mm n 44 j*£t ; \-„ J- PRACTICAL CHEMISTRY. ^ gi^y= PRACTICAL CHEMISTRY: INCLUDING THE TIIEOHY AND PBACTICE OF ELECTRO-DEPOSMON ; PHOTOGRAPHIC APT ; THE CHEMISTRY OF FOOD, a Chnutrr nu its Jjlhltrrnfinns : I THE CHEMISTRY OF ARTIFICIAL ILLUMINATION. BY GEOB&E GORE, ESQ., , MARCUS SPIELING, ESQ., BIRMINGHAM JOHN SCOFFERN, M.R., LATE rROf E880R OF CIIEM19TRY kT THE ALT>EIISG ATE SCHOOL Of MEDICINE, AUTHOR OF ELEMENTARY CHEMISTRY, ETC., ETC. LONDON : HOULSTON AND STONEMAN, 65, PATERNOSTER ROW; U Wm. S. ORR AND CO., AMEN CORNER. qj $f^& MDCCCLVI. PREFACE. The present Volume of the Circle of the Sciences embraces, to a very limited extent only, the great subject of Practical Chemistry ; but it includes those subjects which are invested with a popular character, and on which the public mind may be considei-ed more particularly open to information. Most of the other branches of the subject j become either manufacturing or professional ; and with such subjects it is not intended | to interfere in this scries, which may be considered, as elementary in its character, re- serving such subjects for treatment in another series with which it is intended to follow | the present one. The chief value of the present Volume arises from its strictly practical treatment of its subjects. In the important branch of Electro-deposition, Mr. Gore has given his formuloc in a plain and practical manner, the information being drawn from his own practice, and from the practice of the establishments most advanced perhaps in the world in this department of science. Mr. Sparling has treated that most fascinating of all pursuits, Photography, in the j same spirit, giving in detail his own practice ; but, from the nature of the subject, and j from the number of accomplished and scientific men who have written on the siibject | in a desultory form, and from the circumstance of some of these having experimented with him and mixed up his own investigations with theirs, more of the general litera- ture of tho subject has been included in his Treatise than in the former. Of Professor Molcschott's Treatise " Lehre der Nahrunga Mittcl," it is only necessary to say that it has gone through four large editions in Germany, and has been translated into two foreign languages, in each case with great success. The Conductor trusts the present translation, with Dr. Scoffern's Chapter on Adulterations, will be found useful in directing the public mind, as well as legislature, to an adjustment of that question. The translator regrets that, owing to his absence on the Continent while the work was passing through the press, the errata are unduly numerous. A list of the principal is given, and the student is requested to correct them in the margin. X PREFACE. In respect to the Treatise on Artificial Illumination, the Author prefers for the present to remain unknown ; it must, therefore, rest on its own merits. The Conductor can only add his own testimony to the internal evidence it offers, that there is no writer of the day more competent from position and experience to write on the subject. How far, however, the objects have been fulfilled it remains for the reader to decide. Actuated by the conviction that no chemical author, of whatever talent, fame, or ex- perience, could treat of subjects so diverse as the numerous branches of technological chemistry, otherwise than as a compiler and a theorist in some, we have confided each separate branch to the responsibility of some one author who had previously devoted to it his special attention, and was favourably known in relation to it : hoping by this means to produce a series of Treatises full of practically useful information. Amen Corner, Paternoster Row, September, 1856. CONTENTS. ELECTRO-METALLURGY. The Theory of Electro-Deposition . PAGE 7 Alternation of the Electro-Circuit . PAGE 31 Arrangement of the Subject . . . 8 Effects of Electro- Deposition of Method of Studying the Subject . 9 Liquids 32 Theoretical Division of the Subject 10 Electrical Terminology .... 33 Depositing Arrangements 11 Electro-Chemical Scale .... 33 Deposition by One Metal and One- Influence of Temperature and Light 12 34 Deposition by Two Metals and One Dynamic or Mechanical Conditions 35 13 Theory of Electrodes 36 Deposition by One Metal and Two Mathematical Conditions of Depo- Liquids . 15 37 Deposition by Two Metals and Two Definite Chemical Action . . . 38 17 Binary Theory of Electrolysis . . 39 Deposition by Liquid, Magnet, and The Practice of Electro-Depo- Coil 18 40 Compound Deposit Arrangements . 19 General Arrangement of Apparatus 41 Conditions of Deposition .... 20 The Magneto-Electric Machine . . 42 Chemical Conditions of Deposition . 21 44 Degrees of Chemical Affinity of 45 21 46 Difference of Chemical Affinity ne- Arrangement of Dissolving Plates . 47 cessary to Deposition .... 22 Scouring and Cleaning Apparatus . 48 Various modes of Deposition Force 23 Liquids for Adhesive Deposits . . 49 Acid and Basic Affinities necessary . 24 50 Fluidity essential to Electro- Depo- Depositing Solutions and Liquids . 51 25 Tests for Depositing Liquids . . 52 Altcrnation and Circulation of Che- Effects of Different Forces of Depo- mical Affinities necessary . . . 25 sition 53 Circulation of Chemical Affinities . 26 54 27 Bismuth and Zinc Solutions . . . 56 Free Acids in Deposition .... 28 Cadmium and Tin Solutions . . . 57 Electrical Conditions of Deposition 29 58 Electrical Polarity of the Metals . 30 Lead and Iron Solutions .... 59 xii CONTENTS. Cobalt and Nickel Solutions . . . PAGE 60 83 61 General Rules for Working Solu- Patented Processes for Brassing . . 62 84 Mercury and Silver Solutions . . 63 Applications of Copper Depositions 85 Silvering by Immersion .... 65 Copy Woodcuts bv Deposition . 86 Silver Plating Solutions .... 66 Deposition Substituted for Stereo- Silver Solutions by Battery Process 67 87 Electro-Plating Liquids .... 68 Copying Daguerreotypes by Depo- Bright Silvering Solutions . . . 69 88 70 Coppering Cloth by Deposition . 89 Gold Depositing Solutions . . . 71 Management of Silver Solutions 90 72 Management of Plating Liquids 91 Gold Deposition by Battery Process 73 75 Management of Gilding Solutions . 93 Preparing Metal for Keceiving a Extracting Gold and Silver from 76 Exhausted Solutions .... 94 77 Methods of Recovering Silver . . 95 Moulding Manipulation .... 78 Methods of Recovering Gold . . . 96 Rendering Moulds Conductible . . 79 List of Electro-Deposition Patents . 98 Regulating Battery Power . . . 80 List of Books upon Electro-Depo- Intensity of the Electric Current . 81 99 Regulating the Quantity of Metal . 82 100 PHOTOGRAPHIC ART. Introductory Remarks .... 101 126 "Wedgwood's Discovery .... 102 127 Berard and Daguerre's Discoveries . 103 Dispersive Powers of Prisms . . 128 Fox Talbot's Discovery .... 104 Combinations of Lenses .... 129 The Chemistry of Photography 105 130 Photographic Chemicals .... 106 The Photographic Apparatus . 131 110 131 Recovering Silver in Solutions . . 113 Construction for different purposes . 132 The Optics of Photography . . 115 133 116 Major Halkett's Portable Camera . 134 117 Detached parts of the Camera . . 135 Actinic Chemistry 118 137 Refractive Power of Different Sub- The Calotype Process .... 138 119 M. Biot's Report on Calotype . . 139 Refractive Power of Different Li- The Talbotype Process .... 141 120 143 Various shaped Lenses .... 121 Improved Process of Calotype . . 144 Construction of Lenses .... 122 Sir W. J. Newton's Process . . . 145 123 Mr. Llewellyn's Process .... 147 Qualities of a Convex Lens . . . 124 M. Gustave Le Gray's Process . . 149 Magnifying Power Explained . . 125 Dr. Diamond's Process . . . . 151 CONTENTS. xiii PAOE 155 Mr. Hardwich's Formula) 211 161 Defects and their Remedies . . . 217 The Wax-paper Process . . . 161 Nitrate Bath and Apparatus . . 220 162 Spiller and Crookes' Process . . 221 Advantages of the Wax-paper Pro- Nitrate of Magnesia 222 163 Mr. Shadbolt's Honey Process . . 223 MM. Le Gray and Fabre's Pro- Capt. Caron's Dry Collodion Process 224 164 Mr. Mansell's Improved Process . 225 Mr. Teasdale's Process .... 166 Mr. Mayall's Dry Collodion Process 226 Tabular View of Wax-paper Pro- Tests of the Strength of Acids . . 229 170 Dissolving the Collodion .... 230 M. Geoffroy's Process 171 Washing and Drying the Pyroxy- Lcspiault's Process 172 231 Manipulatory Summary . . . . 174 On Purifying Ether 233 Causes of Failure, and Remedies . 175 234 Photography of Glass .... 177 Collodio- Albumen Process . . . 235 177 Coating the Collodion with Albumen 236 Preparing the Glass Plate . . . 179 Exciting the Albumen 237 Developing the Image 181 Exposure in the Camera . . . . 237 Mr. Ncgretti's Albumen Process 182 Focussing in the Albumen Process . 238 Application of the Albumen . . 183 Fixing and Varnishing . . . . 239 Developing by Mr. Negretti's Pro- 240 184 Gutta-percha as a Sensitized Me- 185 241 186 Mr. Archer's Patented Process . . 242 Preparing the Collodion . . . . 187 Mr. Archer's Field Camera . . . 243 Photographic Properties of Collodion 188 Mr. Clarke's Siphon 244 Experiments on Collodion . . . 189 Fixing and Colouring of Positives . 244 Chemical qualities of Collodion . . 190 245 Effects of Iodizing Collodion . . 191 Preparation of Plain Salted Papers . 246 Fogging, and its Remedies . . . 192 Serum of Milk Solution . . . . 247 Mr. Ash Iladow's Experiments . . 193 Remarks on the Various Formula) . 24S Mr. Ilardwich's Researches . . . 195 249 Mr. Ilardwich's Nitrate Path Ex- Time of Exposure to the Light . 250 198 Mr. Ilardwich's Developing Expc- Washing and Drying .... 252 199 252 201 Mr. Sutton's Toning Bath . . 253 Mr. Home's Exciting Process . . 202 Positive Printing by the Negative Mr. Home's Nitrate Bath Process . 203 254 Mr. Home's Developing Process . 204 SirW. Newton's Negative Process 255 Mr. Home's Fixing Process . . . 205 Fading of Positive Proofs . . 256 Positive Pictures by Mr. Home's Results of Imperfect Washing . 257 206 Enlarging and Reducing Positive The Author's Process 207 208 Enlarging and Reducing Photo- The Author's Manipulation . . . 209 graphs 259 xiv CONTEXTS. T'ttT? ~V) A flTTT , "R"R"FnTVPT! PXGE 259 Stereoscopic Photography . . PAGE 278 "Prorinrino* flip "PI at OS 260 279 C*f\<*4-Tr\(r +110 ~PlfltpQ*hv rjlpotTA-rlpT^n- V_> U ft L HI LHU J. iaLLo U j UlC^HU UCjJU 280 261 228 The Polishing Table 262 The Stereoscopic Camera .... 284 Polishin 0, Process 263 Production of Stereoscopic Pictures 287 Polisliin 0- Powders 264 Pictures of Microscopic Objects 288 M Claudet's Process 265 Imperfectly Developed Processes 289 The Polishin"- Lathe 266 Photographs in Natural Colours 290 Polishin 0, Vice 267 Application of Photography to As- TTftnfl TnHsTiino* 268 291 Applying the Bromine and Iodine . 269 Photographs from the Electric-light 292 Accelerating Materials .... 270 Photographs from the Oxyhydrogen 271 293 Mercurializing the Plate .... 272 Form of Oxyhydrogen Microscope . 295 The Colouring Process .... 273 Photographs on "Wood 296 274 Colouring Photographs .... 297 Mr. Bingham's Process . . . . 276 Glycerined Collodion 299 Chlorine in Combination with Bro- New Printing Processes .... 300 277 Preparations for Sensitizing Paper . 301 THE CHEMISTRY OF FOOD. Book the First. On the Formation of Blood . . . 305 The Alimentary Principles . . .306 Combinations of Inorganic Elements 307 Organic Elementary Substances , 308 Compound Elementary Substances . 309 Digestive Processes 310 The Chyle 311 Production of Blood 312 Colouring Matter of the Blood . .313 Solid Substances of the Body . . 314 Albuminous and Horny Substances 316 On Secretion 317 Fatty Acids of the Body . . . .318 Dissolving Juices 319 On Excretion 320 Transformations of Matter . . .321 Excreting Glands 322 Constituents of the Secretions . . 324 External Excreting Organs . . . 325 On Hunger and Thirst .... 326 Effects of Abstinence 327 Effects of Hunger 329 Book the Second. On Food 330 Alimentary and Digestive Principles 331 Principles of Nutriment .... 332 Tests of Digestibility 333 On Solid Food 334 Constituents of Beef 335 Effects of Cooking 336 Difference of Taste 337 Comparative Qualities of Flesh . . 338 Fish, Flesh, and Fowl Compared . 339 Nutritive Qualities of different Animals 340 Nutrition Increased by Animal Diet 341 Composition of the Cereals . . . 342 Constituents of Bread 343 Indigestibility of Cakes .... 344 Peas, Beans, and Lentils . . . 345 Vegetables 346 Potatoes and Edible Boots . . . 348 Fruit 350 Degrees of Acidity 351 CONTENTS. PAGE P VO R 352 Breakfast, Dinner, and Supper . . 373 352 Requisite Combinations of Food 374 Milk 354 Solid and Liquid Food .... 375 Coffee, Tea, and Chocolate . . . 355 Diet of Childhood 376 Wine, Beer, and Spirits .... 358 Milk as Food for Children . . . 378 Milk of different Animals Compared 379 Ox Condiments 363 Diet of Youth, Maturity, and Age . 380 364 Requirements of Youth and Age 382 366 Diet of Woman 384 367 The Nursing Mother 386 368 387 369 Diet of the Artist and Literary Man 388 Book the Third. Diet in Winter and Summer . . 390 393 371 394 ON FOOD ADULTERATIONS. 396 Adulterations of Coffee . . . . 416 Constituents of Bread 400 Chicory 418 Tests of Adulteration in Bread . . 402 419 Basis for Legislation on Bread . . 405 Tea and its Adulterations . . . . 421 Sugar and its Adulterations . . . 407 Cocoa and Chocolate 423 411 424 411 Adulteration of Spirits . . . . 425 Water and its Impurities . . . . 426 Adulterations of Vinegar . . . . 414 428 CHEMISTRY OF VRTIFICIAL ILLUMINATION. 429 Transparency, Transcalcncj', and 431 446 History of Street Illumination . . 432 Disperson of Light and Heat . . 447 433 * 448 On Combustion and Flame . . 434 Their Manufacture 449 Theories of Combustion . . . 435 Tallow . 450 Nature and Cause of Flame . . 436 Palm and Cocoa-nut Oil . . . . 452 Colour and Heat of Flame . . 439 Chevreul's Discoveries . . . . 454 Relative Value of Combustibles . 440 456 Effects of Cold on Flame . . . 441 458 442 459 On the Laws of Light and Ra Condensed Coal-gas Candles . . . 461 diant Heat . 443 Lamp-oils and Spirits . . . 462 Instruments for Measuring Light . 444 464 Reflection and Reflecting Instru 465 445 Causes of Spontaneous Combustion 466 xvi CONTENTS. TAGB PAOE Animal Oils . 471 517 521 Peat and Coal-tar Gases .... 522 Coal-Naphtha . 480 Apparatus Required tor Con- sumption of Gas 523 Lamps of Antiquity .... . 483 524 The Argand Lamp . 487 527 . 489 Pressure of Gas 532 Fountain and Car eel Lamp . . . 491 Glover's Governor 534 Camphine and Naphtha Lamps . . 493 Self-regulating Burner . . . . 535 Gas or Vapour Lamps .... . 494 Management op Gas and Ven- History of Gas Lighting . . . . 497 537 Action of Heat on Organic Matter . 499 Gas Ventilation 538 539 .Relative Value of Coals . . . . 503 Innocuous Illuminating Agents 541 . 504 The Oxyhydrogen Light . . . . 542 V alue oi the lieiuse Matter . . . 505 The Electric Light 543 lests ot Impurities m Coal-gas . . 506 Mode of Obtaining the Light . . 544 Commercial V alue oi Coal-gas . . 509 Apparatus for Sustaining the Light 545 Specific Gravity of Gas . . . . 512 The Charcoal Electrodes . . . . 548 Relative Value of Gas .... . 513 Intensity of the Electric Light . . 550 Oil Gas ..." 514 Cost of Producing the Electric Light 551 Portable and Resin Gases . . . . 516 The Steel Mill of the Miner . . . 552 Index, Explanatory and Referential, 553. 1 I PEACTICAL CHEMISTRY. ♦ When the theorist has explained the doctrines of a science and enunciated its laws, the task proposed to himself is usually considered at an end ; for rarely does it happen that the discovery of scientific truths, and their application to the wants of mankind, is the lot of one and the same individual. Considered as an intellectual exercise of the mind, and apart from its usefulness, there has always existed a preference in favour of non-applied or abstract science. The origin of this prejudice it is not difficult to traco. It took its rise at a period of the world's history when the industrial arts were rude and undeveloped, held to he unworthy the attention of free men, and practised only by slaves ; nor, apart from social prejudices, was there much inducement for a philosopher of Greece or Rome to cultivate the application of science to the common affairs of life. Such application pre-supposes the existence of numerous material aids — the growth of centuries, the happy issue of countless tentative approaches and empirical trials. Aids of this kind the ancient philosopher had not at his com- mand, and, not having them, he little dreamed of the magnificent truths which they were destined to make known in a future age. The far-penetrating acumen of a Plato or a Socrates, in tracing to their first sources the sentiments of the human mind, started from equal vantage ground with the phi- losopher of our own days ; and the ancient student of pure mathematics had, perhaps, an advantage over one of our own age to this extent, that being absolved from the trammels of the device of place in numbers, and the extended symbolisation consequent on its adoption, he was driven more to cultivate the ideas of mathematical abstraction. On the other hand, if there had chanced to live in the time of Plato or Pythagoras a philo- sopher, who, disregarding the conventional stigma associated with science in its appli- cations to the affairs of life, should have attempted to surmount the prejudice by personal PRACTICAL CHEMISTRY. — No. I. 2 ANCIENT PREFERENCE FOR ABSTRACT SCIENCE. example, how unsatisfactory and dispiriting would have been the issue ; checked and thwarted as the philosopher would have been at every step for lack of co-operation of material aids ! It might have been that Archimedes could prove, in a manner satisfactory to his own mind, that, given a material having a certain degree of tenacity and hardness equivalent in these respects to wrought iron, a tubular bridge might be constructed like the one which spans the Menai Straits ; but so long as the material aids for reducing this theory to practice were wanting, the theory would present itself to the philosopher's mind so much in the guise of an unfulfilled dream, that it would probably create a distaste for the application of science in general, and he would hasten back with redoubled delight to the regions of philosophic abstraction. It is not difficult, therefore, to refer the partiality in favour of abstract science amongst the ancients to its cause ; and even at this time, enormous though the accumula- tion of material aids has become, the position must be conceded that the intellect of man enjoys a far wider range into the realms of abstract number and quantity, than it has necessary means of applying to practical purposes ; hence we can at the same time explain and justify the higher status, intellectually regarded, given to pure mathe- matics. The distinction between the terms abstract and applied, probably evident enough as concerns mathematics, has been extended rather as the result of popular usage, than in deference to a well-recognized truth, to chemistry, and other experi- mental sciences. No strictly chemical deduction can be arrived at by an effort of the unaided reasoning faculties. Material aids must be sought in the form of apparatus ; they must be applied in the way of experiment ; and experiment is but another name for practice. To recognize, then, a distinction between abstract and applied chemistry, in the sense of the distinction between abstract and applied mathematics, would be absurd. A spiritual being might range through the whole extent of mathematics, though material things were annihilated, or had never been ; but chemistry without matter is an impossibility, and without experiment it is, at the most, no better than a philosophic dream. Chemistry applied on a large scale for the advancement of any of the civilised arts — or, in other words, that which is usually understood as technological or applied chemistry — has other claims on the philosopher's regard than those of mere utilitarianism. Many of the most important laws of the science had never been adequately proved and illustrated, before the branch of chemistry to which they refer had been practised in the workshop or manufactory ; and not a few unsound theories would have been received on authority as truths, had it not been for the searching scrutiny of the technological appeal. Some of the most remarkable properties of light, or at least its associated agencies, have been made known to us by means of photographic operations ; some recondite facts of electrical induction have been disclosed in the course of experi- ments made on insulated subaqueous telegraphic wires ; and some remarkable calorific phenomena have received their most forcible illustration through the agency of the steam-engine. Technological or applied chemistry having these powerful claims to the appreciation of mankind, the fact may seem strange that even the bpat systematic treatises on that subject have been received with less favom- than the treatises on non-applied or theoretical chemistry. Though strange, the cause does not seem inexplicable ; on the contrary, we conceive it to admit of ready and satisfactory solution. It appears to have depended on a fallacious principle which the editor will endeavour to steer clear of in the preparation of tbe present volume. One individual can scarcely do justic3 to any one branch of technological chemistry indiscriminately, much less can he SKETCH OF SUBJECTS TO BE DISCUSSED. 3 do justice to all. The indispensable qualification for writing a treatise on each branch of applied chemistry, appears to us to be a practical acquaintance with the particular manipulation it requires. No mere general acquaintance with chemistry in the labo- ratory of research, no name, or fame, or authority, or acquaintance with books, would, in our opinion, compensate for the want of this essential, much less would we be influenced in such a matter by eloquence of expression or grace of style. A practical subject, to be usefully dealt with, must be treated of by a practical man. This is the sentiment on which the monographs contained in this volume are based ; each subject is treated of by an author practically conversant with the subject which bears his name ; and such is the peculiarity of the present volume. It remains for the chemical public to testify its appreciation of the result. Six leading topics of applied chemistry will be discussed in the following pages. They are Photography ; Electro- deposition; the Production of Artificial Light; the Chemistry of Food and Drink ; the Chemistry of Explosive Compounds, more particu- larly in their relation to their warlike uses ; and the Chemistry of the Fatty Acids, involving the two most important processes of the manufacture of soap and of candles. Contemplating the beautiful productions of heliographic art as now prosecuted, it is interosting to compare them with the rude attempts of the philosophers who, about half- a- century ago, first tried to give permanence to the discolouration effected on certain metallic solutions by the sunbeam. Every tyro in chemical experiment has noticed the darkening effected by the sun's rays on the white chloride of silver, and on tissues imbued with solution of the nitrate of that metal. In 1803, "Wedgwood, the porcelain manufacturer of Etruria in Staffordshire, employed the chloride and the nitrate of silver, spread on paper and white leather, for copying the designs of painted glass windows. He could not succeed, however, in giving permanence to the impressions produced, and hence the beautiful art of photography remained in abeyance. Argenti- ferous salts, though they first suggested the possibility of heliographic painting, and had even been rudely applied by Wedgwood and others, were not associated with the first artistic triumphs of sun-painting. During a long series of years, between 1814 and 1827, M. Niepce, of Chalons-sur-Saone, had engaged himself in the study of certain curious results consequent on the exposure of metallic surfaces to light. He ultimately made the discovery that resin, spread upon metallic or glass tablets, was sensitive to solar agency, the parts exposed becoming more soluble than those in shadow. He was thus enabled to obtain pictures upon silver plates by means of the camera obscura. Further than this the heliographic art had not progressed until 1829, when Niepce associated himself with M. Daguerre of Paris, who had also been carrying on similar investi- gations of his own, having a similar result for their object. Eventually the combined experiments of MM. Niepce and Daguerre led to the discovery of the process of hdlio- graphy upon metallic surfaces, to which the term Daguerreotype has since been applied. The year 1839 was doubly important in relation to heliography. In the course of it Mr. Fox Talbot made known the result of his investigations, commenced in 1834, for the discovery of some efficient method of giving effect to the discoloura- tion of metallic solutions, which had already been practically applied to a certain extent by Mr. Wedgwood. Numerous investigations followed. Sir John Herschcl and Mr. Robert Hunt were amongst the first successful cultivators of this beautiful art, which has tended, perhaps, more than any other to make the study of chemical operations generally popular. 4 HISTORY OF ELECTRO-DEPOSITION. The progress of heliographic art furnishes a striking example of the advancement of a science through one of its technological applications. Most of the improvements in calotype and daguerreotype processes are directly referable to chance operations taking place in the practice of these arts. These chance improvements suggested principles, and began to be the means in their turn of illustrating some of the most remarkable laws of radiant matter. The practice of heliography has enabled us to trace the influence of solar rays upon plants, to register variations of terrestrial magnetism, the oscillations of the barometer and thermometer, making the sunbeam to indicate the aberrations which are due to its own influences. It has even been made to relieve the watching astronomer at his nightly vigil, apparatus being now constructed to mark the phenomena of the heavens as they pass the night-glass to which it is attached. The sister art of heliography is that of electrotyping, more correctly designated, perhaps, by the term which the author of our treatise has chosen of electro-deposition. Unlike heliography, it does not enable us to copy shadows, but by its aid we can copy every solid form and line. None, we presume, will be inclined to underrate the importance of this branch of applied science, or to doubt the propriety of including it in the list of our first technological monographs. As is the case with many other branches of applied science, the credit of the original discovery of this is keenly disputed ; and we do not think it necessary to enter upon the particulars of the dispute on this occasion. It will suffice to indicate the phases which the invention might have assumed under given conditions of voltaic science and experiment. The great impediment to the continuous working of all voltaic batteries anterior to those of Daniell and Mullens, was the deposition of zinc on the copper plates. This deposition, in batteries as ordinarily constructed, it was impossible to prevent, and no sooner did it take place than the voltaic action ceased. The proposition was then made to deposit copper upon copper. The problem was solved, and worked success- fully in the arrangements of Daniell and of Mullens ; and this solution must be considered tht germ of the art of electro-deposition in all its numerous details and ramifications. If it was possible to deposit a layer of copper on a flat surface of that metal, it was obviously possible to effect a similar deposition upon an irregular surface ; and the irregularity in question might obviously vary from the merest scratch or indenta- tion impressed at random, to the most elaborate productions of the etching point or graver. Then followed another step, and it possesses a double interest, an interest both theoretical and practical ; not only does it illustrate the beautiful deduction of Faraday, that every portion of a voltaic arrangement, conductors and all, are active portions of that arrangement, but it advances the art of electro-deposition beyond the narrow limits to which it would have been restricted, if copper of neces- sity must have remained the metal to be deposited, thrown down upon a surface of the same metal. The next obvious improvement consisted in transmitting the voltaic energy from a battery externally to the surface to be operated upon. This improve- ment once effected, almost any conducting surface could be rendered amenable to the will of the electro-depositor — it was henceforth easy to distribute the precious metals — gold, silver, and platinum — over surfaces thickly or thinly, at tic will of the operators, to whom the laws of definite proportionalism, and of electro-chemical equivalents, furnished an unerring guidance. Not only has the practice of electro-deposition ministered to the elegant and the useful arts to an extent which those persons who have not closely watched its progress would be totally unprepared to believe, but it has ameliorated in a hygienic sense the condition of artisans employed in at least one | CHEMISTRY OF ILLUMINATION AND OF EXPLOSIVE BODIES. 5 branch of the arts. The operation of electro-gilding may now be pronounced altogether innocuous to health, whereas the process of amalgam-gilding is most injurious. Already has electro-gilding been applied to purposes formerly accomplished by the amalgam process, and to a corresponding extent has human health been ameliorated ; and it is not, perhaps, unreasonable to believe that the injurious operation will here- after be totally discontinued. The branch of applied chemistry involved in the development of light for illumina- ting purposes, is one which so intimately concerns the public that no words are neces- sary to enforce its importance. By a necessary condition of human existence, we can only lighten our darkness by processes involving the generation of poisonous and in- jurious results. Such is a broad statement of the case, and it is applicable to every practical condition of artificial illumination. Whether we select for illustration the smallest night light, or the most brilliant flame of coal gas, the results of combustion are poisonous ; and for the most part, especially in the case of coal gas, not only is there a poisonous emanation, but one, water, which, distributed through our rooms in vapour, is also injurious. Yet how comparatively small is the number of those to whom these facts are known ! How frequently do we see the most obvious indications of science violated in the employment of sources of artificial light ! The author who will treat of this branch of applied chemistry is one who, from his professional con- nection and the close study which he has given to the subject of artificial illumination, brings to the task all the qualifications necessary to its satisfactory fulfillment. The next branch of applied chemistry to be treated of in this volume, is one the importance of which is attested by the instinct of human self-preservation. It is one long recognized and fostered by the governments of most civilized countries except our own ; but it is one that, strangely enough, has only been appreciated in these isles as it deserves, subsequently to the analytical sanitary commission instituted by the con- ductors of the Lcrncet. In our article on that important subject, care will be taken to impose limits to the words "adulteration," " contamination,' and "impurity;" all three now so indiscriminately applied in the double sense of a mere lowering of strength or quality of a given substance, and of the incorporation therewith of a material actively injurious. Passing from the means of preserving health and promoting human longevity, our article next in succession will present indeed a wide divergency; it is one, nevertheless, which the political condition of the times in which we live invest with an interest scarcely less absorbing. The chemistry of explosive bodies and their application to modern warfare, is a subject which at the present epoch must possess an interest for all ; and it must be of prime utility to that large elass of inventors who are at this time endeavouring to supply the government with improvements on existing means of human destruction. The application of explosive bodies to the purposes of warfare is one to which our nation has contributed with no sparing hand. Although the first discovery of gunpowder is lost in the mazes of antiquity, and the first application of it to projectile uses is a question scarcely less in dispute ; yet the claim of England to the greatest modern discoveries in connection with the use of gunpowder (understanding by that name every possible combination of nitre, charcoal, and sulphur) is clear and unquestioned. To Shrapnell we owe the invention of the destructive shell which bears his name, otherwise called spherical case shot. To Congreve is due the merit of imparting to the rocket length of flight and depth of penetration ; removing it from the category of a weapon of mere oriental annoyance, to that of a missile vieing 6 CHEMISTRY OF THE FATTY ACIDS. with, shot and shell in their most terrible effects, whilst superadding its own specific peculiarities. To Hale is attributable the merit of inventing the rotatory, or rifled rocket, thus abolishing the stick or tail which is a necessary source of so much incon- venience in the rocket of Congreve, and of adopting the force of hydraulic pressure for filling the iron cases, instead of the old and dangerous process of ramming. To Lancaster we owe the solution of the problem of rifling a piece of ordnance so that it may be adapted to the projection of an iron shell. Then we have the incipient application of a liquid spontaneously combustible, more terrible than the Greek fire of the middle ages, by Captain Disney. Passing from improvements adopted or contem- plated in ordnance and their projectiles, what vast strides have been made during the last ten years in the development of the rifle principle of revolving and breech-loading fire-arms. Not descending to particulars in this place, it will suffice to insist on the one great principle common to the practice of all modern rifled arms, the substitution of conoidal for spherical projectiles. This substitution, now that it has been adopted, seems so obvious — one so palpably suggested by the conditions involved in the con- struction of rifle guns — that our wonder is that it was not adopted at some anterior period- In short, the chemistry of explosive bodies, and their application to warfare, illus- trates more forcibly, perhaps, than any other to which reference could be made, the strides which a branch of applied science may make when stimulated by the laws of demand and supply. Whilst England was at peace, this branch of chemistry was cold and torpid ; now that England is at war, it has become endowed with a sudden vitality — a vitality so active and impetuous that it is suggestive of the theme which called it forth. The subject wherewith this volume will conclude is one which, perhaps, furnishes the most brilliant illustration of results not fortuitously achieved, but worked out as the consequence of a problem specially proposed. Before the masterly researches of M. Chevreul on the fatty acids, the chemical nature of these important bodies was totally misunderstood. Beyond the somewhat arbitrary division of oils and fats into fixed and volatile, no general principle of classification had been deduced. It remained for M. Chevreul to prove that all fixed oils were made up of various salts, veritable com- binations of acids with a base, to which latter the term 11 glycerine" from its quality of sweetness, was applied. Of these fatty acids, some were discovered to bo solid at ordinary temperatures ; others liquid, so that by separating them from each other, or in some instances effecting the separation of the associated salts having glycerine for a base, fatty bodies might be obtained of various densities and fusing points, adapted to all the various wants of arts and manufactures, from the solid substitute of our wax and spermaceti to the most limpid lubricating oils. In the chemistry of the fatty acids these masterly developments will be fully detailed, and the various stages of the soap and candle manufacture will be elucidated. Thus, in a few preliminary pages, have we endeavoured to lay before the reader a sketch of the contents of the present volume. To what extent our objects will have been fulfilled it remains for our readers to decide. We shall at least have brought to the task a definite scheme of action and rule of guidance. Actuated by the conviction that no chemical author, of whatever talent, fame, or experience, could treat of subjects so diverse as the numerous branches of technological chemistry, otherwise than as a compiler and a theorist in some, we have confided each separate branch to the respon- sibility of some one author who had previously devoted to it his special attention, and was favourably known in relation to it. ELECTRO-UEPOSITING KOOM. ELECTRO-META LLTJRGY. THE THEORY OF ELECTRO-DEPOSITION. Introductory Remarks.— As the ultimate object of this treatise is to enable the reader to work in an electroplate manufactory in a commercially successful manner ; we shall endeavour to include within its pages, as far as the limited space will allow, every portion of the subject calculated to assist him in obtaining that result, excluding from it every other portion which does not contribute towards that object. With this view we shall include the principles or theory of electro-deposition, because every workman in an electroplate manufactory is certain to meet with difficulties, which no amount of practical knowledge or experience will enable him to overcome without a perfect knowledge of the theoretical principles. These difficulties may be new ones, such probably as he has never seen before, and no doubt, in some cases, such as no one else has ever seen ; a knoM'ledge of the theory will here enable him to apply its principles to the difficulties, and suggest remedies, some of which are almost sure to be successful. We shall also include the practice of the subject ; because, after all, its success depends on careful manipulation; for with ever so perfect a knowledge of prin- ciples, without a perfect knowledge of the application of those principles in the form of practical rules and practical manipulation, success cannot possibly be attained. With the same end in view, we shall avoid saying anything about the history of the 8 ARRANGEMENT OF THE SUBJECT. subject, or the claims of rival discoverers or inventors, these being subjects for the historian. Neither shall we say much about the electro-deposition of rare metals, or about any collateral branches of the subject, excepting only so far as they are capable of illustrating the subject in a direct manner, or of otherwise furthering the object in view. To enable the reader to master each portion of the subject as he proceeds, we have so arranged it that every portion shall be, so far as it goes, 8 ' complete in itself, requiring no anticipatory knowledge of more advanced parts to enable him to under- stand it. The only arrangement of the subject which admits of this important object being attained, is to treat of the theory before treating of the practice, and by arranging tbe theory in an inductive order. In other words, we shall commence with the various classes of facts on which electro-deposition is founded, and ascending from these to the general laws or principles, chemical or electrical, which govern them. Proceeding from the theory to the practice, arranging all in a deductive order, and applying theo- retical principles in the form of practical rules, the results cannot be other than successful. Beginning with the more general rules which apply to all electro-depo- sition processes, and to the electro- deposition of all metals, and proceeding, step by step, to those more special rules of manipulation which are required for the working of particular metals and solutions, the necessary requirements for the production of the more difficult substances and more complicated works of art will be attained. Through the whole treatise, the reader will thus be led gradually from the most common and well-known facts to the most complex and difficult applications of electro- deposition . In accordance with this plan, the subject will commence with a review of such facts of electro-deposition as every man possessing the few necessary materials, which are easily procurable, may readily verify for himself. On these facts the whole subject throughout will be based. From them we shall proceed to the circumstances or con- ditions under which they occur, namely, the causes of electro-deposition. The principles will be inferred from the facts as we proceed, until we arrive at the more abstract conditions of the phenomena. The facts will be based on numerous experiments, in which instances where depo- sition does occur, as well as experiments in which it does not occur, will be cited. These investigations will satisfy the reader that in all cases where deposition does occur, certain conditions are invariably present ; and where it does not occur, one or more of those conditions is invariably absent ; and, therefore, that the conditions observed are the causes of the phenomena. Another and more ultimate reason for mentioning negative as well as positive instances is, that in practical working it is nearly as important to know what will prevent deposition, as to know what will produce it. The following table exhibits the phenomena of electro -deposition arranged in an inductive order, suitable for learning the subject theoretically, and without immediate reference to its practical applications. The first portion of the table contains the facts of electro-deposition, divided into seven classes, under which may be ranged the whole known facts of the science. The second portion contains the principles or conditions under which those facts are manifested ; these are also divided into seven classes, which are capable of including all the known conditions or causes of electro- deposition. METHOD OF STUDYING THE SUBJECT. 9 A. — Facts. 1. Deposition by one metal and one liquid. 2. Deposition by two metals and one liquid. 3. Deposition by one metal and two liquids. 4. Deposition by two metals and two liquids. 5. Deposition by connecting cither of the foregoing arrangements (except the first) with a separate depositing liquid. 6. Deposition by connecting other sources of depositing power with a separate depositing liquid. 7. Deposition by combinations of the foregoing. B — Principles. 1. Chemical conditions of deposition. 2. Electrical conditions of deposition. 3. Thermic conditions of deposition. 4. Mechanical conditions of deposition. 5. Mathematical conditions of deposi- tion. 6. Logical conditions of deposition. 7. Ontological conditions of deposition. This arrangement has been used with much success in teaching the theoretical part of electro-deposition, enabling the pupils to understand each portion clearly as they proceeded. The plan adopted was — . First, to exhibit before the pupils numerous experiments of each class of facts in succession, including positive cases in which deposition did occur, as well as negative ones in which it did not occur. Second, to place each of the theoretical principles in succession before them in the form of a hypothetical question, referring them to the various facts on which it is founded, and leaving them to observe for themselves whether or not the principle there stated was borne out, allowing them to draw their own conclusions. By this method they were soon led to observe, that, wherever deposition occurred, certain oonditions were present, and that where it did not occur those conditions were absent. When treating of the laws and principles of deposition, the reader will be referred back to the facts upon which they are based ; so, when describing its practical appli- cations, he will in like manner be referred to the laws or principles for his guidance ; his knowledge of the practice will thus be based in a great measure upon the prin- ciples, as the principles will be deduced from the facts, which, as we have said before, are within the reach of every one to repeat and prove for himself. The practical part will treat in succession of the general rules for working all the different processes of washing or dipping, whether by single cell, battery, or other process. The requisites for preparing good depositing solutions, both for simple metals and alloys, with methods of making solutions generally, and of working them ; suitable sources of electricity, either by the magneto-electric machine or other bat- teries, together with their construction ; as well as instructions for regulating the quantity and intensity of the current; regulating the quantity and quality of the deposited metal ; cleaning and preparing metallic surfaces for receiving adhesive and non-adhesive deposits ; copying works of art in various substances ; elastic moulding ; preparation of non-conducting surfaces to receive a deposit ; multiplication of works of art by deposition ; deposition, by the various processes, of such metals and their alloys as antimony, bismuth, zinc, cadmium, tin, lead, iron, cobalt, nickel, copper, brass, ger- man silver, mercury, silver, gold, platinum, and palladium ; chemical relations of the cyanides of gold and silver ; manufacture of cyanide of potassium ; recovery of gold and silver from damaged solutions ; and a full list of patents upon electro-deposition. 10 MODES OF DEPOSITION. Fig. 1. fur Fig. 2. THEORETICAL DIVISION OF THE SUBJECT. 1. Facts* — One Metal in One Liquid. — There are various modes in which, deposition of one metal upon another may take place, and they may be classed as follows : — 1st. By the simple immersion of one metal in one liquid (Fig. 1), namely, by putting the metal to be coated into a solution of the metal to be deposited, and allowing it to remain a longer or shorter period of time, the liquid being at a suitable temperature ; for instance, if we immerse a piece of clean iron in a solution of sulphate of ^copper, it will become coated with copper, but if we immerse a piece of silver in that liquid it will not become so coated. 2nd. Two Metals in One Liquid. — By the immersion of two metals in one liquid (Fig. 2), the two metals being in contact with each other ; instance, if we connect a piece of silver A and a piece of iron B together, and immerse them in a solution of sulphate of copper C, the silver will become coated with cop- per as well as the iron ; but if a piece of silver in contact with a piece of gold or platinum is immersed in the same liquid, it will not become coated. We have already seen that silver immersed alone in such a liquid will not receive a deposit of copper. 3rd. One Metal in Two Liquids. — By the immersion of one metal (i.e., one land of metal) in two liquids D and E (Figs. 3 and 4), the liquids being prevented from mixing with each other either by a porous partition F (Fig. 3) of bladder, thin wood, unglazed earthenware, or other porous material which will allow the two liquids to touch each other through its pores ; the piece of metal being either bent so as to dip into each liquid, or cut into two portions, and its two ends united by a wire C, the end or piece to receive the deposit being immersed in one liquid, and the other piece in the other liquid (Fig. 3) ; or the two liquids being- put in a deep narrow vessel, the heavier one being poured in first, and the lighter one poured carefully above it so as not to mix them together, and the piece of metal being in the form of a rod or wire placed vertically in the two liquids (Fig. 4) ; for instance, if the lower liquid consists of -a solution of sulphate of copper and the other of dilute sulphuric acid, and a piece of copper is immersed in both liquids, that part of it which is in the sulphate solution will become coated with copper, whilst that in the 'acid liquid will be partly dissolved; but if, instead of copper, we use a piece of platinum, it will neither be dissolved nor receive a me- tallic deposit. 4th. Two Metals in Two Liquids. — By the immersion of two metals A and B (Fig. 5) in two liquids, D and E, the two being, as in the last arrangement, either separated by a porous diaphragm F, or poured one above the other, the two metals being immersed one in each liquid, and connected together by a wire C ; for instance, if one liquid is DEPOSITING ARRANGEMENTS. 11 Fig. 5. dilute sulphuric acid, and the other a solution of sulphate of copper, and a piece of copper is immersed in the dilute acid and a piece of silver in the metallic solution, 3ing thus in mutual contact, the pi the silver receive a deposit of copper ; but if we immerse a piece of pla- tinum in the dilute acid with the silver in the sulphate solution, the platinum will not dissolve, nor the silver receive a metallic deposit. 5th. Separate Depositing Liquid with all the others. — By connecting any one of the foregoing arrange- ments by means of wires with two separate pieces of metal of a similar kind immersed in a separate and suitable liquid (Fig. 6) ; for instance, if we take the arrangement of two metals in one^ygpid, such as iron B and copper C, in a solution of sulphate of copper A ; ^dHMp B and silver C in dilute sulphuric acid A, and connect them by two scpawtte' v.-hvs D and E with two pieces of copper D and E immersed in a solution of sulphate of copper F contained in a separate vessel, the piece of copperjE connected with the silver will dissolve, whilst the other piece D which is conaMKed with the zinc will receive a deposit of copper; but if we. substitute a solution of aulphate of zinc, freely acidulated with sulphuric acid, for the solution of sulphate of copper F, and two pieces of platinum for the pieces of copper, the one piece of platinum will not dissolve nor the other receive a metallic deposit. 6th. Separate Depositing Liquid with any other Source of Dower. — By connecting the pieces of metal in the separate depositing liquid with any other source of depositing power, such as a magneto-electric machine, cell, or battery. 2. In these arrangements it will be observed that we have— 1st, deposition by one metal and ono liquid ; 2nd, by two metals and one liquid ; 3rd, by one metal and two liquids ; 4th, by two metals and two liquids ; 5th, by a separate depositing liquid and metals connected with either of these ; and Gth, by a separate depositing liquid and metals con- nected with any other source of depositing power. These six classes and their combinations arc capable of including all the known cases of electro-deposition. 3. Under the head of each of these classes will be mentioned a number of experiments with various metals and liquids, and it would be advisable for the student to try a few experiments, as he proceeds, both of deposition and non-deposition of each class, in order to fix the facts more firmly in his memory, and give him a fuller comprehension of the principles. 4. Depositing Arrangement No. 1. — Deposition by one metal and one liquid (Fig. 7) takes place in the following instances : — Hydrochlorate of Terchhride of Antimony. — In a solution of hydrochlorate of ter- chloride of antimony (the ordinary chloride of antimony, as prepared for pharma- ceutical purposes), bismuth, zinc, tin, lead, brass, and german silver become coated with antimony ; whilst antimony, iron, nickel, copper, silver, gold, and platinum do not become coated. Chloride of Bismuth.'