the action of the solution ; but since the whole plate is insulated with the exception of the lines of the etching, it follows that along these only the copper is attacked. The lines may thus be bitten- in to any required depth ; the depth may be determined at the will of the operator by adjusting the distance between the elec- trodes, the points of nearest approach being those which receive the deepest cut. The resulting plate is used precisely as an ordinary etched copper plate ; and, indeed, it is such, the processes employed to produce them being identical except in the method of biting-in the hues. To obtain anything but crude results, however, by these processes demands much experience and atten- tion, as it is frequently necessary to stop-out some of the finer lines to prevent further action at different periods of the process, and practice and artistic skill alone can guide the operator in this matter. An ingenious process for obtaining nature-prints of leaves and similar bodies has sometimes been used. The leaf is placed between two plates, one of polished steel, the other of soft lead, and is then passed between rollers which exert a considerable pressure. The leaf thus imparts an exact impression of itself, and of all its veins and markings, to the surface of the lead ; and this impression may be electrotyped and the produced copper plate used for printing in the ordinary way. MANUFACTURE OF REFLECTORS. 191 The subject of reducing copper from its ores and refining the crude metal will be dealt with in Chapter XV. Manufacture of Eeflectors. — The increasing use of powerful electric search-Ughts for naval and military purposes has led to a demand for a substitute for the heavy, costly and frangible glass lenses hitherto employed. The substitute patented by Cowper- €oles ^ is interesting, not only as an example of a special process of copper deposition, but as illustrating a successful appHcation of electro-metallurgy in a new field. Instead of being refracted through a glass lens, the rays of light are rendered parallel by reflection from an accurately formed parabolic reflector. To accomphsh this, a convex glass matrix is prepared of such shape that its convexity would exactly fit into the concavity of the reflector required. This matrix may be made mathematically true by grinding and polishing, and is ready to serve as the mould for a whole series of reflectors. It is first coated with silver by immersion for half an hour, face downwards, in a silver- ing liquid, made up of equal quantities of a 0*5 per cent, solution of silver nitrate, a 0*5 per cent, solution of caustic potash, and a 0-25 per cent, solution of glucose. The film of silver is then burnished with cotton wool and chemically precipitated peroxide of iron, and the mould, which is handled by means of a * sucker ' attached to the reverse side, is clamped to a metal ring, through which connection is made between the silver film and the cathode wires. The ring is supported horizontally from a rotating frame, so that the mould is plunged face downwards in the electrolyte tank containing an acid copper-sulphate bath (copper-sulphate 13, sulphuric acid 3, water 83 per cent.) and suitable anodes. The frame is so attached to its support that the mould may be tempo- rarily tilted, in order that the periphery of the silver shall be immersed before the centre portion. Here it is rotated at first with an apphed E.M.F. of 9 volts, and then with a current density of 19 amperes per square foot, until the silver is well covered with a layer of conducting copper. The edges of the mould are then stopped off" by contact with a ring, which prevents deposition on the copper beyond it, and so gives a sharply-defined border to the reflector. When the layer of copper is sufiiciently thick, the mould with the reflector adhering to it is removed from the bath, and placed in cold water, the temperature of which is gradually raised to 120° F., whereupon the difference in the expansion causes the separation of the copper reflector, which is then electro-plated on the face with palladium for use, leaving the mould ready for a fresh coat of silver and copper. * Jour. Inst, of Electrical Eiigineering, 1898, vol. xxvii. p. 99. CHAPTEE IX. THE ELECTRO-DEPOSITION OF SILVER. The electro-plating of articles with silver was one of the earliest applications of electrolysis, because it produced a material analogous to, but cheaper than, the older * silver-plate,' in which the base metal was covered mechanically with a layer of silver ; and even at the present day, when electrolysis is used to obtain coatings of so many different metals for such varied pur- poses, the deposition of silver must, perhaps, take the foremost place, both in respect of universality of practice and value of results. Deposition by Simple Immersion, or Whitening. Silver, as compared with most metals, is very electro-negative^ and hence all the base metals are capable of exchanging places with it when dipped into a solution of one of its salts. This process is, however, used only to impart the thinnest possible wash of silver, more especially to small articles such as nails and hooks; so thin, indeed, is the film that the name whitening is thoroughly descriptive of the process. It is clear that the coat of silver can. be but of the thinnest, because, as soon as the metal is covered with the slightest covering of silver, it becomes protected, partially at least, from further contact with the solution. There are two principal methods of silvering by simple im- mersion — first, by dipping the article into a solution of silver, either hot or cold ; second, by rubbing a semi-solid paste of a silver compound over the surface of the object. For both pro- cesses the objects must be clean, and must present bright metallic surfaces to the action of the depositing compound. Formulse for- making-up such silver mixtures are numerous ; those principally used are included in the following table, in which they are arranged under the respective class-headings of solutions and. pastes. SIMPLE-IMMERSION MIXTURES. 193 P p !^ 1^ o o E- X s ^ o t-i CO g H ^« ^ Pi O Ph o O w O w 02 CO H O P^ O W H 05 o CO CO o d > . O cc .?e fH 'i § ft 2 I- 1> ^'^ fcH t O Si o o ■ § 2>? - S CO O Water. 1000 1000 1000 1000 OOOT AAAT q. s. 1000 1000 1000 •D •0 6< T.PViP'fl.tpfl nhfilt . . o • • 00 6h Amiuoiiium Chloride. , . o . ! (M 8 00 H (P Ammonia. S own.! Lilli Thiosulphate. .oo . . . , I : : ; : o oo : : r-l Sodium Chloride. \a rH o • 00 00 42001 Sodium Carbonate. O CO 1— 1 Alum. : : : : : : o H Potassium Ferrocyanide. o . . ! I ! I o tH • I-l r*1 • • • o rH Silver Chloride. rH o 00 : «5 rH ^ rH • 2000 o Most Suitable Tempera- ture. 1 Boiling - • o Boiling -o ft o O . □ o • c3 P^ ft^d o o O O 02 ri OS a (DO)-; j:3 o o rH M CO Ttl N 194 ELECTRO-DEPOSITION OF SILVER. Immersion Solutions. — It is obvious that since the deposition of the silver is due (and is also proportional) to the amount of the base-metal which dissolves from the object under treatment, the solution gradually becomes exhausted of the former and contami- nated with the latter ; and if the articles are of copper or brass, as they most frequently are, the fact of the contamination, and in some degree its extent, are rendered apparent by the blue colour imparted to the solution by the dissolved copper. Most of the liquids are used hot, and a momentary dip suffices to effect the required purpose. One of the best is I^To. 3 (Roseleur's), pre- pared by making up into a paste 1 ounce of silver chloride with 4 pounds each of powdered potassium bitartrate (cream of tartar) and sodium chloride (common salt), then adding a proportion of this to boiling-water, contained in a copper vessel, immediately before it is required for use. The articles, held in a copper sieve or porcelain colander, are plunged into the solution, where they become coated instantaneously ; but for the sake of security they should be stirred around with a piece of wood or with a porcelain or glass rod ; they may then be removed, thoroughly washed by rinsing in two or three vats of water, and dried in hot boxwood sawdust. As this bath works best when old, and consequently highly charged with copper, care must be taken that no pieces of iron or zinc or other very electro-positive metal be clinging to the goods, or a certain proportion of copper will be deposited with the silver, which will in consequence acquire a pinkish coloration. Cyanide solutions may be made to give a good whitening effect, as indeed may any of those specified in the above table. An interesting process of Roseleur's is not included in this table ; the liquid is prepared by slowly adding a solution of silver nitrate to one of sodium bisulphite, until the precipitate, which forms upon admixture, begins to dissolve but slowly in the solution on shaking. The copper or brass objects are dipped into the bath, cold, and immediately become covered with silver by simple exchange; but after this, unlike the behaviour of other solutions, the film continues to increase in thickness, not, however, on account of any further solution of the base-metal, but owing to a chemical action inherent in the bath itself, which causes the deposition of the silver, not only on the metallic objects immersed, but even on the walls of the bath, on glass, or on any substance introduced. This is due to the ready decomposability of the silver salt employed, and to the tendency of sulphurous acid to absorb oxygen, which it does at the expense of a portion of the silver oxide, depositing an amount SILVERING BY SINGLE-CELL AND BATTEKY. 195 of silver corresponding to that of the oxygen used up. This reaction occurs but slowly in the cold, so that there is time for a gradual building up of the silver into a coherent and adhesive deposit. If the liquid be heated, the action becomes too rapid and the quality of the coat suffers accordingly. Pastes. — The use of pastes is especially applicable to the wash-gilvering of comparatively large and flat surfaces, such as the dials of barometers, and for the application of local deposits, or even of preliminary protective films to bodies which are subsequently to be plated with an electro-negative metal. The simplest paste is that made by rubbing together 1 part of silver chloride with 2 or 3 parts of potassium bitartrate (cream of tartar) until they are in a condition of the finest powder, and then working the mixture into a creamy paste by the addition of water. Many operators vary these proportions, or add other ingredients, but the mixture, as it stands, will be found to give excellent results. Roseleur's paste for silvering lamp-reflectors (No, 12 on above list) is rubbed on to the surface with a wad of soft rag, allowed to dry in situ, and is then rapidly removed with a fresh piece of soft linen. In applying the pastes generally, a piece of soft cork or a pad of wash-leather may conveniently be employed. Many of the so-called ' plate-restoring powders ^ used for restoring a white €olour to worn electro-plate, which shows the brass foundation in places, consist of one or other of these mixtures or of modifica- tions of them. Occasionally plate-powders containing mercury are sold ; they are, however, fraudulent, for they purport to give a film of silver to the discoloured objects, but instead impart one of the less expensive mercury, which is in every way to be condemned, for not only is the mercury itself objectionable, but it is gradually absorbed by the base-metal, leaving the surface dull, while repeated applications cause the object to become brittle and useless. The thickness of silver on whitened goods is usually so infinitesimal that they will not bear scratch-brushing or any of the ordinary methods of polishing ; but friction by contact with dry sawdust in a rotating barrel may be satisfactorily substituted. Single-Cell Process. This process is not largely used for silver-deposition, and is quite unsuitable to establishments where there is much work in hand. It may, however, be efl"ected by using an ordinary 196 ELECTRO-DEPOSITION OF SILVER. cyanide plating-solution, containing a porous cell with a zinc rod or plate immersed in potassium cyanide solution, with the usual connections between the zinc and the objects which are being coated in the outer cell. Steele prepared a solution for single- cell work by converting one part of silver into silver chloride, washing and dissolving it in 60 parts of water, in which was also placed the mass resulting from the fusion of 6 parts of potassium ferrocyanide with 3 of potassium carbonate. Na porous cell was used ; the object to be plated was simply con- nected with a plate of zinc, and both together were plunged into- the prepared solution. In a similar manner articles immersed in hot silver-baths have been sometimes treated by simply binding zinc wire around them, so that a greater thickness of deposit would be given than that impartible by simple immer- sion. The separate-current process may be said to be universally applied to electro-silvering, as the plant may be made of any size, and the process is under perfect control. The Separate-Current Process. In working silver solutions with a battery or dynamo-electric machine, the solutions must be well watched, and the current prevented from becoming excessive, as a good fine-grained minutely-crystalline deposit can never be yielded with a high current-density. Resistance-coils should, therefore, be at hand, or some other suitable means of regulating the current under all conditions of the bath, and under all dispositions of the- electrodes within it. The Battery. — The Smee- or Daniell-cells are, perhaps, to be most recommended ; the former being arranged in groups of two- in series, when more than one cell is employed, so that the electro-motive force may be twice that given by a single pair of the plates. A single Daniell-element gives an electro-motive force very suitable to the work (1 volt), and if several cells are- used they should be placed in parallel. Some operators- prefer the original copper-zinc cell, probably because its prime cost is less than that of Smee's, owing to the absence of platinised silver. It is, however, less effective, and becomes very badly polarised as soon as its action commences, but by using a number of couples and plates of large size, it is quite possible to obtain excellent results with it. If a dynamo be used, it should have a very low electro-motive force, because it is less, convenient to arrange the silver-baths or the individual elec- SEPAKATE-CUERENT PROCESS. 197 02 xn o o < C I— ( si >^ 2 CO 05 00 I— ! " O >H O Ph :^ o O w H O o W pq < CO Water. Q 2 O O 03 ^ C2 > I— I K I— I I O"-' O.l CO O t« , 2> r-l O o COCO O o a o 03 > . ft Si ft 03 03 o CO oo 03 CO to S ft Silver Cyanide Silver Chloride, o o o o o o o o o o o o o o o ooooooooo o ooooooooo o ooooooooo o o o o o o o o o o Ammonia. Sodium Chloride. CO • • lb I— I — Sodium Carbonate. : : :g : : : : : co co • Potassium Iodide. . o • : o o Potassium Cyanide (95 7o). Q CO Co ^ ^ o ^'^'Povo 00000^2 ^Sin+ + + ^ ^: y-t M ^OoioCM i-l 00 C!l O r-l CM CO ^ CO 00 03 03 f-i bO 03 cd' 03 r=5 03 pd H I 1^ <1 H o H 1^ H O 198 ELECTRO-DEPOSITION OP SILVER. trodes in each series-fashion, than it is in the electrotype-copper vats; the current, moreover, must be under absolute control by the use of measuring-apparatus and resistances. The Solution. — Most of the solutions used largely in practice have the double cyanide of silver and potassium for their basis ; " and doubtless the solution of this body in a liquid containing an excess of potassium cyanide constitutes the simplest and best plating-bath for general work. The composition of the principal mixtures suggested is embodied in the foregoing table. Silver-Baths. — The silver-baths are generally prepared, as re- quired, by dissolving metallic silver in nitric acid, precipitating it with potassium cyanide, washing thoroughly, and dissolving it in excess of the potassium salt. Ten parts of pure silver yield 12*4 parts of pure silver cyanide. The water and all the chemicals used in preparing the solutions must be pure, as the presence of much foreign matter acts injuriously upon the deposit j the potassium cyanide especially should be examined, as it frequently contains only 50 or 60 per cent, of the pure salt (see p. 394). For a like reason it is better to prepare the silver cyanide separately, and to wash it thoroughly, before mixing it with the remaining ingredients of the solution ; by simply adding potassium cyanide in excess to the nitrate or chloride of silver, a clear bath is pre- pared, but it contains, in addition to the silver cyanide, a quantity of the potassium salt corresponding to the silver compound used, and this is generally objectionable. Thus, for example, on adding potassium cyanide to silver chloride, the silver cyanide is formed which is required for plating, but with it is an equivalent of potassium chloride produced by exchange. AgCl _ -f KCX = KCl + AgCN. Silver chloride. Potass, cyanide. Potass, chloride. Silver cyanide. Again, the proportion of potassium cyanide to silver in the bath, although variable between wide limits, is by no means an indeterminate quantity. Having produced the insoluble silver cyanide, as in the above equation, by the use of one equivalent of potassium cyanide, a second equivalent of the latter is neces- sary to form the double cyanide of silver and potassium, which alone is soluble in the bath ; and in addition to this an extra pro- portion of the potassium salt {free cyanide) must be employed to ensure the perfect solution of the anodes, for a reason which may be stated as follows : — In passing the electric current through a solution of silver-potassium cyanide KAg(CX)o, the double cyanide is broken up into the cation, K, and the anion, Ag(CN)2. The potassium is deposited at the cathode, but, by chemical exchange, ELECTRO-SILVERING BATHS. 199 displaces from the surrounding solution of the double cyanide an equivalent of silver (which deposits on the cathode) and forms po- tassium cyanide in the liquid [thus : KAg(CN)2 + K = Ag + 2KCN]. Thus there is a concentration of potassmm cyanide around the objects which are being plated. The silver travels in the anion, Ag(CN)2, to the anode, where the ion decomposes into AgCN + CN, and the cyanogen (CN) set free attacks the silver anode to form another molecule of AgCN. Hence at the anode there is formed a double quantity of the insoluble compound AgCN. It is there- fore necessary that a good excess of free potassium cyanide be pre- sent, to combine with the silver cyanide, which would otherwise form an incrustation on the anode, and thus to form the soluble double cyanide, whereby the silver is carried into solution and the anode is left bright throughout. The following two equations show the re- quirement of the minimum two equivalents of potassium cyanide :— 1. AgN03 + KCN = AgCN + KN03 (washed away). 2. AgCN +KCN = KAg(CN)2. Thus 108 parts of silver, or 108-1-14 + 48 (AgNOg) = 170 of silver nitrate, require 2 (39 4-12 + 14) = 130 parts of potassium cyanide (two equivalents = 2KCN) ; while 108 + 12 + 14 = 134 parts of the pure silver cyanide (AgCN) require one equi- valent, or 39 + 12 + 14 = 65 parts of the potassium salt, to form the double compound. Thus 108 parts of silver require a minimum of 130 parts of potassium cyanide, and should have, m addition, at least 50 to 75 per cent, extra cyanide, supposing the latter to be pure; when the commercial salt, containing (say) trom 50 to 70 of pure KCN, is employed, the minimum would range from 180 to 250 parts, and the added quantity from 70 to 150. So also 134 parts of silver cyanide call for 65 of the pure potas- sium cyanide as the minimum allowance. But, although a certain amount of free cyanide is necessary, a great excess must be avoided, because it would dissolve the anode too freely and increase the strength of the bath, and, worse than that, would tend to produce a somewhat scaly and non-adhesive deposit upon the cathode. The condition of the bath in respect of free cyanide may be readily tested by withdrawing a little of the liquid m a glass vessel and adding to it a few drops of silver nitrate solution ; a precipitate is thus produced which shoald at once redissolye in the liquid. If it dissolve but slowly even on stirring, it is an indication of a deficiency of cyanide, and the time that elapses before it vanishes completely affords a rough gauge of the amount of free cyanide present. In practice the appearance of the anodes 200 ELECTRO-DEPOSITION OF SILVER. is itself indicative of the condition of the bath; if they are covered with a black deposit during the passage of the current, there is insufficient cyanide present, while if they are quite bright and white, the cyanide is in excess. The best results are obtained when the anodes present a greyish appearance while the current passes, but immediately become white and brilliant when it ceases, showing a complete solution of the thin film upon the surface. A further precaution which may be taken as a check upon the work- ing of the bath is occasionally to w^eigh the electrodes separately both before and after the process ; the loss of weight shown by the anode at its second weighing should be just balanced by the gain upon the plated objects or cathodes. Any departure from this equilibrium indicates either an incorrect ratio of anode to cathode surface, or a wrong proportion of cyanide in the bath ; or, thirdly, an unsatisfactory adjustment of the one to the other. But if the surfaces of the electrodes are well arranged (see further on, under anodes), a loss of anode-weight unaccounted for by the increase in cathode- w^eight clearly points to an excess of cyanide in the bath, or vice versa. Such an abnormal action brings about an alteration in the character of the bath, and must be rectified by the addition of cyanide of silver or potassium, as the case may be. Another very useful test may be made by dipping a strip of bright copper into the bath ; if it become coated with silver by simple immersion, the liquid contains too much cyanide, and the deposit yielded by it wall be bad, especially upon copper articles, or upon coppered objects. Baths are also liable to alteration by exposure to the air, gradually absorbing carbonic acid, which takes the place of an equivalent of hydrocyanic acid in the bath, so that a certain pro- portion of the cyanide is removed, and the electrical resistance of the solution is increased. Fresh cyanide must, therefore, be added from time to time, and these frequent additions, coupled with the gradual accumulation of other substances dissolved from impure anodes, or from the cathodes, give rise to a corre- sponding increase in the density of the solution. Accompanying the increased density is a greater sluggishness of the solution, and hence a greater tendency to separate into layers during electrolysis, the heavy silver-laden liquid from the anode sinking to the bottom of the vat and accumulating there, while the lighter potassium cyanide finds its way to the top. Thus the electrolytic action becomes irregular, a greater quantity of silver is deposited upon the low^er portions of the cathodes, while the coating upon the upper part is not increased, but may even be dissolved, after the manner described on p. 103. A similar ELECTRO-SILVERING BATHS. 201 action is observed in working concentrated, and, therefore, sluggish baths, with an exceedingly weak current from a battery which has 'run down.' Under these circumstances the anode is immersed in a liquid saturated with silver, the cathode in one containing little silver but much free cyanide, and an opposing electro-motive force is thus set up which tends to re-dissolve the deposit. Nevertheless, a moderately (one or two years') old solution will be generally found to give a better deposit than one newly made-up, provided that the ratio of free cyanide to silver be rightly maintained; hence a certain proportion of an old plating-liquid is commonly used in making-up a new bath. When this is impracticable, the effect of age may be imitated by boiling the liquid for two or three hours, or by the addition of a few drops of ammonia solution. The evils attending excessive concentration may be remedied by appropriate dilution, except when the extreme density is due to the accumulation of foreign matter; or, within certain limits, by maintaining the cathode- objects in gentle motion, which exposes changing surfaces to the liquid, and also prevents the separation of the latter into layers ; or, thirdly, by stirring the Hquid well every night after work is over. Even a concentrated solution, however, may conduct well, and may be made to give a good deposit ; but a dilute bath has a lower conductivity — and is, therefore, more tardy in action — while the metal will have a characteristic dead-white lustre. The specific gravity of the solution should lie between 1*05 and 1*10, pure water being regarded as unity ; Gore lays down the limits between which a good deposit is attainable as 1*036 and 1*116. But the dissolved bodies are not the only impurities which find their way into the solution ; insoluble matter from the anodes and dust from the air gradually collect, and when the liquid is kept well stirred, remain in suspension in it, and, becoming entangled in the precipitating silver, produce an uneven deposit. It is, therefore, advisable to filter the solution through blotting-paper from time to time as required. The worst enemy to the plating-solution is organic matter : little by little it accumulates, and although exerting no pre- judicial influeace in small quantities, it is fatal to successful work when a certain limit is reached. The cyanide solution, most unfortunately, is capable of readily dissolving many organic bodies, and the most jealous examination must be made of all objects which are to be introduced into it. Gutta-percha in any form is especially to be avoided ; moulds or stopping-out varnishes containing this body should, therefore, be excluded when silver- plating is to be effected in the cyanide bath. 202 ELECTRO-DEPOSITION OF SILVER. The cyanide solution is generally used cold ; for coating small articles, however, or objects made of iron, tin, zinc, or lead, upon which a film of copper has been first deposited, it is occasionally heated. The bath may be prepared electrolytically, although it is rarely so treated on a large scale, by dissolving 1 J to 2 ounces of pure potassium cyanide in a gallon of distilled or rain water, and passing a current through it from a large weighed silver anode to a small silver or platinum cathode, the weight of which also should be known, until the excess of silver dissolved into the bath from the former over that deposited upon the latter shows the bath to be sufiiciently charged with the precious metal. For this purpose the electrodes are removed from time to time, rinsed, dried, and weighed, until at last the desired strength of solution is reached. Here the large anode is used with a small cathode that the action of the current upon them respectively may be disproportionate ; it is required to add to the weight of silver in solution, so that the case is different to that of an electro-plating bath, in which the solution is to be maintained of uniform density, and which, there- fore, demands approximate equality of electrode-surface. There is no doubt that the cyanide solution possesses many advantages over other possible baths, and with ordinary care will give but little trouble. The main objection to its use is its highly poisonous character, w^hich always involves risk to the operator, and which renders the atmosphere unwholesome, even in fairly- ventilated rooms. Many attempts have been made to substitute safer solutions, but in no case yet with sufficient success to pro- claim the introduction of a serious rival to the cyanide bath. One inventor has used a silver salt dissolved in thiosulphate solution ; but this bath, although it is said to give good results, gradually decomposes, especially on exposure to light, and, becoming brown at first, gradually deposits its silver in the form of a black sulphide. Zinin has more recently found that the solution of silver iodide in potassium iodide (I^o. 18 in the table of solutions) could give very good results with a small current of low electro-motive force. He recommends it especially for the production of thick deposits, as in electrotypy. One practical objection to its use is, of course, the high price of the iodides. This is not a fatal objection, if the process be a good one, but is certainly adverse to its general adoption. The metal obtained from the solutions named has a frosted appearance, due to its being built up of an incalculable number of minute crystals, the facets of which disperse the rays of light falling upon them, instead of reflecting them uniformly ; but the ELECTKO-SILYEKING BATHS. 203 slightest friction upon the surface suffices to unite the crystals into an even plane, which reflects the Hght perfectly, and has the lustre of polished silver. It was early found, however, that the presence of a minute quantity of carbon bisulphide in the plating-vat caused the pre- cipitation of the metal in the bright condition, and although the use of the hright jplating-solution entails greater difficulties than are met with in the ordinary processes, yet it is largely used for certain classes of work ; for example, in those to which it is not easy to apply friction, such as those carrying remote or sharp angles or interior surfaces. In no case, however, is the whole plating-process conducted in the brightening- vat, but the bright deposit is given finally, when an almost sufficient weight of silver has been deposited in the usual way. Of all the substances which have been recommended as brightening agents for the silver-bath (and these include, inter alia, silver sulphide, collodion, a solution of iodine and gutta- percha in chloroform, chloride of carbon, and chloride of sulphur), the only reagent practically employed is carbon bisulphide. A mere trace of this suffices to effect its object, while a slight excess produces spotted deposits and brown stains, probably of silver sulphide, and a large excess overshoots the mark altogether, and often gives a dead-white film. To prepare the bright-bath, place a quart of an old plating- solution in a large bottle (a Winchester quart bottle, for instance) and add to it 3 ounces of carbon bisulphide with, or without, I or 2 ounces of ether, shake vigorously for a minute, and add IJ pints more of the old solution. Again agitate thoroughly, and allow it to stand for two or three days ; there is, at most, of an ounce of carbon bisulphide in each ounce of this liquid. A separate plating-bath must be used for brightening; then every night, after the day's work is done, one ounce of the mixture just described is added to every 10 gallons of an ordinary plating-solution, specially devoted to this class of work and con- tained in the special vat. An ounce of old plating-liquid may be added to the mixture in the Winchester bottle in place of that removed. This quantity (1 ounce per 10 gallons) is the maximum amount of brightening-mixture permissible. If the required effect can be produced with less, so much the better ; but on no account should a larger proportion be used, as the risk of spoiling the whole bath would amount almost to a certainty. The bright-bath requires a stronger current than the ordinary solution, and it deposits a harder metal, the bright film beginning to show itself at the bottom and gradually extending upwards. 204 ELECTRO-DEPOSITION OF SILVER. Any disturbance of the solution during electrolysis may cause it to yield a dull deposit; a whole batch of objects should, there fore, be prepared for immersion simultaneously, and introduced consecutively with the utmost rapidity possible ; then, when a sufficient deposit has been given, w^hicU usually requires from ten to tw^enty minutes, the current is stopped, and the pieces are removed and are at once well washed in clean water. These liquids have a great tendency to deposit the black sul- phide of silver, to check which the cyanide solution is added, as described, to replace the portion added to the bath ; the brown stains above alluded to are doubtless traceable to the same cause. If the pieces are not thoroughly washed immediately upon removal from the brightening-bath, they will become rapidly tarnished. Gore has shown, too, that the deposited metal contains appreci- able quantities of sulphur, which may, in part, account for the variation in the physical characteristics of the metal. The Anodes. — The silver anodes must be of the purest silver obtainable; the ordinary standard silver for coinage contains 7*5 per cent, of copper (most foreign currency has even a larger pro- portion of base metal) ; it should not, therefore, be used as such, but if it be the only form readily available at any time, fine silver should be prepared by the method described on p. 392. The anode may be of cast or rolled metal, but if the latter be selected, it should be annealed by heating to a dull-red heat, and subse- quently cooling it before immersion in the vat, so that it may be softer and more readily soluble. As a general rule, the anode should present an area of surface equal to that of the cathode ; but, as will now be readily understood, a somewhat smaller anode surface must be employed when the bath contains excess of cyanide, and a greater area when the cyanide is deficient, the object being always to equalise the action at the electrodes, in so far as it is represented by solution or deposition of metal. The anode should never be suspended in the solution by means of copper-wires (unless they can be so arranged that the copper never comes into contact with the bath) because they dissolve under -the action of the current, and passing into the solution render it impure. Silver wire has not the same objection ; but as it dissolves rapidly, especially at the surface of the liquid, it gradually becomes weakened until it is no longer capable of supporting the weight of the anode. Platinum wire, being quite insoluble, is free from both these objections, and is the best material to use. If copper or even silver supports are adopted, they should be kept from contact with the liquid in the manner explained in Chapter V. (p. 111). CHARACTER OF THE METAL DEPOSITED. 205 The Vat. Any of the ordinary vats described in Chapter Y. may be employed, except those Hned with gutta-percha mixtures, which are more or less soluble in the cyanide liquid. Enamelled iron lined with thin wood is, perhaps, mostly to be preferred, necessarily so if the solution be heated. The disposition of con- ducting wires and the manner of imparting a reciprocating motion to a frame, from which the various objects are suspended, so that they may be kept in constant motion, has also been explained in Chapter Y. The vats should be considerably larger than the objects to be plated, and may indeed be made of any reasonable size, remembering that a large bulk of solution generally gives a better deposit than a small one, especially in the case of bright- plating baths. When several vats are to be worked from the same battery or dynamo, they should be coupled in parallel, because, as a rule, the work to be silvered is very irregular in shape and size, and under these conditions the series arrangement is less satisfactory. A well-fitting cover may be made for the vat, to preserve it from atmospheric dust when it is not in actual use. The Character of the Metal Deposited.— Like most other metals, silver, which is deposited by a current strong enough to evolve hydrogen simultaneously, is dark in colour, powdery, and non-adherent ; it is in the spongy condition, and is useless as a coating. A weak current, on the contrary, gives a strong, malle- able metal, adherent and coherent, and minutely crystalline. Some operators consider the commonly-employed current-strength of 0*032 ampere per square inch (0*5 ampere per square decimetre) too high, and prefer to reduce it to 0*013 ampere (0*2 ampere per square decimetre) ; but for all ordinary work the larger current-volume will be found satisfactory, and will, of course, deposit a given weight of metal in a shorter period of time. The metal should have a pure white colour, any departure from this indicating the presence of impurities. A pinkish shade prob- ably points to the existence of copper in the precipitate. A yellowish shade or tarnish, which is apt to appear upon surfaces that have been for some time exposed after removal from the vat, is probably due to a small percentage of a sub-cyanide of silver deposited with the metal, which gradually changes colour on exposure to light. It is found that a dip into potassium- cyanide solution, or even a stay of two or three minutes in the plating-bath after the current is cut off, suffices to prevent this, doubtless by dissolving the objectionable sub-salt. It has already been stated that the silver deposited by the carbon bisulphide brightening-solution contains a small proportion of sulphur, which 206 ELECTEO-DEPOSITION OF SILVER. is possibly accountable for the alteration of structure indicated by the different nature of the deposit. The thickness of a coating of silver may vary from an almost imperceptible film to a depth of of an inch on electro-plate, or of -~o ^^^^^ silver electrotypes. Owing to its open crystalline nature,' the deposited silver, if peeled from the surface on which it is precipitated, lacks the metallic ring emitted by the rolled metal when struck. The Process of Electro-Silvering. Brass, copper, bronze, German silver, and similar alloys are best adapted to the electro-silvering treatment ; the softer metals — lead, tin, Britannia metal and pewter — though sometimes plated, are less well suited because they are not structurally so capable of resisting the final mechanical treatment of polishing and burnishing ; iron and steel, zinc and other metals may also be silvered. But whenever the metal is attacked by the cyanide bath, so that silver is deposited by simple exchange and without the aid of the current, it should receive a thin coating of copper or be subjected to the process of quicking ; this coating with mercury is, however, often used, even when the metal has no action on the bath, to render adhesion doubly sure. The explana- tion of the process is given on p. 131. Organic matter must as far as possible be eliminated for reasons already given; hollow sheet-metal objects, therefore (brass candlesticks, for example), which are often filled up with pitch-composition, as a support to the thin metal of which they are made, must be gently heated to effect its thorough removal prior to electro-plating; this operation must be conducted with care, because cheap articles are frequently made in several pieces, which are held in place by the composition, and, therefore, become separated when it is removed. All non-metallic handles or appurtenances should be, if possible, detached from objects before plating, because there is not only a risk of their being damaged by the solution, but liquid is sure to penetrate into the sockets and interstices, from which it can afterwards be removed only at the expense of much trouble. The processes preliminary to the actual electro-deposition are — (a.) Stripping, or removal of an old coat of silver, if any exist. (b.) Polishing, if necessary. (c.) Cleansing, consisting of — 1, boihng in caustic potash to remove grease; 2, dipping in sulphuric acid to remove oxide; STRIPPING. 207 and 3 scouring with sand or a dip into a mixture containing nitric acid according to the nature of the metal. (See Chapter VI.) {d.) PreUminary coating with copper, if necessary. \e.) Quicking, if required. Stripping. — When old goods are to be re-plated, every trace of the original coating must be removed, in order that the new deposit shall be regular and uniformly adhesive. In the choice of a stripping solution, the operator must be guided by the character of the basis metal, from the surface of which the silver is to be dissolved, as it is essential that the Hquid should not be able to attack this to any serious extent, when it is laid bare by the removal of the precious metal. For brass, copper, or German silver, a mixture of concentrated sulphuric and nitric acids is generally employed. The most rapid method consists in heating a sufficiently large quantity of strong sulphuric acid in a stoneware vessel, and adding to it, immediately before use, a small quantity of potassium nitrate (saltpetre) or sodium nitrate (Chili saltpetre) ; this is at once decomposed, a small proportion of the sulphuric acid being neutralised and a corresponding quantity of nitric acid being liberated in the liquid. Such a mixture when used hot is capable of dissolving the silver from the articles, which should be suspended in it by copper hooks or preferably by copper tongs, b)ut should show no very great corrosive effect on the copper or I)asis-metal. Nevertheless it is not entirely without action : and the process must, therefore, be watched most carefully, especially towards the end, when most of the silver has been dissolved, so that on the disappearance of the last trace of covering metal the article may be removed and plunged into a large volume of water without loss of time. With this object in view the pieces under treatment should be frequently removed from the liquid for inspection. The extremities of long articles which have not been properly reversed during their first electro-silvering, and the more prominent portions of every object having a thicker coat than the remainder, are denuded last ; and it is often advisable so to place the goods towards the end of the stripping- process that only these portions are immersed in the liquid. It is essential that the concentration of the bath be well maintained; any dilution increases its tendency to attack the base-metal, a comparatively small addition of water sufficing to render the action even violent. For this reason the mixture, which absorbs water vapour from the air with great avidity, must be stored in tightly -closed vessels when not actually in use, and the objects to be stripped must be dry when placed in the vat ] indeed the 208 ELECTKO-DEPOSITION OF SILVER. introduction of water in any way into the hot sulphuric acid would cause a sudden generation of steam, almost explosive in its violence, so that it is alike dangerous to the operator and destructive to the bath. In course of time the accumulation of potassium bisulphate in the bath (from the decomposition of the saltpetre added each time before use) is rendered evident by the deposition of crystals. When this is observed it will generally be found that so much acid is neutralised that the liquid is no longer serviceable; a fresh quantity of sulphuric acid should then be prepared, the old bath being reserved to recover from it the silver" which it contains. On account of the great care necessary in conducting this process, only one object should be treated at a time, if at least it be of moderate size, for the operation proceeds with great rapidity. When this is inconvenient, by reason of the number of pieces to be treated, extra precautions must be taken to guard against too prolonged action in any individual case. With a cold solution the action is slower, and consequently under better control ; hence a mixture of 10 parts of concentrated sulphuric acid (specific gravity 1*84) and 1 part of strong nitric acid (specific gravity = 1*39) is often preferred, this being applied at the ordinary temperature of the room. Except that it is not heated, and that a greater number of pieces may be treated with safety, the method of use is the same as that just described ; and water and moisture must be equally rigorously excluded. A difi'erent sttipping-process must be employed for zinc, iron, lead, tin, Britannia metal or pewter, or for any alloys of these, which would be vigorously attacked by the acid mixture suitable for copper. This process consists in suspending the articles as the anodes in a strong solution (say 10 per cent.) of potassium cyanide, opposite a plate of platinum, copper, or brass, and connecting the former with the positive or copper pole of the battery, so that the process of electro-plating is reversed, and the current flows in the electrolyte from, instead of to, the pieces. Thus, the goods being the anode, the silver is dissolved from them and deposited upon the platinum cathode after a time, at a rate depending upon the volume of the current which is being applied. An old silver-bath may be utilised for this purpose, the metal deposited upon the cathode plate being, of course, recoverable. In any case the same solution may be used repeatedly ; and the current may be stronger than that permissible for plating, because the object is no longer to produce a good deposit, but to dissolve an old one with the utmost rapidity. But when a strong POLISHING, WASHING, AND COPPER COATING. 209 current is employed, the silver may be deposited in the pulveru- lent condition, so that particles frequently become detached and fall into the liquid, to prevent which the cathode plate may be enveloped in a case of parchment paper or even of fine muslin. Silver baths in current use must never be employed as stripping- solutions, because they would gradually dissolve small quantities of the base metals from which the silver had been removed, and would thus become too impure to yield a good deposit ; only disused baths are permissible. As soon as the pieces are com- pletely stripped, they are removed from the vat, plunged into water and well washed. The process is, of course, equally applicable to copper and those alloys which are often treated by the more rapid acid method. Polishing, Washing, and Copper Coating. — After the original silver case has been removed, it is often necessary to pass the goods to the polishers to buff and finish, prior to the cleansing and quicking operations, after which they are transferred to the plating-vat without loss of time. Iron or steel cannot be quicked, because this metal is one of the few which refuse to amalgamate or alloy with mercury ; Britannia metal also is not usually quicked ; but copper, brass, or nickel-silver are fitted in this way to receive an adherent deposit. Zinc should receive a preliminary wash of copper in the alkaline copper-bath, and it was at one time customary to submit the tin-lead alloys (pewter, Britannia metal, and the like) to the same treatment, but there is no great difficulty in directly silvering them with good results. Steel, which is, in some hands, still coppered before silvering, may also take a perfectly sound and adhesive deposit by dipping the cleaned articles at once into a striking-solution. Suspension of Objects in the Bath. — The suspension of objects in the silver-bath is effected by thin copper wires (commonly of about No. 20 of the standard wire-gauge). New wire should be used each time, because the deposit upon an old wire is apt to be loosened by re-bending, and to crumble off in the bath. The manner of attaching the wire depends upon the nature and shape of the goods. Some articles afford natural places of vantage from which they can be slung; such as cream ewers, cups, and the like, which carry metallic handles ; or perforated objects ; or those which, being unfinished, have rivet holes, through which the wire may be threaded. Spoons and forks are best supported in slings made by forming the wire into a loop around the shank, or by bending it into three-quarters of a circle at the end, and at right angles to the wire itself, leaving a hori- 0 210 ELECTRO-DEPOSITION OF SILVER. zontal space (the remaining quarter of the circle), through which the shank may be slipped, but which is not wide enough to allow the handle or bowl to pass (fig. 99). Plates, salvers, and the like should be hung by wires bent lightly around them, these having- their ends joined by twisting together. . Obviously the methods of attaching wires are innumerable, and must be determined by the circumstances of the case ; the guiding rule in making the connection is that the wire shall be so arranged that the object cannot escape from its hold ; yet, on the other hand, it should be so loosely fixed that the relative position of wire and object may be shifted at any moment without difficulty, so that fresh surfaces are brought in contact, and wire-marks are not formed on the deposited metal. The wiring is best done before the final potash and acid cleansing processes. Many firms place a piece of glass-tubing over that portion of the wire which is in contact with the solution between the cross cathode-rod of the vat and the suspended object, so that silver may not be uselessly deposited upon it; others use gutta- percha or india-rubber as an isolating medium, but as these are slowly attacked by the cyanide liquor, glass is preferable, and there is no difiiculty in adapting it. Having ascertained the length necessary to protect the portion of wire which is to be immersed, this distance is measured off on a piece of narrow glass-tubing; a fairly deep mark is then made at the desired point with a triangular file; now placing a hand on either side of Sling for nick, with the thumbs immediately beneath it on spoons, the other side of the tube, a steady bending pressure is so applied that the file mark is on the outside, the thumbs on the inside of the bend ; almost immediately the tube should break with a clean even fracture at the place of the file- mark. In experienced hands accidents are not likely to happen, but in early attempts at breaking tube in this way it is perhaps safer, though even then scarcely necessary, to envelop the hands with a thick cloth. Arrangement of Objects.— The objects must be introduced into the bath gently without disturbing the sediment; they should be arranged in rows alternating with, and midway between, the anode plates, so that each side will receive the same weight of deposit ; thus whatever the size of the bath, the number of anodes will always be one in excess of that of the rows of cathodes, and all will be in parallel circuit. If there be only one row of cathodes there will be two anodes ; if two rows, THE STRIKING BATH. 211 then three anodes, and so on. Several objects may, of course, be suspended side by side from the same cathode-rod, and will be influenced by the same pair of anode-plates — provided that free space is left between adjacent objects. The anode- and cathode-rods should be parallel to one another, in order that the spaces of conducting liquid between the different pairs of electrodes may be equal. It is preferable* also that, as far as practicable, only goods of the same shape, or, at least, of the same diameter^ should be suspended from the same rod. On account of the great ir- regularities in form of objects to be plated, a considerable distance must be left between the electrodes, so that the portions nearest to the anode may not be so near, as compared with those more remote, that they receive an undue share of the deposited metal. Allowance must also be made for the motion imparted to the objects in the bath, so that the opposing electrodes may not make contact at the end of each swing. Again, two different kinds of metal should not be suspended from the same rod (for example, copper and Britannia metal), as the local current set up between them, both being immersed in the same exciting liquid, and being in metallic connection through the suspending-rod, will tend to cause the gradual solution of the more electro-positive metal, and will diminish the deposition of silver upon it, until it is quite pro- tected by a perfect layer of the precious metal. The solution is thus injured by the introduction of foreign matter. If the electro- chemical difference between the two metals be so great as to cause an electro-motive force greater than that of the depositing current (which could rarely, if ever, happen were ordinary care bestowed on the process), no deposit would occur on the more positive metal, which would rapidly dissolve and cause a double thickness of coating to be given to the object made of the more negative metal. Otherwise the local back electro-motive force simply retards the action of the current in regard to the positive metal, until it is sufficiently covered to prevent further action ; and unless this retardation be very protracted, the difference between the weights of metal deposited on the plates and that which it was desired to precipitate will not be very appreciable, so that the chief injury is done to the bath. Use of the Striking-Bath. — An additional reason for guarding against this contingency is that lead and its alloys conduct elec- tricity less satisfactorily than brass or nickel-silver, and far less so than copper, and hence they need a somewhat greater length of time to acquire the thin wash of silver which suffices to protect them. Many operators, therefore, prefer as a preliminary step to dip the 212 ELECTRO-DEPOSITIOX OF SILVER. articles, immediately after quicking, into a silver bath worked bv a stronger current until, almost immediately, a thin film of this metal has been imparted, when they may be transferred to the ordi- nary Tat, in which the remainder of the deposit is to be built up. The first, or striking-hath, may contain less silver than the usual solutions (half-an-ounce to the gallon commonly suffices), but the proportion of free cyanide is often greater. Large silver anodes are used ; and, indeed, everything must be done which tends to reduce the resistance and increase the rapidity of deposit, in order that the action may be almost instantaneous, and that a momen- tary dip into the vat may be sufficient to give the required deposit. But, on the other hand, it need scarcely be remarked that the volume of current must not be so great that a pulverulent or spongy deposit results. The electrical connections of the striking- vat may be similar to those recommended for the plating-bath ; but a smaller bath with two large anodes, one at either end, and with a single cathode-rod, to which the negative battery-wire is attached, and which is lowered into the bath by hand, is really all that is required. How to Ensure Uniformity of Coating. — After the transference of the goods to the plating-vats, they may be left with less con- stant attention until a sufficient thickness of film has been ob- tained, provided that the current is constant and that the an-ange- ment for imparting motion to them during the action is working satisfactorily. All that is necessary is slightly to shift the position of each piece relatively to its supporting-wires from time to time, to ensure uniformity of deposit at these points, with an occasional momentarv removal from the bath for an examination as to the regularity of the action. Should spots appear upon the surface, the article must be removed from the bath, rinsed, scratch-bmshed, and then cleansed by a dip into hot potassium cyanide or caustic potash solution. Finally, after rinsing once more, they are re- quicked and introduced again into the bath. Since, in spite of the gentle motion imparted to the objects, the solution is certain to vary in density, and to produce a thicker deposit upon the lower portions of articles, long objects, such as spoons and forks, sus- pended upright in the bath, should be reversed at intervals of (say) half-an-hour, so that if the bowls were downwards at fii'^t, the handles would be so after the first shift. In immersing the quicked and struck articles into an empty vat, that is, into one which contains only anodes suspended within it, those first immersed would receive too strong a current, unless their superficial area were very considerable, and would be covered with a spongy silver precipitate, until, at last, the cathode^ UNIFORMITY OF COATING. 213 surface had been increased by the introduction of more objects, .sufficiently to produce the right proportion of current-strength per unit of area. Meanwhile, however, the original pieces would have suffered serious injury. To obviate this, either the current-volume may be reduced at first by the interposition of wire-resistances, which are gradually lessened as fresh objects are introduced, or one or more of the anode-plates (according to the size of the vat) are hung upon the cathode-rods at the outset, and are transferred to their proper places, one by one, as each batch of objects is im- mersed which presents a total area equal to that of one plate. In this way a large proportion of the current is at first occupied in transferring silver from one anode-plate to another ; and the bath from the very beginning is under the same conditions as it is when filled with goods undergoing the silvering process. Thus even a small object introduced alone should receive a normal current throughout. Some electro-platers, in order to secure a more perfect coat, are in the habit of removing the articles after a certain amount of silver has been deposited and submitting them to a preliminary scratch-brushing, after which they are well rinsed and cleansed and again returned to the bath ; but it is very doubtful whether any real advantage accrues from this practice. When a sufficient thickness of metal has been deposited, which may be known, as explained in Chapter V., by ascertaining the mean strength of current, the total cathode-area, and the time occupied, or by the use of the plating-balance, the pieces are re- moved from the vat, transferred (if necessary) to the brightening- bath, where they are left undisturbed for a few minutes, and are then plunged into a slightly warm solution of potassium cyanide to remove any silver subcyanide left in the pores of the metal, and thoroughly washed in several w^aters held in successive tubs {vide instructions for washing coppered goods on p. 152) ; they are next dipped momentarily into a vat containing water mixed with 2 or 3 per cent, of sulphuric acid, and are again rinsed in water, and taken to the scratch-brush for preliminary polishing, and to the burnishers, for the final treatment. The potassium cyanide dip may be dispensed with, if the objects are left in the plating-solution for a few minutes after disconnecting the current. How to Thicken the Coat Locally. — An extra thick coating of silver may sometimes be imparted to those portions of goods which will have to stand the chief amount of wear in use. This may be effected in many ways according to the appliances and ingenuity of the plater. The application of stopping-out varnish after a certain time to the parts which are to receive a thinner 214 ELECTRO-DEPOSITION OF SIL^T:R. coat is rarely admissible, because, although it would have the desired effect, it gives too defined an outline of the thicker deposit, and this has to be obliterated mechanically, an imperceptible gradation being generally required. This method would be suitable if the whole of one side of the object had to be thickened, but an equally good result would be attainable by the use of only one anode (adjacent to this side). Another plan is to introduce, towards the end of the operation, a subsidiary anode, correspond- ing in shape to the part which is to receive the greater deposit, and which is placed in greater proximity to it, in proportion to the increase of substance to be acquired. Yet another system may be adopted in plating the bowls of spoons and similar objects, which are worn most largely in use at the most prominent parts of the curve. After plating in the usual way the spoons are so placed in a shallow bath that only the convexity of the bowl is immersed, and receives a coating ; the depth of immersion must be frequently altered in a slight degree to ensure that no distinct boundary marks are produced on the surface. It is true that these marks may be mechanically removed, but there is no reason why they should occur at all. Thickness of Deposit. — The thickness of the deposit and, con- sequently, the duration of the process is, of course, governed solely by the class of work under treatment. Many common goods, especially those made of white metal, receive a film so thin as to be beyond the range of practical measurement. This is naturally useless to resist even the slightest wear ; it gives simply an orna- mental covering for a short time. As a general rule for ordinary electro-plating, a deposit of from 1 to 2 ounces per square foot of surface-area may be deemed a good well-wearing coating ; a single page of this book represents approximately the thickness of a film equal to 1 ounce per square foot. This will occupy from three to nine hours in coating, according to the strength of the current. A thinner deposit than that of 1 ounce per square foot is not to be recommended, as even two or three years of ordinary wear would suffice to lay bare the base metal at the edges and in all the more prominent parts. In working, however, for the trade, the crafts- man is rarely allowed to decide what, in his judgment, is best fitted for the work, but must do as he is ordered by his customer, and will be paid at the rate of so much per unit weight of silver deposited. SILVER ELECTROTYPING. ' -215 Silver Electrotyping. Silver is occasionally used in special cases for copying works of art or even valuable engraved steel-plates. Ordinary wax and gutta-percha moulds, such as are used for copper electro- typing, are not admissible for silvering, because they are to some extent attacked by the cyanide solutions. The simplest method of obtaining replicas of works of art in silver is to obtain first a thin electrotype shell of copper from the intaglio-mould, and then to deposit silver upon this in the cyanide-bath. The copper protecting-film may be of the thinnest, so that it shall not destroy the sharpness of the lines, but it must, of course, be subsequently removed, after the required thickness of silver has been deposited, and the whole electro separated from the mould. This solution of the copper may be effected by treatment with warm hydro- chloric acid, or (better) with a warm solution of iron perchloride, either of which will attack the copper but leave the silver un- touched. On the removal of the copper, the pure silver surface has the required form in practically undiminished sharpness and brilliancy. The silver may be built up to a thickness of one- eighth of an inch or more. It is rarely, however, that this process is required ; and practically the sole application of electro- silvering is to be found in the coating of other metals to endow them with properties which they do not of themselves possess. Ornamenting Silver Surfaces. There are many ways of altering the appearance of electro- silvered goods ; but to give a description of these, many of which are purely mechanical, is beyond the scope of this work. Let it suffice then to say that — A dead lustre may be obtained by depositing upon the silver a thin film of copper, which has a slightly roughened surface of excessively fine grain, and then again upon this a thin layer of silver. Oxidised silver, which is an entirely misleading term, inasmuch as oxygen plays no part in its formation, is made by dippirig the object into, or painting it with, either a solution of platinum, which covers the whole surface with a thin layer of that metal by 'simple immersion,' or one containing sulphides which imparts to the silver a superficial film of black silver sulphide. This latter solution is made up by dissolving three-quarters of 216 ELECTRO-DEPOSITION OF SILVER. an ounce of potassium polysulphide ( ' liver of sulphur or of ammonium sulphide, in each gallon of water ; it is applied to the silver at a temperature of 150° F. The potassium compound is to be preferred; some operators add to it about twice its weight of ammonium carbonate. A few seconds' immersion in either of these liquids usually suffices ; the articles are then rinsed in water and dried. Antique silver is produced by rubbing into, and leaving upon, the parts of an object which are not in relief a thin layer of black-lead, finely crushed and stirred into spirit of turpentine ; some prefer to add a little ochre to the mixture in order to pro- duce a warmer ground-tone of colour. Niello-icork is prepared by tracing a pattern upon bright silver with silver sulphide or with mixtures of lead, copper, and silver sulphides, prepared artificially; when placed in position the object is heated to their fusing point in order to ensure adhesion. It is, in fact, a process of enamelling. Clearly, however, it is quite inapplicable to many classes of electro-plate, while to any it must be applied with the utmost care, in order to avoid the stripping or buckling of the coated object or the fusing of the basis metal. Satin finish may be produced, according to Wahl, by the application of fine sand propelled forcibly upon the surface of an object with the aid of an air-blast — a process analogous to that largely used at present for decorating glass. Any process which will destroy the polisti upon the silver with equal fineness and regularity would, of course, answer the same purpose ; but the sand-blast is probably the simplest and most economical extant. Obviously the enumeration of the methods of decorating silver surfaces is by no means exhausted in the few words given above ; they are innumerable, and capable of infinite variation according to the taste and skill of the artificer. CHAPTER X. ; THE ELECTRO-DEPOSITION OF GOLD. Advantages of Gold-Plating. — Owing to its high power of resisting atmospheric influences, combined with the richness of its colour, and the brilHancy of the pohsh which it is capable of receiving, and to the fact that all these properties are manifested even by the thinnest imaginable film of the metal, gold is very frequently deposited ; none the less so, perhaps, because, being a costly material, gilt objects of low value may pass for articles of much higher worth. ; ^ Gold is a very electro-negative element, so that any of the common metals is capable of replacing it in any of its compounds. It may, therefore, be readily deposited by simple immersion, although the electrolytic process is more satisfactory. Deposition by Simple Immersion. Solutions. — Many baths have been used at various times, the principal of which are included in the following table. Roseleur's Process. — Of all these, Roseleur's solution (No. 4) is the best for treating small articles — of jewellery, for example — made of copper, bronze, or brass. The gold chloride crystals should be dissolved in a small proportion of the water, and added to the solution of sodium pyrophosphate in the remainder, and the mixture warmed until the yellow colour of the liquid has disappeared. The solution thus made up, however, : is too readily decomposable, as, indeed, is indicated by the gradual change of colour to a dark red purple which it undergoes on standing ; hence the hydrocyanic (prussic) acid is added as a check upon the rapidity of the spontaneous reduction. It is omitted by some gilders, but the bath is under better control when it is used ; when working too slowly, more gold chloride is added, or when it becomes deep purple in colour, fresh hydrocyanic acid is introduced. ' 218 ELECTRO-DEPOSITION OF GOLD. •-7 m KH O §1 1^ o o O o CO > X < 00 CO CO c: o o s II CO » o o . t3 o o to Potassitun Cyanide. Water. 00 00 00 00 00 00 00 00 00 Ammonium Sulphide. s« . . . . . Sodium Pyrophosphate. 0 00 Potassium Bicarbonate. 0 0 . 0 ; 0 0 ; 0 JO 0 la Caustic Potash. 1-1 PARTS BY Hydrocyanic Acid. go 0 Gold Sulphide. « I : I I I Gold, as Gold Chloride. =^ mP II 2 -1 i Zinc goods Copper, Ik'onze, and Brass Large Bronzes before ]!]lectro- gilding Authority. Braun . . . Elkington . . Gore .... Roseleur . . | Wahl . . . ;5 rH S<1 CO 00 IMMERSION GOLD SOLUTIONS. 219 In using this bath, the object must present a clean bright surface such as may be imparted to it by pickling, scratch- brushing, and cleaning; it is then quicked by a momentary plunge into a dilute solution of mercuric nitrate, rinsed in water, and immediately transferred to the gold-bath, which should be nearly boiling. It is more economical and satisfactory to use three gold dips in succession, each solution being richer in gold than that previously applied ; this is readily effected by using old baths for the first two operations, and by arranging a system in which, as soon as the final vat ceases to yield a good deposit, it is made the second instead of the third bath ; that which had been the second being now the first, and the old first, now practically exhausted, being discarded. Thus the pieces are most thoroughly washed before they enter the last bath ; and no gold is lost, as the small quantity left in the third solution, when it is no longer serviceable, is used up during the time that it is acting as the second and the first. An immersion of a few seconds in each liquid should suffice ; and the resulting deposit, which is, of course, very thin, should have a good yellow colour and require only slightly scratch-brushing or burnishing to impart a final polish, rinsing, and lastly, drying in hot white wood sawdust ; for this resinous woods, oak, or walnut, which tend to discolour the work, are to be avoided. It may sometimes be necessary to improve the appearance of the gold by colouring methods, which will be described at the end of this chapter. If it be desired to obtain a thicker coating by this method, it is only necessary to repeat the process several times, re-quicking each time before passing the articles through the gold-baths. The deposit is thus gradually built up, because at each quicking- stage a small proportion of mercury deposits upon the surface, and then exchanges for an equivalent of gold, when placed in the gilding solution, the latter gradually accumulating mercury in place of the more precious metal. A really thick coating, however, cannot well be built up by this tedious process, and the electrolytic process is more convenient and more expeditious. Elkington's Process. — Elkington's process of luater-gilding (No. 2) employed potassium bicarbonate in place of sodium phosphate ; but its use is more troublesome, and it permits only a semi-exhaustion of the bath, leaving the remainder of the gold to be recovered from the residual liquids by chemical means. Roseleur's Process for Large Objects. — Roseleur's bath (No. 5) rapidly precipitates gold upon articles which have not been previously quicked ; the deposit is not of high quality, but the process is well adapted and largely used for coating large objects 220 ELECTRO-DEPOSITION OF GOLD. with a wash of gold, prior to submitting them to the electrolytic process. Other Solutions. — Of other solutions for simple immersion gilding, perhaps the most interesting are : — that of gold chloride in ether, which is appUed to gilding iron .and steel goods ; and that of Braun's, a solution of gold sulphide in ammonium sulphide, which is adapted to the direct gilding of zinc, because the latter metal would dissolve but slowly in such a liquid, and the coating is, therefore, the more Hkely to be adherent. This sulphide solution is quickly oxidised, and should be preserved from unnecessary exposure to the air. Dbpositiox by the Single-Cell Process. The Elkington bicarbonate process, above alluded to, when used to deposit upon silver or German silver, demanded that a piece of zinc or copper should be attached to the objects, and thus became practically a single-cell process. Steele also, in the specification of a patent granted to him, claimed the use of a cyanide bath in which the object to be coated was immersed in contact with a piece of zinc ; but, inasmuch as a considerable proportion of the gold w^as found to deposit upon the zinc itself, owing to the wide difference between the electro-chemical relations of the two metals, zinc and gold, the method is not practically used. Deposition by the Separate-Current Process. The Battery. — Almost any of the ordinary battery-cells may be used. A current of fairly high potential is required, but no great volume is essential. The Bunsen-cell is well adapted for the work, but the resistance in the circuit should be sufficient to reduce the current-intensity to 0'006 ampere per square inch (0*1 ampere per square decimetre). The Solution. — As with silver, the double cyanide solution will generally be found to give the best results, provided that due care is paid to all the details of the process. The number of other solutions prepared with the object of supplanting the poisonous cyanide compounds, and of the modifications of the cyanide-bath itself are, as usual, innumerable. The chief of them are included in the following table. For ordinary use, a bath containing three-quarters of a troy ounce of gold, dissolved and converted into cyanide, together SEPAEATE-CUERENT PROCESS. 221 05 00 CO CO U5 cc B IT CO :^ 1" o ^ Pi ,H CO "3 s rrt CO O P as l-H O »2 >S -r; 2 ^ ^ o CQ . 'o i| .S 2 So U5 SI-:! r-l o !>• --I '^^'-'■^ i-T'ft o SSI S Water. Ammonium Sulphide. Ammonium Chloride. Sodium Phosphate. Sodium Hyposulphite. Sodium Bisulphite. Potassium Carbonate. Potash. Potassium Ferrocyanide. o o o ooooo 2 2 S o o o ooooo o o o ooo ooooo o o o o o o o o o oo o ooooo o oo oo o ooooo o oo oo o ooooo o oo Potassium Sulphocyanide. Potassium Cyanide. ^ Ammo- niuret. Oxide. .S Cyanide. o o o o O 00 o to r-l CN Chloride. CO 5 O C<1 . rH 1?^ • G 03 fl. ft- CO Most Suitable Tempera- ture (F). ^ CO CO o O .^r=i . O . ^-^ rH . . i> .O . rH O .° tgo ^ °o b ^ Special Application of Bath. Siiver Copper, Brass, Silver Silver, ) Copper, V Ger. Silv.) Iron & Steel Watch Movements Authority. Becquerel . de Briant . . Fizeau . . . Gore . . . »> ... Kick . . . { Levol ... Lerebour . . M.J.L.(Gore)-| Pfanhauser . Roseleur . . de Ruolz . . Wagner . . Watt . . . Weiss . . . Wood . . . No. iHOlCO TjikDCOt-OO Oi O rH (NCO iOCOt^00050rHG • • ... 15 25 ... 3 26 15 Eoseleur, ... 40 ... 16 Volkmer, ... ... 111 ... 17 Watt, . . Tin, Britannia Metal, &c. ... ... 33-3 ... ... ... 18 Iron 40 19 Weiss, . . 50 20 ) > • • 50 21 3 3 • • Iron and Steel 50 22 5 3 • • Zinc 42 17 23 55 • • ... 42 24 J) ♦ • Hard deposit ... 1 40 10 25 Weston, . . 1 ... 1 1 50-67 ... the ordinary temperatur The heavy figures in the last column refer to the numbers of the vertical columns representing NICKELING SOLUTIONS. 239 Separate- Current Process, as recommended by various Authorities. 14 15 16 17 18 19 20 21 22 23 24 25 OF ingredients. Ammonia. Ammonium Carbonate. Ammonium Cliloride. Ammonium Sulphate. Ammonium Tartrate. Calcium Acetate. Acetic Acid. Benzoic Acid. Boric Acid. Citric Acid. Tannic Acid. Water. 1000 J.UUU 1 000 1000 0*25 1000 1000 ... 19-22 ... ... 4-5 1000 ... 25-30 • 1000 190 ... ... 1000 ... ... ... ... ... 1000 1 000 9 Pi (l.s. 1000 ft litmus-paper, and rendered faintly acid, if necessary, with sul- phuric acid. It is advisable to agitate the bath at intervals in order to maintain an equality of density throughout ; but this is not so necessary as in the case of copper deposition, because the duration of the nickeling process is so much less than that of the other, rarely requiring an exposure for more than two or three hours. Some operators have endeavoured, with considerable success, to prevent the formation of the basic salt of nickel by the addition of a small proportion of an organic acid (citric or tannic), or of a Q 242 DEPOSITION OF NICKEL AND COBALT. feeble inorganic acid such as boric acid. This last-named solution, as formulated by Weston (No. 25), certainly gives admirable results, and although the simple solution previously recommended may be made to yield a most excellent deposit without difficulty by careful attention to the working of, the vat, Weston's bath allows more latitude in working, and may perhaps be preferred by many. It is made by dissolving 8 or 10 ounces of a double sulphate of nickel and ammonia per gallon of water, and adding 2J to 5 ounces of boric acid. Various other nickel salts have been substituted for the double sulphate, among them the chloride and acetate ; but in nearly every case a double rather than a single salt is preferred. When the single salt is used in making up the bath, a quantity of the corresponding compound of ammonia is added at the same time. In one bath (No. 24) a small proportion of cobalt is added, which causes a joint precipitate of the two metals that is said to possess greater hardness than nickel alone. The Anodes. — The nickel anodes must be as pure as it is possible to obtain them. They are to be had either cast or rolled, of almost any shape. The cast plates are less dense and, as a rule, are more readily soluble than the others ; either kind may be used, but the latter may be procured thinner than the former, and the prime cost of the nickeling plant is thus reduced, moreover, they are more uniform and reliable in composition, and are less liable to become spongy during treatment owing to unequal solution. They should present a total area in excess of that of the cathodes, in order that the bath may be kept saturated with nickel, which otherwise would not be possible, owing to the inferior solubihty of the metal. A bath that requires a difference of potential of 2 volts between cast anodes w^ill probably work well and give even better results with a difference of 3*5 or 4 volts between rolled anodes. Cast anodes being the more readily soluble are more likely to neutralise the acid set free by electrolysis at the anode, and hence the natural tendency of the (ammoniacal) nickel bath to become alkaline asserts itself. Rolled anodes are more Ukely by insuffi- ciently neutrahsing the acid to cause the bath to become acid. Carbon anodes are not to be recommended, partly because, sooner or later, the carbon is sure to disintegrate into the bath, and partly because, as it does not dissolve in the solution, the acid radical deposited at the anode is not neutralised, and there- fore causes the bath rapidly to become acid. The anodes should be supported by nickel hooks, and may with advantage be made with lugs at the upper corners, as ELECTRO-NICKELING. 243 indicated in fig. 61, so that the hooks do not enter the solution ; even, however, with these anodes, the use of brass or copper supports is to be avoided, for, becoming splashed with the solu- tion, they would in time become corroded, and the copper passing into the vat would seriously damage the nickel-bath. The Vats. — Any glass, earthenware, enamelled iron, or lined wood tank may be used. If the solution is to be heated, the enamelled iron is, of course, preferable. The vat should always exceed in size that of the work to be treated by 15 or 20 per cent. The Process of Electro-Nickeling. — Stripping, — It is even more important in nickeling than in silvering or gilding that an existing film of nickel shall be entirely removed, or the new deposit will most certainly lack adhesive properties. Small articles may be treated by persistently rubbing with fine emery-cloth until the desired end is accomplished. More often a chemical method is employed, which consists in dipping the articles for a brief period into an acid bath that will readily attack nickel. Such a bath may be prepared as recommended by Watt, by gradually and carefully adding one volume of nitric acid and two of sulphuric acid to one volume of water, constantly stirring meanwhile with a porcelain or wooden rod to prevent the action from becoming too violent. The liquid is allowed to cool, and should be transferred for use to a glazed earthenware pan or dish sufficiently large to hold any object which it will be required to treat in it. It is employed cold or only very slightly warm, and should be placed outside the operating-room, in a well- ventilated place, for example, in the battery-cupboard, because unwholesome and irritating acid fumes are evolved during the process. The author has frequently used this bath with success. The plated article is suspended from a copper wire and plunged beneath the liquid in the bath, where, however, it should not be permitted to remain for more than a few seconds at a time, for a thin coating is almost instantaneously dissolved, and the acid is then free to attack the basis metal beneath. It is, therefore, frequently removed from the vat and closely examined, so that the action may be stayed at the moment when the last trace of nickel has disappeared ; it is then transferred to a large volume of cold water, and after washing twice or thrice in fresh water, is ready for the subsequent stages of the process. Some operators prefer to strip by electrolysis, by making the object the anode in an old nickel-bath. Attention is equally necessary in conducting this process to guard against any attack upon the basis metal ; but since it is impossible entirely to pre- 244 DEPOSITION OF NICKEL AND COBALT. vent all action, no bath which is to be afterwards employed for depositing the metal should be used for this purpose, as it will become gradually charged with impurities. A 10 per cent, solu- tion of sulphuric acid in water may be equally readily adapted to the electrolytic stripping. - r ^ n ^ ^ Iron or steel articles are best treated by either of the first two processes, brass or copper by either of the two last-named, yet with due care any of the three methods may be applied to all classes of work. Preliminary Preparation for the Bath.— It has already been pointed out that, by reason of its extreme hardness, the nickel deposit cannot be burnished. Ordinary methods for imparting the final polish to electro-plated goods are not, therefore, appli- cable to nickel-coated wares. It is thus essential that the highest possible polish* should be given to the objects prior to immersion in the plating-vat, remembering that as they are when placed m the bath, so they will be when finished. Even traces of pre- existing scratches, or tool-marks, cannot be obliterated, except with the greatest difficulty, when once nickel has been precipi- tated upon the surface. If, therefore, the goods passing into the hands of the plater are iu any degree rough or unfinished, they must be most carefully polished until every scratch has ceased to show ; extra care should be taken with large blank areas of surface, unbroken by the lines of a design or by a change of shape in the article itself, because even the slightest flaw becomes more visible on such surfaces than upon smaller articles. More than usual care must also be bestowed upon the cleans- ing operations. In silver- and gold-plating, especially with warm solutions, the cyanide liquor compensates for any slight inade- quacy of cleansing, by its power of dissolving the offending grease, so that the surface is finally cleansed by the electrolytic bath itself (which, however, is gradually spoiled by the absorp- tion of organic matter), but the nickel-bath has no such solvent action ; so that it cannot be too strongly impressed upon beginners that the success of their work is dependent upon the absolute chemical cleanhness of the pieces to be plated. After polishing every trace of grease is first removed in the potash-vat and ot tarnish in the acid dip (for iron goods) or the cyamde-bath (for brass, copper, or zinc). Then after a thorough rinsing in water, the goods are transferred without loss of time to the platmg-vat. After passing the potash-bath the surface of the article should be handled with brass tongs or with clean rags, and must on no account be touched with the hands. * See, however, p. 138. ELECTRO-NICKELING. 245 Copper and brass articles always, wrought-iron and steel com- monly, are at once nickeled without further preliminary treatment ; zinc is also occasionally treated in the same way, but inasmuch as it is readily attacked by the nickel solution, and the latter is rendered worthless when contaminated with zinc, it is advisable to protect the objects with a covering of copper before immersion. Meidinger has suggested a covering of the zinc sheet with mer- cury, which would answer the same purpose, but care is neces- sary to guard against over-amalgamation, which only renders the plate very brittle without affording any corresponding advantage. Cast-iron also should be covered with copper ; it is a common practice first to bestow upon the surface a wash of tin, then upon this one of copper, and, finally, the layer of nickel. The articles having been made perfectly bright and clean, and, if necessary, covered with copper, are ready for suspension in the bath. Nickel-Depositing. — It has been pointed out that the current should be somewhat stronger at first than subsequently ; but it must not be so intense that the metal becomes burnt, as explained on p. 237, a fault which is by far the more serious of the two. Therefore, in introducing the cleaned objects, although the cur- rent must pass immediately they enter the bath, the objects first suspended must be protected from receiving an excessive current by the interposition of resistances, or by hanging one or more anodes from the cathode rods, as explained in speaking of electro- silvering on p. 213. The surface of the article should almost mmediately be completely covered with a grey deposit of nickel. The goods are suspended by copper wires, which should be used ■only once, because, from the want of adhesion of nickel to old nickel surfaces, the metal deposited upon old wires is liable to strip off in flakes, which, falling into the solution, may, in course of time, form a metallic bridge or connection between the elec- trodes, and thus produce a short circuit, or they may adhere to projecting portions of the cathode surface and interfere with the regularity of the coating. An alkaline bath is apt to give a darker tinge to the deposited nickel than is the acid bath ; the same effect, however, is produced if the current density is not suitable. The nickel solutions are usually inferior conductors of elec- tricity, and there is in consequence a more marked difference than usual in the rate of deposition upon portions of a given object placed at different distances from the anode ; and there is even less tendency for a current to pass through great lengths of solution when the basis metal is also a poor conductor of electricity; coating bad conductors with copper is, therefore, to 246 DEPOSITION OF NICKEL AND COBALT. be recommended as a distinct assistance in starting a deposit of nickel. Objects which are to be coated on all sides with nickel should therefore be quite surroimded with anodes, and should be placed as nearly as possible equidistant from them ; and if they have an irregular form, they should be systematically inspected to ensure that all the deeper hollows are covered at once. While, then, on the one hand, the pieces must be very carefully examined after they have been struck (i.e., first completely covered with nickel), they must not, on the other hand, be kept too long out of the solution, so that they tend to become dry, because in that time they will have acquired an imperceptible film of oxide, which will efi'ectually prevent the adhesion of the nickel afterwards deposited. The thickness of the nickel need not, as a rule, be very great on account of its extreme hardness. Generally speaking, from half-an-hour to four hours will sufiice for the deposition. Thick deposits are very liable to peel off, occasionally spontaneously in the bath, but more often during the period of administering the final polish ; this is especially the case with iron and steel goods, which take a thick deposit less satisfactorily than those made of brass or copper. This peeling of the metal, whenever it happens, is annoying, because it necessitates stripping the remainder of the deposit with a recommencement of the process de novo ; but if it occur in the bath, the separation of loose fra.c^ments may give trouble in a manner already described. When the thickness of coating is sufficient, the pieces are removed from the bath and thoroughly washed in cold water, then plunged into boiling water, so that evaporation may take place more rapidly, and dried completely — small objects in hot sawdust, large articles in a stove heated to, or very slightly above, the boiling point of water. They must then receive their final polish, and are ready for the market. The nickeling of larger or irregular surfaces is conducted after the same manner as that of smaller objects : the conditions to be observed most particularly are — that the goods shall be thoroughly polished and absolutely clean ; that they shall be as far as possible surrounded by anodes, and equidistant from them ; that the whole surface is in fair conductive connection with the negative pole of the battery ; and that the solution is in good order, being neither alkaline nor more than feebly acid. To secure good connection it is often desirable to employ more than one wire, especially when considerable lengths, such as chains or rods of a feeble conductor, are under treatment ; these should be supported from the cathode-rods at intervals by copper ELECTRO-COBALTING SOLUTIONS. 247 hooks, so that several starting-points are offered, instead of one, for the formation and spread of the deposit: there is thus a greater uniformity of coat. The hooks must be shifted from time to time, to avoid the formation of surface markings. Small objects should not be coated in the perforated porcelain pans recommendable in plating with other metals, because of the difficulty in arranging the anodes. It is, indeed, possible to effect the nickeling in this manner, and the method is sometimes practically adopted ; but it is safer either to attach each article individually to a copper wire, or to rest several together on a very shallow and narrow metal tray which may be suspended between the two anodes. The Smith Deakin process (p. 119) may with advantage be employed. Small articles are especially liable to receive a burnt deposit when first placed in an empty vat, and for this reason either a large number of articles should be intro- duced into the solution simultaneously, or one of the anodes should be made a cathode for the time being, as previously explained, or small pieces may be immersed while the current is already coating objects with larger surface. Finishing.— When the goods are to be left as they come from, the bath without further polishing, and, therefore, with a slightly deadened surface, they must not be touched with the hands upon any exposed surface, as the coating in this condition is peculiarly susceptible to grease-markings, and the stains will inevitably show after drying. The pieces should be lifted from the vat by the suspending wires, plunged first into two or three cold wash-waters, and then into hot clean water. If the pieces are at all thick or substantial, the heat energy stored up in them, by a short immersion in the boiling water^ will suffice on removal to evaporate the small proportion of hquid clinging to them ; but if the surface be large as compared with the mass, it may be necessary to finish the drying in a stove. To this end the objects are placed on a tray, the suspending wires unhooked, and the tray transferred to the oven, so that from first to last they are not touched with the lingers. So treated the dead surface presents an extremely attractive appearance. Distilled water should be used for the final washing-bath (heated) if possible, because it leaves no residue. Britannia metal and zinc should be coppered before nickeling, and some prefer thus to treat even German silver ; this done, the objects are coated with nickel in the manner described. Applications.— Among the many useful appHcations of electro- nickeling, that of coating the comparatively soft copper printing surfaces demands especial notice. A thin film of nickel, so thin 248 DEPOSITION or NICKEL AND COBALT. CO * CO ~ 2 lO * CO 1 and 00 1—* a CO •'— 3 O ed P- rH CO 0 gcd s • I— I P C rz O ^ 31^ Water. 1000 O O O 1000 1000 1000 iMagnesiiuii Sulphate. • O (M 1— 1 o Ammonium Chloride. o o Ammonia. CO o o o t— i Potassium . Cyanide. Cobalt Sulphate. < Cobalt >(itrate. Cobalt Chloride. r-H 1 CO O o 27-41 a; C3 o :0 ^ CO r£3 n3 O o GQ O S CO ^ c o ^ o — a: CO ELECTRO-COBALTING SOLUTIONS. 249 that the size of the printing surface is not affected, will increase the hardness, and consequently the life of the plate enormously ; indeed it would appear that a nickel-coated copper plate will give about four times as many impressions as one coated even in the usual way with iron. A special advantage also attaches to its use — it enables copper type to be used with a red pigment (vermilion) which cannot be done without such protection, because the copper alone decomposes the mercury sulphide, which is the basis of the pigment, and thus destroys its colour, and at the same time tends to become brittle by the absorption of the reduced mercury. Electro-Deposition of Cobalt. The chemical properties of nickel and cobalt are so nearly allied that this chapter would appear to be the most appropriate place for introducing the subject of electro-plating with the latter metal. The deposit of cobalt is similar to that of nickel ; it is equally brilliant, but is somewhat harder, and has been found by Professor Silvanus Thompson to possess a higher resisting power for organic acids, which renders it more suitable for the internal coating of copper or other cooking utensils. It is only lately, however, that cobalt anodes have been procurable, and have thus enabled the process to enter practically upon the field of electro- metallurgical competition. Even now, its comparatively high price tends to check its adaptation to many purposes to which its application may be desirable. The battery should consist of two Bunsen-cells, and it is well to place an ammeter in the circuit and to have resistance-coils at hand, because the deposit yielded by a powerful current is defective in adhesion ; a fairly high electro-motive force but a low intensity of current is required. Many solutions have now been used, of which some are indicated in the preceding table. The double sulphate of cobalt and potash, corresponding to the nickel-potassium sulphate, above recommended, may also be used in 10 per cent, solution. Professor Thompson, however, has obtained perfect results from the solution No. 5, made by mixing 20 volumes of a nearly saturated solution of magnesium sulphate with 1 volume of a similar solution of cobalt sulphate or chloride. This bath should be used hot, and will then yield a film which, if deposited with all due precautions on iron, brass, or German silver {inter alia), is extremely adherent and good. Instead of making up this solution in the manner described, it may be prepared by electrolysis, if the current be passed from a 250 DEPOSITION OF NICKEL AND COBALT. large cobalt anode into a magnesium sulphate solution with a temporary cobalt-cathode, which should be removed as soon as- sufficient metal has dissolved in the electrolyte to yield a good deposit. Anodes of pure rolled cobalt are now obtainable, and should be used like those of nickel, of large superficial area as compared with that of the cathodes : they must not be supported by copper wire. The vats and the general treatment, as well as the character of metal deposited, are, indeed, regulated exactly ' as in the case of nickel. And because the film is harder even . than that of the sister metal, it follows that the same precautions- must be taken in regard to previous polishing as well as cleans- ing. It is even more difficult to impart a smooth polish to a rough cobalt surface than it is to one of nickel. So also the same care is necessary in stripping old deposits previous to plating; corresponding methods may be employed. CHAPTER XII. THE ELECTKO-DEPOSITION OF IRON. The electro-deposition of iron (or of steel, as it is sometimes wrongly termed) upon the surface of engraved copper plates has long been practised, in order that its superior hardness may enable the printer to obtain a greater number of sharp impres- sions from the same plate ; and the extreme ease with which the worn film may be removed from the copper renders it possible to renew the protective coating again and again. All that is necessary is to suspend the printing operations as soon as the first tinge of the red foundation metal appears through any portion of the iron cover, and then removing the iron by means of dilute acid, to re-immerse it. in the iron-plating vat. In this manner the copper need scarcely be appreciably worn, and may be made to yield many thousand impressions without losing any of the sharpness or accuracy in definition of the lines. This, however, is practically the only use for electro-deposited iron; as a metal it is too readily oxidisable to render its use as an external protective coating serviceable in any other way. Its hardness qualifies it for printers' work, and enables copper to compete with steel as a material for steel-plate engraving ; but even in this field it is outrun by nickel, by which it will probably be superseded in course of time. Nevertheless, for plates which may require alteration from time to time (maps, for example) the iron is preferable, on account of the greater difficulty experienced in removing the nickel coat ; yet, if the alterations are likely to occur frequently, it may be better not even to coat the plate with iron, although the stripping is in this case so comparatively simple. Iron is never deposited by immersion, but always by the separate-current process, for which purpose the Bunsen-cell is best adapted. The Solution. — Two kinds of iron salts are commonly known—-; the ferric or j9er-salts, and the ferrous or j^ro^o-salts, which are combinations of, the metal (Fe) with a greater (FcgOg) or smaller 252 ELECTRO-DEPOSITION OF IRON. (FeO) proportion of oxygen respectively ; and, as the heat of formation of any ferric compound is considerably higher than that of the corresponding ferrous body, the proto-salts exhibit a great tendency to absorb oxygen from the air, or from any substance in which it is loosely combined, and thus to become converted into the per-salt of the same class. But the ferrous salts are alone suited for electro-depositing the metal, hence the iron solutions employed must be carefully protected from the action of the air by retaining them in closed vessels when not in actual use. While the current is flowing, it tends to correct any peroxidising action ; because the newly-deposited iron upon the cathode is attacked by ferric salts in the solution in its immediate vicinity, and is re-dissolved, while the ferric salt is simultaneously reduced to the ferrous condition, thus^ — FegCle + Fe = SFeClg Ferric chloride with iron give ferrous chloride. And newly-precipitated iron or, if we may use the expression, nascent iron, exhibits a much greater activity in this respect than metal which has been cast or rolled, and which is usually in a more dense, as well as a more stable, condition. The oxygen which, meanwhile, is being deposited upon the other pole by a moderate current, such as is required for iron coating, acting upon a sufficiently large anode and, therefore, upon an excess of metal, forms only the protoxide. Thus, in course of time, the peroxide originally present will become reduced, and the bath will be brought into the best condition for yielding a good deposit ] but in effecting this, a considerable amount of time and of battery-power may have been wasted. Another effect of oxidation of ferrous salts in neutral solutions is the formation of basic salts (containing an excess of iron), which being insoluble in the liquid give rise to a rust-coloured precipitate or turbidity. This happens because the iron in the ferric condition requires not only more oxygen, but more acid to form salts, than it does in the ferrous state ; for example 2 atoms of iron in the state of ferric oxide require 3 mole- cules of sulphuric acid to dissolve them and form the sul- phate (Fe203-h3H2SO^ = Fe,(S04)3 + 3H,0), while in the form of ferrous oxide only 2 molecules of the acid are needed (Fe202-i-2H2S04=2FeSO^-t-2H,0). The addition of oxygen from the air may thus suffice to produce peroxide, but there may not then be sufficient acid in the bath to combine with it, and it thus appears as a solid precipitate. The following equation sums up the reaction in a general way, and shows how IRON SOLUTIONS. 253 peroxide is formed along with the persulphate, by the action of oxygen upon the protosulphates : — 6FeS04 + 30 = 2Fe2(S04)3 + Fe^- ' The remedy for this precipitate or cloudiness is obviously the addition of a sufficient quantity of free acid to combine with the peroxide or basic precipitate, and form the soluble persulphate. When much of the ferric compound has formed, just sufficient acid should be added to dissolve the precipitate, and a current of electricity should be passed through it between iron electrodes, until the pure pale green colour of the ferrous liquid has been restored and the metal is depositing well upon the cathode. Bub it must be remembered that all acid which is added to dissolve the rust-coloured precipitate becomes free again in the solution as soon as the bath is reduced to the ferrous condition, which requires less acid to form its compounds. Prevention in this case is, therefore, better than cure, and the bath should be kept as far as possible out of contact with air ; Klein adds a small proportion of glycerin to the liquid to hinder the formation of ferric compounds. The compositions of the principal baths used in depositing are quoted in Table XIX. One of the best of these solutions is that of Klein (No. 3), which is especially suitable to the production of thick deposits. A solution of ferrous sulphate in water is first prepared in a sufficiently large jar ; a solution of ammonium carbonate is now gradually added to it until no further quantity of the precipitate which forms at first is produced ] the precipitate is now allowed to subside, the liquid is poured off, fresh water is added, from which the ferrous carbonate is again allowed to separate by subsidence. The water is once more poured away, and sulphuric acid, diluted with twice its volume of water, is added little by Uttle, with constant stirring, until the precipitate is exactly re-dissolved and yet no excess of free acid is present. Towards the end, the acid must be added by a few drops only at a time, after which a few seconds' pause must be made to give oppor- tunity for it to attack the precipitate before introducing a further quantity. A blue litmus-paper suspended in the solution will indicate the condition of the liquid at any time ; it should become purple, but never red. Only recently-boiled water should be employed to dissolve and wash the ferrous sulphate and precipitate, for natural water contains a quantity of dissolved oxygen,' which would peroxidise the iron, but which is expelled by boiUng. This solution should be used as concentrated as 254 ELECTRO-DEPOSITION OF IKON. O O m 3 o 05 ■^3 O 1 go CQ o? c3 o3 Water. o o o o o o o o o o o o o o o o o o o o o o o o o o o Sulphuric Acid. Ammonium Chloride. o CO o o o o CM T-H o o Ammonium Carbonate. CO Caustic Soda. Rochelle Salt. Potassium Ferrocyanide. Iron Alum. o CO Ammonio- Ferrous Sulphate. Ferric Sulphate. Ferrous Sulphate. Ferrous Chloride. o O CN O CO 03 £3 O S S > ^ ^ rH (M CO OO Oi IRON ANODES. 255 possible and preferably warm ; it must never be allowed to become acid. In order to guard against this latter evil, Klein recommended the use of anodes presenting an aggregate area equal to eight times that of the cathode surface, so that any free acid should have every facility for becoming saturated with iron ; and he even attached slips of a more electro-negative element, such as platinum or copper to the anode plates, so that a slight local current might be set np, which would assist in the solution of the iron, without affecting the current passing through the bath The deposit from Klein's solution should not show any tendency to crack upon the surface and peel off in the form of spangles as its inventor observed happened frequently when other baths were used for producing thick coatings, especially the double chloride of iron and ammonium. The double sulphate of iron and ammonia, which is obtainable in the market in a very pure condition, is capable of yielding extremely good films upon engraved copper plates. If it be at all acid, a little chalk should be added, or better still, a little washed ferrous carbonate prepared as above described, until no further quantity is dissolved by the liquid. Perhaps the best solution of all is one recommended by Klein, and made by dissolving sufficient equal proportions of ferrous sul- phate and magnesium sulphate, to form a concentrated solution, neutralising acidity, not by means of chalk, but by means of magnesium carbonate suspended in a tray. The bath should be used with a very low current density. In using any of these iron-baths, the separation of hydrogen with the iron upon the object is to be strenuously avoided, because the gas bells clinging to the surface form pin-holes which are fatal to the impression taken in the press from a plate so affected ; and further than this the character of the iron is pre- judicially influenced by the absorbed hydrogen. The conditions, therefore, which are least favourable to hydrogen production must be fulfilled ; these are mainly — the use of a concentrated solution, the absence of free acid, and the application of a sufficiently weak current. The Anodes. — The anodes should be made of the purest iron available. Electro-deposited iron would theoretically be most suitable ; next to this the softest wrought-iron or so-called mild steel sheet are preferable. Hard steel, and above all cast-iron plates, are to be avoided, because they contain comparatively large percentages of carbon and other impurities which, being insoluble in the liquid, remain for a time suspended in the bath and are apt to attach themselves to the objects under treatment. 256 ELECTRO-DEPOSITION OF IRON. Cast-iron may contain as little as 93 per cent, of iron (or some- times even less). The anode surface should be much larger than that of the cathodes (eight times as large) for reasons already given. The anodes should be removed from the vat now and again, brushed to detach the insoluble matter which is left upon the surface in a spongy or pulverulent condition, and returned to their position. The Vat. — The vat is best constructed of iron which may be heated when the solution is to be used warm ; it must be kept thoroughly well cleaned, and, for this cause, enamelled iron is to be preferred. It must, of course, be larger than the work to be plated on account of the increased surface which is given to the anodes. The Character of the Deposited Metal. — The iron deposited in thick coatings is of a bright grey-white colour, is extremely hard and brittle, and demands the most careful attention if it is to be removed from the matrix. After annealing at a low red heat it becomes softer, and after a fair cherry-red it is as soft as steel which has been similarly treated. The fracture of unannealed deposited iron resembles that of cast-iron ; it, of course, contains no carbon, but is usually highly charged with hydrogen, which it occludes during the process of formation. Cailletet has found as much as 240 volumes of this gas in 1 volume of a sample of iron, which was sufficiently hard to scratch glass, and was extremely brittle. The annealino: has the double effect of softenino; the deposited iron and of removing the hydrogen which it had previ- ously contained ] but the inference is not safe that the hydrogen is the sole cause of the brittleness, which is more probably mainly due to the particular arrangement of the molecules of the metal. Its extreme hardness has procured for the application of deposited iron to engraved copper plates the misleading title of steel-facing^ a designation which implies the presence of combined carbon within it, whereas excepting the hydrogen contained in it (which may be removed by heating), it is one of the purest forms of iron obtainable. In its relation to magnetism, deposited iron is comparable with mild steel : but Beetz has shown that when deposited under powerful magnetic influence, as between the poles of a strong electro-magnet^ and from solutions containing ammonium chloride, it will itself act as a powerful magnet, retaining its magnetism for a considerable period of time. Iron is so electro-positive a metal that it has a great tendency to combine with oxygen, that is, to rust ; the electro-deposited film must, therefore, be dried very thoroughly as soon as possible.. ELECTKO-PLATING WITH IKON. 257 It, however, generally contains within its pores a distinct quantity of the solution from which it was precipitated, and this must be perfectly removed by washing two or three times in boiling • water, or the liability to rust will be greatly increased. The Process of Electro-Deposition. — In coating copper plates with iron (which is the chief application of the process), the copper must first be cleansed carefully, so that the sharpness of the lines may not be diminished. Klein dips the plate first into benzene and then into potash to remove grease, but it is usually sufficient first to rinse and then to boil the plate in a solution of caustic potash, then, after washing twice or thrice in clean water, to pass it through a bath of dilute sulphuric acid (containing from 2 to 5 per cent, of the acid), and after a second thorough wash to transfer it at once to the iron- vat, without touching the surface at any point. In the bath it is suspended by suitable hooks or by holders such as those mentioned on p. Ill; two plates may be used with one anode by placing the engraved faces of the copper fronting the latter. The current from the Bunsen's cell (or cells, arranged in parallel, if much work is in hand) is then passed through the solution ; an ammeter and set of resistance-coils should be placed in position. Usually from five to six minutes suffice for the process of deposition; but if a thicker coating be required, remembering that the last portion deposited forms the printing surface, and hence must be perfect in character, it is advisable to remove the plate, rinse, rapidly examine, and brush it well with a hard brush under water, so that no extraneous matter may cling to the surface, then replacing it for five or six minutes in the iron-bath, the alternation is again and again repeated until the desired thickness is obtained. After the final removal from the bath, the plate is dipped into a large volume of cold water, is then immersed in boiling water for the space of half a minute, and is again rinsed in cold water. It may then be lightly rubbed with dilute potash or soda solution, sponged dry, and rubbed with oil, the excess of which is subsequently removed by means of benzene. Thus treated, the plate will not be greatly liable to rust ; but if it is to be stored for any length of time, it must be treated like an ordinary engraved steel plate and covered with a protective film of wax. Stripping. — When an old iron-coated plate is to be re-plated, the residue of the first coat must be stripped, after removing all grease in a potash-bath, by immersing it in dilute sulphuric acid (5 to 10 per cent.) until the copper surface is left completely bare; the plate, after washing, is then ready for the iron-bath as usual. E 258 . ELECTRO-DEPOSITION OF IRON. Electrotyping with Iron. — Thick deposits also may be made, and may be obtained direct from the mould, but in this case the matrix should be first covered with a thin sheath of copper, upon which the iron is precipitated, because iron refuses to deposit well upon the black-leaded surface of the gutta-percha or other non-conducting mould. The copper may be afterwards dissolved away by making the iron sheet the anode in a copper cyanide bath, or (but with greater risk) by simple treatment with the strongest nitric acid, the excess of which must be quite washed away immediately the copper is removed. CHAPTER XIII. THE ELECTRO-DEPOSITION OF PLATINUM, ZINC, CADMIUM, TIN, LEAD, ANTIMONY, BISMUTH, AND PALLADIUM; ELECTRO-CHROMY. Electro-Deposition of Platinum. Platinum, one of the most insoluble and acid-resisting metals known, would form an excellent protective coating to metals could it be readily applied ; Eoseleur, indeed, has stated that he had twenty times evaporated nitric and sulphuric acids alter- nately in a platinum-plated copper crucible without finding the basis metal to be sensibly attacked until the last operation. But the very insolubility of the metal constitutes one of the difficulties in electro-plating with it; for the anodes resist the solvent action of any solution which may be safely used as an electrolyte without injuring the objects suspended as the cathode. It IS very rarely used, however, as a covering metal. Platinising.— It is so electro-negative an element that nearly all the other metals are able to decompose its solution, and thus deposit the platinum by simple immersion; but the coating so formed is usually black, granular, and non-adherent. Silver, copper, and brass are the most readily treated, while lead, tin, zinc, iron, Britannia metal, and the like present great difficulty unless previously protected by a substantial film of copper. For platinising copper by simple immersion, Boseleur recommends the use of a boiling solution containing 10 parts of platinum converted mto the neutral chloride, and 120 parts of caustic soda in 1000 of pure (distilled) water. Another solution may be made by dissolv- ing 25 parts of the double chloride of platinum and ammonium, and 250 of ammonium chloride in 1000 parts of water; this also IS used at the boiling temperature. When silver is platinised, and a simple solution (not too strong) of platinum tetrachloride m water will suffice to effect this, the object should be afterwards rinsed, first in dilute ammonia, and then in water, because silver chloride is formed by the exchange of metals with the platinum chloride; and this silver compound, being insoluble in water. 260 ELECTRO-DEPOSITION OF PLATINUM. requires the treatment with ammonia, in which it readily dis- solves, to effect its complete removal from the plated goods. Tin, brass, bronze, copper, and tin plate have also been platinised by rubbing upon their siu'face, with a woollen or Imen rag, a solution of 1 part of platinum chloride in 15 parts of spirits ot wine and 50 of ether, and then washing well with water, after allowing the ether to evaporate. Platinum may also be deposited by the single-cell process. Lesmondes' method consisted in placing the articles m a per- forated zinc tray and immersing the whole in a solution made by adding sodium carbonate, m the first place, to a strong solution of platinic chloride until effervescence ceases, then a little glucose, and afterwards sufficient sodium chloride to yield a white precipi- tate. This bath is to be used at a temperature of 140° F., and is most suitable for treating copper and brass, the deposition being mainly due to the current set up by the solution of the zmc ot which the trav is composed. The method adopted by Smee for coating the silver plates required for use in his battery is also a single-cell process. The plate, slightly roughened by mechanical means or by a momentary immersion in nitric acid, is placed in a solution containing dilute sulphuric acid with a few drops of platinum chloride solution added to it. A porous cell containing a rod of zmc standing m dilute sulphuric acid is then introduced into the bath; on making metalhc connection between the silver and the zmc, a current is set up, the zinc dissolves, and a proportionate amount of platinum is deposited in the form of a dark-grey or black powder, which, nevertheless, holds fairly tenaciously to the roughened silver surface. Platinating.— But for general pm-poses the separate-current pro- cess should be emploved. Of the various solutions recorded in the appended table, those of Langbein (Xo. 3) and Eoseleur (So. 4) are the most reliable. To prepare a gallon of the latter, three-quarters of an omice of platinum is dissolved in aqua regia and converted into chloride, which is then dissolved in a quart of chstilled water : in the meantime half a pound of ammonium phosphate should have been dissolved in a quart of pm-e water in one vessel, and two and a half pounds of sodium phosphate in the remaining two quarts contained in a second vessel. The solution of the ammonium salt is now to be added to the platinum liquid, with which it produces a dense precipitate: disregarding this, the sodium phosphate solution is next added with constant stirring, and the whole bath is boiled until no more smell of ammonia is observed, but on the contrary it is shown to be faintly acid by PLATINATING SOLUTIONS. 261 Ah O 02 o I o o O P B O O o TZ2 O O OQ o p- o o W c o I >< X < rH .22 M to l-H fl o „ oo» o o O " o O . ^02 ^ s CD ^ C3 03 r-l eg M OJ I— I o 03 Water. Citric Acid. Acetic Acid. Sulphuric Acid, Aramonium Phosphate. Ammonium Chloride. Sodium Citrate. Sodium Pyrophosphate. Di-Sodium Orthophosphate. Sodium Chloride. Sodium Carbonate. Caustic Soda. o o o o o o 000 000 000 Potassium Cyanide. Ammonium- Platinic Chloride. Platinum as Platinic Chloride. •o 03 •a c Hi B o 262 ELECTRO-DEPOSITION OF PLATINUM. blue litmus-paper. During the period of boiling, water will have been evaporated, which must be restored before using the liquid. When in active use, a little of the fresh solution must be added at intervals to supply the place of the platinum which has been lost by deposition upon the cathode ; because, as already ex- plained, the anodes are not attacked, and cannot, therefore, replenish the exhausted solution. A process similar to this,^ but with the addition of a small proportion of common salt, has been recently patented by Thoms. The method of preparing Langbein's excellent solution is given in the Table. The objects to be plated should be well polished before de- positing, because electrolytic platinum is hard, so that greater difficulty is involved in polishing after deposition than before. The platinum-coated surface may be left dead, or it may be brightened by means of iron-wire scratch-brushes (brass is too soft and, itself becoming rubbed, leaves a yellow stain upon the goods), or by careful rubbing with very finely-powdered pumice. When old platinum-covered goods are to be re-plated, the stripping of the previous coat presents a difficult problem ; it cannot well be removed electrolytically because the baths do not attack the metal, although a long exposure in a gold-stripping bath may sometimes effect the desired object, especially if the platinum be not in its densest and hardest condition ; but at best it is a very tedious operation. The chief solvent for platinum is aqua regia, but this cannot be applied because it would vigorously attack the basis metal beneath; and, in fact, as soon as any of the latter metal became uncovered, it would dissolve all the more rapidly on account of its contact with the remainder of the platinum coat, and, being more electro-positive, would even serve to protect the latter from further action. The surest and most rapid method, whenever practicable, is to apply mechanical means and simply rub off the platinum by means of emery-cloth, and then re-polish the metal beneath. This system cannot, of course, be applied when any delicate pattern or design is traced upon the object, in which case the chemical methods must be tried. A greater loss is caused by the use of emery, but this may be minimised by saving the dust produced, and subsequently working it up to recover the platinum. The name usually applied to platinum-coating by simple immer- sion is platinising^ as in the case of the Smee-battery silver plate, - while the electrolytically-covered object is said to be platinated. Watt, however, raises an objection to this latter term, and suggests the use of the expression ^platined' to denominate this class of work. His objections to the other term are doubt- / USE OF DEPOSITED ZINC. 263 lesi wortliy of consideration, but the older word is perhaps more euphonious, and will probably hold its ground ; while if it be regarded as a contraction of the compound word ' platinum- plating,' it is not, after all, unscientific. Electro-Deposition of Zinc. Only in special cases is electrolytic zinc deposition resorted to ; the metal has not a fine colour or lustre, and is readily dulled with the thin film of tarnish, which forms Very soon upon exposure to the atmosphere. Its highly electro-positive character certainly readers it suitable to the protection of iron surfaces from destruc- tion by rust; because when submitted together (in metallic con- tact) to the same corrosive influence, the zinc is the earlier of the two metals to be attacked. But, like tin, zinc is usually more satis- factorily deposited upon iron by dipping the latter into a bath of the molten metal. Such a process yields a perfectly homogeneous and continuous coat, which is applied at such a temperature that it is impossible for water to exist between the two surfaces, or in, cavities ; while the electrolytic method deposits a crystalline, and,; therefore, to some (however slight) extent, porous cover, in which small quantities of the solution from which it has been deposited may be locked up, and facilitate the oxidising action. Thus the dry or fusion method of coating the iron; is preferable whenever; its application is possible; the meaningless title galvanised iron given to this product is manifestly a misnomer, and is distinctly misleading. But for internal or other surfaces which cannot well be I coated by the fusion method, electro-deposition is frequently used with great advantage ; it is especially useful in coating hardened steel ware, which is to be used in the hardened condition, and of which the temper would be drawn by the heating necessary for the treatment with melted zinc. Pattery Process. — On account of its very electro-positive nature, zinc cannot be precipitated upon ordinary metals by simple immersion, nor is it practically deposited by single-cell methods.; By separate current it may be obtained irom the neutral sulphate,; chloride, acetate, or other soluble salt of zinc (except the nitrate), or from the corresponding double salt of zinc and ammonia. The current-strength required for most zinc solutions is considerable, and demands the use of two or three Bunsen-cells. The solutions are, as usual, various, but good results may be obtained from a 10 per cent, solution of zinc sulphate with a current of from 0*06 to 0-13 ampere per sq. in., or 1 to 2 amp. per sq. decimetre ; other formulae are given in Table XXI. 264 ELECTEO-DEPOSITION OF ZINC. CO CO CO LO CO in Water. CO 2^ o o •3 a pi ^1 rH fl, cc c o CO CO 1—1 CO §•2 CO ^ R .r-< . o ^ & 3 CO O pj Q o o o o o o o o o o o o o o o o o o o o o SiilnTiiirip Acid. ... little Ammonium Sulphate. r : : : : o • • . • . »o . Ammonium ^ : Ammonia. : : : : >o • • • - . . (N : : : Alum. . o : Potassium Carbonate. : : : : lo • • . . . . : : : Potassium Cyanide. Potash, excess : o . to Zinc Sulphate. : ^ o . lO • CN • o : 00 . 00 rH Zinc Chloride. ^ : : o . i-H Zinc Hydroxide. • • : o • rH Zinc. ... 18-75 m (-H . 2 •i-t O ' O O hS* c*2 ^ .tS CO *^ to S m PI a .2 eS O o H CO CO CO Water. o o o o o o o o o o o o o o o o o o ! Sodium Pyrophosphate. Ammonia- Alum. o Potassium Bitartrate. Caustic Potash. Tin Tetrachloride. Tin Bichloride. Tin Binoxide. o . O o C3 ^ AO Ph O O O o o o O o o 03 •73 tS3 o 4J CO a pi o o CO o u :3 GO o3 a> P o CD o 6 ^ CO eg Pi o o Pi {=^^ r-lO ^.2 CO c c o rHCq O (£> - = o rH^ ^ 'o c ^ Q § Is . rH -ta cq gcqj o CO c6 , "5 > ^ ^ 'o ^ ^ -^^^ ^ ^ $ C ^ i "c '^t; g rH r; "o K ^ OC 5; G Pi CD ^3 "Water. o o o o o o O O CZr o o o o o o o o o Tartaric Acid. Sulphuric Acid. Zinc Acetate. | Sodium Pyropho-sphate. Sodium Carbonate. • ■ : : ?o : c3 2 o ■4^ »-> O c3 2 o r ■■ -(_> 4^ TINNING BY SINGLE CELL AND BATTERY. 