— -In a solution of acid hydrochlorate of bismuth oxide (chlo- 12 DEPOSITION BY ONE METAL AND ONE LIQUID. ride of bismuth), zinc, tin, lead, and iron deposit the bismuth upon themselves ; whilst antimony, bismuth, copper, brass, german silver, gold, and platinum do not. Sulphate, Chloride, Nitrate, or Acetate of Zinc. — In a solution of either sulphate, chloride, nitrate, or acetate of zinc, neither antimony, bismuth, zinc, tin, lead, iron, nickel, copper, brass, german silver, silver, gold, or platinum become coated with zinc. Protochloride of Tin. — In a solution of protochloride of tin, zinc and lead become tinned ; whilst antimony, bismuth, tin, iron, nickel, copper, brass, german silver, silver, gold, and platinum receive no deposit. Hyponitrate, Nitrate, or Acetate of Lead. — In a solution of hyponitrate, nitrate, or acetate of lead, zinc receives a coating of lead ; whilst antimony, bismuth, tin, lead, iron, nickel, copper, brass, german silver, silver, gold, and platinum receive no deposit. Ferrous Sulphate. — " Zinc," as Fischer says, " immersed in a perfectly neutral solution of ferrous sulphate (protosulphate of iron) contained in a stoppered bottle, throws down metallic iron, which is deposited partly on the zinc ;" but in this solution neither antimony, bismuth, tin, lead, iron, nickel, copper, brass, german silver, silver, gold, or platinum receive any metallic deposit. Sulphate of Copper. — In a solution of sulphate of copper, zinc, tin, lead, and iron become coated with copper ; whilst antimony, bismuth, nickel, copper, silver, gold, and platinum do not. Chloride of Copper. — In a solution of chloride of copper, bismuth, sine, tin, lead, and iron receive a copper deposit ; whilst antimony, nickel, copper, silver, gold, and platinum do not. Nitrate of Copper. — In a solution of nitrate of copper, zinc, tin, lead, and iron become coated ; whilst antimony, bismuth, nickel, copper, silver, gold, and platinum receive no deposit. Bichloride of Copper. — With a solution of dichloride of copper in liquid ammonia, or of oxide of copper in a solution of sal-ammoniac, zinc receives a deposit ; whilst antimony, bismuth, tin, lead, iron, nickel, copper, silver, gold, or platinum do not. Mercurious Salts. — Solutions of mercurious salts have their metal deposited by arsenic, antimony, bismuth, zinc, cadmium, tin, lead, iron, copper, and brass, also by the alloys of silver with zinc, tin, lead, or copper. Nitrate of Mercury. — A solution of nitrate of mercury yields its metal to bismuth, zinc, cadmium, lead, iron, or copper, and, if acidulated with nitric acid, to antimony also ; but not to silver, gold, or platinum. Acetate of Mercury. — Iron deposits mercury from a solution of acetate of mercury. Silver Solutions. — The following metals, viz., manganese, arsenic, antimony, bismuth, zinc, cadmium, tin, lead, iron, copper, and mercury, deposit silver from its solutions in the metallic state ; an aqueous solution of nitrate of silver yields its metal to manganese, arsenic, antimony, bismuth, zinc, tin, lead, iron, nickel, copper, brass, and german silver ; but not to silver, gold, or platinum. Lead and tin deposit the silver most quickly ; then follow the other metals in this order, cadmium, zinc, copper, bismuth, antimony, arsenic, mercury. Arsenic deposits silver from the alcoholic solution of nitrate of silver ; antimony receives a coating of silver either in the aqueous sulphate or alco- holic nitrate ; bismuth deposits silver from the alcoholic nitrate, but not from the aqueous sulphate ; zinc receives a silver deposit in the alcoholic nitrate ; tin becomes silvered in the alcoholic nitrate, but more quickly in the aqueous sulphate ; iron deposits silver from the sulphate of silver ; but not from the alcoholic nitrate ; copper DEPOSITION BY TWO METALS AND ONE LIQUID. 13 depo«its it from the aqueous sulphate or alcoholic nitrate ; brass and the alloys of silver, with zinc, tin, or lead, deposit silver from silver solutions completely. In a solution of the double cyanide of silver and potassium (the ordinary plating liquid), zinc, lead, and copper become silvered; also brass and german silver, but more slowly ; whilst antimony, bismuth, tin, iron, nickel, silver, gold, and platinum do not. Gold Solutions. — From an acid solution of terchloride of gold, most of the base metals, likewise mercury, silver, platinum, and palladium deposit gold, generally in the metallic state, but not always ; arsenic rapidly deposits gold in this solution ; antimony, tellurium, and bismuth become gilded ; zinc, cadmium, lead, iron, cobalt, mercury, silver, platinum, and palladium deposit the gold ; whilst titanium, tungsten, molybdenum, and chromium do not. In a solution of the double cyanide of gold and potassium, zinc quickly becomes gilded, and copper, brass, and german silver slowly ; whilst antimony, bismuth, tin, lead, iron, nickel, silver, gold, and platinum do not. Bichloride of Platinum. — Platinum is deposited from a solution of its bichloride by arsenic, antimony, tellurium, bismutb, zinc, cadmium, tin, lead, iron, cobalt, nickel, copper, brass, german silver, mercury, and silver ; but not by gold or platinum. 5. Observations upon Class of Instances No. 1. — In reviewing all these instances, we may make the following observations : — 1st, that various metals by mere immersion in solutions of other metals, at the ordinary temperature of the atmosphere, sometimes become coated with a deposit of metal, and sometimes not ; 2nd, that no metal becomes coated by mere immersion in a solution of the same metal — for instance, zinc does not become coated with zinc in a solution of sulphate of zinc ; copper with copper in a solution of its sulphate, gold with gold in its chloride ; 3rd, that tbe baser metals, especially zinc, cadmium, tin, lead, and iron, become coated more frequently than the noble metals, especially gold and platinum ; 4th, that solutions of base metals, especially of zinc and iron, yield their metal less frequently than those of the noble metals, especially those of gold and platinum ; 5th, that of all the ordinary metals mentioned in the foregoing instances, zinc deposits metal from the greatest number of solutions, and appears to have the strongest depositing power ; 6th, that the coherent and adhesive deposits obtained are in all cases exceedingly thin; and 7th, that oftentimes the deposited metal, whatever its kind may be, has the appearance of a black or dark-coloured powder on its surface, especially when it has been deposited very rapidly ; and that sometimes it exhibits its ordinary colour and appearance, especially if its outer portion is rubbed off. 6. To this mode of depositing belongs the process of tinning brass articles (wash tinning), by boiling them in water containing a salt of tin and bitartrate of potash ; the process of silvering brass nails, buttons, hooks and eyes, buckles, &c, by rubbing them with any of the well-known silvering compositions moistened with water ; also the water gilding process, &c. 7. Depositing Arrangement No. 2. — Deposition by Tico Metals and One Liquid. — The following instances belong to the class of deposition by two metals and one liquid, the two metals being either in mutual contact (touching each other either above or beneath the liquid), or connected together by a wire. Chloride of Antimony. — For instance, if we immerse a piece of an- timony A, in contact with a piece of zinc B, in a solution of the ordinary chloride of antimony C, it will receive a coating of anti- mony ; or if we immerse a piece of platinum in contact with a piece of tin in this 14 DEPOSITION BY TWO METALS AND ONE LIQUID. t liquid, it will receive a deposit of antimony; but if we immerse a piece of antimony in contact with a piece of platinum, or a piece of platinum in contact with a piece of silver in this liquid, it will receive no metallic deposit. Chloride of Bismuth. — In a solution of chloride of bismuth, brass in contact with a piece of zinc, copper in contact with tin, or german silver with iroiij receives a deposit of bismuth ; but brass in contact with a piece of gold, gold in contact with silver, or german silver with platinum, receives no deposit. Sulphate, Chloride, or Nitrate of Zinc. — "With a solution of either sulphate, chloride, or nitrate of zinc, no metal of any pair of metals selected from amongst the following, will receive a deposit of zinc : antimony, bismuth, zinc, tin, lead, iron, nickel, copper, mercury, silver, gold, platinum, or palladium. Protochloride of Tin. — With a solution of protochloride of tin, either antimony, tin, or copper, immersed in contact with zinc or lead, will receive a coating of tin ; but antimony in contact with tin, tin with silver, copper with iron, or either gold or platinum with copper will not receive a deposit. Hyponitrite of Lead. — With a solution of hyponitrite of lead, either tin, copper, or brass, in contact with a piece of zinc, will receive a deposit of lead ; but tin in contact with copper, copper with lead, or brass with platinum, receives no deposit. Nitrate of Lead. — With a solution of nitrate of lead, either copper, brass, or silver, in contact with zinc, receives a coating of lead ; but copper in contact with iron, brass with tin, or silver with copper, receive' no such coating. Protosidphate of Iron. — With a saturated solution of protosulphate of iron, platinum in contact with zinc receives a deposit of iron ; but in contact with copper it receives no metallic deposit. Chloride of Nickel and Ammonia. — In a solution of the double chloride of nickel and ammonia, copper in contact with zinc receives a deposit of nickel ; but in contact with silver it does not receive such a deposit. Sulphate of Copper. — In a solution of sulphate of copper, brass in ~ contact with zinc ; or tin, german silver, silver, or platinum, in contact with iron, receives a deposit of copper ; whilst silver in contact with antimony, or platinum in contact with brass, receives no deposit. Oxide of Copper in Ammonia. —In a solution of oxide of copper in ammonia, plati- num in contact with zinc receives a deposit ; but silver in contact with iron does not. Nitrate of Mercury. — In a solution of nitrate of mercury, silver in contact with either zinc or iron, or platinum in contact with copper, receives a metallic deposit ; but platinum in contact with silver does not. Nitrate of Silver. — In a solution of nitrate of silver, gold in contact with zinc receives a deposit of silver ; but in contact with platinum it does not. Bichloride of Platinum. — In a solution of bichloride of platinum, platinum in contact with zinc becomes coated with platinum ; but in contact with gold it receives no such coating. 8. Observations upon Class of Instances No. 2. — The following general observations may be made upon the foregoing facts : — 1st, that in some instances deposition does, and in others it does not, occur ; 2nd, that no metal will cause another metal to be coated by this method, unless it can coat itself in the same liquid by simple immersion — for instance, zinc cannot coat itself with zinc in solutions of zinc, neither can it cause other metals to become coated with that metal in those solutions ; copper cannot coat itself with zinc in a solution of sulphate of zinc, or with tin in a solution of chloride DEPOSITION BY ONE METAL AND TWO LIQUIDS. 15 of tin, neither can it cause silver, gold, or any other metal, to become coated with zinc or tin, in those liquids ; 3rd, that one of the two metals which receive a deposit by this method, derives its power of receiving the deposit by virtue of its contact with the other metal ; 4th, that any metal which has the power of coating itself hy simple immersion in a given liquid, can by this method cause other metals which do not coat themselves by simple immersion in that liquid to hecome coated — for instance, zinc, tin, and iron coat themselves with copper by simple immersion in a solution of sulphate of copper, and silver, gold, and platinum do not ; but if either of the former metals be con- nected with either of the latter, and the two immersed together in that liquid, the latter metals as well as the former will become coated with copper ; 5th, that base metals, and especially zinc, have generally the power of causing other metals to become coated by this method ; whilst the noble metals, and especially gold and platinum, rarely possess this power ; 6th, that by this method metal is deposited much more frequently from solutions of the noble metals, than from those of the base ones ; and 7th, that thick deposits of metal may be obtained hy this method, provided the action is continued sufficiently long, and the liquid properly renewed. 9. Depositing Arrangement No. 3 — Deposition by One Metal and Two Liquids— Chloride of Antimony. — The following instances belong to deposition hy the immersion of one metal in two liquids, D and E (Fig. 9), sepa- rated by a porous diaphragm F, the metal being either in two pieces connected together by a wire or wires C, or in one piece, and bent so as to dip into both liquids ; the diaphragm may be dis- pensed with, as already explained (1), hy pouring the lighter liquid carefully ahove the other, and placing the piece of metal vertically in the two liquids ; if two pieces of antimony, A and B, connected together by a wire or wires C, are immersed, one in dilute nitric acid D, and the other in a solution of chloride of antimony E, the piece in the dilute acid will dissolve, whilst, that in the chloride solution will receive a metallic deposit. Chloride of Bismuth.— If two pieces of antimony are immersed in the previous manner, one in hydrochloric acid, and the other in a solution of chloride of bismuth, that in the acid will dissolve, and the other receive a coating of bismuth. Sulphate of Copper. — With antimony, in dilute hydrochloric acid on one side, and in a solution of sulphate of copper on the other, a deposit of copper is obtained. Chloride of Bismuth.— With bismuth in hydrochloric acid on one side, and in a solution of chloride of bismuth on the other, a free deposit of bismuth is soon ohtained. Chloride of Zinc. — If a piece of zinc is hent so as to dip into dilute hydrochloric acid on one side, and into a neutral solution of chloride of zinc on the other, a free deposit of zinc will he found upon the end in the metallic solution after a period of twelve hours. Solution of Acetate of Zinc,— With zinc in a solution of acetate of zinc on one side, and in dilute sulphuric acid on the other, that in the dilute acid will dissolve, whilst the other end will receive a metallic deposit. Tig. 10. 16 DEPOSITION BY ONE METAL AND T WO LIQUIDS. Iron in Chloride of Antimony. — With iron in dilute sulphuric acid on one side, and in a solution of chloride of antimony on the other, the end in the metallic solution will receive a deposit of antimony, whilst that in the dilute acid will dissolve. Iron in Sulphate of Zinc. — "With iron in dilute sulphuric acid on one side, and in a solution of sulphate of zinc on the other, no deposit of zinc is obtained in twelve hours ; similarly with iron, dilute sulphuric acid,' and a solution of protosulphate of iron, no deposit occurs in twelve hours. Tin in Chloride of Tin. — With tin in dilute hydrochloric acid on one side, and in a solution of chloride of tin on the other, a deposit of tin is obtained. Zinc in Sulphate of Zinc. — With zinc in dilute sulphuric acid, and in a solution of sulphate of zinc, a free deposit of zinc occurs in twelve hours. Bismuth in Nitrate of Bismuth. — With bismuth in dilute nitric acid, and in a solution of acid nitrate of bismuth, a thin deposit of bismuth is found in twelve hours. Copper in Sulphate of Zinc. — With copper in dilute sulphuric or dilute nitric acid on one side, and in a solution of sulphate of zinc on the other, no deposit of zinc occurs in twelve hours. Brass or Copper in Sulphate of Copper. — With brass or copper in dilute sulphuric acid on one side, and in a solution of sulphate of copper on the other, a deposit of copper is obtained in twelve hours ; similarly with copper in dilute hydrochloric acid, and in a solution of chloride of copper, a metallic deposit occurs. Silver in Plating Liquid. — With silver in either dilute sulphuric, or dilute nitric acid on one side, and in a solution of sulphate of copper on the other, no deposit of copper takes place in twelve hours ; but with silver in a solution of cyanide of potassium on one side, and in the double cyanide of potassium and silver on the other, a free deposit of silver takes place upon the end or piece in the latter solution. Platinum in Nitrate of Copper. — With platinum in aqua regia on one side, and in either a solution of nitrate of copper, the ordinary cyanide gilding solution, or a solution of bichloride of platinum on the other, no deposit of copper, gold, or platinum occurs. 10. Observations on Class of Instances No. 3. — 1st, it appears, that in this class also we obtain negative as well as positive instances ; 2nd, that by this arrangement unlike the previous classes, almost any metal may cause the same metal to be deposited — for instance, zinc may deposit zinc, copper deposit copper, and silver deposit silver ; 3rd, that by it even a noble metal may cause the deposition of a base metal, provided we have a suitable combination of liquids ; for instance, if a piece of gold or silver is im- mersed in a strong solution of cyanide of potassium on one side, and in a solution of sulphate of copper or chloride of antimony on the other, the end in the free cyanide solution will dissolve, whilst that in the copper antimony solution will receive a deposit ; 4th, that the metal or end which receives a deposit, derives that power from its contact with the metal in the other liquid ; 5th, that, as a general rule, base metals have a greater power of causing deposition by this method than the noble ones ; 6th, that the noble metals are more readily and more often deposited than the base ones ; and 7th, that we may produce thick and coherent deposits by this method. 11. Depositing Arrangement No. 4. — Deposition by Two Metals and Two Liquids. — The following instances belong to the class of deposition produced by the immersion of two metals, A and B (Fig. 11), in two liquids, D and E, the metals being in mutual contact or connected together by a wire C, and the liquids sepai-ated by a porous partition F. DEPOSITION BY TWO METALS AND TWO LIQUIDS. 17 Zinc Depositing Antimony.— If a piece of antimony A be immersed in a solution of chloride of antimony D, and a piece of zinc B is immersed in dilute sulphuric acid E, and the two metals are connected together by a wire or wires C, a free deposit of antimony upon the metal A will take place in twelve hours. Tin Depositing Zinc. — With tin in hydrochloric acid, and zinc in a neutral solution of sulphate of zinc, a deposit of zinc is obtained in the metallic solution. Iron Depositing Antimony. — With iron in dilute hydrochloric acid, and antimony in chloride of antimony, a copious deposit of antimony takes place in twelve hours. Copper Depositing Zinc— With, zinc in dilute sulphuric acid, and zinc in a solution of sulphate of zinc, a deposit of zinc occurs. , Zinc Depositing Copper— With zinc in dilute sulphuric acid, and brass in a solution of sulphate of copper, copper is deposited. Bismuth Chloride of Antimony. — With bismuth in dilute hydrochloric acid, and anti- mony in chloride of antimony, no deposit of the latter takes place in twenty-four hours. Iron and Chloride of Tin. — With iron in dilute hydrochloric acid, and tin in a solution of chloride of tin, no deposit of tin took place in eighteen hours. Copper and Chloride of Antimony. — With eopper.in dilute hydrochloric acid, and anti- mony in chloride of antimony, or tin in chloride of tin, no deposit of antimony or tin took place in twenty hours. 12. Observations upon Class of Instances No. 4.— 1st, It appears that negative as well as positive instances occur in this arrangement in common with the others ; 2nd, that by using suitable metals and liquids, deposition may be effected more rapidly by this method than by the preeeeding ones ; 3rd, that the metal which receives the deposit derives its power from its contact with the other metal , 4th, that base metals in strong acids have the greatest power of causing a deposit upon the other metals, and noble metals the least ; 5th, that the noble metals are more readily deposited than the base ones ; and 6th, that thick and coherent deposit may be obtained. In all the above instances, instead of using one vessel divided into two parts by a porous diaphragm, it will be found convenient to put one of the liquids in an unglazed earthenware porous cell, and immerse the cell in the other liquid (see j vessel A, Fig. 13). In this case, either liquid may be in the outer vessel. This ! last arrangement (No. 4) is usually termed the " single cell" process. 13. Depositing Arrangement No. 5. — Deposition by Separate Liquid. — The next class of instances are those in which either of the foregoing arrangements, except the first, may be connected by wires with two pieces of similar metal immersed in a separate liquid. For instance : — 1st. With Two Metals and One Liquid (Fig. 12).— If we take a vessel A containing either dilute sul- phuric acid or a solution of sulphate of copper, and immerse in it a piece of zinc B and copper C, with copper wires D and E attached to them, and either immerse the free ends of those wires in a sepa- rate solution of sulphate of copper F, or connect them with two pieces of copper immersed in that liquid, the piece of copper E in liquid F will dis- solve, whilst the opposite piece D, connected with the zinc, will receive a deposit of copper. 2nd. With One Metal and Two Liquids.— It we take a vessel A (Fig. 13) containing Fig. 12. PRACTICAL CHEMISTRY. — No. I. * 13 DEPOSITION BY LIQUID, MAGNET AND COIL. a porous cell B, with a neutral solution of sulphate of zinc C in the outer vessel, and dilute sulphuric acid D in the inner, and immerse two pieces of zinc E and F, with copper wires G and H attached, into D and C respectively, and immerse the ends of those wires in a separate solu- tion of sulphate of copper I, the end of the wire H will dissolve, whilst that of G will receive a deposit of metallic copper. 3rd. With Two Metals and Two Liquids (" single cell" arrangement, Fig. 13). — If we substitute a piece of copper for the piece of zinc F in the last- mentioned instance, and a solution of sulphate of copper for that of sulphate of zinc, similar effects will take place at the ends of the wires in the liquid I, except that the action will he much more rapid ; but if in either of these three instances we use a solution of sulphate of zinc freely acidulated with sul- phuric acid, instead of the solution of sulphate of copper I, and platinum wires in place of the copper ones to be immersed, neither of the pieces of platinum will dissolve or receive a metallic deposit. 14. Remarks upon Class of Instances iVb. 5. — In this class of instances the method or arrangement differs from the three preceding ones, simply by the wires which con- nect the two pieces of metal being cut in two, and its free ends either immersed in a separate liquid or connected with two pieces of metal dipping into that liquid. It is not necessary to have the depositing vessel perfectly separated ; it may even be attached to the same piece of apparatus, provided the liquid in it is perfectly separated from the other liquids and metals. The pieces of metal in the separate liquid possess no power of deposition of themselves in that liquid, even if they were connected together, but derive their power of dissolving and receiving a deposit wholly from the other metals and liquids by means of the wires. 15. Depositing Arrangement No. 6. — Deposition by Magnet and Coil (Fig. 14). — "We may produce deposition in the separate liquid by connecting the two pieces of immersed metal with any other source of depositing power— for instance, if a long copper wire A, covered with silk or cotton, is coiled upon a large bar of pure soft iron B, and its ends C and D are immersed in a solution of sulphate of copper E, and the poles of a powerful horse-shoe magnet F are brought in contact very many times with the end of the bar, and every time before removing the magnet from the bar one of the ends of the wire is taken out of the liquid, and re- placed before returning the magnet, one end of the copper will slightly dissolve, and the other receive a thin copper de- posit ; but if each of the ends is allowed to remain constantly in the liquid, no such effects will occur. 16. Compound Depositing Arrangement No. 7.— Any of the foregoing combinations of liquids and metals (except the first), or the magnetic arrangement, with or without separate depositing liquids, may be connected together in a series of any number, and may include each of the arrangements in the same series, or include any Fig. 14. COMPOUND DEPOSIT ARRANGEMENTS. 19 number of depositing liquids, and deposition may be obtained either in the whole or in any portion of them at the same time ; for instance, the vessel A (Fig. 15) eon- tains a piece of zinc B, and copper C, immersed in dilute sulphuric acid ; vessel D Fig. 15. contains zinc E, in dilute sulphuric acid, and copper F, in a solution of sulphate of copper ; vessel G contains a series of separate depositing liquids, consisting of solu- tions of sulphate of copper, connected together by bent pieces of copper; the extreme pieces being attached to zinc B and copper F ; here deposition takes place upon every alternate piece of copper throughout the wbolc series, except that in vessel A. 17. General Observations. — We may make the following general observations upon the whole of the foregoing facts : — 1st, that negative as well as positive instances occur in all classes of facts of electro-deposition ; 2nd, that almost any of the ordinary metals, both noble and base, may be deposited by each of the Methods or arrangements described ; 3rd, that the particular result of deposition or non-deposition occurring appears to depend chiefly upon the particular combination of liquids and metals, their arrangement and connections ; 4th, that the size or shape of the containing vessels, the bulk or depths of the liquids, the size, form, or position of the metals, appear to exercise little or no influence upon the result ; 5th, that in all cases of deposition there is a difference in kind, either of the metal, of the liquid, or of both ; 6th, that in all such cases a metal dissolves in a liquid ; for instance, in Arrangement No. 1, with a piece of iron immersed in a solution of sulphate of copper, a portion of the iron is dissolved as the copper is deposited ; in Arrangement No. 2. with iron and copper together in a solution of sulphate of copper, the iron dissolves and the copper receives a deposit ; in Arrangement No. 3, with copper in dilute sulphuric acid, and in a solution of sulphate of copper, the copper in the acid dissolves, whilst that in the metallic solution receives a deposit ; in Arrangement No. 4, with zinc in dilute sul- phuric acid, and copper in sulphate of copper, the former dissolves, whilst the latter receives a deposit ; in Arrangements Nos. 5 and 6, one piece of metal in the separate depositing liquid dissolves, whilst the other receives a metallic deposit, and the same with the compound Arrangement No. 7 ; 7th, that in Arrangements Nos. 1 and 2, the same piece of metal which dissolves also receives a metallic deposit, and in Arrange- ments 3, 4, 5, 6, and 7, the pieces of metal which receive a deposit do not dissolve ; 8th, that as a general rule in all methods, and in all solutions except alkaline metallic cyanides, zinc among common metals has the greatest, and platinum the least power of producing deposition ; 9th, that zinc generally deposits metals most rapidly from their solutions, and most frequently in the state of a dark-coloured or black powder ; and 10th, that among solutions of the salts of ordinary metals, those of the salts of noble 20 CONDITIONS OF DEPOSITION. metals yield their metal most easily, and those of the base metals, zinc especially, with the greatest difficulty. 18. In looking over the foregoing instances, we also observe : — 1st, that when several metals are used, they must either touch each other or be connected together by wires or other pieces of metal ; 2nd, that when several liquids are used, they also must touch each other, either by means of a porous diaphragm, or otherwise ; 3rd, that when a series of metals and liquids are used, they must together form a complete circuit, and all their points of contact be perfectly clean ; 4th, that a separate depositing liquid possesses no power of deposition by itself, but derives its power by means of the wires from the other arrangements with which it is connected ; and 5th, that the length of the connecting wire, has no very great influence on the result. These observations have led m to conclude that deposition is caused by some force which is generated in some part of the apparatus, and circulates through the liquids, metals, and wires, which compose the circuit. 19. Principles. — Conditions of Electro-Deposition. — From the simple facts of electro- deposition, and the general observations made upon them, we proceed to consider the causes of deposition, and the conditions or circumstances under which deposition occurs, in the following order :— 1st. The Chemical Conditions. — If we immerse a clean iron wire in a solution of nitrate of mercury, it receives a deposit of that metal ; but if we immerse it in perfectly dry metallic mercury, it receives no deposit, because in the former case the necessary chemical conditions of deposition are present, whilst in the latter case they are absent. 2nd. The Electrical Conditions. — If we connect together a piece of iron and a piece of copper by means of a metal wire, and immerse them in a solution of sulphate of copper, the copper will receive a metallic deposit ; but if we connect them together by a cord of gutta-percha or rod of glass, no deposit will take place, because in the former case all the electrical conditions are present, whilst in the latter case one of them, viz., a complete conducting circuit, is absent. 3rd. The Thermic Conditions remain unknown. 4th. The Mechanical Conditions. — If a piece of iron be immersed in a solution of sulphate of copper, it receives a copper deposit ; but if a piece of platinum be so immersed it receives no such deposit, because, for one reason, in the former instance the mechanical conditions of attraction and repulsion at the dissolving and depositing surfaces are present, but in the latter they are absent. 5th. The Mathematical Conditions. — If we immerse two piece of carbon in fused ^rofo-chloride of tin, and connect them with a voltaic battery, tin will be deposited ; but if we immerse them in fused fo'-chloride of tin no deposition will occur, because in the first instance all the mathematical conditions are present ; the fluid salt contains one atom of chlorine for each atom of tin ; whilst in the latter case one of them is absent, the salt contains two atoms of chlorine to one atom of tin, and, according to Faraday's law, " only those substances of the first order are directly decomposible which contain one atom of one of their elements for each atom of the other." These several heads are capable of including all the known circumstances or con- ditions under which deposition occurs ; and under the head of each of them will be given a few instances, both of deposition and of non-deposition, to illustrate the principle ; and it would be advisable for the reader to try for himself most of the experiments given, in order to fix the principles more firmly in his memory. CHEMICAL CONDITIONS OF DEPOSITION. 21 20. Chemical Conditions of Deposition.— The first chemical condition to he observed is, that in every case of deposition the depositing liquid contains acid and basic elements, namely, a salt, the acid of which is to dissolve or combine with one metal, and its metal or base to be deposited upon the other. 1st. Deposition by One Metal and One Liquid. — With the first class of facts, if we immerse a piece of clean iron in a solution of nitrate of mercury, it will receive a deposit of that metal; but if we immerse it in mercury alone, it will receive no deposit ; in the first instance an acid as well as a basic substance was present, and deposition took place ; but in the second instance the metal or base alone was present, and no deposition occurred. 2nd. With Two Metals and One Liquid. — If zinc and platinum are immersed in mutual contact in a solution of nitrate of mercury, the platinum will receive a metallic deposit, but if they are immersed in pure dry mercury, no deposit will occur. 3rd. With One Metal and Two Liquids.— If we immerse one end of a platinum wire in a strong solution of cyanide of potassium, and its other end in a solution of nitrate of mercury, the two liquids being in mutual contact by means of a porous partition, the end in the metallic solution will soon receive a deposit of mercury ; but if perfectly dry mercury is substituted for the nitrate solution, no such deposit will occur. 4th. With Two Metals and Two Liquids.— -If we immerse zinc in dilute sulphuric acid, and platinum in a solution of nitrate of mercury, the two liquids touching each other by a porous partition, and the metals connected together by a wire, the platinum will quickly receive a deposit of mercury ; but with dry mercury instead of the metallic solution, the platinum will not receive a deposit. 5th. With a Separate Depositing Liquid.— -If we take two pieces of platinum wire, connect them, as already described (13), with either of the foregoing Arrangements, or with a magnet and coil (15), and immerse their free ends in a solution of nitrate of mercury, one of the wires will receive a deposit of mercury ; but if the separate liquid consist only of dry mercury, no deposit will be obtained. It is evident from these facts, that in every case where deposition occurs, the depositing liquid contains both acid and basic substances, and that without the presence of both no deposition takes place. 21. Degrees of Chemical Affinity of Metals and Liquids. — At the present point it is necessary to mention a few instances of the different degrees of chemical affinity manifested by different metals and liquids, that the reader may be able to understand their general chemical relations in electro-deposition more clearly 1st. If we immerse a piece of potassium in almost any liquid, very violent chemical action takes place, which is stronger in mineral and vegetable acids than in water or organic liquids ; if we place a small piece of it upon water, violent chemical action occurs, the water is decomposed, heat is produced, gas is evolved and it takes fire, the metal melts and rolls about on the surface of the water, oxidates and dissolves. 2nd. If we'immerse a piece of zinc in any of the strong mineral acids (sulphuric, hydrochloric, hydrofluoric, or nitric acids), strong chemical action takes place, gas is freely evolved, and the metal oxidates and dissolves ; with solutions of the ordinary vegetable acids, i.c , oxalic, tartaric, citric, formic, and acetic acids, the same effect^ occur in a much weaker degree ; but with water there is no visible decomposition, no gas evolved, nor any perceptible chemical action ; these instances shew that pot- 22 DIFFERENCE OF CHEMICAL AFFINITY. assium has a much stronger affinity for liquids than zinc, and that both potassium and zinc hare a stronger affinity for acids, especially mineral ones, than for water. 3rd. If we immerse a piece of zinc successively in each of the ordinary mineral and vegetable acids diluted with water, it will be quickly dissolved, with evolution of hydrogen gas in nearly all of them ; but if we immerse a piece of copper in those liquids, it will be quickly dissolved in only one of them, viz., nitric acid, and from this we conclude that copper has generally a much weaker affinity for acids than zinc. 4th. If we immerse either gold or platinum" in any of the strong mineral or vegetable acids, or even in cold aqua regia, it will be quite unaffected in all of them, whilst copper would be rapidly acted upon by nitric acid or by aqua regia, and slowly by several of the others, thereby shewing that the affinity of gold or platinum for acids, is generally much weaker than that of copper. From the foregoing, and many other instances that might be mentioned, we con- clude that the general order of affinity of the metals for acids occurs in the following order, namely, potassium, zinc, copper, gold, and platinum. Hydrocyanic acid, and cyanogen appear to differ in one respect, in their chemical relations towards ordinary- metals, from oxygen, chlorine, and the ordinary mineral and vegetable acids, in having a much stronger affinity for noble metals, and weaker for the base metals ; this is probably one of the chief reasons for the extensive adoption of cyanogen compounds in electro-deposition"; those compounds are highly suitable for the deposition of noble | metals, because of the great affinity of cyanogen for those metals, but not, as some | persons assert, for the deposition of many of the base metals, on account of its com- ' paratively weak affinity for them. 22. Potassium is an alkali metal ; zinc, cadmium, tin, lead, iron, cobalt, nickel, and copper, are base metals ; and mercury, silver, gold, platinum, palladium, &c, are noble metals ; and of these three classes, the alkali metals possess the strongest affinity for acids, base metals intermediate, and noble metals the least. The same order prevails in their degrees of depositing power ; potassium and the alkali metals generally deposit nearly all metals from their solutions ; zinc, and the base metals generally, deposit a smaller number ; and gold, platinum, and the other noble metals deposit very few from their solutions ; thus we perceive that those metals which have the strongest chemical affinity for acids, possess the greatest depositing power, and those which have the least affinity for acids, have also the least depositing power. 23. difference of Chemical Affinity Necessary to ^Deposition. — The second chemical condition which Ave have to observe upon is, that in every case of deposition there is a difference of chemical affinity at the dissolving and receiving surfaces for the different elements of the liquid, and that the dissolving metal has a stronger affinity for the acid elements of the liquid than either the metal in solution or the receiving metal ; for instance : — 1st. With One Metal and One Liquid. — If we immerse a piece of iron in a solu- tion of sulphate of copper, a deposit takes place upon it, but if we immerse a piece of platinum in the liquid, it receives no deposit ; in the first case, the iron having a stronger affinity for the sulphuric acid of the salt than copper, combines with it and dissolves, and the copper thereby set free from the acid is deposited upon the iron ; whilst in the second case, platinum having a much weaker affinity for the acid than the copper, cannot separate the acid and copper, and therefore cannot cause deposition. 2nd. With Ttvo Metals and One Liquid. — If we immerse copper and iron in mutual con- ! tact, in a solution of sulphate of copper, the iron dissolves, and deposition of copper takes I ) ' VARIOUS MODES OF DEPOSITION FORCE. 28 place upon both metals ; but if we immerse copper and platinum in mutual contact in this solution, no deposition occurs. In the first instance, the iron possessing a stronger chemical affinity for sulphuric acid than the copper, combines with it, and sets the copper free ; by this action, a current of depositing force is generated, which circu- lates through the iron, liquid, and copper, at their points of contact, and causes the metal of the liquid to be deposited upon the piece of copper ; but in the second case, neither the copper nor platinum possessing a stronger affinity for the acid of the salt than its associated metal, there is no copper set free, no current of depositing force generated, and consequently no deposition takes place. 3rd. With One Metal and Two Liquids. — If we immerse one end of a piece of copper in dilute sulphuric acid, and the other in a solution of sulphate' of copper, the two liquids touching each other, copper will be deposited upon the end immersed in the metallic solution, whilst the other end will combine with the acid and dissolve ; hut if* a, piece of platinum or gold is substituted for the copper, neither of its ends will dissolve or receive a metallic deposit ; in the first instance, the dilute sulphuric acid, having a stronger affinity for copper than a solution of sulphate of copper, combines with it, causes it to dissolve, develops a current of depositing force which circulates through the metals and liquids, and a deposit of copper is produced ; whilst in the ' second instance, the platinum or gold having a weaker affinity for the acid of one liquid i than copper for the acid of the other liquid, cannot separate the copper, or cause deposition. 4th. With Two Metals and Two Liquids. — If we immerse a piece of silver in a i strong solution of cyanide of potassium, and a piece of copper in a solution of the double cyanide of copper and potassium, the liquids touching each other by a porous partition, and the metals mutually touching by a wire, the silver will dissolve, and the copper receive a metallic deposit ; but if a piece of iron is substituted for the silver, no deposit will occur. In the first instance, the one solution has a stronger affinity for the silver, than the other has for the copper, consequently the former is dissolved, a current or depositing force is generated, and copper deposited ; but in the second case, the one liquid has a weaker affinity for iron, than the other has for copper, and therefore no iron is dissolved, no depositing force generated, and no copper deposited. 5th. With a Separate Depositing Liquid connected with any Source of Depositing Power— If we connect two pieces of silver with any of those sources of power, and immerse them in a solution of the double cyanide of silver and potassium, one piece will quicklj' dissolve, and the other receive a deposit of silver ; but if pieces of iron arc substituted for those of silver, neither will dissolve or receive a metallic deposit. In every case where a separate depositing liquid is used, the two pieces of metal im- mersed in it have a difference df chemical affinity imparted to them hy virtue of their con- nection with some arrangemant which develops depositing force, and this difference of affinity is manifested most wl en the liquid has a strong affinity for the immersed metal, and least when it has a weak affinity for that metal ; so in the first of the two imme- diately preceding instances, he liquid having a strong affinity for silver ; allows this difference of affinity to be fre ly exercised at the immersed surfaces of the two pieces of metal, and consequently one d ssolves, and the other receives a deposit ; hut in the second of these instances, the liquid laving a very weak affinity for iron, does not admit of the exercise of this difference of (affinity, and hence neither piece dissolves or receives a deposit. From these instances it is manifest that whenever deposition occurs, there is a difference of chemical affinity between the dissolving and receiving surfaces for 24 ACID AND BASIC AFFINITIES. the different elements of the liquid ; that the dissolving metal has a stronger affinity for the acid elements than the receiving one ; and that -without this condition no deposition occurs. 24. It must be mentioned also that the metals which have the greatest difference in their degrees of affinity for acids, are those which evolve the greatest strength of depositing power ; for instance — 1st. With One Metal and One Liquid. — If we immerse a piece of silver in a solution of terchloride of gold, it slowly becomes gilded, but if we immerse a piece of zinc in it, gold is almost instantaneously deposited ; because, in the former case, the difference of affinity between gold and the immersed metal for the acid of the liquid is very much smaller than in the latter case. 2nd. With Two Metals and One Liquid. — If we immerse a piece of platinum and a piece of copper, in mutual contact, in a solution of nitrate of silver, the platinum will become silvered, but much more slowly than if iron or zinc were used in place of the copper, because there is a greater difference of affinity between platinum and iron or zinc for the acid of the liquid, than between platinum and copper; or, if we immerse a piece of zinc and a piece of platinum or platinized silver in dilute sulphuric acid, and connect them separately with two pieces of copper immersed in a separate solution of sulphate of copper, copper will be dissolved and deposited in the separate liquid more rapidly than if we used zinc and copper in place of zinc and platinum, and much more rapidly than if we used iron and copper , because the difference of affinity be- tween zinc, platinum, or platinized silver for dilute sulphuric acid, is more than zinc and copper for that liquid, and much more than between iron and copper. 3rd. With Two Metals and Two Liquids. — If we immerse a piece of zinc in dilute sulphuric acid, and a piece of copper in a solution of sulphate of copper, the two liquids touching each other, and the two metals connected with two pieces of copper in a separate solution of sulphate of copper, the amount of metal dissolved and deposited in a given time in the separate liquid will be much smaller, than if we used zinc in dilute sulphuric acid, and platinum in strong nitric acid; because the difference of affinity between the two metals in the two liquids, in the first instance, is less than in the second instance. 25. Acid and Basic Affinities Necessary. — Chemical affinity differs not only in degree but also in kind ; basic substances, such as metals, alkalies, alkaloids, and most metallic oxides, have a great tendency to combine with acids ; whilst acid substances, such as the metalloids (oxygen, sulphur, chlorine, phosphorus, fluorine, &c), mineral and vegetable acids, and some metallic oxides, tend to combine with bases. In a similar manner, the difference of affinity between the dis- solving and receiving surfaces in electro-deposition is not wholly one of degree, but is also one of kind ; the dissolving metal in a separate depositing liquid (13) acquires, by virtue of its connnexion with some source of depositing power, an affinity for the acid elements of the liquid ; whilst the receiving metal acquires, by the same means, an affinity of an op- posite kind; for instance (Fig. 16), if a piece of platinum A and a piece of zinc B are immersed in dilute sulphuric acid C, some mercury D placed at the bottom of a separate solution of protosulphate of iron E, a piece of iron F immersec i a this liquid and connected ALTERNATION OF CHEMICAL AFFINITIES. 25 by a wire with the platinum, and the mercury connected by an iron or platinum wire G which is prevented from touching the liquid by a tube of glass or gutta-percha, the immersed piece of iron will exercise one kind of affinity, combine with the acid of the dissolved salt, and form a definite chemical compound (protosulphate of iron), containing one equivalent of iron and one equivalent of sulphuric acid ; while the mercury will exercise an opposite kind of affinity, and combine with the metal or base of the salt, forming likewise a definite chemical compound (Fe Hg), containing one equivalent of iron and one of mercury. If a solution of sulphate of copper is substi- tuted for that of sulphate of iron, and a piece of copper for the piece of iron ; a similar definite compound of copper and sulphuric acid is formed at the dissolving plate, and of copper and mercury (Cu Hg) at the receiving metal. These experiments prove in a most satisfactory manner that, in the act of deposi- tion, the surface of the dissolving metal possesses one kind of affinity by virtue of which it tends to attract acid substances, and combine with them in definite proportions ; and the surface of the receiving metal possesses an opposite kind of affinity, by •virtue of which it tends to attract and combine with basic substances, also in definite proportions. Mercury is the only metal which has been observed to manifest this definite affinity at the receiving surface, probably because it is the only metal fluid at ordinary temperatures, fluidity being an essential condition of such affinity ; but it is likely that other metals would also manifest this tendency, if kept in a melted state in contact with suitable fused salts, and properly acted upon by depositing force. 26. Fluidity Essential to Electro-Deposition. — The affinities of electro- deposition, like those of ordinary chemical action, require, generally speaking, one at least of the combining bodies to be in the liquid state ; and they act, like them, wholly at in- sensible distances, being confined in their exercise to the immediate surfaces of mutual contact in the opposed substances, and the compound formed at those surfaces becomes diffused through the fluid masses by capillary cohesion and mechanical mixture. This affords a reasonable explanation why fluidity of the receiving metal is essential to the formation of definite compounds at its surface, as well as why fluidity of the metallic salt is essential to its decomposition, and for the formation of definite compounds at the dissolving surface. If chemical action took place at sensible and considerable distances, i.e., throughout the whole mass of the opposed fluid oodics, combination would pro- bably be in all cases violent and instantaneous ; and if fluidity were not essential to combination, the substances deposited upon the receiving surface would probably, in most cases, enter into the mass of the receiving metal, and combine with it. 27. Alternation and Circulation of Chemical Affinities Necessary. — The next chemical condition to be observed upon is, that in every case of deposition the surfaces at which the acid and basic affinities are manifested alternate with each other in the circuit, and that the acid affinity circulates one way in the circuit, while the basic affinity circulates in the opposite direction ; for instance — 1st. With One Metal and One Liquid. — If we immerse a piece of copper in a solu- tion of double cyanide of silver and potassium, it becomes silvered ; but if we immerse a piece of iron therein, it receives no deposit. In the first of these instances it is considered that, immediately upon the immersion of the metal, the superior affinity of copper for cyanogen over that of silver, causes it to combine -with that substance, and set the silver free ; at the same time, an immense number of minute currents of depositing force or chemical affinity are developed all over the immersed surface of the pioce of copper, leaving it at innumerable minute points, passing a very small 23 CIRCULATION OF CHEMICAL AFFINITIES. distance into the liquid, and re-entering the copper at numberless other points ; and thus the affinities circulate, the copper dissolves, and receives a deposit simultaneously. In each of these atomic circuits as they are termed, acid affinity is exercised where the depositing force leaves the metal, and basic affinity where it re-enters it ; but in the second instance, where iron is used, there is no circulation of those affinities, no dissolving of metal at one point, or deposition of it at another. 2nd. With Two Metals and One Liquid. — If we immerse a piece of iron and a piece of copper, in mutual contact, in a solution of sulphate of copper, a deposit of copper takes place upon the iron and upon the copper ; but if we immerse a piece of gold in place of the iron, no deposit occurs upon either. In the first of these instances, in addition to the circulation of atomic currents of affinity all over the immersed sur- face of the iron, as already explained, and which cause it to dissolve and receive a deposit, there are separate and distinct currents of the same force circulating through the liquid, and the two metals by their points of mutual contact, cause the surface of the iron to combine with the acid, and that of the copper to receive a metallic deposit ; but in the second case there is no circulation of affinities, and no solution of the gold or deposition of the copper. 3rd. With One Metal and Two Liquids. — If we immerse two pieces of silver, one in a solution of cyanide of potassium, and the other in a solution of double cyanide of silver and potassium, the two pieces being connected together by a wire, and the liquids touching each other by a porous partition, chemical affinities will circulate through the metals, wire, and liquids, and silver will be dissolved and deposited ; but if we substitute pieces of iron for the pieces of silver, there will be no circulation of affinities and no deposition. 4th. With Separate Depositing Liquids (Fig. 17), consisting of solutions of sul- Fig. phate of copper, with separate pairs of copper wires C D, E F, G H, immersed in them, and with end pieces of copper 13 and I, B being connected with a piece of zinc J immersed in dilute sulphuric acid, and I connected with a piece of copper A immersed in a solution of sulphate of copper, the two liquids being separated by a porous diaphragm K, chemical affinities will circulate in opposite directions through the whole of the circuit, every alternate piece of immersed metal, J, I, G, E, and C, will exercise one kind of affinity and dissolve, and every other alternate piece, A, B, D, F, and H, will exercise an opposite kind of affinity, and receive a deposit of copper. If in either of the foregoing instances, where deposition occurs, we break the con- tinuity of the circuit, either by separating the metals from each other at their points CURRENT OF AFFINITY. 27 of contact, lifting them out of the liquid, or by cutting through the connecting wires, no deposition will occur ; but if we immerse the free ends of the divided wire in a suitable liquid, such as a solution of sulphate of copper, if the wires are of copper, deposition will immediately recommence throughout the circuit. 28. A consideration of such facts as these, leads us to conclude that the process which causes every alternate metal in a series to combine "with acids and dissolve, and every other alternate metal in the circuit either to combine with bases, or receive a deposit, are of a chemical character, and circulate in opposite'directions through the circuit, and has led to the application of the term " current affinity," to designate the depositing force, when viewed only in a chemical aspect. 29. Two or any other even number of vessels containing combinations of liquids and metals, each similar, and capable of generating current affinity, may be so arranged in a complete conducting circuit, that no current affinity will circulate or deposition occur ; for instance, if we take two separate vessels A and B (Figure 18), each con- taining dilute sulphuric acid, and a separate piece of zinc Z, and silver (S), and connect them to- gether thus, zinc, silver, silver, ziDc, no depositing power will be manifested if we immerse the free ends of their copper wires C C in a separate solu- tion of sulphate of copper D ; but if they are connected thus, zinc, silver, zinc, silver (Figure 19), and the copper wires im- mersed as before, current affinity will circulate, and deposition willl" proceed in the separate liquid. In the first of these cases the affinities set in motion by the metals and liquid in the vessel A are opposite in direc- tion to those generated in vessel B, and the two arrangements, being equal in power, exactly neutralize each other, preventing the currents of affinity and their effects ; but in the second case the direction of the affinities evolved by the vessel A coincide with that of those from vessel B, the currents circulate, and deposi- tion is effected. In the case of an unequal portion of the metals of a series being con- nected the opposite way, those which are wrongly connected will neutralize and be neutralized by an equal number of the remainder, provided that all the pairs of metals andt he liquids arc similar; for instance, if three out of twelve arc wrongly connected, they will neutralize the power of three more, and only the remaining six will act in the desired direction. 30. During the act of deposition, a salt is generally formed at the dissolving surface by the union of the metal with the acid elements of the liquid, which is dis- 23 FREE ACIDS IN DEPOSITION. solved in the liquid ; at the same time the acid, which combines with the dissolving surface, is generally set free at the receiving surface by the deposition of the metal ; for instance — 1st. With One Metal and One Liquid. — When iron coats itself with mercury by simple immersion in a solution of nitrate of mercury, nitrate of iron is formed by the union of the iron with the nitric acid or of the nitrate of mercury, and nitric acid is at the same time set free by the deposition of the mercury. 2nd. With Two Metals and One Liquid. — "When copper receives a deposit of copper by immersion, in contact with a piece of iron in a solution of sulphate of copper, sulphate of iron is formed at the immersed surface of the iron, by the union of the sulphuric acid, or the sulphate copper salt, with the iron ; at the same time sulphuric acid is set free at the copper surface by the deposition of the copper. 3rd. With One Metal and Two Liquids. —"When copper receives a deposit of copper in a solution of sulphate of copper, by connection with another piece of copper im- mersed in dilute sulphuric acid, the two liquids touching each other by means of a porous partition or otherwise, sulphate of copper is formed at the surface of the metal in the dilute acid, by the union of the copper with the acid, and, at the same time, sulphuric acid is set free at the surface of the other piece by the deposition of the copper. 4th. Silver.— "When silver or any other metal receives a coating of silver in a solution of double cyanide of silver and potassium, by connection with a piece of zinc in dilute sulphuric acid, the two liquids being separated by a porous diaphragm, sulphate of zinc is formed at the surface of the zinc, by the union of the acid and zinc, and cyanogen (a substance of acid character) is at the same time set free at the receiving surface, by the deposition of the silver. 5th. With a Separate Depositing Liquid, connected with any source of current affinity— for instance, when two silver plates, immersed in a solution of the double cyanide of silver and potassium, are connected by wires with a piece of zinc, and a piece of copper is immersed in dilute sulphuric acid in a separate vessel ; cyanide of silver is formed at the surface of one of the pieces of silver by the union of that metal with the cyanogen of the liquid ; and, at the same time, either cyanogen or hydro-cyanic acid is set free at the surface of the other piece by the deposition of the silver. 81. Proper Proportion of Free Acid in the Depositing Liquid. — If a solution contains a large excess of uncombined acid, metallic deposition will not always occur ; for instance, if two pieces of zinc are immersed in a neutral solution of sulphate of zinc, and connected by wires with another piece of zinc, and a piece of silver is immersed in dilute sulphuric acid, in a separate vessel, one piece of zinc will dissolve and the other receive a deposit of metal ; but if a rather large quantity of sulphuric acid is added to the depositing liquid, no deposit of zinc will occur. In the same manner, if we connect two pieces of silver, immersed in a strong solution of cyanide of potassium, with the zinc and silver in dilute sulphuric acid, as just described, one of the pieces of silver will combine with the cyanogen of the cyanide of potassium, and form cyanide of silver, which will combine with a portion of the remaining cyanide and then dissolve ; but the other piece of silver will not receive a deposit of silver, until the remaining uncombined cyanide of potassium has decreased to a certain proportion by the working of the process. If, on the other hand, a depositing solution contains no free combining substance, deposition will either ELECTRICAL CONDITIONS OF DEPOSITION. 29 proceed very slowly, or be entirely stopped, in consequence of an insoluble salt being formed upon the surface of the dissolving metal, and impeding the action; for instance, when two silver plates, immersed in a solution of double cyanide of silver and potassium, are connected with some source of depositing power, one of the plates will receive a deposit of silver, whilst the other will gradually become covered with a white layer of insoluble cyanide of silver, which impedes and eventually stops deposition. 32. Necessity of a Proper Proportion of Water.— If a depositing solution is diluted with water to a very large extent, deposition will progress very slowly, but if, on the other hand, it contains insufficient water, crystals of metallic salts will collect upon the dissolving metal and gradually stop the action ; for instance, if two pieces of copper, immersed in a saturated solution of sulphate of copper containing free acid are connected with a piece of zinc, and a piece of silver immersed in dilute sulphuric acid in a separate vessel, one piece will receive a deposit of copper, wbllst the other will slowly dissolve and gradually become covered with crystals of sulphate of copper, first at its lower part and then at the edges, which will gradually stop the action. 33. All the foregoing chemical facts prove that chemical affinity plays a very important part in the phenomena of electro-deposition. 34. Electrical Conditions of Deposition Positive and Negative Substances Necessary — The first electrical condition to be observed is, that in every case of deposition the liquid contains both substances of an electro-positive and of an electro-negative character; metals and alkalies are electro-positive, and metalloids (oxygen, sulphur, chlorine, iodine, bromine, &c.) and acids are electro- negative ; cyanogen is also electro-negative. 1st. With One Metal and One Liquid. — In the first instance, if we immerse a piece of copper in a solution of nitrate of mercury, deposition occurs, mercury being deposited, but if we immerse it in dry mercury, there is no deposition ; in the former case the liquid contains both electro-positive mercury and negative nitric acid, but in the latter case it only contains the positive mercury. 2nd. With Two Metals and One Liquid. — Immerse zinc and platinum in mutual contact in a solution of nitrate of mercury, the platinum receives a deposit ; but if we immerse them in dry mercury, it receives no deposit. 3rd. With One Metal and Two Liquids.— If. one end of a platinum wire is immersed in a solution of cyanide of potassium, and the other end in a solution of nitrate of mercury, the two liquids touching each other by a porous partition, the end in the mercurial solution will receive a metallic deposit ; but if dry mercury be substituted for the nitrate solution, no deposition will occur. 4th. With Two Metals and Two Liquids. — If we immerse zinc in dilute sulphuric acid, and platinum in a solution of nitrate of mercury, the metals touching each other, and the liquids separated by a porous partition, the platinum receives a mer- curial deposit ; but if dry mercury is substituted for the nitrate solution, it receives no deposit. 5th. With a Separate Depositing Liquid. — If we immerse two platinum wires in a solution of nitrate of mercury, and connect them with any source of depositing power, one of the wires will receive a metallic deposit ; but if we use dry mercury instead of the nitrate solution, there will be no deposit: These and many other instances prove that unless the depositing liquid contains both electro-positive and electro-negative substances, no deposition occurs. 30 ELECTRICAL CONDITIONS OF DEPOSITION. 35. Electric Polarity of the dissolving and Heceiving Metals. — The second electrical condition to be observed upon is, that in every case of deposition there is an electrical difference between the dissolving and receiving metals, and that the former is always electro-positive, the latter being electro -negative, relatively to each other ; the dissolving metal, consequently, has the strongest electrical attractions for the electro-negative or acid elements of the liquid, and the receiving metal has the strongest electrical attraction for the electro-positive or metallic elements ; for instance — 1st. With One Metal and One Liquid.— Iron in a solution of sulphate of copper, being electro-positive to the copper of the salt, has a stronger electric attraction for the acid or electro-negative elements than the copper, and combines with them in setting the copper free ; but in a solution of sulphate of zinc, iron being electro-negative to the zinc of the salt, has a weaker attraction for the acid than the zinc, and therefore does not combine with or set the zinc free. 2nd. Tivo Metals and One Liquid. If we immerse a piece of iron and a piece of copper in a solution of sul- phate of copper, and connect them by wires with a galvanometer (Fig. 20), the copper will receive a deposit^ and the needles N of the instru- ment will be strongly deflected in such a direction as to indicate that i the iron is positive, and the copper | negative ; but if a piece of iron and a piece of zinc are immersed in a solution of sulphate of zinc, and connected with the instrument, no deposit will take place upon either needles will be produced. 3rd. With One Metal and Two Liquids. — If one piece of copper is immersed in dilute sulphuric acid, and another in a neutral solution of sulphate of copper, the two liquids touching each other by a porous partition, and the pieces of copper connected by wires with the galvanometer, deposition will take place upon the piece of copper in the metallic solution, and the instrument will indicate, by the direction of the deflection of its needles, that the dissolving piece of metal is positive, and the receiving piece negative ; but if we substitute two pieces of platinum for the pieces of copper, no deposition of copper will occur, and scarcely any difference of electric condition between the two pieces of metal will be indicated by the galvanometer. 4th. With Two Metals and Two Liquids. — If a piece of zinc is immersed in dilute sulphuric acid, and a piece of copper in a solution of sulphate of copper, the two liquids being separated by a porous partition, and the metals connected with the galvanometer, deposition will take place freely upon the copper, and the needles of the instrument will be powerfully deflected, indicating, by the amount and direction of their move- ment, the zinc to be strongly positive and the copper negative ; but if a piece of plati- num is substituted for the zinc, there will be no deposition, and scarcely any deflection of the needles. 5th. With a Separate Depositing Liquid. — If we immerse two pieces of silver in a solution of the double cyanide of silver and potassium, and connect them with any metal, Fig. 20. and (scarcely any deflection of the ALTERNATION OF THE ELECTRO-CIRCUIT. ol .source of depositing power, and interpose a galvanometer in the circuit, deposition of silver will occur, and the needles will be strongly deflected in such a direction as to indicate the dissolving piece of silver to be positive, and the receiving piece negative ; but if we substitute two pieces of iron for the pieces of silver, there will be scarcely any deposition of silver, and very little deflection of the needles, the receiving piece being feebly negative. From these and many other similar cases, we conclude that in every case where deposition occurs, the dissolving metal is electro- positive, and the receiving metal electro-negative, relatively to each other, and that the former has the strongest electric attraction for the negative elements of the liquid, and the latter for the basic or metallic element. In all electric phenomena, positive sub- stances repel positive and attract negative, and negative substances repel negative and j attract positive. 36. Alternation and Circulation of Electro-Polarities Necessary. — The third electric condition is, that in every case of deposition, the positive and negative surfaces, or metals, alternate with each other in the circuit, and electric forces circulate through the circuit ; for instance — 1st. With One Metal and One Liquid. — "When a piece of iron is immersed in a solution of sulphate of copper, innumerable minute electric currents arc generated upon the surface of the metal, and circulate to a very minute depth within the opposed surface of the liquid, while the positive electricity passing out of innumerable points of the iron into the solution causes it to dissolve — pass through the solution, re-enter the iron at innumerable other points, and deposit the copper ; but when a piece of gold is immersed in this solution, there are no currents of electricity generated, and the gold is neither dissolved nor does it receive a deposit. 2nd. With Two Metals and One Liquid.— "When a piece of zinc and a piece of silver are immersed in mutual contact in a solution of nitrate of silver, the zinc dissolves and the silver receives a deposit, and an electric current is developed which circulates through the metals and liquids, and may be rendered further evident by connecting the metals with a galvanometer (Fig. 20) ; the zinc is positive and the silver negative. 3rd. With One Metal and Two Liquids.— When two pieces of copper are immersed, one in dilute sulphuric acid, and the other in a solution of sulphate of copper, the two liquids touching each other through a porous diaphragm, and the pieces of copper connected with a galvanometer, an electric current circulates through the circuit, the piece of copper in the acid is positive and dissolves, and the piece in the metallic solution is negative and receives a deposit. 4th. With Two Metals and Two Liquids.— When a piece of zinc is immersed in dilute sulphuric acid, and a piece of silver in a solution of double cyanide of silver and potas- sium, and the two are connected with a galvanometer, the two liquids touching each other by a porous partition, electricity circulates through the circuit, the zinc is positive and dissolves, and the silver is negative and receives a deposit. 5th. With a Separate Depositing Liquid.— "When two pieces of antimony immersed in the ordinary chloride of antimony, are connected with any source of electricity, one piece becomes positive said dissolves, and the other negative and receives a deposit, and an electric current circulates through the circuit, as may easily be proved by the galvanometer, as in the foregoing instances. 6th. With a Series of Depositing Liquids. — If a series of depositing vessels (Figs. 15 and 17), containing solutions of sulphate of copper, and pieces of copper, are connected by their extreme pieces with a sheet of zinc, and a sheet of copper immersed 32 EFFECTS OF ELECTRO-DECOMPOSITION OF LIQUIDS. in a solution of sulphate of copper, every alternate piece of metal in the series -will be electro-positive and dissolve, and every other alternate piece electro-negative and receive a deposit, and a current of electricity will circulate through the whole series. 37. Electrical Conducting Circuit Necessary. — In every case where deposi- tion is proceeding, the whole of the circuit is capable of conducting voltaic electricity ; and this is one important condition of the result, for if in any such case we interpose an imperfect electric conductor in the circuit, such as a long iron wire, or a short column of water, the process will be greatly impeded, and if we interpose a non-conductor of electricity, such as a rod of glass or of gutta-percha, or allow the least film of air to break the continuity of the circuit, deposition will be completely arrested. 38. Electric Conductivity, — The following is the general order of the con- ductivity of metals and alloys for voltaic electricity at 60° Fah., beginning with those which conduct most freely, viz., silver, copper, gold, cadmium, zinc, brass, tin, palla- dium, iron, steel, lead, platinum, german silver, antimony, mercury, bismuth, potas- sium. The order is somewhat different at other temperatures. 39. Direction of the Electric Currents. — In every case where a current of electricity is developed by the mutual contact of liquids and metals, or where it merely passes through them, as in a separate depositing vessel, the current of positive electricity invariably passes from the positive or dissolving metal, through the liquid, to the negative or receiving one ; and the negative electricity passes similarly in the opposite direction. When we speak of " the current" without stating which is meant, — the positive or negative electricity,— it is always intended, for the sake of simplicity of expression, to indicate the positive electricity ; when we speak of the positive metal or plate, the metal which is positive and dissolves is meant ; but when the positive pole is mentioned, the metal from which the positive electricity proceeds out of the arrangement or apparatus into the wires, and which is invariably the negative or receiving metal, is intended ; for instance, if a piece of zinc and a piece of silver or copper are immersed in dilute sulphuric acid, the zinc is the negative pole and the positive plate, whilst the copper or silver is the positive pole and the negative plate. 40. In a separate depositing vessel it is the dissolving metal which is called the positive plate, because it corresponds to the zinc or dissolving plate of the battery, while the receiving metal is called the negative plate ; the term pole is also sometimes applied to those plates, but in an irregular manner. 41. Electrical Decomposition of Liquids, — In all cases of electro-deposition, the elements of the liquids are split asunder by electric action at the surfaces of the metals ; the electro-negative elements, such as metalloids and acids, either combine with, or are set free at the surface of the dissolving or positive metal, and the electro- positive elements, such as metals and alkalies, either combine with, or are set free or deposited, at the surface of the receiving or negative metal ; for instance, if a piece of silver and a piece of copper are immersed in a solution of sulphate of copper, and a piece of zinc and a piece of platinum immersed in dilute sulphuric acid, the silver connected with the zinc by one wire, and the copper with the platinum by another wire, the negative elements of the liquid, namely, the sulphuric acid of the sulphate of copper solution, will be split from its associated copper, and will combine with the positive metal, the copper, causing it to dissolve in the liquid ; while the positive element of the liquid, namely, the copper of the salt, will be deposited at the surface of the negative or receiving metal, the silver, but will not combine with it ; but if we substitute a piece of platinum for the piece of copper, and mercury for the silver (Fig. 16), the effects ELECTRICAL TERMINOLOGY. 33 will be reversed, the acid or negative element will collect around the positive platinum, but will not combine with it, whilst the positive element of the liquid, the copper, will be deposited and combine with the negative mercury. Fused salts yield the same substances by electric decomposition as the same salts yield dissolved in water. 42. Electrical Terms in Deposition. — For the more clear remembrance of the different parts of the circuit and of the direction of the electric forces, ami for the better understanding of the action of the currents, Faraday has proposed the following terms, which have come into general use : — The liquid undergoing decomposition he terms an " electrolyte," from two Greek words, "electron," meaning "electric," and u luo," to " set free ;" the act of electric decomposition he terms "electrolysis;" the metallic or other surfaces at which the electric forces enter and leave a liquid he terms " electrodes" from two Greek words, " electron" and "odos," meaning a " way ;" the positive electrode, or that point at which the positive electricity enters a liquid, is termed " anode" from two Greek words, " ana," meaning " upwards," and " odos," a " way," — the way in which the sun rises ; and the negative electrode, or that by which the positive electricity leaves a liquid, is termed " cathode" from two Greek words, " cata," meaning " downwards," and " odos," a " way," — the way in which the sun sets ; the ele- ments of the liquid set free by electrolysis he terms " ions" from a Greek word meaning <' going ;" those which combine with, or are set free at the anode, arc termed " anions ;" and those which combine with, or are set free at the cathode, are termed " cations" 43. Anions and Cations. — Under the head of Anions may be classed — oxygen, fluorine, chlorine, bromine, iodine, and cyanogen, probably also sulphocyanogen, also the various mineral acids. Cations include — hydrogen (and ammonium), the alkali metals^ magnesium, manganese, arsenic, antimony, bismuth, zinc, cadmium,tin, lead, iron, cobalt, nickel, copper, mercury, silver, gold, platinum, palladium, and the salifiable bases. 44. Electro-chemical Scale. — The various elementary substances have been arranged by Berzelius according to their relative degrees of positive and negative electro-chemical character, in a table or scale like the accompanying one, commencing with those substances possessing the strongest electro-positive properties, and ending with those of the strongest electro-negative properties: — POSITIVE END. XAME. SYMBOL. NAME. SYMBOL. , . Te. NAME. SYMBOL. Potassium . . . . K. Tin .... . . . Sn. Carbon .... . . C. . . . Bi. . . B. Lithium . . . L. Tungsten . . . . . w. Molybdenum . . . . Mo. Strontium. . . . Sr. . . . Ag. . . Va. Mercury . . . . . Hg. Chromium . . . . Cr. Magnesium . . . Mg. Palladium . . . Pd. Glucinum . . , * • G. Phosphorus . . . . P. . . Y. Platinum . . . . . Pt. . .1. . . . Os. Br. . . Th. . . . Au. . . CI. Cadmium . . . . Cd. Fluorine . . . . . Flj Manganese . . . Mn. Zinc. . . . . . Zn. i S. Tit. Tantalum . . . . • Ta. NEGATIVE END. PRACTICAL CHEMISTRY.— No. II. 34 INFLUENCE OF TEMPERATURE. It -will be observed that the division indicated in the above table between gold and hydrogen is, in a great measure, an arbitrary one ; useful to assist one's memory in recollecting the general electro-chemical character of the substances, but not really existing in nature ; for instance, sulphur and chlorine, two of the most negative of substances, must be viewed as positive in relation to oxygen ; a still more negative sub- stance when combined with that element, as hyposulphurous or hypochlorous acid, but negative in relation to arsenic, hydrogen, zinc, potassium, when combined with those elements, in the various metallic sulphides and chlorides ; in fact, each substance throughout the scale may be viewed as both positive and negative, — positive in relation to those below it, and negative in relation to those above it ; those of the upper end being strongly positive and feebly negative, and those of the lower end strongly negative and weakly positive. It has been objected that sulphur and nitrogen occupy a position too near the negative end of the scale, they being generally less negative than chlorine and fluorine ; also that hydrogen should be placed higher up in the positive division. 45. A consideration of all the foregoing facts leads m to conclude, that current electricity acts a most important part in all the phenomena of electro-deposition. 46. Thesrmic Conditions. — Under this head comparatively little can be said, because it is that portion of the subject which has been the least investigated ; but it is highly probable that, as heat is generally evolved by the chemical combination of metals with metalloids or with acids, there exist thermic as well as electric and chemical conditions of deposition. In a series of experiments with two plates of anti- mony immersed in a conducting liquid, the two plates being maintained at different temperatures, and numerous liquids being tried, a weak current of electricity was developed, which passed from the hot metal through the liquid to the cold one, the hot metal thus being positive : in all cases except with liquids containing uncom- bined nitric acid. 47. Influence of Temperature on the depositing Liquid. — The strength of affinity between the different elements of a liquid undergoing electrolysis, varies with the temperature of the liquid ; being almost invariably diminished by elevation of temperature. Rise of temperature increases the electric conductibility of an electrolyte, and decreases that of the metal plates immersed in it ; but the decrease of conducti- bility of the latter is small in proportion, at moderate elevations of temperature, compared with the increase of the former ; consequently the general effect of heating a depositing liquid is to increase the rapidity of deposition. 48. "We have repeatedly observed, that with some solutions used at a high tempera- ture for depositing, if the cathode was immersed in the liquid at the ordinary atmos- pheric temperature, and the liquid then heated to the desired point, no conduction or deposition took place ; nor did it occur if the receiving metal was taken out, washed in cold water, and re-immersed ; but if the temperature of the liquid was first raised^ and then the cold cathode suddenly immersed, deposition took place freely, and the liquid might be cooled down many degrees without stopping the action. In coating iron with tin in some solutions, if the iron was immersed before heating the liquid, no deposition took place even at 150° Fah. ; but if the liquid was first heated, deposi- tion occurred below 100° Fah. 49. Influence of light upon deposition. — Light appears to exercise much less influence upon electro-deposition than heat ; in some cases, however, where the elements of a depositing liquid are held together by unstable affinities, it decomposes MECHANICAL CONDITIONS OF DEPOSITION. 35 the liquid and renders it unfit for deposition ; for instance, a solution formed by dis- solving hyposulphate of silver in a solution of hyposulphite of soda, has a tendency to be decomposed in this way. 50. — Dynamic o* Mechanical Conditions. — The various phenomena occur - ing in a liquid undergoing electrolysis, may be viewed, in a mechanical or dynamic aspect, as a series of minute movements (attractions and repulsions) occurring between the various particles of matter composing the opposed surfaces of the liquids and of the metals immersed in them ; for instance, if we immerse a piece of zinc in a solution of sulphate of copper and connect it with some mercury in the same liquid, by a platinum wire, protected from the solution by a tube of glass or gutta-percha, the particles composing the surface of the zinc, being all electro-positive, will tend to repel each other, and the particles of the liquid surface in contact with it, being rendered electro-negative, will also tend to repel each other, whilst the particles of acid near? being electro-negative, will attract the particles of zinc, which are electro-positive, and the two will combine together and form a salt ; at the same time, the particles of the liquid surface in contact with the mercury, being made electro-positive, will tend to repel each other ; while the particles of the opposed mercury surface, being rendered electro- negative, will also tend to repel each other, whilst the particles of the mercury, being electro-negative, and the contiguous particles of copper in the liquid being electro- positive, the two will combine together and form an alloy. The deposition of copper upon the xine in this case must be wholly disregarded, because it is quite a separate and distinct phenomenon. To put this in a clearer form, suppose (Fig. 21) the vertical row of particles Cu, Cu, Cu, Cu, to represent the copper anode of a sulphate of copper depositing liquid, and the row of particles Hg, Hg, Hg, Hg, the mercury cathode. Pt, Ft> being the connecting wires from the battery, and the double row representing the particles of acid SO 4 , and copper Cu, composing the intervening liquid ; tne par- ticles of the anode surface, being all electro-positive, tend to repel each other ; and the contiguous particles of Cu and SO 4 , being Fi? 2 i. all negative, also tend to repel each other, whilst the particles of the copper anode, being positive, attract the nearest particLs cf negative acid, and combine with them and form a salt; at the same time, the particles of Cu and SO 4 nearest the mercury, being all positive, tend to repel each other, whilst the contiguous particles of mercury being negative, and the copper in the iquid being positive, attract and combine with each other and form an alloy. By this combination of simultaneous movements, the copper anode dissolves, and the mercury (or any other conducting substance which forms the cathode) receives a deposit, and the particles of copper of the liquid are gradually removed and replaced by those from the anode. 51. These attractions and repulsions like ordinary chemical actions, are all supposed 36 THEORY OF ELECTRODES. to take place at insensible distances, at the mutually opposed surfaces of the liquids and metals, and not to extend into their masses, except so far as they are mixed with each other by capillary attraction or ordinary mechanical motion, and can therefore only take place where one or both of the substances are in a liquid state ; if it were otherwise, the combinations and decompositions of the whole masses would probably occur instantaneously. 52. In addition to those minute and invisible movements of the particles, there are other and sometimes visible movements produced by capillary attraction and by differ- ence of specific gravity in the liquids ; for instance, the salt formed at the anode, if it is soluble in the liquid, is dissolved and gradually diffused through it by capillary attraction or adhesion, whilst from its greater specific gravity than the remainder of the liquid when dissolved, it tends to sink towards the bottom ; at the same time, the acid set free at the cathode is likewise gradually diffused through the liquid by similar means, and from its less specific gravity tends to rise to the surface. These movements are of general occurrence during deposition, and in some solutions, especially if they are very dense and possess a colour, are plainly visible to the unassisted eye ; and their occur- rence explains why the substances set free by deposition are not instantaneously transferred from one electrode to the other, but occupy, especially in dense liquids and electrodes far asunder, a considerable period of time in their transference ; it also explains why, if depositing solutions are not occasionally stirred, their upper portions become exhausted of metal, whilst their lower parts become deficient of acid. Motion of the cathode is generally considered necessary to make the deposited metal harder. 53. Position of the Electrodes. — The position of the electrodes has a consider- able influence upon the phenomena of electro-deposition. For instance — 1st, If the two electrodes in a depositing liquid are horizontal, with the anode above and the cathode below, the salt formed at the anode will, by virtue of its greater specific gravity, sink in the liquid, whilst the acid set free at the cathode will, by its less specific gravity, rise upwards, and thus the anode will be constantly supplied with fresh uncombined acid, the cathode will receive a constant supply of metallic salt, and deposition will continue without interruption. 2nd, If the two electrodes are vertical in the liquid, similar differences of specific gravity will cause the lower part of the liquid to become saturated with metallic salt, and its upper part to consist of free acid mixed with the water ; in consequence of this, the current of electricity will almost wholly pass from the upper part of the anode diagonally downwards, through the liquid to the lower part of the cathode, and thus the upper part of the anode will dissolve rapidly, whilst its lower part will dissolve but slowly, and the cathode will receive a rapid deposit at its lower part and but very little at its upper part. In this position, vertical lines, and even deep grooves, are sometimes produced in the deposit (especially if the position of the cathode is slightly overhanging), by the ascent of streams of the lighter acid liquid from which the metal has been exhausted by deposition ; if the solution is nearly a saturated one, and has been freely worked without stirring or disturbance for some time, crystals of the metallic salt are apt to form all over the lower part of the anode, which will be dissolved very rapidly at the surface of the liquid, and appear as if cut by a knife ; in addition to these effects, if the solution is a very deep one, with much free acid, two independent currents of electricity will be developed, one in each electrode, by the unequal action of the two different strata of liquid upon their upper and lower parts. An instance of this will be found in u One Metal in Two Liquids" (9), MATHEMATICAL CONDITIONS OF DEPOSITION. 37 in which these currents leave them at their upper parts, pass down through the liquid, and re-enter them at their lower extremities. 54. Form of the Electrodes.— If either of the electrodes he of an irregular form, or have unequal projections or hollows, the anode will dissolve most freely, and the cathode receive the greatest deposit of metal at those parts at which they are nearest each other, and least in the hollows and more distant parts. If the surface of the oathode be rough, it greatly increases the tendency of the deposit to become crystalline. 55. Mathematical Conditions.— Definite Chemical Action.— In the combination of different elementary and compound substances with each other, by ordinary chemical affinity, it has been observed, and accurately proved by analysis, that their combinations invariably take place in certain definite proportions, or in very simple multiples of those proportions ; and as in the first of these cases it is inferred that their combinations with each other occur, one atom with one atom, the numbers which represent those simple proportions represent also the relative weights of those atoms to each other. The following table contains the names, symbols, and atomic weights of nearly all the simple or elementary substances : — Name. Symbol. Atomic weight. Name. Symbol. Atomic ■weight. Name. Symbol. Atomic weight. Hydrogen . . II . .. l-o Cobalt . . . Co . .. 29-5 Uranium . U .. 60-0 Beryllium . . Be .. 4-7 Nickel . . . Ni . .. 29-6 Tellurium . Te .. 64-2 Carbon . . . C .. G-0 Copper . . Cu .. 317 Barium . Ba .. 68-5 Lithium . . Li .. 6-5 Phosphorus P . .. 32-0 Vanadium . Va .. 68-6 Oxygen . 0 . .. 8-0 Zinc . . . Zn . .. 32-6 Arsenic . As . .. 750 Boron . . . B . .. 10-9 Chlorine . . CI . .. 35-5 Bromine . . Br . .. 80-0 Magnesium . Mg .. 12-2 Potassium K . .. 39-2 Tungsten . \V . .. 950 Aluminium . Al .. 137 Selenium . Se . .. 39 5 Platinum . . Pt . .. 98-7 Nitrogen . . N .. H-0 Strontium . Sr . .. 40-8 Iridium . . Ir . .. 99-0 Sulphur . S .. 1G-0 M olybdenum Mo . .. 46-0 Osmium . . Ob . .. 99-6 Fluorino . . f\ .. 18-9 Lanthanum . La . .. 47-0 Mercury . Hg . .. 1000 Calcium . Ca .. 20-0 Cerium Cc . .. 470 Lead . Pb . .. 103-7 Silicium . Si .. 21-3 Didymium D .. 50-0 Silver . • Ag .. 108-1 Zirconium . . Zr ... 22-4 Rhodium . R .. 522 Iodine . . I .. 127-1 Sodium. . . Na .. 23-0 Ruthenium Ru .. 52*2 Antimony . Sb .. 129-0 Titanium . . fl ... 25-0 Palladium . Pd .. 53-3 Tantalum . Ta .. 184-0 Chromiun . . Cr ... 26-7 Cadmium . Cd .. 56-0 Gold . . . Au .. 197-0 Maganese . . Mn ... 27-6 Tin . . . Sn .. 59-0 Bismuth . . Bi .. 213-0 Iron . . . . Fe ... 28-0 Thorium . Th .. 596 50. As there arc a number of compound substances used in electro-deposition, such as sulphuric acid, cyanide of potassium, &c, and it will be useful to the practical depositor to know their combining proportions in making the different salts, used in the art, we have selected those which are likely to be required, and give their names, symbols, and atomic weights or combining proportions : — Water, HO ; 9-. Common Oil of Vitriol, specific gravity, 1-848 ; SO 3 , HO ; 49-. Strongest Hydrochloric Acid, sp. gr. T21 ; HC1, GHO; 90-5 (i.e., 90 and 5-tenths). Strongest Nitric Acid, sp. gr. 1*52 ; NO 5 , 2HO; 72-. Sesquicarbonate of Ammonia (Sal- vola- tile) ; 2 NIP, 3C0 2 , 2HO; 118\ Hydrochlorate of Ammonia (Sal-ammo- niac) ; NH 3 , HC1 ; 53-5. Hydrate of Potash (fused Caustic Potash ; KO, HO ; 50 2. Crystallized Carbonate of Potash ; KO, GO 2 , HO ; 51-2. Carbonate of Soda, (ordinary washing Soda) ; NaO, CO 2 , 10HO ; 143-2. Chloride of Sodium ; Na CI : 58*5. 38 DEFINITE CHEMICAL ACTION. Caustic Lime ; CaO ; 28*. Calcined Magnesia ; MgO ; 20*2. Ordinary Carbonate of Magnesia ; MgO, CO 2 , HO ; 51.2. Oxide of Zinc ; ZnO ; 4-6. Commercial Sulphate of Zinc (White Vitriol) ; ZnO, SO 3 , 7HO ; 143 2. Sesquioxide of Iron (Crocus, Colcothar) ; Fe 2 0 3 , 80-0. Commercial Sulphate of Iron (Green Vitriol) ; FeO, SO 3 , 7HO ; 138*. Protoxide of Copper (Black Oxide of Copper) ; CuO ; 39-7. Oxide of Silver; AgO ; 116*1. Commercial Sulphate of Copper (Blue Vitriol) ; CuO, SO 3 , 5HO ; 125\ Chloride of Silver ; Ag CI ; 143-6. Nitrate of Silver; AgO NO 5 ; 170*1. Oxide of Gold ; AuO ; 205'. Terchloride of Gold; Au CI 3 ; 303-5. " Bichloride of Platinum ; Pt CI 2 ; 169 '7. Cyanogen; Cy ; 26*. Cyanide of Potassium ; KCy ; 65-2. Cyanide of Zinc ; Zn Cy ; 58 6. Sesquicyanide of Copper ; Cu 3 , Cy 2 ; 148*. Cyanide of Mercury; Hg Cy ; 126". Cyanide of Silver; Ag Cy; 134-1. Cyanide of Gold ; Au Cy ; 22-3. 57. Definite Electro- Chemical Action. — The chief mathematical condition is, that in every case of electro-deposition, all the actions, both of combination and decom- position, take place in certain mathematical proportions, that is, according to the relative atomic weights of the substances combining or being decomposed. For instance : — 1st. With One Metal and One Liquid. — If a piece of pure iron is immersed in a solution of sulphate of copper, it is dissolved, and copper is deposited ; and for every 28 parts, or one atomic combining equivalents of iron dissolved, 317 parts, or one atom, of copper are deposited, and 49 parts (one equivalent) of hydrate of sulphuric acid (common oil of vitriol) are separated from the copper, and combine with the iron, forming therewith one equivalent of protosulphate of iron. 2nd. With Two Metals and One Liquid. — If a piece of zinc and a piece of silver in mutual contact are immersed in a solution of nitrate of silver, the zinc will dissolve and the silver receive a deposit; and for every 108-1 parts of silver deposited, 32*6 parts of zinc will be dissolved. 3rd. With One Metal and Two Liquids. — If one piece of copper is immersed in a solu- tion of sulphate of copper, and another in dilute sulphuric acid, the two being connected together by a wire, and the liquids touching each other by a porous partition, copper will be dissolved in the dilute acid in the proportion of 31-7 parts for every 31*7 parts of copper deposited in the metallic solutions ; and for every 49 parte, or one equivalent, of hydrate of sulphuric acid set free at the cathode by the deposition of the equivalent of copper, one equivalent of acid will combine with a like amount of copper at the anode. 4th. With Two Metals and Two Liquids. — If we immerse a piece of zinc in dilute sulphuric acid, and a piece of silver in a solution of double cyanide of silver and potassium, the two metals being connected by a wire, and the liquids touching each other by a porous partition, one equivalent, or 32 - 6 parts, of zinc will combine with one equivalent of the acid, and one equivalent, or 108*1 parts, of silver will be deposited, setting one equivalent, or 26 parts, of cyanogen free. 5th. With a Separate Depositing Liquid. — If the plates of a battery are connected by wires with two pieces of gold in a hot solution of double cyanide of gold and potassium, for every atomic equivalent, or 197 parts, of gold deposited and one equiva- lent of cyanogen set free, one equivalent of gold will combine with one equivalent BINARY THEORY OF ELECTROLYSIS. 39 of cyanogen, and dissolve; and not only this, but for each of these actions one equi- valent of zinc will combine with one equivalent of acid, and one equivalent of hydrogen will be evolved in each of the battery cells supplying the current of electricity. 6th. In a whole Series of Depositing Liquids. — Such, for instance, as solutions of sul- phate of copper with electrodes of copper, — arranged as in Figs. 15 and 16,— connected with a piece of zinc immersed in dilute sulphuric acid, and a piece of copper immersed in a solution of sulphate of copper, the two liquids touching each other by a porous diaphragm, the whole of the combinations, decompositions, and depositions of metal, throughout the series, will take place in the proportions of their atomic weights or chemical equivalents. These, and many other instances which might be adduced, prove that all the electro-chemical actions taking place in any given circuit, occur in certain definite proportions, and that this definite electro-chemical action is one very- important condition of electro-deposition. 58. Binary Theory of Electrolysis. — The law of definite electro-chemical action was first established by Faraday, and in addition he has advanced what is termed the binary theory of electrolysis — that " only those compounds of the first order are directly decomposible by the electric current, which contain one atom of one of their elements for each atom of the other ; for instance, compounds containing one atom of hydrogen or metal with one atom of oxygen, iodine, bromine, chlorine, fluorine, or cyanogen;" whilst " boracic acid (B O 3 ), sulphurous acid (S O 2 ), sulphuric acid (S 0 :i ), iodide of sulphur, chloride of phosphorus (P CI 3 ) and (P CI 5 ), chloride of sulphur (S 2 CI), chloride of carbon (C 4 CI 9 ), bichloride of tin (Sn CI 2 ), terchloride of arsenic (As CI 3 ), quintochloride of antimony (Sb CI 5 )," are non-conductors of elec- tricity, and incapable of electrolysis. Some substances, which are not of the simple binary character mentioned, are decomposed by 'current electricity, and yield their positive and negative elements in equivalent proportions at the respective electrodes ; but, according to this theory, they are indirectly decomposed, i.e., they are decomposed by the chemical action of some of the elements set free by the direct action of the current upon other substances present. For instance, " fused borax (biborate of soda NaO, 2B0 3 ) yields oxygen gas at the anode and boron at the cathode ; now, since fused borax is not decomposible by the electric current, the separation of the boron must bo attributed to indirect action ; the current resolves the soda (NaO) into oxygen and sodium, and the latter separates boron from the boracic acid." (Faraday.) Again, an aqueous solution of ammonia (IIO, NH :! ) yields, by electrolysis, nitrogen gas at the anode and hydrogen at the cathode. In this case, according to the theory, it may be supposed that only the water (IIO) is directly decomposed, and that its oxygen, set free at the anode, combines chemically with some of the hydrogen of the ammonia, again forming water, and thus indirectly its nitrogen is set free. 59. Mathematical Idea of Electro-depositing Force. — A consideration of the law of definite electro-chemical action, and the binary theory of electrolysis, leads us to view the electric current or depositing force in a mathematical aspect, ns 44 an axis of forces equal in power, but opposite in direction," became, for every atom of an electro-positive substance attracted or transferred in one direction, an atom of an electro-negative character is attracted in the opposite direction. It also suggc sts to us the idea that an intimate connexion exists between those equivalents or mathematical relations of matter and the development of current electricity by chemical action, and its transference through liquids by electrolysis, because these phenomena only occur when the mathematical conditions are present. 40 PRACTICE OF ELECTRO-DEPOSITION. 60. Sizes of Electrodes, Liquids, and Wires.— The rapidity of deposition is influenced by the area of the electrodes, the length and area of the intervening liquid, and of the connecting wire ; the larger the immersed surfaces of the metals, the shorter the length and the greater the transverse area of the liquid between them, and the shorter and thicker their connecting wires, the more rapid is the process of deposition. If the anode is very large and immersed in the lower part of the liquid, and the cathode very small and suspended near the surface, much more metal will be dissolved than is deposited, gas being generally evolved at the cathode in place of some of the metal deposited. 61. Rapidity of Deposition. — The character of the deposited* metal is very much influenced by the rapidity of deposition : if it is deposited very rapidly, it will be in the state of a perfectly black, soft, non-coherent powder ; if deposited more slowly, it will possess the ordinary characters of the particular metal ; and if deposited very slowly, it will be crystalline, because the atoms are then allowed sufficient time to arrange themselves in the crystalline form. 62. Logical Conditions. — The logical conditions of deposition are — 1st, that in all cases there is a difference either in the " material substratum," of metal, of liquid, or of both ; or in the forces involved, both of chemical affinity and electricity, pro- bably also of heat and of motion ; and, 2nd, that in all cases of deposition there are certain chemical, electrical, thermic, dynamic, and mathematical conditions invariably present, and certain other conditions of each of those kinds invariably absent, both of which classes of conditions are necessary to the production of the phenomena ; the whole of the necessarily present circumstances constituting its causes, and all the necessarily absent ones constituting its preventives ; and if all the causes of deposition are present, and all its preventives absent, deposition will invariably occur ; but if only one of its causes is absent, or one of its preventives present, deposition cannot take place. 63. Ontological Condition. — The last, the most necessary, most evident, and most simple condition of all deposition is— that metals, liquids, and forces are required in order to produce it. THE PRACTICE OF ELECTRO-DEPOSITION. 64. Objects of Practical Deposition.— In the theoretical division we have brought forward a large number of instances, both of deposition and non-deposition ; and from a consideration of them we have drawn conclusions both as to what sub- stances and arrangements, and what conditions of those substances, really existed in cases of deposition ; and, in similar cases of non-deposition, we have pointed out in what respect the conditions varied. Our object in pursuing this course was to impress the reader with a perfect knowledge of the theoretical principles on which deposition proceeds, that he may feel himself perfectly able to apply his knowledge to remove the difficulties certain to arise in his practice. Now, however, our object is different ; it is to instruct the reader how he is to apply those principles in daily working, to give him practical rules, recipes, and directions for carrying out the various minute points in workshop manipulation; and to enable him to obtain the greatest degree of practical success. 