271 simple; and the principal solutions are enumerated in the pre- ceding table. Deposition by Single-Cell Process. — Solutions recommended by Eoseleur are given in Table XXII. (Nos. 4 and 5). In use, small articles are most conveniently placed upon a tray of per- forated zinc and lowered into the liquid, which should be kept warm. The tray should be lightly shaken from time to time in order to bring different points into contact with the zinc, and the surface of the latter must be scraped at intervals to remove the white incrustation which forms upon it and destroys metallic contact. Large objects are immersed in the liquid in contact with fragments of zinc, which should have a surface equal in the aggregate to about the one-thirtieth part of that which is to be coated with tin. At the end of an immersion lasting from one to three hours, Roseleur recommends that the pieces should be removed and scratch-brushed, while at the same time the bath, which has been impoverished by the deposition of tin, is re- generated by the addition of fresh tin and alkaline bitartrate or pyrophosphate. After a further immersion of about equal dura- tion, the goods are well washed, scratch-brushed, and dried. Deposition by Separate Current. — For this purpose the current should have a fairly high electro-motive force, such as would be yielded by two Bunsen-cells in series. Of all the baths quoted in Table XXIII., that of Roseleur will probably be found to give the most universally good results. Roseleur's bath (No. 8, Table XXIII.) is made by placing the pyrophosphate and water in a tin-lined wooden tank, the lining serving also as anode ; afterwards the stannous chloride is suspended in the liquid in a copper sieve. The first result is to produce a cloudiness in the liquid, which, however, subsequently becomes clear; and after complete solution of the tin salt is ready for use. The liquid may have a yellowish colour, but should be perfectly transparent and clear. This bath requires an addition of tin from time to time, because the metal is de- posited from it at a greater rate than that at which the solution of the anodes replenishes it, in spite of the relatively large surface of the latter exposed. The solution already mentioned as being made by dissolving the binoxide of tin in caustic potash may be used as an electrolytic bath for coating iron. Maistrasse ensures the complete continuity of the tin-covering given by his bath (No. 6) by heating the coated object to the melting point of the tin, thus causing the latter to fuse over the surface and alloy with the basis metal ; this solution is said to be especially applicable to the tinning of cast-iron. 272 ELECTRO-DEPOSITION OF LEAD. No special remarks are called for upon the practical electro- depositiou of tin. A current of moderate volume at from 3 to 5 volts' pressure is required. The time to be expended varies with the process and with the thickness of metal to be deposited, from one or two up to twenty-four hours, which latter period i& recommended for Maistrasse's solution. The objects, as usual, are carefully cleansed before immersion, and are subsequentl}^ most thorouo-hlv washed. Thev mav be left in the ' dead ' con- ~ 1/ 1/1/ dition, if preferred, but are more generally polished, either by^ scratch-brushing or by friction with bran. The anodes should be of pure metal. So-called tin-plate, whick is only sheet-iron covered (in the dry way) with the thinnest- possible coating of metallic tin, is, of course, useless ; and tin-foil must be used with caution, for many samples of the foil are made from alloys of lead and tin, but these are generally duller' in their outer aspect ; while others may consist of the thinnest lead-foil, covered on one or both sides with tin, the two surfaces being miited together by rolling ; and such samples in external appearance have all the characteristic appearance and brightness of the pure metal. Only pure tin-foil, or plates cast from the best grain ti7i, should be employed. Electro-Deposttiox of Lead. With lead, as with tin, the low fusing-point renders the coating- of an object more simply effected by immersion in the melted metal than by electro-deposition. The old experiment of grow- mg a ' lead tree ' by suspending a fragment of metallic zinc in a dilute solution of lead acetate (sugar of lead) is simply a case- of deposition by ' simple immersion ' ; the peculiar, largely- crystalline, spongy formation of the resulting lead illustrates very well the difficulty of getting a good solid adherent metal by simple exchange with a more electro-positive element. \Yhen, however, it is necessary to deposit lead in the wet way, a simple dilute solution of the acetate may be electrolysed by separate current ; but the alkaline bath, prepared by boiling 5 parts of lead oxide (litharge) in a solution of 50 parts of caustic potash in 1000 of water until it is completely dissolved, is preferable. With either liquid lead anodes are used, and the objects are^ carefully prepared for the bath, and polished afterwards as usual. The methods cannot, however, be relied upon to give a thick deposit, nor are they largely used in practice. Many lead solutions tend to form an insoluble higher oxide (a peroxide = PbOo) at the anode, which thus receives a coating ELECTRO-DEPOSITION OF ANTIMONY. 273 as well as the cathode, but of a different kind ; and the principal interest attaching to the process of lead electrolysis centres in the possibility of producing films of oxide which present different colours to the eye by reason of their extreme tenuity. Electeo-Deposition of Antimony. The deposition of antimony, again, is a process of no commercial importance, although the metal, which has a fairly bright lustre when polished, but is rather grey in colour, resists well the tarnishing action of the atmosphere. It is a very brittle metal, and would be useless as a coating upon any thin article, or upon one which is liable to be bent in any degree when in use. Immersion Process. — It is fairly electro-negative, and will, therefore, give a deposit upon many metals by simple immersion. For example, brass will receive a lilac-coloured surface tint, varying in depth of shade according to the time of immersion, by dipping it in a boiling dilute solution of antimony terchloride (butter of antimony), made by adding much water to a little of the antimony compound, and boiling until the dense white precipitate formed on mixture has re-dissolved; and then, after a further addition of water and a second boiling, filtering and heating for use. The coated pieces must be well dried in hot sawdust or in a stove, and must be protected by a varnish of lacquer, if the lilac colour is to be preserved. TABLE XXIV. — Showing the Composition of Solutions for the Electro - Deposition of Antimony, recommended by various Authorities. 1 2 3 4 5 6 7 8 9 PARTS BY WEIGHT OF INGREDIENTS. No. Authority. Antimony. Antimony Terchloride. Antimony- Potassium Tartrate. Antimony Tersulphide. Sodium Carbonate. Ammonium Chloride. Hydrochloric Acid. Tartaric Acid. ■ Water. i Special Method of Preparation. 1 Gore, . X y 1000 2 3 )> • j> • 4000 2000 1000 1000 Electrolyse pure strong hydrochloric acid with antimony anode until solution is charged. 4 83 124 83 1000 5 Roseleur, 50 100 1000 (Use boiling ; it deposits kermes mineral on cooling.) s 274 ELECTEO-DEPOSITION OF AXTIMOXY. Battery Process. — But, as usual, the separate-cmTent process is to be preferred, for which the sokitions given in Table XXlV., inter alia, have been recommended. Of these solutions Xo. 3 will probably give the best results in workshop practice. It is made by dissoMng four poimds of the double potassium-antimony tartrate (tartar emetic) in a mixtiu'e of two poimds of strong hydrochloric acid with one of water. This solution is particularly useful for producing thick deposits, as considerable latitude in cuiTent-strength is permissible. A cun-ent of 0'06 to 0*1 ampere per square inch, or 1 to Ih amperes per square decimetre, will be fomid most suitable, but the Toliune may be gTeatly increased, and the rate of deposition coiTespondingly humed, without danger. Tartar emetic itself is a feeble conductor, and cannot alone be made to give good deposits, for, as Gore has shown, even a very weak ciuTcnt brings down the metal in a pulvemlent form ; hence the addition of hydrochloric acid, which sutiiciently increases the conductivity. The other solution containing tartar emetic has less hydi'ochloric acid, is a poorer conductor, and must be used with a weaker current, not exceeding 0*0 13 ampere per square inch, or 0*2 ampere per square decimetre. The bath prepared by Roseleiu- by boiling together for the space of one hoiu\ and subsequently filterhig the solution. 1 oimce of antimony tersulphide, 2 of sodium carbonate, and 1 pint of water, tends to deposit antimony oxysulphide (kermes mineral) on cooling, as above noted ; and must, therefore, be always used hot. The pale orange precipitate of oxysulphide is soluble in the mother-liquor as soon as the boiling-point is reached. The anodes may be made of platinimi, but preferably of antimony, which must be cast into slabs of the required shape and size, as it is far too brittle to allow of mechanical work, such as rolling or hammering. Xo special treatment is required in depositing antimony ; the pieces must be cleansed thoroughly, and the strength of the bath must be maintained by adding a fiu'ther quantity of the solution from time to time. After coating, the pieces are rinsed, dried in a stove (or in hot sawdust), and brightened in the usual way. But if the chloride solution has been employed, the object must be rinsed once or twice in hydrochloric acid immediately it is removed from the bath, and then in water, because water added to the original solution produces a dense ciu'dy- white precipitate of antimony oxychloride. So that, if the pieces, with a portion of the bath liquor clinging to them, were dipped into water at the outset, they would be covered with this white deposit : but if EXPLOSIVE ANTIMONY DEPOSIT. 275 fir^t washed in a menstruum such as hydrochloric acid, with which the solution mixes without decomposition, the original liquid is safely removed, and the final cleansing may be effected in water without risk. Character of Deposit.— The metal deposited by too strong a current is, as usual, black, powdery, and non-adherent ; and that yielded by some solutions may be so, even when a weak current is employed. But the metal is capable of being thrown down in two different modifications of the reguline or solid form — one of k grey-slate colour in the dull condition, but taking a good polish and resembling cast-iron when scratch-brushed, having a crystal- line fracture, and being hard and very brittle ; while the other is darker and more steely, but somewhat softer, non-crystalline, or amorphous, and with a lustre which resists atmospheric influences for quite a lengthened period. Explosive Antimony.— The most curious and interesting pheno- menon in connection with antimony deposition is the production of an explosive variety, which has been fully studied and described by Gore. He found that the amorphous antimony deposited from a solution of 1 part of antimony terchloride in 5 or 6 parts of hydrochloric acid (of specific gravity 1*12), or in 10 of hydro- bromic acid (specific gravity 1*3), or in 15 parts of hydriodic acid (specific gravity 1*25), would, under certain conditions, undergo a physical change and become crystalline ; and that this change was attended by an increase of density, and with an evolution of heat so considerable that, if evolved instantaneously by a con- siderable mass, it may develop almost explosive violence. The heat is so great that, if a sufficient body of metal undergo the change, paper in contact with it is burned, and wood is scorched brown; the 'explosion' is often accompanied by a flash of light, but always by a slight cloud of vapour expelled from the interior. The three varieties (from the chloride, bromide, and iodide) differ in their sensitiveness, as well as in other particulars. None of them are pure, but retain within their pores about 6, 20, and 22 per cent, respectively of the depositing liquor, which may be expelled by heating, as, for example, at the moment of explosion ; the cloud of vapour observed is thus accounted for. The presence of this liquor in the metal gives rise to an apparently abnormal excess ^ of deposited metal over that which should be yielded according to the electro-chemical equivalent. The alteration of condition proceeds gradually on keeping, but more quickly in the case of powder or of thin pieces than with larger masses of metal ; and the heat is then evolved almost imperceptibly. But freshly- 276 ELECTRO-DEPOSITIOy OF BISm^TH. deposited material mav be caused to undergo the change, m a rapid or explosive manner, by any physical means, which is cap- able of sufficiently affecting the molecular arrangement of the body. With the chloride Tarietv the action begins when it is heated to 170° F., becoming sudden and complete at about 205° F. : with the bromide deposit the explosion occurs at 320° F. ; with that from the iodide at a still higher temperatm^e. A similar descending order of sensitiYeness was observed when other means were employed to initiate the action ; a touch with a red-hot wire caused immediate conversion of the chloride variety, while the bromide metal was merely locally affected by contact with the hot wire, the action only spreading through the whole when it was- raised to 250° F. throughout, and through the iodide specimen when it was heated at 338° F. The heat developed by the alteration in the fii'st-named case was so great that a tin rod ^th of an inch in diameter, upon which the amorphous antimony was electrolytically built up to a total diameter of haK an inch, melted, flowed awav from the antimonv, and remained fluid for some time. A sudden blow, or even mbbing with glass or metal, is liable- to convert the amorphous into the crystalline variety, so that if it is required to break up the imexploded metal into smaller pieces, it shotdd be fractm^ed imder cold water by a comparatively soft material, such as wood. Provided that they are kept under iced-water meanwhile, very thm pieces may even be crushed to a fine powder in a mortar ; and this 230wder may be dried in the cold over sulphuric acid ; and, remaining in the original condi- tion, will evolve subsequently the same proportion of heat as the thicker imtouched dejDosits. The Electeo-Depositiox of Bismuth. The deposition of this metal possesses at present little beside a scientific or theoretical interest. It may be thrown down from a weak and very slightly acidi- fied solution of the nitrate, either by simple immersion upon certain more electro-positive metals, such as tin, or by the separate-current process. Bert rand uses for the latter method a solution of 30 parts of the double chloride of bismuth and ammonium in 1000 of water, containing a small proportion of hydrochloric acid. With one Btmsen-cell he succeeded in obtain- ing a coat which, although black exteriorly, exhibited the well- known slightly pink shade of the metal, and was susceptible- ELECTRO-DEPOSITION OF PALLADIUM. 277 ^of a very high poUsh. Like antimony, the brittle nature of the metal renders it unfit for coating objects which are at all strained *or altered in shape subsequently. The Electro-Deposition of Palladium. Palladium is one of the rarer metals, belonging to the platinum group. Having a silver-white colour and lustre, and being also untarnishable at ordinary temperatures by oxygen or (unlike silver) by sulphur compounds in the air, it is sometimes substituted for silver. Occasionally, but very rarely, silver-plated goods are given a thin final coat of palladium. Cowper-Coles,* in his interesting electrolytic process for the manufacture of parabolic reflectors, which are required to be always bright, and which, if used for electric search-lights may be exposed to high temperature and to influences which would rapidly tarnish silver, covers the copper of which the reflectors are made with a coat of palladium having a thickness corresponding to 70 to 80 grains of palladium per square foot. The bath that he uses is that recommended by Bertrand, namely, a solution of the double chloride of ammonium and palladium but with excess of ammonium chloride added. He dissolves 6*2 parts of this compound with 10 parts of ammonium chloride in 1000 of water, and uses it at a temperature of 75° F. with a current density of about 0*15 ampere per square foot and an E.M.F. between the electrodes of 4 to 5 volts. He uses an anode of carbon ; Bertrand, however, employs one of palladium. Of the remaining metals there are none which render necessary in this work a description of the means by which they may be electrolysed. With regard to some of them, indeed, many published processes would appear on thermo-chemical grounds to be visionary. In regard to aluminium especially ; the extreme popularity of this metal, combined with a great want of knowledge, on the part of the public, as to its properties, have led to a demand for its electro-deposition. Many solutions have been proposed which it was claimed should give good deposits of the metal, but have been found by various experimenters to be worthless. In our own experience, the brilliant grey deposit, which has been afforded by some of these methods, but which has never exceeded in thickness that of a mere film, has consisted principally of iron, * Jour, Inst, of Electrical Engineers J 1898 (xxvii.), p. 105. 278 PEODUCTION OF METALLO-CHROMES. a metal which is almost universally present in commercial aluminium compounds. The deposit has been often found, on testing, to contain aluminium ; this may have been due to traces of the solution remaining in the pores of the coat, or it may have resulted from aluminium which had actually been deposited with the iron as an alloy ; but in all cases the iron was found to be vastly preponderating. That it is possible to deposit aluminium by the electrolysis of fused compounds is no doubt^ true, but further evidence is necessary to prove the satisfactory deposition of the pure metal from aqueous solution. Colouring of Metallic Surfaces. Advantage has been taken of the fact that lead and certain other metals tend to deposit as peroxide upon the anode, instead of, or sometimes in addition to, precipitating as metal upon the cathode, to obtain certain colours upon metal surfaces. The most interesting application of this is to be seen in the formation of metallo-chromes by the deposition of an infinitesimal film of lead peroxide upon a polished steel surface. It has long been known that colourless transparent substances, if sufficiently thih^ are capable of displaying a series of colours by reflected light by the optical phenomenon known as the interference of luminous, waves (where the wave of light reflected from one side of the film passes so near to that reflected after refraction from the other that their respective vibratory influences interfere witL one another). The play of colours upon the soap-bubble or upon oil floating on water are instances of this phenomenon, which was first studied by Newton. A momentary immersion of a. bright steel or platinum plate as anode in a lead solution suffices, to deposit a fllm of peroxide, which answers the requirements for the production of these iridescent colours. Nobili was the first to observe this action with acetate of lead ; Becquerel's solutiou is now used for this purpose ; it is made by dissolving 1 4 ounces, of caustic potash in half a gallon of water, adding to this 10^ ounces of lead oxide (litharge) and boiling for from half-an-hour to an hour, allowing it to stand for some time, then decanting^ the clear liquid from the subsided precipitate, and making up the whole to a gallon in volume. The electrolytic action must be continued for exactly the right period of time, an. insufficient exposure does not give time for the development of sufficient thickness to allow of interference, while an excessive action causes an opaque dirty brown deposit; intermediately between the two, a very beautiful play of colours may be secured. PKODUCTION OF METALLO-CHROMES. 279 The cathode may be of copper-sheet. Gassiot produced patterns upon the anode by interposing a cardboard disc, with a perforated design upon it, between the electrodes, so that the deposit chiefly occurred on the portions unshielded by the solid portions of the card. Watt uses copper wire bent into various shapes ; this system has the advantage that there are varying distances between the difi'erent portions of the anode and the cathode, and, therefore, a varying thickness of film is produced, which adds to the beauty of the iridescence. The film is fairly adhesive, but should not be handled more than is necessary. CHAPTER XIV. THE ELECTRO-DEPOSITION OF ALLOYS. The principles upon which the possibility of electro-depositing alloys may be said to depend have already been explained in Chapter II. (p. 37). Brass, bronze, and German silver are practically the only alloys deposited, if we except the mixtures used in producing coloured gold coatings, and of these the first- named alone has any widespread use. The Electro-Deposition of Brass (Copper and Zinc). A brass coating may be given to a copper article by covering it electrolytically with a thin film of zinc, and then, after wash- ing and drying, applying to it a heat just sufficient to cause the two metals to form an alloy superficially; and similarly an object made of any other material, which will withstand the necessary heating, may be brass-surfaced by depositing alternate layers of copper and zinc, and alloying them in situ as before. But in practice this could not well be done. Nor is brassing usually efi'ected by simple immersion, although Watt has shown that a zinc rod, dipped into a mixed solution of copper and zinc acetate, becomes covered with a yellow deposit of the alloy. But for practical purposes the production by the separate-battery process is alone adopted. The Bunsen form of battery is the best, and should generally be arranged with two cells in series. The Solution. — The solution may be greatly varied, and, indeed, no absolute and unalterable rules can be laid down as to its con- stitution. The basis of most of the liquids is the mixed cyanides of copper and zinc, as in this combination zinc does not dis- place copper from its solution, and there is in consequence a better chance of obtaining a simultaneous coating of the two bodies ; but the relative proportions of these may require to be varied in working, according to the behaviour of the solution, which depends upon several inconstant quantities — strength of ELECTRO-BRASSma SOLUTIONS. 281 current, resistance of solution, and the like. The following table (XXV.) summarises the principal electro-brassing solutions. One of the best of these solutions is that recommended by Koseleur (No. 8), which is somewhat complex, but will be found to give good results — to prepare 1 gallon, 2J ounces of copper sulphate and a like weight of zinc sulphate are dissolved in water; and a solution of 6|- ounces of sodium carbonate in a convenient quantity of water is added to the mixture. A dense precipitate of copper and zinc carbonates is thus formed. It is allowed to settle, water . is poured on, -and it is thus washed several times by decantation ; the clear liquor is finally siphoned .away from the precipitate, which is then filtered off from the residual liquid, and mixed with a solution of 3^ ounces of sodium carbonate, and 3 J ounces of sodium bisulphate, in 7^ pints of water. To this is added 3 J ounces of potassium cyanide and 20 grains of arsenious acid (white arsenic) in f pint of water. After filtering, the clear liquid contains the copper and zinc, and should be quite decolorised ; any blue colour indicates the presence of unaltered copper salt, and calls for the addition of a further quantity of potassium cyanide solution. It may be taken as a general rule that all the cyanide brass- baths should be free from blue colovir ; and that if they are not so initially, they must be made so by the introduction of sufficient •extra potassium cyanide, while if they become blue when in use, the same remedy is to be applied. It is to be recommended that .a 10 per cent, solution of the cyanide should be kept for this purpose, so that a little may be added , whenever necessary. The bath (Roseleur's) is used cold and with brass anodes, but as the composition is liable to variation by unequal solution of the brass, a further addition of copper or zinc may be required from time to time. This should be eff'ected by adding separate, solu- tions of copper or of zinc cyanides, made by dissolving their carbonates in solutions of potassium cyanide. Sodium or potas- sium arsenite may be used in place of arsenious acid, but a proportionately larger quantity must of course be employed. With the addition "of a little arsenic the solution gives a brighter deposit than is yielded by the copper-zinc solution alone ; a large quantity, however, is found to give to the deposit a tem- porary increase in whiteness, which is objectionable. Baths may be prepared electrolytically either, as in Gore's solution (No. 3), by passing the current through a suitable solution from a brass anode, so that the constituents of the ;anode alloy may dissolve simultaneously into the liquid, or, as in Yolkmer's bath (No, 14), by passing the current from a copper 282 ELECTRO-DEPOSITION OF ALLOYS. TABLE XXV. — Showing the Composition of Electro- 1 2 3 4 5 6 7 8 9 10 11 12 13 PARTS BY WEIGHT No. Authority. S Copper Acetate. Copper Carbonate. Copper Chloride. Copper Cyanide. Copper^ Sulphate. Zinc Acetate. Zinc Carbonate. Zinc Chloride. Zinc Cyanide. Zinc Sulphate. Caustic Potash. Potassium Carbonate. 1 Brunei, .... ... 10 20 250 2 J, ..... 5 10 80 3 Gore, ..... X ... ... ... ... 4 Heeren, * . . . 3 ... ... ... 26 ... 5 X 6 Japing, .... 7 8 Morris & Johnsonj . Ecseleur, .... ... 10 .... ... 12-5 ... ... ... ... ... 10 ... ... 6-2 ... ... ... ... ... ... 9 .... 12-5 ... ... ... ... 10 ... ... 10 J) .... 14 ... ... 14 ... ... 11 12 Russell & Woolricli, De la Salzede, . . ... 150 5 ... ... 15 ... ... ... ... 9-5 ... 120 13 . . . 3 7 100 14 Volkmer, .... 15 Watt, 4 8 56 16 Weiss, 4 6-8 40 17 Wood, 14 1 7 Notes to Table. — The heavy figures in the last column refer to the numbers of the vertical Nos. 9 and 17 to iron and steel. No. 8 is recommended to be used cold. ELECTRO-BRASSING SOLUTIONS. 283 Brassing Solutions 14 15 16 17 AS RECOMMENDED BY VARIOUS AUTHORITIES. ; 19 20 21 22 23 24 OF INGREDIENTS. go 12 125 60 6-5 100 20 40 30 excess 2- 5 10 125 6-5 3- 4 82 en -1-3 150 20 100 42 Is If 20 20 28 '3 o s S 62-5 16 31 24 100 125 5^ 27 a P <5 125 14 61 to •il 03 -^q 0-2 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 Special Method of Preparation. Dissolve together in 24 ; add 14 last. Form electrolytically. Dissolve 6 in 12 of 24 ; 11 in 52 of 24 ; 14 in 120 of 24 ; mix ; add r(;st of 24. Form electrolytically with brass anode. ^Form electrolytically, with brass anode supplemented by copper or zinc anodes, if necessary, to ^ improve colour. f Precipitate 3 and 8 from CUSO4 and I ZnS04 with Na2C03, and wash ; \ add 16 and 18 in 900 of 24 ; add I 14 and 23 in 100 of 24 ; filter. Add more 14 if solution remain blue. j Dissolve 14, 16, and 18 in 800 of 24 ; \ add 2 and 9 in 200 of 24. Dissolve 14 and 18 in 800 of 24 ; add to solution of 2, 9, and 19 in 200 of 24. ( Add 14 last, until precipitate re- ( dissolves. ( Dissolve 14 in 24 of 24 ; dissolve 4, < 11, 13 in rest of 24 ; add 22 ; ( stand for some days and decant. Form electrolytically ; with copper anode till saturated ; then with \zinc anode till deposit is brass colour. { Dissolve 2 in 65 of 24 and add 15 of I 19 ; dissolve 11 in 125 of 24 and \ add 16 of 19 ; mix ; add 12 in 125 I of 24, then 14 in 125 of 24. Stir, L add rest of 24 ; stand and decant. ( Dissolve 2 in 19 ; add 12 ; then add 1 11, 14 and 24. columns indicating the various reagents. Solutions Nos. 6 and 10 are especially applicable to zinc ; No. 14 at 86° to 100° ; No. 7 at 150° ; No. 17 at 160° F.; and Nos. 3 and 4 boiling. 284 ELECTEO-DEPOSITION OF ALLOYS. anode alone at first, until sufficient of that metal has been taken up ; and next from a zinc anode alone, until the requisite shade of brass-colour appears on the sheet metal cathode ; a brass anode is then substituted for that of zinc, and the plate at the cathode pole is replaced by the object to be coated. Anodes. — The anode used for the various solutions above given jnay be made of brass, and this should have approximately the same composition as the metal which it is proposed to deposit, and should be prepared from the pure virgin metals ; but instead of using the copper and the zinc combined together in the form of ^n alloy, they may be used separately by suspending alternate strips of the two metals from the anode rod; and this method, although less usually adopted, presents the advantage that the composition of the bath may be controlled by altering the relative numbers of the two kinds of strips, so that a greater area of copper anode surface may be presented to the liquid when the deposit is becoming too pale in colour, or of zinc when it grows too red or yellow. The anodes are supported in the usual way, either completely immersed and supported by stout brass hooks, or partially immersed only. The vats for cold and for hot solutions are similar to those used for the copper (cyanide) depositing process. Nature of Deposit.— The character of the metal deposited is entirely dependent upon the conditions of current and of bath, but chiefly upon the composition of the solution. If the liquid contain an excess of either constituent beyond the normal, the deposited metal will also contain an excessive percentage of that metal, and its properties and colour will be influenced accord- ingly. The composition of the anodes vastly influences the deposit yielded by a solution, inasmuch as it modifies the con- stitution of the bath itself. ^ A weak current, or the imparting of motion to the suspended ^ objects (which is in some respects equivalent to a reduction in ■ current volume), tends to produce a deposit containing a greater proportion of the more electro-negative element copper, while a stronger current yields a larger percentage of zinc. Then again — given the same battery, solution, and anodes — heating the bath, by increasing its conductivity, raises the current- strength, and thus also tends to increase the percentage of zinc. The deposition of hydrogen must as usual be avoided, in order to obtain a good adherent deposit. But, to sum up the preceding observations, within the limits of current-volume that permit the production of a good coat, the conditions which favour the precipitation of an alloy rich in copper, and, therefore, a red or KEGULATION OF COLOUR OF BRASS. 285 yellow colour, are — a solution and anode containing a high per- centage of copper, a weak current, a cold bath, and the movement of the articles under treatment ; while the opposite effects of a. greater proportion of zinc with a corresponding whitening of the deposit are yielded by a strong current, by anodes and solution, richer in zinc than in copper, and by maintaining the objects motionless in a hot liquid. It will now be understood that the various relations of current- strength and other conditions of work are mutually so inter- dependent that it is impossible to lay down inviolable rules for working brass solutions; but with the requisite constant obser- vation and care, there is no difficulty in obtaining any desired nature of deposit. If the colour of the metal is too red, an excess of copper is indicated, which may be rectified by increasing the current- volume or adding more zinc to the liquid; if it is too white, showing an excess of zinc, the current is reduced or copper is added to the bath. The alteration of current may often be conveniently effected, as mentioned in another chapter, by increasing or decreasing the surface of the anodes according as its volume is required to be greater or less. It is evident, therefore, that baths of even comparatively widely-differing compositions may be caused to yield deposits of the same alloy by adjusting the various conditions of work. But since the proportion of the constituents in the deposited metal, and hence its colour, are so susceptible of alteration by a change in any of the conditions, it becomes necessary to watch the progress of the electrolysis most carefully, not only to prevent the variation of the alloy, but to prevent local alterations, which may give rise to spotted or unevenly coloured deposits, such as would be produced if any portion of the cathode object were receiving more or less current than the remainder, or if the solution were not properly mixed. It is, therefore, advisable to stir the solu- tion very thoroughly before commencing work, and at intervals while the deposition is in progress; and again to observe that the pieces forming the opposite electrodes are as nearly as possible equidistant from one another. The observance of this latter precaution, which is necessary enough in depositing single metals, where the main question is one of thickness of deposit, becomes greatly exalted in importance when it is seen that not only the thickness but also the colour of the coating is influenced, the portions more remote from the anodes receiving less cur- rent, and, therefore, having a redder shade than those in closer proximity. Similarly, the difference in specific gravity of the various strata in imperfectly mixed liquids produces a variation in colour between the deposit at the top and that at the bottom of an article. 286 ELECTEO-DEPOSITIOX OF ALLOYS. The Process. — In the practical application of the process, the objects to be brassed must be first carefully cleansed and polished after the orthodox fashion; they are then immersed in the electrolytic bath until the required thickness of metal has been attained, and, provided that it be of the right colour, it is then rinsed, scratch-brushed, well washed' in hot water, and dried in hot sawdust or in a stove. In some of the liquids used the cyanide solution does not exert sufi&cient solvent action upon zinc oxide, which thus becomes separated in the solid form upon the surface of the anode, finally crumbling away and collecting on the bottom of the bath. Such a formation is objectionable, first, because it impedes the action by yielding a film upon the electrode surface : then, because it becomes detached, and so introduces into the bath solid matter that may be held for a time in suspension in the liquid, which is always dangerous, because fragments are hable to become attached to the suiiace of the object being plated: and, thirdly, because a deficiency of zinc passing into solution from the anode is equivalent to a relative increase in the amount of copper in the bath. In cases where such a precipitate is observed upon the anode (which should, therefore, be inspected from time to time), a small quantity of liquor ammonige mixed with a slight excess of potassium cyanide should be added to the liquid; any oxide already formed will thus be dissolved, and the anode ''surface will be kept clean, owing to the non-formation of fresh quantities of the substance. When ammonia is one of the constituents of the bath, a further quantity must be added at intervals, because it is constantly evaporating by exposure to the air. AVhen an object is foimd to be taking a bad deposit at first, it should be removed from the vat, scratch-brushed and returned, the defect in the electrolytic arrangements having been made good in the meantime. A dirty yellowish or earthy-looking deposit is often the result of insufficiency of cyanide, and may be corrected accordingly. The chief use of brass composition is to coat zinc or iron surfaces with a rich-coloured material that may be subsequently bronzed or otherwise ornamented, or which may be simply lacquered and left with the original brass colour. Large quanti- ties of iron tube for bedstead, fender, and other work are thus coated electrolytically with brass. It is sometimes employed for facing typographic matter, but presents no advantage over nickel for this pm-pose ; it is especially apjDlicable to coating book- binders' type, which should have a hard face, and which must bear heating, as these tools are frequently used hot. ELECTRO-BRONZING SOLUTIONS. 287 O Pi tz; o «H H *A O e o 6 H O o o > ^. >< o o o « O {z; t-H o W pa CO 1-1 00 CO CO cq o o 3 'o (33 P4 .5o5 Is CO iH OJ o . CQ R O rH^ o' ^ CQ .S .2 2 S II S C3 05 tH © t> © o o pi .2 - 02 ^3 Water. 1000 1000 1000 1000 1000 1000 Rochelle Salt. o r-i Soda-Lime. : ' • : : o : : : . . oo T^n Q c G 1 n in X U tdooi Lllli Cyanide. ■■ n s S S : 5m i-H Carbonate. ... o o . o o • rH i-H • Caustic Potash. ^ I • • CO • 5m * • • 5m Sodium Stannate. : : : : : : : : : : ^ Tin Binoxide. J (M : : : : Tin Bichloride. : : ^ rH : : Tin Tetrachloride. o oo Cuprous Cyanide. J »o : : : : Cuprous Chloride. . . . Cupric Chloride. . o • : : : A H o o W O o O S Pu O O Approximate H.-P. Hours used per 1 lb. Copper (calculated). 666 66666 Thickness of Copper Deposited per Hour in Inches. 0-000105 0-000045 0-000096 0-000056 0-00045 0-00056 0-00045 to 0-00049 0-00056 to 0-00112 Total Copper Deposited per Hour. cc Cq (MCO q 00 Number of Vats in Series. 0 0 0 0 00 0 0 § ^ ^ ^ 0 0 Total Number of Vats. 000000000 Type of Dynamo. Gramme Do. No. 1 >> Wilde Cliamberlain surfaces being united by pressure. A very low E.M.F. is used, and it is found that to join two 1 inch bars end to end would require the expenditure of about 22 H.P. for one minute. (2) In a process of which Burton's liquid forge and Lagrange and Hoho's systems are types, the metal to be heated is connected with the negative pole of the generator (which must give a current of over 110 volts) and is then plunged into a lead-lined vat containing dilute sulphuric acid, or of a suitable saline solution {e.g.^ sodium carbonate), the lead being connected with the positive pole. The result is that a very powerful current passes through the bath from the lead which acts as anode, to the immersed object on which hydrogen is generated in large quantities. The resistance at the surface, in part covered as it is with hydrogen, becomes so great that the metal is very rapidly raised to a welding heat ; it may then be removed and welded on the anvil in the usual way. In Burton's process the piece is only in part immersed, so that the evolved hydrogen burns and contributes to the heating of the bar. It is obvious that these processes, rather than Thomson's, are the equivalent of the forge, and they may be employed for heating metal otherwise than for welding — for example, for bending or for annealing. But it is questionable whether there are many cases in which they would prove more economical than ordinary forge- heating. It must be remembered, however, that the metal is perfectly clean, being protected from oxidation by the hydrogen in contact with it whilst in the bath. (3) That in which the electric arc is used. This may be done^ as in the Benardos process, by placing together the surfaces to be welded and connecting both with the positive pole of a generator giving a current of 110-120 volts, and from 200 to 300 amperes- or more (200 amperes for joining metal \ inch thick), while the negative pole is connected to a rod of carbon which may be held by insulated tongs, and brought into contact with the surfaces.. The moment contact is made the current passes, and the carbon is withdrawn, so that an arc is formed, which is commonly 2 or 21 inches in length. The metal in the immediate neighbourhood 320 EXTRACTION AND REFINING PROCESSES. of the arc is rapidly melted, and the surfaces are thus burned together (as in the autogenous soldering of lead), the carbon is then shifted along the joint until the next part is similarly united, and so on, the metal being hammered where possible, in order to improve its quality, but in any case some hammer- ing is given by means of a ' former ' ' adapted to the joint in hand. In the Zerener process, an ingenious adaptation of the deflection of the arc by magnetic action is used : two carbons form the electrodes, and a very powerful electro-magnet, actuated by the current used in welding, is placed with its two poles so arranged that they would be on either side of the arc that forms between the carbons. The result is that the arc- is deflected downwards, and may thus be made to play upon the surfaces to be joined. The whole arrangement forms a kind of electric blow- pipe. A current of 60-65 volts pressure and from 3 to 250 amperes may be used, according to the character of the work. Electric Annealing or Softening. — In the use of hardened steel plate it is frequently necessary to make additional rivet holes, which cannot ordinarily be done without softening the plate and re-hardening. The Thomson Welding Company of America have adapted an electrical heating process for drawing the temper of such plates locally. This is accomplished by resting a coupler of copper bars on the same surface of the plate on either side of the part to be softened, and passing through them (and so through the stretch of plate between them) a current of about 3500 amperes at 4 volts. Gradually the current is increased to 6000 amperes. The steel is thus rapidly raised to a dull red heat (sufficient for the purpose), and the current is then gradually re- duced again, so that the steel may cool very slowly until it is below about 1100° F., or else it may chill by contact with the mass of cold metal around it, and so become hardened again. CHAPTER XVI. THE RECOVERY OF CERTAIN METALS FROM THEIR SOLUTIONS OR FROM WASTE SUBSTANCES. When a solution has become spent, and is too highly charged with impurities to be any longer serviceable for electrolytic work, it is desirable so to treat the liquid that the valuable metal is recovered in a form suitable for re-application to the plating work. Such methods as are applicable to the more usual solutions are briefly sketched in this chapter; but it must be understood that they are merely general methods, and modifica- tions of baths by the addition of fresh substances may render recovery by these means incomplete or perhaps impossible ; and m many cases the metal may not be entirely regained, even under ordinary circumstances, the amount that is left depending upon the solubility of the precipitated compound in the solution from which it is to be separated. Cobalt. The solution should be boiled down mitil fairly concentrated ; if acid (reddens blue litmus-paper), sodium carbonate should be added until it is neutral, or nearly so, but it must still remain clear. The addition now of finely-divided barium carbonate, stirred up in water, will cause the separation of all the iron as per- oxide; this is allowed to subside, and the clear liquid containing nearly the whole of the cobalt is poured ofi*. This liquid must be neutral or slightly acidified with acetic acid, and to it must be added an excess of strong solution of potassium nitrite mixed with sufficient acetic acid to prevent it from turning red litmus- paper blue; the whole should be left in a warm place for one or two days. The precipitated double nitrite of cobalt and potassium is then separated from the supernatant liquid, and dissolved in hydrochloric acid; the addition of caustic alkah (potash or soda, not ammonia) to this solution brings down the 322 EECOVEEY OF METALS FROM THEIR SOLUTIONS. cobalt practically pure as hydrated oxide ^vhlcll may be filteied off and dissolved in any acid, the cobalt salt of which it s desired to use. From a simple solution of cobalt, free from uon and other metals, the pure cobalt may be at once precipitated as hydrated oxide by caustic alkali; but the solution must be boiled, if necessary, until all odour of ammonia has vanished, as the presence of this body interferes with complete precipitation. Copper. From acid solutions, fragments of metallic iron suspended in the solution will precipitate all the copper in the metallic state by simple exchange. Subsequently the copper may be dissolved in warm dilate sulphuric acid containing a small proportion of nitric acid, more of the latter being added by degrees as the action is observed to become less; and m this way it will become converted into copper sulphate, which may be afterwards separated in the form of pure crystals by slow evaporation The p/ecipitation by iron may be effected by single-cell deposition as expired in an earlier chapter, by connecting ^^^^ fragments of iron contained in sulphuric acid within a porous cell, with copper strips placed in the copper solution. This will cause the precipi- tated metal to deposit upon the copper instead of_ upon the iron so that there is no risk of introducing impurities into the spongy copper. . Gold. From the cyanide solution the gold may be recovered as follo^vs :-Hydrochloric acid is first added in excess, until no further precipitate of gold cyanide is produced ; the precipitate s allowed to subside, and is washed twice by pouring water upon it allowing it to settle, and then pouring away and renewing the water again; then, after filtering, it is dried -d fused with an excess of dry sodium carbonate m a clay crucible But this process is not to be recommended on account of the intense y ^oSnous nature of the hydrocyanic acid gas so evolved during the first treatment with hydrochloric acid but if t be adopted,"this portion of P--- be conduc^^^^^^^^^^ special draught-cupboard, or m the open air with the operator well to the windward of the vessel. Bbttger's method is preferable: he evaporates the whole solu- tion to^ryness in a porcelain or enamelled dish, placed over a RECOVERY OF LEAD AND MERCURY. 323 saucepan containing boiling water. The residue is then to be crushed in a mortar and mixed with an equal weight of lead oxide (litharge) and a thirtieth part of its weight of charcoal powder introduced into a fire-clay crucible and heated to bright redness in a small pot-furnace ; a portion of the lead is reduced to the metallic state and with it will be alloyed all the gold contained in the charge. This alloy is then boiled with nitric acid, in which the lead will dissolve together with any silver or copper that may be present ; the gold is left in a finely divided condition, and after washing may be fused into a single homo- geneous mass, or it may be re-dissolved at once to form a fresh gold-bath. A plate of zinc attached by wire to one of gold, and placed in the original gold-bath, gradually deposits the precious metal upon the gold strip, forming, in fact, a single-cell arrangement, which may be used for recovering the gold directly from the spent solution ; but the operation occupies a long time, and other metals are liable to be precipitated subsequently. These latter, however, may be partly separated by boihng with nitric acid in which gold is insoluble (the separation can never be absolute, because portions of them are locked up within the gold so as to be out of the reach of the acid solvent). Lead. From the solution of the oxide in potash, insoluble lead carbonate is produced by bubbling carbonic acid gas through it ; this gas is evolved by the action of hydrochloric (muriatic) acid upon chalk or marble contained in a separate vessel which must be closed with a cork through which a tube is passed to conduct the gas to the required spot. The addition of acetic acid to the liquid until it is nearly neutral, followed by the introduction of sodium bicarbonate, will produce the same effect. From the lead acetate solution, sodium carbonate precipitates lead car- bonate. This latter substance, however produced, may, after washing, be dissolved in acetic acid, or in any other solvent suited to the particular form of bath which is to be prepared. Mercury. This metal is used largely in amalgamating the zinc plates of the galvanic battery, from the residue of which it may sub- sequently be recovered by distillation. To obtain it pure, it EECOTEEY OF METALS FROM THEIR SOLCTIOXS. Should be distilled in vacuo, or at least under ^f^^'^f J^^^^f^^ but to recover it fairly clean, m a conclition suitable for applica- tion once more to amalgamation, tbe tragments ot zmc may be XducedTnto an ii-on retort, A (fig. 105), with a tube passmg lk-tShrthrou.h one neck of a WovilfFe^s bottle, B, ^to water : the other neck of the bottle is connected with a second tube dso d^p^L beneath water in a second vessel, ^, as a ^teguard against the escape of mercury. On applymg heat the retou bv a suitable fii-e the mercury slowly vaporises, and di^tiUm^ Fig. 105.— Mercury distillation apparatus. over shotdd be entirelv condensed in B, any escapmg_ this being cdlected in d. The i^sidue in the retort stiU contams a lit le r^ercaU together with the bulk of the lead and copper origmally piesent in the whole zinc plate, for being more electro-negative San the zinc, the latter dissolves in preference, and leaves the other metals to accumtilate by the amalgamatmg effect of the inercurT. Nickel. From solutions of the double sulphate of_ nickel and ammonia the <^reen double salt itself is precipitated m a poetically pure condition in the form of minute crystals by the addition of succes- sive quantities of a s^iturated solution of ammonium sulphate, mtiir standing, and after complete subsidence of the precipi- tate the solution is quite decolorised. Advantage ,s thu. taken o the insolubility of the double salt in strong solutions of am- monium sulphate, a fact which was first observed by L nwin The cTstals have onlv to be filtered, drained free from the liqvud IlS^ n^ to them, and washed once or twice with a strong soki- Sin of^lmmonium sulphate, and they are ready to form a ne^ bath The original liquid may with advantage be boiled down to half its bu]k,^or even less, before applymg this treatment. EECOVERY OF PLATINUM AND SILVER. 325 Platinum. By adding to the bath an excess of a ferrous sulphate solution, and then a caustic alkali, the resulting dark-green coloured pre- cipitate of hydrated ferrous oxide (FeO) will reduce the plati- num to the metallic state, and itself become converted to a pro- portionate extent into ferric oxide (Fe203). This precipitation should be effected in a covered or corked flask, which should then be allowed to stand for some hours in a warm place, with occa- sional agitation. On the addition subsequently of an excess of hydrochloric acid, the iron oxides will re-dissolve, but the platinum powder precipitated with them will remain untouched. This finely-divided platinum may then be filtered, washed, and re-dissolved in aqua regia for further use. Silver. As in the case of gold solutions, the addition of hydrochloric acid precipitates silver cyanide ; but the use of an excess of the acid effects its further conversion into silver chloride. If this precipitate be red in colour the presence of copper cyanide is in- dicated, which may, however, be effectually removed by boiling it with moderately concentrated hydrochloric acid. The washed silver chloride should then be dried, mixed with soda ash or dried sodium carbonate, and fused in a clay crucible. The acidifying process has the same objections on the score of danger to health, and, therefore, requires the same precautions as the similar method above described for the treatment of gold solutions. A second method, allied to the dry process for recovering gold, is also applicable to silver residues. The solution is evaporated to dryness, and the residue is transferred to a crucible, l)ut alone, not mixed with litharge, and fused at a bright red heat. If sufficiently heated, the silver compound, mixed with excess of potassium cyanide and carbonate, will be broken up, and the liquid silver will melt and sink through the fluid slag to the bottom of the pot. When effervescence has ceased, the crucible is removed from the fire, and its contents poured into a hemi- spherical cast-iron ingot-mould, from which the solidified mass is detached when cold, and broken by a light blow with a hammer, to separate the metal from the slag. The crucible may be used again and again, provided that an examination after each opera- tion reveals no sign of a crack. The fluid contents of the pot 326 KECOVERY OF METALS FROM THEIR SOLUTIONS. may, of course, be allowed to cool in situ, by setting the crucible aside on a level surface ; but to extract the silver button in this case involves the loss of the pot by fracture. If the heat is insufficient, the silver will remain as a grey spongy mass, from which the slag may be removed by boiling with water containing a small proportion of hydrochloric acid. The silver should be practically 'fine'; but to ensure the perfect purity of the product, it may be re-fused with carbonate of soda mixed with about one-tenth of its volume of nitre, which will attack the base metals (but not the silver), and cause them to pass into the slag ; it will not affect gold or platinum, which are scarcely likely, however, to be present. If small quantities of these metals should by any chance become mixed with the silver, the button must be boiled with pure nitric acid (free from hydrochloric acid), which dissolves silver (and even platinum when present in small proportion with much silver), but leaves the gold as a dark-brown or black powder. But if more than 40 per cent, of gold be present, a certain proportion of silver (increasing with the percentage of gold) will be left with the latter. The silver solution is then filtered from the residue of gold, and is mixed with an excess of hydrochloric acid. In this way the insoluble silver chloride is formed, which must be filtered, washed, dried, mixed with an equal bulk of sodium carbonate and a httle charcoal, and fused at a red heat. Pure silver Avill thus result, the gold having been left undissolved, while the platinum which had passed into the solution would not be precipitated by the hydrochloric acid, and must, therefore, remain to be extracted from the hquid filtered from the silver chloride precipitate. CHAPTER XVII. THE DETERMINATION OF THE PROPORTION OF METAL IN CERTAIN DEPOSITING SOLUTIONS. In this chapter it is proposed to give outlines of methods by which the depositing solutions of the more important metals in electro-metallurgical use may be assayed, to give at least an approximate idea of the quantities contained. For a full ex- planation of this subject, or for methods of dealing with compli- Fig. 106 —Balance. cated analytical problems, works on quantitative analysis must be consulted, as it is impossible and undesirable to treat of so wide a subject in detail within the limits of this book. For this purpose, a fairly delicate balance is essential. It should be enclosed within a glass case, as in fig. 106, to protect it from dust and corrosive fumes, and should be capable of indi- 328 ASSAY OF DEPOSITING SOLUTIONS. eating a weight of of a grain. For eleetrolytie analysis, a platinum dish, about three-quarters of an inch to an inch in height, and about 3 inches in diameter, forms a convenient cathode, at once holding the solution and receiving the deposited metal. The anode consists of a circular plate of stout platinum foil about 2 J inches in diameter, with several perforations to allow gas to escape from beneath it. The platinum sheet is fastened horizontally without solder to the end of a vertical platinum wire attached to the positive pole of the battery, the platinum dish making contact externally with a copper wire attached to the negative pole. Instead of this, a cylinder of platinum foil may be used as cathode, being suspended, with its main axis vertical, within a small beaker, the anode consisting of a coil of platinum wire placed within the cathode. The object of the electrolytic method is to continue the action of the current until every trace of the required metal is precipitated on the platinum cathode ; and, as the latter should have been weighed previously, the increase of weight shown after the deposition gives the number of grains of the metal in the quantity of solution taken. The platinum dish should be lightly covered with the two halves of a broken clock glass 4 inches in width — that no splashings be lost ; these glasses should be rinsed into the solution about a couple of hours before stopping the current. After deposition, the metal must be washed well with water, then rinsed with alcohol, and dried rapidly by heating to the temperature of boiling water. Antimony. For the sulphide solution, measure out very accurately half a fluid ounce of the liquid, transfer it to the weighed platinum dish ; dilute it with from 1 to 2 ounces of water ; introduce the platinum anode, and pass a current from two Bunsen-cells through the liquid, until a drop of the solution, removed by a glass rod and placed upon a watch-glass, gives no yellow-coloured precipitate (but only white), when mixed with a few drops of hydrochloric acid. Electrolytic experiments of this nature may 1)6 conveniently started in the evening, and will be found to be finished on the following morning. When completed, the current is stopped, the anode is removed, the contents of the dish poured into a beaker ; the dish rinsed four or five times with pure water, then with alcohol, and finally with a, fevv drops of ether ; it is, lastly, deposited in a warm place for a few ASSAY OF COBALT. 329 minutes until the ether has quite evaporated, and is then re-weighed. The increase in weight shows the number of grains of metalHc antimony in half-an-ounce of the original solution. For the tartar emetic solutions, half-an-ounce should be taken and mixed with 1 oimce of yellow ammonium sulphide, and a like volume of water. It is then electrolysed as before. Cobalt. This is a difficult assay for untrained hands — and, indeed, even the simplest methods of dealing with any metal can scarcely be expected to give more than approximate results, unless the operator has had some previous training. Probably the best method will be to measure half-an-ounce of the solution and proceed to obtain the cobalt as the cobalt-potassium nitrite, after the manner described in the last chapter (p. 321). The experi- ment must, of course, be conducted with great care ; the solutions should be maintained fairly concentrated, and a considerable excess of the potassium nitrite must be used for the precipitation, because the precipitate is less soluble in a liquid containing this salt than in pure water, so that the time required for it to rest in a warm place is shortened — usually one night suffices. The precipitate may then be treated as described, and the second precipitate of hydrated cobalt oxide may be filtered through blotting-paper (see p. 56), washed well, dried on the paper, brushed off from the filter, by means of a camel's-hair brush, into a small weighed porcelain crucible (about an inch in height), heated to dull redness in a smokeless Bunsen-burner or spirit lamp, cooled and weighed in the crucible. The excess of weight over that of the clean crucible gives the weight of the precipitate which is an oxide of cobalt having the formula C03O4. Multi- plying this weight by 0*73 gives the weight of metallic cobalt in the half fluid-ounce of solution taken. If preferred, the nitrite precipitate may be dissolved in a little hydrochloric acid, the solution rendered neutral with ammonia, and a good excess of ammonium oxalate added to produce a clear solution of the double cobalt-ammonium oxalate, which may then be electrolysed in the platinum dish, and the deposited cobalt washed, dried, and weighed as above described for antimony. The solution should be kept warm during electrolysis by resting the dish upon a sauce-pan of water heated over a gentle flame. The electrolysis must be continued until a drop of the solution gives no black precipitate, or even brown colour, when mixed with a drop of ammonium sulphide in a watch-glass. 330 ASSAY OF DEPOSITING SOLUTIONS. Copper. The simple acid copper solution lends itself so readily to the electrolytic assay, that this would certainly seem to be the most suitable. It is possible to separate every trace of copper from such a solution, so that the method may be made to give absolutely accurate results. The cyanide solution should be acidified with sulphuric acid and boiled until there is no longer any smell of prussic acid, resembling that of bitter almonds. Both these operations must be conducted cautiously and in a well-ventilated place, on account of the poisonous character of the evolved gas. The liquid is then ready for electrolysis. Half-an-ounce of either solution may be employed, and electro- lysis is continued until the liquid is decolorised, and a drop removed from it strikes no blue colour with an excess of ammonia. The excess-weight of the platinum dish is that of metallic copper in the volume of solution taken. Gold. One fluid-oimce of the solution is boiled with an excess of hydrochloric acid in a well-ventilated spot until there is no further smell of hydrocyanic acid ; an excess of a clear solution of ferrous sulphate is then added, and the mixture is allowed to stand all night in a warm place. The precipitated gold powder is then filtered from the solution, which should now be quite free from precious metal. It is washed many times on the filter with pure water, and, after drying, is placed with the filter-paper in a small weighed porcelain crucible, and heated over a spirit- lamp or non-luminous gas-flame, until the paper is completely burnt to a white ash. The crucible is re- weighed with its con- tents, on cooling, and thus the weight of metallic gold in one fluid-ounce of the liquid is determined. The precipitate is pure gold, so that no further calculation is necessary. Lead. Add to half-an-ounce of the solution 3 ounces of pure water, an excess of sulphuric acid (until a further addition no longer pro- duces a white precipitate) and an equal volume of spirits of wine. Allow the mixture to stand for a few hours ; filter ofi" the white lead sulphate precipitate, and wash the precipitate on the filter ASSAY OF NICKEL, PLATINUM, AND SILVER. 331 several times with a mixture of equal volumes of spirits of wine and water. This washing must be very thorough, or the trace of residual acid left in the precipitate will char or weaken the paper when it is dried. A drop from the last washing but one should not redden blue litmus-paper. The paper and contents are dried, the heavy white powder is brushed into a weighed porcelain crucible, heated over a non-luminous flame, cooled and re-weighed. The increase of weight multiplied by 0*683 gives the weight of metallic lead in the sample taken. Nickel. To half-an-oimce of the solution add a fair excess of ammonium sulphate, together with about 4 ounces of water, and then oxalic acid. If this should produce a precipitate, more ammonium sul- phate must be added until the solution is clear. Excess of am- monia is now introduced, any precipitate is filtered off and washed with water containing ammonia, the washings being added to the filtrate. The filtered solution and washings are then mixed with a further small portion of ammonium oxalate, evaporated to con- venient bulk in a porcelain dish, and electrolysed for metallic nickel as above described. Platinum. To half-an-ounce of the solution add an excess of a ferrous sul- phate solution, and then excess of caustic potash. The mixture is kept in a warm place for an hour or two with occasional stir- ring; an excess of hydrochloric acid is now added, so that the liquid reddens blue litmus-paper, and the precipitated metallic platinum, which remains undissolved, is filtered and burned with the paper after the manner described under the heading of Gold in this chapter. The final precipitate should be pure metallic platinum. Silver. The solutions may be treated electrolytically, using the cur- rent from one Bunsen-cell only, as a stronger current tends to deposit pulverulent or flaky metal which peels ofl" during the pro- cess of deposition. The silver is, of course, deposited and weighed in the metallic state. CHAPTER XVIIL POWER REQUIRED FOR ELECTROLYTIC WORK. Calculation of Power required for Electrolysis.