65. General Arrangement of Electro-Deposition.— The practical part will be best arranged by beginning at the very commencement; and assuming the reader to be without materials or apparatus of any kind— that he has to provide a workshop, GENERAL ARRANGEMENT OF APPARATUS. 41 prepare his solutions, batteries, scouring and cleaning apparatus ; that he has to clean his articles for receiving deposits, and prepare his materials for moulds, before he can commence the process of deposition, gradually acquiring a knowledge, as he proceeds, of the kind of depositing process best fitted for his purpose ; of the most suitable source of depositing power; the best solutions; the best recipes for making solutions ; the construction of voltaic batteries and of magneto-electric machines ; the rules for regulating the currents of electricity, and the character of the deposited metals ; rules for depositing metals generally, as well as for cleaning and preparing metal articles to receive deposits ; copying works of art by moulding ; preparation of moulds for receiving deposits ; making solutions for ordinary coppering, silvering, and gilding ; and for the management of those solutions ;— all these it will be our task to supply. 66. First Considerations. — The first step in practice is, to consider the proba- ble magnitude of the operations to be carried on, and to provide rooms of suitable size. These should be upon the ground floor (except for electro-gilding), well lighted and ventilated, with conveniences for the erection of boilers and drying flues, for placing washing troughs, depositing vats and batteries, and for the escape of un- wholesome vapours ; there should also be ready access to a plentiful supply of clean water. The establishment should consist of at least three rooms, and an open yard with an outhouse ; an upper or more private room for gilding, a ground-floor room for silvering, and another ground -floor room for the coarser work, such as coppering, brassing, and the preparation of the larger and coarser articles for receiving deposits. The outhouse is for the batteries, and the yard for washing the battery cells. If a magneto-electric machine is employed, an additional small, dry, and clean apartment will be required, which should be reserved for it alone. 67. Boilers, Furnaces. — For the purpose of general deposition, several large iKon boilers, with furnaces beneath, either in the coppering-room or in close proximity to the silvering-room, are required ; these arc to contain solutions of caustic potash for cleaning articles. A low furnace should be erected between those rooms, having a long horizontal flue covered with plates of iron, for drying deposited or plated articles upon j the room for coppering should be furnished either with a low furnace or stove for heating the solutions used for coppering or brassing iron. Each room, whether for coppering, silvering, or gilding, should be provided with a tap of running water, and a leaden trough beneath, for washing the smaller articles; and the coppering-room should be furnished with one or two large wooden tubs or troughs, filled with water, for washing articles of larger size ; both this room and the outhouse should contain a number of large stoneware pans and jars, oval and round, of different sizes and propor- tions, to contain the various " pickling" and " dipping" liquids, acids, or spent solu- tions. Several large iron trays, filled with sawdust, should also be provided and fitted to the furnace flue, for drying plated and deposited articles upon. Each of the rooms should be provided with a " scratch-brush lathe," for scouring the various articles. The gilding-room should have several small stoves for heating gilding solutiens, or, in lieu thereof, several iron tripods, with large gas burners beneath. The silvering and coppering-rooms should each be provided with one or two pairs of large and well- insulated copper wires, proceeding from the depositing vats to the batteries outside. The gilding-room will not require these, small batteries only being used in it, which are kept in the same room. 68. Source of Electricity.— A point for early consideration will be whether a 42 THE MAGNETO-ELECTRIC MACHINE. magneto-electric machine or voltaic batteries are to be used as the source of depositing power. The choice of the former will depend very much upon the degree of confidence the operator possesses in each source of electricity, upon its expense, and whether or not motive power to drive it is readily and constantly available at a moderate cost. Voltaic batteries are readily obtained and worked in almost any situation. "We will suppose, for the purpose of explanation, that the operator has resolved to use both a magneto-electric machine and voltaic batteries ; and, therefore, we will explain the construction of each. 69. Construction of Magneto-Electric Machine The simplest form of apparatus for generating current electricity by the joint influence of magnetism and motion has been already de- scribed (15), but the appara- tus required for practical purposes is far more elaborate and costly in its construc- tion. A (Fig. 22) is a strong framework of wood ; B, B, B, B are four bundles of powerful horse-shoe mag- nets, firmly fixed to the wooden frame ; C is an axle driven very rapidly by steam- power ; the axle carries two brass circles, upon which, at right angles, are firmly fixed four round bars of pure soft iron, equal in length to the distance asunder of the poles of each magnet, upon each of which is coiled a long piece of thick copper wire, the wire being covered with cotton to insulate its coils from each other ; the ends of these wires are connected with four semicircular pieces, formed of a piece of brass tube, fixed upon the axle C, but insulated from it and from each other (Fig. 23) by a tube of hard wood or gutta-percha ; by rotation of the iron armatures past the poles of the magnets, currents of electricity are generated in the wires in one direction as they are approaching the magnets, and in an opposite direction as they recede from them ; and to collect those currents and convey them to the depositing vat, also to throw them all into one uniform direction, two brass springs, D E press against the semicircles of brass during their revolution, and are so arranged at their points of mutual contact that, just at the moment that the currents are changed in direction by the armatures passing the magnets, ttie revolution of the axle causes the points of contact of the springs to pass from one pair of the semicircles to the other ; and thus, by reversing the connection at the moment the direction of the current is changed, an uniform direction of currents is obtained in the wires beyond. To explain the action of this machine more fully, we will suppose NS, NS, Fig. 24), to represent those poles of the compound horse-slioe magnets which are THE MAGNETO-ELECTRIC MACHINE. 43 Fig. 24. small arrow surrounding towards the observer in the elevation (Fig. 22), A is the axle, and B, C, D, and E are the ends of the four horizontal round iron bars or armatures, moving in the direction of the large arrows. When a bar of soft iron, having a coil of insulated wire wound upon it, is moving towards a pole of a magnet, a current of electricity is de- veloped in the wire in one direction by the piece of iron within it gaining mag- netism ; and when the bar is moving from the pole of the magnet, a current is produced in an opposite direction by the iron bar losing its magnetism ; and these currents are both reversed in di- rection, if the pole of the magnet is reversed ; so that the current developed in a wire, coiled upon an iron bar, by moving that bar towards the south pole of a magnet, is the same in direction as that produced by moving it from the north pole. "We will now suppose that the current produced in the coil of wire upon bar B, moving from S to N, is in the direction of the that bar, viz., fe/U-handed motion; if so, the current of the wire of bar D will also be /c/^-handed in direction, because it also is moving from a south to a north pole ; whilst the currents in the wires of bars C and E will be rey/^-handed, because both those bars and coils are moving from north to south poles. From these remarks, by careful attention, it may be perceived :— 1st, that the currents of electricity, in all the coils, are reversed in direction every time the bars pass the centres of the poles of the magnets, i.e., four times in every revolution ; 2nd, that to obtain a current of one uniform direction from all the four coils (by conducting the whole four into one stream), during only one quarter of a revolution, i.e., from one pole to the next one, it is necessary to connect the ends of the wires of the coils of B and D in an opposite manner with the semi-cylinders of the commutator or break-piece (see Figure), to those of the coils of C and E ; and, 3rd, that as the currents in all the coils are reversed in direction every time the bars pass the centre of the poles of the magnets, the two springs which press upon the semi-cylinders, must, by some means or other, be reversed in their order of connection with all the wires every time, and at the same moment that the bars pass the centres of the poles, in order to throw the whole of the currents during a complete revolution into one uniform direction in the springs and in the wires which proceed to the depositing vessel. To enable us to understand how the opposite currents of the different coils are thrown into one uniform direction during one quarter of a revolution, and how the whole of the currents are alternately conducted into one uniform stream during rapid revolution, we will suppose A (Fig. 25) to be the axle, and 1, 2, 3, and 4, the semi- cylinders insulated from the axle by a tube of gutta-percha. In the first place 1 and 2 are connected together by a short piece of thick copper or brass wire, or a strip of sheet copper behind (not shown in Figure), and 3 and 4 are connected together by another and similar piece of metal ; next, the ends of the wires A, A, A, A are gathered into one bundle, and connected by soldering with the wire or strip of metal of the semi-cylinders 44 VOLTAIC BATTERIES. 1 and 2, and the ends B, B, B, B are connected in like manner with the semi-cylinders 3 and 4; and by this arrangement, on careful attention, it will be perceived that the whole of the currents are thrown into one direction during one quarter of a revolution. In the sketch, the bars are just approaching the centres of the poles, and the semi-cylinder 1 is just about breaking contact with spring C, and semi-cylinder 4 is about making contact with it ; semi-cylinder 3 is about passing from spring D', and semi- circle 1 is about making contact wich it. In this position of the bars, it will be perceived that all the A ends of the wires, the semi- cylinders 1 and 2, and the spring C, are positive, as indicated by the arrows ; whilst the B ends, the semi-cylinders 3 and 4, and the spring ■ Fig * 25, D' are negative ; but immediately the bars pass the centres of the poles, the currents are all reversed in direction— all the B ends of the wires, and the semi-cylinders 3 and 4 become positive, and the A ends, and the semi-cylinders 1 and 2 negative ; but as the rotation of the axle shifts the points of contact of semi-cylinder 1 to spring D', and of semi-cylinder 4 to spring C, at the same moment, the spring 0' still remains positive, and D' negative, as before. The connections of the springs with the semi-cylinders in the other two quarters of the revolution, alternate in a similar manner ; and these alternations take place regu- larly, and synchronously with the reversals of the currents, no matter how rapidly the axle revolves ; and, notwithstanding the incessant and indefinitely rapid changes of the currents, one uniform stream of electricity is obtained. In the practical machine, the bars of iron rotate as closely as possible to the ends of the magnets without abso- lutely touching them, in order to obtain the greatest amount of power ; and the power is regulated by placing the soft iron keepers which are upom the magnets (Fig. 23) nearer or further from the poles ; the keepers are secured to the magnet, that they may not fall off by the vibration of the machine. Two wires proceeding from the springs are connected, one with the dissolving plates and the other with the receiving articles in the vat. The machine-may contain either eight revolving armatures or eight magnets, according to the amount of work to be effected. 70. Voltaic Batteries. — Each of the arrangements of metals and liquids which have been described under the head of " Facts," and which develop a current of electricity, constitutes an elementary voltaic battery— a battery in all its essential parts in principle, but not in outward form. All voltaic batteries consist of one or other of those theoretical arrangements modified and adapted for practical use, and are com- posed either of two metals and one liquid, or two metals and two liquids; because these two arrangements develop the greatest amount of electricity, and are most convenient in use. In the theoretical form any kind of metal, of any size or shape, with almost any conducting liquid, and with any kind, shape, or size of containing vessel, will develop a current and produce deposition ; but a true voltaic battery— the practical instrument- consists of particular metals and liquids (those which evolve the greatest power), of SINGLE CELL BATTERIES. 45 particular sizes, shapes, and proportions, and at certain distances apart, with suitable screws attached for connections, and with containing vessels made of particular mate- rials, and of special forms and sizes. 71. The kinds^of batteries most in use for electro-deposition are 1st. The old zinc and copper battery (Fig. 26), each pair of which consists of a cross piece of wood at the top, with a groove in it for passing down the zinc plate, and two copper plates, one on each side of the zinc, which are pre- vented from touching it by a slight frame of wood attached to the cross piece ; the zinc plate is move- able vertically, and has a strong cramp screw attached to support it at any given height, and with which to form connection ; the copper plates are connected together by a strip of sheet copper, they have also a screw for forming connections ; they do not touch the bottom of the vessel by several inches, being supported by the cross head upon the upper edges of the vessel. 2nd. Smee's battery of zinc and platinized silver, is siniila* in arrangement to the one de- scribed, except that the sheets of platinized silver, being exceedingly thin, are stuck upon the board by shellac varnish, the board being previously saturated with that substance to prevent the battery liquid from acting upon it. 3rd. JDaniell's battery (Fig. 27), consisting of a piece of zinc, either in the form of a round bolt or thin cylinder ; this is immersed in dilute sulphuric acid, and a cylinder of 6heet copper immersed in a solution of sulphate of copper, the two liquids being separated by a porous diaphragm ; the zinc is generally contained with the dilute acid in a porous vessel, which is immersed in an outer vessel combining the cylinder of sheet copper and the sulphate solution, or the outer vessel itself is formed of copper, and constitutes the negative metal. In each case a small perforated shelf is placed near the upper end of the battery, containing crystals of sulphate of copper for supplying the outer liquid. r *£- 2/ ' 4th. For purposes where a current of consider- able intensity is required, a battery of cast-iron and zinc is provided. 72. Battery Cells. — The form of cells for these batteries is generall} 1- cither round or square ; for small batteries, either of the old zinc and copper kind, or of Mr. Smee's arrangement, square ones are generally used, but for small Daniell's batteries, or for large batteries of either of these kinds, round vessels arc almost invariably adopted. They are made of stoneware, glass, or gutta-percha ; the first of these is universally used for large batteries of all kinds ; glass is too expensive for large vessels, but it possesses the great advantage of enabling the operator to watch the action of the batteries, and is now being moulded into large vessels for the use of some of the electro-plate manu- 46 DEPOSITING VESSELS. facturers ; gutta-percha has been also used, but possesses rather less advantage upon the whole, than the other materials, being opaque and expensive, while the zinc salt of the battery liquid passes rather rapidly over its edges by capillary action. 73. Porous Cells. — When Daniell's or any other battery with two liquids is used, porous vessels are required also to allow the two liquids to touch each other without mixing ; they are of three kinds— unglazed earthenware, wood, and bladder ; the first of these is the only kind in use by manufacturers; they should always be kept in clean water when not in use, to remove the salts of the battery liquids from them, to prevent their cracking, and to preserve them always fit for immediate use. 74. Zinc for Batteries.— The best kind of zinc for batteries, and the kind chiefly in use by electro platers, is the German or Liege zinc, known as " Mosselman'e," from the name of a firm who manufacture it. The thickness of the plate should vary with the size of the battery ; the smallest should not be less than one-eighth of an inch thick, on account of its brittleness when amalgamated ; large ones are generally about one quarter or three-eighths of an inch in thickness. Zinc bolts for Daniell's batteries are generally made by melting together a number of old worn-out pieces of battery I plates, and casting it in a suitable mould. The wholesale price of unrolled (cake) zinc is about twenty-five shillings per hundredweight. 75. Amalgamation of Zinc. — Zinc plates or bolts are best amalgamated by im- mersing them about a quarter of an hour in a mixture of about one part of sulphuric acid and ten or twenty parts of water ; then pouring mercury upon them, and rubbing it all over them with a hare's foot or piece of old cloth, using a small hard brush for the refractory places ; they are then washed in water, and drained for half an hour, and brushed to recover the superfluous mercury. 76. Copper and Platinized Silver for Batteries. — Ordinary sheet copper answers very well for this purpose, and platinized silver may be obtained of most philo- sophical instrument makers, or it may be easily prepared by any one by the following means :— immerse a piece of zinc in dilute sulphuric acid contained in a porous cell, place the cell in an outer vessel, and fill the outer space with water, to which a few drops of sulphuric acid have been added ; add to this a sufficient quantity of a solution of bichloride of platinum to render it of a brown colour ; immerse the piece of silver to be platinized in the outer liquid, and connect it by a wire with the piece of zinc ; gas will soon be evolved from the surface of the silver, and the silver will gradually become black with a deposit of platinum ; it may then be removed, dipped several times in water, and afterwards dried, care being taken not to rub off the platinum. The porous cell requires to be immersed a short time beforehand. The solution of bichloride of platinum may be easily and cheaply formed by adding scraps of platinum foil to a hot mixture of one measure of nitric acid and two and a half measures of hydrochloric acid, as long as gas is evolved from them ; the liquid will then be of a deep red colour. Silver alone is not nearly so effective for the negative metal of voltaic batteries as platinized silver, because the hydrogen gas evolved adheres very strongly to it, and greatly reduces the amount of its surface in contact with the liquid, whereas the pla- tinum being a very negative metal, and being deposited in the state of a fine powder, causes the hydrogen to be thrown off very rapidly from its surface, and thus increases the action. Copper is still less effective than silver, because the battery liquid acts chemically upon it and forms a salt of copper, which dissolves in the liquid and re-acts upon the zinc plates, causing them to waste rapidly ; for this reason they cannot, like silver, be ARRANGEMENT OF DISSOLVING PLATES. 47 safely left in the liquid any great length of time when the battery is not at work In addition to this, when they are taken out and exposed to the air they soon become covered with a film of oxide, which considerably weakens the electric current on their re-immersion. 77. Depositing Vessels, Vats, &c.-The depositing vessels are made of various materials lor small operations nothing is so suitable as glass vessels or a stoneware pan ; but for ordinary manufacturing purposes, vats containing from twenty to several thousands gallons are used ; they are generally made of wood lined with sheet lead ■ but very large ones, for containing sulphate of copper solution, such as are used for deposit ing hfe-sized figures in copper, have in some instances been built of bricks coated with cement, and lined with gutta-percha. Vats used to contain cyanide solutions should not be lined with this substance, because the cyanide of potassium acts upon it The vats used for ordinary silver-plating are about twenty-four or thirty inches deep from two to three feet wide, and from three to twenty feet long ; their dimensions vary greatly m different manufactories, and depend upon the number and size of the articles to be plated in them. Some electro-depositors use vats formed of sheets of wrought iron riveted together; but there is always a slight salt or sediment found on their sides which settles at the bottom of the liquid. ' 78. Arrangement of Dissolving Plates in Vat.-In the vats used for silver ing general articles, such as spoons, knives, forks, teapots, plates, &c, the dissolving Big. 2?. plates are sometimes fixed all round the sides of the vessel just beneath the surface of the liquid ; in addition to this, vertical wooden frames are fixed at intervals of about two feet, across the vat, (Fig. 28), with dissolving plates upon them, and aU the dissolving plates arc connected together. The articles to be plated are suspended by small copper wires, from brass or copper tubes resting across the vessel upon two other and longer tubes passing all along the upper edges of the vat, and connected by a large copper wire with the negative pole of the battery, whilst the dissolving plates are connected by another large copper wire with the positive pole ; by this arrange- ment each row of the articles has dissolving plates all round it, which greatly facilitate the rapidity of deposition. The wooden cross frames arc moveable, so that when large articles are to be plated, one or more of them may be removed to make room. 79. The large vessels used for depositing solutions which require to be worked, hot such a3 the cyanide coppering or brassing liquids, arc formed either of cast iron, wrought 48 SCOURING AND CLEANING APPARATUS. iron, or iron coated with enamel ; and the smaller vessels, such as are used for gild- ing, are oftentimes of stoneware or glass ; enamelled iron pans are also used for this purpose. 80. ' Sciratch-brush Lathe."— The deposition will require" several' "[scratch- brush. " lathes, one in each depositing room, for scouring and preparing the surfaces of metal articles to receive a de- posit. This instrument (Fig. 29) consists simply of an ordinary lathe A, with a wooden chuck B, to the sides of which are firmly secured four horizontal bundles of fine brass wire ; above it is a vessel C, containing stale beer, which is allowed to drop constantly, by the pipe and tap D, upon the revolving brushes whilst working ; the sides E E are to prevent splashing, and the tray F and pipe G are to collect and convey the waste liquid. The workman stands op- posite the end of the machine in using it, working the treadle with his foot, and pressing the article against the ends of the revolving wire brushes, exposing in succession different parts of the article to their action. Wire i Yig„ 29. of different degrees of fineness is used with different articles. 81. Connecting Wires.— The depositor should provide a large number (several pounds weight) of pieces of copper wire, of about the size No. 18 or 20 of the Birming- ham brass wire guage, and about fifteen or twenty inches long, for suspending the smaller and more numerous articles to be coated in the depositing solutions ; a few other pieces of a larger and stronger kind should be provided for the heavier articles. Copper is the most suitable metal for connecting- wires, and the most generally used, because it is one of the best conductors of electricity, being also flexible and not expensive ; next to it we should select brass ; silver is the best conductor, but is too expensive. 82. Dipping Liquids, Pickling Liquids, &c— The depositor will next pre- pare his various liquids for cleansing articles for plating. For cleaning iron articles he will require large stoneware pans and jars, containing a mixture of one part of sulphuric acid, and twenty parts of water, or weaker, according to the kind and condition of the metal ; smooth wrought iron requires a weaker liquid than rough cast iron. For cleaning either copper, brass, or german silver, he will require several stoneware pans, one containing strong nitric acid, another filled with " dipping " liquid (a mixture of G4 parts of water, 64 parts sulphuic acid, 32 parts nitric acid, and 1 part of hydro- ) chloric acid), and a third containing " spent" liquid, i.e., either nitric acid or dipping | LIQUIDS FOR ADHESIVE DEPOSITS. 49 liquid the power of which has been partly exhausted. In addition to these liquids 5 he Will require some glass-cutters' fine sand; with several small hand brushes and pieces of old cloth, for brushing and rubbing the sand upon the more rusty and refractory parts of the metals to be cleaned ; also a file and scraper to further assist in cleaning them. Hydrofluoric acid, contained in a small leaden or gutta-percha bottle, should be at hand, to apply to the « glazed" patches occasionally met with upon cast iron 83 Battery Liquids—The only kind of acid used by electro-platers to' excite their batteries is sulphuric acid ; it is obtained in large quantities at about one penny or three-halfpence per pound. For the negative solution of Daniell's batteries a stock of sulphate of copper should be provided; its price varies from fourpence to fiveoence per pound in large quantities. Ll ^ uids fo * Causing Adhesive Deposits.-Solutions of nitrate, or of cyanide of mercury, will be required for preparing the surfaces of copper, brass, and german silver, for receiving adhesive deposits of silver. The nitrate solution is prepared by adding one ounce of mercury to sufficient nitric acid, diluted with three times its bulk of water to. dissolve it ; no more mercury must be added than the liquid will dissolve; when thoroughly dissolved, dilute it with about one gallon of water To prepare the cyanide solution, dissolve one ounce of mercury as stated, dilute it with water, and add a solution of cyanide of potassium to it, just as long as a pre- cipitate is produced ; filter it, add a small quantity of water to the precipitate in the filter, and when thoroughly drained, take it out and add to it a strong solution of cyanide of potassium until it is all dissolved, then add a little more cyanide solution, and finally dilute it with water until the whole measures one gallon. The solution when prepared is kept in a large stoneware pan, a pan of dipping liquid and another of water being near it, and each placed near the scratch-brush lathe and depositing vats in the silvering-room. b 85. Materials for Moulding.-The electro-depositor who includes in his busi- ness not only the ordinary electro-plating, but also the manufacture of works of art by deposition, requires a number of substances for moulding, and preparing the surface of the moulds to receive a deposit. For moulding ordinary metal objects, he often uses wax m a composition consisting of equal parts of white wax, spermaceti, and gutta-percha ; but one of the best substances we have used for this purpose has been a composition of our own, consisting of two parts of gutta-percha, and one part of Jeffery's marine glue ■ the glue is cut up into small pieces and melted at a gentle heat in an iron ladle! the gutta-percha also cut very small, is then added, and the mixture constantly and vigorously stirred at a gentle heat until the two are thoroughly incorporated. In this state it is poured over the object to be copied. This substance possesses several im- portant advantages over gutta-percha alone as a moulding material ; it is softer when heated, and takes a sharper impression ; it contracts more in cooling, and is therefore more easily removed from the original ; and in taking the blacklead it is very superior to gutta-percha. With ordinary care many copies may be taken by deposition off one ot these moulds ; we have taken upwards of ten from one of them. 86. Elastic Moulding Composition.-When the objects to be copied are much under-cut, or when we wish to take a mould of a bust aU in one piece, elastic moulding composition is required. The best substance of this kind, and almost the only one used is composed of four parts of best Russian glue and one part of treacle ; the glue is broken into small pieces and soaked for one or two hours, or until it is quite soft, in sufficient cold water to cover it; when it i s soft the superfluous water is PRACTICAL CHEMISTRY. — No. II. ~ ~~ I 50 CONDUCTING MATERIALS. thrown away, and the glue, together with the treacle, is heated in a common glue-pot, like ordinary glue, to nearly a boiling heat, and stirred until the two substances are thoroughly mixed ; the use of the treacle is to prevent the mould drying and shrink- ing too rapidly. 87. Blacklead, Phosphorus Liquid, &c. — For rendering the surfaces of non- conducting substances, such as gutta-percha, wax, marine glue, &c, conductible, the application of several substances will suffice : — 1st. The common powder blacklead for ordinary non-elastic moulds ; there is the greatest difference, however, between different specimens of blacklead in their value for this purpose, some causing the deposit to spread over the moulds very quickly, whilst others scarcely cause it to spread at all ; the best we have found, and it has been very good, is " Dix's," sold in twopenny packets, one or two of which will serve the operator a long time for this purpose. 2nd. For the moulds made of elastic composition (86) he will require the following liquids, patented by Mr. Alexander Parkes : — A, the phosphorus solution — to make nearly three ounces of which, melt 64 grains of bees-wax or tallow ; then dissolve eight grains of india-rubber cut up very small, in 160 grains of bisulphide of carbon, and when it is dissolved add to it very carefully (as it is highly inflammable) the melted wax, and shake the mixture thoroughly ; then dissolve 64 grains of phosphorus in 960 grains (about 2 \ ounces) of bisulphide of carbon, and add to it 80 grains of spirit of turpentine, and 64 grains of asphalte in fine powder ; when dissolved, add this solution to the previous one of india-rubber and wax, and thoroughly mix them by shaking. B, the silver solution — to make twenty ounces (one pint) of this liquid, dissolve about 18 or 19 grains of pure silver in about 20 or 25 grains of the strongest nitric acid, and then dilute it to the required volume with distilled water. And C, the gold solution — to make 20 ounces of which, dissolve about 5 or 6 grains of pure gold in about 20 or 25 grains of a hot mixture of one measure of nitric acid, and about two or three measures of hydrochloric acid, and, when dissolved, dilute the solution with 20 ounces of distilled water. The same patentee includes in his patent a phosphorus moulding composition, by the use of which the immersion in the phosphorus solution is dispensed with, the moulds themselves containing the required amount of phosphorus ; to make about one pound of this composition, melt together half a pound each of wax and deers' fat, then dissolve about 19 or 20 grains of phosphorus in about 300 grains of bisulphide of carbon ; keep the wax mixture barely melted, and add the phosphorus solution slowly | to it, and with brisk stirring of the fat, pouring it in at the bottom of the melted ! mixture by a vessel with a long spout, to prevent its inflaming. It is highly j dangerous to leave spilled portions of the phosphorus solution or composition about in i contact with wood, paper, rags, &c, as after a lapse of some time (even hours) they i will often burst into flame. 38. Selection of Depositing Processes. — For very small articles of which there { are a great number, such as buttons, hooks and eyes, pins, &c. ; and which require ! only a very thin deposit, the simple immersion or wash process will answer very well, I being both easy of execution and cheap. For the multiplication of numerous small 1 articles in copper, such as medallions, &c, the single cell process is very advan- tageous ; it is quicker than the battery process, and considering the time occupied, and j the other elements of expense, it is to be preferred to that method. But for all j ordinary deposits, plating, &c, the battery process is by far the best, because coatings DEPOSITING SOLUTIONS AND LIQUIDS. 51 of any thickness, in all ordinary metals, may be obtained by it, and the solutionis not, as in the other processes, require renewal. 89 Methods of Making Depositing Solutions—The operator will next consider about making depositing liquids. They may be made by two methods, the one called the chemical and the other the battery process. The chemical process consists of the various ingredients by the usual chemical means, and adding them together in suitable proportions to form the complete liquid ; for instance-lst, the ordinary sulphate of copper solution is prepared by dissolving a certain proportion of com- mercial sulphate of copper in water, and adding to it a certain proportion of sulphuric acid to form/™ acid ; and, 2nd, to form the ordinary cyanide of silver and potassium plating liquid, silver is dissolved in dilute nitric acid ; the solution of nitrate of silver formed is precipitated by addition of a solution of cyanide of potassium ; the white precipitate of cyanide of silver is washed, and then added, as much of it as will dis- solve, to a solution of cyanide of potassium ; after that an additional portion of cyanide of potassium is added to form free cyanide. The battery process consists in taking some water and dissolving in it a certain proportion of acid or salt, as the ease may be, then placing a large anode of the given metal at the lower part of the liquid and a small bright cathode at the upper part, and, if necessary, applving heat, and connecting them with a suitable battery until the required quantity of metal is dis- so ved, which is indicated by the cathode receiving a good deposit; in making gold solutions the cathode is generally placed in a small porous cell filled with the same liquid, and immersed nearly to its edge in the outer liquid, and by transferring the cathode occasionally to the gold solution, and absorbing; if it receives a good deposit, "?,™, ay T W suffi cient metal is dissolved; the liquid of the cell may then be added to the outer solution. If it is wished to make sulphate of copper solution by this method (which we should not advise, however, the salt being so cheap), take the same quantity of water as we prescribed for the chemical method, and add to it as much acid as was contained in the salt of copper, with the free acid as before, and then pass a current from a battery of one or two pairs by a large anode and small cathode, until sufficient metal was dissolved ; or if it is desired to make some cyanide ot silver and potassium solution by this method, which is sometimes done, take the same proportions of water, cyanide of potassium, and free cyanide as in the chemical process, and pass the current by a large silver anode, until the same proportion of silver is dissolved as required in the chemical method. 90. Selection of Depositing Liquids.-The following rules should be observed in selecting a suitable depositing liquid for the battery process. 1st. It should act strongly upon the anode, and hold abundance of metal in solution. 2nd. It should possess good electrical conducting power 3rd. It should yield its metal freely, and in a reguline state. 4th. It should not act chemically to any great extent upon the base metals, because it is that we generally wish to coat, and chemical action upon it would endanger the adhesion of the deposited metal. 5th. It should not decompose by contact with the atmosphere, nor should light influence it in such a way as to injure it for depositing purposes. ■ rJl El ? ^ i ' d0 ° S DOt ° V0lve * as at the surfaco of the receiving a hole whilst depositing, because that generally indicates a waste of battery power attended by oxidation of the liquid. 52 TESTS FOR DEPOSITING LIQUIDS. 91. Testing a Depositing Liquid. — To test a depositing liquid, pass a cur- rent of electricity through it, from about two pairs of Smee's batteries, with a suitable clean anode of proper size, and a clean piece of iron, brass, or copper, of about the same size, to receive a deposit, observing how much gas is evolved in the battery ; if the deposit appears quickly, and is of a bright and proper colour ; and if it adheres to the metal ; if the cathode evolves gas from" its" surface, and the anode dis- solves freely, cleanly, and without escape of gas, work it at intervals, keeping it exposed to light and air ; observe if it continues to work well, or whether, on the contrary, it shows a decrease of conductibility, deposits a sediment, or if the anode becomes covered with an insoluble crust (this last may arise either from deficiency of free acid, or from impurities in the metal) . If but little gas is evolved in the battery, it is a bad con- ductor, and will neither dissolve nor deposit the metal freely at that temperature, or it is deficient in free acid or free salt. If the deposited metal is of a bad colour, either the battery is too strong, the receiving article too small, or the liquid is incapable of yielding good metal. If the immersed metal or article is coated by simple immersion without the aid of the battery, it shows that, to adapt the liquid to articles made of that particular metal or alloy, they must receive some previous preparation, in order to make the deposit adhere. If it deposits a sediment, or alters^in conducti- bility by exposure to the air and light, the greater probability is that those influences alter either its chemical composition, or the arrangement of its particles. If it evolves gas at the receiving surface during deposition, it shows either that there is too much battery power, too little metal in solution, too much free acid, or that it is a wasteful liquid, in which one part only of the current is employed in depositing metal, whilst another part of it is employed in depositing gas and oxidating the liquid. 92. Testing Solutions for Depositing Alloys. — With solutions in which alloys are to be deposited the most important condition is, that neither of the metals to be deposited are electro-positive to each other in that liquid. This is best tested by taking a wire of each metal, connecting them with a galvanometer, and simul- taneously immersing their free ends in the liquid ; if either is electro -positive, the needles of the instrument will be deflected, and the direction of the deflection will in- dicate which is positive, while the amount of deflection will indicate the amount of their electric difference in that liquid. It may also be tested by immersing a wire of each metal (not in mutual contact) in the liquid ; if either becomes coated with metal in an hour, that one is positive ; but if neither becomes coated in six hours, there is no per- ceptible electric difference between them. 93. The following experiments bear upon this part of the subject, and show that if a liquid contains two metals in solution, and a wire or other piece of each of those metals is immersed in the liquid, and one becomes covered with a deposit of metal, while the other does not, the one so covered is electro-positive to the other in that liquid. 1st Experiment. With an alloy solution consisting of equal measures of a strong solution of protochloride of tin, and terchloride of antimony, with an anode either of tin or antimony (the latter is the most proper, because it does not coat itself by simple immersion in the liquid), a copper cathode, and one pair of small Smee's battery, only antimony was deposited ; tin coated itself with antimony in this solution by simple immersion, and was found by the galvanometer to be strongly positive to that metal. 2nd Experiment. With a liquid composed of equal measures of a solution of proto- chloride of tin and chloride of bismuth, and either a bismuth or tin anode (the former is the best), a brass cathode, and one pair of small Smee's batteries, only bismuth was EFFECTS OF DIFFERENT FORCES OF DEPOSITION. 53 deposited ; tin was positive to bismuth, in this liquid by the galvanometer, and coated itself quickly with that metal by simple immersion. 3rd Experiment. With a mixture of equal measures of terchloride of antimony, and chloride of bismuth, antimony anode, copper cathode, and a feeble Smee's battery, only antimony was deposited ; bismuth coated itself slowly with antimony in it by the simple immersion, and was, by the galvanometer, moderately positive to the latter metal in it. 4th Experiment. "With 100 grains each of protochloride of tin and chloride of zinc dissolved together in an ounce of distilled water, tin anode, copper cathode, and one pair of small Smee's batteries, only tin was deposited ; zinc was positive to tin in this liquid by the galvanometer, and deposited tin upon itself by simple immersion. 5th Experiment. "With equal measures of strong solutions of nitrate of zinc, and tcrnitrate of bismuth, and a little nitric acid, bismuth anode, copper cathode, and a feeble one pair battery, only bismuth was deposited ; zinc was strongly positive to bismuth in this liquid by the galvanometer, and coated itself quickly with that metal by simple immersion. 6th Experiment. With a solution of the mixed sulphates of zinc and copper, copper anode and cathode, and a single small battery, copper alone was deposited ; zinc was strongly positive to copper in this liquid by the galvanometer, and coated itself immediately with copper in it by simple immersion. 94. Further, if wc take some distilled water, and caustic potash is dissolved in it, passing a moderately strong current through it by platinum electrodes, hydrogen gas will alone be set free at the cathode ; in this case also hydrogen — the least positive of the two positive elements of the liquid — potassium and hydrogen — is set free or deposited. If we now add a little sulphuric acid to the liquid to convert it into a solution of sulphate of potash, add some sulphate of zinc besides, and pass a weak current through ; we shall obtain a deposit of .zinc on the cathode, but no hydrogen or potassium ; in this case we cannot determine by the galvanometer which is the most positive in this liquid, hydrogen or zinc, because the former is a gas ; but it is probable that hydrogen is the most positive, because zinc does not evolve it by simple immersion in this liquid. If we further add to the liquid a small quantity of sulphate of copper, and treat it as before, neither potassium, hydrogen, nor zinc will be deposited, but only copper ; and we find by the galvanometer that copper is less positive than zinc in such a liquid, and that zinc coats itself with copper in it by simple immersion ; in this case also the least positive of the positive elements of the liquid is alone deposited. From these and many other expqrinients, which we have tried with similar results, we deduce the following rule : — If a liquid contains several metals or other electro-positive sub- stances dissolved, and a weak electric current is passed through it, only that substance which is the least electro-positive, and which has the weakest affinity for the acid or negative elements of the liquid, will be separated from its acid and be deposited. 95. With regard to the influence exercised by the proportions of the ingredients of the liquid, and the strength of the current, we may observe, that, if a liquid contains several metals dissolved in equal quantities, and only one is being deposited by the passage of a weak current, a considerable increase in the strength of the current will cause a portion of the next more positive metal to be deposited along with the less positive one ; but this alloy deposit will not be very coherent, because the power required to deposit the second metal in the rcguline state will be so great as to deposit the first as a soft powder. This holds most true when the difference of electric power required is the 54 ANTIMONY SOLUTIONS. greatest ; for instance — 1st, if small and equal quantities of sulphate of zinc and sul- phate of copper are dissolved together in a large quantity of water, and a feeble current passed through the solution, only reguline copper will be deposited ; but if the ! battery power be considerably increased, either by a greater number or larger surface j of the battery plates, the deposit of copper will cease to be reguline, and zinc will be deposited with it. If the power be still further increased, hydrogen gas will also be evolved at the surface of the deposited metals. 2nd, If we dissolve a small quantity of sulphate of copper, and a large quantity of sulphate of zinc, in a large quantity of ! water, and pass a strong current through the solution, copper, zinc, and hydrogen will be set free at the cathode. 3rd, If we slightly^ moisten a lump of caustic potash ! with pure water, and pass a weak electric current through it by platinum electrodes, hydrogen alone will be set free at the cathode, but if a very powerful current is em- ' ployed, potassium also will be deposited. In each of these cases we find, that when the current is weak, the least positive of the positive substances is alone deposited ; but if the power is sufficiently increased, and there is only a small proportion of the less positive substance present, the more positive substances, even though they are much more positive, will also be deposited. Thus the weaker affinities are overcome first, and to the greatest extent ; the current of electricity exercising its influence first, and in the greatest porportions, upon the salt of the least positive metals. 96. Depositing Liquids.— For the benefit of the practical depositor, to whom a ' : general knowledge of all solutions from which ordinary metals may be deposited, with their respective advantages and disadvantages, is of considerable importance, we will give a description of those solutions in regular order, making such remarks in our j progress as will be likely to assist him in the selection of those most suitable for his ! particular purposes. 97. Antimony Solutions. — The most common salts of antimony are the sulphide, ! terchloride, and potassio-tartrate. The hydrochlorate of terchloride of antimony, j i.e., the ordinary chloride or butter of antimony, as prepared for pharmaceutical purposes, ' is formed by chemical means thus :— take one pound of black sulphide of antimony, j add to it four pints of hydrochloric acid, gently heat the mixture with constant stirring, until the gas evolved decreases, then boil it slowly down to two pints, keeping- it partly covered all the time ; cool it, filter it through calico, and keep it in a stop- pered bottle. It is now a yellowish red liquid, of specific gravity 1-47, but becomes nearly colourless by depositing antimony from it by the battery process ; the price of the commercial article is about ninepence per pound. A similar solution may be made by the battery method ; this consists in passing j a current from several pairs of batteries through strong hydrochloric acid by a large anode of antimony, until a good deposit is obtained ; this solution is nearly eolourless. ! The chloride of antimony is an excellent conductor of electricity, it dissolves the anode freely, yields plenty of bright reguline metal if the battery power is sufficiently weak, and its depositing power does not deteriorate by exposure to light or the atmosphere, but it appears to be gradually exhausted by working ; it is also decom- posed more or less rapidly by zinc, tin, lead, iron, brass, and german silver, each, of which coat themselves in it with antimony by simple immersion, and articles immersed in it require to be washed with hydrochloric acid before washing them with water, other wise the latter decomposes the adhering liquid and covers them with a white insoluble powder. 98. The mixed chlorides of antimony and ammonia form a very good depositing ANTIMONY SOLUTION'S. 85 liquid. It may bo formed either by the battery process, by mixing one measure of a saturated solution of sal-ammoniac with one measure of hydrochloric acid, and working antimony into it by means of a battery and a large antimony anode ; or by simply mixing together equal measures of a saturated solution of sal-ammoniac and commercial chloride of antimony. This solution conducts easily, yields its metal freely and of good quality, and does not act 30 strongly upon base metals as chloride of antimony alone ; but in other respects it is similar to the chloride. The mixed chlorides of antimony and manganese, or of antimony and bismuth, yield a reguline deposit easily, but do not appear to possess any special advantages. 99. The potassio-tartrate of antimony is a salt not very soluble in water ; its ! aqueous solution is a very bad conductor of electricity, and is not to be compared to j the chloride for depositing purposes ; we have never been able, either with strong or weak batteries, to deposit from it anything better than a small quantity of antimony in the state of a perfectly black powder ; — on the other hand its solution in hydrochloric acid (which dissolves it very freely), or hydrochloric acid and water, is by far the best ! solution for depositing antimony that we have tried ; it is a most excellent conductor of electricity ; it is not impaired by long working or exposure to light or the atmosphere : (we have deposited antimony from it constantly during many months) ; it will bear a ] very great amount of battery power without the deposit passing into the state of a loose powder ; it deposits reguline metal very rapidly and in great thickness ; we have ' obtained such deposits from it upwards of two inches in thickness ; articles immersed ' in it wash clean in water alone without the previous use of hydrochloric acid ; it may be made by mixing together about two pounds of water, four pounds of hydrochloric ' acid, and eight pounds of potassio-tartrate of antimony ; a greater proportion of water may be used if desired. 