— It has been shown in Chapter II. that the power expressed in watts absorbed in electrical work is found by multiplying the quantity of current used, expressed in amperes by its pressure in volts. Hence the power absorbed in any electrolytic vat may be readily deter- mined, provided the current conditions are known. Thus, suppos- ing a P.D. of 0-3 volt is necessary to force an electric current through a certain copper electrotype bath, with a current density of 10 amperes per sq. ft. of cathode surface, an expenditure of 10 X 0-3 or 3 watts will be required per sq. ft. of surface, and if the total area of the cathode were 249 sq. ft., there would be needed 3 x 249 or 747 watts, which is practically 1 H.P. In other words, in such a copper bath as that described, 1 H.P. will be absorbed in the vat itself in the deposition of ten times the electro-chemical equivalent of copper per second on each of the 249 sq. ft. of cathode surfaces used. So that , , , P.D. X CD. H.P. required per unit area of cathode surtace = where P.D. is the potential difference expressed in volts between the anode and cathode, and CD. is the number of amperes em- ployed per unit arep. of cathode. Using this formula for an alkaline copper bath, in which a P.D. of 4 volts is driving a current of 0*025 amperes per sq. in. of cathode surface through the bath, 4 X 0'025 the H. P. required per sq. in. of surface would be — ; but, as the square inch is a very small unit for calculations of this magnitude, the equivalent in amperes per sq. ft. may be more conveniently taken (see Table XXIX., p. 400) ; the H.P. required per sq. ft. of surface would then be - ^ = , BOARD OF TRADE UNITS REQUIRED FOR ELECTROLYSIS. 333 which is approximately 0*02 H.P. ; but as the H.P. required per sq. ft. of the acid copper bath is H.P. ( = 0*004 H.P.), it is evident that the alkaUne bath absorbs almost exactly five times as much power as the acid bath."^ In the same way it may be shown that in nickel baths using a P.D. of 2 volts and a CD. of 3 amperes per sq. ft., the H.P. required per sq. ft. of cathode is ^ ^ ^ r= A ^ 746 746' or about 0.008 H.P., which is approximately double that used in the acid copper bath. In silver baths with a P.D. of 0-6 volt and a CD. of 3 amperes per sq. ft., the H.P. per sq. ft. of cathode is ^'%p ^ = or about 0*0024 H.P. 746 746 In all these calculations only the actual power required for the bath has been taken into account. The power required from the generator will be greater, inasmuch as the loss of power in over- coming the resistance of the copper cables and wires used to con- duct the current to the baths and that of the various joints and electrical connections in the circuit have been neglected. But with a good arrangement of the plant, so that stout copper leads are used and the baths are in close proximity to the generators, and all joints and connections are thoroughly clean, this loss may be practically negligible. Where, however, a dynamo is used, the conversion of mechanical into electrical energy absorbs an amount of power varying with the size and construction of the dynamo. As a rule the efficiency of the small dynamos used in electro- plating is less than that of the larger machines employed in the heavier work of smelting or refining. If the efficiency be taken in round numbers at 85 per cent., it is obvious that the H.P. re- (^uired from the engine driving the dynamo will have to be in excess of that required for the vat itself (neglecting resistance of leads) in the proportion of 100 : 85, or, in other words, the H.P. of the engine will have to be 1^ times as great as that found by the calculations indicated above. Board of Trade Units required for Electrolysis.— Electricity * It is obviously unnecessary to work out these calculations to obtain simply a comparison of the power required to work different baths : it suffices to compare the respective products of P.D. xC.D. Thus the H.P. required for the acid is to that for the alkaline bath in the ratio 0*3 x 10 • 4x3-7 = 3 : U-8. 334 POWER REQUIEED FOR ELECTROLYTIC WORK. is charged for by supply companies by the Board of Trade unit, which, as already stated, is one kilowatt-hour or a current of 1000 watts passing for one hour. Since the horse-power hour is equal to 746 watt-hours, it is evident that the number of Board of Trade units required per hour for any given work will be approximately times the horse-power hours required for the same work; and the calculation may be made by the formulse given above, if 1000 be substituted for 746 in the denominator of the fraction. The watt, how^ever, is the unit of activity, and expresses the rate of doing electrical work, just as the horse-power is the mechanical unit of activity, and it is easy to eliminate, if required, the question of time. The term ' horse-power ' conveys the idea that work is being done at a rate equivalent to the raising of 550 lbs. through the height of 1 foot in each second of time during which the power is applied; or, of course, to the lifting of 1 lb. through 550 ft. in each second. Hence a 'horse-power-hour' means that an amount of work has been done equivalent to 550 lbs. being raised through the space of one foot during every second for an hour; i.e., since there are 3600 seconds in an hour, to 550 lbs. being lifted through 3600 feet, or 550 x 3600 lbs. being lifted through 1 foot during that interval of time. Hence 550 x 3600 ft. lbs. represents the total energy equivalent to 1 H. P. -hour. Now a current of 1 ampere is one in which 1 coulomb of elec- tricity passes every second. Hence a watt is used to describe a current in which 1 coulomb of electricity flows per second under a pressure of 1 volt, or one in which 2 coulombs fiow^ per second at a pressure of ^ volt, or 3 coulombs at J volt, ^ coulomb at 4 volts, or any number of coulombs at such a pressure that the number of coulombs per second multiplied by the number of volts is unity. The term ' kilowatt-hour ' expresses the idea that a current of 1000 amperes, i.e., of 1000 coulombs per second, has been flow^ing for an hour under a pressure of 1 volt, so that it is equivalent to 1000 x 3600 coulombs of electricity at a pressure of 1 volt, or 3,600,000 volt-coulombs. It may there- fore be 1,800,000 coulombs at 2 volts, 7,200,000 coulombs at | volt, 3600 coulombs at 1000 volts, or any other number of coulombs, provided that the product of coulombs x volts = 3,600,000. From this the quantity of any metal that may be deposited by a Board of Trade unit of electricity may be found as follows : — Weight in grammes of any metal deposited by B. o. T. unit = ^Q.- ^ 3^,600,000^ where Eq. is the electro-chemical equivalent in grammes and P.D. is the voltage between the electrodes of the vat. BOARD OF TRADE UNITS REQUIRED FOR ELECTROLYSIS. 335 Since the last column but two in Table XXVIII. (p. 399) shows the weight in grains of each element deposited by a current of 1 ampere in an hour, the figures there given may be utilised for the purpose of calculation to obtain the required result in pounds in- stead of in grammes. These numbers (which we will term Dep.) are found by converting the electro-chemical equivalent from grammes into grains and multiplying the product by 3600 ; hence Dep. is the equivalent of Eq. x 3600 in the above formula, and so weight in grains of any metal deposited by B. o. T. unit = ^^V-^x^^OO ^ ^j. weight in lbs. deposited by B. o. T. unit = Dep. x 1000 ^ Dep. P.D X 7000 P.D. X 7' Thus, in the case of the acid copper bath, where a voltage of 0*3 is used, lbs. of Cu deposited from the acid bath by 1 B. o. T. unit — = 8*7 ^ 0-3x7 With the alkaline bath worked at a pressure of 4 volts, and remembering that the copper is in the cuprous condition, so that its electro-chemical equivalent is double that of the cupric copper in the acid bath, lbs. of Cu deposited from^the alkaline bath by 1 B. o. T. unit = Ml^^ = 1'3 4x7 From this it is seen that a given amount of electrical energy will deposit ^ ( = 6*7) times as much copper from the acid bath as from the alkaline bath. That this must be so is obvious from the fact that although the E.M.F.'s apphed respectively in the two cases are in the ratio of 0*3 : 4 (or 1 : 13*3), yet a given volume of current deposits twice as much copper from the alkaline as it does from the acid bath, so that the ratio of the energies required is 13*3 1 : , or nearly 1 : 6*7. At first sight, however, this may seem to be in contradiction to the results of the calculations at the beginning of the chapter, by which it was shown that the power absorbed per sq. ft. of cathode surface in the alkaline bath was 5 (not 6*7) times that required in the acid bath. But the discrepancy is apparent, not real. It must be remembered that the earlier numbers (ratio 5:1) represent power absorbed per unit of electrode area, while the later (ratio 6*7 : 1) represent energy required per unit of weight of metal deposited, and the rate of 336 POWER REQUIRED FOR ELECTROLYTIC WORK. deposition in the acid bath was taken as 10 amp. per sq. ft., whilst in the alkahne bath it was only 3-7 amp. per sq. ft. Making- allowance for the fact that in the latter case each ampere deposits twice as much copper as in the former case, the ratio of copper deposited in the two baths is 10 : 7*4 (not 10 : 3*7). That this is correct may be seen from the fact that 'the ratio 10 : 7*4 is practi- cally the same as the ratio 6*7 : 5. It must not be forgotten that in these latter calculations, as in the former, the loss of energy in the dynamo or in the motor- dynamo and on the leads has been neglected ; and due allowance must be made, as before, for these losses in estimating the gross current that would be required. It should also be noted that, on account of subsidiary reactions, the amount of metal deposited by a given quantity of current is never quite in accordance with theory, although, under favourable conditions, it should be nearly so. Cost of Electricity. — The actual cost of electricity varies enormously, according to the system of generation and distribution. The battery is one of the most costly methods of producing electricity, and by estimating the consumption of material in the battery and calculating the cost there is no difficulty in determining the expenditure required per unit. Professor Ayrton, in his Practical Electricity, has worked this out for several cells, takmg the lowest cost of materials, at wholesale rates, and excluding prime cost of battery or expenditure on renewal of porous cells or other relatively durable parts. The numbers are as follows, the value of the copper deposited in the battery whilst in use being deducted in the case of the Daniell cell v — Cost of 1 Board of Trade Unit (Kilowatt-hour). Using Daniell cells, ...... lid. Grove cells, ...... Is. Potassium bichromate cells, . . . Is. 3d. Leclanche cells, ..... Is. 5d. When the materials are bought at retail prices, the cost per unit of electricity delivered at the terminals of the battery would range from Is. 6d. to 2s., or even higher. The cost per unit of public electricity supply at the consumer's premises varies in different towns, and in some towns according to the quantity required and the purpose to which it is applied. Under favourable circumstances it may be as low as 2d. per unit (or less), in other cases it may be 6d. per unit (or more) j but in the worst case it is far less costly than battery-current. It must be remembered that a considerable addition is made to the cost by COST OF ELECTRICITY. 337 the necessity to convert the current at the town pressure to that at which it is to be used, using a combination of a small motor and dynamo with, let us say, an efficiency of 80 per cent, each ; the combined efficiency is only 64 per cent., and the cost of the current as supplied must be multiplied by say (roughly) 1*5, in order to find the cost per kilowatt-hour as ready for distribution to the vats. In public electricity supply the cost of distribution (that is, the cost of the mains and of their upkeep and the loss of energy by resistance) forms a very large item in the consumer's bill ; more- over, at present, while the major portion of the current generated is still used for lighting purposes, the cost of generation is enormously enhanced by the necessity to put down a plant sufficiently large to cope with the maximum demand for current in the early hours of the evening, when light is most wanted, much of the plant standing idle for the remainder of the day. If the load could be equalised, so that it should be practically con- stant at all hours of the day, a much smaller plant could be made to yield the same total output. Hence in many districts the day charge for power is less than the night charge for lighting purposes. In large electrical works, where current is constantly required (night and day) — as, for example, in copper refineries — the condi- tions are much more favourable. In large establishments the gen- erating plant may be larger than that of a ' central station,' so that the greater economy of large plant is utilised to the full, and at the same time, a steady current is in continuous use, and the cost of distribution is minimised, because the generating station is close to the place where the current is to be used. Under such circum- stances it is probable that the cost per unit need not exceed |d. (or in some cases ^d.) where coal is cheap and good. It has been calculated that with continuous (night and day) running, where coal is about 4s. or 5s. a ton, the cost per H.P.-year need not, with the best possible plant, exceed £10, which is equal to 0'27d. per H.P. per hour, or 0*36d. per kilowatt-hour. In- America steam power has been produced with coal at 6s. per ton, and with large engines of over 1000 H.P., at a cost of between £10 and £11 per H.P.-year; and with a plant one quarter of this size, and with coal at 12s. 6d. per ton, at a cost of less than £15 per H.P.-year, or 0'4d. per H.P.-hour, or 0'53d. per kilowatt-hour. The cost per unit of electricity will, of course, be greater than this by from say 10 to 20 per cent., according to the efficiency of the dynamo. In electric lighting supply stations, where large plant is used, Y 338 POWER REQUIRED FOR ELECTROLYTIC WORK. but where, from the irregularity of the load, the conditions are unfavourable to economical working, the cost of generation alone per unit has in a few cases been reduced to a little over Id. ; in the majority of undertakings,"^ however, in England the total cost per unit, including all charges for generation but none for distribution, ranges from 2d. to 4d. per unit. The cost of production by water-power is very variable, owing to the different conditions under which the power is utilised. It is, of course, impossible to generalise, but it may be stated that in the Niagara district power can be obtained at a cost of about £3 or less per H.P.-year, or at less than half the cost of steam power, and at Schaffhausen on the Ehine, it is also said to be as low as £2, 10s. per H.P. per annum. Absorption of Power in Conductors. — In order to ascertain the actual loss of power in conductors, and so to form a true estimate of the waste due to the employment of unnecessarily long or thin leads, it is only necessary to know the resistance of the conductor and the volume of current flowing. When a current flows through a circuit, it is well known that the loss of pressure in any section due to overcoming electrical resistance is proportional to that resistance, so that the amount of power absorbed in any part of the circuit may be found (as in the case of the electrolytic vats described earlier in the chapter) by multiplying the current volume in amperes by the potential diflerence at the two ends of the section under examination. So if the difference of potential between the two ends of a wire (say between one terminal of a dynamo and the corresponding terminal of the vat) be known, the loss of power will be C x E, where C = amperes flowing, and E the potential difference in volts between the two ends of the wire. E But since, according to Ohm's law, C = --, and since, therefore, E = CE, E may be eliminated from the expression C x E, and the expression OR may be substituted for it. Thus the loss of power in watts is equal to C x OR or to C^R, where R is resistance in ohms, or, the loss in any rod or tvire is found by multiplying its resistance in ohms by the square of the current passing through it Thus if a current of 100 amperes be passed through 50 yards of No. 00 (Imperial standard wire gauge) copper wire, with an area of 0*095 sq. in. in cross section, and a resistance of ^ See Hammond, "The Cost of Generation and Distribution of Electrical Energy," Jour. Inst. Electrical Eng., 1898, vol. xxvii. p. 246. (Mr Hammond treats exclusively of the so-called Central Stations.) INFLUENCE OF SIZE OF CONDUCTOR ON LOSS OF POWER. 339 O-0128 ohm in all, the loss of power will be 1002x0-0128 = 128 ivatts, or almost exactly ^th of a horse-power. Again, with a current of 30 amperes passing through 50 yards of No. 8 (standard W.G.) copper wire with a total resistance of 0-06 ohm, the loss of power will be 30^ x 0*06 = 54 watts, or about 0-07 H.P. Influence of Size of Conductor on Absorption of Power. — It will be noticed that the loss of power in conductors varies as the square of the current strength. Independently, therefore, of the danger that would accrue through the overheating of a conductor if it be used for too large a volume of current, there is a great and increasing loss of power. Thus to take two concrete examples : — It has been shown in the last paragraph that when a current of 30 amperes is passed through 50 yards of No. 8 copper wire, the loss of power is 54 watts or 0*07 H.P. ; if 60 amperes were passed, the loss would not be 108 watts or 0*14 H.P., but 60^ x 0*06 ■or 216 watts or 0*28 H.P. ; and if 90 amperes were passed the loss would be, not 158 watts or 0*21 H.P., but 486 watts or 0*65 H.P. As has been mentioned above, the overloading of a wire with ■current means a rise of temperature, which if the current were in very great excess of normal might cause risk of fire, but even where there is no such danger to be apprehended, there is not only the greater loss of power already described, but there is an in- creased resistance and therefore further loss of power, because the conductivity of a wire when warm is inferior to that of the same wire when cold. A general rule at one time adopted was that the sectional area of copper conductors should be calculated for the current which they were to carry in the proportion of 1 sq. in. per 1000 amperes. The Institution of Electrical Engineers, in their revised rules for wiring for the supply of electrical energy, have recommended a formula by which the sectional area of copper wire or cable may be calculated, according to the current to be transmitted, so that the increase in temperature of the wire shall not exceed 20"" F., the conductors being insulated and laid in casing or tubing — bare wire being more free to radiate heat could, of course, take a somewhat higher current-density. The formula given by them as the result of careful experiment is — LogC = 0-82log A-l-0-415, or C = 2'6A0'82, where C = current in amperes and A = area in thousandths of a square inch. 340 POWEE EEQUIEED FOE ELECTEOLYTIC WOEK. Table XXXYIIa on page 407 shows the maximum current in am- peres allowable for wires or cables of different sizes, in Col. 2 when external temperatures are high (the formula used is in this case different, viz., C = 2A0"' and the rise of temperature allowed for only 10° F.); and in Col. 4 (using the formula given in the last paragraph) when the external temperature is normal. Col. 5 gives the total length of wire or conductor of the size specified in Col. 1, along which there will be a P.D. of 1 volt, when the maximum current allowed in Col. 4 is passing. From this the loss of power is readily foimd, because the number of amperes in Col. 4 multi- plied by 1 volt gives the number of watts converted into heat in overcoming the electrical resistance of the wire along the length named in Col. 5. Col. 3 gives the corresponding length for the same wire with the current given in Col. 2. To take two examples by w^ay of illustration : — (1) A No. 18 wire (standard wire gauge) may, under normal conditions of temperature, be allowed to take a current of 4*2 amperes, and with this current passing, the potential of the wire at any one point will be one volt higher than it is at a point 18 yards nearer the negative pole of the dynamo or battery j and in every 18 yards of the wire there will be a loss of 4*2 watts with the current of 4*2 amperes flowing. (2) A cable of 19 strands of No. 20 wire may carry a current at normal temperature of 30 amperes, and there will be a P.D. of 1 volt between any two points 26 yards apart, so that the loss of power will be 30 watts for every 26 yards of cable with the maximum ciuTent allowable. Influence of Material of Conductor on Absorption of Power. — It is obvious that, if the loss of electrical energy in a conductor when a constant current is flowing through it varies with the resistance of the conductor, the absorption of power in wires of the same sectional area but made of different metals will vary in inverse proportion to the conductivities of the metals used. Thus, for example, since the relative conductances (see p. 34) of copper and iron are approximately 100 : 16, an iron wire will 100 absorb -y^- or rather more than 6 times as much power as a copper wire of the same diameter and length ; and the current of 100 amperes which causes a loss of nearly |th H.P. in 50 yards of copper wire of No. 00 gauge (see pp. 338, 339) will cause a loss of about 1 H.P, in 50 yards of an iron wire of the same gauge. CHAPTER XIX. MODERN THEORIES OF ELECTROLYSIS. Modern Theories of Electrolysis. —Within the last few years much work has been done in the field of electro-chemistry, tend- ing to explain the phenomena of electrolysis. It would be im- possible, even if it were desirable, in the short space here avail- able to describe this work in detail ; and they who would study the subject exhaustively must, therefore, be referred to some of the larger text-books dealing with the theories of electro-chemistry. A short survey of one or two of the principal observations and theories is, however, here necessary. It should be observed that the theory of ionic dissociation is by no means universally accepted as the true explanation of the phenomena of electrolysis, and many eminent electro-chemists dis- pute the premises upon which it is based, while recognising that it is a valuable working hypothesis, and that it has done much to advance electro-chemical knowledge and its practical applications. It is, however, accepted by so large a number of thinkers, especially on the Continent, that the author has endeavoured to give a few broad outlines of the theory. Solution Pressure. — It is well known that many substances, such as gold and silver, are insoluble in water; whilst others, such as copper sulphate, silver nitrate, and potassium cyanide are readily soluble, and that the degrees of solubility of the different members of the latter class are very varied. Each soluble com- pound has its own constant degree of solubility under constant conditions— thus 37 parts of crystallised copper sulphate, and no more, are always soluble in 100 parts of pure water at 10° C. But the solubility of each substance is largely dependent on temperature. Now this phenomenon is comparable with certain phenomena connected with vaporisation. Water, at all temperatures, tends to evaporate, and the higher the temperature the greater is the tendency to do so. Nevertheless, at any one temperature the tendency is always constant and can be measured ; it is known as the vapour-pressure of water. The 'vapour pressure' of a 342 MODEEX THEOEIES OF ELECTEOLYSIS. liquid expresses, then, its tendency to pass into the form of rapour. The corresponding tendency of a soHd to pass into sokition, when immersed in a solvent such as water, is known as. its solution-pressure, and this solution-pressure is constant under any given set of conditions, but varies definitely ^-ith a definite change of the conditions. Osmotic Pressure.— It is well known that when a liquid is- placed in contact with air in a closed vessel, evaporation goes on steadily until the air becomes 'saturated'; thus, water intro- duced into the bottom of a dry bottle evaporates imtil the air in the bottle is saturated with water vapour, and then the evapora- tion ceases. As the vapom- passes into the air, it exerts a pres- siu^e in the atmosphere within the bottle, and the partial pressm-e of this vapour tends to prevent further evaporation, until at last the pressure of the accumulated vapour in the air is equal to the pressure or tendency of the water particles to vaporise, and equi- hbrium is attained. Similarly, when a soluble substance is placed in ^ (say) pure water, it passes, at first, rapidly into solution, owing to its solution pressure, but, as the dissolved molecules accmnulate in the water, they exert a back pressure, similar to the back pressm^e of the water vapour in air, and so gradually lessen the tendency of the sohd to dissolve, until at last the point of equihbriimi is reached, when the back pressure of the dissolved particles equals the solution-pressure of the substance. This back pressure is known as Osmotic Pressure, and is suscep- tible of accurate measurement. Just as the pressure of the particles of any gas, when mixed with another gas, causes the one to diftuse into the other, until they, are uniformly mixed, so the osmotic pressure of a dissolved substance causes difiusion until the mixture is miiform ; but in the case of solutions, owing to the greater density of the solvent, difiusion is vastly more slow than in the case of gases. ^ If a cylmder with a counterpoised piston (fig. 107), sliding air- tight within it, be so arranged that there is air in the space A beneath the piston, while the space B above it is rendered vacuous, the pressure of the air in A will cause the balanced piston to rise until it touches the top of the cylinder, but if weights are placed on the tray C, they will counteract the expansive pressure of the air, and the piston will be forced doTOward to some extent, the greater the weight on C the greater being the compression of the air in A. ^ Equilibrium will be attained only when the expansive pressure in A exactly equals the compressive pressure exerted by the weights on the piston, for the rising of the piston is due to the expansive force of the air in A. OSMOTIC PRESSURE. 343 If, instead of air, a solution, say of sugar in water, is placed in A, and B is left vacuous, there can be no expansion, because the liquid in A is incapable of expanding like a gas, yet the dissolved sugar has the same tendencij to expand— that is, to diffuse through a larger volume — as had the air in the former experiment. To show this tendency, it is only necessary to provide, in place of the vacuum, a substitute which will enable the sugar to diffuse as air expands into a vacuum. This can be effected by filling the space B with pure water and using a piston made of a porous material, through which water, but not dissolved sugar par- ticles, can pass freely. Water can now pass from A into B or vice versa, but the sugar, being unable to pass through o the piston, must all remain in A. The expansive force {i.e., the osmotic pressure) of the sugar in the solution in A now becomes effective (but, of course, very slowly), because water passes freely through the piston as the latter rises, and the pure w^ater passing into A from above plays the part of the vacuum through which the sugar particles can diffuse ; the diffusion w^ill be found to take place gradually until the material is evenly distributed through the liquid and the piston has risen to the top of the cylinder. By placing weights on C, the osmotic pres- sure of the dissolved sugar could be counterbalanced, and this pressure, at any position of the piston, i.e., at any degree of dilution, could be measured. Not exactly in this way, but by an analogous method, the osmotic pressures of various substances have been accurately determined. Speaking generally, and referring mainly to very dilute and therefore unsaturated solutions, it is worthy of note that such substances when in solution seem to obey the ordinary law^s of gases. It is even true that the actual pressure exerted by each molecule of the substance in the dilute solution is the same as if it were in the gaseous state diffused through a space equal to the volume of the solution. Further, Boyle's law is obeyed, for the c Fig. 107. 344 MODEKN THEOEIES OF ELECTROLYSIS. osmotic pressure exerted in a given volume of the solution is proportional to the number of molecules present. Thus, the osmotic pressure exerted by 20 grains of sugar in a cubic inch of water is just double that exerted by 10 grains in the same volume. So also is Avogadro's law applicable, for equal numbers of mole- cules of different substances exert equal osmotic pressures ; thus the pressure exerted by 500 molecules of sugar in a given volume of water would be the same as that caused by 500 molecules of alcohol, although the actual weights present in the two cases would be in the proportion of 342 : 46, which are the respective molecular weights of the two compounds. Electroljrtes. — The osmotic pressures exerted by certain substances do not, at first sight, obey the law last quoted. Solutions of organic substances in solvents such as benzene or ether, or of many soluble substances in water, behave generally in accordance with the law. But the whole class of substances known as electrolytes, when dissolved in water, behave abnormally. The osmotic pressures of their solutions are in excess of the normal. In very dilute solutions, the pressure exerted by such substances as sodium chloride (NaCl), hydrochloric acid (HCl), potassium nitrate (KNOo), and silver nitrate (AgNOg), give about twice the pressure that an equal number of molecules of (say) sugar or alcohol dissolved in the same volume of liquid would show, and it is remarkable that these substances are the salts or compounds of monovalent atoms. Similarly, such substances as copper chloride (CuClg) and potassium sulphate (K^SO^) give nearly three times the normal pressure ; and these are' the compounds of divalent metals or acids. It is supposed, then, that each molecule of the salt in the very dilute solution breaks up, in the case of those substances which exhibit twice the normal pressure, into two parts and in the case in which the pressure is thrice the normal into three parts, each of the dissociated parts then producing as high an osmotic pressure as the whole molecule would ^produce. Analogies for this are to be found— for example, the vapour density of heated ammonium chloride (NH^Cl) is only half the normal, owing to the fact that the vapour of the salt dissociates at high temperatures into HCl and NH3, and thus occupies twice the volume that would be required for the un- dissociated vapour at the same temperature. Ions.— The theory, then, assumes that sodium chloride, NaCl, when dissolved in a large volume of water, becomes broken up into the two sepai-ate parts Na and CI ; that hydrochloric acid dissociates into H and CI ; potassium nitrate into K and NO3, and silver nitrate into Ag and NO3, each molecule forming two separate ELECTKOLYTIC CONDUCTION. 345 simpler entities, which are known as Ions, and are recognisable as the materials that are deposited at the electrodes of a solution undergoing electrolysis. Again copper chloride (CuClg) breaks up into three parts (Cu + Cl + Cl), as does also potassium sulphate K SO (K + K + SO4). At first sight it seems contrary to all the teachings of chemistry that two such substances as Na (sodium) and CI (chlorine) could exist uncombined in the same aqueous solution, especially as sodium is known to decompose water with great violence, and chlorine is distinguished by a most unpleasant smell, that is entirely lacking in solutions of pure sodium chloride (common salt). But it is certam that even if they be diffused as ions through the liquid, they do not exist m the condition familiar to chemists. It is assumed that the 10ns carry heavy charges of electricity, those of metals carrying positive charges, and those of non-metals (chlorine and the like) or acid radicals— ^'.e., certain non-metallic groups of atoms (such as NO3 or SO4)— carrying negative charges. It is supposed that the charges of all monovalent ions are equal, and that the charge of every divalent ion is double, and that of each trivalent ion is three times that of a monovalent ion, and so on. Hence sodium chloride is assumed to dissociate into the sodium ion, Na, with a positive charge, and the chlorine ion, CI, with an exactly equal (but opposite) negative charge, the resulting electrical effect in the solution being nil, because the free ions are diffused equally through the solution, and the positive electricity of the one set exactly neutrahses the negative electricity of the other. ^ So also with silver nitrate, the positive charges of the silver 10ns, Ag, exactly neutralise the negative charges of the NO3 ions. In the case of copper chloride, the divalent copper ion, Cu, carries a double charge of positive electricity, which is neutralised by the two singly charged monovalent chlorine ions, (CI 4- 01) separated simultaneously with the Cu at the moment of dissociation. The actual weight of an ion must be exceedingly small, but cannot yet be ascertained. Since, however, its chemical formula or symbol is known, it is convenient in making calculations to think of it as having a gramme-equivalent weight ; the weight of a silver ion would thus be its atomic weight in grammes, viz., 108 grammes (nearly), and that of a NO3 ion would be (N = 14) + ^0^3^48) ==62 grammes. The actual electrical charge of such a monovalent ion is regarded as 96,540 coulombs, which is the exact quantity of electricity necessary to deposit precisely one equivalent weight of the ion in grammes, viz., 1 gramme of hydrogen or 108 grammes of silver. Electrolytic Conduction.— It is especially to be noted that 346 MODERN THEORIES OF ELECTROLYSIS. solutions of substances which do not show some sign of dissociation in solution are non-conductors of electricity. Water alone does- not dissociate, and is, therefore, when pure, practically a non-con- ductor j and the addition of sugar (which does not dissociate) cannot render it a better conductor, whereas the presence of a- mere trace of an acid or a salt (such as sodium chloride, or copper sulphate) capable of dissociation into ions, renders it to some extent- a conductor at once. There would appear, therefore, to be a direct connection between the separation of the salt into ions, or lonisa- tion as it is termed, and its power of conducting electricity. It would seem that when a current of electricity is led through a. solution, it is conducted onward by the separate ions existing in the solution, each carrying its own charge and conveying the electricity piecemeal, so to speak, from one electrode to the other. Every ion takes a definite charge, and the transfer of electricity is. accompanied by a migration of the ions. As the ions carry definite- charges and have definite weights, it is evident that the transport of electricity is accompanied by, and proportional to, the transport of matter, every 96,540 coulombs of electricity being accompanied by 1 ion (or atom) of a monovalent element such as sodium, (weighing 23 grammes), or of silver (weighing 108 grammes), or of chlorine (weighing 35*5 grammes); or by one gramme equiva- 63 *5 lent of a divalent ion such as copper (weighing - — grammes) or Q A of sulphur tetroxide (SO^) weighing — grammes. The electrical charge is essential to the existence of an ion, and if it be re- moved, the ion at once becomes converted into the corresponding element (or group of elements) with the properties so familiar in chemistry. Electrolytic Solution Pressure.— It has been stated above that a soluble salt passes into solution when placed in a solvent, and that it has a certain solution pressure. A salt is made up of a combination of a positive with a negative ion, and the solution of the salt, accompanied by a partial ionisation, does not aff^ect the apparent electrical condition of the solution. If, however, a metal such as zinc be placed in water, and zinc ions be formed in the solution, each of such ions must take a definite charge of electricity, so that positively charged ions would then exist in the solution without any balance of negatively charged ions. But if any portion of zinc did so pass into solution as an ion, there would necessarily be liberated at the same moment an equal and opposite charge of negative electricity in the zinc plate, because it^is not ELECTROLYTIC SOLUTION PRESSURE. 347 possible to generate either positive or negative electricity alone. The solution would thus be positively charged, and the zinc negatively, and this would counteract any tendency for more zmc ions to take up charges and pass into solution. ^ The^ special tendency of any metal to ionisation when placed in a liquid is known as the Electrolytic Solution Pressure, to distinguish it from the * solution pressure' of salts. But as in the case of solution pressure, electrolytic solution pressure is opposed by osmotic pressure ; thus, if a metal were placed in a solution of one of its own salts, the osmotic pressure of the ions of that metal already in the solution would tend to prevent the passage of fresh ions of the same kind into the liquid. If the osmotic pressure were less than the electrolytic solution pressure of the metal, ions would tend to pass into solution and to charge the solution positively, while the metal itself w^ould, up to a certain point, be charged negatively, as already shown ; but if the osmotic pressure were greater than the electrolytic solution pressure, it would not only prevent the passage of fresh metalHc ions into the solution, but would contrariwise tend to deposit some of the positive ions from the liquid on the metal, so that the metal would become positively charged, and the negatively charged ion set free in the solution would impart to the liquid a negative charge; whilst, if the osmotic pressure were exactly equal to the electrolytic solution pressure, no change at all would be observable, as the system would be in stable equiUbrium. It has actually been observed that the so-called electro-positive metals, such as potassium and sodium, zinc, cadmium, and iron, are charged negatively when placed in solutions of their salts, whilst such metals as copper, mercury, gold, and platinum, having electrolytic solution pressures that are low as compared with their osmotic pressures, usually become charged with positive electricity. It is thus possible to arrange the metals in an order cor- responding to their behaviour in this respect. In this way we obtain again the electro-chemical series, as it has been given above, and find a new explanation of the series. Some idea of the actual electrolytic solution pressures of certain common metals is gained from the following numbers, taken from Le Blanc : — Atmospheres Zinc, =9-9x1018 Cadmium, =2*7x10^ Iron, =1-2x10^ Cobalt, =1-9 Nickel, =1-3 Atmospheres. Lead, =1-1x10-3 Hydrogen, = 9-9xl0-4 Copper, =4-8x10-2^ Mercury, =1*1 xlO'^^ Silver, =2-3 xlO-^^ 348 MODEEN THEOEIES OF ELECTEOLYSIS. But there appears to be a close connection between the heat that would be generated by the chemical changes taking place in such a cell, if no current were produced, and the electro-motive force of any current that is generated. It thus appears that, in the case of electro-positive metals, heat is generated when a metal passes into the ionic condition, and 'this is termed the heat of ionisation. It is not rendered sensible in electro-chemical opera- tions because it is converted into electrical energy, and this is expended to a large extent outside the cell in which the action has taken place. The heat of ionisation for certain metals has been calculated by Ostwald, as shown in the following Table : Metal. Symbol and Valency. Heat of Ionisation of 1 Equivalent Weight. Metal. Symbol and Valency. Heat of Ionisation of 1 Equivalent Weight. Potassium, Sodium, Magnesium, . Iron, Zinc, I^a' Mg" Fe" Zn" Grm. cals. 61,000 56,300 53,400 10,000 16,300 Cadmium, Cobalt, . Nickel, . Copper, . Silver, . Cd" Co" Ni" Cu" Ag' Grm. cals. 8,100 7,300 6,800 - 8,800 -26,200 Thus a given quantity of electricity is conducted equally by equivalent weights of all metals ahke, but the electro-motive force necessary to set the particles in continuous motion varies with different ions, and depends upon conditions intimately connected with the heats of formation of the compounds in use. So, also, the force with which an ion clings to its electrical charge varies with, and is measurable by, the heat of ionisation, those metals which evolve the most heat in becoming converted into ions requiring the greatest expenditure of energy to make them relin- quish their charges and reassume the metallic state. This must evidently be so, the heat absorbed in the latter process being exactly equal to that evolved in the former. Simple Exchange of Metals.— It has been shown that when a metal {p.g., zinc) is placed in water it tends to form ions, but that the tendency is checked by the fact that the positive charges necessary to the independent existence of the ions can only be derived from the rest of the zinc, which must, therefore, become charged negatively, so that ionisation stops almost as soon as it has begun. It is evident also that no appreciable proportion of any ions, whether positive or negative, can exist in a solution without being SIMPLE EXCHANGE OF METALS. 349 accompanied by a number of ions of the opposite kind, carrying in all a charge equal and opposite to their own, otherwise the solution would give evidence of strong electrification. Water is but slightly dissociated into its ions hydrogen (H) and hydroxyl (OH). If now^ zinc be placed in water, and if we suppose it to dis- place hydrogen (as we shall shortly see that it does in sulphuric acid), then zinc (Zn) and (OH) ions would exist side by side in the solution. The chemical compound, zinc hydroxide, Zn(0H)2, represented by this combination, is, however, one which is com- paratively insoluble in water, that is to say, it scarcely dissociates into ions at all. Hence, if zinc is to have any action on water in which it is immersed, it must rob hydrogen ions of their charges and set free the hydrogen as a gas and form, instead, free Zn and OH ions ; but the action must stop almost at once, because solid undissolved and insoluble zinc hydroxide Zd(0H)2 would be formed on the surface of the zinc, and so gradually prevent further contact between the water and the zinc. But when an acid, sulphuric acid for example, is substituted for water, the action is different, for zinc sulphate (ZnSO^) is soluble in water. Sulphuric acid in aqueous solution is largely dis- sociated into the ions H and SO^ ; and hydrogen is an element which has a much lower electrolytic solution pressure than zinc, or, in other words, its ions cling to their charges of positive electricity far less tenaciously than do the zinc ions. Hence the greater electrolytic solution pressure of the zinc tends to carry zinc ions into the solution; and this is now possible, because, by contact with hydrogen ions already in the solution, the metallic zinc particles are able to take up the charges of positive electricity necessary to convert them into ions. In this process the hydrogen ions lose their charges, and, therefore, their existence as ions, and the hydrogen is deposited in the gaseous state. This, then, is the explanation of the observed fact that zinc, when placed in sulphuric acid, evolves bubbles of hydrogen gas. Similarly, zinc in copper sulphate solution deposits copper by exchange ; and any metal tends to ionise when it has the oppor- tunity of taking the charges from the ions of another metal of lower electrolytic solution pressure contained in the solution. Only those metals whose electrolytic solution pressure is higher than that of hydrogen are able to evolve the latter element as a gas, when dipped into a solution containing free hydrogen ions, i,e,, into an acid. It follows, also, that the acids which are the most completely ionised in solution (that is to say, the acids whose solutions contain the greatest number of free ions), are those in which the zinc and hydrogen exchange places most readily — 350 MODERN THEOEIES OF ELECTROLYSIS. l3ecause in such solutions there must be the largest number of free hydrogen ions in contact with the metal. It is found that the so-called strong acids — sulphuric, hydrochloric, and nitric acids— are those which are most completely dissociated into ions when mixed with water, and this, of course, explains the readi- ness with which they act upon metals, such as zinc. Acetic acid and the organic acids, as a class, dissociate but little, and there- fore have proportionately slight action on zinc. It is, then, obvious that the heat evolved by the action of an acid upon a metal is equal to the difference between the heat of ionisation of the metal (as it passes into the ionic state) in solution, and that of the hydrogen ions (as they assume the gaseous or elementary condition). In such a solution zinc will continue to dissolve until the solu- tion is saturated with it ; but the action will go on more and more slowly, because the accumulation of zinc ions in the liquid will cause an increasing osmotic pressure, which opposes the entrance of more zinc ions into the bath, and because the free hydrogen ions are gradually replaced by zinc ions. Simple Cells. — If, now, two isolated plates of metal are im- mersed in an electrolyte, each will exert its own solution pressure and will become negatively or positively charged with electricity, according to the character of the metal. But on joining the two plates with wire, a metallic conductor connects the two into one electrical system, and a new order of things is established. The heat which otherwise would be evolved by the metal ions replac- ing hydrogen ions in the solution may be available for conversion more or less completely into electrical energy, which is employed partly in overcoming the internal resistance in the liquid, or, as it were, the frictional resistance to the passage of the charged ions through the solution, and partly in doing work of some kind in the external circuit or wires outside the liquid. There will be set up a flow of positively charged ions through the solution from the metal which has the higher solution pressure to the other plate, and of negatively charged ions in the opposite direction ; and the electro-motive force which causes this circula- tion will depend mainly upon the difference between the electro- lytic solution pressures of the two electrode-metals, or, in other words, betw^een their heats of ionisation. To take a concrete example. Copper and zinc plates are separately immersed in sulphuric acid. The electrolytic solution pressure of zinc is higher than that of hydrogen, so that simple exchange takes place ; hydrogen ions are converted into hydrogen molecules which escape in the form of gas, and zinc molecules are SIMPLE CELLS. 351 changed into ions, each (divalent) zinc ion taking its positive <)harge from two hydrogen ions and passing into the solution, whilst the excess energy of this change is rendered evident in the form of heat, so that the metal and solution become sensibly warmer. The copper has a negative solution pressure, and so does not displace any hydrogen. Perfectly pure zinc does not deposit hydrogen decompose dilute acid) when simply im- mersed, the action no doubt being stopped after the first instant by the negative charge of the metal as already described, and ionisation can only proceed if by some means sufficient charges of positive electricity can be supplied to satisfy the ions of zinc. This is effected when the zinc and copper plates are united by a wire. The copper has a positive charge when immersed in the -acid, and can now give up some of its charge through the wire to the zinc. The positive charge of the copper is renewed at the expense of hydrogen ions on its surface, which thus become con- verted into free hydrogen gas. In this way it is possible to picture the higher solution pressure of the zinc tending to force the atoms of this metal to take up their proper charges of positive electricity and so to become ions, and to pass through the solution towards the copper, to which the weaker hydrogen ions in front of them give up their charges ; the positive charges of the hydrogen are thus imparted to the copper plate, and, passing through the wire, assist in the ionisation of zinc. This action can evidently go on until all the free hydrogen ions^ have given up their charges through the copper plate and the wire to zinc ions, that is, until all the free acid is neutralised and only zinc sulphate exists in solution. It is obvious that the action could not proceed if zinc ions were to give up their charge at the -surface of the copper and be there deposited, because the expendi- ture of energy required to form zinc ions at one plate would be exactly equalled by that required to discharge the ions and deposit metalUc zinc at the other plate ; and there would be no force available to cause the circulation of the ions. The falling off in the strength of a copper-zinc battery can of course be ex- plained partly by the gradual diminution in the number of free hydrogen ions available at the surface of the copper plate, as they are, little by little, replaced by zinc ions, but chiefly by the polarisation caused by the gaseous hydrogen deposited on the copper having a higher solution pressure than that metal, so that the difference between the solution pressures at the two plates is much lower than it was when the zinc was opposed to copper alone. Thus, at first, the potential-difference between ^inc and copper would be represented by the difterence between 352 MODERN THEORIES OF ELECTROLYSIS. the electrolytic solution pressures of zinc and copper. But as. this pressure is in each case measured by the potential difference between the metal and the solution in which it is immersed, the potential difference (written P.D. for the sake of brevity) between copper and zinc in sulphuric acid would be : — P.D. between copper and sulphuric acid minus P.D. between zinc and sulphuric acid. The P.D. of copper and sulphuric acid is about 0*5, or, as it may be written, +0*5, because the copper becomes charged with positive electricity, and the P.D. of zinc and sulphuric acid is about 0-6, or, as it must now be written, - 0*6 because the zinc is charged negatively. Hence the initial P.D. between copper and zinc is + 0*5 — ( - O'G) = 0*5 + 0*6 + 1 *! volt. Owing, however, to polarisation, the P.D. almost immediately falls off, so- that usually only 0*7 or 0*8 volt is observed. If cadmium had been used instead of copper, the P.D. of the two metals would have been much less, because the electrolytic solution pressure of cadmium is relatively high. The P.D. of cadmium and sulphuric acid is about — 0*2 (cadmium being charged negatively), so that the P.D. of cad- mium and zinc is -0-2 -(-0-6)= - 0*2 + 0*6 = + 0*4. In this, case each metal becomes charged negatively when immersed alone, but the cadmium, having the lower electrolytic solution pressure, takes the place of the copper and acts as the positive pole or cathode. If silver were substituted for copper, the E.M.F. would be even greater than that between zinc and copper, because the solution pressure of silver is lower than that of copper. The E.M.F. of silver and zinc in sulphuric acid would then, at first, be about + 07 - ( - 0*6) = 0*7 + 0-6 = 1-3 volt. In every case, however, the P.D. would rapidly fall off owing to- polarisation. Local Action. — 'l^o further explanation of impure zinc dissolv- ing in acid without being opposed to a plate of a metal of low^er solution pressure is necessary, as the effect of local action described on page 41 can obviously be as readily harmonised with the new theories as with the old. Two-fluid Cells. — It has been seen that no action could be anticipated if zinc and copper be opposed to one another in a normal solution of zinc sulphate containing no free hydrogen ions. If, now, a porous partition be placed across the cell between the two plates, and sulphuric acid be poured into the portion contain- ing the zinc, and normal zinc sulphate solution, free from acid, into the half containing the copper, no appreciable action could be expected, because, although the zinc is immersed in acid, and there is no osmotic pressure tending to retard its solution, yet the- TWO-FLUID CELLS. 353 solution or ionisation of any of the zinc could only occur if deposi- tion of ions took place at the surface of the copper, and the only ions that could be there deposited would be zinc, which it has already been shown will not be thrown down. If, however, the sulphate of zinc be placed in the zinc compartment, and the sulphuric acid on the copper side, action will take place freely : the tendency of zinc to ionise will be accompanied by the tend- ency of the hydrogen ions to give up their charges to the copper, and so the action will proceed, zinc ions from the zinc sulphate travelling through the porous partition into the copper compartment, forcing the hydrogen ions before them into contact with the copper, and making room for fresh zinc ions behind. Other two-fluid cells may also be explained according to the newer theories. Thus in the Daniell cell, the zinc is immersed in sulphuric acid or in zinc sulphate contained in a porous pot, the copper in copper sulphate. Here, in the former case, the zinc can be readily ionised, and can displace some of the hydrogen ions in the acid, which pass through the porous cell into the copper sulphate solution, and there displace copper ions which deposit on the copper plate, and impart their charges through the con- necting wire to the zinc, and so enable it to become ionised. The zinc at the outset has not to contend against any back osmotic pressure, seeing that there are no zinc ions in the solution initially, whilst the deposition of the copper ions is actually assisted by the osmotic pressure of the copper ions in the copper sulphate solution outside the porous cell. The battery is ob- viously constant, as the element which is de-ionised is the same as that on which it is deposited ; and it is, moreover, clear that the solution outside the porous pot should be kept saturated with copper sulphate, not only because there will then be always in contact with the copper a sufficiency of free copper ions to give up the quantity of electricity necessary to charge the zinc as it is ionised, without calling upon any free hydrogen ions that may be at hand (and which are more reluctant to give up their charges than are copper ions), but because the greatest assistance is to be derived from the osmotic pres- sure of the copper salt when the solution is most nearly saturated. It is most important here to observe that the newer theories afford an explanation of the fact that if two pieces of the same metal (connected together) be immersed on different sides of a porous division, in solutions of the same substance but of different strengths, a current is produced. Thus, suppose a strip of copper, Z 354 MODERN THEORIES OF ELECTROLYSIS. bent into the shape of a fl, be immersed in a cell divided by a porous partition into two compartments, so that one of the limbs rests in a strong solution, and the other in a weak solution of copper sulphate, the electrolytic solution pressure of the copper is, of course, the same in both fluids, but in a stronger solution it is opposed by a greater osmotic back pressure than it is in the weaker solution. Hence there will be a tendency for the copper to dissolve in the compartment containing the weaker solution and to send its ions through this solution to the porous partition, and through this to the stronger side. Thus the current flows through the solution from the copper plate m the weak liquid to that in the strong, and thence through the copper strip outside the cell back to the original plate. Meanwhile, anions charged with negative electricity are supposed to be passing from the cathode through the solution towards the anode, a nd are so migrating from the cathode cell through the porous division to the anode cell (^oide inf.). It is true that, even if one solution were one hundred times as strong as the other, the potential difference between the two plates could not amount to one quarter of a volt. But it is a fact that must be borne in mind in practical working, as it shows the necessity for keeping the solutions uniform m strength, and explains why, if a plated article be left in a bath which is not well mixed, the deposit may dissolve off" the portion that is immersed in the weaker solution and thicken on the other portions. This action may, of course, be greatly assisted by any concentration of acid in the one portion, and by the difi'erence m conductance of the solutions, but it must be remembered that it will occur wherever there is a difference in the concentration of ions in different parts of the same liquid, even though only a mere trace of acid may be present. The use of a porous pot is not, of course, essential ; provided the two solutions of unequal concentra- tion are in contact with one another, and each with one of the copper strips, the latter being joined by a metallic connection. The phenomenon is therefore observable if a single piece of copper be so immersed in a solution of copper that one part of the metal rests in a stronger portion of the liquid than does the rest of the Electrolysis.— After what has been said concerning the theory of batteries, the elementary explanation of the mechanism of electrolysis according to these hypotheses should be simple. Two plates are immersed in a uniform solution. The solution must be one in which a neutral salt is wholly or in part dissociated^ into ions, charged respectively with positive and negative electricity, otherwise it could not act as a conductor of electricity. The plates ELECTROLYSIS. 355 are connected to the opposite poles of a generator of electricity, so that one, the anode, is kept supplied with positive electricity at a certain potential, and the other with negative electricity. Immediately the positively charged ions, which had previously heen in motion without any uniformity of direction, will commence to move in a constant direction from the plate which is receiving a constant supply of positive electricity towards that— the cathode — which is supplied with negative electricity, and the ions which carry negative charges will also have imparted to them uniformity of direction, but opposite to that of the positive ions, namely, towards the positively charged anode. Thus there will be a stream of positive ions moving towards the cathode and there giving up their charges and being converted from the ionic to the neutral or elementary condition, and a stream of nega- tive ions moving towards and giving up their charges to the anode. Thus to picture the simplest case first : — Copper sulphate dis- solved in water is partly dissociated into positively charged Cu ions and negatively charged SO^ ions homogeneously distributed through the liquid. Two copper plates are immersed in the solution; the plates, being alike, have equal tendencies to dis- solve (or the same electrolytic solution pressure), and the copper sulphate solution, being homogeneous, exerts the same osmotic back pressure on the two plates. Even, therefore, if the plates be joined by a wire, no action can take place. Meanwhile it may be supposed that the ions of both kinds have more or less motion in all directions throughout the solution; possibly even the free ions may be constantly changing places with atoms of the same kind existing in as yet undissociated molecules. ISTow it may be supposed that the copper plates are connected severally to the two poles of a battery. One plate becomes an anode, the other a cathode. At once there is an increase in the potential difference between the positively charged anode and the surround- ing solution, and this is equivalent to establishing an increased electrolytic solution pressure for the anode plate. Thus the copper tends to be converted into ions, which derive their charges from the positive electricity of the anode plate, and so pass into the solution as positive ions. The supplying battery being supposed constant, the potential at the anode remains the same, or, in other words, as fast as a part of the positive charge is withdrawn from it by the newly-formed ions, it receives an additional supply from the battery, sufficient to maintain the same potential difference between the anode and the solution. At the same time the potential difference between the cathode 356 MODERN THEORIES OF ELECTROLYSIS. connected with the negative pole of the battery and the surround- ing solution is altered, but of course in the opposite direction to that of the anode, for while the anode becomes markedly positive to the solution, the cathode becomes markedly negative. This is equivalent to the creation of a strong, negative solution pressure,, that is to say, of a tendency not for the copper of the plate to be ionised into the solution, but to attract to itself positively charged ions from the solution. Thus while one portion of copper is receiving charges from the battery, and so being ionised, at the anode, another portion is giving up charges and so becoming neutral metallic copper at the cathode, and there is a constant flow of copper ions in a steady stream from the anode to the cathode ; moreover, each ion is of definite weight and is charged with a definite quantity of electricity. So copper dissolves into the solution at the anode and is deposited at the cathode, the positive electricity led in, so to speak, from the battery at the anode is carried by the copper ions through the solution to the cathode, and the quantity dissolved from the one must be equal to the quantity deposited at the other, and each quantity must be dependent entirely on the volume of current passing through the solution. The volume of current carried depends partly on the number of free ions in the solution — a very weak solution cannot conduct as well as a stronger solution, because it contains fewer of the ions at any moment in contact with the electrode — and partly on the potential diff'erence between the electrodes, that is, between the anode and the solution at one side, and the cathode and the solution at the other — because the higher the P.D. the greater the added solution pressure at the one pole and negative pressure at the other. But while Cu ions are travelling from anode to^ cathode, negative SO4 ions must be passing in the opposite direction, delivering up their negative charges at the anode. Since there is no electrification of the solution observable any- where except at the surfaces of the electrodes, it follows that although the Cu ions are migrating steadily from anode to cathode, there are an equal number of oppositely charged SO4 ions near- them, otherwise the solution at some places would show an excess of positive electricity owing to the excess of Cu ions at those points, and at other places an excess of negative electricity due to« free SO4 ions. Simultaneously with the formation of one Cu ion at the anode, another is discharged at the cathode ; at the same time an SO4 ion is delivered at the anode, where its negative charge is exactly equal to the positive charge imparted to the copper ion at that moment launched from the anode into the solution, so that the two constituents of the molecule Cu and S04^ ELECTROLYSIS WITH INSOLUBLE ANODES. 357 Ibecome free ions in the liquid at the anode surface at the same instant, and the result is the same as if a molecule of copper sulphate, CuSO^, were there added to the solution in the dis- sociated condition; whilst the Cu ion deposited at the cathode leaves there an unneutralised SO^ ion at the very moment when it is required to take the place of an SO^ ion which has set off on its migration towards the anode. Hence the transport of ions, and, therefore, of electricity, through the solution is continuous, and practically the only resistance to the flow of the current is the frictional resistance of the solutions to the ions passing through it. As solutions of salts become more mobile when they are heated, they then offer less frictional resistance to the ions, and so the conductivity of salt solutions is higher as the temperature is raised. The balancing of the equal and opposite pressures at the anode and cathode, when anodes of the same metal as that undergoing deposition are used, accounts for the fact that with pure copper solutions an exceedingly low E.M.F. sufiices to cause copper to be deposited. Electrolysis with Insoluble Anodes. — The principal difference between this case and that last described is that with insoluble anodes the anion does not meet with a positively charged metal ion passing into the solution from the anode. Its negative charge is therefore given up to the anode, and the ion ceases to be an ion, and becomes an ordinary uncharged element or group of elements. Groups of elements which are supposed to exist together in the ionic state are usually unstable when their charges are removed, so that a decomposition is then observed. Thus the anion, SO^, of sulphuric acid and sulphates cannot exist as an uncharged chemical group, and so breaks up in contact with the water, forming free oxygen and sulphuric acid thus : — SO4 + H2O = H2SO4 + 0. But since no metal is being ionised at the anode, the liquid, at least so long as metal is being deposited at the cathode, is gradually becoming weaker. Hence electrolysis with insoluble anodes, in cases where metals are deposited at the cathode, is characterised by the deposition at the anode of the element constituting the anion, or the evolution of oxygen gas or one of the constituents of the anion, and also by the gradual removal of the cations from the solution. If sulphate of copper solution be electrolysed between a platinum anode and a copper cathode, copper will be deposited at the latter, and since the platinum does not dissolve, SO^ will be deposited at the anode, and will there break up, in the presence 358 JSIODEEX THEOEIES OF ELECTROLYSIS. of water, into sulphuric acid and oxygen ; the nett result of the experiment will be the deposition of metallic copper on the copper plate and bubbles of oxygen on the platinum, and the accumulation of sulphuric acid in the solution. Gradually all the copper w^ill be deposited, and then, if the current pressure be sufficient to cause hydrogen ions to give up their 'charges at the cathode, the sulphuric acid will itself be electrolysed, and hydrogen ions will be caused to discharge at one pole, and oxygen will, as before, appear at the other, re-forming sulphuric acid, so that the result is the same as if w^ater only were decomposed. In this case the copper and the platinum exert their own solution pressures, and these in such a way that the copper tends to become ionised and to pass into the solution, so that the action of electrolysis is opposed to the tendency of the metals forming the electrohi;ic cell, and therefore the current passed into the latter must be able to exert sufficient pressure to overcome the difference between the solution pressures of platinum and copper in a solution of copper sulphate, and to deposit oxygen upon the platinum electrode. Even if electrolysis be commenced with two platinum electrodes, the cathode soon receives a sufficient deposit of copper to cause it to act as a copper plate, and after that time the electrolysis is carried on as between a platinum anode and a copper cathode. Secondary Actions. — It has been stated above that when an unstable complex anion, such as the group SO^, is deposited, it breaks up by chemical action unless it is re-absorbed into the solution along with an equivalent cation detached from a soluble anode. But, to consider the electrolysis of a sulphate wdth an insoluble anode (for example, platinum) : — some water may be electrolysed (but not much, as it is but slightly dissociated or ionised), and there is in consequence a little hydroxyl formed, but chiefly SO^ is deposited, and this forms free oxygen, as already described. Should there be contained in the liquid in contact with the anode a substance that is capable of a higher degree of oxidation, as, for example, ferrous sulphate, some or all of the free oxygen will be absorbed in converting this substance into the corresponding compound containing a larger proportion of oxygen, ferric sulphate, or, if suitable organic substances are present, they may be oxidised, or, as it were, burned by a process of liquid com- bustion. The evolution of oxygen would then be wholly or in part suppressed, and the heat that, in an ordinary chemical experiment, would be produced by this oxidation, would be rendered available for reducing the total E.M.F. required for the reactions at the electrodes. These anode reactions are actually employed in many electro-chemical operations. But chemical ELECTROLYSIS OF MIXED SOLUTIONS AND DOUBLE SALTS. 359 chaiip-es may also occur at the cathode, if any metal or cation be there deposited, that is readily capable of decomposmg water. Thus when potassium or sodium salts in solution m water are electrolysed, potassium or sodium ions become converted into metal at the cathode; and as either metal is capable of attacking water with great energy, the metal itself is not seen, but only the hydroo-en that results from its action on water. When mercury is employed as cathode, the deposited alkali metal becomes dissolved to some extent in, and diffused through, the mercury, and so removed from the possibility of attack by water except on the exposed surface of the mercury. The formation of a true amalgam of mercury and the alkali metal testifies to the reality of this deposition, and the reaction is utihsed m certain electrolytic processes for the production of caustic soda from solutions of common salt and the like. ^ ^ , ^ ^ Electrolysis of Mixed Solutions and Double Salts.— When a current is passed through a mixed solution of several electrolytes, the conductivity of the liquid is found still to be dependent on the number of free ions. It thus appears that all the free ions m the liquid are engaged in the transport of electricity from one pole to the other, cations from anode to cathode, and anions m the opposite direction. But it is well known that if a current ot moderate density be used with a solution contammg several metals, only the one with the lowest electrolytic solution pressure (the most electro-negative) will be deposited at first, and then the others in turn, in the reverse order of their solution pressures. Thus in a solution of cadmium, copper, and silver, silver alone would be deposited first, then silver with an increasing percentage of copper, then copper, next copper with an increasing percentage of cadmium, and lastly cadmium alone, supposing always that the E M F. applied is sufiicient to allow of the deposition of the cadmium with its relatively high solution pressure. Remembering that a metal with high solution pressure is able to become ionised at the expense of the ions of metals with lower solution pressure, this action may be thus explained :— All the free ions alike — cadmium, copper and silver— are migrating and carrying charges from anode to cathode, but at the cathode surface the current deposits that ion which requires least E.M.F. to overcome its electrolytic solution pressure, and to cause it to give up its positive charge to the cathode ; even if the ion of another metal with higher solution pressure were deposited, the deposited metal would exchange with an equivalent of the metal with lower solution pressure, and would pass again into solution. So long, then as there is a sufficient number of ions of the metal with 360 MODEEX THEORIES OF ELECTKOLYSIS. lowest solution pressure {e.g., silver) in contact with the cathode to carry to the cathode the whole volume of the current passing, only that metal, silver, will be deposited. But as, in course of time, more and more of the silver in the solution is deposited at the cathode, the quantity left will sooner or later be insufficient to carry the whole of the current to the cathode' surface ; then ions of the metal with the next lowest solution pressure (copper) will begin to be deposited. At first only very little copper will deposit with the silver, but as the silver ions become fewer, the proportion of the copper ions discharged must increase, until, at last, all the silver is thrown down, and only copper and cadmium re- main in the solution. Just in the same way the gradual ex- haustion of the copper ions will lead to the co-deposition of cadmium, and, finally, when all the silver and all the copper are deposited, cadmium ions only will be available to carry the electricity through the solution, and give up their charges at the cathode plate. Electrolysis of Solutions of Complex Acids. — Where, however, the solution is not merely a mixture of two electrolytes or a solu- tion of a double salt, but a chemical compound, the action of the current is different. A mixture of copper sulphate and nickel sulphate would act as described above ; copper first, and then (if the solution were kept neutral) nickel, would be deposited at the cathode, and SO^ (and hence oxygen) only at the anode. But if gold cyanide be dissolved in potassium cyanide solution, the resulting liquid does not behave in the same way. It may be shown that the gold travels with the ion which migrates from cathode to anode, and only the potassium migrates from anode to cathode ; yet the gold is deposited only at the cathode. This is due to chemical reaction. The ions of such a cyanide (KAuCy.^) are apparently K and AuCy2, K-ion giving up its charge at the cathode, but immediately attacking the solution in contact with it and depositing not hydrogen but gold from the liquid around, because potassium can break up the complex aurocyanide of potassium that is present. KAuCys + K + HgO = 2KCy + Au + H2O. Thus gold is deposited at the cathode not because ions of free gold exist in the liquid, but because potassium is deposited and exchanges with the gold in some of the complex substance KAuCyg in contact with the cathode. At the same time gold does not form at the anode, because the ion AuCyg cannot exist alone in the liquid, but breaks up into cyanogen, Cy, and gold cyanide AuCy, which re-dissolves in the free potassium cyanide in the EFFECTS OF THE MIGEATION OF THE IONS. 361 solution, re-forming potassium aurocyanide. The free cyanogen is then able, with its negative charge, to allow of the passage of another atom of gold to withdraw its positive charge from the anode and to pass into the ionic condition. It will thus be under- stood that in a gold bath there is a great tendency for gold €yanide to deposit on the anode, and also that, as there is no con- stant migration of gold to the cathode, but rather contrariwise, the deposition of potassium must be made to take place so slowly that the natural diffusion of the salts in the solution may ensure that there is always sufficient of the aurocyanide in contact with the cathode to allow of the exchange of the whole of the potassium for gold without water being decomposed and hydrogen deposited. Thus, also, silver travels in the anion in the electrolysis of the double cyanide of silver and potassium, and the silver is deposited by exchange with the potassium which, as cation, is deposited at the cathode. The double chloride of platinum and sodium ISTagPtClg behaves isimilarly, breaking up into the anion PtClg (which decomposes) and the cation Na ; so that it is not merely a double salt, but a platino-chloride of sodium, and platinum is deposited at the cathode only by secondary action. The ferrocyanides and ferricyanides, and many other complex salts, behave similarly, the iron or other metal being present in a complex anion. If it be true that in the case of these complex salts some of the metal to be deposited is travelling in the anion, and, therefore, away from the cathode, it is obvious that the current density used for electrolysis should be low, otherwise the liquid around the ^. — A monobasic weak organic acid, obtained as colourless plates very slightly soluble in water, w^hich should leave little or no residue when burnt GLOSSARY. 371 upon a sheet of thin platinum held over a flame. Its salts are termed henzoates. Boric Acid, H3BO3. — Commonly known as horacic acid ; a mineral body, and the acid basis of borax ; it is a white crystal- line solid, fairly soluble in w^ater. Dissolved in warm spirits of wine and ignited, the liquid burns with a brilliant and charac- teristic green flame. It is tribasic, and its salts are called borates. Citric Acid, C3H4(OH).(C02H)3, or CgHgO^. A white trans- lucently-crystalline, solid, tribasic, organic acid, readily soluble in water (100 parts in 75 of cold or 50 of hot water). It forms • citrates with metallic oxides. Hydrochloric Acid, HCl. — The pure body is a gas under ordinary conditions. It is intensely soluble in water, and its solution is sold under the above name or that of r)iuriatic acid. The stronger solutions emit pungent white fumes in air, owing to a partial evaporation of the contained gas, which recondenses in the form of clouds in contact with atmospheric moisture. A saturated solution contains about 40 per cent, of the pure gas, and has a specific gravity of 1*2; the actual saturation percent :age depends upon the temperature, as the acid is less soluble in hot water than in cold. The acid of commerce has usually a .-specific gravity of about 1*150, equal to 30 per cent, of pure HCl ; the acidum hydrochloricum of the British Pharmacopoeia €ontains about 32 per cent., with a specific gravity equal to 1*16. The liquid should be colourless, but the commercial muriatic acid is generally yellow, owing to the presence of iron and other impurities ; only the pure acid, however, should be used for most electro-metallurgical processes, though the crude liquid may often be employed for cleansing purposes. The acid is monobasic, and its salts are termed chlorides. Hydrocyanic Acid, HCN. — Commonly known as prussic acid. This also is bought as an aqueous solution of HCN, which is a liquid of low boiling point. It smells strongly of bitter almonds and is a deadly poison, so that its use in the arts is to be strongly deprecated whenever it can be avoided. The British Pharma- copoeia solution contains 2 per cent, of the pure acid. When used, the greatest care must be exercised, as the vapour evolved by a strong solution is itself extremely poisonous, and even when 372 GLOSSAEY. diluted considerably with air, produces giddiness and headache. Its salts are designated cyomides ; the acid is monobasic. Nitric Acid, HXO3. — Commercially known as aqua foHis, It is a very powerful and corrosive monobasic acid, which must be handled with great care. It stains the skin and other animal substances a bright yellow, which becomes intensified by the application of an alkali or of soap ; its oxidising power is so intense that the accidental fracture of a carboy of the acid ha& been known to set fire to straw which happened to surround it. The pin-e acid (HNO3) ^^^^ ^ specific grayity of 1'52. The acid commonly sold in commerce has a specific grayity of 1*45, equal to 77 per cent, of pure HNO3, while a weaker acid, containing 70 per cent, (specific grayity = 1*42) is also to be had, and con- stitutes in fact the acidum nitricum of the British Pharmacopoeia, while others even less concentrated are likewise to be procured. The stronger solutions fume in the air. ^Yhen pure, the liquid should be colourless, but owing to partial decomposition into lower oxides of nitrogen, it frequently possesses a straw-coloured or yellow tint, becoming, in the commoner kinds, orange and, finally, green, when it is commercially termed nitrous acid. The salts of the acid are called nitrates. When mixed with hydrochloric acid in the proportion of 1 to 3 (HXO3 : HCl) the liquid is known as aqua regia, and assumes an orange colour due to the presence of the gas nitrosyl chloride (NOCl) formed by the imion. This liquid is endowed with the highest oxidising powers, dissolving even the precious metals. which resist the attack of either acid sino-ly. <^ t/ Oxalic Acid (COoH)^ or C^H.p^.— A dibasic organic acid, which forms a white crystalline solid containing 2 molecules of water (C2H2O4.2H0O). It is readily soluble in water or alcohol, and is a powerful poison. It forms oxalates. Sulphuric Acid, HgSO^. — Known as oil of vitriol. A dibasic and most powerful mineral acid. When perfectly pure, it is a colourless and odourless, oily liquid of specific gravity 1*842. It combines most energetically with water, and in doing so generates much heat, so that dilution even of the ordinary commercial acid with water must be eff'ected with the greatest care. Large volumes must never be thoughtlessly mixed, nor should the water be added to the acid, lest a sudden generation of steam of explosive violence result, and the dangerously corrosive liquid be scattered in all directions. The acid is in all cases to be added GLOSSARY. 373 to the water in a very gentle stream, and with constant stirrmg. The acid of commerce is diluted in various degrees, but is always concentrated. The salts formed from this acid are sulphates. Tannic Acid, Ci.HioOg.-A pale yellow solid and weak organic acid, readily soluble in water. It burns completely away when heated upon a metallic plate, and yields a blue-black colour when added to solutions containing iron. It forms tannates. Tartaric Acid, G.R^O^U^^O^'I^^ C^HeOe.-It is a colourless crystalline solid, which readily dissolves m water. It is a weak dibasic organic acid, forming salts which are known as tartrates. Alcohol, CM.iOB) or C^H^O.-Pure or absolute alM has a strong affinity for water, so that the alcohol usually bought generally contains a small quantity of the diluent. It should be Colourless and have a specific gravity of 0-7939. It is commonly sold under the name of spirits of wine, or rectified spirit, which contains about 16 per cent, of water, and has a specific gravity ot 0-838 Proof spirit is a mixture carrying 49-24 per cent, ot pure alcohol, and is the standard with which alcoholic liquids are com- pared in commerce. Methylated spirit contains a certam amount of methylic alcohol or wood spirit, and frequently has a quantity of resinous matter added to it to render it xmfit for dnnkmg, so that it may not be liable to excise duties, and shall yet be available for most of the purposes for which spirit is required in the arts. Such a product should on no account be used tor electro-metallurgical work without previous thought as to the probable consequences of using it. For example, the etteot ot washing an object in the sophisticated spirit and then plunging it into the electrolytic bath would be the introduction of undesir- able organic impurities into the latter, and even the formation ot a non-conductive coating upon the surface of the article itself ; because water added to the impure alcohol throws down the resinous substances in the form of a white precipitate, buch spirit tends to burn with a smoky flame. Alcohol should therefore, be tested by burning, when the flame should be almost non-luminous, and by the addition of water to a sample, which addition should produce no turbidity. Aluminiiun, Al.— A white silvery element with an almost imperceptible bluish shade. It is extremely light (the specific gravity being only 2-58), is very malleable and ductile, takes a 374 GLOSSAEY. high jpolish, and is not liable to tarnish in air. It melts at about 1160" F. It is largely used in alloys, and for manufacture of objects where combined lightness and strength are required. Its principal common impurities are iron and silicon. Aluminium Chloride, Al2Clg.— A ' white, sometimes yellowish substance, which in the anhydrous condition absorbs water with great avidity; and, having once absorbed it, cannot be induced to part with it, the action of heat upon the hydrated crystalline salt (Al^Clg . I2H2O) causing decomposition, with the formation of alumina and hydrochloric acid. It must, therefore, be stored in perfectly^ air-tight jars. In small quantities it volatilises at 356° to 365° F. without fusion, but in a large bulk it may be induced to melt first. Almninium-Sodimn Chloride, Al2Clg.2NaCl.— This salt is made by^ heating the simple aluminium chloride with common salt. It is more useful than the latter for electro-reduction by the fusion method, because, melting at 365' F., it volatilises only at a red heat; moreover, it is less readily attacked by aqueous vapour. Aluminium-Potassium Sulphate, Al2(S04)3.K,S04.24H20.— Commonly known as potash alum. It is a crystalline substance, with an astringent taste, and is readily soluble in water, 12 parts of alum dissolving in 100 parts of water at the ordinary tempera- ture, 357 parts at the boiling point. It is a double sulphate of alumina and potash. Ammonia alum is exactly analogous, the potassium sulphate being simply replaced by ammonium sulphate {^h{^0,\.Q^ll,\^0^,2mp\ and is for most purposes inter- changeable with potash alum. Soda alum is similar, but is more readily soluble in water. Ammonium Hydrate, NHg.H^O.— Commonly termed ammonia or spirits of hartshorn. It is simply water saturated with ammonia gas (NH3). It is a powerful alkali, and is, therefore, useful to neutralise the acid properties of any substance. As it has an overpoAveringly pungent odour, care must be taken in using the stronger solutions. It is obtainable in the market as ammonia fortiss., with a specific gravity of 0*880, which is almost saturated with the gas and contains about 36 per cent, of pure Bottles of the liquid must be opened cautiously in hot weather, because the warmer water cannot dissolve so large a volume except under pressure ; and the excess is given ofi", GLOSSARY. 3'^^ sometimes with considerable violence, directly the pressure is the removal of. the stopper. A weake^ solut^^, amvwnicB. liquor fortior, contammg ^^'S Per cent, of NH^^^^^^ P'ravitv = 0-891) is that recognised by the British 1 harmacopoeia, f nd is safer for use in the heat of summer. By exposure, Iminia gas s gradually evolved, so that it must be stored in "y lpperedlottles fn order to preserve the strengh oUhe solution u^mpaired. By regardmg the f^^^^^^^^, ^f, f ^ta^ce becomes the hydrate of the hypothetical ^^e^aUike si^^^^^^ ammonium (NH,). This radical or group of elements, ^a.^, bSvesTits chemical relations exactly as a monovalent metal, combining with acids to form ammonium salts. Ammonium Carbonate, (NH,)2C03, and Bicarbonate, (NH,) HcCare white solid substances, soluble in -ter -d smelling strongly of ammonia. The commercial salt is ^P^^^^/.'f bonaS,Vit is in part carbamate; it is, ho-jer, suitable fo^^^^^^ nurnoses to which it is generally applied. As an alfcaUne Sona e Vt takes the placi of ammonia itself m neutralising acSs carbonic acid gas being evolved while the ammonium salt of the stronger acid is formed. Ammonium Chloride, NH,C1.-A white substance occurring intmmerce in the form'of to V fibrous crystals odo.nless and soluble in water (100 parts of cold water dissolve 35 parts, and of boiling water 77 parts of the salt). Ammonium Nitrate, NH,N03. - A colourless crystalline bodv 200 parts of which dissolve in 100 parts of cold water Heated gentry it melts and is afterwards decomposed into nitrous oxSe gaf (laughing gas = xXp) and water, leaving no residue. Ammonium Sulphide, {^^,\^, and ^ydrosulphide NH HS may be prepared by passing hydrogen sulphide gas mto a Stion oi ammonia' It is at first colourless but gra^ua^ decomposition becomes yellow. It is generally bough a^ an amber-coloured, frequently almost orange, A^^d Jhe many products of decomposition do not seriously interfere with its general utility except in so far as they weaken the solution of the sulphide itself. Ammonium Sulphate, {^B.,\SO^.-k white crystalline solid of wht^ToO parts of cold ;ater dissolve 50, of hot water 100 parts. 376 GLOSSARY. Ammonium Salts of the acids just described should leave no fixed residue when heated over a flame upon a piece of sheet metal. Antimony, Sb.~A white, highly crystalline and extremely brittle metal. The commercial ingots have usually a very crystalline surface, which resembles the fern-like appearance of frost upon window-glass, from which the name star antimony is derived, and which is regarded by many as an infallible criterion of the purity of the metal, but which must not be relied upon f^^^\ ^^'^ "^^^^^ ^ ^P^^'^^ gravity of 6-8, and melts at 800 F. The principal objectionable impurities likely to occur m ordinary antimony are sulphur, arsenic, tin, lead, silver, bismuth, and iron. It is only procurable in the cast condition, because on account of its brittleness it cannot be rolled. Antimony Chlorides.— There are two chlorides— the trichloride or antimomous chloride, 8hG\„ known as butter of antimony which IS that more usually required for electro-metallurgical work : and the pentachloride or antimonic chloride, SbCL. The former is a colourless crystalline substance melting at 164° F It may be prepared in crude form, mixed with excess of acid, by dissolving antimony or the sulphide in hydrochloric acid to which a little nitric acid has been added to increase its oxidising action. The pentachloride is a colourless fuming liquid having' a most unpleasant odour. Antimony Sulphide, 8h,8^^ occurs in nature as stihnite or antimony glance. It is usually bought as grey needle-shaped crystals sub-metallic in lustre. The pentasulphide, Sb,S., requires no notice here. Antimony-Potassium Tartrate, KSbC,H,0„ commonly known as tartar emetic, A white crystalHne substance, of which 100 parts of cold water dissolve 5 parts, while a like volume of hot water dissolves 50 parts. Aqua Fortis.— See Acid, Nitric. Aqua Regia.— See under Acid, Nitric. Bees^wax.— The substance of which the honey-comb is built up. It IS usually of a yellow or brown colour, the specific gravity ranging from 0*958 to 0-960, and the melting point from GLOSSARY. 377 144** to 156° F. in different samples. It dissolves readily in oils and in ether, but not in water. Its chief adulterants are- mineral matter, starch or flour, and water, the presence of either of these being detected on melting the sample ; and resinous or fatty substances, vegetable waxes and paraffin which influence the specific gravity and the melting point. When melted it should give a clear liquid free from any cloudiness, and should not indicate the presence of water by the formation of two layers of fluid. Bismuth, Bi. — A highly crystalline and very brittle metal, resembling 'antimony, but having a faint pinkish colour. Like antimony it cannot be rolled or drawn into a wire, but, on the contrary, may be crushed into a powder with an ordinary pestle and mortar. It melts at 515° F., and has a specific gravity of 9-759. It is one of the most useful constituents of fusible metal. The commonest objectionable impurities are — sulphur, iron, lead, copper, arsenic, and silver. Bismuth Nitrate, Bi (^03)3 + 3H2O.— Made by dissolving the metal in dilute nitric acid; it is precipitated as a white basic bismuth sub-nitrate by the addition of much water. Black Lead. — See Plumbago. Brass is an alloy of copper and zinc, the percentage of the former varying from 70 to 60. Special alloys are made for certain purposes, many containing tin and lead. Sterro metal is a brass to which a small percentage of iron has been added, while other complex alloys are made containing iron and man- ganese in addition to other bodies, to give additional strength or stiffness to the metal. Ordinary brass, unless specially made from the purest virgin metal, generally contains notable pro- portions of iron and lead and sometimes of tin. Bronze.— This term embraces the class of alloys of copper and tin, and includes bell-metal and gun-metal. The proportion of tin varies from 5 to 20, the average sample containing about 10 per cent. Some samples have a small percentage of zinc added, others manganese and iron ; in fact the remarks made in regard to brass apply equally to bronze in this matter. Certain alloys are wrongly called by this term, for example, aluminium bronze, which contains 10 per cent, of aluminium, but no tin. Some forms of manganese bronze also contain only a nominal percentage 378 GLOSSARY. of tin, and the alloy is then really a complex brass with a very small, but sufficient, percentage of manganese. Cadmimn, Cd. — A soft and very malleable bluish- white metal not unlike zinc, with which element it is commonly associated in nature. Its specific gravity should lie between 8*6 and 8*8, while its melting point is about 608° F. It is rarely used in the Arts in the metallic form, except in the manufacture of fusible alloys. Cadmimn Bromide, Cd.Er2. — A white crystalline substance soluble in water. Cadmium Chloride, CdClg.SH^O.— A similar body, of which 140 parts are soluble in 100 of water. It is made by dissolving the metal in hydrochloric acid and evaporating. Cadmium Sulphate, 3CdS04 . 8H2O.— A white crystalline substance, 59 parts dissolving in 100 parts of water. Calcium Carbonate. See Chalk. Chalk is a natural rock composed of calcium oxide (lime) and carbonic acid, CaCOg. On strongly heating it the carbon dioxide (CO2) is driven ofi", and the pure calcium oxide remains as quick- lime (CaO). Limestone has the same composition as chalk, and behaves similarly. A particular kind of lime, selected from that burnt in the neighbourhood of Sheffield, has found especial favour among metal-polishers. As the burnt lime, by contact with the air, rapidly absorbs first water, and thus becomes slaked (Ca (011)2), and then carbon dioxide, and so becomes reconverted into calcium carbonate, the lime, which is to be stored for any length of time, must be preserved in air-tight cases until it is required for use. For polishing purposes it must be uniformly soft and free from gritty particles, which would give rise to scratches ; treated with dilute hydrochloric acid, a sample of quicklime should dissolve with but slight eff"ervescence, and leave no residue undissolved. Chalk or ivhiting should dissolve with brisk eff*ervescence, but this also should leave no appreciable residue. . Cobalt, Co. — A metal similar to, and generally occurring in nature with, nickel. It has a specific gravity of from 8*5 to 8*7, and a high melting point, approximating that of iron. It is GLOSSARY. 379 readily soluble in sulphuric (dilute), hydrochloric, and nitric acids, forming cobalt sulphate, chloride, and nitrate respectively. Cobalt Chloride, CoCl^.eH^O.— A dark red crystalline body readily soluble in water; it may be prepared by dissolvmg the metal, its oxide or carbonate, in just sufficient hydrochloric acid. It is well to use a deficiency of the latter in order to ensure the neutrality of the solution. Cobalt Nitrate, Co(N03)2.6H20.— A pink crystalline soluble substance prepared like the chloride but with nitric acid. It is readily procurable in the market. Cobalt Sulphate, CoSO^.TH.O.— It resembles the two last- named salts in the manner of preparation (but using sulphuric acid) ; 100 parts of cold water dissolve 35 parts of the salt. Cobalt-Ammonium Sulphate, CoSO,.(NH4)2S04.6H20.— A pink salt which may be made by adding the right proportion of ammonium sulphate to cobalt sulphate (47:100), and then dissolving them and evaporating them together until a good crop of crystals has separated. Copper, Cu.— A red metal with a fusing point of about 1920° F , and a specific gravity of 8-9 to 8*95 according to its condition —whether it is simply cast or has been afterwards rolled or hammered. By reason of its extreme malleability and ductility it may be readily obtained in the form of rolled plate or foil, and of rod or wire. It is most readily attacked by nitric acid, but is slowly dissolved when immersed in heated hydrochloric or sulphuric acids. The metal is frequently found native (that is in the metallic state in nature), but is most usually smelted from ores in which it is combined with sulphur as sulphide, or with oxygen and perhaps other acid substances as oxide, or oxidised compounds. Such metal often contains small percent- ages of sulphur, lead, bismuth, arsenic, antimony, and iron, with sometimes traces of silver and gold, and, more rarely, of nickel, cobalt, and tin. There are two classes of copper salts— one, cupric, containing more oxygen or other electro-negative element, formed from the oxide CuO, and yielding generally blue- or green-coloured salts and solutions ; the other, cuprous, prepared from the sub-oxide, CU2O, and giving nearly colourless solutions. The former, which are more usual, alone need be referred to here. Copper Acetate, Cu(C2H302)2.H20.— Dark green crystals. 380 GLOSSARY. moderately soluble in water, formable by dissolving cupric oxide or carbonate in acetic acid. Copper Carbonate, CuCOg.— Occurs in nature as malachite and allied minerals. The artificial carbonate is a green sub- stance, insoluble in water, but readily decomposed by mineral acids (as well as by many of organic origin) yielding the copper compound of the added acid, and carbon dioxide gas, the evolu- tion of which gives rise to great effervescence. The so-called carbonate is usually mixed with hydrated oxide and has the formula— CuCOg. Cu(0H)2. Copper Chloride, CUCI2.2H2O.— Blue-green crystals, readily soluble in water. May be prepared by treating an excess of oxide or carbonate with hydrochloric acid, filtering, and evaporat- ing the resulting solution. Copper Cyanides.— The cupric cyanide, Cu(CN)2, precipitated by potassium cyanide from copper sulphate solutions, is very unstable, rapidly changing by exposure into cupro-cupric cyanide, Cu(CN)2.Cu2(CN)2, and cyanogen gas; the form^er when solid, forms green crystals, which are again decomposed at the tempera- ture of boiling water into cuprous cyanide, Cu2(CN)2, and cyanogen; and this latter forms several double cyanides with potassium— for example, Cu2(CN)2.KCN.H20,Cu2(CN)2.2KC]S^ and others, which are for the most part very soluble in water. Copper Nitrate, Cu(N03)2.3H20.— Blue crystals, very soluble m water, and rapidly absorbing moisture from the air. Excess of metal, oxide or carbonate treated with nitric acid yields the salt. Copper Sulphate, CUSO4.5H2O.— Commercially known as blue vitriol ; it is the commonest compound of copper. It forms blue crystals, of which 100 parts of cold water dissolve about 40, and of hot water about 200 parts. It is so largely used in the Arts that it may be procured everywhere ; it may be made the starting point for making other compounds of the metal. By adding to an aqueous solution of the salt a quantity of sodium carbonate, dissolved in water, a green solid precipitate of copper carbonate is produced ; this may be allowed to subside, filtered, washed well, and dissolved in any acid which will produce the required salt. Glue.— The ordinary best glue in the market, made from GLOSSAEY. 381 bones, should be used for moulding. It should be quite trans- parent, although perhaps dark in colour, hard, and brittle when sharpi; struck, and must be free from particles of foreign matter. When soaked in water it should swell and absorb about five or six times its weight of the water. Gelatine is only a specially pure and clean form of glue, and isinglass is similar m com- position. Glue Marine.— There are several descriptions of this useful cementing material. A commonly employed glue is made by dissolving a little india-rubber very carefully and with the aid ot heat in twelve times its weight of coal-tar naphtha, adding to it twenty times its weight of shellac, and finally pourmg it upon a flat cold surface to solidify and harden. It is only necessary to warm the glue and to apply it to the gently-heated surfaces that are to be united. It also makes a good waterproof and non-conductive lining when painted thickly upon the interior surfaces of tanks for cold solutions. Gold Au.— A yellow metal of high specific gravity (19-26) and fusing point (1915° F.). It is the most malleable and ductile of metals, and combines with these properties that of a very fair electric conductivity. In nature it occurs m the metallic state, almost invariably associated with silver, and often with copper and iron. In commerce it is met with as fine gold, and in various alloys of which the principal has the standard value ot the British sovereign gold-91-67 of gold to 8-33 of copper These alloys are described as being so many carats fane ; thus, it an alloy contain 22 parts of pure gold in 24, it is said to be 22-carat gold, if it contain if of its weight of the pure metal it is 18-carat gold, and so forth; the remaining metal may be copper or silver, separately or together, according to the oolotir which the metal is required to have ; the sovereign is made ot /22 91-6\ 22-carat gold \^24"'T00/. Gold is insoluble in nitric, hydrochloric, or sulphuric acid alone, but readily dissolves in a mixture of the two former (aqua regia), and in a very finely-divided condition may be made to dissolve in a mixture of the first and third. To prepare pure gold from the alloy on a small scale is a simple matter It me alloy contain less than 40 (or more safely 30) per cent, of the precious metal, mere prolonged boiling in nitric acid will dissolve the copper and silver, but leave the gold untouched in the form of a black powder, which is very heavy, and requires only to be 382 GLOSSARY. washed several times, by stirring it up with repeated additions of cold water, allowing it to settle and pointing off one batch of water each time before adding fresh, and then to be dried and fused to yield the metal practically in a state of purity. The solution must be effected in a glass or glazed earthen- or stone- w^are vessel, which will not be attacked by the strong acid ; and should be carried on in the open air or in a well-ventilated fume-cupboard. But if the alloy contain a larger percentage of gold, the other metals are not completely removed by the acid, and the original mixture must be treated with aqua regia. The gold will now be in solution ; any undissolved white residue is pro- bably silver chloride, which is formed by the agency of the hydro- chloric acid and is insoluble in the hquid. It should be allow^ed to subside, and should be washed once or twice by decantation and filtered. The solution should now be transferred to an evaporating dish or other vessel, in which it may be evaporated to the consistency of a thick ^ syrup, by placing it over a saucepan of boiling water. By this time the bulk of the nitric acid will have been boiled away, and the residue will be a strong, but more or less impure, solution of gold chloride containing hydrochloric acid. The liquid is now diluted, and a quantity of a solution of ferrous sulphate is added, and the mixture is allowed to stand for a day or two in a warm place. The ferrous salt becomes converted into a ferric compound at the expense of the gold oxide, and the gold should thus be liberated completely as pure precipitated metal, of dark brown or black colour, which may be washed, dried, and fused as before. The fusion may be made in a small clay crucible under a cover of a few grains of borax by way of flux for residual impurities. Oxalic acid is sometimes substituted for ferrous sulphate as a precipitant. Gold Chloride, AUCI3.2H2O.— A most soluble and deliquescent yellow crystalline substance, which may be prepared as described in the latter portion of the last paragraph, but using pure gold instead of alloyed metal as the basis. Gold Cyanides. — On adding a neutral gold chloride solution to one of potassium cyanide, there is produced potassium auricyanide, 2KAu(CX)^.3H20, which in the solid condition forms colourless tabular crystals, that are very soluble in hot water, but decompose at about 400° into lootassium aurous cyanide, AuCX.KCX, and cyanogen gas. This latter body, potassium aurous cyanide,' is formed by^ dissolving aurous oxide, AugO, or even finely- divided gold in potassium cyanide solution; it is a colomdess GLOSSARY. 383 crystalline and very soluble salt. Simple aurous cyanide, AuCN, is an insoluble lemon-yellow substance. Gold, Fulminating, Au,03.(NH3),.-A brown or green powder obtainable bv adding ammonia or ammonium carbonate to a SXn of goM chloride. It should never be allowed to become iTiov in this condition it is liable to explode with great violence. So long as it is moist, there is no danger attending its use. Gold SulpMde, Au2S3.-Obtahied by passing sulphuretted hydrogen gas into a solution of the chloride ; it then appears as a black precipitate, soluble in alkaline sulphides. Graphite.— See Plumbago. Gutta-percha.— A gum prepared from the exudation of certain trees in the Malay Peninsula and Islands. It is usually pro- curable in sheets. As a moulding material it is valuable, because at the temperature of boiling water it is extremely plastic and maybe worked into any required shape, which it will retain on cooling, when it again becomes hard yet somewhat elastic, it is a non-conductor of electricity. Iron Fe —The fusing point of the pure metal is very high The iron of commerce is never pure. In the condition m which it is smelted from the ore as pig-iron or cast-iron it is more readily fusible, but is highly charged with impurities, derived from the ore, fuel, and flux, and contains varying proportions of carbon, sulphur, phosphorus, silicon, and manganese, frequently accompanied by other elements also the foreign matter in the aggregate amounting to from 4 to 7 or more per cent. In this condition it is hard, brittle, and unworkable. By refinmg away the greater proportion of these impurities, the melting pomt is greatly raised, and at the same time the metal becomes soft ductile, and malleable, and is known as malleaUe- or wrougU- iron, which may be rolled into sheet of any degree of thinness. Wrought-iron is the purest form of marketable iron, but even this is not pure, containing perhaps O'S per cent of foreign substances. Between wrought- and cast-iron is another form— steel-the characteristics of which are chiefly governed by the percentage of carbon, which may range from 1| per cent m the harder varieties of tool steel to practically nothmg m mM-steel. The latter of these alone, from its greater purity, enters into 384 GLOSSAEY. competition with wrought-iron as a rival in the manufacture of anodes. The metal is soluble in either of the three common mineral acids, and foiTQS two classes of salts, one ( ferric) with more oxy- gen, of which the peroxide, Fe.Og, is tjpicai, the other {ferrous) of which the protoxide, FeO, is the basis. The latter are readily converted into the former by the addition of oxvgen, even bj absorption from the air ; but unless there be an exc'ess of acid in the bath the effect of the peroxidisation will be the precipitation of basic salt (see p. 252). It is for this reason that neutral ferrous solutions rapidly become turbid with a yellowish shmv deposit. Iron GYilovi6.es.— Ferrous Chloride, reCl,.4H,0, and femo chloride, FeX1.3.12H,0, are both very soluble crystalline bodies — the former bluish, the latter yellow in colour. ^ Iron Sulphate.— 7/'o?^ protosulijhate, ferrous sulphate, or green Vitriol, FeSO^.THoO, is a green crystalline substance, often yellowish on the exterior, owing to the formation of feme com- pounds with the aid of atmospheric oxygen. On account of this tendency to peroxidation, this and other ferrous compounds should not be exposed more than necessary to the air. 100 parts of cold water dissolve about 70 parts of the salt, of hot water 330 parts. The ferric sulphate, Fe,(S0j3, demands only casual mention in this place. Iron-Ammonimn Sulphate, FeSO,.(XH,),SO,.6H20, is a body similar to the last, but with a bluer shade of colour, and IS much less liable to alteration by exposure to air, and is, there- fore, preferable to the former for many reasons. 100 parts of cold water dissolve 16 parts of this salt. Lard.— The pure white lard is used; as it is frequentlv adulterated with water and solid substances, it should be melted and allowed to stand for some time in this condition. The bulk of the water and heavier matter will sink to the bottom, and the purified fat, which should now be quite transparent, may be drawn off from above, or removed by ladles into vessels wherein It may be allowed to solidify. It should melt at a temperature of about 110-5° F. Lead, Pb.— One of the softest of metals, it is very malleable, but, having a low tenacity, is deficient in ductihty; it may be GLOSSARY. 385 rolled into sheet, but not drawn into wire. Its fusing point is 625° F., and its specific gravity 11-25 in the cast state, or 11*39 when it has been condensed by rolling. On account of its ready fusibility it may be cast into slabs, or it may be rolled into sheet for use as anodes. Commercial lead, frequently very nearly pure, is never absolutely so. It always contains at least a trace of silver, often with varying proportions of antimony, tm, copper, iron, and sulphur. It is readily dissolved in nitric acid, and slowly in boiling hydrochloric acid. Sulphuric acid, except of the most concentrated description, is almost without action on the metal. Both chloride and sulphate of lead are practically insoluble in their respective acids, so that very soon the metallic surface becomes coated with a deposit which prevents further action. The salts of lead are formed from the basis of the monoxide (litharge = PbO), in which the metal is divalent, although two other oxides, red lead, PbgO^, and peroxide, PbOg, are known. Lead Acetate, Pb(C2H30,)2.— A readily soluble white crystal- line substance, easily formed by dissolving lead oxide or carbonate in acetic acid. It is very commonly known as sugar of lead. Lead Nitrate, Pb(N03)2, forms soluble white crystals (100 parts of water dissolve about 54 parts in the cold, or 135 parts when heated). r-^ , ^ r Lime. — See Chalk. / Magnesium, Mg.— A white divalent /Inetal, readily becoming dull in moist air. When ignited at a.-^lightly elevated tempera- ture, it continues to burn with a mc/t brilliant white light, until it is completely converted into oxide. It has a very low specific gravity (1-75), and fuses at 1380° F. It forms one oxide, magnesia, MgO, which is the basis of the various salts of the metal. Magnesium Chloride, MgCl2.6H20.— A most soluble and deliquescent crystalline salt (100 parts of water dissolve 280 parts in the cold, or 782 parts of the body when heated). It must be stored in a closely-stoppered bottle. Magnesium Sulphate, MgS04.7H20.— Commonly known as Ejpsom salts. It forms white crystals, easily procurable, of whicxi 100 parts of cold water dissolve about 70. 2 B 386 GLOSSAKY. Mercury, Hg. — Frequently called quicksilver. It is the only known metal which is liquid at ordinary temperatures ; it solidifies at - 38° -9 F., and boils at 680° F. Its specific gravity at the normal temperature is 13 "59. Mercury has a great tendency to dissolve other metals, and so to form amalgants ; it must not, therefore, be stored in, or allowed to come into contact with, clean surfaces of any metal commonly in use except iron or platinum, with which it does not combine. Gold and silver are especially liable to be dissolved, and as articles of jewellery are thus readily spoiled by mercury, the greatest care must be taken in using it" Gold becomes white and dulled by it, and requires the application of strong heat to effect its removal; and the surface is then left "dead," so that it must be re-polished. Mercury, therefore, should not unnecessarily be introduced into the workroom containing electro-plated goods awaiting treatment ; but if used for any purpose it should be carefully preserved from contact with any article liable to be spoiled by it. In consequence of its proneness to combine with other metals, mercury is rarely quite pure. If it be required clean, it may be spread in a shallow dish and covered with dilute nitric acid, with which it should be stirred from time to time. The base metals, such as zinc, copper, and lead, being more electro-positive than mercury, tend to dissolve first ; but a certain amount of the mercury itself dissolves also, and forms mercurous nitrate. This sub-nitrate assists in the removal of the other metals by simple exchange; gold and silver, which are more negative than mercury, are, of course, unaffected by the treatment. The solu- tion which has been used for cleaning mercury may be used again and again to treat fresh samples, so long as it contains either an excess of acid, or an appreciable quantity of mercury in solution. This may be ascertained by the blue litmus-paper test in the former case, or in the latter, by adding a drop of hydrochloric acid to a little of the solution placed in a test tube, when a dense white precipitate of mercurous chloride (calomel) is at once produced if mercury be present. Mercurous nitrate solution may be substituted for nitric acid at the outset if pre- ferred. The most satisfactory method of purification, however, is to distil the mercury from a glass retort, and, preferably, under diminished atmospheric pressure, effected by adopting a system of hermetically-joined retort and condenser connected to an air pump. In this way the boiling point of the mercury may be greatly lowered, and the probability of simultaneous distillation of small quantities of zinc and lead is diminished, k rough test, commonly applied to indicate the presence of any considerable GLOSSARY. 387 percentage of base metal, is conducted by placing a drop of the mercury upon an inclined surface of smooth glass or glazed porcelain; if pure, it should retain its spherical shape and roll over the surface, leaving no trace behind; but if impure, it assumes an elongated form and tends to leave a grey trail behind it, or, in other words, it is said to tail. Mercury may be mono- valent or divalent, and thus forms two oxides, mercurous (Hg20) and mercuric (HgO), with their corresponding salts. ^ As these salts deposit mercury on base metal by simple ^ immersion, and the reduced mercury then amalgamates with the re- mainder of the other metal, their solutions must be used in the operating room with as much circumspection as quicksilver itself. Mercury Chlorides. — Mercurous chloride or calomel^ Hg2Cl2 and mercuric chloride or corrosive sublimate, HgCl2. — The former is a heavy white powder insoluble in water; the latter an extremely poisonous white, crystalline body, of which about 7 parts dissolve in 100 parts of cold, 53 in a like weight of hot water. Mercurous Nitrate, Hg2(N03)2.2H20.— A white, crystalline, very poisonous substance, which may deposit basic salt when treated with water, but is readily soluble in water containing a little nitric acid. It is best made by treating an excess of mercury with cold dilute nitric acid. The hot concentrated acid tends to produce mercuric nitrate, Hg(N03)2. Nickel, Ni.— A white metal of specific gravity 8*9, and very high fusing point. Formerly it could be obtained only in the cast condition, but by improved methods of treatment it is now readily procurable rolled into sheet of any required size. The chief impurities affecting its use are iron, copper, cobalt, and arsenic. It forms two oxides, but the chief salts belong to the monoxide group (NiO), in which it is divalent. Nickel is slowly dissolved by sulphuric or hydrochloric acids, rapidly by nitric acid, the attack being always favoured by heating. Nickel Carbonate, NiCOg. — An insoluble pale apple-green powder. An impure carbonate containing an excess of oxide is produced by adding potassium or sodium carbonate to the solu- tion of a nickel salt. Nickel Chloride, NiCl2.6H20.— Green soluble crystals, re- 388 GLOSSARY. suiting from the solution of oxide, metal, or carbonate in liydro- chloric acid. Nickel Citrate, Is^i(C^H-0-)o . UK.O.— A soluble green body formed by dissolving nickel oxide or carbonate in citric acid. Nickel Nitrate, Xi(X03)2.6HoO. — Green crystals, of which 50 parts are soluble in 100 of cold water. May be formed like the chloride, substituting nitric for hydrochloric acid. Nickel Sulphate, XiSO^.THoO. The most generally known and used salt of nickel. It is full green in coloiu', and is soluble in water to the extent of 37 parts in 100. Nickel- Ammonium Sulphate, XiS04.(XH^),S0^.6H,0.— Re- sembles the last, but 100 parts of water dissolve only 5*5 parts of the salt. It may be made by dissolving together nickel sul- phate and ammoniimi sulphate, and evaporating the solution until crystals are obtained. Phosphorus, P. — A non-metallic elementary substance procur- able in two modifications — vitreous and amorphous. The vitreous pliosjjliorus is sold in colourless or yellowish translucent sticks which gradually become slightly opaque, especially upon the sur- face. It is poisonous, and is insoluble in water, but dissolves very readily in certain liquids, of which carbon bisulphide is a ty^Q. It is a most oxidisable body, and takes fire spontaneously when exposed to the air ; it must, therefore, be preserved under water, and should only be removed from it when required for use, and then all operations must be conducted rapidly and carefully. If it is required to cut the blocks into smaller frag- ments, they should be ^^laced singly in a shallow dish containing suflicient water to cover them completely ; they may then be cut with a penknife, but on no account should they be so cut except under water, as the friction of the knife may suffice to inflame the phosphorus when in contact with air. Fragments must be prevented from clinging under the finger nail, as should they inflame subsequently, very troublesome sores may be produced. The pieces should be rapidly dried between pieces of blotting- paper and used without delay, being handled as little as possible ; it is safer for those unaccustomed to work with chemical sub- stances to hold them with light brass tongs. Phosphorus is soluble in oils and in carbon bisulphide ; its solution in the latter substance is used occasionally to assist in GLOSSARY. 389 the metallisation of electrotype moulds (see pp. 160, 1G4), but it is a most dangerous liquid to work with, owing to the readi- ness with which the solvent evaporates and leaves upon any object a thin film of phosphorus which often takes fire spontane- ously. This solution and its destructive properties have long been known under the name of Greek fire. This, and indeed all operations involving the use of stick-phosphorus, should be under- taken only by experienced persons, and should, if possible, be excluded from common Avorkshop use. The amorphous (or red) phosphorus, which is prepared by heating the vitreous variety to 464° F. with suitable precautions for the exclusion of air, is not spontaneously inflammable at ordinary temperatures, and is not poisonous; but as it is insoluble in carbon bisulphide, it is useless for the purposes to which this element is usually applied in electro-metallurgy. Plaster of Paris, from the mineral gypsum.— Thi^ is a more or less pure calcium sulphate, CaSO^. Its use as a plaster depends upon the property possessed by the anhydrous substance (all trace of water having been expelled by the application of heat) of taking to itself a quantity of water in chemical combina- tion, to form the hydrated salt, CaS04.2H20. In doing this a considerable amount of heat is evolved, expansion ensues, and the cream formed by the admixture of water and the powdered material sets into a substance, which rapidly hardens ^ as the combination becomes complete and the excess of liquid is absorbed. Since the value of the plaster is dependent upon its power of absorbing water, it must never be allowed to remain exposed to the moisture of the air, from which it would slowly extract its full measure of water of hydration, but must be preserved in well-closed vessels. Gypsum, which has been over- burnt, refuses to absorb water, and is, therefore, useless. A sample of the plaster when made into a cream with water should become warm, and in the course of half-an-hour set into a firm, solid, but porous mass. For moulding purposes the plaster must be free from foreign matter, especially from gritty particles. Platinum, Pt. — A heavy, brilliantly-white metal, unalterable in air, very ductile and malleable, and of extremely high fusing point. Its specific gravity is 21*5. It dissolves only in aqua regia, being unafi'ected by either hydrochloric or nitric acid alone, and forms two sets of salts, corresponding to the oxides PtO and Pt02 respectively. 390 GLOSSARY. Platinic Chloride, PtCl^.SHgO. — Eecl crystals soluble in water ; but the substance usually known by this name is liydro- jplatinic chloride^ PtHgClg.GHgO. It results from evaporating a solution of the metal in aqua regia, together with a good excess of hydrochloric acid, and thus forms red brown, very soluble — and, indeed, deliquescent — crystals. Plumbago, sometimes known as gra]jMte or hlack-lead. — It is an impure natural variety of carbon ; and is found very abund- antly in Cumberland and in Ceylon. Being a conductor of electricity, it is largely used for facing non-conductive surfaces, which are to receive an electro-deposit of any metal. It should be very finely crushed, even to an impalpable powder. As some varieties are very inferior conductors, samples should be tested for efiiciency in this respect before final selection for use. Potassium Acetate, KC2H3O2. — White soluble crystals; 100 parts of cold water dissolving about 230 of the salt. Potassium Carbonate, 2K2CO3.3H2O. — White crystals very soluble in water; often used in the anhydrous state, when 100 parts of water dissolve about 105 parts of the solid. It is decomposed, with effervescence, by the addition of acid. Potassium Bicarbonate, KHCO3. — A much less soluble salt (25 in 100) which may be formed by passing carbon dioxide (carbonic acid gas) through a strong solution of the normal carbonate. Potassium Citrate. — White soluble crystals formed by just neutralising citric acid with potassium carbonate. Potassium Cyanide, KC]^. — A white opaque solid, generally bought in irregular lumps or in sticks. It is very soluble in water ; and owing to its becoming decomposed by even the weakest acids, carbonic acid among the number, it gradually alters by exposure to the air, especially in large towns where the atmosphere is laden with carbon dioxide, slowly evolving hydrocyanic acid, which imparts to it the peculiar and charac- teristic faint smell of bitter almonds. It is a deadly poison, and must be used with the utmost caution. Taken internally in minute quantities it may cause instant death, while the solution passing into the blood through cuts in the hand gives rise to painful sores and blood-poisoning ; even the fumes, in a badly- GLOSSARY. 391 ventilated room, cause headache and depression. The commercial cyanide is rarely pure, that known as gold cyanide is the best, the silver cyanide is inferior. It should always be tested before use, as it frequently contains less than half its weight of the pure salt. Potassium Ferrocyanide, K4FeC,N^.3H20. Yelloio prussiate of potashv—Y qWov^ crystals, of which 25 parts dissolve m 100 of water. A very commonly procurable substance. It gives a deep blue precipitate of Prussian blue when mixed with a solution of ferric chloride. Potassium Hydroxide {Potassium Hijdrate), KRO— Caustic Potash.— k most powerful caustic alkali bought, like the cyanide, in sticks or cakes. It is soluble in water with evolution of much heat; and substances, moistened with the solution, give rise to a peculiar slimy sensation of the skin when touched. It should never be allowed to enter the mouth, as even dilute solutions almost immediately remove the lining of tender skm Should such an event happen, the mouth should be at once rinsed several times with water and then with very dilute acetic acid. This body, whether in the solid state or in solution, must be carefully stored in well-closed vessels, as it rapidly becomes converted into carbonate by absorption of carbonic acid from the air, and thus loses its caustic properties. Potassium Iodide, KI.— An intensely soluble, white crystalline substance, 150 parts of which dissolve in 100 of cold water. It is decomposed by nitric acid with separation of lodme, which colours the solution yellow if dilute, or produces a dark almost black precipitate if it be concentrated. Strong sulphuric acid has a similar effect. Potassium Binoxalate, Y^G^HO^ — Salt of Sorrel. —WhiiQ crystals, not largely soluble in cold water, but imparting to it an acid reaction, turning blue litmus-paper red. Potassium Thiocyanate {Potassium Sidjpliocyanide), KCNS.— A very soluble white salt, absorbing much heat (or, as it is more commonly said, producing great cold) when dissolved in water. Its sohition gives a blood red colour when mixed with ferric chloride. Potassium Bitartrate, ^G^rO— Cream of Tartar.