100. Both the black and red sulphides of antimony dissolve in cold hydro-sulphate of ammonia, and the resulting solutions conduct very freely with an antimony anode and one pair of Smee's battery, but yield no deposit of metal even wi*h a battery of twenty-five pairs intensity. Aqueous solutions either of canstic potash, tartrate of potash, or oxalate of potash, scarcely conduct at all with an anode of antimony, and a battery consisting of one or two pairs. Cyanide of antimony dissolved in a solutiou of cyanide of potassium has been proposed as a depositing liquid, hut we have found a solution of cyanide of potassium to be a very bad conductor with an anode of antimony. 101. Antimony is one of the easiest metals to deposit in the reguline state ; its appearance when deposited, from the chloride solutions and from the solution of the potassio-tartrate in hydrochloric acid, is very beautiful, and when deposited slowly, it has much the appearance of highly-polished steel. Some of its properties when thus deposited are very peculiar and interesting, especially with regard to heat :— "If, during any part of the time the deposit is progressing, the deposited antimony be taken out and struck gently or rubbed with any hard substance, such as metal or glass, an ex- plosion occurs, accompanied with a small cloud of white vapour, sometimes with a flash of light, and nearly always with considerable heat, sufficient to burn one's fingers, melt gutta-percha, burn paper, and even scorch deal wood quite brown, especially if the deposit is thick; and invariably accompanied by fracture of the deposited metal ; sometimes, if the process of deposition has been interrupted and the deposited metal is not homogeneous the fracture extended quite through the metal to upwards of one. eighth of an inch in depth. This phenomenon has been observed many times both 56 BISMUTH AND ZINC SOLUTIONS. before and since its first publication ; in several instances the explosion took place even in the liquid, by striking the deposit against the glass containing vessel ; and in one instance it occurred after the metal had been well washed with dilute hydrochloric acid, dried, and had remained out of the liquid several hours." On one occasion, a deposit had been well washed, dried, and out of the solution many hours, and a friend, in course of conversation, was unconsciously breaking small portions off it with his fingers, when it became suddenly heated and exploded, causing t a slight noise like the lighting of a congreve match, and burning his fingers. On other occasions a deposit has been progressing, and has been removed an instant for exami- nation, and the battery liquid strengthened by the addition of acid ; upon examin- ing the deposit a few hours afterwards, it has been found cracked in various directions, as if an explosion had occurred in the interval, although the apparatus has been undis- turbed. A French writer has suggested that this deposit is a compound of antimony and hydrogen ; and from the fact that the explosions occurred when the metal was depositing rather rapidly, we are inclined to think this explanation correct ; the extra power, as we have seen in other cases of hydrogen deposit, which being in its nascent state instead of being evolved, might combine with the metal and form an explosive compound. Another suggestion we would make is, that the metal is deposited in a peculiar condition of unequal mechanical tension, similar to that of unannealed glass, and that, by breaking, the closer aggregation of the particles may develop light and heat. 102. Another peculiarity in depositing antimony from the potassio-tartrate solu- tion is, that if the solution be a very dense one, and the process long continued without disturbance of the liquid ; the deposit occurring upon the cathode will slowly spread out in the form of a thin sheet upon the surface of the liquid, until it touches j the anode ; whilst the deposit beneath progresses very slowly. We have a button of j antimony formed in this way upon a vertical copper wire, one and five-eighths inches in diameter, the deposit beneath the surface of the liquid having been only half-an-inch thick; it occupied about eighteen days with a small one pair Smee's battery in forming. Deposits of antimony formed in the above solution do not spread over blackleaded surfaces of gutta-percha, nor do they adhere with any great degree of firmness to copper, brass, or iron. 103. Bismuth Salts. — The most usual compounds of bismuth are the chloride, mononitrate (pearl white), and ternitrate. The chloride is formed by digesting bis- muth filings a long time in a warm place with hydrochloric acid. The mononitrate is formed by dissolving bismuth to saturation in warm dilute nitric acid, and then adding a large quantity, say fifty or a hundred times its volume of water ; the precipitate pro- duced, when well washed with water, is the substance required. The ternitrate is formed by dissolving the metal in hot nitric acid, evaporating the solution, and leaving it in a cold place to crystallize. Bismuth may easily be deposited from a solution formed by dissolving either mononitrate or ternitrate of bismuth in dilute nitric acid, but requires an exceedingly feeble current to deposit it in a reguline state ; its appear- ance when so deposited is very beautiful, white with a faint pinkish tint, and with a fine silky lustre ; it does not spread over blackleaded surfaces of gutta-percha in this liquid. A bismuth anode does not dissolve readily in a hot solution of cyanide of potassium. 104. Zinc Salts. — There are a variety of salts of this metal in ordinary use, the most common of which are the sulphate, chloride, nitrate, and acetate. The sulphate CADMIUM AND TIN SOLUTIONS. 57 may be formed by dissolving zinc to saturation in a mixture of sulpburic acid and water, filtering and evaporating the liquid, and setting it in a cold place to crystallize. The chloride is formed by saturating hydrochloric acid with zinc, filtering, evaporating, and crystallizing. The acetate may be made either by dissolving zinc in strong acetic acid to saturation, then evaporating and crystallizing the solution ; or by adding a solution of acetate of lead to a solution of sulphate of zinc as long as it produces a precipitate ; filter, evaporate, and crystallize the liquid. 105. Zinc Solution. — The sulphate of zinc solution for depositing may be formed by dissolving two pounds of the salt in a gallon of water, and filtering the mixture ; but the best sulphate depositing solution we have used has been the spent battery liquid taken from a cell of a Smee's battery, in which there had occurred a very good deposit of zinc upon the platinized silver plate ; but with this and with other solu- tions of zinc there is a great tendency to the evolution of hydrogen gas at the cathode during deposition ; they require, therefore, to be worked very carefully and with very feeble battery power. Zinc may be readily deposited, either by the single cell or by the battery process, from a neutral solution of the sulphate ; but the single cell is less adapted for its deposition than the battery, because the acid, set free by the deposi- tion of the metal, re-acts upon the deposit, and diminishes its cohesion. The other solutions, such as the chloride, nitrate, acetate, or the various double salts of zinc, with ammonia, or potash, do not appear to possess any general advantages over the sulphate. Amongst other liquids, that of cyanide of zinc dissolved in a solution of cyanide of potassium has been recommended ; but it is a bad conductor with a zinc anode, and requires to be used hot to make it conduct at all freely, or make the anode dissolve. This might easily have been foreseen from a knowledge of the fact, that the affinity of cyanogen for all, or ncaidy all, the base metals is comparatively feeble. "We have found by experiment that a solution of cyanide of potassium will dissolve only about one half as much cyanide of zinc as it will of cyanide of copper. Zinc oxide dissolves somewhat freely in a boiling solution of cyanide of potassium. Cyanide of zinc dis- solves freely in a [solution of sesquicarbonate of ammonia. Ferrocyanide of zinc is but feebly soluble in a boiling solution either of ferrocyanide (yellow prussiate), or of ferrid-cyanide (red prussiate) of potassium, but it is freely soluble in a boiling solution of cyanide of potassium. Zinc deposits may be made to spread over blackleadcd surfaces by the battery process. 106. Cadmium Solution. — A patent was^taken out March 19, 1849, by Messrs. Russell and "Woolrich for the electro-deposition of cadmium, and the following is their description of the process :— " Take cadmium and dissolve it in nitric acid diluted with five or six times its bulk of water, at a temperature of about 80 3 or 100° Fah., adding the dilute acid by degrees until the metal is all dissolved ; to this solution of cad- mium a solution of carbonate of soda (made by dissolving one pound of the ordinary crystals of soda in one gallon of water) is to be added until the cadmium is all pre- cipitated ; the precipitate thus obtained is to be washed four or five times with, tepid water ; next add as much of a solution of cyanide of potassium as will dissolve the precipitate ; after which, one-tenth more of the solution of cyanide of potassium is to be added, to form free cyanide. The strength of this solution may vary, but the patentees prefer a solution containing six troy ounces of metal to the gallon. The solution is worked at about 100° Fa! 1 .., with a plate of cadmium as an anode." 107. Tin Salts. — The most common salts of tin are the peroxide and proto- 58 TINNING SOLUTIONS. chloride ; in addition to these there are two others used extensively in Manchester, and the cotton printing districts, viz., the bichloride and the stannate of soda, i.e., oxide of tin combined with caustic soda. Peroxide of tin is formed by dissolving pro- tochloride of tin in water containing a few drops of hydrochloric acid, and then adding liquid ammonia or a solution of carbonate of potash as long as a precipitate can be produced ; the precipitated peroxide of tin should be washed and dried. Protochloride of tin is easily made by adding grain tin to strong hydrochloric acid, and keeping it at 150° or 200° Fah., until gas ceases to be evolved from the metal; the resulting solution should then be evaporated and crystallized. Aqueous bichloride of tin may be made by dissolving tin in aqua regia not containing too much nitric acid ; a mixture of nitric acid with sal-ammoniac or common salt may likewise be used. Stannate of potash may be formed by fusing together one equivalent (75 parts) of freshly precipi- tated peroxide of tin, and one equivalent either of caustic potash (56'2 parts), or of crystallized carbonate of potash (87*2 parts). 108. Tin Solutions. — M. Eoselau has patented the following liquids for the depo- sition of tin : — 1st. For simple immersion or wash process, which may be used for small articles generally :— dissolve 17| ounces of aramoniocal alum in 22 pounds of boiling water, and, when dissolved, add one ounce of protochloride of tin ; the articles to be coated should be well cleaned and then immersed in the liquid, and moved about in it until they are sufficiently white. 2nd. For depositing tin upon lead, iron, steel, copper, or brass, by connecting the articles with a piece of zinc and immersing them in the solution : — dissolve 10| ounces of bitartrate of potash in l7f pints of water, then add three-quarters of an ounce of protochloride of tin, and boil it a few minutes ; the articles to be coated are immersed in the solution in contact with a piece of zinc of proportionate size. 3rd. For coating zinc, iron, copper, and many other metals by the battery process : — dissolve 11 ounces of pyrophosphate of potash or soda in 17i pounds of water, then add Ah, ounces of protochloride of tin, and operate by the battery process with an anode of tin. By this process M. Roselau states that he can tin metals beautifully and to any thickness. Pyrophosphate of soda is easily formed by heating to redness the common diphosphate of soda. 109. A protochloride of tin-depositing liquid may easily be formed by dissolving the ordinary commercial protochloride in water, and adding a little hydrochloric acid to remove any cloudiness or white precipitate which may be formed ; a similar liquid may be made by the battery process, by passing a current through dilute hydrochloric acid by means of a large tin anode until sufficient metal is dissolved. This or any other from chloride of tin is not a good solution to obtain reguline metal from ; it has a very great tendency to deposit the tin in the form of long crystalline needles, of a fern- like appearance, which often project from the corners and edges of the cathode to a distance of upwards of half an inch. A solution composed of eleven ounces of water, one ounce of hydrochloric acid, and eighty grains of protochloride of tin' admits of this effect being produced in a striking degree ; nearly all the compounds of tin, and especially those formed with mineral acids, exhibit this tendency in a greater or less degree when acted] upon: by electrolysis, rendering the deposition of tin in thick layers of fine white coherent metal a matter of considerable difficulty. 110. The stannate of potash solution is easily formed either by dissolving the crystallized salt in water, or by dissolving freshly precipitated peroxide of tin (109 ) LEAD AND IF.ON SOJ.TTIOJVS. 59 whilst still moist, in a boiling solution of caustic pot:ish. It may also be easily formed by the battery process, by passing a strong current of electricity by a large tin anode through a strong and boiling solution of caustic potash, until the immersed cathode receives a free white deposit. This solution, if worked at loO 3 Fah., yields a good deposit of fine white metal ; but it decomposes by exposure to the atmosphere, and soon deposits all its metal as oxide of tin, at the bottom of the vessel. A solution of cyanide of potassium and tin has been proposed as a depositing liquid ; but it is a bad conductor with a tin anode, even if hot, and does not dissolve tin freely. 111. Mr. Joseph Steele coats zinc, iron, steel, copper, ond brass, with tin, in his patent solution, by the battery process, thus :— dissolve 60 pounds of common soda, IS pounds of pearl-ash, 5 pounds of caustic potash, and 2 ounces of cyanide of potassium, in 75 gallons of water, at 75° Fah., and filter the resulting solution ; then add 2 ounces I of acetate of zinc, and 16 pounds of bioxide of tin ; stir the resulting solution until all is dissolved ; it is then ready for use. Work it by the battery process with an anode of zinc or tin, and with the liquid at 75" Fah. 112. Electrical Relations of Tin and Iron.— Tin is feebly negative to iron at all temperatures between 62° and 203"" Fah. in distilled water, and positive to it at 212° Fah. It is positive to iron at all temperatures between 62° and 212° Fah. in a saturated solution of boracic acid ; also the same between those temperatures in a strong solution of phosphoric acid in distilled water ; or in one measure of oil of vitriol mixed with either nine or ninety-six measures of distilled water ; or in a mixture of one measure of this acid, and 192 measures of distilled water, from 73° to loS" Fah., and negative to iron above that to 212° Fah.; it is positive to iron from 72° to 212° Fah. in a mixture of equal measures of hydrochloric acid and water ; it is negative to iron from 70°to77° Fah., and positive above that to 212° Fah., in a mixture of one measure of hydrochloric acid and nine measures of distilled wator ; it is negative to iron from 70 1 to 212° Fah. in a mixture of one measure of hydrochloric acid, and ninety measures of distilled water, and positive tit iron from 68° to 212° Fah. in one measure of hydrofluoric acid and nine measures of water ; it is positive to iron in one measure of nitric acid and nine measures of water, from 70° to 111° Fah., and negative from 111 3 to 21T Fah. ; and it is positive to iron from 82° to 212° Fah. in a mixture of one measure of nitric acid and ninety-six mea- sures of water. 113. Lead Salts.— The most common salts of lead arc the nitrate and the acetate. The nitrate is formed by dissolving lead in dilute nitric acid, taking care that no more lead is added than the acid will dissolve ; the resulting solution irmst be filtered, evapo- rated, and crystallized; it is a hard, white salt, soluble in water. Acetate of lead is made by digesting oxide of lead in vinegar or acetic acid ; filtering, evaporating, and crystallizing the liquid ; it is soluble in water. 114. Lead Solution. — Lead may be deposited from an aqueous solution, either of nitrate or acetate of lead, or from a solution of plumbit of potash, — the latter is formed by dissolving litharge in a boiling solution of caustic potash ; zino and tin articles (but not iron ones) decompose this liquid and coat themselves with lead in it by simple immersion. It is difficult to deposit any considerable thickness of rcguline metal from either of these liquids. 115. Salts of Iron. — Among the salts of iron in most common use, are the sulphate, chloride, and nitrate ; they may be respectively formed by dissolving metallic iron in dilute sulphuric, hydrochloric, or nitric acids, evaporating and crystallizing the solu- tion as much as possible out of contact with the air. GO COBALT, NICKEL, AND COPPER SOLUTIONS. 116. Iron Solutions. — Iron may be reduced from a solution of its protosulphate (green copperas), made by dissolving metallic iron in dilute sulphuric acid ; or from its protochloride, which is preferable, and which is made by dissolving iron in hydro- chloric acid. "We have deposited it in the state of reguline white metal, by passing a current of considerable intensity (15 or 20 pairs), for one hour, through an anode of iron immersed in a saturated aqueous solution of sal-ammoniac ; its appearance when deposited from this liquid is rather white, with an appearance very similar to that of freshly broken cast iron. By the same means it may also be deposited, using a saturated solution, either of carbonate of ammonia, acetate of ammonia, or acetate of potash. We have also obtained good metal from a saturated aqueous solution of a mix- ture of two parts of protosulphate of iron and one part of sal-ammoniac. We have deposited it from an aqueous solution of ferrate of potash, which may be formed either by igniting peroxide of iron (crocus) very strongly for some minutes with caustic pot- ash and saltpetre ; or we may make a very strong solution of caustic potash, immerse in it a large iron or steel anode, and a small copper or platinum cathode, and pass a strong current from fifteen or twenty pairs of Smee's batteries through it until it acquires a deep amethyst or purple colour ; by that time the cathode will have obtained a coating of iron, which will be in the state of a dark powder if the power has been too great, or it will have the appearance of white cast iron, or intermediate between that and the appearance of reguline deposited zinc, if the power has been sufficiently weak ; this solution rapidly decomposes without any very apparent cause, becoming colourless, and depositing all its metal in the state of peroxide at the bottom of the vessel. Iron may be very easily deposited from its sulphate, thus : — dissolve a little crystalline sulphate of iron in water, and add a few drops of sulphuric acid to the solution ; one pair of Smee's batteries may be used to deposit the iron upon copper or brass. The metal in this pure state has a very bright and beautiful silvery appearance. An aqueous solution of cyanide of potassium is a very bad conductor with an iron anode, even if it be maintained hot. 117. Cobalt Solution. — A solution of protochloride of cobalt maybe easily formed by digesting commercial brown oxide of cobalt in hot hydrochloric acid ; it is a deep blue liquid, but changes to a reddish-brown colour on the addition of water; by slow cooling, fine red crystals of the chloride may be obtained. We have not tried this liquid for electro-deposition. 119. Nickel Solutions.— The nitrate of nickel solution may be formed by dis- solving nickel in nitric acid, slightly diluted with water, and, when dissolved, diluting it with much more water ; it is a solution which does not yield its metal freely. We have deposited nickel in the state of reguline white metal from a solution of the double I chloride of nickel and ammonia, by making a lump of metallic nickel the anode in a ! strong aqueous solution of sal-ammoniac, and passing a strong current of electricity ! through it for several hours, until the liquid acquired a pale greenish-blue colour. We have also obtained a similar deposit by treating a solution of one part of arseniate of potash and five parts water in a similar manner. It has also been deposited from a solution formed by dissolving pure nickel in nitric acid, then diluting and precipitating it by a solution of carbonate of pot ish, or cyanide of potassium ; washing the precipi- tate and dissolving it nearly to saturation in a solution of cyanide of potassium, and operating upon this liquid, by the battery process, with an anode of pure metal. Its appearance, when deposited from this solution, is said to be nearly equal in whiteness to silver, and its deposition has been proposed to be applied to the production of an inferior class of plated articles. COPPER SOLUTIONS. 61 119. Copper Salts. — The ordinary salts of copper are the protoxide (black oxide of copper), sulphate, chloride, nitrate, acetate, and cyanide. To make the pro- toxide, heat either the carbonate or nitrate to a moderate red heat, or the sulphate to intense redness. The sulphate may be formed by heating one equivalent (31*7 parts) of copper filings, and at most two equivalents (98 parts) of oil of vitriol, until the residue is quite dry, then dissolving the product in water, filtering, evaporating, and crystallizing the solution ; but it is much molfc convenient to purchase it, on account of its low price (fourpence to sixpence per pound). The chloride and nitrate may be formed, the first by dissolving copper in aqua regia, or by saturating hydro- chloric acid with protoxide of copper, and evaporating and crystallizing the liquids ; and the second, by dissolving copper in nitric acid, evaporating and crystallizing the solution. Acetate of copper is most conveniently purchased ; its commercial name is crystallized verdigris. Cyanide of copper may be made by adding a solution of cyanide of potassium to a solution of sulphate of copper (each liquid being cold), as long as a precipitate can be produced ; filtering, and washing the precipitate, which is the required compound ; it is a fine powder of a pale green colour. In the operation a large quantity of cyanogen gas is evolved, which if freely inhaled is dangerous to the health ; in consequence also of this escape of cyanogen the cyanide of copper is not a protocyanide, but contains two equivalents of cyanogen for every three equivalents of copper ; it is freely soluble in a solution of cyanide of potassium ; it is also soluble in aqueous ammonia and in a solution of carbonate of ammonia.' The proportions of materials we have used in making it has been 65 parts of cyanide of potassium and 125 parts of sulphate of copper. The precipitating solution is invariably greenish- blue, and contains much copper after all precipitation ceases, but no use has hitherto been made of this remainder. 120. Copper Solutions. — Copper may be easily deposited either by simple immersion (wash process), by the single cell, or by the battery process. According to Keinsch, iron may be coated with a durable and polishable layer of copper of any thickness (?). The process by simple immersion is thus :— mix together one measure of hydrochloric acid, three measures of water, and a few drops of a solution of sulphate of copper ; clean and immerse the iron ; wash it, rub it with the ropper solution, and re-immersc it repeatedly, adding a few drops of the copper solution occasionally. In depositing copper by the single cell process, a nearly saturated solution of sulphate of copper answers very well ; but for the battery process an excellent solution may be made by dissolving four parts, by weight, of finely divided sulphate of copper (best quality), and one part of sulphuric acid, in about eighteen or twenty parts of water, and then filtering it ; neither of these solutions, however, is fit to deposit copper upon iron, steel, or zinc ; because the electrical relations of these metals in the solution are unsuitable ; these metals decompose the solutions rapidly, and deposit the copper upon themselves by simple immersion. To effect an adhesive deposit of copper upon iron, a solution composed of cyanide of copper dissolved in a solution of cyanide of potassium may be used. It is formed thus : — dissolve cyanide of copper to saturation in water containing about two pounds of water to the gallon, and then add about one- eighth more of the cyanide of potassium solution to form free cyanide ; the liquid is then ready, and should be used at a temperature of about 150° Fah. 121. Copper is electro-positive to iron in the following liquids at 60° Fah. :— powerfully in a solution of hydrosulphuret of ammonia ; feebly in a saturated solution of ammonia ; in a solution of oxide of copper in liquid ammonia ; in aqueous 62 PATENTED PROCESSES FOR BRASSING. ammonia, or in a saturated solution of ferrocyanide of potassium, each but for a short time — it then becomes negative ; in a saturated solution of bichromate of potash ; iu a strong aqueous solution of sulphide of potassium, it is increasingly positive up to the boiling point of the liquid. This last liquid has a similar effect on brass. 122. Brassing Solutions. — Much interest and importance was long attached to the discovery of solutions whereby alloys, and especially brass, might be deposited in the reguline state, and various liquids have been used and patented for this purpose. M. de Ruolz, in 1841, deposited brass from the cyanides of zinc and copper, dissolved together in a solution of cyanide of potassium. Copper articles may be superficially brassed by boiling them in a solution of bitartrate of potash with zinc amalgam, or by boiling them in dilute hydrochloric acid with some bitartaite of potash and zinc amalgam. 123. The same object was effected by Eussell and Woolrich's patent, dated March 19, 1849, which is as follows :— Take 10 pounds of acetate of copper, 1 pound of acetate of zinc, 10 pounds of acetate of potash, and 5 gallons of hot water ; dissolve the salts in the water, then add as much of a solution of cyanide of potassium as will precipitate it and will re-dissolve the precipitate, and in addition add about one-tenth more of cyanide of potassium in excess. Use a brass anode, or else two anodes, one of zinc and one of copper. 124. Joseph Steele's patent, dated August 9, 1850.— Dissolve 2| pounds of American potash in 6 gallons of hot water, and filter the solution ; also dissolve 2£ ounces of acetate of copper in half a pint of strong liquid ammonia, and add it to the first solution with stirring ; also add 4 or 5 ounces of sulphate of zinc, and stir till dis- solved ; and, finally, add 2 ounces of cyanide of potassium, filter the resulting solution, and use it at 100° Fah., with a brass anode. To obtain a dark coloured brass add more acetate of copper ; and to obtain it of a lighter colour, add more sulphate of zinc. 125. Salzede's patent, dated September 30, 1847.— Take 5000 parts of water, dissolve 12 parts of cyanide of potassium in 120 parts of it, then add 610 parts of sub-carbonate of potash, 48 parts of sulphate of zinc, and 25 parts of chloride of copper to the remainder of the water, and heat the mixture from 144° to 172° Fah. ; and when the salts are entirely dissolved, add 305 parts of nitrate of ammonia, allow the liquid to remain undisturbed for twenty hours, and then add the solution of cyanide of potassium ; allow it to remain again till clear, and then draw off the transparent liquid, which is ready for use ; work it with a large brass anode and a strong battery. Another liquid which he uses for brassing consists of 5000 parts of water, 500 parts of sub-carbonate of potash, 35 parts of sulphate of zinc, 15 parts of chloride of copper, and 50 parts of cyanide of potassium. For a bronzing solution he uses 25 parts of chloride of tin in place of the sulphate of zinc of the first brassing liquid, and proceeds as with that process , for a second bronzing solution he uses 12 parts of chloride of tin in place of the sulphate of zinc, of the second brassing liquid, using the solution from 77° to 97° Fah. " 126. Brunei, Bisson, and Gauguin's formula. — Required fifty parts of carbonate of potash, 2 parts of chloride of copper, 4 parts of sulphate of zinc, and 25 parts of nitrate of ammonia, dissolved together in cold water, and used with a brass anode and a strong battery. 127. Morris and Johnson's patent, dated December 11, 1852. — According to this process, dissolve 1 pound of cyanide of potassium, 1 pound of commercial 03 carbonate of ammonia, 2 ounces of cyanide of copper, and 1 ounce of cyanide of zinc, in 1 gallon of water, and use the solution at 150° Fah., with a large anode of brass and a powerful battery. Or a solution may be taken of 1 pound of cyanide of potassium and 1 pound of carbonate of ammonia, dissolved in 1 gallon of water, and saturated with copper and zinc to the requisite degree by means of a strong- battery, a large brass anode, and small cathode, until the latter receives a good deposit of brass, the solution being at a temperature of 150° Fah. To increase the proportion of copper in the deposit, either add cyanide of potassium, or raise the temperature of the liquid ; and to increase the proportion of zinc in it, either add carbonate of ammonia, or lower the temperature. 129. Of the numerous solutions that have been tried for depositing brass, the one just mentioned is much the best. By it reguline and thick deposits of brass, of uniform colour, and of any desired composition, may be obtained. It is not an expen- sive liquid; it acts with average strength upon the anode; it holds a sufficient quantity of the alloy in solution ; it conducts electricity with moderate facility ; and it yields its metal in the reguline state very uniformly ; while it bears a great variation in the electric power without injury to the character of the deposit: is, therefore, very easily managed; does not act perceptibly upon cast iron, wrought iron, steel, or even zinc, so as to injure the adhesion of the deposit ; and it is not decomposed b)' exposure to the atmosphere, to light, or heat, in such a way as to elfect its depositing power. Its defects are, that it requires to be worked hot, and with considerable battery power, in order to make the anode dissolve rapidly, the solution conduct copiously, and to cause a rapid deposit ; while it evolves an abundance of gas at the cathode when working, whether the solution is hot or cold, which indicates that part of the battery power is expended in decomposing the water of the liquid, depositing its hydrogen with the mctalic alloy, and oxidizing the solution. But all the brassing solutions are, theoreti- cally considered, very imperfect. 129. Mercury Solutions. — The ordinary compounds of mercury are the bioxide (red precipitate), bisulphide (vermilion), bichloride (corrosive sublimate), nitrate, and bicyanide. The nitrate is formed by dissolving mercury in nitric acid diluted with three times its bulk of water, the mixture being cold, and no more metal added ;han the acid will dissolve. The solution when diluted with water may be used for depositing by the battery process, a layer of mercury at the bottom of the liquid being used as the anode, and connected witli the battery by a platinum wire passing through a tube of glass or gutta-percha. The bicyanide is made by taking eight parts of prussian blue and 16 parts of peroxide of mercury, both in the state of fine powder, in 30 parts of water, boiling the mixture for about a quarter of an hour, filtering the liquid, and evaporating, and crystallizing the solution ; the resulting salt is the bicyanide, and to form it into a solution fit |pr depositing, it must be dissolved in a solution of cyanide of potassium ; the solution may be used with a mercury anode and battery as already described. 130. Silver Salts. —The most common salts of silver are the oxide, chloride, nitrate, and cyanide ; the oxide, chloride, and cyanide, are sold retail at about seven shillings per ounce, and the nitrate at five shillings per ounce. Oxide of silver is pre- pared by adding a solution of caustic potash to a solution of nitrate of silver, as long as a precipitate can be produced ; the brown precipitate, when washed and dried, is oxide of silver. jChloride of silver is made by adding either hydrochloric acid or a solution of common salts to a solution of nitrate of silver, until a precipitate ceases to he formed ; 64 SILVER SOLUTIONS. the white precipitate of chloride of silver should be washed dried, and preserved out of the influence of light. Nitrate of silver is easily formed by adding grain silver, in small quantities at a time, to a warm mixture of one measure of distilled water and four measures of the strongest nitric acid ; if the liquid is too hot, or too much silver is added at a time, the action will be very strong, and loss of materials may be occasioned ; in such a case add a small quantity of cold distilled water. When the liquid ceases to dissolve more metal, it should be evaporated and crystallized, or else kept, protected from the light, until required to be used ; nearly all the compounds of silver are formed by means of this salt. Acetate of silver is made either by adding a solution of acetate of potash or acetate of soda, to a solution of nitrate of silver, as long as a precipitate occurs, or by digesting the oxide or the carbonate of silver in hot and strong acetic acid ; it is freely dissolved by a solution of cyanide of potassium. Cyanide of silver is generally prepared by adding a solution of cyanide of potassium to one of nitrate of silver as long as a precipitate occurs ; the white precipitate, which is cyanide of silver, is insoluble in water, and is not perceptibly soluble in commercial hydrocyanic acid ; it dissolves very freely in a solution of cyanide of ammonium, potassium, or sodium, and in hyposulphite of soda ; it ia also said to be soluble in solutions of ammonia, carbonate of ammonia, sal-ammoniac, nitrate of ammonia, and ferrocyanide of potassium. 131. M. Brandeley, a French experimentalist, makes the following remarks upon the preparation of cyanide of silver : — " To obtain a beautiful and easy deposit of silver we choose, among all the salts of silver, the cyanide, as giving the best results ; but as the dealers sell this at a high price,both amateurs and manufacturers reject it. Others, for the sake of economy in procuring it, purchase hydrocyanic acid ; but this also is of too high a price, independently of our being obliged to use it immediately, it being both dangerous and difficult to preserve, as the air and light decompose it. If we take commercial hydrocyanic acid which has been prepared fifteen days, and pour it into a solution of nitrate of silver, consisting of one part of the nitrate to six parts of water, cyanide of silver is formed, but it is more or less yellow, and much ammonia and hydrocyanic gases are evolved. On the other hand, if we make a solution of cyanide of potassium, filter it, and dissolve cyanide of silver in it, immediately the solution, which was clear and colourless, becomes troubled and black, and betrays an odour of ammonia and hydrocyanic acid. This odour will continue as long as the solution exists, and a deposit of carbon will be found in the containing vessel. This sediment arises from the decomposition of one part of the cyanide of potassium, caused by the presence of the cyanide of silver. Having occasion to use considerable quan- tities of the cyanide of silver, I dissolve pure silver in pure nitric acid, evaporate just to dryness, dissolve the nitrate of silver thus obtained in distilled water, and pass hydrocyanic (prussic acid) gas through it, from a mixture o^ pounded ferrocyanide of potassium and sulphuric acid diluted with twice its weight of water, continuing thus as long as a precipitate is formed. Wash the cyanide of silver, and preserve it below water, away from the light. Thus precipitated, the salt dissolves without residuum or colour, and gives splendid results." With regard to this process we may remark, that six parts of sulphuric acid should be mixed with from thirty to forty parts of water, and the mixture allowed to cool ; then put into a glass vessel (Fig. 30), together with ten parts of coarsely- powdered ferrocyanide of potassium ; and heat applied until gas is evolved from the mixture, and continued as long as gas is evolved, or as long as a precipitate is pro- duced in the silver solution, the gas being passed into the liquid by a suitable tube SILVERING SOLUTIONS. Go (see Fig 31). This process may yield a purer product than when the nitrate solution is precipitated by cyanide of potassium ; but it cannot be very economical, because only half of the cyanogen of the fer- rocyanide passes over as hydro, cyanic acid; the remainder is left behind, and is contained in the yellowish-white residuum in the gas-generating vessel. 132. Silver Solutions. — Silver has been deposited by the ordi nary dipping or wash process, by the single cell, and by the bat- tery process. The following are recipes for solutions taken from various sources, adapted for sil- vering articles by the simple im- mersion or wash process, chiefly applicable to small articles, such as pins, buttons, buckles, coffin-nails, hooks and eyes, &c, where only a very thin coating of silver is required. The solutions in the propor- tions indicated, are used by adding a small quantity of water, sufficient to form the ingredients into a pasty liquid of the consistence of cream, stirring the articles thoroughly about in it, or rubbing them over with it until they have acquired the desired degree of whiteness : — 1st, take equal parts of chloride of silver and bitartrate of potash ; 2nd, take chloride of silver one part, ahini two parts, common salt eight parts, and tartar eight parts ; 3rd, take chloride of silver one part, prepared chalk one part, common salt one and a quarter parts, and pearl- ash three parts ; 4th, a " novar- gent" solution for re-silvering old plated goods, consists of one hundred parts of hypo- sulphite of soda and chloride, or any other salt of silver, fifteen parts. Compounds of this description are also used for silvering clock-faces, thermometer and barometer plates, and many other articles of copper and brass. 133. Silvering by Immersing. — Mr. Joseph Steele took out a patent, dated August 9, 1850, for silvering articles by immersing them in a silver solution in contact with a piece of zinc of proper size. The process is as follows : — Dissolve four ounces of pure silver in twenty ounces of nitric acid ; also dissolve separately one and a half pounds of common salt in one and a half gallons of water ; mix the two solutions together, allow the mixture to remain till clear, pour away the clear liquid, and wash the preci- pitate, which is chloride of silver ; next fuse together twenty-four ounces of fcrroeyanidc of potassium and twelve ounces of carbonate of potash, and when the mass is cold, add it, together with the chloride of silver, to one gallon and a half of water ; boil the mixture, and filter it ; it is then ready for use. 134. Silvering Solution for Battery Process. — Many solutions have been proposed and tried for depositing silver by the battery process, but none have stood the test of time and experience like tho one composed of double cyanide of silver and potassium dissolved in water, and a little free cyanide of potassium added. It may be made "of various strengths, from half-an-ounce of silver to tho gallon of water, to two, four, six, or more ounces ; and from an ounce of cyanide of potassium to several pounds per gallon, and still be e*ffective in working. The formula of M. de Ruolz is as follows :— dissolve one part of cyanide of silver and ten parts of cyanide of potassium in one PRACTICAL CHEMISTRY— No. III. 66 SILVER PLATING SOLUTIONS. hundred parts of water, and dilute the resulting liquid with water to the required j strength. 135. Silver Plating Solution. — The following is the most practical method of. making a large quantity of the ordinary cyanide of silver plating liquid : — Take four parts of grain silver, add it, in small portions at a time, to a warm mix- ure of about five parts by weight of strong commercial nitric acid (the acid varies greatly in strength), and one part of water, contained either in a glass or stoneware vessel. Gas will be evolved from the surfaces of the pieces of silver, and brown fumes of nitrous acid will arise from the mixture, which should be conveyed out of the apartment by means of the chimney. The action should be maintained moderate and uniform, and if it should become too strong, a little cold water should be added, and the mixture kept more cool ; when the whole of the metal is dissolved, apply a greater heat, and evaporate the solution nearly to dryness, which will drive off any excess of acid that may be present ; the resulting salt, nitrate of silver, may then be dissolved in a large quantity of water, in the proportion of half a gallon (more o r less) to each ounce of the silver used ; at the same time a solution should be made of from 3 to 3j parts (according to its quality) of cyanide of potassium in 30 or 40 parts of water, which is to be added gradually to the solution of nitrate of silver as long as it produces a precipitate ; if too much be added, it will cause some of the precipitate to re-dissolve and be wasted ; it will also make the liquid appear clear where it passes ; in such a case the liquid should be stirred, then allowed to settle clear, and a small quan- tity of nitrate of silver dissolved in distilled water should be added as long as it pro- duces a white cloud. By conducting the operation in a glass vessel, adding the liquid towards the latter period in small quantities at a time, and at intervals of a few minutes each, with gentle stirring immediately upon each addition, carefully observing when it ceases to produce a precipitate, the point of neutralization may be very accurately '' determined. The liquid must now be allowed to remain undisturbed until quite dear, the clear portion poured steadily away from the precipitate of cyanide of silver, and the precipitate washed five or six times in a large quantity of water, by simply adding the water briskly to it, allowing it to settle, and then pouring away the clear portion. Next dissolve from 3 to 3| parts (according to its quality) of cyanide of potassium in 20 parts of water, adding it in portions at a time to the wet cyanide of silver, with free stirring, until the whole is dissolved, then add about 3 parts more of cyanide of potassium to form free cyanide, and sufficient water to reduce the whole to the proportion of about one ounce of silver to the gallon ; finally, when all the free cyanide is dissolved, filter the solution through a piece of unglazed calico. On the small scale, distilled water is used in the various parts of the process, except the washing ; but, on the large scale, clean rain water or spring water is used in all the operations. 136. Another Solution. — The cyanide of silver plating solution may be made by other modifications of the chemical method than the one described ; for instance, some depositors make the solutions by adding oxide, carbonate, or even chloride of silver to a solution of cyanide of potassium, as long as it will dissolve, and then adding an amount of free cyanide ; by this process the depositor is enabled to use caustic potash, carbonate of potash, hydrochloric acid, or common salt, instead of cyanide of potassium, for precipitating the nitrate of silver ; nevertheless it still requires two equi- valents of cyanide of potassium to be used as before, viz., one to convert the salt of silver into cyanide, and the other to dissolve the cyanide of silver formed, because in all such cases, according to the researches of Messrs. Glassford and Napier (" Philoso- SILVER SOLUTIONS BY BATTERY PROCESS. 67 phical Magazine," 1844), when any salt of silver is added to a solution of cyanide of potassium, it is first converted into cyanide of silver at the expense of one portion of the cyanide of potassium ; it then combines with the remaining cyanide to form double cyanide of silver and potassium, which dissolves in the water ; therefore, by this modification of the chemical method, no cyanide of potassium is saved, and the carbo- nate of potash, hydrochloric acid, &c, are wasted. This modification has a still greater disadvantage ; it introduces substances into the depositing liquid which arc injurious. "We have before said (90) that a good depositing solution should dissolve the anode freely, hold abundance of metal in solution, and not act chemically upon base metals, because it is such metals we generally wish to coat ; now, if instead o* cyanide of silver we add oxide of ailver to the cyanide of potassium liquid, it converts part of the cyanide into caustic potash ; if wo add carbonate of silver, it converts it into carbonate of potash ; and if chloride of silver, it converts it into chloride 0; potassium ; and each of these substances, especially the last, diminishes the action of the liquid upon the dissolving plate, decreases its solvent power for cyanide of silver, makes its particles less mobile, and causes it to act in some degree upon base metals, and thus endangers the adhesion of the deposits upon them. Some electro-platers think the presence of these salts not injurious, but most consider them highly detrimental. 137. Solution by Battery Process. — The same silver solution may be formed by the battery process (89) as well as by the chemical method, and this process has its own advantages and disadvantages ; it is very convenient in making a small quantity of liquid, because it enables the operator to make it quickly, to avoid the trouble of making the nitrate solution, of precipitation, of washing, and of the attendant risk of loss of materials ; but it has the disadvantage of converting a large proportion of the cyanide of potassium into caustic potash, by taking its cyanogen to form cyanide of silver, and setting the potassium free, which immediately combines with the oxygen of the water, forming caustic potash, which dissolves in the liquid ; whilst the hydro- gen of the water is evolved at the cathode, and the dissolved potash gradually becomes converted into carbonate of potash by absorption of carbonic acid from the atmo.-pherc. Neither caustic potash nor carbonate of potash arc so injurious in the liquid as chloride of potassium ; still they diminish the action of the liquid upon the dissolving plate, render it a worse conductor, reduce its solvent power for cyanide of silver, and make its particles less mobile. 138. Solid Deposition of Silver.— -Mr. Alexander Parkes took out a patent, March 29 5 1841, for improvements in the solid deposition of silver. lie converts an ounce of silver into oxide of silver, by first dissolving it in nitric acid, and then precipitating it by caustic potash ; he then dissolves the oxide together with sixteen ounces of cyanide of potassium in two gallons of water, and uses the resulting liquid for depositing solid articles in silver. 139. Mr. Edmund Tuck took out a patent, June 4, 1842, for "improvements in depositing silver upon german silver." For plating the [commoner qualities of this alloy he uses a solution composed of sulphate of silver dissolved in a solution of car- bonate of ammonia, and for the best quality he uses cyanide of silver dissolved in a solution of carbonate of ammonia. The solutions are formed by dissolving 70 parts of carbonate of ammonia in distilled water, then adding 156 parts of sulphate of silver, or 134 parts of cyanide of silver, and boiling the liquid until the salt is dissolved ; for coating common german silver he adds half an ounce of sulphate of silver to 107 grains of bicarbonate of ammonia. 68 ELECTRO-PLATING LIQUIDS. 140. For depositing purposes, a solution composed of water 20 parts, cyanide of potassium four parts, and acetate of silver one part, conducts very freely, and yields'a fine white deposit of silver. A solution composed of water 25 parts, prussic acid 65 parts, "black" cyanide of potassium 12 parts, and cyanide of silver 10 parts, is also a very good one. 141. Many electro-platers use a cyanide solution containing about half an ounce of silver to the gallon, and add a very large proportion of free cyanide to make it con- duct freely ; such a solution has the advantage of being comparatively inexpensive in its first formation, quick in working, and yields metal of an average character ; but it is rather difficult to manage in hot weather, and dissolves the anode very rapidly, on account of the large proportion of free cyanide. In practice, the amount of silver to the gallon varies from half an ounce to about four ounces, but ordinary solutions con- tain about one or two ounces to the gallon ; the amount of free cyanide of potassium also varies from about half the weight of the silver dissolved in the liquid to five or ten times this quantity ; a very good proportion is about three-fourths of the weight of the dissolved silver ; but there is no rule generally recognised in the trade upon this point ; some manufactures use a very large and others a very small proportion. 142. A good plating liquid should contain one equivalent (65 parts) of pure cyanide of potassium, and one equivalent (134 parts) of cyanide of silver, besides free, cyanide, and sufficient water to form a thin liquid. It is necessary to have free cyanide, because in working the solution insoluble cyanide of silver is formed and requires free cyanide of potassium to combine with it and form the soluble double cyanide, at the same time cyanogen and cyanide of potassium are set free at the cathode or receiving surface by the deposition of the silver ; and as it requires some time for those substances to mix with the liquid and reach the dissolving plate, free cyanide must be provided. The necessity of having sufficient water to form a thin liquid arises from the double cyanide formed at the dissolving plate, being specifically heavier than the liquid, having a tendency to sink to the bottom, whilst the cyanogen and cyanide of potassium set free at the surface of the articles, being specifically lighter, tends to rise to the surface ; at the same time each of them mixes more or less with the surrounding liquid by capillary attraction or adhesion, and the more dilute the liquid is, the more mobile are its particles, and the more rapidly does this mixture take place. This explains why strong silver solutions require more frequent stirring than weak ones to keep them uniform. In some manufactories, where they have steam-power at command, the articles are kept in constant motion by machinery swinging them gently to and fro ; but in ordinary electro-plating establishments, the silver solutions are stirred every evening. 143. If a solution contains but little water and a large supply of free cyanide, and from any cause the battery current becomes suddenly weak towards the evening, the silver deposited upon the articles will be re-dissolved, in consequence of the liquid about the dissolving sheets having by the day's work become saturated with silver, and that about the articles become full of free cyanide ; the two electrodes (i.e., the dissolving plates and the articles) form a kind of voltaic battery (one metal in two liquids), which develops a current of electricity in an opposite direction to the original one, and thus re-dissolves the deposited silver. 144. Bright Silver Solution. — Much practical interest was for a long time attached to the anticipated discovery of a solution by which silver might be deposited in a bright condition, and the labour of burnishing be thereby avoided or lessened ; and BRIGHT SILVERING SOLUTIONS. G9 this discovery was at last effected as follows : — Some operators at the electro-plate works of Messrs. Elkington and Mason, Birmingham, were engaged m some experiments on moulds containing bisulphide of carbon ; whilst these moulds were being coated with silver in the depositing vats, the depositor observed some very peculiar ap- pearances upon the various articles receiving a deposit in the vat, some having very bright patches upon them like burnished metal. From the known presence of j bisulphide of carbon, experiments were tried of adding that substance to a quantity of silvering liquid, which ultimately resulted in success, and a patent was taken out by Messrs. Lyons and Millward, March 23, 1847, in which they give the following in- structions for forming a " bright solution :" — " Add to the usual solution of silver in cyanide of potassium, bisulphide of carbon, terchloride or other chloride of carbon, I sesquichloride of sulphur, or hyposulphite of either potash or soda. The bisulphide of ( carbon may be used alone or dissolved in sulphuric ether ; or it may be used in con- : junction with any of the other substances mentioned above, but the patentees prefer | using it as follows : — Six ounces of bisulphide of carbon are put into a stoppered bottle, j and one gallon of the usual plating liquid added to it, the mixture is then shaken and j set aside for twenty-four hours ; two ounces of the resulting solution are then added to every twenty gallons of the ordinary plating solution in the vat, and the whole stirred together ; this proportion must be added every day, on account of the loss by evaporation, but when the mixture has been made several days, less than this propor- tion may be used at a time ; when hydrocarbons are used instead of the bisulphide, a much larger quantity must be added. This proportion gives a bright deposit, but by adding a larger proportion a dead surface may be obtained, very different to the ordinary dead surface." This substance is generally employed throughout the trade, although few are licensed to use it. Other compounds are also used, but to a very limited extent; among these arc sulphur and collodion. A solution of iodine and gutta-percha in chloroform is said to be more permanent in its effect than the bisul- phide of carbon. The liquid is generally added to the vat in the evening after the work has been taken out. A method of bright gilding has also been recently brought into use in the trade. 145. Method of Making Cyanide of Potassium. — As nearly all the solutions which arc used for electro-silvering and gilding contain cyanide of potassium, and as this substance is used extensively in electro-deposition generally, it will be necessary for the practical depositor to understand how it is made, and to possess information re- specting its impurities, and the method of testing its quality. It is nearly always made by the following process :— Take ferrocyanide of potassium (yellow prussiate of potash), pound it fine, and gently heat it in an iron pan, with constant stirring until quite dry ; treat a quantity of the best quality of carbonate of potash in a similar manner. When they are perfectly dry, add about three parts of the carbonate to eight parts of the ferrocyanide, and thoroughly mix them ; heat the mixture rapidly in au iron ladle or crucible, until it melts into a clear liquid, when gas will be evolved from its surface. It should be maintained at a moderate or dull red heat aboiit fifteen or twenty minutes, or until the end of a cold iron rod dipped into it shows a white sample. The fusion should not be continued until the evolution of gas ceases, or the product will be of a gray colour. It should be kept covered as much as possible. By allowing it to stand undisturbed a few minutes at the latter part of the' operation, and occasionally tapping the sides of the ladle or crucible, the iron of the ferrocyanide will settle at the bottom as a fine black powder; the 70 CYANIDE OF POTASSIUM. colourless cyanide of potassium may then be poured off into a cold iron pan, or upon a thick and cold iron plate ; it should be broken up whilst still warm, and preserved in a well-stopped jar. The black sediment, which contains much cyanide of potassium, should be scraped out of the vessel while still hot, and preserved, as water will at any time dissolve the cyanide. If the process has been well conducted, the product will be of a clear white colour, or at most but very slightly gray. A larger proportion of cyanide of potassium is obtained by this process than when ferrocyanide alone is employed, because in the former case one-third of the cyanogen (that which was combined with the iron) combines with the potassium of the carbonate of potash, whilst in the latter case it is lost ; the cyanide produced by the fusion of the ferrocyanide of potassium alone is of a grayish-black colour, and is termed " black cyanide." 146. Cyanide of Potassium. — Commercial cyanide of potassium varies very much in price ; the best quality of white cyanide is sold retail from 2s. 3d. to 2s. 6d. per pound in the United States of America ; in England it varies from 2s. Gd. to 4s. 6d. per pound, according to quality ; and black cyanide may be obtained retail at 3s. lOd. per pound. By dissolving several specimens of commercial black cyanide in water, and filtering the solution, we found the proportion of black impurity in them varied from one-fourth to one-sixth of their weight ; and by experiments with the commercial white cyanide we found that one part (200 grains) of it, dissolved in about one and a quarter parts (230 grains) of distilled water at 60° Fah., and that it dissolved much more freely in water containing hydrocyanic acid. 147. Impurities in Cyanide of Potassium. — According to the researches of Messrs. G-lassford and Napier (" Philosophical Magazine," 1844), commercial white cyanide generally contains about thirty-five per cent, of impurities, and often as much as fifty per cent., in the form of carbonate and sulphate of potash, chloride of potassium, cyanate of potash, ferrocyanide of potassium, and silica ; and if the mixture of salts from which it is made is not dry, ammoniacal compounds are also formed. The sulphate of potash and chloride of potassium occur in the commercial carbonate of potash ; the silica is present when we operate with an earthen crucible ; and even when the process is well conducted and pure materials used, the product contains twenty per cent, of cyanate of potash, produced partly by the contact of the air with the melted mixture. 148. Testing Cyanide of Potassium. — According to the same experimentalists, the quantity of pure cyanide in any given sample of cyanide of potassium may be cor- rectly ascertained thus : — Make two solutions, one of the given cyanide and one of nitrate of silver, each containing known weights of the salts, say one ounce of the cyanide dissolved in six ounces of distilled water in a graduated glass vessel, and 175 grains of the crystallized nitrate dissolved in about two or three ounces of distilled water ; add the cyanide solution carefully and slowly to the nitrate of silver liquid, until the precipitate first formed is all re-dissolved. The quantity of the cj^anide solution required to effect this (with the above quantity of nitrate of silver) will have contained 130 grains of pure cyanide, and from the quantity used we may easily cal- culate the amount of pure cyanide in the whole ounce. It is said, that " when nitrate of silver is added to a solution of cyanide of potassium, so long as the precipitate formed is all re- dissolved, we obtain the whole of the cyanide of potassium in com- bination with the silver ; none of the other salts in solution take any part in the action, even though they be present in a large proportion. This enables us to test the exact quantity of cyanide of potassium in any sample." GOLD DEPOSITING SOLUTION. 71 149. Chemical Characters of Cyanide of Silver. — In the presence of cyanide of potassium, as we are informed by Messrs. Glassford and Napier, cyanogen lias a greater affinity for silver than anything else has, decomposing every salt of silver except the sulphide, and forming cyanide of silver. In dissolving the oxide, carbonate, chloride, or ferrocyanidc of silver, in a solution of cyanide of potas- sium, they are all decomposed, and cyanide of silver always formed. Cyanide of silver should be dried below 260° Fah. ; hydrochloric acid decomposes it with a solution of hydrocyanic acid gas ; cold nitric acid has no action upon it ; a boiling mixture of sulphuric acid and -water decomposes it, with escape of hydrocyanic acid gas, and formation of sulphate of silver ; it is soluble in the alkaline chlorides, but its best solvent is an aqueous solution of cyanide of potassium, of which it requires one equivalent (65 parts) to dissolve one equivalent (134 parts). The resulting solution, when evaporated, yields crystals of double cyanide of silver and potassium, which are soluble in eight parts of cold and in one part of boiling water. The solution of this double salt, which is nearly the same as the ordinary plating solution, may be boiled for any length of time without being decomposed, and it is very little affected by light ; it is decomposed by all acids, and they precipitate the silver as cyanide of silver ; the hydro-acids — hydrochloric acid, for example — decompose the cyanide of silver also ; sulphuretted hydrogen precipitates the silver as sulphide of silver. 150. Gold Solution.— Various salts of gold have been used for electro-deposition, among which are the hyposulphite, sulphite, iodide, bromide, terchloride, cyanide, and sul- phocyanide. Finely-divided gold, which is sometimes used for dissolving, may be obtained by adding a solution of protosulphate of iron to a solution of terchloride of gold, as long as a greenish-brown precipitate occurs ; this is gold in a state of vary minute division. Oxide of gold is obtained by adding to a solution of terchloride of gold a cold solution of caustic potash, until it ceases to produce a precipitate ; or by digesting terchloride of gold with magnesia; washing the latter precipitate, first with dilute nitric acid, and then with water only. Iodide of gold is formed, either by digesting oxide of gold in hydriodic acid, or by adding a solution of iodide of potassium to a solution of terchloride of gold as long as a precipitate is produced, washing the precipitate with water ; it is of a yellow colour, insoluble in cold water, but freely soluble in a solution of iodide of potassium. Bromide of gold may be formed cither by digesting finely-divided gold, or oxide of gold, in'liquid bromine contained in a stoppered bottle. It is a salt of a rich red colour, and is soluble in water. 151. Chloride of Gold. — Terchloride of gold, commonly known as chloride of gold, is the most usual salt of the metal, and its preparation requires separate explanation. It is formed as follows : — Take a mixture of either two or three measures of hydrochloric acid, and one measure of nitric acid ; make the mixture hot, and add pure grain gold to it as long as it will dissolve ; it evolves gas whilst dissolving. "When it is all dissolved, 6lowly evaporate the liquid until it crystallizes to a dark ruby red mass, or it may be of a yellow colour, according to the proportions of the ingredients. This is terchloride of gold, and contains one equivalent (197 parts) of gold, and three equivalents (106-5 parts) of chlorine ; if it is too much evaporated, chlorine gas will be evolved, the gold will be set free and he mixed with the salt, and will precipitate on dissolving the salt in water. To produce the yellow chloride, mix together, in a glass or stoneware vessel, one part by weight of nitric acid, three parts of hydrochloric acid, one part of fine gold, and one part of water ; cover the vessel with a glass dish, make the liquid quite hot, and maintain the heat until the red vapours cease ; if some of the gold remain undissolved, add more 72 GILDING BY IMMERSION. of the liquid mixture, and treat as before ; when the vapours cease, remove the glass cover, and replace it by folds of blotting-paper, and evaporate until it crystallizes on cooling into yellow chloride of gold. The red chloride is formed in the same manner, except that the liquid mixture should be composed of one part of nitric, and two parts of hydrochloric acid, more being added than is necessary to dissolve all the gold. One ounce of gold will dissolve in about four ounces of the mixture, and when crystallized into the red mass, will weigh about one ounce and 165 grains. 152. Cyanide of Gold. — " Cyanide of gold is formed by cautiously adding a solution of cyanide of potassium in six parts of water, to a normal solution (i.e., not containing any free acids) of terchloride of gold, consisting of one part of the chloride and five parts of water, until a copious yellow precipitate settles down ; if more cyanide of potassium is added, the precipitate becomes dirty yellow, and is more quickly deposited ; a still larger quantity renders it orange yellow. It is a crystalline powder, permanent in the air. By ignition it is resolved into gold and cyanogen gas ; it is not decomposed by sul- phuric, hydrochloric, or nitric acid, nor by aqua regia, unless freshly precipitated, and then only slowly. It is not decomposed by sulphuretted hydrogen ; hydrosulphate of ammonia dissolves it slowly but completely, forming a colourless solution, from which, by the addition of acids, sulphide of gold is precipitated. It dissolves in aqueous solu- tions of ammonia, hyposulphite of soda, or alkaline cyanides ; but not in water, alcohol, or ether." 153. " Gold precipitated from a solution of chloride of gold by protosulphate of iron, dissolves in a boiling solution of cyanide of potassium ; a hot solution of cyanide of potassium will also dissolve ordinary metallic gold if air be present. Both oxide of gold and aurate of ammonia, dissolve completely in a solution of cyanide of potassium, and form double cyanide of gold and potassium. Cyanide of gold requires 23 parts of cyanide of potassium dissolved in water to dissolve it. For every one part of gold to be dissolved by the battery process, six parts of cyanide of potassium dissolved in from two to four times their quantity of water, at 100 D Fah., is required; two electrodes of gold being connected with a suitable battery, and immersed in it until the required quantity of gold is dissolved."—" The crystallized cyanide of gold and potassium dis- solves in seven parts of cold, and in half a part of hot water (Himly) ; in four parts of cold and in 0-8 parts of hot water (Glassford and Napier). It dissolves very sparingly in alcohol. Its aqueous solution gilds copper and silver by simple immer- sion, especially if hot, and the copper and silver dissolves in it." 154. Gildina bv Immersion.— The following solutions have been used for gilding by the simple immersion, or " water-gilding" process : — First, dissolve five troy ounces of grain gold in fifty-two avoirdupois ounces of hot aqua regia, until vapours cease to be evolved. Decant the clear liquid when cool, dilute it with four gallons of distilled water, add twenty pounds of purified bicarbonate of potash, and boil it for two hours. The articles to be gilt are immersed in the liquid from a few seconds to one minute, according to the kind of metal immersed, and the temperature and newness of the liquid ; warmth assisting the action, and a new liquid acting more quickly than an old one. Second, for gilding silver articles, dissolve equal parts of bichloride of mercury (corrosive subli- mate) and sal-ammoniac in nitric acid ; add pure gold to it, and evaporate the liquid by heat to half its volume. Apply the liquid while hot to the surface of the article. 155. Joseph Steele's patent, dated August 9, 1855. — Dissolve an ounce of gold in a mixture of four ounces of hydrochloric acid and eight ounces of nitric acid, and evapo- rate the solution to dryness. Fuse together 24 ounces of ferrocyanide of potassium, GOLD DEPOSITION BY BATTERY PROCESS. 73 and 12 ounces of carbonate of potash ; when it is nearly cold, dissolve it in two or three gallons of pure boiling water, cool and filter the solution, then add the chloride of gold, and boil it for a quarter of an hour. The articles to be coated are connected with a piece of zinc, of suitable size, and immersed in the liquid, the latter being at a tem- perature of 80° or 85* Fah. 156. Gold Solution for the Battery Process.— For the battery process there are many gold solutions, though but few good ones : — 1st, [The hyposulphite of gold and soda, which is formed by dissolving chloride of gold in a solution of hypo- sulphite of soda ; it is not considered a good liquid for practical purposes. 2nd, The sulphite of gold and potash, used by Mr. Woolrich ; his solution was made by adding sulphite of potash to water, saturating five-sixths of the resulting liquid with oxide of gold, and then adding the other portion of solution to form free sulphite. 3rd, The terchloride of gold dissolved in water ; it is a very inferior liquid for practical purposes, because all the common metals decompose it. 4th, The bromide of gold, proposed by Mr. Spencer ; his solution was made as follows : — " Make a mixture of equal parts of bromine and alcohol, and of this mixture take one part, of acetic acid one part, and four parts of water containing a few drops of sulphuric acid." The resulting liquid is then nearly saturated with gold by suspending it in two elec- trodes of gold, and connecting them a sufficient time with a suitable battery ; when nearly saturated, add to the solution three times its vulume of water containing a few drops of sulphuric acid. 157. Electro- Gilding Liquid.— The best liquid that has yet been tried for practical electro-gilding, consists of the double cyanide of gold and potassium dissolved in water. It was first patented by Messrs. Elkington, and may be formed, either by dis- solving finely-divided gold, or any salt of gold, in a solution of cyanide of potassium ; or by the battery process, by suspending two electrodes of gold in a solution of cyanide of potassium, and passing a current from a small battery until the cathode receives a proper deposit, the liquid being at about 100° or 150° Fah. 158. Cyanide of Gold by Chemical Process. — The cyanide of gold and potassium gilding solution is often made by the chemical method, as follows : — Form some terchloride of gold (151), and dissolve it in water ; then either add a cold solution of caustic potash as long as a precipitate is produced, filtering and washing the precipitate with distilled water ; or by digesting the chloride solution with magnesia; filter, and wash the precipitate first with nitric acid and then with distilled water; or add to the chloride liquid a, solution of carbonate of ammonia, until a precipitate ceases to be formed ; filter, and wash the precipitate with water. The precipitates produced by potash or by magnesia, consist of oxide of gold ; whilst those pro- duced by ammonia, or its carbonate, arc aurate of ammonia (fulminate of gold), a very explosive compound. The precipitate, after being well washed by the successive additions of clean water, should be added, whilst still wet, to a solution of cyanide of potassium, containing the proportion of one pound of cyanide to one gallon of water, and then about one-fifth more of the same solution should be added to form free cyanide. A very good proportion of the ingredients is, one ounce of gold, one pound of cyanide of potassium, and one gallon of water. The wash-waters should not be thrown away without first being tested for gold, by immersing a piece of bright zinc in them, and observing if it receives a yellow deposit ; if it does, a solution of protosulphate* of iron should be added as long as a precipitate of a greenish-brown powder, which is metallic gold, is produced. If this fails to precipitate the whole of the 74 GOLD DEPOSITION BY BATTERY PROCESS. gold, a sheet of bright zinc should he immersed in the liquid, taken out occasionally, and the deposit of gold brushed off by a hard brush in water containing a little sul- phuric acid. The greater the quantity of free acid contained in the original chloride solution, and the larger the excess of potash, ammonia, or carbonate of ammonia added, the greater is the amount of gold dissolved in the wash-waters. If, when we dissolve the terchloride of gold in water, a yellow powder remains undissolved at the bottom of j the vessel, it indicates that there is no free acid in the salt, and may be redissolved by | the addition of a small quantity of the mixture of nitric and hydrochloric acids and the j application of heat. 1-59. Solution by Battery Process. — The same solution may be made by the battery pro- j cess, thus : — Dissolve some cyanide of potassium in hot distilled water, in the proportion ! of from one to two pounds to the gallon ; nearly fill a small porous cell with the liquid, j and immerse it nearly to its edge in the solution ; place a large gold anode in the outer liquid and a small bright copper cathode in the liquid of the porous cell, and connect them with about three pairs of Smee's batteries — the gold anode with the platinized silver by one wire, and the copper cathode with the zinc by another wire, allowing the current to pass, until, by transferring the cathode for a short time to the outer liquid, it receives a good deposit of gold, the solution being maintained at a temperature of about 150° Fah. ; the liquid of the porous cell should then be transferred to the^outer solution and the process stopped. The amount of gold dissolved is not of material consequence provided the deposit is good, as a solution may contain from half an ounce to four ounces to the gallon and be a good depositing liquid. 160. Gilding Solution of 31. Ruolz. — "Dissolve ten parts of cyanide of potassium in 100 parts of distilled water ; filter it, and add one part of cyanide of gold, prepared with care, well washed, and dried out of the influence of light ; keep the mixture in a closed glass vessel, at the temperature of 60° to 77° Fah. for two or three days, out of the presence of light,~with frequent stirring." 161. M. BecquereW s Gilding Liquid. — " Dissolve one part of terchloride of gold and ten parts of ferroeyanide of potassium in one hundred parts of water ; filter the liquid, to remove the separated cyanide of iron ; add 100 parts of a saturated solution of ferro- eyanide of potassium, and dilute the mixture with once or twice its volume of water. In general the tone of the gilding varies according as the solution is more or less dilute; the colour is most beautiful when the liquid is most dilute, and most free from iron. To make the surface appear bright it is sufficient to wash the article in water, acidulated with sulphuric acid, rubbing it gently with % piece of linen cloth." 162. Gilding Liquids of M. Fizeau.—" 1st, Dissolve one part of dry chloride of gold, in 160 parts of distilled water ; then add, little by little, a solution of carbonate of potash in distilled water until the liquid begins to become cloudy : we may use this liquid immediately. And, 2nd, used by M. Lerebour i — Dissolve one gramme of chloride of gold and four grammes of hyposulphite of soda in one litre of distilled water." 163. M. Levol's Solution for Gilding Silver.—" Dissolve neutral chloride of gold, then add an aqueous solution of sulphocyanide of potassium, until the precipitate first formed is redissolved. The liquid will retain a slightly acid reaction ; if it has lo«t it, it must be renewed by adding a few drops of hydrochloric acid." 164. By M. De Briant.— U Dissolve thirty-four grammes of gold in aqua regia, and evaporate the solution until it becomes neutral chloride of gold ; then dissolve the chloride in four kilogrammes of warm water, and add to it 200 grammes of commercial I magnesia, carefully sifted. The gold is precipitated in union with the magnesia; filter SOLID DEPOSITING. 75 and wash with pure water ; digest the precipitate in forty parts of water mixed with three parts of nitric acid to remove the magnesia; then wash the oxide of gold remaining with water, until the wash -water exhibits no acid reaction. Next dissolve 400 grammes of ferrocyanide of potassium, and 100 grammes of caustic potash, in four litres of water, add the oxide of gold, and boil the solution about twenty minutes. I When the gold is dissolved there remains a small amount of iron precipitated, which J may be removed by filtration, and the liquid, of a fine gold-yellow colour, is ready for use. It may be used either hot or cold." 165. Formulas of M. J. L. — "1st, Take thirty-one grammes and twenty-five centi- grammes of oxide of gold, five hectogrammes of cyanide of potassium, and four litres of water, and boil them together half an hour. The resulting solution must be worked hot, and may be used to gild coppor, brass, and silver. 2nd, Dissolve ten parts of ferrocyanide of potassium and one part of dry terchloride of gold in 100 parts af water; oxide of iron will be precipitated. Boil the solution two or three hours, in a porcelain or glass vessel, until a precipitate collects at the bottom and the supernatant liquid I is transparent and of a canary-yellow colour ; filter the solution, and dilute it with i three times its volume of water." 166. A process or branch of trade termed "solid depositing" has of late years been i gradually extending itself. It consists in making solid articles of gold and silver, by j electro-deposition, upon gutta-percha or other moulds — such, for ins'ance, as watch I and clock faces, ornamental snuff-boxes, and other articles elaborately chased or engraved, or which have very complex or undercut ornaments upon them; the expense of multiplying these by the electro-process being less than by the ordinary means. Mr. Alexander Parkes took out a patent, dated March, 1841, for a solution for depositing solid articles in gold ; it is formed thus : — Dissolve one ounce of pure gold in aqua j regia, and evaporate the solution to dryness; then add two gallons of water and sixteen ounces of cyanide of potassium, and work the resulting liquid at a temperature of about 120° or 130° Fah. 167. Salts o f Platinum. — The only common salt of platinum is the bichloride ; it is formed by adding pieces of platinum-foil to hot aqua regia as long as gas is evolved from them ; the solution, which is then of a deep red colour, should be evaporated nearly to dryness and left to cool. 168. Platinum Solutions. — For platinizing silver by simple immersion process, we ; may use a solution consisting of bichloride of platinum, dissolved in water containing I one-fourth its volume of nitric acid, or we may use simply a very hot aqueous solution | of the bichloride alone. Nearly all metals decompose the bichloride solution, and become coated with platinum in it by simple immersion. For the battery, process we I may use solutions of the iodide, bromide or bichloride, or the double chloride of j platinum and 6odium. The solution of the double chloride of platinum and sodium is ! made by dissolving one equivalent (1697 parts) of bichloride of platinum and one j equivalent (58*5 parts) of common salt, in water; it requires a small anode of platinum I and a very weak battery to obtain a reguline deposit. A good solution for depositing j reguline platinum may also be made by dissolving bichloride of platinum and common salt in a solution of caustic potash. 169. Palladium Solutions. — A solution of double cyanide of palladium and potassium may be used for depositing palladium. It may be made by chemical means, by dissolv- ing palladium in nitric acid, precipitating the solution by a solution of cyanide of potassium, washing the precipitate, and dissolving it in a solution of cyanide of 76 SELECTED LIQUIDS FOR DEPOSITING. potassium to saturation, and then adding a little free cyanide ; or it may be easily made by the battery process, by passing a current through a large palladium anode in a solution of cyanide of potassium, until a clean smooth cathode receives a good deposit. This is an excellent solution for depositing reguline metal ; it acts upon the anode with uncommon energy, conducts freely, and deposits^ plenty of reguline metal by easy management ; a thin deposit of palladium obtained in this solution has been used for fixing Daguerreotype pictures, instead of gold, and is said to give them a finer tone. 170. Selected Practical Liquids. — Having enumerated nearly all the solutions which have been used for depositing different metals, and described the modes of forming them, we now offer a selection of those which are in most general use in the trade : — 1st. For depositing zinc, which is not very often attempted, the sulphate solution (105) may be used. 2nd. For depositing copper upon all ordinary metals except zinc, tin, lead, iron, and steel, and upon gutta-percha, wax, and elastic moulds, after being made conductible by the battery process, the sulphate solution (120), is in general use; but for surfaces of tin, lead, iron, or steel, the solution of cyanide of copper and cyanide of potassium (120) is used. 3rd. For the deposition of brass upon all ordinary metals, the patented solution of Messrs. Morriss and Johnson (127) is very successfully used ; the solutions of Brunei (126) and Salzede (125) are also in practical use ; and, in the United States of America, a solution very similar to that of Messrs. Morriss and Co. is in use. 4th. For depositing silver upon all common metals, the solution of double cyanide of silver and potassium (135), is almost universally used ; the only exception is where the double sulphite of silver and potash is used, but only to a small extent, the mag- neto-machine being in some cases used with it. 5th. For gilding all ordinary metals, almost the only liquid used is the double cyanide of gold and potassium (157 and 158), the only exceptions being a few minor modifications of it in private use. , 6th. For platinizing, the only solution in practical use is the bichloride (167). 7th. For solid deposition of silver and gold, the patented solutions of Mr. Alexander Parkes (138 and 166), maybe used. 171. Preparing Metal for Receiving a Deposit. — The electro-depositor hav- ing acquired a general knowledge of depositing liquids, as well as of the necessary manipulation, and having made the several solutions necessary for his ordinary use, will require to prepare his articles for receiving a deposit. All metallic articles require to be cleaned and otherwise prepared before they are fit to receive a deposit ; this preparation differs of course according to the nature of the article, whether the deposit is required to adhere firmly to the surface, or is merely used as a matrix, from which the deposited metal is to be removed. 172. Cast Iron and Zinc. — Articles formed of zinc, wrought iron, cast iron, or steel, I are first immersed a few minutes in a boiling solution of caustic potash to remove any grease, tar, or oily substance which may be upon them ; they are then washed in water, and those of wrought or cast iron are immersed in " pickling liquid " (82), until the acid acts rather freely upon them ; rough cast iron requires a stronger liquid than smooth wrought iron ; after being again washed in water, they are " scratched " at the brush (80), and if they are very coarse castings or rusty articles, they may require ' several soakings in the dilute acid, and scourings with sand and a hard brush, and even filing to make them quite clean. 77 173. Copper, Brass, and German Silver.— Those formed of copper, brass, or german silver, should be boiled in the potash liquid, washed in water, dipped into nitric acid or into " dipping liquid" (82), and then washed in water, and dipped into a solution of nitrate or cyanide of mercury (84), before immersing them in the silvering solution. 174. Sometimes, in order to assist in cleaning the articles, they are suspended for a short time in the depositing liquid, in contact with the negative pole of the battery ; this dissolves the surface, and loosens their impurities, unless they are very foul, or the solution is too valuable. In every case they should be well rinsed with water to remove the adhering acid, before dipping them into the mercury solution, or immersing them in the depositing vat. All objects which are to have a defi- nite weight of metal deposited upon them, are weighed, and their weight noted down after they have been cleaned. 175. Wiring Articles. — The articles, having been cleaned throroughly, have wires of copper attached to them, to suspend them by when in the vat. The wires differ in size with different articles ; with small ones, such as spoons, knives, forks, snuffers, teapots, jugs, and such articles, size No. 20 or 22 of the Birmingham brass wire guage, and about eighteen or twenty inches long, are used. Very large articles, such as fire-irons, fenders, hat-stands, and articles of ornamental iron-work, are suspended in the solution by strong copper or brass hooks ; in some cases where a powerful and certain connection is required, the wires are soldered to the articles. 176. Preparing Articles for Adhesive Deposits. — "We have already explained how necessary it is that all articles intended for the depositing vat should be cleaned in the most perfect manner possible, before being immersed in the depositing liquid, other- wise the deposit will not adhere. Articles of copper, bras?, or german silver, which are to be silver-plated, should also be dipped into one of the solutions of mercury (84) ; otherwise the deposit will either not adhere at all, or will vary in appearance in different parts ; and in consequence of this perfect degree of cleanliness required, the cleaning of them often involves more trouble than the de positing. All articles should be plunged while still wet from the cleaning process, into the depositing vat. The practical minutia) of preparing the surfaces of different metals for receiving adhesive deposits of other metals, varies in almost every manufactory, and much information yet remains to be developed upon this point ; for want of this knowledge, the most skilfful operators sometimes fail in producing perfect adhesion, especially upon zinc, cast iron, steel, and Britannia metal. 177. Preparing Metals for Non-adhesive Deposits. — Metal articles which are to receive non-adhesive deposits, such as medallions, of which copies are desired in copper, should be allowed to remain a sufficient time to slightly oxidate after being cleaned, before being plunged in the depositing vat, the oxide preventing adhesion. In some cases they are rubbed over with cotton wool slightly moistened with a very weak solution of bee's-wax dissolved in camphine in the proportion of a piece of wax of the size of a small pea in a quarter of a pint of the spirit ; others use a little sweet oil, which is immediately wiped off with a fresh piece of dry cotton wool. 178. " Stopping -off" to Prevent Deposition.— -Many articles which are to receive deposits require to have portions of their surface " stopped-off," to prevent the deposit spreading over those parts ; for instance, in taking a copy of one side of a metal me- dallion, the opposite side must be coated with some kind of varnish, wax, or fat to prevent deposition ; or in gilding the inside of a cream jug which has been silvered on the outside, varnish must be applied all round the edge on the outside for the | 78 MOULDING MANIPULATION. same reason. For gilding and other hot solutions, copal varnish is generally used ; but for cold liquids and common work, an ordinary varnish, such as engravers use for a similar purpose, will do very well. In the absence of other substances a coating of sealing-wax dissolved in naphtha will answer every purpose. 179. Moulding and Copying Works of Art. — The electro-depositor who includes in his business the multiplication of works of art, as well as the simple plating of metal articles, will require a knowledge of the art of moulding. To copy both sides of a metallic coin or medal in the mixture of gutta-percha and marine glue recommended (85), take a strip of thin sheet copper, brass, or tinned iron about an inch wide, wind it closely round the edge of the medal, and solder its ends together ; wipe the medal and take two balls of the composition, quite hot and soft, and press them simultaneously against the two faces of the medal, working the material from the centre towards the circumference to exclude bubbles of air ; place two thick plates of cold metal, one on each side, and gradually screw up the whole in a vice, or screw press, gently at first, but increasing the pressure to a high degree as the materials become hard. "When it is quite cold, which will be in about two hours, the two copies may be easily removed from the original, by inserting the end of a gimlet in their backs, and drawing them out ; they are easily removed, because the com- position slightly contracts in cooling. They will present fine impressions of the original, and be perfectly free from air bubbles, if the operation has been carefully , performed. 180. Elastic Moulding. — If the medallion is undercut, it must be copied in " elastic moulding" (86), thus :— Encircle its edge by a strip of stout paper, and pour the mixture upon its surface quite hot, and of the consistency of treacle, to the depth of half an inch or more, according to the size of the medal and the depth of its hollow parts, brushing its surface beneath the liquid with a brush having fine and long hairs, to remove air bubbles. Allow the mixture to remain until it is quite firm, which will be from two to twenty-four hours, according to its bulk ; take off the paper, and re- move the mould very gently, carefully stretching and drawing it at the same time in the direction of the overhanging parts, to prevent injury. Should the object to be copied be a hollow metallic bust, proceed as follows : — Partly fill it with sand, to make it heavy and thus prevent its rising in the liquid, and cover its opening by sticking a piece of millboard strongly over it ; then place the bust in the centre of a cylindrical and taper vessel, a few inches deeper and wider than itself, and pour the melted com- position in steadily, until it is a few inches above the top of the head, tapping the bust, and inclining the outer vessel, to facilitate the escape of air bubbles. When the composition is quite firm, which it will be in about twenty hours, it may easily be removed from the vessel by shaking, if the vessel has been previously well oiled ; the mould may then be removed from the bust by previously marking on its lower end the position of the face, passing a knife carefully up the back of the bust nearly to the | crown of the head, and opening the elastic mould with your hands, whilst a second t person lifts out the bust. If the original bust is composed of plaster, it must be pre- viously saturated with oil to prevent the melted composition adhering to it. 181. Rendering Moulds Conductible. — To render the surfaces of non-metallic moulds conductible, there are two methods in use, — first, to cover them with a thin film of blacklead by brushing; and, second, to coat them with a minute film of gold or silver by chemical means. The first of these methods is generally used for | moulds composed of gutta-percha, wax, resinous composition, or plaster saturated with RENDERING MOULDS CONDUCTIBLE. 79 oil, where the parts are not much undercut ; and the second for elastic moulds, because the blacklead cannot be readily applied to all their recesses. 182. Blackleading. — To apply blacklead to a small round or oval medallion formed of gutta-percha, or of gutta-percha and marine glue, first insert the sharp end of a piece of copper wire, size No. 16 or 18, and about fifteen or twenty inches long, into the edge of the mould near its face, then pass a piece of fine copper wire, size No. 28 or 30, once tightly round the edge of the mould close to its face, securing its ends to the other wire. Fix a strip of paper about one inch wide, by means of sealing-wax, tightly round the edge, to prevent the blacklead passing anywhere except upon the face of the mould. Apply the blacklead by a soft camel' s-hair brush with a large and thick body of short hairs, breathing upon the face of the mould occasionally to facilitate the adhesion of the blacklead ; and when the medal is perfectly black and bright, blow off the superfluous blacklead, and remove the paper ; it is then ready, for receiving a deposit, the whole operation occupying about ten or fifteen minutes with a small object the first time of preparing it, but less in subsequent operations. If the mould is very large, and especially if it has deep hollows in its surface, it will require, after being black- leaded, to have several short and fine copper " guiding M ires" carefully attached to the main wire, and their free ends ^ lightly inserted in the face of the mould in the most hollow and distant parts, or to lie in contact with them, in order to cause the deposit to commence and spread there as well as round the edge. If this precaution is not taken, the deposit will be much thinner over those parts, than upon the nearer and more prominent places, and sometimes will not spread over them at all. 183. Preparing the Surfaces of Elastic Moulds. — Elastic moulds are treated in a different manner. First, a stout connecting wire is attached, then a number of fine copper u guiding- wires" are twisted round it, and their free ends slightly inserted in the face of the mould in all the hollow and distant parts ; the mould is then either dipped into the phosphorous solution (87), or its surface is covered with that liquid ; and, after it has been drained clean, it is allowed to remain until perfectly dry ; the silver solution is next applied to it (87), in like manner, for several minutes, until it appears black, with a metallic lustre like black china ; it is then gently rinsed with distilled water, and the gold solution applied in the same way (87), which gives it a yellowish aspect ; after another rinsing in distilled water, it is ready for receiving a deposit. 184. Moulding by Phosphorus Composition. — Some objects which are not much undercut, are moulded in the phosphorus moulding composition (8T) ; and, in some cases, where they are undercut, as well as busts, they are first copied in the elastic moulding, and then the elastic mould copied in this material, the composition being but barely melted that it may not dissolve the elastic moulding. In either of these cases, instead of immersing the mould into the phosphorus, silver, and gold solutions, it is only immersed in the two latter, in the manner already described, the phosphorus contained in the mould itself serving to reduce the silver and gold. 185. Preparing Surfaces of Glass for Deposition.— Glass surfaces may be prepared for receiving a deposit by means of the phosphorus, silver, and gold solutions, but not very satisfactorily. A better method, which we have tried, has been silvering them by Drayton's patent process, thus :— Take one part, by weight, of liquid ammonia, three parts of alcohol, two parts of nitrate of silver, and three parts of distilled water 5 dissolve the silver salt in the water, add the liquid ammonia and the alcohol, shake the mixture, allow it to remain until quite clear, and pour off the clear part into the 80 REGULATING BATTERY POWER. glass vessel to be silvered, which must be perfectly clean ; then add to it one quarter of a part of grape-sugar dissolved in weak spirits of wine, mix the liquid, and heat it to about 150° or 160° Fah. for about twenty or thirty minutes, and the glass vessel will beeome silvered ; the liquid may then be removed, the vessel gently rinsed with dis- tilled water, a connection formed by means of a fine copper wire with the film of silver and the battery, the vessel filled with a suitable depositing liquid, an anode immersed, and the surface deposited upon in the ordinary way. If the process has been success- fully performed, the deposit, whether of silver or copper, spreads instantaneously over the silvered surface. The only way by which we have been able to form an adhesive deposit upon glass or porcelain, has been to send the article to a glass and porcelain gilder, and have gold-leaf burnt into their surfaces, and then depositing upon them in the usual manner. 186. Immersion of Articles in the Vat. — The operator having prepared the various articles and moulds, will immerse them in the depositing solution, having previously immersed the anode and completed all the connections, taking care always to connect the article to receive the deposit with the zinc of the battery, and the metal to be dissolved with the copper or silver. 187. Regulation of Battery Power. — In depositing all metals, it is of the greatest importance that the battery power be properly regulated, and this may be done in a variety of ways, which^collectively consist, either in making alterations in the battery, in the depositing vessel, or in the wires connecting them. The intensity of the current is increased by increasing the number of alternations of the battery plates ; by increasing the conductibility of the battery liquid or depositing solution, which may be effected either by the addition of free acid to the battery or more salt to the solu- tion ; or by increasing the thickness and decreasing the length of the connecting wires. The quantity of the current is increased by immersing all the battery plates deeper in their liquids, also by those means which increase the intensity. Both the intensity and the quantity may be decreased by separating the electrodes in the depositing vessel farther asunder, or by interposing a long and fine iron wire in the circuit. The most usual means adopted of increasing the intensity of the current is to add to the number of the batteries; and for increasing the quantity, to immerse all the battery plates deeper in their liquids. Sometimes, to increase the power, the temperature of the liquid may be raised, or the article placed nearer the dissolving metal ; and, when a smaller quantity of current is required than the ordinary batteries can conveniently supply, a piece of sheet metal may fee hung in the depositing solution with the receiving article, to receive a portion of the deposit, and thus transfer some of the power from the article to itself. 188. Intensity and Quantity of the Current. — The intensity of the current obtained from a series of batteries, depends upon the number of alternations of the metals, whilst its quantity depends upon the amount of immersed surface in each alternation, and it m akes no difference whether that amount of surface is in one battery plate or in many or in one or many containing vessels. A series of similar batteries may be so connected together by wires or other conductors, as to give either an intensity or a quantity of current, provided they have screws or other conveniences for attaching the wires to the plates. For instance — 1st, if we have four pairs of plates, in four separate cells, and connect them alternately, thus (Fig. 32), zinc, silver — zinc, silver — zinc, silver — zinc, silver, with wires from the terminal plates, we obtain a current possessing the intensity of four pairs, and the quantity of one ; but if, 2nd, they are connected, all the zincs by one wire, and all the silvers by another wire (Fig. 33), with one portion of INTENSITY OF THE ELECTRIC CURRENT. 81 each wire left convenient for making connections, we obtain a current possessing the quantity of four pairs, and the intensity of one ; and, 3rd, if they are arranged in two series or rows, two pairs in each, each series being connected intensity fashion, the end silvers of each row facing one way and connected by one wire, and the terminal zincs facing the opposite way and connected by another wire, and these wires left free Fig. 33. for connection with the vat (Fig. 34), we obtain a current possessing the intensity of two pairs and the quantity ot two. By contrivances of connection like thcsc,any number of batteries, provided they arc similar in kind, charged alike, and have suitable connecting screws attached, may be connected together so as to give any desired quantity or intensity of cur- rent within the limits of their power ; thus, a battery of one hundred pairs may be arranged to yield a current having the intensity of one hundred pairs, and the quantity of one ; the quantity of one hundred, and the in- tensity of one; the intensity of fifty, and the quantity of fifty ; or any intermediate degree of each. 189. ReguUting the Quantity of E*epo3i teci Metal,.— The quantity of metal dissolved'and deposited in the vat, is in direct proportion to the quantity of zinc dissolved PRACTICAL CHEMISTRY— No. III. G 82 REGULATING THE QUANTITY OF METAL. and acid consumed in each alternation of the battery. With a perfect depositing liquid, good battery arrangements, and pure materials, for every equivalent of zinc dissolved in each alternation of the battery, an equivalent of metal is dissolved on one side, and an equivalent deposited on the other, in the depositing vessel. For instance, for every equivalent (32*6 parts) of zinc so dissolved, and 49. partsi or one equivalent, of oil of vitriol consumed in the battery, an equivalent (31 7 parts) of copper is deposited in the sulphate of copper solution, or an equivalent (108 parts) of silver in the cyanide silver plating liquid, and similar quantities of copper or silver dissolved at the anode. But in practical working, the materials are rarely or ever pure, or the arrangements perfect ; the zinc nearly always contains a small proportion of other substances, the mercury contains tin or lead, and the sul- phuric acid contains a little nitric acid. The acid liquid of the battery is often too strong ; much acid liquid is frequently wasted, being thrown away before it is com- pletely exhausted. The zinc plates have not kept well amalgamated, or the silver well platinized, or the plates have been suffered to remain too long in the liquid when not in use. The metal of the anode is frequently impure also ; occasionally some of the deposit is allowed to redissolve, in consequence of the battery power becoming low, and from not stirring the solution ; in some solutions, a part of the battery strength is expended in evolving gas at the cathode ; and, finally, the repeated operation of " scratching" removes some of the deposit. Allowing for all these and other unavoid- able sources of loss in practical working, about one pound only of copper can be deposited in the ordinary sulphate solution, by the consumption of from one and a quarter to one and a half pounds of zinc and an equivalent quantity of acid, in each alternation of the battery. To increase the quantity of metal deposited in a given period of time, the battery plates should be sunk deeper in their liquids. 