— k some- 392 GLOSSARY. what insoluble acid salt, 100 parts of water dissolving only 0*5 parts in the cold or 7 at the boiling temperature. It is colourless when pure, but the commercial crude tartar or argol, which is a by-product in the wine industry, is usually stained purple. The pure salt may be made from this by dissolving it in water, filtering it and allowing it to crystallise on cooling. Potassium-Sodium Tartrate, KNaC^H^Og.^H^O — Eochelle- or Seignette-salt, — A very soluble white crystalline substance, which may be made by adding 4 parts of potassium bitartrate and 3 of crystallised sodium bicarbonate, little by little, to 12 parts of boiling water, and then cooling in order to allow crystals to deposit. Rosin, or Colophony. — One of a large series of bodies termed resins, exuded by certain trees. Common rosin is deep amber to brown in colour, and should be translucent and brittle. It becomes slightly but distinctly softened at a temperature of 120° F., and as the temperature rises increases in softness until it becomes viscous, and, finally, semi-liquid at about the tempera- ture of boiling water. Silver, Ag. — A very white and unalterable metal with a specific gravity of 10*4 to 10*5 and a fusing point of 1740° F. It is extremely malleable and ductile, and is at the same time the best known conductor of heat and electricity. It combines readily with sulphur, and is thus rapidly covered with a black tarnish of silver sulphide in the atmosphere of towns. Alloyed with copper it is used for silver coinage, the amount of alloy varying in different countries, the English standard being 92'5 of silver to 7*5 of copper. To prepare silver (^.e., pure silver) from such an alloy, the metal should be dissolved in nitric acid in a glass or glazed earthenware vessel; any black residue is gold, of which there is frequently a small quantity present, and must be filtered off. To the solution (which is blue, owing to the presence of copper) a common salt solution, or better, dilute hydrochloric acid, is slowly added, so long as it continues to produce a white curdy precipitate. The liquid is stirred well to promote the subsidence of the latter, and then allowed to settle ; a [little more of the salt or acid is now added, which should produce no further precipitate (if it should do so, more must be added, until the whole of the silver has thus been thrown down). Any addition of common salt beyond that necessary for complete precipitation only tends to re-dissolve the silver chloride formed. GLOSSARY. 393 which is fairly soluble in brine; thus, hydrochloric acid is to be preferred as a precipitant, because a moderate excess is without action on the silver salt. The blue copper solution is now poured away from the heavy silver chloride, which is then stirred up with fresh water, and allowed to subside. This washmg hy decantation is repeated several times, the wash waters being disregarded. The chloride is then collected on a filter, dried, and mixed with an equal bulk of dried sodium carbonate, transferred to a fire-clay crucible, and heated to a bright-red heat in a clear charcoal- or coke-fire. As soon as fusion commences, effervescence will be observed, due to the mutual decomposition which occurs between the silver chloride and sodium carbonate, whereby sodium chloride and silver carbonate are produced, the latter body being dissociated at the temperature of the operation into carbon dioxide, oxygen, and metallic silver, the two former escaping m the gaseous state, the latter sinking through the slag by virtue of its higher specific gravity, and collecting into a fused mass at the bottom of the pot. When the contents of the crucible are tranquil they are poured, with the aid of a pair of large bent iron tongs, into an iron ingot-mould of cup-shape, from which, when cold, the silver and slag (that is, the fused salt) are readily removed and separated one from the other. Silver forms one set of salts derived from the oxide, Ag20, in which the metal is monovalent. Moist silver salts should not be allowed to come into contact with clean surfaces of base metals, which will de- compose them by simple exchange ; nor should they be exposed unnecessarily to white light, by which many of them are gradually decomposed and darkened in colour. Silver Carbonate, Ag^COg.— A pale yellow, insoluble substance, formed by adding a carbonate of soda solution to one of a soluble silver salt such as the nitrate. Silver Chloride, kgG\—Horn Silver.— k. white substance grad- ually passing through a gradation of shade from violet to black by exposure to white light. It is practically insoluble in water, but dissolves to some extent in solutions of sodium chloride, and readily in ammonia, and in sodium thiosulphate (hyposulphite) or potassium cyanide solutions. It is formed, as described above under the head of fine silver, by adding hydrochloric acid or common salt to a solution of silver nitrate. Silver Cyanide, AgCK.— A white insoluble salt, best formed by gradually adding a potassium cyanide solution to one of silver 394 GLOSSARY. nitrate, carefully watching the formation of the precipitate, and allowing it to subside after each addition of cyanide, so that, immediately another drop of the potash salt fails to produce a further precipitate or cloudiness in the liquid, all further addition is stopped, otherwise the silver cyanide will begin to re-dissolve in the excess of the precipitant. The liquid is then washed several times by decantation, as in the case of the chloride. To obtain the pure cyanide, only distilled water must be used; ordinary spring- or river-water, or even rain-water, contain chlorides, which cause the contamination of the cyanide by silver chloride. The essentials for success are pure substances, and precisely the right proportion between the silver nitrate and potassium cyanide. The silver cyanide dissolves readily in ammonia and sodium thiosulphate as well as in potassium cyanide. Silver Iodide, Agl. — Has a pale yellow^ colour; it is readily formed by adding potassium iodide to silver nitrate solutions. It is insoluble in water, and practically even in strong ammonia ; strong potassium iodide liquor, however, dissolves a fair propor- tion of the salt. Silver Nitrate, kgEO^— Lunar Caustic— K white crystalline body, obtainable readily in crystals, but sometimes fused into sticks. It dissolves readily in water. In making solutions of this or of any other silver salt, only distilled water should be employed ; all other waters, owing to the presence of chlorine, produce a cloudiness or even a distinct precipitate of silver chloride. Silver Oxide, Ag^O. — A deep brown, or almost black, insoluble powder, obtained by adding caustic soda or potash to silver nitrate solution. ^ Silver Sulphate, Ag.,SO^. — Brilliant white crystals, only slightly soluble in cold water, but more so in boiling water : they are also soluble in strong sulphuric acid, from which they are partly reprecipitated by the addition of water. Sodium Carbonate, l^^.fO^.lOYif^— Washing Soda or Soda Crystals. — Very soluble, colourless, alkaline crystals. It behaves chemically like potassium carbonate. An impure kind, contain- ing, inter alia, caustic soda and various foreign salts, is sold as a non-crystalline powder under the name of soda ash, which is suitable for fluxing in obtaining fine silver or gold, but should GLOSSARY. 395 not be employed iu making up electrolytic baths. A similar variety, commonly known as refined alhah, is purer, but still not always safe. Sodimn Bicarbonate, NaHCOg.-A white soluble powder, whose relation to the carbonate is analogous to that between the corresponding potassium salts. Sodium Chloride, IUMI— Common Salt ; Table Salt ; Rock Salt ; or Bmj Salt, the latter are not always F^^e. — The pure salt should form white cubical crystals, of which 100 parts of cold water dissolve 36 parts, hot water taking up slig'htly more. The natural varieties, or rock salt, frequently contain a con- siderable percentage of iron, which imparts a brown or purple tint to the body; while salt obtained from the sea water is often found to contain magnesium compounds and other bodies. Sodimn Citrate.— Soluble colourless crystals formed by neutra- lising citric acid with sodium carbonate. Sodium Hydroxide (Sodium Hydrate), lif MR— Caustic Soda.— White soluble lumps of a highly caustic" character resemblmg potassium hydrate (which see) in properties and ettects. Sodium Phosphate.— There are three principal phosphates— tbe orthophosphate, Na3PO,12H,0; /^e OTftoap/mfe NaPoO..10H„O; and the metapliosphate, NaPOg. All ot them are white bodies soluble in water; ^^V^^ «^*0P^?«P^^*f ' or rather the disodium-odhoplwspliale, iSa^iii U4.1^ti2U, m which one atom of hydrogen takes the place of one of sodium, is that more commonly met with. The pyrophosphate is sometimes used in making up baths. Sodium Sulphite, Na,S03.7H.20. — White soluble crystals, with an alkaline reaction. Sodium Bisulphite, NaHSOj, is a similar body, but with an acid reaction. Both are compounds of soda with sulphurous acid. Sodium Thiosulphate {Sodium Hypomlphite), '^a.^i^fi^^R.iO.^ Colourless soluble crystals, which have the property of dissolving- silver salts by forming a soluble double thiosulphate of silver and soda. 396 GLOSSARY. Sodium Stannate, Na^SnOs.— A white substance soluble in water, formed by fusing either stannic oxide (the dressed ore, tin stone, or cassiterite may be used) with caustic soda; or the metal itself with caustic soda to which sodium nitrate has been added. The aqueous solution, when evaporated, yields crystals containing water of crystallisation. Sodium Tartrate, '^s.^C^^fi ^.211^0. — White and soluble crystals, much more soluble in hot than in cold water. Sulphur, S, formerly better known as brimstone. Obtainable ^^floivers of sulyliur and as stick or roll sulphur, both of which are pale yellow in colour, but the latter variety is crystalline and soluble in carbon bisulphide, while the former is amorphous and insoluble. It melts at 238° F., and will yield sharp impres- sions of objects, so that it is occasionally useful in obtaining casts. Mixed with iron sulphide it forms the basis of Spence's metal, of which medallions and plaques have been sometimes made, and may thus come into the hands of the electrotyper. Being very fragile, objects made of sulphur or Spence's composi- tion, must not be subjected to pressure in taking casts from them with other moulding materials. Sulphur is, of course, very inflammable, and, therefore, requires care in melting. Tin, Sn. — A white, soft, very fusible and malleable metal, too weak to possess any great ductility, with specific gravity 7*29, and fusing point 450° F. It is not readily tarnishable, and, therefore, retains its brilliancy for a long time when exposed to the air. Easily soluble in hydrochloric acid or aqua regia, but converted into an insoluble white oxide by nitric acid. It forms two series of salts, corresponding to the oxides stannic, SnOg, in which it is tetravalent, and stannous, SnO, where it is divalent. Commercially, tin frequently contains traces of lead, tungsten, iron, copper, antimony, or arsenic. Tin Tetrachloride, ^TiG\— Stannic adoride.—K colourless, fuming liquid, boihng at 248° F. It forms several solid hydrates, mostly crystalline, by the addition of water ; such are the butter of tin and oxymuriate of tin. Tin Dichloride, ^^^^.211^} —Stannous Chloride or Tin Salt. — A white soluble crystalline substance, formed by dissolving tin in hydrochloric acid. GLOSSARY. 397 Varnish— Lac^^wer Varnish.— The formulse for this varnish, which is used for protecting metalhc surfaces from tarnish, are almost innumerable. Perhaps the best are those m which seed lac is dissolved in from 8 to 10 parts of the strongest spirits of wine, freed as far as possible from water, and coloured by the addition of dragon's blood or gamboge (say from i to i of a part), or by mixtures of these, according to the particular tint that it is desired to obtain. ^ . . . , . • i Stovping-off Varnish.— Since the object of this class of varnish is to prevent the formation of an electrolytic deposit upon any desired portion of an article, the requirements are evidently— a non-conductive material, easily applied and with facility remov- able which shall not be attacked by the solution m which it is to be immersed, and which should of preference be coloured in such a way that the portions of the surface protected by it may be seen at a glance, for convenience of application. To resist ordinary bath-solutions used cold, almost any varnish is appli- cable, and common copal varnish, mixed with a colouring medium, will be found suitable; or asphalt varnish may be used, as for the back of electrotype plates. But for hot cyanide solutions other materials are required, although copal varnish is often used, and for this work, a mixture of 44 per cent, rosm with 26 of bees'-wax, 17 of seahng-wax, and 13 of jeweller's rouge, may be applied with advantage, provided that the best materials alone are employed in preparing it. Zinc, Zn.— A bluish-white divalent metal, fusible at 783^ F., easily distilled at a higher temperature. Specific gravity, 7*1. It is brittle in the cold, and above 250° F., but near the tempera- ture of boiling water, it is sufficiently malleable to admit of rolling into thin sheets. It forms one class of salts only, from the monoxide, ZnO. The chief impurities in the commercial metal are lead, iron, cadmium, and arsenic. Zinc Acetate, Zn(C,H30,),.3H20. — Very soluble white crystals. Zinc Carbonate, ZnCOg.— A white insoluble substance, pre- pared by adding sodium 'carbonate in excess to a solution of any zinc salt. It is decomposed by acids with effervescence. Zinc Chloride, ZnCl.2.— A very soluble and deliquescent white, opaque, soft mass ; may be prepared by dissolving zmc or its oxide or carbonate in hydrochloric acid. 398 GLOSSAEY. Zinc Cyanide, Zn(CX).,.— A white substance insoluble in water, but readily so in iX)tassium cyanide solutions. Prepared by precipitatiDg a solution of zinc sulphate with one of potassiiun cyanide ; of course, carefully avoiding excess of the latter. Zinc Oxide, ZnO.— A white substance, becoming yellowish when strongly heated, but returning to its original colorn* on cooling. The liydroxide, or hydrated oxide, Zn (0H)o, is preci- pitated when an exact equivalent of caustic alkah is added to a solution of a zinc salt ; an excess of the alkali tends to redissolve the precipitate. Zinc Sulphate, ZnS0..7H.,0— IT7^/fe T'7f/-/o/.— Soluble white crystals; the commonest salt of zinc. It may be prepared like the chloride, of course substituting sulphuric for hydrochloric acid. 100 parts of water dissolve about 50 of the salt in the cold, and nearly 100 at the boiling point. ADDEI^DA. TABLE XXYIII —Giving Data for Calculating the Weight and Thick- NESS OF Deposit Produced by a known Current-Volume in a Given Time for certain of the Commoner Mktals. METAL. Atomic Weight. Equivalent Weight. Specific Gravity. ' Electro-Chemical ! Equivalent. : Weight de- 30sited per hour by current of 1 Ampere. Thickness of Deposit produced in 1 hour by current of : — Grammes. Grains. 1 Ampere per Square Decimetre. 1 Ampere per Square Inch. Mm. Inch. Aluminium, . . . 27 9-0 2-6 0-00009317 0-3354 5^175 0-0129 0-007875 Antimony, .... 122 40-6 6-8 •00042025 [•5130 •23-350 •0222 •013584 Arsenic, . . . • 75 25 5-8 •00025880 0-9317 14-378 •0161 •009806 Bismuth, .... 210 70 9-8 •00072464 2-6087 40-258 •0266 •016251 Cadmium, .... 112 56 8-6 •00057971 2-0870 32-207 •0243 •014811 Chromium, .... Oil 17*5 J. 1 o 7*0 •00018116 0^6522 10^052 •0093 •005687 Cobalt, 59 29-5 8-7 •00030538 1-0994 16-966 •0126 •007715 Copper (Monovalent), 63-5 63-5 8-9 •00065735 2-3665 36-520 •0265 •016208 (Divalent), . 63-5 31-7 8-9 •00032867 1^1832 18-260 •0133 •008104 Gold, 196-6 65-5 19*3 •00067806 2-4410 37-670 •0127 -007721 Iridium, .... 197 49-2 21-1 •00050932 1-8335 28-296 •0087 -005291 Iron (Divalent), 56 28 8-1 •00028986 1-0435 16-103 -0128 •007826 Lead, 207 103-5 11-4 •00107140 3-8571 59-525 •0338 -021134 Magnesium, . . • 24-3 12-1 1-7 •00012526 0-4^09 6-959 •0-265 •016190 Manganese, . . . 55 27-5 8-0 •00028468 1-0248 15-816 •0128 -007821 Nickel, 59 29-5 8-5 •00030538 1-0994 16-966 •0129 •007894 Palladium, .... 106-5 26-6 11-4 •00027536 0-9913 15-298 -0087 •005291 Platinum, .... 197 44-3 21-2 •00045859 1-6509 25-478 -0078 •004743 Silver, 108 108 10-6 •00111800 4-0249 62-113 •0380 •023142 Tin (Divalent), . . 118 59 7-3 •00061077 2-1988 33-932 •0302 •018414 Zinc, 65 32-5 71 •00033644 1-2112 18-691 -0171 •010415 Note —These figures are based on Lord Rayleigh's number for the electro-chemical equivalent of hydrogen = 0-000010352. This (0*000010352 grams H. per second) is equal to 6-9503 cc. H. or 10*4255 cc. mixed gas {'R^ + 0)per mmute. 400 ADDENDA. TABLE XXIX.— Showing the Value of Equal Cueeent Volumes as Expressed ix Ampeees pee Squaee Decimetre, pee Squaee Foot, AXD PEE Squaee Inch of Electeode Sueface. «22 o 00 O £^ O o II o & ac — ^ 'J 03 « y o 2^ g s 11^ 0 2^ X p < — S § 2 S"x .E l|| -4^ m 0 II ^ 0 - Amperes per S(iiiare Inch. 005 0-46 0-0082 0-8 7-43 0-0516 6-20 57-6 0-4 0-054 0-5 0-0035 0-86 8 0-0555 6-46 60 0-4167 0-077 0-72 0-005 0-9 8-36 0-0581 7 65-0 0-4516 0-1 0-93 0-0064 0-93 8-64 006 7-53 70 0-4861 0-11 1 0-0069 0-97 9 0-0625 7-75 72-0 0-5 0-15 1-44 001 1 9-29 0-0645 8 74-3 0-5161 0-2 1-86 0-0129 1-08 10 0-0694 S-61 80 0-5555 0-22 2 0-0139 1-09 10-08 007 9 83-6 0-5806 0-3 2-79 0-0193 1-24 11-52 008 9-30 86-4 0-6 0'31 2-S8 002 1-39 12-96 009 9-69 90 0-6250 0-32 3 0-0208 1 '55 14-4 01 10 92-9 0-6452 0-4 3-71 0-0258 2 18-6 0-1290 10-76 100 0-6944 0-43 4 0-0278 2-15 20 0-13S9 10-85 100-8 0-7 0-46 4-32 003 3 27-9 0-1935 12-40 115-2 0-8 0-5 4-64 0-0323 3-10 28-8 0-2 13-95 129-6 0-9 yj Orx o ou 10 ou i-i-i U 1 0-6 5-57 0-03S7 4 37-1 0-2581 20 185-8 1-2903 0-62 5-76 004 4-30 40 0-2778 21-53 200 1-3889 0-65 6 0-0417 4-60 43-2 0-3 30 278-7 1-9355 07 6-50 0-0452 5 46-4 0-3226 31-0 288 2 0-75 7 0-0486 5-38 50 0-3478 32-3 300 2-0833 0-77 7-20 005 6 55*7 0-3S71 46-5 432-0 3 By this table the current density may be expressed iu amperes per square decimetre, square foot, or square inch, any one of them being given. Thus a current of 1 ampere per square decimetre has the same electrolytic value as one of 9"29 amperes per square foot, or 0-0645 per square inch. To find the value of intermediate numbers not shown above, add together the various numbers representing the hundreds, tens, units, and decimals of the given quantity. Thus 27-5 amperes per square decimetre ( = 20-r7-^0-5) is equivalent to lS5-S+65-f-'l -64 = 255-44 amperes per square foot, or l-2903+0-4516+0-0323=l-7742 amperes per square inch. ADDENDA. 401 TABLE XXX.— Showing the Specific Gravities of Sulphuric, Kitric, AND Hydrochloric Acids corresponding to varying Percentages OF Pure H.SO4, HNO;}, and HCl in the Liquids respectively. % H2S04. HNO3. HCl. 7o H2SO4. HNO3. HCl. 7o H2SO4. HNO3. HCl. 100 1-8426 1*500 66 1 -568 1*3783 33 1*247 1*1895 X XOi/t/ 1 -1 fi40 X X u ^vy 99 1-8420 1*498 65 1-557 1*3732 32 1*239 1*1833 1 -1 584 X xOO'± 98 1-8406 1-496 64 1-545 1*3681 31 1*231 1*1770 X X 1 4 V 1 -1 536 X X o^\/ 97 1-8400 1*494 63 1*534 1*3630 30 1*223 1*1709 X X < \j 0 1 -14-84 96 1*8384 1*491 62 1*523 1*3579 29 1*215 1*1648 1 -1 433 95 1-8376 1*488 61 1*512 1*3529 28 1*2066 1*1587 1 -1 882 94 1-8356 1*485 60 1*501 1*3477 27 1*1980 1*1515 1 -1 888 X xOOO 93 1*8340 1*482 59 1*490 1*3427 26 1*190 1*1467 1 -1 982 92 1*8310 1*479 58 1*480 1*3376 25 1*182 1*1408 X X TtUtJ 1 -1 989 X XZiO^ 91 1*8270 1*476 57 1-469 1*3323 24 1*174 1*1345 1-11 89 X X xo^ 90 1*8220 1*473 56 1*4586 1*3270 23 1*167 1*1286 1 -1 181 X X i 0 X 89 1*8160 1*470 55 1*448 1-3216 22 1*159 1*1227 1 -1 081 88 1*8090 1*467 54 1*438 1*3163 21 1*1516 1*1168 1*1081 X X\J'J X 87 1*802 1-464 ' 53 1*428 1*3110 20 1*144 1*1109 X X X v/c 1 -0Q81 86 1*794 1-460 ^ 52 1-418 1*3056 19 1*1 86 X X tJU 1 '1051 1 -0Q81 85 1*786 1*457 ! 51 1-408 1-3001 1 18 1 '1 9Q X X Zit7 1 *0Q98 1 -0889 84 1*777 1*453 50 1*398 1-2947 17 X 1 1*1 91 ■i X ^ X 1*0935 1 '0882 83 1*767 1*450 49 1*3886 1*2887 16 1*11 ,8fi XXX ou 1 *0878 1 -0788 82 1*756 1-446 48 1-379 1*2826 ... X *J 1 '1 06 X xuu 1 -08-21 X VOii X 1 -0784 81 1*745 1*4424 47 1-370 1*2765 14 1 'OQS 1 -0764 1*0684 80 1*734 1-4385 46 1-361 1*2705 1 'OQl 1*0708 1*0635 79 1*722 1*4346 45 1-351 1*2644 1 2 X ^ 1 -08^ i uoo 1*0651 1*0586 78 1*710 1*4306 44 1-342 1*2583 1 1 X X 1 -0756 1*0595 1*0537 77 1*698 1*4269 4'3 1-333 1*2523 10 1*068 1*0540 1*0487 76 1*686 1*4228 42 1*324 1-2464 Q £7 1 -061 X vU X 1*0485 1*0438 75 1*675 1*4189 41 1*315 1*2402 8 1*0536 1*0430 1*0389 74 1*663 4-0 1 ouo 1 -9^4.1 X ZiO i J- i lyOD 7 4 1 'C\Af\A i U'10'1 1*0375 1*0340 73 1-651 1*4107 39 1*297 1*2277 1*1922 6 1*039 1*0320 1*0292 72 1-639 1*4065 38 1*289 1*2212 1*1878 5 1*032 1*0267 1-0244 71 1*627 1*4023 37 1-281 1*2148 1*1840 4 1*0*256 1*0212' 1*0196 70 1*615 1*3978 36 1*272 1*2084 1*1786 3 1*019 1*0159 1*0148 69 1*604 1-3945 35 1*264 1*2019 1-1738 2 1*013 1*0106 1*0098 68 1*592 1-3882 ... 34 1*256 1*1958 1*1689 1 1-0064 1*0053 1-0049 67 1*580 1*3833 Note. — The sulphuric acid numbers are quoted from Otto, those for nitric acid from Ure ; while the hydrochloric acid figures are compiled by inter- polation from Ure. Liquid hydrochloric acid is practically saturated with 40 per cent, of HCl gas. 2 c 402 ADDENDA. TABLE XXXI —Showing the Specific Gravities of Solutions CORRESPONDING TO THE DEGREES OF THE BaUME HYDROMETER ^ Degree Baume. = Specific I Degree I = Specific Gravity. ; Baume. | Gravity. Degree = Specific Degree! = Specific Baume. Gravity. Baume. Gravity. 10 11 12 13 14 15 16 17 18 1-000 19 ' 1-147 37 1-007 20 1-157 38 1-014 21 1-166 39 1-020 22 1-176 40 1-028 , 23 1-185 41 1-034 1 24 1-195 42 1-041 ! 25 1-205 43 1-049 26 1-215 44 1-057 27 1-225 45 1-064 i 28 1-235 46 1-072 ' 29 1-245 47 1-080 : 30 1-256 48 1-088 ; 31 1-267 49 1-096 1 32 1-278 50 1-104 : 33 1-289 51 1-113 1 34 1-300 52 1-121 ! 35 1-312 53 1-130 1 1-3-24 54 1-138 1 1 1-337 1-349 1-361 1-375 1-388 1-401 1-414 1-428 1-442 1-456 1-470 ■485 ■500 - •515 -531 •546 -562 1-578 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 1-596 1-615 1-634 1-653 1-671 1-690 1-709 1-729 1-750 1-771 1-793 1-815 1-839 1-864 1-885 1-909 1-935 1-960 XoTP — The sDecinc gravity oi a somiioii is rapmi^ aa«.ci tai^ic^i ..^.*v.^a wPi^htPd alai tX closed at lioth ends, with a bulb in the centre and a long thin glass rubfabovl ThS scale upon it. This instrument sinks deeper m Stions ohow deS^^^^ of high gravity ; and the actual specific gravity is fmind iw thrie^^l at ^ the liquid stands on the graduated portion when the In^aratus is floatin- freely in it. Hydrometers of this kind are sometimes graduated so Kthe specmc^^^^^^^^^ off direct from the scale, others are graduated by Baume s method ;"he^^eading may then be converted into the number representnig the true density, by reference to the above table. TABLE XXXII.— Densities of Solutions of Crystallised Copper AND Zinc Sulphates. Copper Sulphate. Zinc Sulphate. Percentage by Weight of CUSO4. 5H2O. Density. Percentage by Weight of CUSO4. 5H2O. Density. 2 1-0126 14 1-0923 4 1-0254 16 1-1063 6 1-0384 18 1-1208 8 1-0516 20 1-1354 10 1-0649 22 1-1501 12 1-0785 24 1-1659 5 10 15 20 25 30 Density. 1-0289 1-0588 1-0899 1-1222 1-1560 1-1914 ESI 35 40 45 50 55 60 Density. 1-2285 1-2674 1-3083 1-3511 1-3964 1-4451 NOTE—The pure salt is supposed to be dissolved in pure distiUed water. ADDENDA. 403 TABLE XXXIII.— Showing THE Specific Electrical Resistances* of Different Sulphuric Acid Solutions at various Temperatures {Fleeming Jeiikin). Specific Gravity TEMPERATUHES (FAHRENHEIT). of Acid. 32° 39° -2 46° -4 53° -6 60° -8 68° 75°'2 82° -4 1-10 1-37 1-17 104 0-92 0-84 079 074 071 1"20 1-33 1-11 0-93 0-79 0-67 0-57 0-49 0-41 1-25 1-31 1-09 0-90 0*74 0'62 0-51 0-43 0*36 1-30 1-36 1-13 0-94 0-79 0-66 0-56 0-47 0-39 1-40 1-69 1-47 1-30 1-16 1-05 0-96 0-89 0-84 1-50 2-74 2-41 2-13 1-89 1-72 1-61 1-32 1-43 1-60 4-32 4-16 3-62 3-11 2-76 2-46 2-21 2-02 170 9-41 7'67 6-25 5-12 4*23 3-57 3-07 271 TABLE XXXIV. — Showing the Specific Electrical Resistances* of Different Copper Sulphate Solutions at various Temperatures {Fleeming Jenkin). JSTo. of parts of Copper Sulphate dissolved in 100 parts of water. TEMPERATURES (FAHRENHEIT). 57° -2 60° -8 64° -4 68° 75° -2 82° -4 86° 8 457 437 41-9 40-2 37-1 34-2 32-9 12 36-3 34-9 33-5 32-2 29-9 27-9 27-0 16 31-2 30-0 28-9 27-9 26-1 24-6 24-0 20 28-5 27-5 26-5 25-6 24-1 227 22-2 24 26-9 25-9 24-8 23-9 22-2 207 20-0 28 247 23-4 22-1 21-0 18-8 16-9 16-0 * Note. — By the term Specific Resistance in the above tables is meant the absolute resistance in ohms of a column of the liquid 1 square centimetre in cross-section, and 1 centimetre long ; in other words, it is the resistance of a cubic centimetre of the liquid. The diminution of resistance accom- panying a rise of temperature should be especially marked. 404 ADDENDA. CQ P P3 63 O H o H 50 CO 62 P5 S H O XI XI x: •J <1 H S-g p §^ = 5 p o o o GO 1— < 1—1 Oi So § ^ «4-l Ci CM O I O CO o I 1—1 CO I ^ lO o o o o CO Ci CO i>. o IX> -t^ -rtl CO C ^ ^ rH 1;^ I— 1 CO o o CO CM 1—1 1-1 1^ O 1-1 CM (M u n ^ G CO o CO OO lO CO CO i 5 2 .2 rl O 03 c+_i — |55 GO CO i_ c p ^ ^ CO id CO o C O 53 c3 O C ^«*-<^ .2 r-l O pO 155 (M CO CO CO CO o Oi !>. CO CO CO CO C5 o OO CO o 1—1 CO o CO o o o CO OO CO GO o CO (M CO CM o o o 1— 1 1—1 1—1 CM CM CO o o o o o o o o o o o 00 CO CO rH CO Oi 1—1 OO CO CO 1—1 CO CO Ci CO o CO CO o Oi OO Oi CO Oi OO CM CO ■rri wo l:^ OO o o o o o o 1—1 1—1 CM CM o o o o o o o o o o o o o o o o o o CO CO 1—1 1—1 Oi <3i CO CM lO o Oi o i>. o OO VO CO CO OO o 00 1^ CO vo CO CO Oi CM 00 rH CO CO CM CO (M Oi OO o )0 o !>. Oi rH CO CO • CM iO o o o o 1— < rH CM CM o o o o o o o o O o o o o o o o o o o o o o o o o o o o o CM CO ADDENDA. 405 TABLE XXXYL— Actual Diameters Corresponding to the Numbers OF the Old Birmingham Wire Gauge. Gauge Number. DIAMETER. Gauge Number. Diameter. Gauge Number. Diameter. Inch. Milli- metres. Inch. Milli- Inch. metres. 0000 f\ -A KA U 404: 1 ± oo 11 0*120 3*05 24 0-022 0-56 000 •425 10-79 12 •109 2^77 25 •020 •51 00 ocU i? oo 13 •0Q5 2^41 26 •018 •46 0 •340 8-63 14 •083 2^11 27 •016 •41 1 •300 7-62 1 15 •072 1^83 28 •014 •36 2 •284 7-21 16 •065 1-65 29 •013 •38 3 •259 6-58 17 •058 1^47 30 •012 •305 4 •238 6-04 18 •049 1-24 31 •010 •254 5 •220 5^59 19 •042 1^07 32 •009 •229 6 •203 5-16 20 •035 0-89 33 •008 •203 7 8 •180 4-57 21 •032 •81 34 •007 •178 •165 419 22 •028 •71 35 •005 •127 9 •148 3-76 23 •025 •63 36 •004 •102 10 •134 3-40 TABLE XXXYIa.— Diameters Corresponding to the Numbers of the American (Brown & Sharpe's) Standard Wire Gauge. Gauge Number. Diameter. Gauge Number. Diameter. Gauge Number. DIAMETER. Inch. Milli- metres. Inch. Milli- metres. Inch. Milli- metres. 0000. 0-46 11^68 12 0^0808 2^05 27 0^0142 •361 000 •4096 10-44 13 •0720 1^83 28 •0126 •320 00 •3648 9-26 14 •0641 1-63 29 •0113 •287 0 •3249 8-24 15 •0571 1-45 30 •0100 •254 1 •2893 7-35 16 •0508 1^29 31 •00893 •227 2 •2576 6^54 17 •0453 1^15 32 • ^00795 •202 3 •2294 5-83 18 •0403 1^02 33 •00708 •180 4 •2043 5-19 19 •0359 0-912 34 •0063 •160 5 •1819 4-62 20 •03-20 •813 35 •00561 •142 6 •1620 4^11 21 •0285 •724 36 •005 •127 7 •1443 3-66 22 •0253 •642 37 •00445 -113 8 •1285 3^26 23 •0^226 •574 38 •00397 •101 9 •1144 2-90 24 •0201 •510 39 •00353 •09 10 •1019 2-59 25 •0179 •455 40 •00314 •08 11 •0907 2-30 26 •0159 •404 406 ADDENDA. o f4 W o H O Ph . cc W W O Ph P Ph ^ oo O t-H § § o ^ 02 P^ H !^ -< t-H P I XI X S 3^ 2 S fcJD . O as;?; CO rH Oil— (C0?O0iC000'-t<(MCi00?O»O'^C^(Mi— lOO lO^'^COCN'MrHi— (t— lOOOOOOOOOO ooooooooooooooooooo CO I— iCXDOiOkOCOOOr— iOOiO(NOOOO'^COC.<:D»o-xficoc^ CMCqCMC^T— (I— IrHi— ii— (i—irHOOOOOOOO COO(M'^?OOOOC^OO'^O^C<100-^OCO'>10 000500t^?OCOO-^'*-^COCO(M(MC^i-Hi— It— I T— It— lOOOOOOOOOOOOOOOOO ooooooooooooooooooo (^^co^locol>.oooi07-^(^^co^ocox:^ooo:>o ^ '^l OO 00 O:i"^C0 1^Oit^i— l<:0C.--cioo t-^cocococn C^G.x>.ooot^rHOCcx)j.^cokr;iTtico:M)CM(Mi— ii— ii—ir— I coTHcot^t^r-iTtii>-ooa;'rc(:oocoi— icoo^ico'^ C03^C0vO00kO0JOai05(NOCi-^OC0(MO0D OiCO'^CslOOiOOi^vO-^^CO'MC^C^r— li— IrHO TJ-lr- It— li— It— (OOOOOOOOOOOOOO o OOit^COO^OOCMr- foasoooot^cocovoiOT^ i~^i^^o:>,-H-^iXioqcoO'^oocooO'^ococ^05CO .X>.CO»OvO'^'^-<#COCOCQC— lOil^CO'^CMr-iO iOTl^^coco(^0(^:)c^^(^^(^lc^^I— It— ii— 11— ii— It— It— I o o o o o o 1^ CO CM T— IC^It^D-^OCOt^OOOSOr— f(M ADDENDA. 407 TABLE XXXVIIA.--SHOWING Maximum Cubkents ^ok Copper Conductors Insulated and Laid in Casing or Iubing. Wire or Cable. Size S.W.G.t Approximate Area in sq. in. 18 or 62/38 or 97/40 3/22 17 or 130/40 3/20 16orll0/38or 172/40 15 7/22 14 or 172/38 or 7/21^ 3/18 7/20 7/18 19/20 7/16 19/18 7/14 19/16 19/14 37/16 19/12 37/14 61/15 61/14 37/12 61/12 91/12 91/11 < 0*0018 >0-0018 0-0024 0-0031 0*0032 0-0041 0-0043 0-0050 0*0054 0*0071 0-013 0-019 0-022 0-034 0-035 0-061 0-096 0*12 0-16 0-19 0-25 0-31 0-31 0-52 0-77 0-9^ Maximum Current in Amperes for High Ex- ternal Tem- perature. Total Length in Yards of Lead and Keturn giving 1 Volt Drop. 3*1 3- 3 4- 0 4-8 4-9 5*9 6-2 7*0 7*5 9-3 14-0 20-0 23-0 31'0 32*0 49*0 70*0 83-0 100-0 120-0 150*0 170-0 180-0 260-0 350-0 420-0 23 23 25 26 27 28 28 29 30 31 37 39 40 45 45 51 56 59 66 64 67 74 73 82 90 94 Maximum Current Allowable. 4-2 4*4 5*4 6*6 6*8 8-2 8*7 9*8 11*0 13*0 21*0 30*0 34*0 48-0 49*0 77-0 110*0 130-0 170-0 190-0 240-0 290-0 300*0 450*0 620-0 740-0 Total Length in Yards of Lead and Return giving 1 Volt Drop. 18 17 19 19 19 20 20 21 20 22 25 26 27 29 29 32 35 37 39 40 42 43 44 47 51 53 * This Table with the exception of the approximate ar^^s in Col. 1, which have beeVa'te part oL table given in the Wiring Rules of the InsUu- tion of Electrical Engineers. . , v. ^ c+vanrqc, nf thp e-iven t The double figures in this column refer to 7™^^„%f,^^^rof N^^ number of wire in a cable : thus 3/22 means a cable of 3 strands ot JNo. wire. W5 ADDENDA. TABLE XXXYIII. — Comparison of Centigrade and Fahrenheit Thermometers. Fah. Deg. 32 33 33-8 34 35 35 6 36 37 37-4 38 39 39-2 40 41 42 42-8 43 44 44-6 45 46 46-4 47 48 48-2 49 50 51 51-8 52 53 53-6 54 55 55-4 56 57 57-2 58 59 60 60-8 61 62 62-6 63 64 64-4 65 Cent. Deg. 0 0- 5 1 1- 1 1- 7 2 2- 2 2- 8 3 3- 3 3- 9 4 4- 4 5 5- 5 6 6- 1 6- 7 7 7- 2 7- 8 8 8- 3 8- 9 9 9- 4 10 10- 5 11 11- 1 11- 7 12 12- 2 12- 8 13 13- 3 13- 9 14 14- 4 15 15- 5 16 16- 1 16- 7 17 17- 2 17- 8 18 18- 3 Fah. Cent. Fah. Cent. Fah. Cent. Fall. Cent. Fah. Cent. Deg. Deg. Deg. Deg. Deg. 'Deg. Deg. Deg. 74-4 Deg. Deg. 66 18*9 100 37-8 133 56-1 166 199 4 93 66-2 19 100-4 38 . 134 56-7 167 75 200 93-3 67 19-4 101 38-3 134-6 57 168 75-5 201 93-9 68 20 102 38-9 135 57-2 168-8 76 201-2 94 69 20-5 102-2 39 136 57-8 169 76-1 202 94-4 69-8 21 103 39-4 136-4 58 170 76-7 203 95 70 21-1 104 40 137 58-3 170-6 77 204 95-5 71 21-7 105 40-5 138 58-9 171 77-2 204-8 96 71-6 22 105 8 41 138 2 59 172 77-8 205 96-1 72 22-2 106 41-1 139 59-4 172-4 78 206 96-7 73 22-8 107 41-7 140 60 173 78-3 206-6 97 73-4 23 107-6 42 141 60-5 174 78-9 207 97-2 74 23-3 108 42-2 141-8 61 174-2 79 208 97-8 75 23-9 109 42-8 142 61-1 175 79-4 208-4 98 75-2 24 109-4 43 143 61-7 176 80 209 98-3 76 24-4 110 43-3 143-6 62 177 80-5 210 98-9 77 25 111 43-9 144 62-2 177-8 81 210-2 99 78 25-5 111-2 44 145 62-8 178 81-1 211 99-4 78-8 26 112 44-4 145-4 63 179 81-7 212 100 79 26-1 113 45 146 63-3 179-6 82 213 100-5 80 26-7 114 45-5 147 63-9 180 82-2 213 8 101 80-6 27 114-8 46 147-2 64 181 82-8 214 101-1 81 27-2 115 46-1 148 64-4 181-4 83 215 101-7 82 27*8 116 46-7 149 65 182 83-3 215-6 102 82-4 28 116-6 47 150 65-5 183 83-9 216 102-2 83 28-3 117 47-2 150-8 66 183-2 84 217 102-8 84 28-9 118 47-8 151 66-1 184 84-4 217-4 103 84-2 29 118-4 48 152 66-7 185 85 218 103-3 85 29-4 119 48-3 152-6 67 186 85-5 219 103-9 86 30 120 48-9 153 67-2 186-8 86 219-2 104 87 30-5 120-2 49 154 67-8 187 86-1 220 104-4 87-8 31 121 49-4 154-4 68 188 86-7 221 105 88 31-1 122 50 155 68-3 188-6 87 250 121 89 31-7 123 50-5 156 68-9 189 87-2 302 150 89-6 32 123-8 51 156-2 69 190 87-8 400 204 90 32-2 124 51-1 157 69-4 190-4 88 482 250 91 32-8 125 51-7 158 70 191 88-3 572 300 91-4 33 125-6 52 159 70-5 192 88-9 752 400 92 33-3 126 52-2 159-8 71 192-2 89 932 500 93 33-9 127 52-8 160 71-1 193 89-4 1112 600 93'2 34 127-4 53 161 71-7 194 90 1292 700 94 34-4 128 53-3 161-6 72 195 90-5 1472 800 95 35 129 53-9 162 72-2 195-8 91 1652 900 96 35-5 129-2 54 163 72-8 196 91-1 1832 1000 96-8 36 130 54-4 163-4 73 197 91-7 2282 1250 97 36-1 131 55 164 73-3 197-6 92 2732 1500 98 36-7 132 55-5 165 73-9 198 92-2 3182 1750 98-6 37 132-8 56 165-2 74 199 92-8 3632 2000 99 37-2 ADDENDA. 409 TABLE XXXIX.— AvoiEDUPOis Weight. = Ounces. = Drachms. = Grains. = Grammes. 1 Pound, .... 16 256 7,000 453-25 1 Ounce, . . . . 1 16 437-5 28-33 1 Drachm, 0-062 1 27-34 1-77 TABLE XL.— Tkoy Weight. = Ounces. = Pennyweight. = Grains. = Grammes. 1 Pound, . . . • 12 240 5,760 372-96 1 Ounce, .... 1 20 480 n-os 1 Pennyweiglit, 0-05 1 24 1-55 TABLE XLL— Apothecaries' Weight. 1 = Ounces. 1 = Draclims. = Scruples. = Grains. = Grammes. 1 Pound, . 12 96 288 5,760 372-96 1 Ounce, 1 8 24 480 31-08 1 Drachm, . 0-125 1 3 60 3-88 j 1 Scruple, . 0-042 0-33 1 20 1-29 TABLE XLIL— Imperial Fluid Measure. =^ Quart. = Pints. Fluid Ounces. = Fluid Drachms. = Minims. = Weight in Grains. = Cubic Inches. = Litres. = Cubic Centimetres. 1 Gallon, . . 4 8 160 1,280 76,800 70,000 277-276 4-541 4,541 1 Quart, . . . 1 2 40 320 19,200 17,500 69-319 1-135 1,135-2 iPint, . . . 0-5 1 20 160 9,600 8,750 34-659 0-567 567-6 1 Fluid Ounce, . 0-025 0-05 1 8 480 437-5 1-733 0-0284 28-34 1 Fluid Drachm, 0-0031 0-0062 0-125 1 60 54-7 0-217 0-0035 3-55 1 Minim, . . 0-00005 0-0001 0-0021 1 0-0167 1 0-91 0-0036 0-00006 0-059 410 ADDENDA. 2? IM c PliSpliiiiliSpHiliiiiiSiiigiP w iiiiiiiiiiiPPiiliiiililiPililii iii! iiiig|Siiiiii?Piii|p|IHiiiigip m 9 9 99 T ^'t^r ^r^T^r^-y ^rt-^T-^^ 9 Vr--P^ 99 99 9? O > 1 ill Wl ""^^=^/^«-^^^;^;^??-i2xg2gg||§|g| 1— W W < Or-c^T^wt^c;0(>iTOu^^:c!r^t-ccxTi-ri.-'roL-ou~0'--t-o»--toooooo < ipj pg??ppp?pgg£SSpiS3SSSpgS£5SgSgS^ W H fill Hi ill III 9rT'v'?"{^9^=c=r^9 >— 1 ADDENDA. 411 From this table any ordinary conversions up to 2000 units may be Teadily made. For. example It is required to find the number of cubic -centimetres equal to 1728 cubic inches. 1728 = 1000 + 700 + 20 + 8 cubic inches. But a reference to the sixth column shows that 1000 cubic inches = 16,385*92 cubic centimetres *700 „ „ - 11,470-10 20 „ = 327-72 8 „ „ - 131-09 „ Add together: 1728 „ - 28,314*83 „ The Bronzing of Copper and Brass Surfaces. It is often desired to give newly deposited copper the appearance of age and to destroy the brilliant metallic lustre which it possesses at first. ^ The methods of accomplishing this end are numerous. In all cases it is desirable to start with a clean metallic surface, freed from grease by immersion in potash, or by any other suitable cleansing process. To obtain a red bronze tone, the metal is brushed over with finely-powdered crocus, or a mixture of crocus and black-lead, made up into a paste with a little water, and is then heated on a metal plate above a clear Hre until the powder has become dark. After cooling, the whole surface is thoroughly brushed ; if necessary the process may be repeated to produce a darker colour. The bronzing is due to the oxidation of the copper superficially by the heated crocus; a l^etter lustre is obtained by finally rubbing persistently with a brush which is from time to time passed over the surface of a cake of bees'-wax. Slightly wetting the clean surface of copper articles with very dilute nitric acid, or with a solution of ferric chloride and nitrate, or with a solution of copper nitrate, followed by drying and heating, also effects the required oxidation and produces a brown bronze. Dark brown or black bronzing has sometimes been effected by merely brushing the surface with plumbago or vegetable black, conveyed in a suitable medium followed by a varnish or lacquer. The formation of the black copper sulphide on the surface of the metal, by painting with dilute alkaline sulphide solution (such as ammonium sulphide), gives the same appearance. The superficial precipitation of other metals, such as platinum, gold, or arsenic, is often adopted also ; a very weak solution of platinic chloride, or of gold chloride, or a solution of 1 ounce of arsenious a.cid {white arsenic), and 1 ounce of ferrous sulphate in 12 ounces of water answering the purpose well. After applying any of these solutions, the object must be well washed and dried, and finally lacquered. A mere bronze-coloured varnish, recommended by Button for bronzing brass work, is made by dissolving 5 parts of aniline purple and 10 parts of fuchsine in 100 parts of methylated sj)irit, and then adding 5 parts of benzoic acid, and boiling until the liquid has attained the desired colour. * Note that the equivalent of 700 cubic inches is found by multiplying the figure for VO by 10. 412 ADDENDA. An excellent black bronzing for brass is obtained by dissolving copper carbonate in an aqueous solution of ammonium carbonate. The best results are obtained with gildiug metal, which after cleansing thoroughly need only be immersed for a short time in the liquid, after which the object is swilled in water and dried out. One of the secrets of success is to avoid the presence of even traces of chlorides in the bronzing solution. In their presence the black coating is liable to scale or to lighten in colour after a short time. Green bronzes are made by converting the surface of the article into the green basic acetate or carbonate of copper, and may be produced by exposing the article for a time to the vapour of acetic acid. Any acid vapour in moderation will atibrd the same result. One metliod, recommended by Napier,, consists in enclosing the object immediately above a little dry bleaching-powder contained in a closed vessel, until the required effect is produced. ANTIDOTES TO Poisons. Most of the metallic-plating and the cleansing solutions are extremely poisonous, and stress has already been laid upon the danger both of using domestic drinking utensils for any purposes connected with the work of electro-plating, and of dipping the bare arm or hand into any of the depositing liquids. Unforeseen accidents, however, may occur and may demand the application of speedy remedial measures. Amateur doctoring is to be strongly deprecated, and medical aid should be sought at once ; but upon a sudden emergency it may be necessary to administer relief, pending the arrival of the physician. In any case of poisoning by swallowing, simple emetics should at once be given — for example, luke-warm water, mustard and water, ipecacuanha, or even zinc sulphate (of the latter from 10 to 30 grains are often given), the two first-named are better for domestic applica- tion ; while these are preparing, the patient may often induce vomiting by thrusting the fore-linger as far as possible down the entrance to the throat. The nature of the subsequent remedies will depend upon the character of the poison, thus : — Acids. — Mineral acids, such as sulphuric, nitric, hydrochloric, or glacial acetic acids, require an alkali to neutralise them : magnesia, chalk, whiting, lime water, or carbonate of soda may be administered stirred up with water. Failing these, the acid must be diluted by copious draughts of water ; olive oil, milk, or white of egg may then be given. Alkalies. — Caustic alkalies demand neutralisation with a mild acid, as vinegar, or the juice of an acid fruit, such as the lemon or lime, or by extremely dilute acetic, citric, or tartaric acids. Then oil or white of egg may be taken. Antimony. — For the chloride solution magnesia or sodium carbonate are used ; for tartar emetic, a vegetable astringent is to be applied ; very strong tea may answer the purpose : then barley water or the like ; small doses of stimulants being given from time to time. Arsenic. — Freshly made hydrated ferric oxide with magnesia are often employed. Coirper. — White of egg mixed with water, plenty of milk, water, or barley- water or the like should be taken. Some have used calcined magnesia stirred with water. Cyanides. — Freshly precipitated peroxide of iron with an alkaline car- bonate, such as potassium carbonate ; plenty of fresh air should be available ; the coldest possible water should be poured over the head and down the ADDENDA. 413 spine ; and the atmosphere around the patient may with advantage contain .a little chlorine ; for example, a little dilute acid may be ])oured upon bleaching powder in a saucer placed at some distance to windward of the patient. Lead.—K very dilute solution of sulphuric acid, or a solution of magnesium or sodium sulphate may be administered ; some use sodium phosphate ; the object in each case being the formation of an insoluble salt of lead. This should be followed by an active purgative. Milk or white of egg may be plentifully taken. Mercury.— MhmsAnovi^ fluids (white of egg) should be given in sujficient {quantity, mixed preferably with milk ; a large excess of the albumen is not advisable, the quantity generally recommended being the white of one egg to each 4 grains (about) of mercuric chloride taken. Then barley-water or its equivalent is allowable. Oxalic Acid and Oxalates. — Lime-water or chalk may be used ; but alkaline carbonates should not be applied, because they form intensely poisonous oxalates. Silver Nitrate. — Common salt in solution forms insoluble; silver chloride. Zinc Salts.— Wsivm demulcent drinks, such as barley-waier, to be given. In all the above cases the application of the special remedy must be pre- ceded by the use of strong emetics, except perhaps in the case of strong acids, when water should be taken to effect dilution before inducing the vomiting. Acids which have been spilled upon the hands or upon the floor of the room should be neutralised with chalk after dilution. The vapour of acid in the atmosphere of a room may be neutralised by the vapour of ammonia. ERRATUM. Page 15, line 3— for Chapter XX. read Chapter XIX. INDEX A. Accumulators, electrical, 86. Accuracy of electrotypes, 156. Acetic acid, 370. Acid-bath for copper deposit, 144. effect of, in copper-baths, 148, 151. final cleansing in, 127. nitric, as depolariser, 50. ,, effect of, in quicking- bath, 131. ,, strength for Grove's battery, 52. ,, -resisting composition (vat- lining), 109. Acids, opening bottles of, 369. ,, organic, resistance of cobalt to, 249. ,, use in nickeling, 241. ,, specific-gravity tables, 401. „ various, 399-410. Activity, unit of, 39. Adams, metallisation of moulds, 165. nickeling solution, 238. Adhesion of copper surfaces pre- vented, 167. ,, low, of nickel deposits (cause), 246. non-, of deposits (cause), 89. Agate burnishers, 136. Ageing of silver-baths, 201. Agitation of solutions, necessary, 104. Agitator, von Hiibl's, 112. for baths. 111. Air, circulation of, maintained, 107. effect of, on iron-baths, 252. ,, pure, need of, 105. ,, use of, in circulating solutions, 300. Alcohol, 373. Alkali, action of, on grease, 124. without action on mineral, oils, 124. Alkaline cleansing liquid, 126. ,, copper-baths, 144. Alkalinity of nickel-baths, cause of,. 240. Alloy, backing-, electrotypers', 180. ,, lead-tin-, for polishing steel,. 138. standard silver-coinage, 204. ,, thermopile, 70. Alloys, aluminium, produced, 314. ,, copper, cleansing of, 128. - deposition of, 36, 280. ,, fused, electrolysis of, 35. fusible, 162. „ gold, 224. lead, stripping silver from,, tin, cleansing of, 130. Alternate-current dynamo, 75. Aluminium and its compounds, 373. ,, carbide, 314. ,, chloride, 374. ,, deposition of, 277. ,, -nickel alloy, 290. ,, reduction of, 12, 314, 315.. ,, smelting of, 315. Alums, 374. Alu-ni, 290. Amalgamation of battery zincs, 42. gold, 226. Amalgams, 386, American Postal Telegraph Co.'s plant, 155. Ammeter, 97. ,, advantages of, 90, 93. ,, use of, in art-el ectrotyping,, 187. INDEX. 415 Ammeter, position of, in electrotype circuit, 170. Ammonia, 374. alum, 374. ,, solution, opening bottles of, 374. use of, in hot copper- baths, 147. Ammonium compounds, 374. sulphide, use in electro- typing, 185. Amorphous phosphorus, 389. Ampere, value of the, 38. Ampereage, best, for electro-deposi- tion, 90. surface-, interconver- sion of units, 400. Amperemeter, 97. Animal forms, reproduced in copper, 188. Anion, meaning of term, 30. Annealing, effect of, on hammered metals, 125. ,, on iron deposit, 256. on nickel ,, 238. ' ^„ electric, 320. Anode, meaning of term, 30. ,, plate, form of. 111. ,, slime (copper), 147. Anodes, antimony, 274. ,, arrangement of, in art-work, 185. „ brassing, 284. carbon, use of, 242. ,, choice of, 101. coating on, by lead, 272. ,, cobalt, 250. ,, copper, 147. ,, behaviour in refining, 293. „ refinery, 297. ,, -regulus, use of, 303. ,, size for, 148. „ gold, 224. incrustation on, in brassmg, 286. ,, insoluble, use of, 31, 32, 102, 357. ,, iron, 255. ,, lead, effect of, in copper- bath, 186. ,, nickel, 242. Anodes, nickel, arrangement of, 246. ,, silver, 204. ,, ,, appearance during electrolysis, 200. ,, ,, arrangement of, 210. size of, 102. „ effect of, 147. ,, soluble, effect of, 30. „ use of, 32, 101. ,, supplementary, use of, 188. ,, suspension of. 111, 112. tin, 272. unlike, eff'ect of, 32. , , various, effect of, 32. ,, wire-skeleton, for statuary,, 184. ,, zinc, 265. Antidotes to poisons, 412. Antimony and its compounds, 376. anodes, 274. ,, behaviour in copper re- fining, 294. ,, deposited, nature of, 275. deposition of, 273. ,, explosive, 275. ,, extraction of, 310. ,, solutions, assay of, 328. Antique silver, 216. Apothecaries' weight, 409. Aqua fortis, 128, 372. „ regia, 372. Areas, sectional, of standard wire gauge, 406. Argol, 392. Armature, dynamo, 76. direction of current in,, 73. ^ ,, varieties of, 78. Arrangement of baths, copper-re- fining, 297. ,, ,, electro typing, 168. plating, 98. ,, of rooms, 105. Arsenic, behaviour in copper-refining, 295. ,, effect of, in brassing-bath,. 281. Art-electrotyping, 182. Articles, cleansing of, 123. Assay of depositing solutions, 327. ,, electrolytic, 328. Astatic galvanometer, 96. Atomic-weight, definition of, 17. 416 INDEX. Atomic weights of elements, 21. Atoms, meanint]^ of term, 16. Autogenous soldering of lead, 108. Avoirdupois weight, 409. B. Backing of copper electrotypes, 180. ,, -metal for electrotypes, 180. Balance, plating-, 114. Balance, plating-, correction in use of, 116. 5, sensitive, 327. Barometer dials, dead-gilding of, 230. silvering of, 195. Barrel, rotating-, for plating small goods, 119. Base-bullion, refining of, 305. Basis-metal, influence of colour on gilding, 229. ,, use of term, 131, Baths {see Solutions). ,, arrangement of, in copper- refining, 297. ,, cyanide, spontaneous altera- tion of, 200. electrotype, arrangement of, 168. ,, old, recovery of metal from, 327. plating, arrangement of, 99. Battery, bichromate, 53, ,, Bunsen's, 51. ,, costliness of, 40, 336. Cruickshank's, 4. ,, Daniell's, 46. ,, ,, Breguet's, 48. „ ,, gravity, 49. ,, Kuhlo's, 48. ,, ,, Meidinger's, 48. ,, post-office, 49. ,, depolarisation of, 44. ,, direction of current in, 26. economical arrangement of cells, 58. ,, effect of size of plates, 57. ,, for brassing, 280. ,, ,, cadmium-plating, 267. ,, ,, cobalt-plating, 249. ,, ,, copper-depositing, 143. ,, ,, electrotype, 170. „ gilding, 220. Battery for iron-depositing, 257. ,, ,, nickel-plating, 240. ,, ,, silvering, 196. ,, ,, tinning, 271. ,, ,, zinc-depositing, 263. Grove's, 50. ',, injurious fumes from, 53, 105. 3, invention of, 3. ,, Leclanche's, 54. local action on zinc, 41. ,, maximum efficiency of, 59. ,, parts of, 43. ,. polarisation of, 43. ,, position of, in plant, 106. ,, practical hints on, 65. ,, principle of, 25, 41. ,, screws, 61. ,, secondary, 86. ,, single and two-fluid, 45, 351, 352. ,, size of, effect on current, 57. ,, Smee's, 45. ,, switch-board for, 62. ,, theory of, 350. ,, . thermo-electric, 64. ,, Clamond's, _ 69. ,, direction of current in, 66. ,, ,, Noe's, 71. ,, reversal of current in, 67. ,, wasteful- ness of, 71. ,, weakening of, 43. ,, Wollaston's, 42. -zincs, amalgamation of, 42. ,, ,, need for purity, 41. Baume's hydrometer, value of de- grees, 402. Bay salt, 395. Beardslee's cobaltiug solution, 248, Becquerel's cobalting solution, 248. „ electro-chromy, 278. ,, „ -gilding, 221. ,, electrolytic works, 5. ,, ore-treatment, 304. Bedstead tubes, brass-coated, 286. Bees'-wax, 376. cracking of, on cooling, 159. INDEX. 417 Bees'-wax, use of, in moulding, 159. Benardos' electric welding process, 319. Benzene, use of, in cleansing, 123. Benzoic acid, 318. Benzoline, use of, in cleansing, 123. Bertrand's bismuth solution, 276. ,, cadmium solution, 267. ,, palladium solution, 277. Bessemer's copper-plating, 5. Bichromate battery, 53. Binding-screws, 61. Birmingham wire-gauge, value of numbers, 405. Bisulphide of carbon for bright-plat- ing, 9, 203. Bismuth, 377. behaviour of, in copper- refining, 294. deposition of, 276. ,, use of, in fusible alloys. 162. Black deposit in bright-silver bath, 204. ,, ,, on silver anodes, 200. gold deposit, 225. -lead, 390. ,, ,, application to moulds, 164. water-repelling action of, overcome, 179. Black-leading machine, 177. ,, process, invention of, 8. wax (type) moulds, 177. Bias and Miest's ore-treatment, 304. Blende, roasting of, 309. ,, treatment of, 308. Blocks, wood-, electrotyping of, 181. Blood-poisoning from plating solu- tions, 117. Board of Trade Unit, 39, 334. ,, used in electro- lysis, 333. Bobs for polishing, 133. Boden's nickeling solution, 238. Bookbinders' type, brassing of, 286. Boracic acid, 371. Borchers' antimony extraction, 310, refining of lead, 305. ,, system of copper-refining, 300. Boric acid, 371. Boric acid, use of, in nickeling, 242. Bottger's cobalting solution, 248. ,, iron-plating solution, 254. ,, platinating solution, 261. silvering solution, 197. Bottle-form of bichromate cell, 53. Box-wood sawdust, use of, 152. Brass, 377. ,, anode, 284. ,, ,, cause of incrustation on, 286. bronzing of, 411. , , cobalting of, 249. coppered by immersion, 141. deposit, colour of, controlled 284. ,, nature of, 280. depositing-solutions, 280, 282. ,, deposition of, 36, 280. ,, final cleansing of, 128. ,, gilding of, by immersion, 218. ,, nickeling of, 245. platinising of, 259, 260. ,, silvering of, 193, 209. stripping of nickel from, 243. ,, ,, silver irom, 207. ,, wire scratch-brushes, 134. Brassing of small goods, 119. Braun's immersion gilding, 218. Breguet's Daniell-cell, 48. Briant (de), gilding solution, 221. Bright-dipping of metals (in acid), 129. Bright-silver bath, use of, 203. plating, 9, 203. Brightness of silver anodes during electrolysis, 200. Brimstone, 396. Britannia metal, cleansing of, 130. ,, gilding of, 231. nickeling of, 238, 247. , , silvering of, 209. , , stripping silver from, 208. ,, unsuited for silver- ing, 206. Bronze, 377. ,, depositing solutions, 287. deposition of, 288. gilding of, by immersion, 218. Bronzing of copper and brass, 411. Brown copper-deposit, cause of, 171. 2 D 418 INDEX. Brown gold, cause of, 223, 225. Brunei's brassing solutions, 282. Brush dynamo, 84. Brushes of dynamo, 74. }>osition of, 75. regulation of, 86. scratch-, 134. Building tools for wax moukls, 177. Bullion, base-, lefining of, 305. Bunsen's cell, 51. ,, electro-smeltiug of magne- sium, 317. Burnishing, 136. ,, aud scratch - brushing, contrasted. 137. deposited copper, 154. etfect of, on metals, 136. nickel, difficulty in, 244. Burnt nickel, 237. Burton's liquid forge, 319. Busts, moulding from, 163, 183. Butter of antimony, 376. „ tin, 396. Buttons, silvering of, 193. C. Cadmium, 378. deposition of, 267. use of, in fusible alloys, 162. Calamine, treatment of, 308. Calcium carbide, production of, 315. ,, sulphate, 389. Calculations as to disposition of vats, 168. as to thickness of de- posit, 92. Calomel, 387. Calorie, value of the, 38, 39. Campbell's platinum-silver bath, 289. Carbon anodes, use of, 242. ,, bisulphide for bright-plating, 9, 203. chloride, use in bright-silver- iug, 203. Cast-iron, 383. as anode, 102, 255. ,, nickeling of, 245. ,, preliminary cleansing of, 151. .. tiuning of, 271. 'i Cast-metal anodes, 102. Cast-nickel anodes, 242. Castner's sodium smelting process, 317. Casts, electrotype, 157. i Cathode for copper-retiniug, 297. meaning of term, 30. ,, motion imparted to, 113. ,, secondary actions at, 358. ,, suspension of, 111, 167. Cathodes, unlike-, on same rod, effect of, 211. Cation, meaning of term, 30. Caustic alkali cleansing-baths, 126. ,, lunar, 394. ,, potash, 391. ,, soda, 395. Cell {see Battery). ,, direction of current in, 26, 350. ,, economical arrangement of". 61. ,, principles of voltaic, 25, 350. . ,, size of, effect on current, 57. I Cells, porous, preservation of, 55. Centigrade thermometer scale, 38. ,, and Fahrenheit scales compared, 408. Chalk, 378. Charcoal rendered non-conductive, 312. Chases for use in electrotyping, 174. Chemical combination, heat of, 20, I 23, 348. ., energy, relation of, to j ' electrical, 24, 348. ! ,, formuh-e, 19. ,, symbols, 18. Chlorides, 371. ,, of carbon and sulphur for I bright-plating, 203. ] Circuit, current-, 44. divided-, distribution of current in, 44. Citric acid, 371. ! ,, use of, in nickeling, 241. Clamond's thermo-electric battery, 69. Cleanliness, necessity for, 89, 123. ; Cleansing finished electrotypes, 188. j ,, for nickeling, care in, 244. ! ,, liquids, acid, 127. I ,, ,, alkaline, 124, 126. cyanide, 129. objects for plating, 123. Clock-dials, dead -gilding of", 230. INDEX. 419 Coal and zinc as electrical generators, 39. Cobalt and its compounds, 378. anodes, 250. behaviour of, in copper-refin- ing, 294. characteristics of, 249. depositing solutions, 248, deposition of, 249. -nickel alloy deposited, 239. recovery of, from old baths, 321. ,, resistance of, to organic acids, 241. solutions, assay of, 329. Col3ley's ore-treatment, 304. Coils, resistance-, 93. Coinage, standard silver, 204. Coins, electrotyping of, 182. ,, moulding from, 158. Collodion, use in bright-silvering, 203. Colophony, 392. Colour of brass-deposit controlled, 285. of gold, 223, 225. 5, affected by impuri- ties, 222. ,5 -deposit controlled, 225. Coloured silver-deposit, cause of, 205. Colouring, 137. of gold (dry), 233. Colours, iridescent, produced, 278. Combination, chemical, heat of, 20, 23, 348. ,, -wound dynamos, 78. Commutator, dynamo-, 74. . „ sparking of, 86. ,, tending of, 85. Compass-needle, use of, 94. Composition, moulding-, conductive, 160, 164. ,, elastic, 163, 183. „ ., fluid, mould- ing with, 159. gutta-percha, 158. wax, use of, 159. Composition, moulding-, wax, use of, in elastic, 183. Compound-wound dynamo, 78. Compounds, definition of term, 16. Conditions of electrolysis, 367. Conductance, electrical, 33, 362. ,, of metals, 34. of oxides and sulphides, 303, 304. low, of nickel-baths, 241. ,, of mould ensured, 164. Conduction, electrolytic, 35, 345, 362. Conductors, choice of metals for, 33. copper, maximum currents for, 407. loss of power in, 338. ,, maximum current for, 339, 407. size of, 339. Connecting screws, 61. Consequent poles, magnetic, 80. Continuous current, conversion of alternating current into, 88. Continuous current dynamo, 74. Control of silver-baths by anode appearance, 199. Converter, rotatory, 88. Copper and its compounds, 379. 5, alloys, final cleansing of, 128. anodes, 147. behaviour in refining, 293. ,, ,, for art electrotyping, 185. ,, slime on, 147. ,, use of, in brassing, 284. ,, -baths, acid, 14i. ,, alkaline, management of, 146. effect of anode-size on, 148. J, ,, lead anode on, 186. temperature for, 145, 147. bottoms of ships protected, 30. bronzing of, 411. conductors, maximum current for, 339, 407. 420 INDEX. Copper, crude, impurities in, 292. deposit, character of, 148, 149, 171. ,, relation to current- strength, 148. relation to nature of solution, 148. ,, colours of, 171. drying of, 152. ., ,, spread of, 187. strength of, 149, 150. ,, depositing iron upon, 256. ,, -depositing prior to silvering, 209. deposition by battery, 143. immersion, 139. „ „ (consolida- tion of coat), 139. „ single-cell pro- cess, 141. of, on copper, 167. ,, ,} iron rollers, 153. „ wax, 179. ^ ,, power absorbed in, 332. ,, ,, solutions for, 144, 145. ,, effect of, on colour of gold, 223. ,, „ ,, silver, 205. ,, electrolytic moulds of, 156. ,, electro-plating with, 151. ,, electrotype, backing of, 180. ,j ,, nature of metal required, 149. plates, suspen- sion of, 167. ,, separated from plate 172. J, separated from wax, 180. ,, extraction from ores, 302. ,, gilding by immersion, 217. native, treatment of, 303. -nickel-zinc alloys deposited, 288. ,, nickeling of, 245. Copper-plates, iron-facing of, 245. ,, reproduction of, 167. ,, platinising of, 259. printing-surfaces, nickeled, 247. ,', recovery of, from old baths, 322. ,, ,, ,, w a s h - waters, 152. ,, refined, foreign metals in, 295. form of, 302. -refining, bath arrangement, 297. behaviour of foreign ,, metals, 294. ,, ,, current - strength for, 296. ,, ,, electrolytic, 292. ,, relation of E.M.F. to inter-odal " space, 297. ,, renewing old bath, 301. ,, ,, solution for, 297. „ ,, spacing of electrodes, 296. ,, ,, systems of, 298, 299. ,, reflectors, manufacture of, 191. ,, -regulus, use as anode, 303. ,, silvering of, 193, 209. ,, solutions, assay of, 330. ,, spongy-, used, 165, 178. ,, stripped from iron, 258. ,, stripping of nickel from, 243. ,, ,, of silver from, 207. ,, sulphate as a depolariser, 46. ,, solutions, specific gravity of, 402. „ „ ,, specific re- sistance of, 403. ,, thick deposits of, 153, 154. ,, thickness of coat required, 152, 185. ,, -tin alloys, deposition of, 288. ,, ,, -zinc alloys, deposition of, 288. ,, tubes deposited, 154. INDEX. 421 Copper wires, resistances of, 404. -zinc alloys, deposition of, 280. Coppering metals before frilding, 231. Correction for plating-balance, 116. Corrosion of anodes in refining, 293. Corrosive sublimate, 387. Cost of electricity, 336. Coulomb, value of, 38, 91. Couples, thermal-, arrangement of, 69. Cowles' aluminium - reduction, 12, 313. Cowper-Coles' process for manufac- ture of reflectors, 191. ,, zinc bath, 265. Cream of tartar, 391. Crown of cups, Volta's, 3. Cruickshank's electrolytic experi- ments, 4. Cryolite, 316. Crystalline character of deposits, 154. Crystallisation of battery fluids, 56. Cub. ins. and cub. cms., interconver- sion of, 410. ,, and pints, interconversion of, 410. Current-density, 39. permissible, effect of motion of solution on, 104. Current, alternating, converted into continuous, 88. Current-detector, 94. ,, direction of, found, 95. ,, ,, in armature, 75, in battery, 26, 350. ,, in thermal bat- tery, 66. distribution of, in divided circuit, 44. from public supply, use of, 88. ,, in coils rotating near mag- net, 73. maximum for copper con- ductors, 339, 407. measurement of, 96, 97, 170. ,, regulated by anode, 224. regulation of, in electrotyp- ing, 172. ,, reversal of, in dynamo, 74, 77. ,, in thermopiles, 67. Current, short-circuiting of indicated, 93. sources of, 40. -strength, effect of, in electro- typing, 171. on brass- deposit, 285. on de- posits, 90. J, 5 5 a on silver- deposit, 205. excessive, safe- guarded, 186. for aluminium smelting, 317. for copper-refin- ing, 296. ,, lead-refining, 305. ,, nickeling, 240. zinc - deposi- tion, 265, 266. ,, maximum, for electrotyping, 150. for wax- moulds, 180. ,, measured without ins t r u m e n t s, 170. ,^ relation to nature of copper - de- posit, 148. unit of measure- ment, 38. -value, best, for depositing, 90. weight and thickness of de- posit by, 399. Cut-out for check on current, 187. Cyanide-bath, discovery of, 8. poisonous fumes from, 105. spontaneous alteration of, 200. ,, cleansing-liquid, 129. copper-baths, 144. free, in gold-bath, 222. 422 INDEX. Cyanide, free, in silver-bath, 198. gold-bath, made up, 223. ,, of potassium, 390. of silver, ISO. ,, silver-baths, 197. Cyanides, 372. ,, solution of orojanic matter by, 201, 227. Cylinders, iron-, coppering of, 153. ,, mixing-, 140. D. Daniell's cell, 5, 46. Darcet's fusible alloy, 162. Dead-dipping of objects, 128. „ -gilding, 229. ,, -lustre on silver, 215. ,, -nickeling, 247. Deakin and Smith's rotatory plating apparatus, 119. Decantation, washing by, 393. Dechaud and Gaultier's ore - treat- ment, 302. De la Rue's discovery of electrotyp- ing, 5. Deligny's ore-treatment, 304. Densities of copper and zinc sul- phates in solution, 402. Density, current, 39, Density of silver bath, effect of in- creasing, 201. unequal in solutions, 212, 365. Depierre's copper-bath, H5. Depolarisation of battery, 44. ., by chromic acid, 53. ,, by copper sulphate, 46. by nitric acid, 50. ,, mechanical, 44. Deposits, conditions of formation, 30. ,, crystalline character of, 154. etfect of varying currents on, 90. non-adhesion of, caused, 89, 91. relation to current-inten- sity, 92. rouc;hness, cause of, 301. ruined by want of cleanli- ness, 123. Deposits, slimv, on copper-anodes, 147. ,, striated-, cause of, 103. ,, thickness, calculation of, 92. ,, time required for given, 92. ,, uneven, cause of, 103, 352. ,,' weighing of, in bath, 114. weight and thickness of, 399. ,, of, calculated, 92. ,, ,. per B.O.T. unit, 334. Deposition, electro-, earlv, 4, 5, 8. of alloys, 36, 280. ,, of aluminium, 277. ,. of antimony, 273. ,, of bismuth, 276. of brass, 280. ,, of bronze, 288. of cadmium, 267. of cobalt, 249. of copper, 139. of German-silver, 288. of gold, 217. of iron, 251. ., of lead, 272. j, of nickel, 236. ,, of palladium, 277. ,, of platinum, 259. of silver, 192. „ (bright), 203. ,, ,, (non - electro- lytic), 192. ,, of tin, 267. ,, of zinc, 263. ,, on electro-positive metals, 98, 348. Desmur's nickeling solution, 238. Detector, current-. 94. Diameters, actual, of wire-ganges, 405, 406. Dilute solutions, effect of (coppering), 149. Dipping in acid, need for, 127. potash-vat, 126. Direction of current found, 95. in batter}', 26, 350. ,, in dynamo-ar- mature, 74. ,, ,, in thermal bat- tery, 66. lines of magnetic force, 73. INDEX. 423 Dirt, effect of, in gold bath, 222. ])irty brass-deposit, cause of, 286. Disc-armature of dynamo, 78. Distance between electrodes, 102. Divalent, meaning of term, 18. Divided-circuit, current-distribution in, 44. Doctor, use of, in gilding, 228. ,, Wagener and Netto's, 118. Dolly for polishing, 133. Double salt solutions, electrolysis of, 359. Doucet and Lambotte's zinc-ex- traction, 309. Drainage, system of, 106. Drum-armature of dynamo, 78. ,, for coating small goods, 119, 140. Dry colouring of gold, 232. Dry pile, 42. Drying of coppered goods, 152. ,, nickeled goods, 246, 247. ,, zinced goods, 266. Ductility of deposited copper, 150. Dynamo-armatures, 76, 78. ,, ,, direction of current in, 74. Dynamo, Brush, 84. ,, classed by magnet-wind- ings, 76. ,, driving-power for, 86. ,, efficiency of, 333. field-magnets of, 76. ,, for copper-depositing, 144. ,, refining, 296. ,, Gramme's, 81. ,, invention of, 4, 9. Krottlinger's, 85. ,, management of, 85. ,, necessity of, for large works, 155. position of brushes in, 74, 76. ,, ,, in works, 106. ,, reversal of current in, 74. ,, Schuckert's, 83. ,, Siemens', 83. ,, sparking of, 86. ,, theory of, 72. ''Victoria," 84 Weston's, 83. Wilde's, 80. E. Economy in battery ai rangement, 58. Efficiency, maximum, of battery, 61. Elastic moulding-composition, 163, 183. ,, moulds, invention of, 9. Elasticity of deposited-copper, 150. Electric and chemical energy, re- - lation of, 24, 25. annealing, 320. ,, batteries, costliness of, 40, 336. ,, conductivity of metals, 33. connection-gripper, 175. ,, current, direction found, 95. furnace, Cowles', 312. Siemens', 311. ,, resistance, unit of, 38. welding, 318. " Eiectricias," nickeling solution, 238. Electricity, cost of, 91, 336. ,, derivation of term, 3. generated, 25. ., supply, public, use of, 88. ,, voltaic and static, 27. Electro-chemical equivalents, 399. ,, series, 24. ,, -chromy, 278. -deposition, 28. ,, arrangement of vats, 99. ,, ,, current - strength for, 90. early experi- ments, 4, 5, 8. limit of E.M.F. for, 32. relation of cur- rent and time, 91, 92. ,, -etching, 2, 190. -ore-extraction, scope for, 291. -metallurgy, definition of, 1. ,, scope of, 2. ,, -motive-force, best for deposit- ing, 90. rounter-, 32. limit of, in de- positing, 32. ,, meaning of term, 28. ,, of dynamo, 78. ;> unit of, 37. 424 INDEX. Electro-negative, meaning of term, 24. -j^lating, defiuition of, 2. ,, essentials for baths, 99. ,, of positive metals, 98. -positive, meaning of term, 24. -refining, scope for, 291. ,, -smelting, 311. * ,, of aluminium, 315. ,, of magnesium, 317. of sodium, 317. Electrodes, altering position of, at tirst, 213. ,, distance between, 102. ,, manner of connecting, 110. ,j manner of suspending, 111. ,, meaning of term, 30. Electrolysis, conditions for, 35, 354, 367. ,, meaning of term, 28. ,, of complex acids, 360. ,, of ferric solutions, 252. ,, of mixed solutions, 359. of silver-potassium cya- nide, 198. ,, power absorbed in, 332. theories of (modern), 341. Electrolyte, agitation of, 103, 111. ,, meaning of term, 31, 344. Electrolytic assay, 328. ,, conduction, 35, 345, 362. ,, etching, 190. ,, moulding, 156. ,, preparation of brass- bath, 281. 5, ,, of silver- bath, 202. ,, stripping of gold, 226. ,, ,, of nickel, 243. of silver, 208. Electro-thermal processes, 314. Electrotype-baths, arrangement, 168. ,, -copper for anodes, 186. ,, ,, separation from plate, 172. ,, thickness for, 172, 180. ,, -plate, backing of, 180. Electrotype-plate, final cleansing of, 188. 5, ,, ,, preparation of, 181. ,, -supporters, 168. Electrotyping, 156. ' ,, arrangement of vats, 99. ,, art-, anodes in, 186. ,, excess current prevented, 187. calculation of current- strength, 169. ,, chases, type, &c., to be used for, 173. coins, medals, &c., 182. ,, definition of term, 2. deposit on wax, 179. irregular, 102. earliest experiments in, 6. ,, maximum current- strength for, 150. ,, mechanical finish of plates, 173. ,, moulding - composi- tions, 157. nature of copper re- quired, 149. ,, printers', 165. ,, regulation of current, 172. ,, separating copper from matrix, 172. ,, statuary, 183. ,, supporting of plate in bath, 167. ,, use of guiding-wires, 188. ,, ,, in various printing pro- cesses, 189. ,, ,, measuring in- struments, 170. wood-blocks, 181. Elements, definition of term, 16. ,, electro-positive and -nega- tive, 24. ,, list of, 21. Elkington's early patents, 5. ,, gilding solution, 218. Elmore's solid-deposited tubes, 154. INDEX. 425 Eisner's bronzing solution, 287. coppering solution, 145. silver-bath (battery), 197. ,, (immersion), 193. ,, tinning-bath, 270. ' zincing-batli, 264. Emery-wheels, use of, 138. Emetic, tartar, 376. E.M.F. {see Electro-motive force). Enamelled iron for tanks, 108. Energy, transformations of, 25. Engraved steel plates copied, 166. Epsom salts, 385. Equations, chemical, use of, 18, 19. Equivalent weights, 20. Equivalents, electro-chemical, 91, 399. Etching, electrolytic, 190. Ewers, gilding lips of, 228. Exchange, simple, of metals, 348. Explosive antimony, 275. Extensibility of deposited copper, 150. F. Fahrenheit and Centigrade scales compared, 408. ,, thermometer-scale, 38. Faraday's laws of deposition, 5. Eaure's accumulator, 87. Fearn's tinning solutions, 270. Ferric and ferrous compounds, 251, 384. ,, salts, result of electrolysing, 252. Field-magnet of dynamo, 76. File-marks, removal of, 132. Filigree-work, gilding of, 230. Filter-paper, folding of, 56. Fine gold, preparation of, 381. ,, silver, preparation of, 392. Finishing, 137. Fizeau's gilding solution, 221. Floating typographic formes, 174. Floors, suitable, 106. Flowers of sulphur, 396. Force, electro-motive, 28. ,, physical, 15. Forces, correlation of, 25. Forge, liquid, Burton's, 319. Forks supported in silver- vat, 209. Formes, printers', electrotyping of, 173. ,, floating of, 174. Formes, printers', moulding froni, 159. ,, preparation of, 173. Formulse, chemical, 17. Free cyanide in gold-bath, 222. silver-bath, 198. French weights and measures, 409, 410. Frosted gold, 229. Fulminating gold, 383. Fumes from batteries, injurious, 46, 53, 105. ,, cyanide - bath danger- ous, 105. Furnace, electric, Cowles', 312. electric, Siemens', 311. Furnace, for colouring gold, 233. ,, muffle-, for cleansing, 124. Fused alloys, electrolysis of, 35. ,, salts, electrolysis of, 35, 315. Fusible metals, 161. ,, fusing points of, 162. Fusion by electricity, 311. G. Gallons and litres, in tercon version, 410. Galvani's experiments, 3. Galvanic battery, principles of, 26, 41. Galvanised iron, 30, 263. Galvanography, 190. Galvanometers, 96. ,, astatic, 96. ,, tangent, 97. Galvanoscope, 94. Gaultier and Dechaud's ore-treatment, 302. Gauze, wire-, plating of, 118. Gelatine, 381. ,, hardening of, 163. ,, use of, in moulding, 163. German-silver, cobalting of, 249. ,, deposition of, 288. final cleansing of, 121. ,, nickeling of, 247. ,, silvering of, 197. ,, stripping silver from, 207. Gilding by battery, solutions for 221. ,, ,, immersion, 217. colour of gold in, 223, 225. dead-, 229. 426 INDEX. Gilding, dead, starting of deposit, 230. discoloured patches in, 227. ,, management of process, 226. of electro - positive metals, 231. ,, watch-movements, 234. wire, 117. ,, parcel, 232. quicking prior to, 226. J, stripping of gold before, 225. thin, failure of, 229. use of doctor" in, 228. -vat, 224. ,, water-, 219. Gilt plumbago, 164, 178. Gilt surfaces, ornamentation of, 232. Glass, silvering of, 191. Glass-tube insulator for wires, 210. -vats, cement for, 108. Glazing of steel, 138. Glossary of substances used, 369. Glue, 380. ,, marine-, 381. use of, in moulding, 158. Glycerin, use of, in iron-bath, 253. Glyph ography, 189. Gold and its compounds, 381. ,, amalgamation by mercury, 226, ,, anode, 224. J, behaviour of, in copper-refining, 295. ,, character of deposit, 223, 225. ,, coin, 381. colour, control of, 223, 225. ,, ,, affected by basis metal, 229. ,, ,, by impurities, 222, 223. coloured, 223, 225. ,, colouring of (dry), 232. ,, -cyanide of potassium, 391. ,, dead-, 229. ,, deposition of [see Gilding). properties of, 217. ,, pure, preparation of, 381. ,, recovered in copper-refining. 301. ,, in lead - refining, 306. ,, recovery from old baths, 322. ,, solubility of, in cyanide-bath, 222. ,, -solutions, assay of, 330. Gold, standard, 381. ,, stripping of old coat, 225. Gore's brassing-bath, 282. ,, coppering-bath, 145. ,, experiments with explosive antimony, 275. gilding-bath (battery), 221. ,, (immersion), 218. silvering-bath (battery), 197. ,, (immersion), 193. silvering-pastes, 193. tinning-bath, 268. Graining of surfaces (watch - move- ments), 234. Grains and grammes, interconversion of, 410. Gramme, value of, 38, 410. ■ Gramme's dynamo, 81. Graphite, 390. Gravity, Daniell's, cell, 49. Grease, removal from objects, 123. solubility in cyanides, 227. Greek fire, 389. Green-bronziufT, 412. ,, gold, 223. Grippers, electric connection-, 175. Grounding in burnishing, 136. Grouping of battery -cells, 57. Grove's cell, 50. Guericke's electrical machine, 2. Guiding wires for art-moulds, 187. Gutta-percha, 383. ,, ,, action of cyanides upon, 201. ,, ,, compositions, mould- ing, 158. ,, ,, moulding by, 157, 158. Gypsum, 389. H. Hand-polishing, 132. ,, scratch-brushes, 134. Handling of cleansed objects, 127. Hardening of gelatine, 163. Hardness of deposited iron, 256. ,, ,, nickel, 236. ,, ,, platinum, 262. Hartmann and Weiss, cobaltiug by, 248. Hartshorn, spirits of, 374. Haydn, system of copper-refining, 300. INDEX, 427 Heat evolved in chemical union, 20. ,, unit of, 38. Heating of potash- vat, 125, solutions, 109. Heeren's brassing solution, 282. Hern's tinning solution, 270. Hess' brassing solution, 282. Hides for polishing, 133. Higgins' bichromate cell, 53. Hippopotamus hide for polishing, 133. Hoe's black -leading machine, 177. toggle-press, 176. Hoho-Lagrange electric welding, 319. Homogeneity of solutions, need for, 103. Honeycombing of anode in refining, 293. Hook for anode-suspension, 112. Hooks and eyes, tinning of, 269. Horse-power hour, 39, 334. ,, ,, cost of, 337. ,, relation to Kilowatt, 39, 334. ,, ,, unit, 39, 334. Hospitaller's nickeling-bath, 238. Hot solutions, stoppinf^-olf varnish for, 397. ,, ,, vats for, 109. Hltbl (von), experiments on copper depositing, 149. ,, solution agitator of, 112. Hydrochloric acid, 371. ,, ,, sp. gr. table, 401. Hydrocyanic acid, 371. Hydrogen, 17. ,, absorbed by iron-deposit, 256. ,, co-deposit of, effect, 148, 205. ,, deposition of, in zincing, 266. Hydrometer, Baume's, value of de- grees, 402. Hygienic precautions to be observed, 105. Hyposulphite silver-bath, 202. I. Immersion, antimony deposited by, 273. ,, coppering by, 139. Immersion, copper extraction by, 302. ,, gilding by, 217. ,, platinising by, 259. ,, silvering by, 192. , , tinning by, 269. Imperial wire-gauge numbers, value of, 406. Impurities, behaviour in copper-re- fining, 294. efi'ect on gold-bath, 222. in copper anode, effect of, 147. ,, in refined copper, 295. ,, in silver-bath, effect of, 201. Inches and millimetres, interconver- sion of, 410. Incrustation on anode in brassing, 286. Insoluble anodes, use of, 31, 99. Installation, arrangement of, 105. Institution of Electrical Engineers, rules for copper conductors, 339, 407. Instruments, current - measurement without, 170. Insulation of suspending wires, 210. Insulators, electrical, 33. Intaglios, production of, 156. Intensity of current, meaning of term, 37. Interconversion of amperes per sq. ft., sq. in., and sq. dm., 400. of thermometer scales, 408. ,, of weights and measures, 410. Internal surfaces, gilding of, 228. Iodide silver-bath, 202. Ionic velocity, effect of unequal, 365. lonisation, 346. ,, heat of, 348. Ions, meaning of term, 30, 344. ,, charges of, 345. migration of, 346, 361. ,, rate of, 362. Iron and its compounds, 383. ,, anodes, 255. ,, behaviour of, in copper-refining, 294. ,, cast, cleansing of, 151. ,, unsuited for anodes, 102. 428 INDEX. Iron, character of deposit, 256. cleansing of, 129. cobalting of, 249. coppering-baths for, 144, 145. by immersion, 2, 139. deposit, absorption of hydrogen by, 256. ,, preservation from rust, 257. deposition of, 257. facing of copper-plates, 251. galvanised, 263. ,, use of, 30. ,, gilding of, by battery, 231. ,, by immersion, 220. nickeling of, 245. ., rollers, coppering of, 153. \, silvering of, 193, 197. ,, solutions, 251. red precipitate in, 252. stripping of nickel from, 243. ,, ,, old coat, 257. silver from, 208. 5, tinning of, 268. ,, -vats, use of, 108, 109. Irregular deposits, cause of, 103, 353. ,, solutions, cause of, 103. ,, surfaces, anodes for, 185. gilding of, 228. Isinglass, 381. J. Jacobi, electrotyping by, 6. Jamieson's rule for current-direction, 96. Japing's brassing-bath, 282. ,, copper-baths, 145. ,, zinc-bath, 264. Jewreinolf's platinating-bath, 261. Johnson and Morris' brassing-bath, 282. , , , , German-silver- bath, 289. Jointing of lead-lined vats, 122. Jordan, electrotyping by, 6. Joule, 39. Jugs, gilding lips of, 228. K. Kasalowsky's copper-bath, 145. Keith, refining of lead, 305. Kermes mineral from antimony-bath, 274. Kick's gilding-bath, 221. Kiliani's experiments on zinc-deposi- tion, 266. Kilowatt, 39, 334, ' hour, 39, 334. Klein's iron-bath, 253, 255. Knight's metallisation of moulds, 165. Kopp's immersion copper-bath, 140. Krottlinger's dynamo, 85. Kuhlo's Daniell-cell, 48. Kiihn's silvering-paste, 193. L. Lacquer varnish, 397. Lagrange-Hoho, electric v/elding by, 319. Lake Superior copper, treatment of, 304. Lambotte and Doucet's zinc-extrac- tion, 309. Lamp - reflectors, silvering of, 193, 195. Langbein's nickeling-bath, 238. platinating-bath, 261. Lard, 384. ,, clarification of, 158. Large surfaces, plating of, 119. Lathe for polishing, 133. Law, Ohm's, 59. Lead and its compounds, 384. anode, eti'ect of, in copper-bath, 180. behaviour in copper-refining, 295. ,, -carbonate, used in wax- moulding, 159. cleansing of, 130. ,, coppering-bath for, 145. ,, crude-, impurities in, 305. ,, deposition of, 272. ,, gilding of, 231. lined-vats, jointing of, 122. -peroxide, colours of films, 278. ,, deposit on anode, 272. recovery from old baths, 323. refined-, impurities in, 306. refining of, 305. ,, -sheet anodes for statuary, 186. ,, solutions, assay of, 330. stripping of silver from, 208. INDEX. 429 Lead, tinning of, 270. -tree, 272. unsuited for silvering, 206. ,, use of, in accumulators, 88. ,, fusible alloys, 162. ,, -vats, jointing of, 108, 122. Lead, black-, application to moulds, 164. Lead of dynamo-brushes, 76. Leather bobs for polishing, 133. Leaves, nature-prints from, 190. Leclanche's cell, 54. Leeson's elastic-moulds, 9. Length, unit of, 38. Lenoir's automatic cut-out, 187. ,, statuary-mouldiijg, 184. Lerebour's gilding solution, 221. Lesmonde's platinising cell, 260. Letrange's zinc-extraction, 309. Levol's gilding solution, 221. Lichtenberg's fusible alloy, 162. Light, need for, 105. Lilac colour of antimony-coat on brass, 273. Lime, quick and slaked, 378. Sheffield-, use in polishing, 134. Lines of force, magnetic, 72. Lipowitz's fusible alloy, 162. Lips of ewers, gilding of, 228. Liquid forge, Burton's, 319. Liquids, conductivity of, 35. Litre, value of the, 38. and gallons, interconversion of, 410. Lobstein's tinning-bath, 270. Local action, meaning and effect of, 41. thickening of silver-deposits, 213. Lonyet's zinc-bath, 264. Looseness of deposit, causes of, 89. Lubrication in scratch-brushing, 135. Luckow's zinc-extraction, 308. Ludersdorf's tinning-bath, 268. Lunar- caustic, 394. M. Magnesium and its compounds, 385. ,, electro-smelting of, 317. Magnet, current in coils moving near, 73. Magnet, dynamo-, exciting of, 76. field-, of dynamo, 76. lines of force around, 72. Magnetic iron-deposit, 256. Magnetism, residual, in iron, 77. Magneto-electric machines, invention of, 4. Maistrasse's tinning-bath, 272. Manganese, behaviour in copper- refining, 294. Manufactured goods prepared for silvering, 206. Marchese's ore- treatment, 305. Marine-glue, 381. ,, use in moulding, 158. Marks, striated, cause of, on deposits, 103. Materials for moulding, 157. Matter, definition of, 15. Matthiessen's magnesium - smelting, 317. • Measurement of current, 96. ,, ,, without in- struments, 170. ,, units of, 37. Measures and weights, 409. Measuring apparatus, electrical need for, 89. ,, instruments, position in circuit, 170. Medals and medallions, electrotyping of, 158, 182. Meidinger's Daniell-cell, 48. Melting-points of fusible alloys, 162. Mercury and its compounds, 386. ,, danger to gold, 226. in plate powders, 195. protection of battery-zincs by, 41. ,, recovery of, from old zincs, 323. ,, quicking by, 130. use of, in fusible alloys, 162. Meritens Plating Co.'s silver-baths, 197. Metal, backing-, for electrotypes, 180. ,, Spence's 396. Metallisation of moulds, 164, 165, 178. Metallo-chronies, 278. Metals and metalloids, 20, 21, 24. ,, best current for de])ositing, 90. 430 INDEX. Metals, conductance of, 34. 5, electro-chemical series, 24. 5, -positive and -negative, 24. coating of, 98. fusible, use in moulding, 161. list of, 21. precious-, recovered in copper- refining, 301. ,, recovered in lead- refining, 305. simple exchange of, 348. ,, thermo-electric series of, 64. ,, -electro-motive force of, 65. unlike, effect of hanojing, on same cathode-rod, 211. Methylated spirit, 373. Metre, value of the, 38. Micro-volts, 62. Miest and Bias, ore-treatment by, 304. Migration of ions, 346, 361. rate of, 362. ,, ,, varying velocity of, 365. Millimetres and inches, interconver- sion of, 410. Mil ward's bright-plating solution, 9. Mineral oils, cleansing from, 124. Mixed solutions, electrolysis of, 36, 359. Mixing-drum, 140. Mixture of solution, necessary in brassing, 285. M. J. L.'s gilding-baths, 221. Modern theories of electrolysis, 341. Moebius, silver-refining process, 307. Molecule, definition of term, 16. Monovalent, meaning of term, 18. Mop {moiJinng), 137. Morris and Johnson's brassing-bath, 282. ,, Geiinan-silver- bath, 289. Motion of bath effected, 112. ,, cathodes, 113. Motor-generator, 88. Moulding-box for wax, 174. ,, by electrolysis, 156. ,, -composition, conductive, 160, 164. ,, elastic, 163. 183. Moulding from coins and medals, 158, 182. natural objects, 188. 5, statuary, 183. 5, steel-plates, 158. type, 160, 173. ' undercut models, 157. wax-models, 184. wood-blocks, 181. in sections, 184, 185. materials, 157. ,, with elastic composition, 163. gutta-percha, 157. • mixtures, 158. plaster of Paris, 161. J, 5, sealing-wax, 163. r, wax, 174. -compositions, 159, Moulds, elastic, invention of, 9. guiding- wires in, 187. metallisation of, 164, 165, 178. rendered conductive, 164. ,5 wax-, electrical connection with, 179. ,, maximum current for, 180. 5, parting of copper, from, 180. 5, ,, plumbagoing, 177. ,5 trimming and build- ing up, 177. 5, ,, wetting of surface, 179. Mud, copper refining, composition of, 295. Muffle-furnace for cleansing, 124. Multiple system of copper refining, 298. Munro's tinning solution, 270. Murray's black-leading process, 8. N. Navel's nickeling-bath, 238. Native copper, treatment of, 303.' Natural objects, reproduction of, 188. Nature-prints of leaves, 190. INDEX. 431 Negative-plate and -pole of battery, 43. Net, wire-, plating of, 118. Netto and Wagener's doctor, 118. Neutral-point, thermo-electric, 66. Newton's fusible-alloy, 162. Nickel and its compounds, 387. -anodes, 242. ,,. -baths, 238. ,, cause of alkalinity, 240. behaviour in copper-refining, 294. burnt-, production of, 237. character of metal, 236. -cobalt alloy deposited, 239. -copper-zinc alloy deposited, 288. - -deposit, advantages of, 236. ,, cause of peeling, 245. dark coloured, cause of, 245. ,, hardness of, 237. ,, polishing of. 138. ,, thickness of, 246. ,, -plating, uses of, 236. recovery from old baths, 324. -solutions, assay of, 331. ,, stripping old coats, 243. Nickeling, arrangement of anodes, 245. ,, battery for, 240. careful preparation needed, 138, 244. , , rotatory plating apparatus for, 119. ,, small goods, 119. ,, solutions, 238, 240. ,, suspension of objects for, 119, 245. ,, time required for, 246. -vats, 243. Nickel-silver, gilding of, 231. ,, ,, silvering of, 209. Niello work, 216. Nitric acid, 372. ,, • ,, as depolariser, 50. -dip, 128. ., ,, specific-gravity tables, 401. Nitrous acid, 128, 372. Noe's thermo-electric battery, 71. Non-conductors, electrical, 33. Non-metals and metals, 20, 21, 24. O. Obernetter's iron-bath, 254. Objects, cleansing of, 123. ,, large, plating of, 118. motion of, in bath, 113. ,, polishing of, 133. quicking of, 130. ,, scratch-brushing of, 134. small, coating of, 119. ,, suspension of, in bath, 112. Ohm, value of the, 38. Ohm's law, 5, 59, 338. Oil of vitriol, 372. ,, removal of, 124. Old gold-baths, use of, 224, 227. Opposing electro-motive force, 31. Ore-treatment, Becquerel's," 5. ,, early experiments in, 10. ,, scope for, 291. Ores, antimony-, treatment of, 310. ,, conductivity of, 304. ,, copper-, treatment of, 302. ,, zinc-, treatment of, 308. Organic acids, resistance of cobalt to, 249. use of, in nickeling, 241. dirt, destruction of, 124. matter, danger of, to silver- baths, 201. dissolved by cyan- ides, 201. eff'ect of, on gold- bath, 223. Ornamentation of gilt surfaces, 232. , , silver surfaces, 215. Osmotic pressure, 342. Oxalic acid, 372. Oxidised tilver, 215. P. Pale cop])er-deposit, cause of, 171. ,, gold, cause of, 225. Palladium deposit, advantages of, 277. ,, deposition of, 277. properties of, 277. Paper, filter-, folding of, 56. Paracelsus on coppering iron, 2. Paraffin, cleansing from, 124. Parallel arrangement of battery-cells, 58. 432 INDEX. Parallel arrangement of plating-vats, 99. , , of relining-vats, 298. of refining-vats, limits to, 298. Parchment-paper for porous partition, 143. Parcel-gilding, 232. Paris, plaster of, 337. Parkes', elastic moulding composi- tion, 163. metallisation of moulds, 164. silvering-bath, 197. wax moulding-composition, 160, 164. Patera's treatment of copper-liquors, 303. P. D. {see Potential Difference), 352. Peeling of nickel-deposits, cause of, 245. Pens, steel-, coppering of, 140. Person and Sire's zinc-hath, 264. Petroleum, cleansing Irom, 124. Pewter, stripping of silv^er from, 208. unsuited to silvering, 206. Pfanhauser's gilding-bath, 221. nickeling-bath, 238. J, silvering-bath, 197. Phosphorus, 388. -compositions in type- moulding, 178. production of, 314. -solutions, danger of, 189. use of, in moulding, 160, 164. Pickles for cleansing, 130. Pile, dry, 3, 42. thermo-electric, 62. Clamond's, 69. \ J, direction of cur- rent in, 66. Noe's 71. wasteiulness oi, 71. Pinholes in copper-deposit, cause of, 180. iron-deposit, 255. Pink silver-deposit, cause of, 205. Pins, immersion tinning of, 269. Pints and cubic inches, intercon- version of, 410. Plant, disposition of, 105. Plante-Faure accumulator, 87. statuary anodes, 185. Plaques, moulding from, 158. Plaster of Paris, 389. as porous partition, 143. ,, made water-proof, 16. use of, in moulding, 161, 183. Plastic moulding materials, 15. Plate-restoring powders, 195. Plates, anode-, form of, 111. ,, battery-, 43. copper-, reproduced, 167. ,, steel-, copied, 166. Platinating, 260, 262. Plating apparatus, rotatory, 119. -balances, 114. bright-, invention of, 9. Platining, 262. Platinising, 259, 262. ,, silver, 215. Platinum and its compounds, 389. behaviour in copper-refin- ing, 295. ,, characteristics of, 259. ,, deposition of, 259. dipping-baskets, 128. recovery from old baths, 325. resisting power of deposit, 259. scratch-brushing of, 262. -silver alloy deposited, 289. ,, solutions, assay of, 331. ,, stripping of, 262. -wire anodes for statuary, . 185. Plumbago, 390. ,, application to moulds, 164, 177. ,, increasing conductivity of, 164. ,, water-repelling action of, overcome, 179. Plumbagoing wax-moulds, 177. Poisoning of blood by platiug-baths, 117. Poisons, antidotes to, 412. Polarisation of battery, 43, 55. ,, prevention ofy 44. INDEX. 433 Polarisation of electrolyte, 35. Pole -armature of dynamo, 78. Poles, consequent, of dynamo-mag- net, 80. in electrolyte, 35. ,, of battery, 43. Polishing before nickeling, 244. hides for, 133. ,, -lathe, 133. ,, of surfaces, 133. Porosity of deposits, 154. Porous-cell of battery, 46. ,, preservation of, 55. Positive metals, plating of, 98. -plate and -pole of battery, 43. Post-office Daniell-cell, 49. Potash alum, 374. ,, cleansing solution, 124, 126. -vat, 125, Potassium compounds, 390. ,, cyanide, cleansing by, 129. ,, use in silver-baths, 198. Potential, meaning of term, 27. Pott's nickeling-bath, 238. Powder, silver-, obtained, 234. ,, plate-restoring, 195. Powdery copper-deposit, cause of, 149. Powell's nickeling solutions, 238. Power, cost of, 337. ,, loss of, in conductors, 338. required for electrolysis, calcu- lation of, 332. ,, -spindle for polishing, 133. unit of, 39, 334. Precipitate in washing antimony- deposit, 274. ,, rusty-, in iron-baths, 252. Preparation of goods for nickeling, 244. ,, manufactured goods for plating, 206. Press for moulding from type, 175. Pressure, electrolytic solution-, 346. ,, osmotic-, 342. ,, solution-, 341. ,, of electric current, 27. Primary-battery, 87. Principles of electro-refining (copper), 292. Printers' electro typing, 165. Printers' formes, moulding from, 159, 173. Printing-plates, copper-, iron-faced, 251. ,, ,, nickeled copper, life of, 249. „ -surfaces, nickeled, 247. ,, various, production of, 189. Projections on deposit, cause of, 103. Proof-spirit, 373. Prussiate of potash, yellow, 391. Prussic-acid gas evolved from cyan- ide-baths, 104. Pumice, scouring with, 132. Pure gold, preparation of, 381. ,, silver, preparation of, 392. Puscher's immersion copper-bath, 141. Pyrites, burnt Spanish, treatment of, 302. Quantity and intensity of current, 37. Quicking of objects, 130. ,, before gilding, 226. Quicklime, 378. Quicksilver, 386. R. Rag-gilding, 228. Reaumur thermometer and scale, 38. Red brass-deposit, 285. „ gold, 223. ,, -lead, 385. ,, phosphorus, 389. Refined copper, foreign metals in, 295. Refining, electro-, scope for, 291. „ of copper, systems of, 298. ,, of lead, 306. ,, of silver, electrolytic, 307. Reflectors, electrolytic manufacture of, 191. ,, silvering of, 193, 195. Regulation of current in electrotyp- ing, 172. Regulus, copper-, use as anode, 303. Residual magnetism in iron, 77. 2 E 434 INDEX. Resistance coi^s, 9S. 5, effect of varying, on de- posit, 92. electrical, of copper wires, 404. „ relation of, to power required, 338. specific, of copper-sulphate solutions, 403. ,, ,, of sulphuric acid, 403. ,, unit of measurement, 38. Resistances, use of, 213. Reversal of current in dynamo, 74, 77. ^, 5, thermopiles, 67. Revolving scratch-brushes, 135. Ring-armature of dynamo, 78. Rochelle salt, 392. Rock salt, 395. Rogers' Plating Co.'s silver-baths, 197. Rolled metal anodes, 102. Rollers, iron-, coppering of, 153. Rooms, arrangement of, 105. Rose's fusible alloy, 162. Roseleur's antimony-bath, 273. „ brassing-bath, 281, 282. ,, coppering-bath, 145. ,, gilding-bath (battery), 221." ,, (immersion), 218. „ nickeling- bath, 238. ,, plating-balance, 114. ,, platinising-bath, 259. ,, quicking-bath, 131. ,, silvering-bath (battery), 197. „ ,, (immersion), 193. ,, ,, pastes, 186, 195. ,, tinning-bath (battery), 270. ,, (immersion), 268. ,, wire-gilding process, 117. Roseleur and Lanaux, platinating- bath, 261. Rosin, 392. „ use of, in moulding, 160, 183. Rotatory converter, 88, Rotatory plating apparatus, 119. Rouge, use of, 137. Round's tin-silver-bath, 289. Rules for handling chemicals, 369. Ruolz (de), bronzing solution, 287. ,, gilding-bath, 221. Russell and Woolrich's brassing- bath, 282. ,, ,, cadmium- bath, 267. Rust-coloured precipitate in iron vats, 252. ,, preservation of iron-deposit from, 256. Ryhiner's iron -plating solution, 254, S. Salt, common, 395. Salts, fused-, electrolysis of, 35. Salzede's (De la), brassing-bath, 282. ,, bronzing-bath, 287. Sand, scouring with, 132. „ Trent, use of, in polishing, 133. Sartoria's tinning-bath, 270. Satin-finish to silver, 216. Sawdust for drying, 152. Schmollnitz waters, coppering iron by, 2. ,, treatment of, 302. Schneider-Szontagh system of copper- refining, 301. Scratch-brushes, 134. ,, -brushing, 134. ,, ,, during deposition, 213, 227. ,, ,, effect of, 137. ,, of platinum, 262. Scratches, unobliterated, 103. Screws, binding-, for batteries, 61. Sealing-wax, use of, in moulding, 163. Secondary actions in electrolysis, 358. -batteries, 86. Sectional moulding, 184. Seebeck's discovery of thermo-elec- tricity, 4. Seignette salt, 392. Selective chemical union, 22. Separate current, silvering by, 196# INDEX. 435 Separately excited dynamo-magnets, 76. Series arrangement of battery cells, 58. ,, of plating- vats, 99. of refining- vats, 298. ,, ,, vats, limit to, 298. ,, electro-chemical, 24, system of copper-refining, 298. thermo-electric, of metals, 64. ,, -wound dynamo-magnets, 76. Shape of object, eff'ect, on thickness of coat, 102. Sheffield lime, 378. ,, used in polishing, 134. Ships-bottoms, protection of, 30, Short-circuiting of current, 44. avoided in art-elec- trotyping, 186. ,, indicated, 92. Shttckert's dynamo, 83. Shunt-wires, distribution of current in, 44. -wound dynamo-magnet, 77. Siemens' dynamo, 83. Signets, moulding from, 163. Silver and its compounds, 392. -anodes, 204. ,, ,, appearance during electrolysis, 199. ,, antique, 216. -bath, bright, use of, 213. ,, controlled by anode appearance, 199. ,, ,, cyanide, 198. ,, effect of age on, 201. ,, electrolytic preparation of, 202. iodide, 202. ,, ,, limits of density for, 201. ,, ,, suspension of objects in, 209. ,, ,, thiosulphate, 202. ,, behaviour of, in copper-refin- ing, 295. „ bright deposit, 9, 203. ,, ,, presence of sulphur in, 204. ,, -coin, standard, 204, 392. Silver-cyanide of potassium, 391. ,, cyanide, preparation of, 198, 393. ,, ,, use of, in silver-bath, 198. dead-lustre produced, 215. ,, -deposit, character of, 205. ,, ,, final treatment of, 213. ,, ,, non-adhesive, cause of, 199. ,, ,, thickening of, locally, 213. ,, ,, thickness of, 206, 214. ,, deposition of, by battery, 196. ,, ,, ,, immersion, 196. ,, ,, of, from pastes, 193, 195. ,, on glass, 191. ,, ,, electrode arrange- ment, 210. ,, • ,, non - electrolytic, 194. „ ,, power absorbed in 333. ,, effect of, on colour of gold, 223. ,, electro-refining of, 307. ,, electrotype, use of, 166. ,, electro typing with, 215. German-, deposition of, 288. ,, final cleansing of, 128, gilding of soft-soldered goods, 232. ,, gilt, dead-lustre on, 229. niello-work, 216. oxidised, 215. ,, plate, polishing of, 137. platinised by immersion, 258. ,, -platinum alloy, deposited, 289. ,, -potassium cyanide, electro- lysis of, 199. ,, -powder, obtained, 234. ,, pure, preparation of, 392. , , recovered in copper - refining, 301. ,, in lead - refining, 305. ,, from old baths, 325. , , refining of, 307. solutions, assay of, 331. 436 I^DEX. Silver solutions, for separate current, 197. preparation of, 198. ,, spongy-, prepared, 234. ,, striking-bath, use of, 211. ,, stripping of old coat, 206. ,, sub-cyanide dissolved, 213. -surfaces, ornamentation of, 215. -tin alloy deposited, 289. Silvered plumbago used, 164, 178. Silvering of glass, 191. Silvering-vat, 205. Single-cell deposition of copper, 141. ,, ,. extraction of copper, 303. „ ,; gilding by, 220.' ,, ,, platinising by, 260. ,, silvering by, 195. „ „ tinning by, 271. ,, -fluid battery, 45. Sire and Person's zinc-bath, 264. Size of anode, effect of, 224. battery, effect of, on current, 57. ,, vats, 107. Skeleton wire statuary anodes, 185. Slaked lime, 378. Slate vats, 108. Slime on copper anode, 147. rusty, in iron-baths, 252. Slimes, copper-refining, composition of, 295. Slinging of objects in silver- vat, 209. Small objects, coating of, 119, 151. Smee's battery cell, 45. book on electro-metallurgy, 9. platinising, 260. Smelting, electro-, 311. ,, of aluminium, 315. of magnesium, 317. of sodium, 317. Smith and Deakin's rotatory plating apparatus, 119. Smith's system of copper-refining, 300. Soda alum, 374. ,, ash, 394. cleansing-vat, 126. smelting, 317. Sodium compounds, 394. Softening of deposited iron, 256. Soldering of lead- vats, 1 08* Solder-lines, gilding of, 232. Solenoid, 72. Solution of anode in copper-refining, 293. pressure, 341. ,, electrolytic, 346. Solutions, agitation of, effected. 111. necessary, 103. antimony-depositing, 273. ,, -extraction, 310. ,, brassing, 280, 282. ,, bron2Ang, 287. ,, circulation of, 103, 111, 300. ,, cobalting, 248. ,, copper and zinc sulphates, sp. gi\, 402. ,, -refining, 297. ,, „ ,, renewal of, 301. ,, coppering, 139, 141, 145. depositing-, assay of, 327. dilute-, effect in coppering, 149. for electro-chromy, 278. ,, German -silver depositing, 288. gilding, 217, 220. ,, heating of, 109. ,, homogeneity of, need for, 103. ,, iron-depositing, 254. ,, lead-depositing, 272. ,, -refining, 305. ,, nickeling, 238. , , , , bad conductivity of, 245. ,, platinating, 261. ,, platinising, 259. ,, potash, vat for, 125. quicking-, 131. [321. recovery of metals from, silvering, 197, 198. „ (bright-), 203. ,, stripping for gold, 225. „ nickel, 243. ,, ,, silver, 207. „ tinning, 268, 270. ,, zinc-deposition, 263. ,, ,, -extraction, 308. Sorrel, salt of, 391. Space between electrodes regulated, 102. Sparking of dynamo- commutator, 86. INDEX. 437 Specific gravity of silver-baths, 201. tables for acids, 401. copper and zinc sulphate solutions, 402. resistance of copper sulphate solution, 403. of sulphuric acid, 403. Spence's metal, 396. Spencer, electrotyping by, 6. Spermaceti, use in moulding, 160. Spirits of hartshorn, 374. ,, wine, 373. Sponginess of anode in refining, 293. Spongy copper-deposit, cause of, 171. silver prepared, 234. Spoons supported in vat, 210. [214. ,, thickness on bowls increased, Spots on silver-deposit, 212. Sprague's platinating-bath, 261. [410. Sq. ins. and dms. , intercon version of, Stalmann's system of copper-refining, 300. Standard (coinage), gold, 38. „ silver, 204, 392. Stannate of soda, 396. Stannous and stannic compounds, 396. Star antimony, 376. Statuary, moulding from, 163, 183. Steam-heated plating-vat, 109. potash-vat, 125. Steam-power, cost of, 337. Stearine, use of, in moulding, 160. Steel, 383. ,, burnishers, 136. cleansing of, 129. ,, coppering of, 166. , , -facing of copper-plate, 251 , 256. ,, gilding of, 220, 221, 231. ,, hardened, zincing of, 263. ,, nickeling of, 244. -pens, coppering of, 140. ,, -plates, copying of, 166. ,, ,, moulding from, 158. ,, ,, preservation of, 166. ,, polishing of, 138. ,, silvering of, 197. ,, stripping nickel from, 243. Steele's gilding-bath, 220. ,, tinning-bath, 270. Stein's silver-paste, 193. Stereotyping, 166. Stibnite, 376. Stilography, 189. Stirring of bath eff'ected, 111. ,, need for, 103. Stopping out varnish, 397. Straightening of electrotypes, 181. Strength of deposited copper, 149, 150, 154. Striation marks on deposits, 103. Striking-bath for silver, 211. ,, of nickel, 246. Stripping of old gold coats, 225. ,, ,, iron coats, 257. ,, nickel coats, 243. ,, ,, . platinum coats, 262. ,, ,, silver coats, 207. Sub-cyanide of silver in silver-deposit, 205. Sublimate, corrosive, 387. Suet, use of, in moulding. 160. Sugar, use of, in moulding, 163. ,, of lead, 385. Sulphide of ammonium, use of, in electrotyping, 185. Sulphides as anodes, 303, 305. Sulphur, 396. [203. ,, chloride for bright-plating, ,, presence in bright-silver de- posits, 204. Sulphuric acid, 372. [401. , , specific gravity tables, ,, specific resistance of, 403. Supply, public electricity, use of, 88. Surface ampereage, units intercon- verted, 400. Surfaces, gilt, ornamentation of, 232. ,, internal, plating of, 228. ,, large, plating of, 118. ,, polishing of, 132. ,, scratch-brushing of, 134. ,, silvered, ornamentation of, 215. Suspension of electrodes, 111. ,, objects in depositing vats, 119, 151, 209. ,, objects in potash-vat, 126. Swan's-down used in dollying, 137. Switch-board for battery, 62. Symbols, chemical, 17; of elements, 21. T. Table-salt, 395. Tables, specific gravity, 401. 438 INDEX. Tallow, clarification of, 158. used in moulding, 183. Tangent galvanometer, 97. Tankards, internal gilding of, 228. j Tanks for solutions, 107. Tannic acid, 373. [123. Tarnish to be removed from objects, Tartar, cream of, 391. emetic, 376. Tartaric acid, 373. Telegraph-wires, coppering of, 155. Temperature, effect of, on resistance of baths, 403. [38. unit of measurement, Tenacity of deposited copper, 150, 154. Testing for free cyanide in silver- bath, 199. Tetravalent, meaning of terra, 18. Theories of electrolysis, modern, 341. Thermal (electro-) processes, 314. Thermo-electric battery, 64. ,, ,, ,, Clamond's, 69. ,, ,, ,, direction of current in. \ 66. ,, invention of, 4. ,, ,, Noe's, 71. ,, reversalof current in, 67. [71. ,, ,, wastefulness of, ,, neutral-point, 66. | ,, ,, series of metals, 65. ,, electro - motive force of metals, 66. [tery). Thermopile {see Thermo-electric bat- Thermometer-scales, interconversion of, 408. Thick copper-deposits on iron, 153. iron-deposits, 258. Thick tin -deposits, 270. Thickening of deposits at bottom, 103. silver coat locally, 213. Thickness of deposit, calculation of, 92. ,, of film determined, 172. of metal deposited by any known current, 399. ,, for copper deposit, 152. electrotype, 172. ,, for copper in statuary, 185. for gold, 229. for nickel, 246. „ for silver, 206, 214. Thickness unequal on long articles, 212. Thiosulphate silver-bath, 202. j Thompson's cobalting-bath, 249. ,, switch-board, 62. Thomson's electric annealing process, 320. ,, welding process, 318. Thom's platinating-bath, 262. Time required for electrotyping, 180. ,, for gilding, 227, 229. ,, for iron - depositing, 257. ,, for nickeling, 246. ,, to produce given de- posit, 92. Tin and its compounds, 396. ,, -anodes, 272. ,, behaviour in copper-refining, 294. ,, cleansing of, 130. ,, -copper alloys, deposition of, 288. ,, coppering-bath for, 145. -foil, grades of, 272. ,', nickeling of, 238. „ -plate, 272. ,, platinising of, 260. ,, -powder as conductor, 165, 178. ,, -salt, 396. ,, -silver alloy, deposition of, 289. ,, stripping of silver from, 208. I ,, unsuited for silvering, 206. ,, use in fusible alloys, 162. Tinning solutions for battery, 270. ,, ,, immersion, 268. ,, ,, single-cell, 268. Toggle-press, 175. Tools for burnishing, 136. Tranquil solutions, changes in, 103. Treacle, use of, in moulding, 163. Trent sand for polishing, 133. Tripoli-powder, use of, 137. Trivalent, meaning of term, 18. Troy-weight, 409. Tubes, formation of (copper), 154. Turpentine, Venice-, use of, 160. Two-fluid battery, 45, 352. Type, bookbinders', brassing of, 286. ,, cleansing and preparation of, 173. ,, moulding from, 173. , , to be used for electrotyping, 173. Typographical matter, electrotyping of, 173. INDEX. 439 U. Undercut moulding, 157, 163. Uneven deposits, cause of, 103. Units of measurement, 37. [182. Urquhart's electrotyping of medals, wax-moulding, 160. V. Valency, meaning of term, 18. Yarnish, acid-resisting, 234. ,, for interior of vats, 108. ,, lacquer, 397. ,, non-conductive, 167, 397. use of, 7, 152. ,, stopping-ofF, 167, 234, 397. Varrentrapp's iron-bath, 254. Vat for cobalting, 250 ; gilding, 224 ; iron-depositing, 256; nickeling, 248; potash solutions, 125; silvering, 205. Vats, arrangement of, for copper- refining, 297 ; for electro- typing, 168 ; for plating, 99. description of, 107. ,, lead lined, jointing of, 122. ,, supporting objects in, 111. Vegetable forms reproduced, 188. Velocity of migration of ions, 362. Venice turpentine used, 160. Ventilation, need for, 105. ,, system of, 107. Vermilion pigments, nickeled sur- faces for printing with, 249. Victoria dynamo, 84. Vitreous phosphorus, 388. Vitriol, blue-, 3S0. green-, 3S4. „ oil of, 372. white-, 398. Volkmer's brassing-bath, 282. ,, iron-bath, 254. ,, nickeling-bath, 238. ,, silvering-bath, 197. ,, wax-moulding composi- tion, 160. Volt, value of the, 37. Volta's experiments, 3. [90. Voltage, best, for metal -deposition, ,, reduction of, 88. Voltaic cell, principles of, 25, 41. ,, pile, 3. Voltmeter, 97 ; advantages of, 90 ; position of, in circuit, 170, W. Wagener and Netto's doctor, 118. Wagner's gilding-bath, 221. Wahl's gilding-bath, 218 ; platinating- baths, 261 ; silvering-baths (bat- tery), 197, (immersion), 193 ; tin- ning-bath, 268. Walenn's iron-bath, 254. [160. Walker's wax-moulding composition. Walrus-hide for polishing, 133. Wash-waters, recovery of copper from, 152. [274. Washing of antimony-coated goods, ,, gold-deposits, 227. Watch-movements, gilding of, 234. Water, composition of, 17. „ -gilding, 219. ,, power, cost of, 337. ,, -supply, good, need for, 106. ,, to be used in operations, 98. ,, waste-, removal of, 106. Watt, value of the, 39. Watt's brassing-bath, 282. cobalting-bath, 248. ,, copper-baths, 145. ,, German-silver-bath, 288. ,, gilding-baths, 221. ,, nickeling-baths, 238. ,, platinating-bath, 261. ,, silvering-baths, 193, 197. ,, „ paste, 193. [160. Watt's wax moulding-composition, ,, zincing solution, 264. Wax, bees', 376. ,, -compositions, use of, 160. ,, J J in elastic-moulding, 183. ,, cracking of, on cooling, 159. ,, melting of, 174. ,, -models, moulding from, 184. ,, moulding with, 170. ,, ,, ,, pressure-, 175. ,, moulds, copper deposited on, 179. [with, 179. ,, electrical contact ,, ,, maximum current- strength for, 180. ,, ,, parting of copper from, 180. ,, ,, plumbagoing of, 177. ,, trimming and building up, 177. [179. wetting of surface, 440 INDEX. "Wax, sealing-, moulding with, 163. • Weak currents, effect in coppering, 149. electrotyp- mg, 171. ,, solutions, effect in coppering, 149. Weighing deposit in bath, 114. correction for, 116. Weight, atomic, definition of, 17. of deposit, calculation of, 91. unit of, 38. Weights and measures, 409. [of, 410, „ interconversion ,, atomic, of elements, 21. equivalent, 20, 21. Weil's bronzing-bath, 287. „ copper-baths, 141, 145. [248. Weiss and Hartmann's cobalt-bath, Weiss' brassing-bath, 282 ; bronzing- bath, 287 ; copper-bath, 145 ; gild- ing-bath, 221 ; iroji-bath, 254 ; nickel-baths, 238 ; silvering-baths, 197; tinning-bath,270 ; wax mould- ing-composition, 160 ; zinc-bath, 264. ' Welding, electric, 318. Weston's dynamo, 83. ,, nickeling solutions, 238. White brass-deposit, 285. [159. ,, -lead, use in wax-moulding. Whitening of small goods, 192. Whiting, 378 ; cleansing by, 126. Wilde's first dynamo, 9, 80. ,, coppering of iron rollers, 153. Wine, spirits of, 373. Wire anode for statuary, 185. 5, -gauges, diameters of, 405, 406. ,, -gauze, plating of, 118. -gilding process, 117. marks, to avoid, 151. scratch-brushes, 134. [339. Wires, conducting, loss of power in, ,, maximum cur- rent for, 339. Wires, copper-, resistances of, 404. guiding-, for art-moulds, 187. ,, telegraph, coppering of, 155. Wiring of goods for nickeling, 245. ,, ,, silvering, 209. Wolfe and Pioohe's ore-treatment, 10. Wollaston's battery-eel], 42. coppering of silver, 4. Wood-blocks, electroty ping of, 1 73, 181. Wood's brassing-bath, 282. ,, fusible alloy, 162. „ gilding-bath, 221. Wooden-vats, use of, 108. [282. Woolrich and Russell's brassing-bath, ,, cadmium-bath, 267. Wright's cyanide -bath, 8, Wrought-iron, 383. Y. Yellow gold, cause of, 225. [274. ,, precipitate in antimony-bath, ,, silver-deposit, cause of, 205. Yellow stain on scratch-brushed platinum, 262. Z. [320. Zerener's electric welding process, Zinc and its compounds, 397. -anodes, 265. ,, used in brassing, 284. battery-, amalgamation of, 42. costliness of, 40, 336. mercury from, 55. ,, preservation of, 55. [294. ,, behaviour of, in copper-refining, ,, characteristics of, 263. cleansing of, 130. ^ [280. ,5 -copper alloys, deposition of, ,, -nickel alloy deposited, 288. ,, coppering-baths for, 145. dead gilding of, 231. ,, deposition of, 263. ,, effect of current- strength, 266. [267. ,, dust, use of in zinc-bath, 265, ,, extraction of, 308. ,, gilding of, by battery, 231. J, immersion, 218. local action on, 41. „ nickeling of, 238, 245, 247. ,, quicking of, 131. ,, silvering of, 209. solutions for depositing, 264. spongy deposits, cause of, 267. stripping of silver from, 208. [402. sulphate solutions, sp. gr. of, ,, tinning of, 268. Zinin's silvering-bath> 197. Zosimus on the coppering of iron, 2. PRINTED BY NEILL AND CO., LTD., EDINBURGH. 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ANGLIN, C.E., Master of Engineering, Royal University of Ireland, late Whitworth Scholar, &c With very numerous Diagrams, Examples, and Tables, Large Crown 8vo. Cloth. Second Edition, Revised. 16s. The leading features in Mr. Anglin's carefully- planned ** Design of Struc- tures " may be briefly summarised as follows : — 1. It supplies the want, long felt among titudents of Engineering and Architecture, of a concise Text-book on Structures, requiring on the part of the reader a knowledge of Elementary Mathematics only. 2. The subject of Graphic Statics has only of recent years been generally applied in this country to determine the Stresses on Framed Structures ; and in too many cases this is done without a knowledge of the principles upon, which the science is founded. In Mr. AngUn's work the system is explained from first principles, and the Student will find in it a valuable aid in determining the stresses on all irregularly- framed structures. 3. A large number of Practical Examples, such as occur in the every-day experience of the Engineer, are given and carefully worked out, some being aolved both analytically and graphically, as a guide to the Student. 4. The chapters devoted to the practical side of the subject, the Strength of Joints, Punching, Drilling, Rivetting, and other processes connected with the manufacture of Bridges, Roofs, and Structural work generally, are the result of MANY years' experience in the bridge-yard ; and the information given, on this branch of the subject will be found of great value to the practical bridge-builder. "Students of Engineering will find this Text-Book invaluable."— ^rcArV^c/. "The author has certainly succeeded in producing a thoroughly practical TexXr Book,'— Builder. "We can unhesitatingly recommend this work not only to the Student, as the BEST Tkxt-Book on the subject, but also to the professional engineer as an exchkdinglt ▼ALUABLK book of reference,''— MecJtanical World. "This work can be confidently recommended to engineers. The author has wisely chosen to use as little of the higher mathematics as possible, and has thus made his book of KKAL USE TO THE PRACTICAL KNGiNKKR. . . . Aftcr careful pcrusal, wc havc nothing but praite for the work." — Nature. LONDON: CHARLES GRIFFIN & CO., LIMITED, EXETER STREET, STRAND. ENGINEERING AND MECHANICS. In Large 8vo. Handsome Cloth. 10s. 6d. CHEMISTRY FOR ENGINEERS. BY BERTRAM BLOUNT, and A. G. BLOXAM, F.I.C, F.G.S., A.I.C.E., F I C F C S ^oniultmg Chemist to the Crown Agents for Consulting Chemist*' Head of the ChemUery the Colonies. Department, Goldsmiths' Inst., New Cross. GENERAL CONTENTS.— Introduction- Chemistry of the Chief MaterlalB of Construction— Sources of Energy— Chemistry of Steam-raising— Chemis- try of Lubrication and Lubricants— Metallurgical Processes used in the winning and Manufacture of Metals. K VT'^ SUCCEEDED beyond all expectation, and have produced a work which should give fresh power to the Engineer and Manufacturer."— ^Ae Times bnf f n #v- clear instructions given." — Nature. 8. APPLIED MECHANICS (Elementary Text-Book on). Specially arranged for First-Year Students. Third Edition, Revised and Enlarged. 3 6. " Nothing is taken for granted. . . . The work has very high qualitiks, which may be condensed into the one word ' clear.' " — Science and Art. A POCKET-BOOK of ELECTRICAL RULES and TABLES. FOR THE USE OF ELECTRICIANS AND ENGINEERS. Pocket Size. Leather, 8s. 6d. Thirteenth Edition. See p. 43. LONDON: CHARLES GRIFFIN & CO., LIMITED, EXETER STREET, STRAND. ENGINEERING AND MECHANICS. 33 Third Edition. Very fully Illustrated, Cloth, 4«. QcL STEAM - BOILERS; THEIB DEFECTS, MANAGEMENT, AND CONSTRUCTION. By R D. MUNRO, Chief Engineer of the Scottish Boiler Insurance and Engine Inspection Company. General Contents. — I. Explosions caused (i) by Overheating of Plates — (2) By Defective and Overloaded Safety Valves — (3) By Corrosion, Internal or External — (4) By Defective Design and Construction (Unsup- ported Flue Tubes ; Unstrengthened Manholes ; Defective Staying ; Strength of Rivetted Joints ; Factor of Safety) — II. Construction of Vertical Boilers : Shells — Crown Plates and Uptake Tubes — Man-Holes, Mud- Holes, and Fire-Holes — Fireboxes — Mountings — Management — Cleaning — Table of Bursting Pressures of Steel Boilers — Table of Rivetted Joints — Specifications and Drawings of Lancashire Boiler for Working Pressures {a) 80 lbs. ; [b) 200 lbs. per square inch respectively. This work contains information of the first importance to every user of Steam-power. It is a practical work written for practical men, the language and rules being throughout of the simplest nature. A yaluable comx)Anion for workmen and engineers engaged about Steam Boilers, ought to be carefully studied, and always at hand."— Co^i. Ouardiam^ *' The book is very useful, especially to steam users, artisans, and young engineers." — Engineer. By the same Author, KITCHEN BOILER EXPLOSIONS: Why they Occur, and How to Prevent their Occurrence. A Practical Hand- book based on Actual Experiment. With Diagrams and Coloured Plate, Price 3s. NYSTROM S POCKET-BOOK OF MECHANICS AND ENGINEERING. Revised and Corrected by W. Dennis Marks, Ph.B., C.E. (vale S.S.S.), Whitney Professor of Dynamical Engineering, University of Pennsylvania. Pocket Size. Leather, 15s, Twenty- first Edition, Revised and greatly enlarged. LONDON : CHARLES GRIFFIN & CO., LIMITED, EXETER STREET, STRAND. 34 CHARLES GRIFFIN tfe CO,'S PUBLICATIONS, Large 8vo., Handsome Cloth. With numerous Plates reduced from Working Drawings and 280 Illustrations in the Text. 21s. A MANUAL OF LOCOMOTIVE ENGINEERING; A Practical Text-Book for the Use of Engine Builders, Designers and Draughtsmen, Railway Engineers, and Students. BY WILLIAM FRANK PETTIGREW, M.Inst.C.E. With a Section on American and Continental Engines, By albert F. RAYENSHEAR, B.Sc, Of Her Majesty's Patent Oflace. Co;2?672fs. — Historical Introducrion, 1763-1863. — Modern Locomotives: Simple. — Modern Locomotives: Compound. Primary Consideration in Locomotive Design. — Cylinders, Steam Chests, and Stuffing Boxes.— Pistons, Piston Kods, Crossheads. and Slide Bars.— Connecting and Couplins: Rods.— Wneels and Axles, Axle Boxes. Hornblocks, and Bearing Springs.— Baiancing.—Valve Gear.— Slide Valves and Valve Gear Details.— Framing, Bogies and Axle Trucks, Eadial Axle Boxes.— Boilers.— Smokebox, Blast Pipe, Firebox Fittings.— Boiler Mountings.— Tenders. - Railway Brakes.— Lubrication.— Con- sumption of Fuel, Evaporation and En^rine -Lfflciency.— American Locomotives —Con- tinental Locomotives.— Repairs, Running, Inspection, and Renewals.— Three Appendices. —Index. '* We recommend the book as thoroughly peactical in its character, and meeitinq a PLACE IN ANY COLLECTION ot . . . works on Locomotive Engineering."'— i^azYw^av News. '•The work contains all that can be leaent from a book upon such a subject. It will at once rank as the standard woek upon this nrpoETANT —Railway Magazine. Large Crown 8vo. With numerous Illustrations. 6s. ENGINE-ROOM PRACTICE ; A Handbook fop Engineers and Offieeps in the Royal Navy and Mepcantile Mapine, Including the Management of the Main and Auxiliapy Engines on Boapd Ship. By JOHN G. LIVERSIDGE, Engineer, U.N., A.M.I.C.E., Instructor in Applied Mechanics at the Royal Naval College, Greenwich. Con^enis.— General Description of Marine Machinery.— The Conditions of Service and Duties of Engineers of the Royal Navy. — Entry and Conditions of Service of Engineers of the Leading S.S. Companies. — Raisins Steam"^— Duties of a Steaming Watch on Engines and Boilers. — Shutting off Steam,— Harbour Duties and "Watches.- Adjustments and Repairs of Engines. — Preservation and Repairs of "Tank" Boilers.— The Hull and its Fittings.- Cleaning and Painting Machinery — Reciprocating Pumps, Feed Heaters, and Automatic Feed -Water Regulators. — Evaporators. — Steam Boats. — Electric Light Machinery. — Hydraulic Machinery.— Air-Compressing Pumps.— Refrigerating Machines. — Machinery of Destroyers. — The Management of Water-Tube Boilers.— Regulations for Entry of Assistant Engineers, R.N.— Questions given in Examinations for Promotion of Engineers, R.N. —Regulations respecting Board of Trade Examinations for Engineers, &c. • " The contents cannot fail to be appreciated,"— T/je Steamship. "This VEET USEFUL BOOK, . . . ILLUSTRATIONS are or GREAT IMPORTANCE in a wopk of this kind, and it is satisfactory to find that special attention has been given in this TQW^eat.''''— Engineers' Gazette. LONDON : CHARLES GRIFFIN & CO., LIMITED, EXETER STREET, STRAND. ENOINEBRING AND MECHANICS. 35 WORKS BY W. J. MACQUORN RANKINE, LL.D, F.R.S., Late Regius Professor of Civil Engineering in the University of Glasgow. THOROUGHLY REVISED BY W. J. MILLAR, C.E., Late Secretary to the Institute of Engineers and Shipbuilders in Scotland, I. A MANUAL OF APPLIED MECHANICS : Comprising the Principles of Statics and Cinematics, and Theory of Structures, Mechanism, and Machines. With Numerous Diagram*. Crown 8vo, cloth, 12s. 6d. Fifteenth Edition. II. A MANUAL OF CIVIL ENGINEERING : Comprising Engineering Surveys, Earthwork, Foundations, Masonry, Car- pentry, Metal Work, Koads, Railways, Canals, Rivers, Waterworks^ Harbours, &c. With Numerous Tables and Illustrations. Crown 8vo^ cloth, 16s. Twentieth Edition. III. A MANUAL OF MACHINERY AND MILLWORK : Comprising the Geometry, Motions, Work, Strength, Construction, and Objects of Machines, &c. Illustrated with nearly 300 Woodcuts. Crown 8vo, cloth, 12s. 6d. Seventh Edition. IV. A MANUAL OF THE STEAM-ENGINE AND OTHER PRIME MOVERS: With a Section on Gas, Oil, and Air Engines. « ^By Bryan Donkin, M.Inst. C.E. With Folding Plates and Numerous Illustrations.. Crown Svo, cloth, 12s. 6d. Fourteenth Edition. LONDON: CHARLES GRIFFIN & CO.. LIMITED, EXETER STREET, STRAND. 36 OBARLBB 9RIFFIlf * OO.'B PUBLWATIOKS. Prof. Rankine's Works — (Continued). V. USEFUL RULES AND TABLES : For Architects, Builders, Engineers, Founders, Mechanics, Shipbuilders, Surveyors, &c. With Appendix for the use of Electrical Engineers. By Professor Jamieson, F.ll.S.E. Sev?:nth Edition. 10s. 6d. VL A MECHANICAL TEXT-BOOK: Jl Practical and Simple Introduction to the Study of Mechanics. By Professor Rankine and E. F. Bamber, C.E. With Numerous Illus- trations. Crown 8vo, cloth, 9s. Fourth Edition. *k* The ''Mechanical Text-Book" was designed by Profetsor Eankine as an Inino- aucTion to the above Series of Manuals. VII. MISCELLANEOUS SCIENTIFIC PAPERS. Royal 8vo. Cloth, 31s. 6d. Part I. Papers relating to Temperature, Elasticity, and Expansion of Vapours, Liquids, and Solids. Part II. Papers on Energy and its Trans- formations. Part III. Papers on Wave-Forms, Propulsion of Vessels, &c. With Memoir by Professor Tait, M.A. Edited by W. J. Millar, C.E. With fine Portrait on Steel, Plates, and Diagrams. '* No more enduring Memorial of Professor Rankine could be devised than the publica- tioa of these papers in an accessible form. . . . The Collection is most valuable oo account of the nature of his discoveries, and the beauty and completeness of his analysis. . . . The Volume exceeds in importance any work in the sam« department publisbe<^ in our time. " — A rchitect. SHELTON-BEY (W. Vincent, Foreman to the Imperial Ottoman Gun Factories, Constantinople) : THE MECHANIC'S GUIDE : A Hand-Book for Engineers and Artisans. With Copious Tables and Valuable Recipes for Practical Use. (llustrated. Second Edition. Crown Svo. Cloth, 7/6. LONDON : CHARLES GRIFFIN & CO., LIMITED, EXETER STREET, STRAND. ENGINEERING AND MEGHANI08. 37 SECOJVD JSDITIOJSr, Revised and Enlarged. In Large Svo, Handsome cloth, 34^, HYDRAULIC POWER AND HYDRAULIC MACHINERY. BY HENRY ROBINSON, M. Inst. C.E., F.G.S., FSLLOW OF king's COLLRGK, LONDON ; PROF. OF CIVIL BNGINBBRING. king's college, etc., etc. Mttb numerous TKIloo^cut5, anD Sijti^^snlne plated* General Contents. Discharge through Orifices— Gauging Water by Weirs — Flow of Water through Pipes — The Accumulator — The Flow of Solids — Hydraulic Presses and Lifts — Cyclone Hydraulic Baling Press — Anderton Hydraulic Lift — Hydraulic Hoists (Lifts) — The Otis Elevator — Mersey Railway Lilts — City and South London Railway Lifts — North Hudson County Railway Elevator — Lifts for Subways — Hydraulic Ram — Pearsall's Hydraulic Engine — Pumping- Engines — Three- Cylinder Engines — Brotherhood Engine — Rigg's Hydraulic Engine — Hydraulic Capstans — Hydraulic Traversers — Movable Jigger Hoist — Hydraulic Waggon Drop — Hydraulic Jack — Duckham's Weighing Machine— Shop Tools — Tweddell's Hydraulic Rivettcr — Hydraulic Joggling Press — Tweddell's Punching and Shearing Machine — Flanging Machine — Hydraulic Centre Crane — Wrightson's Balance Crane — Hydraulic Power at the Forth Bridge — Cranes — Hydraulic Coal-Discharging Machines — Hydraulic Drill — Hydraulic Manhole Cutter — Hydraulic Drill at St. Gothard Tunnel — Motors with Variable Power — Hydraulic Machinery on Board Ship — Hydraulic Points and Crossings — Hydrauhc Pile Driver — Hydraulic Pile Screwing Apparatus — Hydraulic Excavator — Ball's Pump Dredger — Plydraulic Power applied to Bridges — Dock-gate Machinery — Hydrauhc Brake — Hydraulic Power applied to Gunnery — Centrifugal Pumps — Water Wheels — Turbines — Jet Propulsion — The Gerard-Barr^ Hydraulic Railway — Greathead's Injector Hydrant — Snell's Hydraulic Transport System — Greathead's Shield — Grain Elevator at Frank- fort — Packing — Power Co-operation — Hull Hydraulic Power Company — London Hydraulic Power Company — Birmingham Hydraulic Power System — Niagara Falls — Cost of Hydraulic Power — Meters — Schonheyder's Pressure Regulator — Deacon's Waste-Water Meter. ** A B»ok of great Professional Usefulness." — Iron %* The Second Edition of the aboTe important work has been thoroughly reTised and brought up to date. Many new full-page Plates haye been added — the number beinjf increased from 45 in the First Edition to 69 in the present. lONDON : CHARLES GRIFFIN & CO., LIMITED, EXETER STREET, STRAND. 38 CHARLES GRIFFIN dfe GO:S PUBLICATIONS. Royal 8uo, Handsome Cloth, 25a. THE STABILITY OF SHIPS. BY SIR EDWARD J. REED, K.C.B., F.R.S., M.P., 4CNIGHT OF THE IMPERIAL ORDERS OF ST. STANILAUS OF RUSSIA', FRANCIS JOSEPH OF AUSTRIA ; MKDJIDIE OF TURKEY ; AND. RISING SUN OF JAPAN ; VICE- PRESIDENT OF THE INSTITUTION OF NAVAL ARCHITECTS. IVtt/i numerous Illustrations an(J Tables. This work has been written for the purpose of placing in the hands of Naval Constructors, Shipbuilders, Officers of the Royal and Mercantile Marines, and all Students of Naval Science, a complete Treatise upon the Stability of Ships, and is the only work m the Engbsh Language dealing exhaustively with the subject. In order to render the work complete for the purposes of the Shipbuilder, ^hether at home or abroad, the Methods of Calculation introduced by Mr. F. K. Barnes, Mr. C'RAT, M. Reech, M. Davmard, and Mr. Benjamin, are aU given separately, illustrated by Tables and worked-out examples. The book contains more than 200 Diagrams, and is iUustrated by a large number of actual cases, derived from ships of aU description*, but especially from ships of the Mercantile Marine. The work will thus be found to constitute the most comprehensive and exhaustive Treatise hitherto presented to the Profession on the Science of the Stability of Ships. " Sir Edward Reed's * Stability of Ships ' is invaluable. In it the Student, new to the subject, will find the path prepared for him, and all difficulties explained with the utmost care and accuracy ; the Ship-draughtsman will find all the methods of calculation at present in use fuUy explained and illustrated, and accompanied by the Tables and i-orms employed • the Shipowner will find the variations in the Stability of Ships due to differences in forms and dimensions fully discussed, and the devices by which the state of his ships under all conditions may be graphically represented and easily understood; the Naval Architect will find brought together and ready to his hand, a mass of information which he would other- wise have to seek in an alm.ost endless variety^ of publications, and some of which he would possibly not he able to obtain at all elsewhere." — Steamship. " This important and valuable work . . • cannot be too highly recommended to all connected with shipping interests." — Iron. " This very important treatise, ... the most intelligible, instructiyk, and COMPLSTX that has ever appeared." — Nature. "The volume is an essential one for the shipbuilding profession.**^ fVestmtnster Rtview. COMPANION-WORK. THE DESIGN AND CONSTRUCTION OF SHIPS. By JOHN HARVARD BILES, M.Inst.N.A., Professor of Naval Architecture in the University of Glasgow. In Preparation. LONDON : CHARLES GRIFFIN & CO., LIMITED, EXETER STREET, STRAND. ENGINEERING AND MEGHANIGS. 39 Fourteenth Edition, Revised. Prioe 21s. Demy Svo, Gloth, With Numeroys UluatrationSy red%bC6d from Working Drawings. A MANUAL OF MARINE ENGINEERING: COMPRISING THE DESIGNING, CONSTRUCTION, AND WORKING OF MARINE MACHINERY. By A. E. S E AT 0 N, M. Inst. C. E., M. Inst. Mech. E.. M.InstN.A. GKNKRAL Part I.— Principles of Marine Propulsion. Part II.— Principles of Steam Engineering. Part III.— Details of Marine Engines : Design and Cal- GONTKNTS. eulations for Cylinders, Pistons, Valves, Expansion Valves, &e. Part IV.— Propellers. Part v.— Boilers. Part VI.— Miscellaneous. This Edition includes a Chapter on Water -Tube Boilers, with Illustra- tions of the leading Types and the Revised Rules of the Bureau Veritas. "In the three-fold capacity of enabling a Student to learn how to design, construct, and work a Marine Steam-Engine, Mr. Seaton's Manual has NO rital."— TfwM. "The important subject of Marine Engineering is here treated with the thorough- ness that It reauires. No department has escaped attention. . . . Gives the results of mucti close study and practical v^ork.^^— Engineering. "By far the best Manual in existence. . . . Gives a complete account of the methods of solving, with the utmost possible economy, the problems before the Marine Knf^meer.—Athenceum. "The Student, Draughtsman, and Engineer will find this work the most valuablb Handbook of Reference on the Marine Engine now in existence."— iIfar«M."— Merchant Service Jieview. Elementary Seamanship. By D. WiLsoN-BARKEJi, Master iviarmer, F.R.S.E., F.R.G.S. With numerous Plates, two in Colours, and JTrontispiece. Second Edition, Revised. 5s. "This ADMIRABLE MANUAL, by Capt. Wilson BARKER, of the 'Worcester,' seems to us PERFECTLY DESIGNED." — AthenCBUm. Know Your Own Ship : a Simple Expianation of the Stability, Con- struction, Tonnage, and ±'reeboard ot Ships. By Thos. Walton, JNaval Ai-chitect. With numerous Illustrations and additional Chapters on Buoyancy, xrim, and Calculacions. i^'ouRTH Edition, Hevised. 7s. 6d. "Mr. Walton's book wiil be found very useful."— Jinyincer, The Construction and Maintenance of Vessels built of Steel. By Thos. Walton, ^^aval Architect. [Shortly. Navigation : Theoretical and Practical. By D. Wilson-Bakker, Master Mariner, dc., and William Allingham. 3s. 6d. "Precisely the kind of work required for the New Certificates of competency. Candidates will find it mYAJMhSLK."— Dundee Advertiser. Latitude and Longitude : How to And them. By W. J. Millar, C.E., late Sec. to the Inst, of Engineers and Shipbuilders in Scotland. 2s. " Cannot but prove an acquisition to those studying Navigation." — Marine Engineer. Practical Mechanics : Applied to the requirements of the Sailor. By Thos. Mackenzie, Master Mariner, F.R.A.S. 3s. 6d. " Well worth the money . . . exceedingly BMiiVeJlh."