190. Regulation of the Quality of the Deposit. — The quality of the deposited metal, i.e., its degree of cohesion, hardness, flexibility, &c, depends upon the intensity of the current. As a general rule, the greater the intensity and the smaller the quantity of the current, the harder and brighter is the deposited metal; and the greater the quantity and the smaller the intensity, the less coherent and the softer it is. To obtain a very hard, bright, and crystalline deposit, we should use a current of small quantity and high intensity ; and to obtain a soft black powder deposit, we should employ large quantity and low intensity ; the combination of moderate quantity and moderate intensity, produces a coherent reguline deposit, possessing all the ordinary characteristics of the particular metal. These results can only be obtained with a good depositing liquid, and with metals, such as copper, silver, gold, &c, which are known to exist in a tough reguline state, and not with those — such as bismuth or antimony, crystalline metals — which are not known to exist in that state. 191. If we are producing a reguline deposit, with a one pair battery, in a good depositing liquid, with electrodes of the same size as the immersed portion of the battery plates, and it is wished to change the deposit to a soft black powder, the plates of the battery must be immersed many times deeper in the liquid, and use a very much larger dissolving plate in the vat ; and if we wish to change the deposit to a crystalline one, several more pairs of battery should be put on, connect them intensity fashion, immerse the plates an exceedingly small depth in their liquids, and use a very small anode. These results have a direct reference to the size of the receiving surface ; for if, with any given battery and anode, we are producing a black powder deposit upon a very small article, a larger article would receive a reguline deposit, and a much larger MANAGEMENT OF THE BATTERIES. 83 one would receive a deposit bordering upon crystalline. Thus it will be perceived that the black powder deposit is a result of too rapid action, and the crystalline one of too slow action. 192. Spread of Deposit.— -If we wish to make a deposit spread rapidly over a metal of inferior conductibility, such as a long iron rod, we must use a current of high intensity and rather small quantity ; this will drive it over the surface without causing it to become soft or non-coherent. The action of such a current appears to consist in conferring upon the particles a kind of polarity, a power of grouping themselves into separate warty nodules or crystals, each of which, as it becomes larger, appears to powerfully repel its neighbour, and thus cause the metal to spread rapidly ; when this action is continued to a considerable thickness of deposit, especially in cold weather, the metal is exceedingly hard and easily broken into a number of distinct grains or nodules, which are in the form of warty lumps with rounded edges ; when the action has been rather too quick, or the liquid not sufficiently cold, and composed of more or less perfect crystals, with edges sometimes beautifully defined, when the action has been very slow, and the liquid very cold and undisturbed. With the intensity of one hundred pairs of Smee's battery, acting for a long period of time in cold weather, and the quantity of the current kept down to the lowest possible degree, we have seen a tough deposit of zinc spread over several square inches of clean gutta-percha ; and in depositing copper with a current of rather high intensity, and small quantity, upon blackleaded gutta-percha medallions, we have repeatedly observed, that where there was a sunk boundary line near the edge, the deposit remained quite thin, as if power- fully repelled, whilst on each side of the line it was very thick, and on the outside edge, accumulated in large warty masses, hard and distinctly separate, and containing as much metal as the whole of the medallion besides. 193. Management of Batteries.— The most suitable strength of liquid for filling the battery cells consists of one measure of sulphuric acid and about twenty measures of water ; a stronger liquid may be used, even to one part of acid to ten or twelve parts of water ; but then the zinc plates require constant watching and frequent amalgamating, to prevent Avaste. If the acid liquid is very streng and the electric action energetic, the zinc plates will require to be examined every day, to see that there is no local action, i.e., chemical action in particular places; and when gas is found to be freely rising from them, as well as from the silver or copper, or when any dull patches appear where the acid has acted too strongly upon them, they should be taken out and amalgamated. They should be frequently amalgamated when new, and afterwards, if much worked, they should be amalgamated every few days ; when they become old they should be rarely amalgamated, because it very much weakens the power. "When they become so thin as to fall to pieces on handling, new ones should be substituted, and the old ones should cither be melted at as low a degree of heat as possible, to p-event loss of mercury, and cast into rods for Daniell's batteries ; or be broken tip, put in an iron retort, and the mercury distilled from them at a strong red heat into a vessel of water. 194. The zinc plates should be taken out of the liquid eveiy evening, unless depo- sition is required to continue all night. After the battery has been at work a few days, a little more acid should be added, and the liquid stirred ; and this should be done as often as the power gets low, until at length the liquid^ becomes thick and nearly saturated with zinc salt, and the salt crystallizes about the [edges of the cells ; it is then time to throw it away and put fresh, or it may be filtered, evaporated, and 84 GENERAL RULES FOR WORKING SOLUTIONS. crystallized, and the resulting salt (sulphate of zinc) preserved for depositing purposes. If any of the silver or copper plates of the batteries become covered with a deposit of zinc whilst working, it shows that all the acid in that cell is exhausted, and that more should be added, or fresh liquid put in. This only happens in a Smee's battery, or in the old zinc and copper battery. The deposit of zinc may be easily and quickly re- moved by the addition of acid to that cell, by a fresh liquid, or by immersing the coated plate in dilute sulphuric acid as long as gas is evolved from it. 195. If copper plates are used in the batteries, they should be heated red hot all over every week or ten days, and quenched in water, and then dipped in " dipping liquid" (82), or in nitric acid ; and if platinized silver plates are used, they should be re-platinized as often as they become light in colour, or their power becomes low, which will happen once in two or three months with constant working. The re-platinizing greatly increases the power of the battery. Great care must be taken that the zinc plates never touch the silver or copper ones when wet, otherwise the mercury will get upon the latter, and much weaken the battery power ; and with the silver plates cause them to drop to pieces if they are very thin. 196. General Rules for Working Solutions. — In working any depositing liquid — 1st, avoid doing anything which will alter the chemical composition of the liquid, or even the proportions of its ingredients, except the water — that may be altered, in proportion, in most liquids without much inconvenience ; 2nd, adapt your electric power to the liquid, rather than the liquid to the power, and regulate the deposit rather by altera- tions in the battery than by alterations in the depositing vessel, except as regards the distances of the electrodes or the temperature of the liquid— these may be altered with safety, and sometimes with convenience ; and, 3rd, as a general rule, let your dissolving metal expose a larger immersed surface than the receiving article. 197. Position of Articles and Dissolving Plates. — The best practical position for the dissolving plate is the vertical, the dissolving plate and the receiving article being suspended in the liquid facing each other, the latter being rather the lowest of the two, and both wholly immersed. The horizontal position, with the dissolving metal above, although the most philosophical arrangement, does not succeed in practical working, because the metal used for dissolving, is never quite pure (with copper, especially), and the impurities from it fall upon the surface of the receiving article beneath, and make it rough ; in addition to this, the position of the article pre- vents our being able to examine it easily or remove it conveniently. If the article to be coated has a very irregular outline, either the dissolving plate should be bent some- what to its form, so that the two may be nearly equi-distant at all parts ; or the article should be often shifted in its position, so as to produce a nearly uniform thick- ness of deposit all over. The nearer the receiving article is to the dissolving plate, the more rapid is the deposit, and a large body of liquid, deposits more rapidly and more evenly than a small one ; large connecting wires are more favourable to quick deposits than small ones. The greatest thickness of deposit always takes place upon the most prominent places, i.e., upon those parts nearest the dissolving metal. 198. Motion of Articles in Vat. — In some solutions — for instance, the double sulphite of silver and potash — if the current is too strong to produce a good deposit, motion of j the articles will prevent the deposit becoming bad. In some plating establishments, I the articles in the vats are kept in constant motion, gently swinging to and fro, the | metal frame upon which they are suspended having four small wheels running upon APPLICATIONS OF COPPER DEPOSITION. 85 four small inclined planes fixed upon the edges of the vat, which are kept in constant motion by steam or other available power. 199. Temperature of Solution— Several solutions, such as the cyanide of copper and potassium, the cyanide of gold and potassium, &c, require to be kept hot, in order to make them conduct freely, and yield suitable metal ; not that they cannot be worked cold, but that they work much better and quicker hot, which more than compensates for the expense of heating them. 200. Protection of Depositing Liquids from Light. — Some liquids, such as the double sulphite of silver and potash and the hyposulphite of silver and potash, require to be protected as much as possible from the influence of light ; and even the ordinary cyanide silver liquid is better screened from an excess of this agent. 201. Clean Connections Necessary. — In every case we must be verj- careful to observe that the circuit is complete, and that it is capable of conducting the current freely throughout ; that the articles to be coated are conductors of electricity ; and that their surfaces, as well as all the ends of the wires, at their various points of contact are perfectly clean. 202. Management of Coppering Liquids. — With the sulphate of copper solution (120) no particular management is required, beyond the general rules already laid down (196). It is not suitable for depositing direct upon zinc, tin, lead, iron, or steel. Articles formed of these metals are first coated with a thin layer of copper, in the cyanide of copper and potassium solution (120), then well washed, and transferred immediately to the sulphate solution, and the remainder of the required thickness of copper deposited upon it. 203. Uses of Copper Deposition.— Among the many uses to which the electro- deposition of copper has been applied, we may mention the following :— To make copper cells for Danicll's batteries ; making copies of stereotype plates, engraved copper plates, and engraved rollers ; coppering the surface of printing type ; coppering steel pens (patented) ; to protect iron and steel goods from rusting, coating telegraph wires, ship's bolts, screws, &c. ; to make copies of Daguerreotype pictures ; to make coppered cloth ; to coat glass chemical vessels ; to coat and protect metal and plaster statues, busts, and sculptured works ; to preserve the form of flowers, fruits, ferns, sea-weed, insects, reptiles, &c. ; to make medallions, busts, and various figures and ornaments in copper ; to etch Daguerreotype pictures ; it has also been applied in the arts of glyphography and electro-tint printing. 201. Making Objects in Copper, Coppering, &c. — To make a cell of Danicll's battery in copper, coat the inside of a glass jar or earthen jelly pot with wax, resin, or stearine, by making the vessel hot, then either blacklcad it thoroughly, or treat it with the phosphorus, silver, and gold solutions ; or, what is more simple, coat it uniformly all over the inside with the phosphorus moulding composition (87), and then treat it with the gold and silver liquids. Make a connection by a fine copper wire with the lower part of the coating ; fill the vessel nearly full of the sulphate of copper depositing solution (120), suspend in it a sheet of copper, and connect the sheet of copper and the fine copper wire with a small battery of one or two pairs. If, instead of the battery process, we adopt the single cell arrangement, containing dilute sulphuric acid and a piece of zinc, placing a porous cell in the sulphate solution, and connecting the fine copper wire with the piece of zinc ; in either case a deposit of copper will soon spread over the entire inside surface of the jar, especially if it has been prepared by the phosphorus method. Engraved steel plates are copied by stopping-off the back with copal var- nish, allowing it to become perfectly dry ; immersing it in the cyanide coppering 80 COPYING WOOD-CUTS BY DEPOSITION. liquid, and depositing a thin film of copper upon it, then washing it well and at once immersing it in the sulphate of copper solution, and depositing the required thickness of copper upon it ; this will require from 24 to 48 hours. The surface of the steel should he previously prepared for a non-adhesive deposit, otherwise the two metals cannot he separated. 2Q5. Copying Wood-cuts in Copper. — For printing purposes, where a large numher of impressions of a particular wood-cut is required, the plan of taking copies of the engraved wooden blcck in copper by the electro-process, and using those copies instead of the original block to print from, has been gradually extending itself for some years, and has now attained a considerable degree of importance ; the vignette at the head of the title-page of the Illustrated News, the title-page of Punch, many of the large engravings in the Illustrated News, and even the illustrations of some of the penny periodicals are regularly produced in this way. To copy an engraved wooden block, the engraved surface is first moistened with water, and firmly enclosed by a shallow frame, a thick piece of gutta-percha, more than sufficient to fill the enclosed space, and made quite soft by heat, is then laid upon it, commencing its contact at the centre of the engraving and proceeding outwards, so as to exclude all air-bubbles ; a plate of cold iron is then laid upon the gutta-percha, and the whole subjected to pres- sure, gentle at first, but increased to a high degree as the substance cools. The block and copy are then separated, and the figured surface of the gutta-percha (with connect- ing and guiding wires previously attached) is treated in the usual manner with black- lead or with the phosphorus, silver, and gold solutions ; copper is then deposited upon it in a solution of sulpbate of copper, until a moderate thickness of deposit is obtained, which will occupy at least twelve or eighteen hours ; when sufficiently thick the deposit is removed, its back made rigid by a layer of solder or type-metal (the surface being previously moistened with a solution of chloride of zinc to make the solder adhere), the back is planed fiat, and mounted upon a block of wood to the height of the type. In London this process is carried on upon a large scale, some of the copies being upwards of two feet square. Engravings upon steel are copied in an exactly similar manner to those upon wood. "We have recently tried some experiments with a view of making the deposit of copper upon gutta-percha and marine glue spread more rapidly than it does, by pre- paring the surface with blacklead, it being a matter of some importance in copying wood-engravings for periodicals of such large circulation as the Illustrated News, that the time occupied in copying them'be reduced. Our experiments have resulted in some success, and we give the following results of them for the benefit of the printing trade. After having formed a reverse copy in gutta-percha and marine glue of the engraved wooden block, and affixed the conducting wires to it, take a mixture of one measure of spirit varnish, and either four or five measures of vegetable naphtha, and apply it very sparingly in a thin layer by a soft camel' s-hair brusb, over the whole surface of tbe mould where the deposit is desired to be spread. "Whilst the surface is still wet, cover it with a mixture of three parts of yellow and one part of white bronze powder, and bring the powder in thorough contact with the whole of the moistened surface by striking it all over with a dry, soft brush, then gently brush off all the superfluous powder. The bronzed mould may now be immersed in the ordinary sulphate of copper solution, and the following actions will occur : — The particles of white bronze powder being composed almost wholly of tin, and those of yellow DEPOSITION SUBSTITUTED FOR STEREOTYPE. 87 j bronze containing much copper, those of tin will dissolve and coat themselves with copper by simple immersion process (4), and those of brass or copper (the yellow ones) will become coated by their contact with those of tin (Two Metals and One Liquid Pro- cess, 7), and thus a thin deposit of copper will almost instantaneously spread all over the bronzed surface. This effect will of course take place without connecting the mould with the battery, but they may be immediately connected together, and a deposit will | spread almost instantaneously over the whole of the bronzed surface by the ordinary battery process, through the medium of the bronze and the thin deposit already mentioned, and may be continued to any required thickness in the usual way. In our earlier experiments with this method, yellow bronze alone was used, which did not reduce the copper by simple immersion j but even then medallions were repeatedly covered with a deposit of copper in from two to five minutes, which would occupy from twenty to forty-five minutes when prepared by blacklead in the usual manner. The addition of white or tin bronze causes the deposit to spread as rapidly as when the surface is pre- pared by the phosphorus solution, but without the disadvantage which occurs in using the latter, of making the deposited metal brittle. The effect of using white bronze alone was not so satisfactory. The surface of the copper copy so obtained is bright and clean, and the character of the deposited metal is good, but the surface obtained is hardly so smooth and fine as that obtained with blacklead ; the difference, however, is very slight, and it is suffi- ciently smooth for all ordinary purposes, and for the object sought, if care be taken to blow off or otherwise remove all superfluous bronze powder before immersing the mould in the vat. We hope that those who have the opportunity will try it upon a larger scale. 206. Copying Sct~up Type in Copper. — The process of electrotyping has also been gradually encroaching upon that of stereotyping, and has, we are informed, almost super- seded that process in America. The plan adopted is similar to that of copying wood- cuts, viz., to lay a sheet of softened gutta-percha upon the surface of the page of type, and subject it to increasing pressure until it is cold ; the gutta-percha copy is then removed, and treated as in copying wood engravings. It would be advisable to try a somewhat softer material for this purpose, such as the mixture of gutta-percha and marine glue, which we have recommended (85). This material takes a sharper and smoother impression than gutta-percha alone, and the deposit spreads over it more rapidly ; and, being softer, it would enter more freely and with less pressure between the fine lines of the letter.-, and still not be sufficiently soft to enter the minute crevices between the body of the types. If a solution of grape-sugar (as used in Drayton's patent process for silvering glass), aldehyde, or other reducing agent, was substituted for the phosphorus solution, for reducing the silver upon the surface of the mould, it would be an advantage, as besides the dangerous character of the phosphorus, it has an offensive odour, and the copper deposited upon surfaces prepared by it, moreover, is in- variably brittle. The mould may also be prepared for a deposit by blaekleading ; it will require a first-rate quality of blacklead, and prolonged and attentive brushing, but will then afford a good result. The air-bubbles may be removed when the mould is in the liquid, by directing a powerful upward current of the liquid against them by means of a vulcanized india-rubber bladder, with a long and curved glass tube with a fine. orifice (Fig. 35) attached to it; but the liquid should be free from sediment. 88 COPYING DAGUERREOTYPE BY DEPOSITION. The advantages of eleetrotyping over stereotyping are numerous ; the metal is t harder, takes a sharper impression of the mould, and delivers the ink much more readily than type metal, besides being a cleaner process ; it also takes up less ink, and consequently the printed pages dry more quickly. Both wood-cuts"and letter- press have also been copied in plaster of Paris, and the deposit of copper formed upon that ; but this material is much inferior to gutta- percha for the process. Messrs. Bradbury and Evans are considered the most suc- cessful manipulators in this branch of electro-deposition, and their apparatus the most perfect of its kind. In this establishment the temperature of the room is carefully attended to, and the vessels containing the solutions have glass covers. The result of this careful manipulation has been that, in some instances, successful deposits of large Illustrated Hews engravings have been formed and taken off in" eight hours ; this can only have been effected by the most perfect blackleading, keeping the solution in excellent condition, and worked with the maximum of battery power. Gutta-percha and marine glue is well worthy of a trial, and the use of bitumen with gutta-percha is also a good idea ; the marine glue would be better, because it is tougher. Iron and steel wire may be coated with an adhesive deposit of copper, by first immersing them, with their surfaces perfectly clean, in the cyanide coppering liquid, and completing the deposit in the ordinary sulphate solution. The coils should be kept separate from each other in the liquid by suspending them upon a horizontal brass rod, turning it occasionally to cause an uniform deposit. Iron screws and nails may be treated in a similar manner except that they should be contained in a wicker basket, and shook about occasionally to produce an uniform deposit. 207. Copying Daguerreotype Pictures in Copper. — The most interesting and beautiful application of the deposition of copper, and, at the same time, one of the easiest to be effected, is that of copying Daguerreotype pictures. First solder a wire to the back of the plate near the edge ; paint over the back and edges, and allow it to dry ; hang it in a clean sulphate of copper solution, which is perfectly free from dust or grease on its surface ; and, in the course of twenty or thirty hours, if about two pairs of small Smee's batteries have been used, the deposit will be sufficiently tbick to be removed ; it should then be taken out, well washed, wiped perfectly dry, and a narrow strip be cut off its edges with a strong pair of scissors ; the two may then be easily separated by inserting the point of a knife, or the end of a thin wedge of hard wood, between them at the edges. If the process has been carefully managed, and the origi- nal picture is a strong one, a most beautiful and vivid copy will be obtained ; and if the picture is not only a strong one, but has also been well fixed by Fizeau's process, a number of successive copies may be taken from it, but their intensity, as well as that of the original, appears to diminish in each succeeding trial. With a vivid original pic- ture, clear solution, very regular and undisturbed action of the battery, and a fine deposit, we have observed a most strange phenomena, viz., the picture has not entirely disappeared, even in twenty hours, although the coating of copper has constantly increased in thickness ; the image has penetrated quite through the deposited metal, and appeared upon the back, even upon deposits as thick as an address card. In some cases the figure was optically positive, and in others negative. 208. Coating Plaster Models, Flowers, and Clay Figures with Copper. — Busts and other similar objects may be coated by saturating them with linseed oil, then well COPPERING CLOTH BY DEPOSITION. 89 blackleading, or treating them with the phosphorus, silver, and gold solutions, attaching a number of guiding wires, connected with all the most hollow and distant parts, and then immersing the object in a sulphate of copper solution containing no free acid, and causing just sufficient copper to be deposited upon them by the battery process to protect them, but not to obliterate the fine lines or features. Flowers, fruits, ferns, sea- weed, insects, &c, may be prepared by the phosphorus, silver, and gold liquids, and the copper deposited upon them, either by the single cell or battery process, in a neutral sulphate of copper solution. 209. Coppering Cloth. — To copper cloth, first stretch it upon a sheet of copper slightly curved, so that it may be in close contact with the metal all over ; then varnish the back or hollow side of the copper, and deposit on the opposite side, by the battery process, from a sulphate solution not containing much free acid, until the meshes of thp cloth are quite filled with copper, and the metal and cloth firmly united together ; the deposit may then be removed and well washed ; the original sheet of copper should of course be properly prepared for a non-adhesive deposit. Mr. J. Schottlaender took out a patent, December, 8th,"1843, for depositing either plain or figured copper upon felted fabrics. He passes the cloth under either a plain or an engraved copper roller, horizontally immersed in a sulphate of copper solution not containing much free acid, and a deposit takes place upon the roller as it slowly revolves ; the meshes of the cloth are thxis filled with metal, and the design of the roller copied upon it, the coppered cloth slowly rolled off, and passed through a second and closely contiguous vessel filled with clean water ; the roller is properly prepared for a non-adhesive deposit. 210. Etching Copper. — In etching a copper plate by galvanism, wc first solder a wire on the back, then varnish the back, and cover the front with a thin layer of engravers' etching ground ; draw the design upon the front surface with an etching needle, cutting through this material to the clean surface of the copper. Having completed the etching, hang the plate as an anode in the ordinary sulphate of copper solution, opposite a suitable cathode of brass or copper. The current of electricity in passing- out of the engraved linos into the liquid, causes the copper in them to dissolve, and thus etches the design in the plate. The different gradations of light and shade' arc produced by suspending cathodes of different forms and sizes opposite the plate to be etched, in different positions and at different distances from it, thus causing the etching to be of different depths in different parts, the deepest action being always at the parts of the electrodes nearest together. 211. Glgphogrophy. — This art consists in varnishing the back of a flat and smooth copper plate, laying fh-st a thin coating of white etching ground upon its front side, and then a layer of black etching ground upon that, engraving the design upon the coating with different engraving tools, then blackleading the whole of the engraved surface, and depositing a thick sheet of copper upon it in a sulphate solution by the battery process; the deposited plate is then removed, its defects corrected, and fixed upon a block of wood in the same manner as a stereotype plate, ready for printing by the ordinary hand-press. This process has been patented, and the patent is worked by Mr. Hawkins, Hatton Garden. 212. Management of Silver Solutions. — Silver plating liquids require much more care and attention than the sulphate of copper solution. Articles formed of zinc, iron, .or steel, require to be coated with a • thin film of copper in the | cyanide of copper liquid, before being immersed in the cyanide of silver solution. \ 90 MANAGEMENT OF SILVER SOLUTIONS. Those formed of Britannia metal, tin, or pewter, are taken direct from the hot potash liquid without rinsing in water, and immersed a short time in a cyanide of silver solu- tion containing considerable supplies of free cyanide, a large anode, and a current of considerable intensity from a strong battery is passed through for several minutes until the articles receive a thin deposit of silver ; they are then transferred to the ordi- nary vat to receive the full amount of deposit. Those of lead are first scraped or otherwise made quite clean and bright by mechanical means, and then treated in the same manner as those of Britannia metal. Articles of copper, brass, or german silver,, after being properly cleansed, are dipped into the solution of nitrate of mercury (84), or of cyanide of mercury and potassium (84), then rinsed in a vessel of water, and immediately suspended in the depositing vat. The preparation of those articles by immersion in a bath of cyanide of mercury was patented by Dr. H. B. Leeson, June j 4th, 1842, and is in use by the electro-platers of Birmingham. If they are immersed without this preparation, the deposited silver does not always adhere firmly. 213. Peculiarities in Practical Silver deposition.— Peculiar phenomena often occur in the electro-deposition of silver, not only upon different metals, but also upon the same metals in different forms or in different conditions of surface for instance : — 1st. If two perfectly similar pieces of thin sheet brass are taken (except that one is perforated all over with small holes), and both be simultaneously immersed in the same solution to be silvered, and with the same battery power applied to each, the latter, although its amount of surface is reduced by the perforations, will become coated with silver much more slowly than the former : — 2nd. If a wire gauze cylinder of a Davy lamp be suspended side by side with a piece of thin tubing of the same metal and of the same dimensions, the latter will become coated much more rapidly than the former. 3rd. If two pieces of the same metal — iron for instance — be immersed to be silvered in the ordinary cyanide solution, or to be coppered in the hot cyanide of copper and potassium liquid, each containing exactly the same amount of surface to be coated, but one being in the form of a thin sheet, and the other in that of a thick plate or solid block of metal, the former will become coated much more rapidly than the latter. 4th. The edges and points of articles, whilst being plated, exhibit a greater tendency to a crystalline deposit than the flat parts, and this tendency is sometimes manifested in depositing silver upon table-knives and forks It is the knowledge of these and many other peculiarities of different metals and articles met with in practical working, and of the means of overcoming their attendant difficulties, which constitutes one of the chief differences between the practical operator and scientific man. 214. Management of u Bright Solution." — A bright solution is much more difficult to manage than the ordinary silvering liquid ; if it is not worked constantly and in an uniform manner, it will lose its power of yielding bright metal. If any of the articles which are being plated in it are disturbed, or removed from the liquid and replaced, that one will not now receive a bright deposit, and the disturbance of the liquid by removing it will oftentimes cause all the neighbouring articles to lose their brightness. If too much "brightening liquid" (144) is added, the solution will be ! considerably injured ; many silver solutions have been irretrievably damaged in this way. A bright solution requires a battery current of large quantity and low intensity i to work it, and the dissolving plates in it are generally of a darker colour than those in 91 the ordinary silvering liquid ; the silver deposited from it is much harder than that deposited from the ordinary plating solution, and has very much the appearance of fused metal ; the bright appearance commences at the upper part of the articles and travels downwards, it soon after commences also at their lower extremities and travels upwards, until the bright portions meet each other. If there are very small holes in the surface of the articles, dull streaks appear above them. 215. Adding Cyanide of Potassium to Plating liquids.— It is neces- sary to add a little cyanide of potassium occasionally to every cyanide of silver plating liquid, probably because the solutions absorb carbonic acid from the atmosphere, which converts some of the cyanide of potassium into carbonate of potash, and sets hydrocyanic acid gas free. A further portion of the potassium salt may also be de- composed by some means, with formation and escape of ammonia ; the necessity of adding a little fresh cyanide is indicated when the dissolving plate begins to change from its ordinary pure white appearance to a dull yellowish gray colour ; it is best added in the evening after plating ; about half an hour before stirring the solution. 216. If the solution is too strong, i.e., if it contains insufficient water, but has silver and cyanide of potassium in their proper relative proportions, it conducts freely ? deposits rapidly, and gives a rich deposit of a fine silky lustre ; but it is more difficult to manage than a weaker liquid, especially in hot weather, because, from the less mobility of its particles, it is very apt to settle by working into strata of different densities, its upper part becoming exhausted of silver and full of free cyanide, and its lower part be- coming nearly saturated with that metal, and destitute of free cyanide ; the consequence of this is, that the upper parts of the dissolving plates waste rapidly, whilst the upper parts of the article receives either very little deposit, or one of a bad quality, being gray, brown, or yellowish, sometimes of a lilac hue, and generally in dull streaky vertical lines ; all these evils may be mitigated by stirring the solution well every night after having finished plating, or it may be entirely prevented by diluting the liquid with water to a proper extent, stirring it every evening, and working it uniformly. All silvering and other depositing liquids exhibit this tendency to settle into strata in working, especially if worked rapidly, but the most dilute ones shew it in the least degree. If the solution is deficient in water, and contains a great excess of free cyanide, the foregoing evils are all greatly aggravated. In hot weather it becomes quite un- manageaole, and tho vapours of ammonia and hydrocyanic acid arising from it are quite overpowering. In this case, the best way to improve it is to add cyanide of silver and water in sufficient quantities to make it of a good composition, keep it in a cool place, stir it daily, and work it constantly in an uniform and careful manner. New solutions, or old ones which have been injured, often improve by daily stirring, with uniform and judicious working. An excess of cyanide of potassium is indicated when the dissolving plates are very strongly acted upon, and the deposit is at the same time either very sparing or of a bad colour. If the solution is too weak, i.e., if it contains too much water, it conducts sparingly, deposits slowly, and the deposit has a dead white appearance. This may be easily remedied by adding cyanide of silver and cyanide of potassium to it in proper pro- portions, and working it uniformly a few days with daily stirring. 217. Washing, Drying, and Ornamenting Silver-plated Articles.— Articles that have been plated with silver are always washed in a running stream of water until evejy trace of the depositing liquid is removed from them ; they are then immersed in hot dry sawdust, moved about in it and gently rubbed with it, until 92 RE-SILVERING OLD ARTICLES. 1 they are perfectly dry. After drying they are, if necessary, weighed, to ascertain how much silver has heen put upon them, then " scratched" (80), and finally finished by burnishing, polishing, &c. Sometimes, for the purpose of ornament, portions of their surface are " oxidized." This is done by applying a hot solution of bichloride of platinum (167) to them, and allowing it to dry. The more platinum the solution contains, and the hotter it is, the deeper black does it produce ; or it may be effected by the application of the solution of " liver of sulphur." . To produce a brownish colour, apply a solution of equal weights of sulphate of copper and sal-ammoniac in vinegar ; and to produce a " dead" appearance, like frosted silver, deposit a mere trace 'of copper upon it in a copper solution, then well wash it, and deposit a very thin layer of silver upon this. In each of these cases the parts which are to remain bright must be stopped-off with varnish. 218. "Stripping" Silver from Copper and Copper from Silver. — Occa- sionally the depositor has sent to him, to be re-plated, old worn-out articles formed of " Sheffield plate," in which the outer layer of silver has been worn away, and exposed portions of the copper base beneath ; these articles generally require to have the remain- ing portions of silver removed, in order to obtain an uniform surface to deposit upon. The removal of the silver is termed " stripping." ' To effect this, add a little nitrate of potash (saltpetre) to a quantity of strong oil of vitriol, and apply heat, until it is all dissolved ; then immerse the articles in the hot liquid, and allow them to remain until all the silver is dissolved. If the action becomes slow, apply more heat or add more saltpetre, the copper will not be much acted upon if the articles are not allowed to remain in too long. A number of such articles are generally done together, and are after- wards washed, and prepared in the usual manner for receiving a deposit. The silver may be recovered from the liquid, in the form of chloride of silver, by diluting it with much water, then adding a solution of common salt to it as long as a precipitate is produced ; the precipitate, when washed and dried, is chloride of silver. By fusing this with carbonate of potash the metallic silver is obtained. To remove copper from silver (which is but rarely required), boil it in dilute hydro- chloric acid, or immerse it in a hot solution of per chloride of iron. This latter solution may be made by adding peroxide of iron (crocus, jeweller's rouge) to hydrochloric acid as long as it will dissolve ; it will remove either tin, lead, or copper, from either gold or silver, without affecting those metals. A solution of chloride of zinc has been used for the same purpose. Copper may also be completely removed from silver or gold, by making it the anode in a sulphate of copper solution until all the copper is dissolved ; the silver will remain unaffected, if the current employed is feeble and has not a greater intensity than one or two pairs. 219. Testing the Purity of Silver. — M. Eunge adopts the following method of testing the purity of silver : — He immerses the article in a mixture of 32 parts of water, 4 parts of sulphuric acid, and 3 parts of chromate of potash. If pure metal, the immersed part quickly assumes a purple colour, which is less deep and less lively in proportion to the amount of alloy contained in the silver. No other metal exhibits the same colour with this liquid. 220. Testing the Amount of Silver and of Free Cyanide in Silver Solutions. — To ascer- tain the amount of silver in a cyanide plating solution, add dilute sulphuric acid to a known quantity of the liquid as long as it produces a precipitate ; wash and dry the precipitate, which is cyanide of silver, containing in every 134 parts, 108 parts of MANAGEMENT OF GILDING SOLUTIONS. 93 metallic silver. To test the amount of free cyanide of potassium, add a solution of crystallized nitrate of silver in distilled water to a known quantity of the plating liquid, as long as the precipitate formed continues to he re-dissolved, and note how much crystallized nitrate is used ; every 175 parts expended, indicate 130 parts of free cyanide, or ahout three parts of free cyanide to four parts of nitrate of silver. 221. Management of Gilding Solutions.— Cyanide gilding solution is gene- rally contained in a glazed iron vessel, and heated either by a stove or by gas jets be- neath ; or it is contained in a stoneware or glass pan immersed in boiling water. On account of the general smallness of the articles to be gilded, the thinness of the deposit required, and the rapidity of the action in a hot liquid, the articles only require to be immersed in the solution for a few minutes ; when a thicker deposit is required, in order to maintain a proper condition of deposit, they should be taken out several times, brushed, and re-immerscd. Articles formed of iron or steel are required to be coated with a thin film of copper in the cyanide coppering liquid before gilding. The strength of battery used for gilding is generallyaboiit two pairs of Smee's, of different sizes, according to the magnitude of the articles to be gilded. The loss of water by evaporation is generally made good by adding a little distilled water, after having finished gilding. 222. Regulation of Colour in Electro -gilding. — Tbe general method now adopted for regulating the colour of electro-gilding is as follows : — After having prepared the solu- tion, work it with a large copper anode until the deposited metal begins to deteriorate in colour ; then replace the copper by a small gold anode. AVith the copper anode can be obtained a rich full colour, becoming deeper as the temperature of the liquid is higher ; to produce a paler yellow, use a small gold anode with the liquid at a lower temperature. 223. Recovery of Gold and Silver from Depositing Liquids. — As both •ilver and gold solutions occasionally get out of working condition and become quite unfit for use, it is very desirable that the operator should understand the chemical action 'of different "substances upon them, and how to recover the metal. Those of silver generally get out of order, either from the addition of too much " brightening liquid ;" from excess of cyanide of potassium, together with the heat of the weather, and, injudicious management; from unsuccessful attempts to improve the condition of the liquid ; from the accidental introduction of impurities ; or from the liquid having been improperly made. Supposing it to be the ustial cyanide liquid, the silver may be recovered in the metallic state thus : — Evaporate the solution nearly to dryness, reduce the resulting salt to powder, mix it with its own weight of a mixture of one part of nitrate of potash and two parts of common salt, and roast the whole in an iron pan to dryness. Fuse the dried mixture at a bright red heat in an earthen crucible, until the silver collects at the bottom of the vessel in a melted state, then pour it slowly into a large quantity of water. The resulting granules of silver should not be used in making a new plating liquid, because they generally contain copper derived from the articles suspended in the plating solution (see 151), but should be exchanged at a silver refiner's for pure silver. 224. " The crystallized double cyanide of gold and potassium fuses and effervesces by heat, and is resolved into cyanogen gas, ammonia, and cyanide of potassium, if air be present; its complete decomposition requires a strong red heat. "When it is strongly ignitad, mixed with an equal weight of carbonate of potash, a button of metallic gold is obtained. "When heated with sulphuric acid, it gives off hydrocyanic 94 EXTRACTING GOLD AND SILVER FROM EXHAUSTED SOLUTIONS. acid gas, and, after ignition, leaves a mixture of gold and sulphate of potash. Iodine sets free cyanogen gas, forms iodide of potassium, and throws down the cyanide of gold . The aqueous solution of cyanide of gold and potassium gilding liquid, when mixe d with sulphuric, hydrochloric, or nitric acid, slowly deposits cyanide of gold ; and ? when boiled with hydrochloric acid, it is completely resolved into cyanide of gold and chloride of potassium. Similar effects are produced by sulphuric or nitric acid, and even by oxalic, tartaric, and acetic acid." 22-5. Extraction of Gold and Silver from Exhausted Solutions. — " To obtain the remaining gold from gilding solutions which have become inactive, they should be evaporated to dryness, the residue finely powdered and intimately mixed with an equal weight of litharge, fused at a strong red heat, and the lead extracted from the alloy button of gold and lead by warm nitric acid ; the gold will then remain as a loose yellowish-brown spongy mass." 226. The following extracts are from the works of Boettger, J. Pr. Chem. 36, 169 ; Eisner, Redtel, Hessenberg, J. Pr. Chem., and other foreign writers : — " I have under- taken a series of researches upon this object, and hasten to communicate the results to the public ; but, before proceeding to the communication, I think it necessary to mention the results of the experiments upon which are based the methods given further on for extracting both the silver and the gold of old cyanide of potassium liquids. 227. " 1st. If we add hydrochloric acid to a solution of silver in cyanide of potas- sium, until the liquid exhibits an acid reaction, we obtain a white precipitate of chloride of silver, which, when submitted to heat, melts into a yellow mass. If this was cyanide of silver, the application of a red heat would have left a regulus of silver. The addition of the hydrochloric acid precipitates all the silver present in the liquid in the form of chloride of silver. 228. "2nd. If we evaporate a solution of silver in cyanide of potassium to dryness, and heat the residue to redness until the mass is in a state of quiet fusion, and has assumed a brown colour, there remains, when we wash the mass with water, metallic and porous silver. The wash- waters, when filtered, still contain a little silver in solution ; because, if hydrochloric acid is added to them, it produces a precipitate of chloride of silver. In evaporating and calcining a solution of gold in cyanide of potas- sium the result is the same — we obtain metallic gold. The wash-waters, acidulated with hydrochloric acid, give, when treated with sulphuretted hydrogen, a brown pre- cipitate of sulphide of gold ; and, with the salt of tin, a violet precipitate purple of cassius, — a proof that these liquids still contain a little gold in solution. 229. " 3rd. If we pour upon finely-divided silver — for instance, silver-leaf, or silver precipitated in the porous state by zinc from a solution of silver — a concentrated solu- tion of cyanide of potassium, at the ordinary temperature, and shake it frequently, the liquid, at the end of a certain time, exhibits silver in solution, and by adding hydro- chloric acid to it, we produce an abundant precipitate of chloride of silver. This expe- riment explains why, in the wash-waters of the various combinations of gold or silver with cyanide of potassium, we can still demonstrate the presence of gold and of silver, after the most minute separation. 230. " 4th. When hydrochloric acid or ordinary sulphuric acid is added to a solution of cyanide of copper and cyanide of potassium, until the liquid exhibits an acid reaction, there results a reddish-white precipitate, which is a cyanide of copper in the anhydrous state. If the precipitate be well washed and boiled in potash lye, protoxide of copper is separated of a beautiful red colour ; and if to the filtered alkaline liquid we add a solu- { METHODS OF RECOVERING SILVER. do tion of green copperas, a dirty blue precipitate is obtained. A solution of carbonate of soda furnishes the same results, and yields, with the copperas, the same dirty blue precipitate. If the reddish- white precipitate is dissolved in pure nitric acid, and a solution of nitrate of silver added to it, an abundant white precipitate is produced, which, -when washed, dried, and calcined, yields silver in the metallic state — a proof that the precipitate is cyanide of silver. 231. "The reddish- white precipitate is soluble in an excess of hydrochloric acid, in nitric acid, and in aqua regia ; it is also soluble in aqueous ammonia and in a solution of cyanide of potassium. 232. " 5th. If we pour hydrochloric acid into a very pure solution of gold in cyanide of potassium, there is slowly formed at ordinary temperatures, and immediately on the application of heat, a yellow precipitate, which is cyanide of gold ; the filtered liquid which has given this precipitate, still contains a little gold in solution. In evaporat- ing to dryness, fusing, dissolving, and filtering a-fresh, there remains upon the filter the remainder of the gold. 233. " When a solution of silver prepared for silvering articles of bronze or of brass, has been employed a certain time for that purpose, the precipitate produced in it by the addition of hydrochloric acid, is not pure white, but reddish, in consequence of the reddish-white cyanide of copper which is precipitated with it ; for we know that those silvering liquids which have been used for sometime, contain copper in solution. The same thing occurs with the solutions for gilding, in which articles of silver, copper, bronze, and brass, have been gilded for a long time ; the liquid contains, after a certain time of service, not only gold, but also silver and copper. This case presents itself especially when gilded articles of silver, containing copper or other alloys of silver, are in the solution of gold ; then, the precipitate of cyanide of gold produced by the addition of hydrochloric acid, does not possess its proper pure yellow colour. It has happened to me to observe a precipitate of this kind, which instead of being yellow, was green ; and, in fact, articles of iron have been gilded in a solution, and the preci- pitate contained, besides cyanide of gold, Prussian blue, so as to be demonstrated in an examination, which consisted in boiling the green precipitate in aqua regia, filter* ing to separate the dirty green residue, evaporating the filtered liquid to dryness, and dissolving the dry salt in water acidulated with hydrochloric acid ; the addition of sulphate of iron to this new liquid gave a brown precipitate, and the salts of tin a reddish-brown precipitate. In treating by aqua regia, the cyanide of gold was then decomposed, and converted into chloride of gold. 234. " Based upon the preceding facts we may found several methods for recovering all the silver and gold of old cyanide of potassium solutions. The extraction of these precious metals may be effected either by the wet or by the dry process. 