— Shipping World. Marine Meteorology: i'or Ofl&cers of the Merchant Navy. By William Allingham, First Class Honours, Navigation, Science and Art Department. [Shortly. Trigonometry : For the Young Sailor, &c. By Rich. C. Buck, of the Thames Nautical Training College, H.M.S. "Worcester." Price 3s. 6d. " This EMINENTLY PRACTICAL and RELIABLE yolume. "—Schoolmaster. Practical Algebra. By Rich. C. Buck. Companion Volume to the above, for Sai?ors and others. Price 3s. 6d. „ ,r ^- 7 »^ It is just the book for the young sailor mindful of progress. —Nautical Magazine. The Legal Duties of Shipmasters. By Benedict Wm. Ginsburg, M.A., LL.D., of the Inner Temple and Northern Circuit; Barrister-at-Law. ^Invaluable to Masters. ... We can fully recommend it."— Shipping Gazette. A Medical and Surgical Help for Shipmasters. Including First Aid at Sea. By Wm. Johnson Smith, F.R.C.S., Principal Medical Officer, Seaman's Hospital, Greenwich. 6s. " Sound, judicious, really helpful."— 27ie Lancet. LONDON: CHARLES GRIFFIN & CO.. LIMITED. EXETER STREET. STRAND. 46 0HARLB9 ORIFFIN S 00*8 PUBLIOATlOm. GRIFFIN S NAUTICAL SERIES. Price 3s. 6d. Post-free. THE British Mercantile Marine. By EDWARD BLACKMORE, MASTER MARINER; ASSOCIATE OF THE INSTITUTION OF NAVAL ARCHITECTS; MEMBER OF THE INSTITUTION OF ENGINEERS AND SHIPBUILDERS IN SCOTLAND; EDITOR OF GRIFFIN'S "NAUTICAL SERIES." G-ENERAL CoxTE^'Ts.— Historical : From Early Times to 1486— Process under Henry YTII.— To Death of Mary— During Elizabeth s Eeign— Up to the Eeign of William III— The 18th and 19th Centuries— Institution ot Examinations — Eise and Erogress of Steam Propulsion — Development of Free Trade— Shipping Legislation. 1862 to 1875— " Locksley Hall'" Case- Shipmasters' Societies— Loading of Ships— Shipping Legislation, 1884 to 1894— Statistics of Shipping. The Eersoxnel : Shipowners— Officers— Mariners — Duties and Eresent Eosition. Education : A Seaman's Education : what it should be — Eresent Means of Education — Hints. Discipline and Duty — Postscript— The Serious Decrease in the Xumber of British Seamen, a Matter demanding the Attention of the Xation. ''Interesting and Instructive . . . may be read with profit and enjotscent."— OJasgow Herald. "Evert bra>ch of the subject i- dealt with in a way which shows that the writer 'knows the ropes' familiarly.'"— iSco^^r/ian. "This ADMIRABLE book . . . TEEMS with useful information— Should be in the han.ls of every ^&\\ot."' —Western Morning Xeics. WORKS BY RICHARD C. BUCK, of the Thames Nautical Training College, H.M.S. ' Worcester,' 1. A Manual of Trigonometry: With Diagrams, Examples, and Exercises. Post-free 3s. 6d. Mr. Buck's Text-Book has been specially prepared with a view to the New Examinations of the Board of Trade, in which Trigonometry- is an obligatory subject. "This EMINENTLY PRACTICAL and RELIABLE \oi.xni^:'—Schoolmaiter. 2. A Manual of Algebra. Designed to meet the Requirements of Sailors and others. Price 3s. 6d. %* These elementary works on algebra and trigonometry are written specially for those who will have little oppormnity of consulting a Teacher. Tney are books for "self- help." All but the simplest explanations have, therefore, been avoided, and answers to the Exercises are given. Any person may readily, by careful study, become master of their contents, and thus lay the fo undation for a further 'mathematical course, if desired. It hoped that to the younger Officers of our Mercantile Marine they will be found decidedly serviceable. The Examples and Exercise > are taken froai the Examination Papers set for the Cadets of the "Worcester.'' *«* For complete List of Griffin's Xautical Series, see p. 45. LONDON: CHARLES GRIFFIN & CO., LIMITED, EXETER STREET, STRAND. NAUTICAL WORKS, 47 GRIFFIN'S NAUTICAL SERIES. Second Edition. Price 5s. Post-free. WLANTJAJLi OF ELEMENTARY SEAMANSHIP. BY D. WILSON-BARKER, Master Mariner; F.R.S.E., F.R.G.S., &c., &c.; YOUNGER BROTHER OF THE TRINITY HOUSE. With Frontispiece, Twelve Plates (Two in Colours), and Illustrations in the Text. General Contents.— The Building of a Ship; Parts of Hull. Masts, &c. — Ropes, Knots, Splicing, &c. — Gear, Lead and Log, &c. — Rigging, Anchors — Sailmaking — The Sails, &c — Handling of Boats under Sail — Signals and Signalling — Rule of the Road — Keeping and Relieving Watch — Points of Etiquette— Glossary of Sea Terms and Phrases — Index. %* The volume contains the new rules op the road. " This ADMIRABLE MANUAL, hy Capt. Wilson- Bakker of the Worcester," seems to ub PERFECTLY DESIGNED, and holds Its place excellently in ' Griffin's Nautical Series.' . . . Although intended for those who are to become Officers of the Merchant Navy, it will be found useful by all yachtsmen. "~^^A#n«jiW. " Five shillings will be well spent on this little book. Capt. Wilson-Barker knows from experience what a young man wants at the outset of his career."— Engineer. Price 3s. 6d. Post-free, :n^avigiation: I^RACTICAX^ ILNJy THEORETIC AX^. By DAVID WILSON-BARKER, RN.R., F.R.S.E., (fee, &c., AND WILLIAM ALLINGHAM, FIRST-CLASS HONOURS, NAVIGATION, SCIENCE AND ART DEPARTMENT. TKaitb IFlumerous 5Uu6tration6 anO :6jaminatlon (SiueBtionB* General Contents. — Definitions — Latitude and Longitude — Instruments of Navigation — Correction of Courses — Plane Sailing — Traverse Sailing — Day's Work — Parallel Sailing — Middle Latitude Sailing — Mercator's Chart— Mercator Sailing — Current Sailing — Position by Bearings— Great Circle Sailing — The Tides — Questions — Appendix : Compass Error — Numerous Useful Hints, &c — Index. " Precisely the kind of work required for the New Certificates of competency in grades from Second Mate to extra Master. . . . Candidates will find it invaluable."— i)«wd!M Advertiser. "A CAPITAL little BOOK . . . Specially adapted to the New Examinations. The Authors are Capt. Wilson-Barker (Captain-Superintendent of the Nautical College, H.M.S. *' "Worcester," who has had great experience in the highest problems of Navigation), and Mr. Allingham, a well-known writer on the Science of Navigation and Nautical Astronomy." —■Shipping World. ***For complete List of Griffin's Nautical Series, see p. 45. LONDON : CHARLES GRIFFIN & CO.. LIMITED, EXETER STREET, STRAND. 4» OHARLBS aRIFPTK A OO.'S PUBLT0ATT0N8. GRIFFIN'S NAUTICAL SERIES. Crown 8vo, with Numerous Illustrations. Handsome Cloth. 3s. 6d. Practical Mechanics: Applied, to the Requirements of tJie Sailor. By THOS. MACKENZIE, Master Marine7% F.R.A.S. General Contents. — Resolution and Composition of Forces — Work done by Machines and Living Agents — The Mechanical Powers: The Lever; Derricks as Bent Levers — The Wheel and Axle : Windlass ; Ship's Capstan Crab Winch — Tackles: the "Old Man" — The Inclined Plane; the Screw — The Centre of Gravity of a Ship and Cargo — Relative Strength of Rope : Steel Wire, Manilla, Hemp, Coir — Derricks and Shears — Calculation of the Cross-breaking Strain of Fir Spar — Centre of Effort or Sails — Hydrostatics : the Diving-bell ; Stability of Floating Bodies ; the Ship's Pump, &c. " This EXCELLENT BOOK . . . contains a LARGE AMOUNT of information."" — Nature. " Well worth the money . . . will be found exceedingly helpful." — Shipping World. " No Ships' Officers' bookcase will henceforth be complete without Captain Mackenzie's ' Practical Mechanics. ' Notwithstanding my many- years' experience at sea, it has told me how much more there is to acquire.'^ — (Letter to the Publishers from a Master Mariner). " I must express my thanks to you for the labour and care you have taken in ' Practical Mechanics.' . . . It is a life's experience. . . . What an amount we frequently see wasted by rigging purchases \vithout reason and accidents to spars, &c., &c. ! 'Practical Mechanics' would save all THIS." — (Letter to the Author from another Master Mariner). Crown 8vo, with Diagrams. 2s. Post-free. Latitude and Longitude: Moiwr to Fincl tlxem. By W. J. MILLAR, C.E., Late Secretary to the Inst, of Engineers and Shipbuilders in Scotland. *' Concisely and clearly written . . . cannot but prove an acquisition to those studying ISTavigation. " — Marine Engineer. " Young Seamen will find it handy and useful, simple and clear."— The Engmeer. For Complete List of Griffin's Nautical Series, see p. 45. LONDON; CHARLES GRIFFIN & CO., LIMITED, EXETER STREET, STRAND. NAUTICAL WORKS. 49 GRIFFIN^S NAUTICAL SERIES. In Crown 8vo. Handsome Cloth. 4s. 6d. Post-free. THE LEGAL DUTIES OF SHIPMASTERS. BY BENEDICT WM. GUSTSBURG, M.A., LL.D. (Cantab.), Of the Inner Temple and ISTorthern Circuit ; Barrister-at-Law. Genepal Contents.— The Qualification for the Position of Shipmaster— The Con- tract with the Shipowner— The Master's Duty in respect of the Crew : Engagement ; Apprentices ; Discipline ; Provisions, Accommodation, and Medical Comforts ; Payment of Wages and Discharge— The Master's Duty in respect of the Passengers— The Master's Financial Responsibilities — The Master's Duty in respect of the Cargo — The Master's Duty in Case of Casualty— The Master's Duty to certain Public Authorities— The Master's Duty in relation to Pilots, Signals, Flags, and Light Dues— The Master's Duty upon Arrival at the Port of Discharge— Appendices relative to certain Legal Matters : Board of Trade Certificates, Dietary Scales, Stowage of Grain Cargoes, Load Line Regula- tions, Life-saving Appliances, Carriage of Cattle at Sea, &c., &c. — Copious Index. "No Intelligent Master should fail to add this to his list of useful and necessary books. The price (4s. 6d.) cannot he quoted as an excuse for non-possession, and a few lines of it may save a lawykk's fee, besides endless vfORRY.'''—Livei'pool Journal of Commerce. " Sensible, plainly written, in cleak and non-technical language, and will be found of MUCH servicb by the Shipmaster."— 5r2Yi5/i Trade Review. FIRST AID AT SEA. With Coloured Plates and Numerous Illustrations. 6s. A MEDICAL AND SURGICAL HELP FOR SHIPMASTERS AND OFFICERS IN THE MERCHANT NAVY. BY WM. JOHNSON SMITH, F.KO.S., Principal Medical Officer, Seamen's Hospital, Greenwich. %* The attention of all interested in our Merchant Na^y is requested to this exceedingly useful and valuable work, it is needless to say that it is the outcome of many years pbactical experience amongst Seamen. Sound, judicious, really helpful " — The Lancet. MARINE METEOROLOGY FOR OFFICERS OF THE MERCHANT NAVY. BY WILLIAM ALLINGHAM, Joint-Author of "Navigation, Theoretical and Practical." [In Preparation. For Complete List of Grtffin's Nautical Series, see p. 45. LONDON: CHARLES GRIFFIN & CO., LIMJED, EXETER STREET, STRAND. OHABLBS ORirWIN A CO.'B PUBLICATIONS. GRIFFIN'S NAUTICAL SERIES. Fourth Edition. Revised throughout, with additional Chapters on Trim, Buoyancy, and Calculations. Numerous Illustrations, Handsome Cloth, Crown 8vo. 7s. 6d. KNOW YOUR OWN SHIP. By THOMAS WALTON, Naval Architect. SPECIALLY ARRANGED TO SUIT THE REQUIREMENTS OP SHIPS' OFFICERS, SHIPOWNERS, SUPERINTENDENTS, DRAUGHTSMEN, ENGINEERS, AND OTHERS. This work explains, in a simple manner, such important subjects as : — Displacement, Deadweight, Tonnage, Freeboard, Moments, Buoyancy, Strain, Structure, Stability, Rolling, Ballasting, Loading, Shifting Cargoes, Admission of Water, Sail Area, &c., &c. The little book will be found exceedingly handy by most officers and officials connected with shipping. . . . Mr. Walton's work will obtain lasting success, because of its unique fitness for those for whom it has been written." — Shipping World. ' * An excellent work, full of solid instruction and invaluable to every officer of the Mercantile Marine who has his profession at heart." — Shipping. " Not one of the 242 pages could well be spared. It will admirably fulfil its purpose . . . useful to ship owners, ship superintendents, ship draughts- men, and all interested in shipping." — Liverpool Journal of Commerce. " A mass of very useful information, accompanied by diagrams and illus- trations, is given in a compact form." — Fair play " A large amount of most useful information is given in the volume. The book is certain to be of great service to those who desire to be thoroughly grounded in the subject of which it treats." — Steamship. " We have found no one statement that we could have wished differently expressed. The matter has, so far as clearness allows, been admirably con- densed, and is simple enough to be understood by every seaman."— Jfarme Engineer, By the Same Author. In Preparation, THE CONSTRUCTION AND MAINTENANCE OF VESSELS BUILT OF STEEL. Illustrated with Numerous Plates and Diagrams. For Complete List of Griffin's Nautical Series, see p. 45. LONDON: CHARLES GRIFFIN & CO.. LIMITED, EXETER STREET, STRAND. GEOLOGY, MINING, AND METALLURGY, 7-8. Griffin's Geological, Prospecting, Mining, and Metallurgical Publications. Geology, Stpatigraphieal, R. Etheridge, F.R.S., . „ Physical, . . Prof. H. G. Seeley, . Practical Aids, 3rd Ed., Prof. Grenville Cole, ,, Open Air Studies, . „ n Griffin's -New Land" Series ) -^^ ^ p^^^ ^ for Prospectors, ( ^ 1. Prospecting* for Minerals, S. Herbert Cox, A.KS.M., 2. Food Supply, . Robt. Bruce, 3. New Lands and their) Prospective Advan->H. R. Mill, D.Sc, F.R.S.E tages, . . . j 4. Building" Construction, Prof. Jas. Lyon, Ore and Stone Mining", 2nd Ed., Prof. Le Neve Foster, Elementary Mining, . . „ „ Coal Mining, 3rd Ed., . H. W. Hughes, F.G.S , Petroleum, .... Redwood and Holloway, Mine-Surveying, 6th Ed., . Bennett H. Brough, A.R.S Blasting and Explosives, O. Guttmann, A.M.I.C.E., Mine Accounts, . . • Rrof. J. G. Lawn, Metallurgy (General Treatise 1 Bauerman, on), 3rd Ed., J (Elementary), Prof. Humboldt Sexton, Assaying, 5tb Ed., . J. J. & C. Beringer, . ^ , n/r X n • lo • (Ed. by Prof. Roberts-Austen, Griffin's Metallurgical Series I qjb^^.R.S., 1 . Introduction to Metal-jp^^^ RobertIausten, lurgy, 4th Ed., . / 2. Gold Metallurgy of, ^^^^^^ ^^^^^ A.R.S.M 3rd Ed., . . . j 3. Iron, Metallurgy of, . Thos. Turner, A.R.S.M., 4. steel, „ . F. W. Harboed, A.R.S.M., 5. Silver and Lead, „ • H. F. Collins, A.KS.M., 6. Metallurgical Machinery, H. C. Jenkins, A.KS.M., Getting Gold, 2nd Ed., . J. C. F. Johnson, F.G.S., Electric Smelting and I Borchers and McMillan, Refining, . . . ( Tables for Quantitative K j^^^^g Morgan, . Metallurgical Analysis, j Electro-Metallurgy, . W. G. McMillan, F.I.C, Goldsmith and Jeweller's Art, Thos. B. Wigley, . PAGE 52 52 53 86- 54 55. 54 54 54 56 56 5T 58. M., 59 59- 60' 61 66 66 62: 63 64 65 65 65 62 55 67 67 68- 68 LONDON: CHARLES GRIFFIN & CO., LIMITED, EXETER STREET, STRAND. 52 OHARLES ORIFFIN ds CO.'S PUBLICATIONS. Demy 8vo, Handsome cloth, 18s. Physical Geology and Palseontology, OJV TEE BASIS OF PEILLIPS. m HARRY G O V I E R S E E L E Y, F. R. S., PROFESSOR OF GEOGRAPHY IN KING'S COLLEGE, LONDON. uattb 4fronti6piece In Cbromo^XUbograpb^t an^ JlluBtrationB* " It is impossible to praise too highly the research which Professoi Seeley*S • Physical Geology ' evidences. It is far more than a Text-book — it is a Directory to the Student in prosecuting his researches." — Presidential Ad- dress 10 the Geological Society^ 1885, by Rev. Prof. Bonney D.Sc. , LL.D., P.P.S. *' Professor Seeley maintains in his ' Physical Geology ' the high reputation he already deservedly bears as a Teacher. " — Dr. Henry Wood- ward^ F.P.S.f in the " Geological Ma^azine^ Professor Seeley^'s work includes one of the most satisfactory Treatises on Lithology in the English language. ... So much that is not accessible in other works is presented in this volume, that no Student of Geology cac afford to be without it.'' — American /oumal ot Engineering. Demy 8vo, Handsome cloth, 34s* Stratigraphlcal Geology & Palaontology, 0^'^ THE BASIS OF FBILLlPS. BY ROBERT ETHERIDGE, F. R. S., OF TMR NATURAL HIST. DEPARTMENT. BRITISH MUSEUM, LATE PALEONTOLOGIST TO Till GEOLOGICAL SURVEY OF GREAT BRITAIN, PAST PRESIDENT OF THE GEOLOGICAL SOCIETY, ETC. TKlttb x^ibap, flumeroue ^Tables, ant) ZUxVq^qxi plates. " No such compendium of geological knowledge has ever been brought together before." — iV^stminster Rexneuu. ** If Prof. Skeley's volume was remarkable for its originality and the breadth of its viewsi, Mr. Ethbridge fully justifies the assertion made in his preface that his book differs ia con- structicm and detail from any known manual . . . Must take high rank amokg womcs OF REFERENCE." — At/ufueufft. LONDON: CHARLES GRIFFIN & CO., LIMiTED, EXETER STREET, STRAND. PRAGTIOAL OEOLOQY AND PROSPEOTINO. 53 Works by GRENVILLE A. J. COLE, M.R.I.A., F.G.S., Professor of Geology in the Royal College of Science for Ireland. PRACTICAL GEOLOGY (AIDS IN): IVITH A SECTION ON PALEONTOLOGY. By professor GRENVILLE COLE, M.RJ.A., F.G.S. Third Edition, Revised and in part Re-written. With Frontispiece and Illustrations. Cloth, los. 6d. GENERAL CONTENTS.— PART I. — Sampling of the Earth's Crust. PART II. — Examination of Minerals. PART III.— Examination of Rocks PART IV. — Examination of Fossils. "Prof. Cole treats of the examination of minerals and rocks in a way that has neret been attempted before . . . deserving of the highest praise. Here indeed are 'Aids' INNUMERABLE and INVALUABLE. All the directions are given with the utmost clear- ness and precision." — Atkenerum. "To the younger workers in Geology, Prof. Cole's book wil be as indispensable as a dictionary to the learners of a language." — Saturday Review. "That the work deserves its title, that it is full of 'Aids,' and in the highest degree 'practical,' will be the verdict of all who use it" — Nature. ** This EXCELLENT Manual . . . will be A VERY GREAT HELP. . . . The scction on the Examination of Fossils is probably the best of its kind yet published. . Full of well-digested information from the newest sources and from personal research." — Annals #/ Nat. History. OPEN-AIR STUDIES; An Introduction to Geology Out-of-doors. By professor GRENVILLE COLE, M.R.I. A., F.G.S. With 12 Full- Page Illustrations from Photographs, Cloth. %s, 6d. For details, see p. 86. Edited by PROFESSOR COLE, The ''New Land" Series for Colonists and Prospectors (See next page). LONDON: CHARLES GRIFFIN & CO., LIMITED, EXETER STREET, STRAND. S4 CHARLES GRIFFiy S CO. '8 PUBLICATIONB. The " New Land" Series Practical Hand-Books Fop the Use of Prospeetops. Exploreps. Settleps, Colonists, and all Intepested in the opening up and Development of New Lands. EDITED BY GEEXVILLE A. J. COLE, ME. LA.. E.G.S., rr::T55:r :: Geology in the Soyal C l'r^r :: ^ iri r : : 'L-^Tze Crown Svo, Cloth or Lear":ier. vr::":i r/.u5:ri:ions. Vol. 1.— PEOSPECTIXCx FuR MINEEALS. By S. Herbeet C.x. Assoc.E.S.M.. AIJr.5:.M.M., E.G.S. Cloth, 5s.: Eearher. rounded c i Ar: -, Vol. 1'. — El'L'E SUEEEY. By Eobert Bruce. Agricul^ura' Smt: erii-Tei-deiit to the Eoyal Dublin Society. With ir-ai^y Ei.^'r:iv:ng5 and EhoT'jgraphs. Clo:h. -Is. ocl. " Bris: r5 ~.: z: ::7Zi.:.:z. '— Farmers' GloseUe. IX PRE PA RA TiOX. YoL. 3.— XEVT EAXES AXE THEIE EE'JSEECTIVE ADVAXTA<>E-. Ey Ht;-z E:E-Er Mill. D.Sc, E.E.S.E.. EiViarian to E i'va^ Gr T^rai .Aoai Society. YoL 4.— BUILEiyG COXSTEUCEIOy :y WnijD. STONE. AND COXCEETE. By Jas. Eton. M.A.. Eroressor o: Encrineerini^ in the Eoval Coh-^r >::ta:^ ::r ^:e-ar.d: sometime SuperinteiAG::-: \ z~P.^ aa^a.-ti I'^t: ar:AirA: in the University o: C aA. a aI^^ : aA^ J, I-^yi:a. A.E.L .:^.!. Other Volumes ^ih- fohow. dealing ^i:':: su; yects of Primary Importa>'ce in t?a Ex_a aa^t av a d Eia' iSATioy of Lands which have not as ye: ; r-A : ;dy level itrd. LI Ml\ : :-:^_lS GRIFFIN & CO.. LiyiTED, EXETER STREET, STRAND. PROSPEGTINQ AND MINING, 55 GRIFFIW^S *^NEW LAND^^ SERIES , Now Ready. With Illustrations. Price in Cloth, 5s. ; strongly hound in Leather, 6s. 6c?. PROSPECTING FOR MINERALS. A Practical Handbook for Prospectors, Explorers, Settlers, and all interested in the Opening up and Development of New Lands. BY S. HERBERT OOX, Assoc.R.S.M., M.Inst.M.M., F.G.S., &c. General Contents. — Introduction and Hints on Geology — The Determina- tion of Minerals : Use of the Blow-pipe, &c. — Kock-forming Minerals and Non- Metallic Minerals of Commercial Value : Rock Salt, Borax, Marbles, Litho- graphic Stone, Quartz and Opal, &c. , &c. — Precious Stones and Gems — Stratified Deposits : Coal and Ores— Mineral Veins and Lodes — Irregular Deposits — Dynamics of Lodes : Faults, &c. — Alluvial Deposits — Noble Metals : Gold, Platinum, Silver, &c. —Lead — Mercury — Copper — Tin— Zinc— Iron — Nickel, &c. — Sulphur, Antimony, Arsenic, &c. — Combustible Minerals— Petroleum — General Hints on Prospecting — Glossary — Index. " This ADMIRABLE LITTLK WORK . . . written with SCIENTIFIC ACCURACY in a CLEAR and LUCID style. ... An important addition to technical literature . . . will be of value not only to the Student, but to the experienced Prospector." — Mining Journal. NOW READY. Vol. II. With many Engravings and Photographs. Cloth, 4s. 6d. By ROBERT BRUCE, Agricultural Superintendent to the Royal Dublin Society. With Appendix on Preserved Foods by C. A. Mitchell, B.A., F.I.C. General Contents. — Climate and Soil — Drainage and Rotation of Crops — Seeds and Crops — Vegetables and Fruits — Cattle and Cattle- Breeding — Sheep and Sheep Hearing — Pigs — Poultry — Horses — The Dairy — The Farmer's Implements — The Settler's Home. "Bristles with infoemation."— /"armers' Gazette. Second Edition. With Illmtrations. Cloth, 35. Qd. GETTING GOLD: A GOLD-MINING HANDBOOK FOR PRACTICAL MEN. Br J. 0. P. JOHNSON, P.G.S., A.I.M.E., Life Member Australasian Mine-Managers' Association. General Contents. — Introductorj^ : Getting Gold — Gold Prospecting (Alluvial and General) — Lode or Peef Prospecting — The Genesiology of Gold — Auriferous Lodes — Auriferous Drifts — Gold Extraction — Secondary Processes and Lixiviation — Calcination or "Poasting"of Ores — Motor Power and its Transmission— Company Formation and Operations — Pules of Thumb : Mining Appliances and Methods— Selected Data for Mining Men — Australasian Mining Kegulations. ' ' Practical from beginning to end . . . deals thoroughly with the Prospecting, Sinking, Crushing, and Extraction of goldi."— Brit. Australasian. LONDON: CHARLES GRIFFIN & CO.. LIMITED. EXETER STREET. STR/iND. 3 S6 0HARLB8 ORIFFIN * OO.'S PUBLIOATIONS. ORE & STONE MINING. BY C. LE NEVE FOSTER, D.Sc, F.R.S., PROFESSOR OF MINING. ROYAL COLLEGE 0F SCIENCE H.M. INSPECTOR OF MINES. Third Edition. With Frontispiece and 716 Illustrations. 34s. " Dr. Foster's book was expected to be epoch-making, and it fully justifies such expec- tation. ... A MOST ADMIRABLE account of the mode of occurrence of practically all KNOWN MINERALS. Probably stands unrivalled f©r completeness."— 7"/^^ Mining Journal. GENERAL CONTENTS. INTRODUCTION. Mode of Oceurrenee of Minerals: Classification: Tabular Deposits, Masses— Examples: Alum, Amber, Antimony, Arsenic, Asbestos, Asphalt, Barytes. Borax, Boric Acid, Carbonic Acid, Clay, Cobalt Ore, Copper Ore, Diamonds, FUnt, Freestone, Grold Ore, Graphite, Gypsum, Ice, Iron Ore, Lead Ore, Manganese Ore, Mica, Natural Gas, Nitrate of Soda, Ozokerite, Petroleum, Phosphate of Lime, Potassium Salts, Quicksilver Ore. Salt, Silver Ore, Slate, Sulphur, Tin Ore, Zmc Ore. Faults. Prospecting : Chance Discoveries — Adventitious Finds — Geology as a Guide to Minerals— Associated Minerals— Surface Indications. Boring: Uses ol Bore-holes— Methods of Boring Holes: i. By Rotation, ii. By Percussion with Rods, iii. By Percussion with Rope. Breaking Ground: Hand Tools— Machinery- Transmission of Power— Excavating Machinery : i. Steam Diggers, ii. Dredges, iii. Rock Drills, iv. Machines for Cutting Grooves, v. Machines for TunnelUng— Modes of using Holes— Driving and Sinking— Fire-setting— Excavating bv Water. Supporting Excavations : Timbering— Masonry— Metallic Supports— Watertight Linings— Special Processes. Exploitation: Open Works :—HydrauUc Mining- Excavation of Minerals under Water— Extraction of Minerals by Wells and Bore- holes—Underground W orkings— Beds— Veins— Masses. Haulage or Transport: Underground: by Shoots, Pipes, Persons, Sledges, Vehicles, Railways, Machinery, Boats— Conveyance above Ground. Hoisting or Winding: Motors, Drums, and Pulley Frames— Ropes, Chains, and Attachments— Receptacles— Other Appiiancea— Safety Appliances— Testing Ropes— Pneumatic Hoisting. Drainage : Surface Water —Dams— Drainage Tunnels — Siphons — Winding Machinery — Puniping Engines above ground — Pumping Engines below ground— Co-operative Pumping. Ventila- tion: Atmosphere of Mines— Causes of Pollution of Air— Natural Ventilation- Artificial Ventilation : i. Furnace Ventilation, ii. Mechanical Ventilation— Testing the Quality of Air— Measuring the Quantity and Pressure of the Air— Efiiciency of Ventilating Appliances — Resistance caused by Friction. Lighting : Reflected Daylight — Candles— Torches— Lamps— Wells Light— Safety Lamps— Gas— Electric Light. Descent and Ascent : Steps and Slides — Ladders — Buckets and Cages— Man Engine. Dressing: i. Mechanical Processes ii. Processes depending on Physical Properties— iii. Chemical Processes— Principles of Employment of Mining Labour —Legislation affecting Mines and Quarries. Condition of the Miner- Accidents. err*' This epoch-making work . . . appeals to men of experience no less than to students.*' — Berg- und Hiittenmdnniscke Zeitung. i /'This splendid ^0'&.vi"—Oesterr. Ztsckrft. fur Berg- und Huttemuesen. ELEMENTARY MINING AND QUARRYING (An Introductory Text-book). By Prof. 0. LE NEYE FOSTEE, F.RS. In Crown 8vo. With Illustrations. [Shortly, LONDON: CHARLES GRIFFIN & CO, LIMITED, EXETER STREET, STRAND. WORKS ON MINING. 57 COAL-MINING (A Text-Book of): FOR THE USE OF COLLIERY MANAGERS AND OTHERS ENGAGED IN COAL-MINING, BY HERBERT WILLIAM HUGHES, F.G.S., Assoc. Royal School of Mines, General Manager of Sandwell Park Colliery. Third Edition. In Demy %vo. Handsome Cloth. With very Numerous Illustrations^ mostly reduced from Working Drawings. Price \%s. "The details of colliery work have been fully described, on the ground that collieries are more often made remunerative by perfection in small matters than by bold strokes of engineering. ... It frequently happens, in particular localities, that the adoption of a combination of small improvements, any of which viewed separately may be of apparently httle value, turns an unprofitable concern into a paying one." — Extract from Authors Preface. GENERAL CONTENTS. Qeolo^ : Rocks -Faults— Order of Succession— Carboniferous System in Britain. Goal : Definition and Formation of Coal— Classification and Commercial Value of Coals. Bearch for Coal : Boring— various appliances used— Devices employed to meet Difficulties of deep Boring— Special methods of Boring— Mather & Piatt's, American, and Diamond ^stems— Accidents in Boring— Cost of Boring— Use of Boreholes. Breaking G-rouncl' Tools— Transmission of Power Compressed Air, Electricity— Power Machine Drills— Coal Cutting by Machinery— Cost of Coal Cutting— Explosives— Blasting in Dry and Dusty Mines— Blasting by Electricity— Various methods to supersede Blasting. Sinking: Position, Form, and Size of shaft— Operation of getting down to " Ston^^-head "—Method of proceeding afterward*— Lining shafts— Keeping out Water by Tubbing— Cost of Tubbing- Sinking by Boring— Kind - Chaudron, and Lipmann methods— Sinking through Quicksands -Cost of Sinking. Preliminary Operations : Driving underground Roads— Supporting Roof: Timbering, Chocks or Cogs, Iron and Steel Supports and Masonry— Arrangement of Inset. Methods of Working : Shaft, Pillar, and Subsidence— Bord and Pillar System- Lancashire Method— Longwall Method— Double Stall Method— Working Steep Seams- Working Thick Seams— Working Seams lying near together— Spontaneous Combustion. Haalage: Rails— Tubs— Haulage by Horses— Self-acting Inclines— Direct-acting Haulage —Mam and Tail Rope— Endless Chain- Endless Rope— Comparison. Winding- Pit Frames — Pulleys— Cages— Ropes— Guides— Engines— Drums— Brakes— Counterbalancing— Expansion— Condensation— Compound Engrines— Prevention of Overwinding— Catches at pit top— Changing Tubs— Tub Controllers- Signalling. Pumping: Bucket and Plunger Pumps — Supporting Pipes in Shaft — Valves — Suspended Hfts for Sinking — Cornish and Bull Engines— Davey Differentisd Engine— Worthington Pump— Calculations as to size of Pumps— Draining Deep Workings— Dams. Ventilation: Quantity of air required- Gases met with in Mines— Coal-dust— Laws of Friction— Production of Air-currents— Natural Ventilation— Furnace Ventilation— Mechanical Ventilators— Efficiency of Fans— Comparison of Furnaces and Fans— Distribution of the Air-current— Measurement of Air- currents. Lighting : Naked Lights — Safety Lamps — Modern Lamps — Conclusions— Lockmg and Cleaning Lamps— Electric Light Underground— Delicate Indicators. Works at Surface; Boilers— Mechanical Stoking— Coal Conveyors— Workshops. Preparation of Coal for Market: General Considerations— Tipplers— Screens— Varying the Sizes made by Screens— Belts— Revolving Tables— Loading Shoots— Typical Illustrations of the arrange- ment of Various Screening Establishments— Coal Washing— Dry Coal Cleaning -Briquettes. "Quite THE BEST BOOK of its kind ... as practical in aim as a book can be . . touches upon every point connected with the actual working of collieries. The illustrations are kxckllknt."— ^ ^>^«^w?«. " A Text-book on Coal-Mining is a great desideratum, and Mr. Hughes possesses ADMIRABLE qualifications for supplying it. . . . We cordially recommend the work," — Colliery Guardian. "Mr. Hughes has had opportunities for study and research which fall to the lot of but few men. If we mistake not, his text-book will soon come to be regarded as the STANDARD WORK of its Vixid."— Birmingham Daily Gazette. LONDON: CHARLES GRIFFIN & CO., LIMITED, EXETER STREET, STRAND. S8 0HARLE8 GRIFFIN GO.'S PUBLICATIONS. AND ITS PRODUCTS: IL JPBL ACTIO AX. TIRE JSLTXSE. BY BOVERTON REDWOOD, F.RS.E., F.I.C, Assoc. Inst. C.E., Hon. Corr. Mem. of the Imperial Russian Technical Society; Mem of the American ChemicRl Society ; Consulting Adviser to the Corporation of London under the Petroleum Acts, &c., &c. Assisted by GEO. T. HOLLOW AY, F.LC, Assoc. R.C.S., And Numerous Contributors. In Two Volumes, Large 8vo. Price 45s. •QClitb mumerou6 mape, platen, anD JUuetratione in tbe XLciV GENERAL CONTENTS. VIII. Transport, Storage, and DiSc- tribution of Petroleum. IX. Testing of Petroleum. X. Application and Uses of Petroleum. XI. Legislation on Petroleum at Home and Abroad. XII. Statistics of the Petroleum Production and the Petroleum Trade, obtained from the most trustworthy and oflacial sources. I. General Historical Account of the Petroleum Industry. II. Geological and Geographical Distribution of Petroleum and Natural Gas. III. Chemical and Physical Pro- perties of Petroleum. IV. Origin of Petroleum and Natural Gas. V. Production of Petroleum, Natural Gas, and Ozokerite. VI. The Refining of Petroleum. VII. The Shale Oil and Allied In- dustries. " The MOST COMPREHENSIVE AND CONVENIENT ACCOUNT that has yet appeared of a gigantic industry which has made incalculable additions to the comfort of' civilised man. . . . The chapter dealing with the arrangement for STORAGE and TRANSPORT of GREAT PRACTICAL INTEREST. . . . The DIGEST of LEGIS- LATION on the subject cannot but prove of the greatest utility."— ^'/le limes. "A SPLENDID CONTRIBUTION to our technical literature."— C/^emica^ News. *'This THOROUGHLY STANDARD WORK ... in every way excellent . . most fully and ably handled . . . could only have been produced by a man in the very exceptional position of the Author. . . . Indispen- sable to all who have to do with Petroleum, its applications, manufacture, STORAGE, or TRANSPORT." — Mining J oumaL " We must concede to Mr. Eedwood the distinction of havmg produced a. treatise which must be admitted to the rank of the indispensables. It con- tains THE LAST word that Can be said about Petroleum in any of its scientific, technical, and legal aspects. It would be difficult to conceive oi a more comprehensive and explicit account of the geological conditions associated with the SUPPLY of Petroleum and the very practical question of its amount and DURATION." — Journal of Gas Lighting. LONDON: CHARLES GRIFFIN & GO, LIMITED, EXETER STREET, STRAND. WORKS ON MJ^ING. 59 MINE-SURVEYING (A Treatise on). For the use of Managers of Mines and Coilieriea, Students at the Royal School of Mines, &c. By BENNETT H. BROUGH, F.G.S., Assoc.R.S.M., Formerly Instructor of Mine-Surveying, Royal School of Mine*. Seventh Edition, Enlarged and Revised. With Numerous Diagrams. Cloth, 7s. 6d. General Contents. General Explanations — Measurement of Distances — Miner's Dial — Variation of ihe Magnetic-Needl-e — Surveying with the Magnetic-Needle in presence of Iron — Surveying with the Fixed Needle — German Dial— Theodohte — Traversing Under- ^ound — Surface-Surveys with Theodolite — Plotting the Survey— Calculation of Areas — Levelling — Connection of Underground- and Surface-Surveys — Measuring Distances by Telescope — Setting-out — Mine-Surveying Problems — Mine Plans — Applications of Magnetic-Needle in Mining — Photographic Surveying — Appendices, ** Has PROVED itself a valuable Text-book ; the bkst, if not the only one, in the English language on the subject." — Mining Journal. ** No English-speaking Mine Agent or Mining Student will consider his technical library complete without it'" — Nature. ** A valuable accessory to Surveyors in every department of commercial enterprise. Fully deserves to hold its position as a standard."— C^^/ZzVry Guardian. In Large Svo, with Illustrations and Folding-Plates, ioj. dd^ B X. A S T I N O : AND THE USE OF EXPLOSIVES. A Handbook for Engineers and others Engaged in Mining, Tunnelling, Quarrying, &,c. By OSCAR GUTTMANN, Assoc. M. Inst. C.E. Member of the Societies of Civil Engineers and Architects of Vienna and Budapest, Corresponding Member of the Imp. Roy. Geological Institution of Austria, &rc. General Contents. — Historical Sketch — Blasting Materials — Blasting Pow- der — Various Powder-mixtures — Gun-cotton — Nitro-glycerine and Dynamite — Other Nitro-compounds — Sprengel's Liquid (acid) Explosives —Other Means of Blasting — Qualities, Dangers, and Handling of Explosives — Choice of Blasting Materials — Apparatus for Measuring Force — Blasting in Fiery Mines — Means cf Ign^iting Charges — Preparation of Blasts — Bore-holes — Machine-drilling — Chamber Mines — Chargmg of Bore-holes — Determination of the Charge — Blasting in Bore- holes — Firing — Straw and Fuze Firing — Electrical Firing — Substitutes for Electrical Firing — Results of Working — Various Blasting Operations — Quarrying — Blasting Masonry, Iron and Wooden Structures — Blasting in earth, under water, of ice, &c. " This ADMIRABLE work." — Colliery Guardian. " Should prove a vade-mecum to Mining Engineers and all engaged in practical work. — Iron and Coal Trades Review. LONDON : CHARLES GRIFFIN & CO., LIMITED, EXETER STREET, STRAND. 6o CHARLES GRIFFIN d: CO:S PUBLICATIONS, NEW VOLUME OF GRIFFIN'S MINING SERIES. Edited by C. LE NEVE FOSTER, D.Sc, F.R.S., H.M. Inspector of Mines, Professor of Mining, Royal School of Mines. Mine Accounts and Mining Bool[-](eeplng, A Manual for the Use of Students, Managers of Metalliferous Mines and Collieries, Secretaries of Mining" Companies, and others interested in Mining. With Numerous Exaiviples taken froiyi the Actual Practice OF Leading Mining Coiyipanies throughout the world. BY JAMES GUNSON LAWN, Assoc.R.S. M., Assoc. Mem.Inst.C.E., F.G.S.^ Professor of Alining at the South African School of Mines, Capeto-wru Kimberley, and Johannesburg. In Large 8ro. Price 10s. M. GENERAL CONTENTS.— Introduction. Part I. Engagement and Payment of Workmen — Data Determininij Gross Amount due to ^Yo^kmen— A. Leng-th of Time Worked — Overtime— B. Amount cf \York done— Sinking and Drivinc:— Exploitation— SHding Scales — Modifications— C. Value of Mineral gotten— Deductions— Pay-Sheet?, Due-Bills, ann Pay- Tickets. Part II. Purchases and Sales— Purchase and Distribution cf Stores — Books and Forms Relating thereto — Sales of Product— Methods of Sale— Contract — Tender — Delivery of, and Payment for, Mineral— Tin Ore — Coal— Silver Ore— Gold Ore Part III. Working" Summaries and Analyses -Summaries of Minerals Raised. Dressed, and Sold; and of Labour— Analyses of Costs — Accounts Forwarded t ) Head Office. Part IV. Ledger, Bal^ anee Sheet, and Company Books— Head Office Books — Ledger— Principal Accounts of a Mining Company— Capital Account— Sale and Purchase Accounts — Capital Expenditure — Personal— Stores— Waires Account— Bad Debts Account— Cash Account— Bills Receivable and Payable Account— Discount and Interest Account— Product Account— Working Accounts- Profit and Loss Account— Joumal-Inventory-BaUnce Sheet— Bibliography— Redemption of Capital — 1. Debentures— 2. Sinking Fund— A. By Equal Annual Sums — B By Annual Sum varying according to a Formula— C. By Annual Sum depending on Mineral worked— 3. En- larg'ed Dividends or Bonuses— Depreciation— Reserve Fand- Biblioirraphy— General Consider- ations and Companies Books— Private Individuals— Private Partnership Companies— Cost-book Companies— Limited Liability Companies— Stocks and Shares— Debentures — Books connected with Shares— Miscellaneous Books— Bibliography. Part V. Reports and Statistics — Inspections of Workings and Machinery— A Colliery Reports, etc.— Inspections— Report Books- — Measurement of Ventilating Current— Permits to fire Shots and carry Safety Lamp Key— B. Ore Mine Reports. &c— C. Miscellaneous Reports. &c.— Reports of Mining Companies- Managers' Reports— Diagrams— Tabular Statements— Reports of Directors— Reports of Cost- book Mines— Mining Statistics— Great Britain— Other Countries— Bibliography. '•It seems impossible to suggest how Mr. Latv^t's book could be made more complete or more valuable, careful, and e^haxiiitiYe.''— Accountants Magazine. Mb. Lawns book should be found of great use by Mine Secretaries and Mine. Managers. It consists of five Parts. Part 1 is devoted to the engagement and payment of workmen, and contains forms of contracts and pay sheets of various descriptions in use by Mining Companies in England and South Africa. Special reference is made to pay sheets and forms employed by the De Beers Consolidated Mises. to the General Manager and the late- Secretary of which Company the author is indebted for the particulars given. Part 2 is taken' up by a description of books' and forms relating to the purchases of Stores, &c.. and to sales of the Products of the mines. Part 3 is the most important section of the book, containing in- structive details of the manner of obtaining summaries of Working Expenditure and Analyses of Costs. The forms used in this connection by the De Beers Company are shown in extenso. Part 4 consists of the Bookkeeping, properly so-called, of a mining company. All details- concerning the ledger, journal, and other books, and the principal working accounts of a mine are given. There are some very interestine: formulie in this section, -howing the manner in which the capital of a company sluaild be kkdkemed and repaid to its Shareholders. According to this manual there are three way> in which this extremely desirable end may be arrived at. . . . The book ic published at half a guinea, a price low enough considering the amount of information and instruction set ioiih.'' —Johannesburg Star. LONDON: CHARLES GRIFFIN & CO., LIMiTED, EXETER STREET, STRAND. ME TA LL URGl OA L WORKS. 6i Third Edition. With Folding Plates and Many Illustrations. Large 8vo. Handsome Cloth. 36s. ELEMENTS OF Metallurgy: A PRACTICAL TREATISE ON THE ART OF EXTRACTING METALS FROM THEIR ORES. BY J. ARTHUR PHILLIPS, M.Inst.C.E., F.C.S., F.G.S., 2g Journal. Dr Rose gained his experience in the Western States of America, but he has securod details of gold-working from all parts of the world, and these should be of gkiat service. to practical men , Tne four chapters on Chlorinaiion. written from the point of Tiew alike of the practical man and the chemist, teek with coksiderationb hithbeto uinKBCOG- KI8ED. and constitute an addition to the literature of Metallurgy, which will prove to be of classical ya.\\ie.''— Nature. " Adapted for all who are interested in the Gold Mining Industry, being free from tech- nicalities as far as possible, but is more particularly of Talue to those engaged m tn& industry— viz., mill-managers, reduction-officers, &Q.''—Cape Times. LONDON: CHARLES GRIFFIN & CO., LIMITED, EXETER STREET, STRAND. METALLURGIGAL WORKS. GRIFFIN'S METALLURGICAL SERIES. 65 THE METALLURGY OF IRON. By THOMAS TUENER, Assoc.R.S.M, F.I.C., Director of Technical Instruction to the Staffordshire County Council. In Large 8vo, Handsome Cloth, With Numerous Illustrations (many from Photographs). Price 16s. General Contents.— E.a.vlj History of Iron.— Modern History of Iron.— The Age of Steel. —Chief Iron Ores.— Preparation of Iron Ores.— The Blast Furnace.— The Air used in the Blast Furnace.— Eeactions of the Blast Furnace.— The Fuel used in the Blast Furnace.— Slags and Fuxes of Iron Smelting.— Properties of Cast iron.— Foundry Practice.— Wrought Iron.— Indirect Production of Wrought Iron.— The Puddling Process.— Further Treatment of Wrought Iron. - Corrosion of Iron and Steel. *' A MOST valuable summary of knowledge relating to every method and stage in the manufacture of cast and wrought iron . . . rich in chemical details. . . Exhaustive and thoroughly up-to-date ."—^i^^^e^m of the American Iron and Steel Association. " This is A DELIGHTFUL BOOK, giving, as it does, reliable information on a subject becoming every day more elaborate. " — Colliery Guardian. "A thoroughly useful BOOK, which brings the subject up to date. Of GREAT VALUE to thosc engaged in the iron industry." — Mining Journal. IN PREPARATION, COMPANION-YOLUME ON THE METALLURGY OF STEEL. By F. W. HARBORD, Assoc.RS.M., F.I.C. Ready Shortly, Important New Work. THE METALLURGY OF LEAD AND SILVER. By H. F. COLLINS, Assoc.E.S.M., M.Inst.M.M. In Two Volumes, Each complete in Itself. Part I.— E A n. A Complete and Exhaustive Treatise on THE MANUFACTURE OF LEAD, WITH SECTIONS ON SMELTING AND DESILVERISATION, And Chapters on the Assay and Analysis of the Materials Involved. To be followed by the Companion-Volume (Part II.) on SILVER. LONDON: CHARLES GRIFFIN & CO., LIMITED, EXETER STREET, STRAND. 66 CHARLES GRIFFIN d: CO.'S PUBLICATIONS. ASSAYING (A Text-Book of): For the use of Students, Mine Managers, Assayers, Ac. By J. J. BERINGER, F.LC, F.C.S., Public Analyst for, and Lecturer to the Mining Association of, Cornwall, And C. BERINGER, F.C.S., Late Chief Assayer to the Rio Tinto Copper Company, London, With numerous Tables and Illustrations. Crown 8vo, Cloth, 10/6. Fifth Edition ; Revised. General Contents. - Part L - Introductory; Manipulation: Sainplmg_; Drying; Calculation of Results-Laboratory-books and Reports^ ' r 'iJ^^r .ieTnc ; Wet Grarimetric- Volumetric Assays : Titrometrxc, Cokrimetnc G^^^ Weighing and Measuring— Reagents- Formulae, Equations, &c.— Specific Gravity. Part H.-Metals : Detection and Assay of Silver, Gold, Platinum, Mercury Co^, Lead, Thallium, Bismuth, Antimony, Iron, Nickel, Cobalt, Zmc, Cadmium, Tm, Tungsten, Titanium, Manganese, Chromium, &c.— Earths, Alkahes. Part III.-Non-Metals : Oxygen and Oxides; The Halogens-Sulphur and Sul- phates— Araenic, Phosphorus, Nitrogen— Silicon, Carbon, Boron— Useful lables. •*A RKAI-LY MERITORIOUS WORK, that may be safely depended upon either for systematic aisfeTiuc^ion or for reference." — Nature. ;„f«^,f;«Ti tKai- -This work is one of the BEST of its kind. . • . Contains all the information that the Assayer will find necessary m the examinatioe af minerals. —Ji,ft£tmer. Handsome Cloth. With Numerous Illustrations. 6s. ELEMENTARY METALLURGY (A TEXT-BOOK OF). Ineluding the Author's Phactical Laboratory Course. Ey a. HUMBOLDT SEXTON, F.LC, F.C.S., Professor of Metallurgy in the Glasgow and West of Scotland Technical College. GENERAL CONTENTS.— Introduction— Properties of the Metals— Combustion -Fuels-Refractory Materials- Furnaces- Occurrence of the Metals m Nature— Pre- paration of the Ore for the Smelter-Metallurgical Processes-Iron: P/eP?^^*^^?|j5 Pig Iron-Malleable Iron-Steel-Mild Steel-Copper-Lead-Zmc and Tm-biljer —Gold-Mercury— Alloys— Applications of Electricity to Metallurgy— Labora- 'TORY Course with Numerous Practical Exercises. "Just the kind of work for Students commencing the study of Metal- lurgy, or for Engineering Students requiring a general knowledge ot it or for Engineers in practice who like a handy work of reference, lo all three clashes we heartily commend the work.''— Practical Engineer. " Excellently got-up and well-arranged. . . . Iron and copper wen fxplained by excellent diagrams showing the stages of the process from start to finish . The most novel chapter is that on the many changes wrought in Metallurgical Methods by Electricity."— C/iemica^ Trade Journal. " Possesses the great advantage of giving a Course of Practical work. — Mining Journal. LONDON: CHARLES GRIFFIN & CO., LIMITED. EXETER STREET, STRAND. ELEGTRO-META LL URO Y. 67 In large 8vo. With. Numerous Illustrations and Three Folding-Plates. Price 21s. ELECTEIO SlELTn& & EEEira&: A Practical Manual of the Extraction and Treatment of Metals by Electrical Methods. Being the Elektro-Metallurgie " of Dr. W. BORCHERS. Translated from the Second Edition by WALTER G. McMILLAN, F.I.C.. F.C.S., Secretary to the Institution of Electrical Engineers ; late Lecturer in Metallurgy at Mason College, Birmingham. CONTENTS. Part I. — Alkalies and Alkaline Earth Metals : Magnesium,, Lithium, Beryllium, Sodium, Potassium, Calcium, Strontium, Barium, the Carbides of the Alkaline Earth Metals. Part II.— The Earth Metals : Aluminium, Cerium, Lanthanum, Didymium. Part III. — The Heavy Metals : Copper, Silver, Gold, Zinc and Cad- mium, Mercury, Tin, Lead, Bismuth, Antimony, Chromium, Molybdenum, Tungsten, Uranium, Manganese, Iron, Nickel, and Cobalt, the Platinum Group. " Comprehensive and authoritative . . . not only full of valuable infor- mation, but give^ evidence of a THOROUGH insight into the technical value and possibilities of all the methods discussed." — The Electrician. " Dr. Borchers' well-known work . . . must of necessity be acquired by every one interested in the subject. Excellently put into English with additional matter by Mr. McMillan."— A^ai?Ldsii\x2^\y zot\x\t'^ —Brit . I OUT. of PhetogTuphy. " For the illustrations alone, the book is most interesting ; but, apart from these, the volume is valuable, brightly and pleasantly written, and most admirably arranged."— " Certainly the finest illustrated handbook to Photography which has ever been .published. Should be on the reference shelves of every Photographic Society." — AmaUur Pho tographer. "A handbook so far in advance of most others, that the Photographer must not fail \o obtain a copy as a reference work." — Photographic Work. **The complktkst handbook of the art which has yet been published." — Scotstrmn. A Standard Work on Photography brought quite up to date." — Photography. This Edition includes all the Newer Developments in Photographic Methods, together with Special Articles on Radiography (the X Rays), Colour Photography, and many New Plates. LONDON: CHARLES GRIFFIN & CO., LIMITED, EXETER STREET, STRAND. CHARLES GRIFFIN A C0:8 PUBLICATIONS. Second Edition, Revised aod Enlarged, with New Section on Acetylene. With Numerous Illustrations. Handsome Cloth. 10s. M. GkS MANUFACTURE (THE CHEMISTRY OF). A Hand-Boob on the Production, Purification, and Testing^ of Illuminating Gas, and the Assay of the Bye- Products of Gas Manufacture. For the Use of Students. BY W. J. ATKINSON BUTTERFIELD, M.A., F.I.C., F.C.S.,. Formerly Head Chemist. Gas Works, Beckton, London. E. " The BEST WORK of its kind which we have ever had the pleasure of re- viewing. The new Edition is well deserving a place in every Engineering Library." — Journal of Gas Lighting. " Amongst works not written in German, WE recommend before all others, . Butterfield's Chemistry of Gas Manufacture."— 07ie?7ii;A;er Zeitung. GENERAL CONTENTS. I. Raw Materials for Gas VI. Final Details of Manu- Manufacture. II. Coal Gas. III. Carl)uretted Water Gas. IV, Oil Gas. V. Enriching by Light Oils. facture. VII. Gas Analysis. VIII. Photometry. IX. Applications of Gas. X. Bye-Products. XI. Acetylene. * * This work deals primarily with the ordinary processes of Gas Manufacture. employed in this country, and aims especially at indicating the principles on which thev are based. The more modem, but as yet subsidiary, processes are fally treated also., the Chapters on Gas Analysis and Photometry will enable the consumer to rasp the methods by which the quaUty of the gas he uses is ascertained, and m the !hapter on The Applications of Gas, not only is it discussed as an illuminant, but also as a ready source of heat aiid power. The Incandescent Gas Light is dealt with in an exhaustive manner, and tn© latest theories of its physical basis, as well as the practical developments of lighting by Incandescence, are thoroughly discussed. In Chapter X. an attempt has been made to trace m a readily-mtelhgibie manner the extraction of the principal derivatives from the crude Bye-products. The work deals incidentally with the most modem features of the industry, in- eluding inter alia the commercial production and uses of Acetylene, to which a special Chapter is devoted in the new Edition, and the application of compressed gas - for Street Traction. The needs of the Students in Technical Colleges and Classes have throughout been kept in view. LONDON : CHARLES GRIFFIN & CO.. LIMITED, EXETER STREET, STRAND. GHEMIS TRY AND TECH NO LOO Y. 75 CASTELL-EVANS (Prof. J., F.I.C., F.C.S., Finsbury Technical College) : TABLES AND DATA for the use of ANALYSTS, CHEMICAL MANUFACTURERS, and SCIENTIFIC CHEMISTS. In Large 8vo. Strongly Bound. {Shoi-tly. *** This important Work will comprehend as far as possible all rules and tables required by the Analyst, Brewer, Distiller, Acid- and Alkali-Manufacturer, &c. &c. ; and also the principal data m Thermo-Chemistry, Electro-Chemistry, and the various branches of Chemical Physics which are constantly required by the Student and Worker in Original Research, Every possible care has been taken to ensure perfect accuracy, and to include the results ©f the most recent investigations. ELBORNE (Wm., B.A., F.L.S., F.C.S.) : PRACTICAL PHARMACY. (See p. io6 General Catalogue,) GRIFFIN (John Joseph, F.C.S.) : CHEMICAL RECREATIONS: A Popular Manual of ExperimentaJ Cheniistry. With 540 Engravings of Apparatus. Tenth Edition, Crows 8vo. Cloth. Parts I. and 11. , complete in one volume, 12/6. Separately — Part L, Elementary, 2/; Part II., The Chemistry of the Non- Metallic Elements, 10/6. MUNRO (J. M. H., D.Sc, Professor of Chemistry, Downton College of Agriculture): AGRICULTURAL CHEMISTRY AND ANALYSIS : A Prac- TICAL Hand-Book for the Use of Agricultural Students. {Griffin^ s Technological Manuals.) In Preparation, IMPORTANT NEW WORK. NEARLY READY. DAIRY CHEMISTRY: A PRACTICAL HANDBOOK FOR DAIRY MANAGERS, CHEMISTS, ANALYSTS. By H. droop RICHMOND, F.C.S., CHEMIST TO THE AYLESBURY DAIRY COMPANY. Contents. — Introductory. — The Constituents of Milk. — Analysis of Milk. — Normal Milk, its Adulterations and Alterations and their Detection. — The Chem- ical Control of the Dairy. — Biological and Sanitary Matters. — Butter. — Other Milk Products. — Milk of Mammals other than the Cow. — Tables. — Appendix, &c. LONDON: CHARLES GRIFFIN & CO., LIMITED, EXETER STREET, STRAND. 76 CHARLES GRIFFIN dfe 00.*8 PUBLICATIONS. Painters' Colours, Oils, & Varnishes: A FRACTICAI. MANUAL. By GEORGE H. H.URST, F.C.S., Member of the Society of Chemical Industry ; Lecturer on the Technology of Painters' Colours, Oils, and Varnishes, the Municipal Technical School, Manchester. Second Edition, Revised and Enlarged. With Illustrations. 12s. 6d'. General Contents. — Introductory — The Composition, Manufacture, Assay, and Analysis of Pigments, White, Red, Yellow and Orange, Green, Blue, Brown, and Black — Lakes — Colour and Paint Machinery — Paint Vehicles (Oils, Turpentine, &c., &c.) — Driers — Varnishes. *' This useful book will prove most valuable." — Chemical News. A practical manual in every respect , . . exceedingly instructive. The section on Varnishes is the most reasonable we have met with." — Chemist and Druggist, ** Very valuable information is given." — Plumber and Decorator. " A THOROUGHLY PRACTICAL book, . . . the ONLY English work that satisfactorilv> treats of the manufacture of oils, colours, and pigments." — Chemical Trades' youmal. ** Throughout the work are scattered hints which are invaluable." — Invention. In Crown 8vo, Extra. With Illustrations. 8s. 6d. CALCAREOUS CEMENTS: THEIR NATURE, PREPARATION, AND USES. Hiri^lx. fsom.e Rem£K,s*lx:s upon Cexnen.^ □Testslm^o BY GILBERT R. REDGRAVE, Assoc. Inst. C.E. General Contents. — Introduction — Historical Review of the Cement Industry — The Early Days of Portland Cement — Composition of Portland Cement— Processes of Manufacture — The V^ashmill and the Backs — Flue and Chamber Drying Processes — Calcination of the Cement Mixture — Grinding of the Cement — Composition of Mortar and Concrete — Cement Testing — Chemical Analysis of Portland Cement, Lime, and Raw Materials — Employment of Slags for Cement Making — Scott's Cement, Selenitic Cement, and Cements produced from Sewage Sludge and the Refuse from AlkaU Works — Plaster Cements — Specifications for Portland' Cement — Appendices (Gases Evolved from Cement Works, Effects of Sea- water on Cement, Cost of Cement Manufacture, &c., &c.) " A work calculated to be of great and extended utility."— C^emtcaZ News. " Invaluable to the Student, Architect, -md ^n^ineev.'''— Building News. *' A work of the greatest interest and usefulness, which appears at a very critical period of the Cement Trade."— J?r«Y. Trade Journal. Will be useful to all interested in the manufacture, use, and testing of Oements."— Engineer. LONDON : CHARLES GRIFFIN & CO., LIMITED, EXETER STREET, STRAND. CHEMISTRY AND TECHNOLOGY. 77 Painting and Decorating: A Complete Practical Manual for Rouse Painters and Decorators. Embracing the Use of Materials, Tools, and Appliances; the Practical Processes involved ; and the General Principles of Decoration, Colour, and Ornament. BY WALTER JOHN PEARCE, LKCTimEK AT THE MANCHESTER TECHNICAL SCHOOL FOR HOUSE-PAINTING AND DECOKATIKO. In Crown 8vo. extra. With Numerous Illustrations and Plates (some in Colours), including Original Designs. 12s. 6d. GENERAL CONTENTS. Introduction— Workshop and Stores— Plant and Appliances— Brushes and Tools— Materials : Pigments, Driers, Painters' Oils— Wall Hangings— Paper Hanging— Colour Mixing— Distempering— Plain Painting -Staining— Varnish and Varnishing— Imitative Painting — Graining — Marbling— Gilding— Sign - Writing and Lettering — Decoration : General Principles — Decoration in Dis- temper — Painted Decoration — Relievo Decoration — Colour — Measuring and Estimating— Coach-Painting— Ship-Painting. "A THOROUGHLY USEFUL BOOK . . . givCS GOOD, SOUND, PRACTICAL INFORMATION in a CLEAR and CONCISE FORM. . . . Can be confidently recommended alike to Student and Workman, as well as to those carrying on business as House -Painters and Decorators."— P/wm6er and Decorator. "A THOROUGHLY GOOD AND RELIABLE TEXT-BOOK. . . . So FULL and COMPLETE that it would be difficult to imagine how anything further could be added about the Painter's craft." — Builders' Journal. Mr. Pearce's work is the outcome of many years' practical ex- perience, and will be found invaluable by all interested in the subjects of which it treats. It forms the Companion-Volume to Mr. Geo. Hurst's well-known work on " Painters' Colours " (see p. 76). LONDON: CHARLES GRIFFIN & CO.. LIMITED, EXETER STREET, STRAND. 78 CHARLES GRIFFIN t. Science Generally: i.e., Societies occupy- f 6, Economic Science and Statistics. ing themselves'with sereral Branches of \ 7. Mechanical Science, Engineering, and Science, or with Science and literature | Architecture jointly. 5 Naval and Military Science. I a. Mathematics and Physics. \ 9- Agriculture and Horticulture. I 3. Chemistry and Photography. § 10. Law, I 4, Geolog^'/Geography, and Mineraio^. §11. Literature, i 5. Biology, including Microscopy and An- § 12. Psychology. thropoioffy. 5 13- Archaeology. S 14. Medicine. '''The Year-Book of Societies' fills a very real want.'' — Engmeertng. " Indispensable to any one who may wish to keep himself abreast of the scientific work of the day/' — Edinburgh Medical /otirnal, " The Ykar-Book of Sociktiks is a Record which ought to be of the greatest use for the progress of Science." — Lord PUyfair, F.R.S., /C.C.B., M.F., Pcut-P resident of the British Associttnon. "It goe* aimost without saying that a Handbook of this subject will be in time one of the most generally useful works for the library or the desk." — Tk^ Titrui. "British Societies are now weu represe&taa In the 'Year-Book of the Scientific and Learned Societies of Great Britain and Ireland,"" — (Art. "Societies'' in New Edition 0/ "Encyclopaedia Britannica," vol. xxii. ■ Copies of the First Issue, giving an Account of the History, Organization, and Conditions of Membership of the various Societies, and forming the groundwork of the Series, may still be had, price 7. 6. Also Copies of the following Issues. The YEAR-BOOK OF SOCIETIES forms a complete index to THE SCIENTIFIC WORK of the year in the various Departments, It is used as a ready Handbook in all our great Scientific Centres, Museums, and Libraries throughout the Kingdom, and has become an indispensable book of reference to every one engaged in Scientific Work. LONDON: CHARLES GRIFFIN & CO., LIMITED, EXETER STREET, STRAND. PRACTICAL MEDICAL HANDBOOKS, 85 Fifth Edition, Thoroughly Revised and Enlarged. With Additional Illustrations. Price 6s. PRACTICAL SANITATION: A HAND-BOOK FOR SANITARY INSPECTORS AND OTHERS INTERESTED IN SANITATION. By GEORGE REID, M.D., D.RH., Fellow, Mem. Council, and Examiner, Sanitarv Institute of Great Britain, and Medical Officer to the Staffordshire County Council. imiitb an 'B.vv^x^'^^l on Sanitary !!Law, By HERBERT MANLEY, M.A., M.B., D.P.H., Medical Officer of Health for the County Borough of West Bromwich. General Contents.— Introduction— Water Supply: Drinking Water, Pollution of Water— Ventilation and Warming — Principles of Sewage Removal — Details of Drainage ; Refuse Removal and Disposal— Sanitary and Insanitary Work and Appliances— Details of Plumbers' Work— House Construction — Infection and Disinfection — Food, Inspection of ; Charac- teristics of Good Meat; Meat, Milk, Fish, &c., unfit for Human Food- Appendix : Sanitary Law ; Model Bye-Laws, &c. "Dr. Raid's very useful Manual . . . abounds in practical detail " — British Medical Journal. "A very useful Handbook, with a very useful Appendix. We recommend it not only to Sanitary Inspectors, but to Householders and all interested in Sanitary matters." — Sanitary Record. Third Edition, Revised. Large Crown Svo. Handsom.e Cloth. 4«. A MANUAL OF AMBULANCE. By J. SOOTT RIDDELL, CM., M.B., M.A., Senior ABst -Surgeon, Aberdeen Royal Infirmary; Lecturer and Examiner to the Aberdeen Ambulance Association ; Examiner to the St. Andrew's Ambulance Association, uiasgow, and the St. John Ambulance Association, London. With Numerous //lustrations and Fu// Page P/ates. ^i.^^m®.''^' 9°"*®"*^-~^^^^^^^^ Human Anatomy and Physiology— The Triangular Bandage and its Uses— The Roller Bandage and its Uses —i»ractures— Dislocations and Sprains— Haemorrhage— Wounds— Insensi- bility and Fits— Aaphyxia and Drowning- Suffocation— Poisoning— Bums, * rost-bite, and Sunstroke— Removal of Foreign Bodies from (a) The Eye ; (6) The Ear; (c) The Nose; {d) The Throat; (e) The Tissues-Ambulance Transport and Stretcher Drill-The After-treatment of Ambulance Patients —Organisation and Management of Ambulance Classes -Appendix • Ex- amination Papers on First Aid. v,ar,H ^^"'^Ki^ • W . directions are bhort and clear, and testify to tne hand of an able surgeon."— ^rfm. Med. Journal. toBonjr lu .uo ,».«J^'^^n®^^"^®^^•^°^® ^» good as it could poss^^^^ . . . Oontaini practically every piece of information necessary to render llrst aid. . . Should find ite place in every household library."— /)at/y Chronicle ►^""um uiiu CoUi^y Guardian^ work , that it is difficult to imagine how it could be better."- LONDON: CHARLES GRIFFIN & CO, LIMITED, EXETER STREET, STRAND. GRIFFIN S "OPEN-A IR" SERIES. ''Boys COULD NOT HATE A MORE ALLURING INTRODUCTION tO scientific pursuit» than these charming-looking volumes."— Letter to the Publishers from the Head- master of one of our great Public Schools. OPEfl-fllR STUDIES IJl BOTiHiY: SKETCHES OF BKITISH WILD FLOWERS IN THEIE HOMES. BY R. LLOYD PRAEGER, B.A., M.R.I.A. lUustrated by Drawings from Nature by S. Rosamond Praeger^ and Photographs by R. Welch. Handsome Cloth, 7s. 6d. Gilt, for Presentation, 8s. 6d. General Contents.— A Daisj^- Starred Pasture— Under the Hawthorns^ —By the Kiver— Along the Shingle— A Fragrant Hedgerow— A Connemara Bog— Where the Samphire grows— A Flowery Meadow— Among the Corn, (a Study in Weeds)— In the Home of the Alpines— A City Rubbish-Heap— Glossary. "A FRESH AND STIMULATING book . . . should take a high placc . . . The- Illustrations are drawn with much skill."— The Times. "BEAUTIFULLY ILLUSTRATED. . . . One of the MOST ACCURATE as well aj£^ INTERESTING books of the kind we have seen."— Athe7iceum. "Redolent with the scent of woodland and meadow."— TAe Standard. "A Series of stimulating and delightful Chapters on Field-Botany. "-^TAe Scotsman. "A work as FRESH in many ways as the flowers themselves of which it treats. The- RICH STORE of information which the book contains . . ."—The Garden. OPEli-fllH STDDIES Ifl GEOLOGY: An Introduction to Geology Out-of-doors. BY GRENYILLE A. J. COLE, E.G.S., M.R.LA., Professor of Geology in the Royal College of Science for Ireland. With 12 FuIf-PagQ Illustrations from Ptiotographs. Cloth. 8s. 6d. General Contents.— The Materials of the Earth— A Mountain Hollow —Down the Valley— Along the Shore— Across the Plains— Dead VolcanoeE —A Granite Highland— The Annals of the Earth— The Surrey Hills— The Folds of the Mountains. ^ "The FASciNiLTiNG ' OpEN-AiR STUDIES' of Peof. Cole givB the subject a glow or ANIMATION . . . cannot fail to arouse keen interest in geology r— Geological Afagazine. Eminently readable . . . every small detail in a scene touched with a sym- pathetic kmdly pen that reminds one of the lingering brush of a Constable."-iV^a^«f«. 'Ihe work of Prof. Cole combines elegance of style with scientific thoroughness."- Petermann s Mittheilungen. ''The book is worthy of its title: from cover to cover it is strong with bracing freshness or the mountain and the field, while its accuracy and thoroughness show that it is the^ work of an earnest and conscientious student. . . . Full of picturesque touches whicb- are most welcome."— iVa^wraZ Science. " A CHARMING BOOK, beautifully illustrated."'— ^M67?«wm. LONDON : CHARLES GRIFFIN & CO., LIMITED- EXETER STREET, STRANDl