235 "Extraction of Silver by the Wet Method. — Adding hydrochloric acid until the liquid exhibits a strongly acid reaction (230). The precipitate of chloride of silver which is thus obtain will be, as we have already said, of a reddish- white colour, because of the cyanide of copper which is precipitated with it when the solution has been used a long time for silvering objects containing copper. In this precipitation by hydrochloric acid there is hydrocyanic acid gas set free, therefore the operation should only be performed in the open air, or in a place where there is good ventilation. If the pre- cipitate is very red it must be treated with hot hydrochloric acid, which will dissolve | the cyanide of copper. The chloride of silver having been washed with water, must be 1 I 96 METHODS OF RECOVERING GOLD. dried, fused with potash in a Hessian crucible, and then coated with borax, in the ordinary manner for obtaining metallic silver. 236. " This method is very simple in its application, and very economical, con- sidering that by the aid of the hydrochloric acid all the silver contained in the solution of cyanide of potassium is precipitated, and there remains no trace of it in the liquid. But the large quantity of hydrocyanic acid gas which is disengaged, is a circumstance which must be taken into serious consideration when operating on large quantities of silver solution, the vapour of which is most deleterious, and nothing but the most perfect ventilation, combined with arrangements for the escape of the poisonous gases, will admit of the process being carried on without danger to the w orkmen ; when, however, we have taken the precautions dictated by prudence, the method in question may be considered as perfectly practical. The liquid should be poured into very capacious vessels, because the addition of the acid produces a large amount of vapour. 237. " Extraction of Silver by the Dry Method. — The solution of cyanide of silver and potassium is evaporated to dryness, the residue fused at a red heat, and the resulting mass, when cold, is washed with water. The remainder is the silver in a porous metallic condition. There still remains in the wash-waters a little silver, which may be precipitated by the addition of hydrochloric acid. 238. " Extraction of Gold by the Wet Method. — A solution of gold and cyanide of potassium, which has long served for gilding articles of silver alloyed with copper, may still contain, as we have already remarked, independently of the gold, both silver and copper, and perhaps iron. In order to obtain these metals we operate in the follow- ing manner : — " The liquid, the same as with the solution of silver, is acidulated with hydro- chloric acid ; in which case there is produced a disengagement of hydrocyanic acid gas, which requires the same careful ventilation. This addition of hydrochloric acid causes a precipitate, which may, according to circumstances, consist of cyanide of gold, cyanide of copper, and chloride of silver. The precipitate, washed and dried, is boiled in aqua regia, which dissolves the gold and copper in the form ©f metallic chlorides, and leaves the chloride of silver unaffected. The solution, containing the gold and the copper, is evaporated nearly to dryness in order to drive off any excess of acid, it is then dissolved in a small quantity of water, and the gold precipitated from it by the addition of protosulphate of iron in the state of a brown powder. The chloride of silver is reduced to the metallic state by the known means. The liquid from which we have precipitated the cyanide of gold, &c, by hydrochloric acid, may yet contain a little gold in solution. I refer to 5th for its further treatment. 239. "This method is distinguished by the great simplicity of the operation, and | we may repeat for it all that we have already said respecting the extraction of silver by the wet method. 240. "Extraction of Gold by the Dry Method.— -The solution of cyanide of potas- sium which contains gold, silver, and copper, is evaporated to dryness ; the residue fused at a red heat, cooled and washed (the wash- waters still contain a little gold and silver, and this occurs most often when the solution of gold or silver contains a very great excess of cyanide of potassium). The residue, after washing, consists of gold and silver in a metallic porous state, and carbide of copper resulting from the decom- position of cyanide of coppei by the heat. The metallic residue is treated by aqua regia, which forms insoluble chloride of silver, and contains the chlorides of gold MEANS OF RECOVERING GOLD AND SILVER. 97 and copper in solution. In order to obtain these metals in the metallic state, we must proceed in the manner previously indicated. 241. " If we operate according to the method of Professor Bcettger, i.e., if we fuse the dried residue with its own volume of litharge, in a covered crucible, the regulus we obtain in this case consists of gold, silver, and lead. In treating this alloy by nitric acid of specific gravity 1*2, and applying heat, the gold remains in the form of a brown powder, whilst the lead and the silver are dissolved in the acid. This solution, after having been diluted with distilled water, may have the silver separated from the lead, by the addition of hydrochloric acid. 242. " These methods of extracting [the silver and gold from old solutions of cyanide of potassium by the dry process present this advantage, that the operator is ' not incommoded, while working, by the disengagement of vapours of hydrocyanic acid. In these operations the poisonous gases are not developed as they are in the processes for extracting the metals by the wet process. 243. "After the experiments here reported, those who are interested in the subject may choose for themselves which of these methods appear the most suitable to the circumstances in which they are placed, and the object which they wish to attain. 244. " Means of Recovering Gold or Silver, by M. Bolley. — Cyanide of gold dissolved in an excess of cyanide of potassium, resists all the means which we have tried to separate them ; and hydrosulphuric acid, for example, does not produce a precipitate. By the wet way we cannot always precipitate the gold completely, and for that reason M. M. Bcottger, Hessenberg, Eisner, and others, propose to evaporate the liquid to dryness ; mix the residue with its own weight of litharge, fuse the mixture at a strong red heat, then dissolve the lead from the alloy by boiling it a long time with dilute nitric acid, which leaves the gold in the form of a light sponge. 245. " M. Wimmer has more recently proposed to evaporate the solution to dry- ness in a water bath, then mix the residue with one and a half times its weight of saltpetre, and introduce the mixture by small portions at a time into a Hessian crucible, heated to redness in order to cause explosions, and to continue this until the entire mass is in a state of quiet fusion. 246. " The first of these two processes does not give room for any objection, except the employment of a great heat, and the use of nitric acid ; the second process is, on the contrary, disagreeable, and very uncertain. "We know that saltpetre never explodes with more violence than with cyanide of potassium ; and, notwithstanding that the inventor of the process advises us not to add more than small portions of the mixture at a time, the explosions are so powerful that they cannot be caused without loss of materials. 247. " The following process is applicable on the small scale with a spirit lamp and a crucible of platinum : — Evaporate the solution to dryness, mix the saline mass with its own weight of sal-ammoniac, and heat it gently ; ammoniacai salts decompose, as we have said, the metallic cyanides, and form cyanide of ammonium, which is itself decomposed by the heat and volatilized, whilst the acid of the ammoniacai salt (the body which salifies the ammonia) combines with the metals (passed to the state of oxides), which were previously united to the cyanogen. The sal-ammoniac then in i this case forms chloride of potassium and chloride of gold, and if the salt contains ferrocyanide of potassium, chloride of iron in addition. The chloride of gold is easily decomposed ; the chloride of iron is partly decomposed, and leaves oxide of iron in beautiful crystalline spangles. The undecomposed portion of the chloride of iron," PRACTICAL CHEMISTRY. — No. IV. B 98 LIST OF PATENTS. like the chloride of potassium, may, after the decomposition is finished (which only requires a low red heat), he washed away by water, leaving the gold in the form of a light coherent mass, and the iron in small spangles, which may be removed by mechanical means. 248. "If we fear that a little of the gold remains mixed with the iron in a pul- verulent state, we may dissolve it in hot aqua regia, and precipitate the gold from the resulting solution by adding to it a solution of protosulphate of iron ; but this appears superfluous ; and I am assured, by evaporation of given volumes of the same solution of gold, the evaporation and calcination of the sal-ammoniac, and other operations? that we have collected in a sufficiently exact manner all the gold of these solutions. 249. " The same process is applicable to the'solution of silver, and independently of the oxide of iron (of the ferrocyanide of potussium) we obtain chloride of silver, which is soluble in aqueous ammonia. 250. List of Patents upon Electro-deposition. — The following is a chrono- logical list of nearly all the patents which have been taken out for subjects connected with electro-deposition. Elkington's Patent.— June 24th, 1836, gilding copper, brass, and other metals, G. E. Elkington. Elkington's Patent. — February 17th, 1837, gilding metals, and coating with platinum, apparatus described, H. Elkington. Elkington's Patent. — December 4th, 1837, gilding and silvering certain metals, apparatus described, H. Elkington. Elkington and Barratt's Patent.— July 24th, 1838, coating and colouring certain metals, G. B. Elkington and 0. W. Barratt. Elkingtons' Patent. — March 25th, 1840, coating or plating certain metals, G. E. Elkington and 'H. Elkington. Parkes's Patent— March 20th, 1841, production of works of art by electro-deposi- tion, Alexander Parkes. Barratt' s Patent.— September 8th, 1841, deposition of metals, 0. W. Barratt. Fox Talbot" s Patent. — December 9th, 1841, coating and colouring metals with other metals, H. H. Fox Talbot. Zeeson's Patent. — June 1st, 1842, electro-deposition, with apparatus, H. B. Zeeson. Tuck's Patent.— Junejith, 1842, coating metals with silver by electro-deposition. E. Tuck. Woolrich's Patent. — August 1st, 1842, coating metals and alloys with metal, J. S. Woolrich. Sturges's Patent. — August 10th, 1842, making plated articles, E. F. Sturges. Fox Talbot's Patent— November 25th, 1842, coating metals with other metals, H. H. Fox Talbot. Moses Poole's Patent.— May 25th, 1843, deposition of metals, with apparatus, Moses Poole. Barratt' s Patent. — June 15th, 1843, gilding and plating metals, O. W. Barratt. Schottlaender' s Patent. — December 8th, 1843, deposition of metal upon felted fabrics, J. Schottlaender. Parkes's Patent.— February 21st, 1844, deposition of metals and alloys, A. Parkes. Parkes's Patent.— October 24th, 1844, depositing metals and their alloys, A. Parkes. Parkes's Patent.— October 9th, 1845, coating metals and alloys, A. Parke?. LIST OF BOOKS. 99 Lyons and Millward's Patent.— March 23rd, 1847, deposition of metals, Lyons and Mill ward. Salzede's Patent.— September 30th, 1847, brassing and bronzing steel, iron, zinc, lead, and tin, Charles de la Salzedc. Fontainemoreau' s Patent. — March 14th, 1849, coating metals and non-metallic substances, Fontainemoreau. Russell and Woolrich's Patent. — March 19th, 1849, coating iron and other metals with metals and alloys, Russell and Woolrich. Parkes's Patent. — March 26th, 1 849, [ deposition of certain metals and alloys, A. I Parkes. Smith's Patent.— June 7th, 1849, dppositing metals, S. B. Smith. Itoseleur's Patent. — March 23rd, 1850, tinning metals, A. G. Roselcur. Steele's Patent.— August 9th, 1850, coating metals, Joseph Steele. Ridgway's Patent.— April 20th, 1852,] coating glass with metal by battery process, J. Ridgway. Lyons' s Patent. — October 7th, 1852, coating surfaces of iron, M. Lyons. Morrias and Johnson's Patent.— December 11th, 1852, deposition of brass and alloy?, Morriss and Johnson. Junot's Patent.— December 28th, 1852, reducing metals by electricity and plating, C. J. E. Junot. Power's Patent. — December 29th, 1852, silvering metals and glass, J. Power. Newton's Patent.— July 29th, 1853, improvements in depositing metals and alloys of metals, W. E. Newton. Newton's Patent.— August 5th, 1853, coating cast iron with metals and alloys, "W. Newton. Person's Patent. — April 27th, 1854, electro-coating with zinc, C. C. Person. 251. List of Published Books, &c, upon Electro-deposition.— Shaw's Manual of Electro-metallurgy ;" Smee's "Electro-metallurgy;" Napier's "Electro- metallurgy;" "Walker's "Electrotype Manipulation;" Sturgeon's '"Art of Electrotyping;" Spencer's " Instructions for the Multiplication of Works of Art by Voltaic Electricity ;" " Manuel Complet de Galvanoplastie," par M. L.de Valicourt, nouvelle edition, 2 vols., 5 francs ; " Traite de Galvanoplastie," par J. L. 2 e edition, revue et augmentee, 2 fr. 50 c, " Manuel de Dorure et D'Argcnture par la Methode Electro-chimique et par Simple Immersion," par M. M. Selmi et De Valicourt, 1 vol., 1 fr. 75 c. ; a paper " On the Cyanides of Gold and Silver," by Messrs. Glassford and Napier, in the Phi- losophical Magazine, 1844 ; and the following papers by G. Gore : — " Inductive View of Electro-deposition," Pharmaceutical Journal, July] 185 3 ; " Electrical Relations of Iron and Copper," Pharmaceutical Journal, September 1853 ; " Deductive View of Electro-deposition," Pharmaceutical Journal, April 1854 ; "On a Peculiar Pheno- menon in the Electro-deposition of Antimony," Philosophical Magazine, January 1855 ; also in Journal de Pharmacie et de Chimie, April 1855; and a scries of articles^ " Practical Rules and Recipes in Electro-deposition," Pharmaceutical Journal, April, May, July, August, September, October, November, and December, 1855. 252. To further assist the reader in remembering the terms applied to the different parts of the electric circuit (see 39 and 42), and in understanding the action of electricity upon liquids, we repeat those terms, and append a diagram of the anode, cathode, &c. S is a wire proceeding from the silver plate or positive pole of a battery ; Z is another wire from the zinc plate or negative pole ; the two wires are attached to two 100 RECAPITULATION. pieces of metal immersed in a depositing liquid. By the direction of the arrows, it will be perceived that the positive electricity circulates in one direction through the circuit, and the negative electricity in the opposite direction. The immersed pieces of metal are termed "electrodes," meaning ways by which the electricity enters and leaves the liquid ; the piece A is called the " anode," and is that Fig. 36. by which the positive electricity enters the liquid ; and C is the " cathode," or that by which the positive electricity leaves the liquid. In the ordinary depositing vat, the anode is that piece of metal which dissolves and supplies the solution, whilst the article or articles which are receiving a metallic coating or deposit constitute the cathode. The liquid is termed an " electrolyte," and whilst the electric current is passing through, it is said to be undergoing " electrolysis," to be " electrolyzed," or suffering " electro-chemical decomposition." The elements of the liquid are termed " ions;" those of them which combine with, or are set free at the u anode," are called " anions," and those which combine with, or are set free at the " cathode," are termed " cathions for instance — 1st, in the electrolysis of a solution of sulphate of copper with copper electrodes, sulphuric acid is viewed as an anion, because it is the element which combines with the anode, and copper the cathion, because it is the element set free at the cathode ; and, 2nd, in the electrolysis of the ordinary cyanide silver-plating liquid, cyanogen is the anion and silver the cathion. GEORGE I GORE, NEGATIVE. POSITIVE. PHOTOGRAPHED UPON THE WOOD "WITHOUT PENCILLING. PHOTOGRAPHIC ART. • — ♦ — . Introduction. — Within the last quarter of a century there has not been a dis- covery more useful, interesting — I may say, more fascinating — than photography. "Whether employed as an |assistant to the artist, or a means of sending home from far- off scenes of war and death the portrait of a friend, or the spot whereon, perhaps, he died or conquered, what can equal its truthfulness ? what can surpass its beauty ? By the aid ©f the sunbeam the physician may now delineate the gradual changes produced by disease, with a faithfulness hitherto unknown ; the architect can obtain the most elaborate details of a building in a few seconds. The art has been made sub- servient to the purposes of the artist, the naturalist, and the mechanic, and even to the antiquary, who, " Bending o'er some mossy tomb Where valour sleeps," may be enabled to preserve a lasting memorial by this art. Viewed in these lights, Photography may justly be considered the most important application of chemical philosophy to develop the powers of nature which modern science has discovered. Premising that the science is at present in its infancy, I shall endeavour to make the present manual as plain, as practical, and as comprehensive as possible, saying nothing, which might require to be afterwards unsaid, nor leaving anything unsaid which may be necessary to elucidate the subject, so far as our present knowledge extends. Wedgwood's Discovery, — The property possessed by the salts of silver, when decomposed by the action of light, was well known to the earlier chemists ; and M. Charles, a well-known French physician, exhibited in his lectures at the Louvre a paper capable of taking silhouette figures by the action of solar light, but he has left no accoxmt of his process. Mr. Wedgwood, therefore, was undoubtedly the first person 102 WEDGWOOD S DISCOVERY. who recorded his attempts to use the sunbeams for photographic printing. In the year 1802 he published a paper in the Journal of the Eoyal Institution, which he describes as " an account of a method of copying paintings upon glass, and of making profiles by the agency of light upon nitrate of silver, with observations by H. Davy," — a gen- tleman afterwards better known as Sir Humphry Davy. From this paper, the earliest we are acquainted with in which the discovery of these processes present themselves, the following extracts are taken : — ■ " White paper, or white leather, moistened with a solution of nitrate of silver, undergoes no change when kept in the dark ; but on being exposed to the day-light it speedily changes colour, and after passing through different shades of gray and brown, becomes at length nearly black. The alterations of colour take place more speedily in proportion as the light is more intense. In the direct beam of the sun, two or three minutes are sufficient to produce the full effect ; in the shade, several hours are re- quired ; and light transmitted through different coloured glasses act with different degrees of intensity. Thus it is found that red rays, or the common sunbeams passed through red glass, have very little effect upon it. Yellow and green are more effective, but violet or blue produce the most powerful and decided effect." " When the shadow of any figure is thrown upon the prepared surface, the part concealed by it remains white, and the other parts speedily become dark. For copying paintings on glass, the solution should be applied on leather ; and in this case it is more readily acted on than when paper is used. After the colour has been once fixed on the leather or paper, it cannot be removed by the application of water, or water and soap, and it is in a high degree permanent. The copy of a painting or the profile, immediately after being taken, must be kept in an obscure place ; it may, indeed, be examined in the shade, but in this case the exposure should be only for a few minutes ; . by the light of candles or lamps, as commonly en; ployed, it is not sensibly affected. No attempts that have been made to prevent the uncoloured parts of the copy or profile from being acted upon by light, have as yet been successful. They have been covered by a thin coating of fine varnish, but this has not destroyed their susceptibility of becoming coloured ; and even after repeated washings, sufficient of the active part of the saline matter will adhere to the white parts of the leather or paper to cause them to become dark when exposed to the rays of the sun. Besides the applications of this method of copying that have just been mentioned, there are many others ; and it will be useful for making delineations of all such objects as are possessed of a texture partly opaque and partly transparent. The woody fibres of leaves, and the wings of insects, may be pretty accurately represented by means of it ; and in this case it is only necessary to cause the direct solar light to pass through them, and to receive the shadows upon leather. " The images formed by means of a camera obscura have been found to be too faint to produce, in any moderate time, an effect upon the nitrate of silver. To copy these images was the first object of Mr. Wedgwood in his researches on the subject; and for this purpose he first used nitrate of silver, which was mentioned to him by a friend as a substance very sensible to the influence of light ; but all his numerous experiments as to their primary end proved unsuccessful. In following these processes, I have found that the images of small objects, produced by means of the solar microscope, may be copied without difficulty on prepared paper. This will probably be a useful application of the method ; that it may be employed successfully, however, it is necessary that the paper be placed at but a small distance from the lens." BERARD AND DAGUERRE's DISCOVERY. 103 Hero we have the first indication of this great discovery. Subsequently, about the years 1810-11, Seebeck made some interesting discoveries as to the production of colour on chloride of silver by solar radiations, the violet rays rendering it brown, the blue producing a shade of blue, the yellow preserving it white, and the red con- stantly giving a red shade to that salt. Berard' s Discovery.— In the year 1812, M. Berard brought the result of some valuable researches before a commission composed of MM. Berthollet, Chaptal, and Biot, who state in their report that M. Berard had discovered that the chemical inten- sity was greatest at the violet end of the spectrum, and that it extended, as Bitter and "Wollaston had previously observed, a little beyond that extremity. When he left substances exposed for a certain time to the action of each ray, he observed sensible effects, though with an intensity continually decreasing in tho indigo and blue rays. Hence they considered it as extremely probable, that if he had been able to employ agents still more sensible, he would have observed analogous effects. To show clearly the great disproportion which exists in this respect between the energies of different coloured rays, M. Berard concentrated, by means of a lens, all that part of a spectrum which extends from the green to the extreme violet ; he also concentrated, by means of another lens, all that portion which extends from the green to the extremity of the red. This last pencil formed a white so brilliant that the eyes were scarcely able to endure it; yet the muriate of silver remained exposed more than two hours to this brilliant point of light without undergoing any sensible alteration. On the other hand, when exposed to the other rays, which were much less bright and less hot, it was blackened in less than six minutes. After some further remarks on the importance of M. Berard' s experiments, they proceed as follows : — u If we consider solar light as composed of three distinct substances, one of which occasions light, another heat, and the third chemical combinations, it will follow that each of these substances is separable by the prism into an infinity of different modifications like light itself ; since we find by experiment, that each of these proper- ties is spread, though unequally, over a certain extent of the spectrum ; and we must suppose, on that hypothesis, that there exist three spectrums one above the other ; namely, a calorific, a colorific, and a chemical spectrum. We must likewise admit that each of the substances which compose the three spectrums, and even each molecule of unetrual rofrangibility which constitutes these substances, is endowed, like the mole- cules of visible light, with the property of being polarized by reflection, and of escaping from reflection in the same positions as the luminous molecules." From that time numerous experiments were conducted by several eminent re- searchers, including the discovery of the more celebrated MM. Niepce and Daguerre. Daguerre and Niepce's Discovery. — To the inventive genius of these gentle- men we are indebted for the first practical application of this great discovery ; but, like most great conceptions of the human mind, this art, as we have seen, advanced by slow steps, and was indicated from time to time by the isolated facts ,we have briefly alluded to. The researches of M. Niepce were commenced in 1814, but it was not till 1826 that he was made aware, by the indiscretion of an optician employed by both, that M. Daguerre was pursuing the same course of experiments. A correspondence between the two philosophers was the result, and henceforth their researches were pursued in common ; and some years later resulted in the discovery of this branch of the art since known as the Daguerreotype. In 1833, M. Niepce died, having communicated all his dis- 10-1 TALBOTYPE DISCOVERY. coveries to M. Daguerre, and, in 1839, that gentleman, with a most laudable abnega- tion of self, communicated his discoveries to the public. Fox Talbot's Discovery. — About the same time Mr. Fox Talbot, stimulated, no doubt, by the patriotic example of M. Daguerre, published the calotype process, thus giving birth to a new branch of the art. That gentleman, it appears, had been carrying on his experiments for five years previously, in perfect ignorance of what Daguerre and others were doing, and had aimed at a method by which the sensitiveness of the salts of silver was increased to a ' marvellous degree. I cannot do better than give an extract from his own communica- tion. After saying how marvellous it seems that, in a few minutes, a picture is produced, displaying the thousand florets of an Agrostis, with all its capillary branchlets, and so accurately delineated, that not one is without its little bivalve calyx, requiring to be examined through a lens, he proceeds : — " And, again, to give some more definite idea of the rapidity of the process, I will state that, after various trials, the nearest valuation which I could make of the time necessary for obtaining the picture of an object, so as to have pretty distinct outlines when I employed the full sunshine, was half a second." He was then speaking of the paper used in the solar microscope. Mr. Fox Talbot also published an account of his first photogenic experiments (for this term was first introduced by that gentleman), and I shall again make use of extracts from it, as they will better convey an idea of his discoveries and their importance than any words of mine : — " In order to make what may bs called ordinary photogenic paper," he says, " I select paper of a good firm quality and smooth surface. I do not know that anything answers better than superfine writing-paper. I dip it into a weak solution of common salt, and wipe it dry, by which the salt is uniformly distributed throughout its sub- stance. I then spread a solution of nitrate of silver on one surface only, and dry it at the fire. The solution should not be saturated, but six or eight times diluted with water. When dry, the paper is fit for use. " I have found, by experiment, that there is a certain proportion between the quantity of salt and that of the solution of silver which answers best, and gives the maximum effect. If the strength of the salt is augmented beyond this point, the effect diminishes, and, in certain cases, becomes exceedingly small. " This paper, if properly made, is useful for all photogenic purposes. For example, nothing can be more perfect than the images it gives of leaves and flowers, especially with a summer sun, — the light, passing through the leaves, delineates every ramifica- tion of their nerves. " Now, suppose we take a sheet of paper thus prepared, and wash it with a saturated solution of salt, and then dry it. We shall find (especially if the paper is kept some weeks before the trial is made) that its sensibility is greatly diminished, and, in some cases, seems quite extinct. But if it is again washed with a liberal quan- tity of the solution of silver, it becomes again sensible to the light, and even more so than it was at first. In this way, by alternately washing the paper with salt and silver, and drying it between times, I have succeeded in increasing its sensibility to the degree that is requisite for receiving the images of the camera obscura. " In conducting this operation, it will be found that the results are sometimes more and sometimes less satisfactory, inconsequence of small and accidental variations in the PHOTOGRAPHIC CHEMICALS. 105 proportions employed. It happens sometimes that the chloride of silver is disposed to darken of itself without any exposure to light ; this shows that the attempt to give it sensibility has been carried too far. The object is to approach, as near as possible, to this condition, without reaching it, so that the substance may be in a state ready to yield to the slightest extraneous force, such as the feeble impact of the violet rays when much attenuated. Having, therefore, prepared a number of sheets of paper with chemical proportions slightly different from one another, let a piece be cut from each, and, having been duly marked or numbered, let them be placed, side by side, in a I very weak diffused light for a quarter of an hour. Then, if any one of them, as frequently happens, exhibits a marked advantage over its competitors, I select the j paper which bears the corresponding number to be placed in the camera obscura." CHEMISTRY OF PHOTOGRAPHY. General Remarks. — The wonderful discoveries announced by M. Daguerre and Niepce, and Mr. Fox Talbot, produced a host of followers, who have brought them to the highest perfection. M. Claudet, one of the earliest, discovered a mode of taking objects by subjecting the plate to vapour of chloride of iodine. Messrs. Fizeau, Caudin, and Leon Foucault, by the aid of divers preparations of bromine, obtained impressions with great rapidity; and, in 1840, M. Fizeau succeeded in fixing the image by means of chloride of gold. Having thus introduced the subject in the words of the respective authors, and given a brief history of subsequent discoveries, I shall now, previous to going into the manipulating branches of the science, devote some little space to the chemical agents which we shall use, and recommend the reader to make himself as much acquainted with their nature and photographic properties as possible ; for I can assure him that if he does not do so, the time spent on the study of the science will be almost thrown away. The first of the photographic chemicals which will come under our notice, taking them alphabetically, will be — Acetic Acid. — The strongest acetic acid, named "Glacial," and sometimes "con- centrated acetic acid," from its property of becoming solid at low temperatures, contains only about one-sixtieth of its bulk of water. The crystals melt, at about 50°, into a pungent limpid liquid, with a smell resembling strong vinegar, of which, in fact, it is the base ; the distilled vinegar of the shops being acetic acid diluted with from 5 to 7 parts of water. It is often contaminated with a trace of sulphuric acid, which may be detected by the addition of a little chloride of barium, when, if any sulphuric acid be present, we obtain a white precipitate. Acetic acid is of the greatest use in all the photographic processes which require development, as it governs or checks the action of pyrogallic and gallic acids, and the sulphate of iron on the salts of silyer undergoing decomposition ; it preserves the whites or parts of the picture not acted on by light ; it also keeps the picture clean by prevent- ing any decomposition, except that caused by the light. I may add, that tartaric and formic acids are sometimes used for the same purpose, but I am inclined to give the preference to the acetic acid. Acetic acid is also the best acid for correcting the alkalinity of the nitrate of silver bath, which will be explained at length as we proceed. 106 PHOTOGRAPHIC CHEMICALS. Albumen, or white of egg, is very much used in preparing the surface of paper for positive printing. It is thoroughly beaten up with water and salt, the action of the nitrate of silver partly coagulates the albumen, and in turn is converted into chloride of sodium by an excess of nitrate of silver — a combination extremely favour- ! able to the production of a picture by the action of light. Albumen, containing small quantities of sulphur and phosphorus, gradually discolours the solution of nitrate of silver used for exciting or making sensitive. This discolouration may be easily re- moved by scraping some pipe-clay into the solution, stirring it up, and allowing it to remain for a day or two, and then filtering carefully. Albumen cannot be used with ammonia nitrate of silver, as the alkaline action of the ammonia would prevent coagulation in the albumen, and cause its separation from the paper. Alcohol. — Alcohol must not be confounded with " spirits of wine," as the latter contains a considerable quantity of water, which would prove almost fatal to the col- lodion process, causing a precipitate of the cotton and a separation from the ether : a proof of this may be seen at once n a collodion picture that has been taken with a collodion containing much water in the alcohol. Upon drying it, and viewing it by transmitted light, you will at once perceive that it has a grain something similar to fine muslin, so that it is of the greatest consequence to obtain the alcohol as free from water as possible. To do so it will be best to mix quick lime, powdered, and alcohol together in equal weights, by distilling both together ; the alcohol will come over pure, leaving the water with the lime, for which it has a great affinity. Ammonia is, or ought to be, only used photographically for the purpose of making ammonia nitrate of silver; for which process see Silver. It should be kept in a stop- pered bottle, as it rapidly absorbs carbonic acid from the air, which converts it into carbonate of ammonia. Bichloride of Mercury, or corrosive sublimate, is a highly poisonous salt, very sparingly soluble in water, unless free hydrochloric acid be present. It is used for the purpose of improving glass pictures — of which more anon — and for removing the yellow- ness sometimes caused in the lights of a print, when the gold colouring bath is used. Bromine. — This is a deep reddish-brown liquid, fuming strongly at common temperatures, and highly suffocating. It exists in sea-water combined with magnesium, and is closely analogous to chlorine and iodine, having stronger affinities than the latter and weaker than the former ; that is to say, bromine would displace iodine, and chlorine would displace bromine. It is sparingly soluble in water, soluble in alcohol, more so in ether. Bromide of Potassium is a mixture of bromine and caustic potash, heated to red- ness to drive off the oxygen from the bromate of potash, the latter becoming bromide of potassium on the loss of its oxygen. It is used for the formation of bromide of silver (see Silver). Chlorine is a greenish-yellow gas, of a pungent and suffocating odour. As has been remarked, closely analogous to the other two of the group, bromine and iodine, the gas has a density of two and a half times heavier than air. It is found abun- dantly in nature in combination with sodium, in solution in sea-water and rock- salt ; and is very useful in the arts for its bleaching properties. It has such a strong affinity for hydrogen that it absorbs it greedily, thus breaking up the structure of the ! organic substance, the latter being bleached by destroying its colour. It can be always discovered, either free or in combination, by a solution of nitrate of silver, with which it forms a white precipitate (chloride of silver). PHOTOGRAPHIC CHEMICALS. 107 Chloride of Sodium, or common table salt, is very useful to the photographer, as it bears the same relation to the positive printing that iodides and bromides do to the negative. Its sources are inexhaustible, being found in large quantities in the ocean as a solution, in the earth as a solid. It is a combination of chlorine and sodium. It fuses -without decomposition at a dull red heat ; but if the heat be pushed too far it sublimes, and the melted salt on cooling becomes a hard white concrete mass. It is sparingly soluble in weak alcohol, but nearly insoluble in absolute alcohol ; it is soluble in three times its weight of water, and crystallizes in cubes which are anhy- drous (witbout water). As chloride of sodium is often contaminated with chlorides of magnesium and calcium, also sulphate of soda, it is best if you can obtain it pure. You must do so by neutralizing hydrochloric acid (spirits of salt) by carbonate of soda. As has been already noticed, it is a very important salt in photography, being used most extensively in the preparation of positive-paper. Chloride of Ammonia, or muriate of ammonia, is a soluble salt formed by the com- bination of chlorine and ammonia. It contains more chlorine than an equal weight of chloride of sodium, and may be used instead of that salt in the preparation of positive printing paper. Chloride of Silver, formed by a combination of chlorine with a solution of nitrate of silver (see Silver). Chloride of Gold. — Take three parts nitro-muriatic acid, put it into a cup, and drop a piece of pure gold into it ono-third its weight ; let it evaporate until chlorine vapour is disengaged, then let it crystallize. An impurity sometimes found in the iodide of potassium (which sec). Chloride of Potassium.— -When there is much carbonate of potash in the iodide, it may be recognised by the crystals being very small and deliquescent (becoming moist when exposed to the air). The carbonate of potash is strongly alkaline to test-paper, and not very soluble in alcohol ; indeed it is a question if it is at all soluble in absolute alcohol, but it is soluble in some degree in the weaker alcohols, to which it communi- cates an alkaline reaction. The next impurity which comes under our notice is the sulphate of potash. This salt is decidedly not soluble in absolute alcohol, and the iodide of potassium may be freed from it by being dissolved in very strong alcohol. The presence of the sulphate of potash maybe detected thus ! —Make a solution of chloride of barium, and add a little to a solu- tion of iodide of potassium ; a slight milkiness need not be noticed, but if a decided white precipitate fall down, then you must either reject the iodide of potassium, or dissolve it in the strongest alcohol, when the sulphate being insoluble will remain undissolved. "We have now to consider the chloride of potassium. The presence of this salt is not so easily ascertainedas either of the others ; indeed it is rather difficult, and to discover an alkaline chloride in the iodide of potassium you must proceed thus : — Make a solution of the iodide suspected, and make a solution of nitrate of silver of the same strength, say twenty grains of each to one ounce of distilled water, mix and add to the iodide of silver a little liquor ammonia; if any chloride of silver (or chloride of potassium, which would form a chloride of silver) be present, it will dissolve in the ammonin, and after filtration may be precipitated by the addition of pure nitric acid ; the iodide of silver must be well washed previous to the addition of the nitric acid, for as the latter often contains traces of chlorine, the presence of any free nitrate of silver, by combining with the chlorine on neutralizing, would very likely cause an error ; it is of great im- 108 PHOTOGRAPHIC CHEMICALS. portance that the iodide of potassium should he pure, as otherwise it may cause a great deal of trouble ; it would not matter so much if the nature of the impurity were known, as in that case we could take steps to counteract its influence. Cyanide of Potassium is a highly poisonous salt, formed by the combination of cyanogen gas and potassium. It generally contains a large per centage of potash, from which it may be freed by boiling in alcohol, which on cooling deposits it in crystals. It has a strong smell of prussic acid, and is freely soluble in water. It dissolves iodide of silver, and is used for that purpose in clearing away all the unaltered iodide of silver from the negative (a positive if on glass). It also removes the stains of nitrate of silver from the skin or linen. "When using it on the fingers, be careful that it does not get into any cuts or sores, as it' is almost as poisonous as prussic acid itself. Ether is a highly volatile, inflammable spirit, obtained by distilling alcohol with sulphuric acid ; the latter in its re-action removes one atom of water, and by so doing converts one atom of alcohol into one of ether. There are three kinds of ether sold in the shops — ordinary rectified ether, washed ether, and washed] and re-rectified etber. Ether is a most important photographic chemical, being the solvent of " gun cotton," with which it forms collodion ; and it is very necessary that it should be pure. It should not have an acid re-action with test-paper ; it should not turn an alcoholic solution of iodide of potassium rapidly brown ; it should not have a high specific gravity from" too much alcohol or water ; and it should be free from any smell of j essential oils or of acetic ether. Provided ether be free from these defects, it matters j very little which ether be used ; if the ordinary rectified ether be pure, it will be the most economical. I shall treat this subject more fully when speaking of collodion. Formic Acid is a fuming liquid with a pungent odour ; it reduces the oxides of gold, silver, and mercury to the metallic state, and is itself oxidized into carbonic acid. The alkaline formiates possess the same properties. It is rather difficult to determine the strength of the commercial formic acid, it being always more or less dilute. It may be obtained in its full strength by distilling formiate of soda with sulphuric acid. It inflames the skin in the same manner as the sting of an ant, from which it gets its name, being originally discovered in the red ant {formica rufa), but is now pre- pared on a large scale by distilling starch with binoxide of manganese and sulphuric acid. Gallic Acid. — This is obtained from gall-nuts, of which the best kind come from Turkey, being called " Aleppo galls." The galls are exposed, after being powdered, to the action of the air for a long time — five or six weeks. The mass must be kept moist during the operation by the addition of a little water from time to time. Thus the gallic acid is gradually formed from the tannic acid first produced, the gallic acid crystallizes in long, needle-like, silky crystals, having an astringent taste, taking about 100 times their weight of cold water to dissolve them ; though, when boiling, three times will be enough. They can be easily purified and separated from the mass by I boiling up in water, filtering the mixture while hot, and setting aside to cool ; the gallic acid will crystallize on cooling. Gallic acid is but feebly acid, and is a very important agent inreducing the silver in the Talbotype process. Although not strictly in its alphabetical order, I shall now introduce a substance of great importance, produced by the action of heat on the gallic acid. Pyrogallic Acid. — At a temperature of about 410° Fah., gallic acid is decomposed, and a white sublimate forms, which condenses in lamellar-crystals. Unlike gallic acid, the new substance is exceedingly soluble in water, and is of the greatest importance in j PHOTOGRAPHIC CHEMICALS. 109 the development of collodion negatives, from the avidity with which it absorbs oxy-en Although termed an acid, it is perfectly neutral. Gelatine.- This is an organic substance, obtained by boiling bones, horns hoof, calves' feet, or similar animal refuse, into a jelly, which, in the mass, is termed " size ■"' or, when dried and cut into slices, « glue." Isinfflass is a similar substance, obtained from the air-bladders of a species of sturgeon, and heretofore has been prepared chiefly in Russia. 3 Gelatine softens and swells in cold water, but scarcely dissolves until the water be heated ; on cooling, it forms a tremulous jelly. An ounce of water will dissolve, when hot, about three grains without gelatinizing on cooling. It is somewhat analogous to albumen, but does not form any compound with the oxide of silver, as the latter does- hence its different action. Gold.— Chloride of Gold when in solution is a bright yellow colour when diluted, but a deep red when concentrated. In the solid state it is a red deliquescent mass, without any apparent regular formation ; and, although chemically neutral, it is acid to test- paper. Its chief use in photography is the property it possesses of blackening the shadows of a positive print, which it does to a wonderful extent. It is easily decomposed by sulphurous acid, charcoal, and many of the vegetable acids ; also by protosulphate and protonitrate of iron. The addition of ammonia to perchloride of gold forms the dangerous compound "fulminating gold." Hyposulphite of gold, or Sel d'Or, is a double hyposulphite of gold and soda, and is formed by the reaction of hyposulphite of soda on chloride of gold. It is very valuable in colouring positives on paper, and is very easily decomposed. It will be more fully noticed under the head of " Th- Colour- ing Bath." Hydrochloric Acid is a volatile gas, exceedingly soluble in water, forming the hydro chloric or muriatic acid of commerce, which contains from thirty to forty per cent, o gas. It is used in the formation of chloride of gold, in combination with nitric acid and for producing the yellow perchloride of iron. Hydrosulphate of Ammonia is formed by passing sulphuretted hydrogen gas through ammonia. It is used for the purpose of separating silver from hyposulphite of soda, to darken negatives, and for testing solutions for silver, &c. Hyposulphite of Soda, and the hyposulphatcs of gold and silver, will be fully described undfcr the head, " The Colouring Bath." Iodide of Fotassium.- This salt, as the reader already knows, and I think there can be no objection to these facts being often repeated, is one of the salts chiefly used for the production of iodide of silver. Iodide of potassium is generally made by dissolving iodine in solution of potash until it acquires a slightly brown colour. This solution contains not only iodide of potassium, but iodate of potash, which may be got rid of by evaporation and heating the residue to redness, when the iodate parts with its oxygen and is converted into iodide of potassium. Iodide of potassium has the following properties : -It forms white cubic and pris- matic crystals, which should be hard, and scarcely, if at all, deliquescent; it is soluble m less than its own weight of water ; alcohol will dissolve from two to eight grains to the drachm, according to its strength-the stronger the alcohol the less it will dissolve : ether will not dissolve it at all. Iodide of potassium as sold in the shops is nearlv always slightly impure from the presence of carbonate and sulphate of potash. Litmus Paper.— This is of the greatest use to the photographer, as it enables him to tell almost at once whether his solutions are acid or alkaline. It is made by soaking of 110 PHOTOGRAPHIC CHEMICALS. porous paper in a solution of litmus, digested in hot water ; the paper when dry is quite blue, but in the presence of any acid becomes red, which changes back again to blue when brought in contact with an alkali ; the red litmus paper is made by dipping i the blue paper in water containing one or two drops of sulphuric acid to the pint. ' Litmus is a vegetable substance procured from various lichens which grow on rocks j near the sea. Protosulphate of Iron is the green copperas of commerce. < It dissolves in about its own weight of water, and when in solution it is used as a developer for positives on glass ; it improves by use and exposure to the air, and is extensively used by photo- graphers. Protonitrate of Iron. — This is another salt of iron, used for the development of positives on glass; but has nothing to recommend it for use in preference to the protosulphate. Nitric Acid is much used in the preparation of pyroxyline, for which purpose it ought to be of the strongest possible description. It is often contaminated with chlorine or sulphuric acid ; the presence of chlorine may be detected by diluting the acid with an equal bulk of distilled water, and then adding a few drops of nitrate of silver solution — a milkiness (chloride of silver in suspension) denotes the presence of chlorine. Sulphuric acid may be detected by the addition of a little chloride of barium, which, with the sulphuric acid, will form an insoluble precipitate of sulphate of baryta. Nitrate of Potash, or saltpetre, is a very abundant natural product. It often con- tains a large proportion of chloride of potassium, which may be detected by dissolving a small portion, and adding a few drops of nitrate of silver ; when, if the chloride of potassium be present, the never-failing chloride of silver will be formed. Nitrate of lead is made by dissolving the metal, or its oxide, in excess of nitric ! acid diluted with two parts of water. It forms, with sulphuric acid or soluble sul- phates, an insoluble sulphate of lead. Silvez Solutions. — Silver and its different solutions are all-important to the photographist. Nitrate of Silver. — Nitrate of silver, or, more correctly speaking, nitrate of the oxide of silver, is made by dissolving pure silver in nitric acid (aqua fortis), which parts with oxygen to the silver, forming an oxide of silver, and that in turn becomes dissolved by another portion of the nitric acid. Nitrate of silver crystallizes in white scales. When the solution has been boiled down nearly to dryness, the crystals have a bitter metallic taste, and are very soluble in water, which will dissolve about its own weight. The nitrate used for photographic purposes should be dissolved in distilled water and re-crystallized, so as to be deprived of all traces of the nitric acid. A solution of nitrate of silver in distilled water is scarcely, if at all, affected by light, unless it be brought in contact with organic matter, when it becomes speedily decom- posed, and thus it becomes so useful in photography ; for if we wash a sheet of paper over with a solution of the nitrate of silver, and place on its surface any opaque figure, such as a coin, a leaf of a tree, or, what is better, a piece of black network, pressed to the surface by a sheet of glass, we shall, by exposure to the rays of the sun for a few minutes, obtain a correct copy of the figure ; but with reversed effects, the parts uncovered being black, the parts covered remaining white. We thus form what we call a negative picture ; for example — If we take three letters cut out of cardboard (Fig. 1), and place them on a piece of paper washed with a solution of nitrate of silver, press them close by means of a sheet of glass, and expose it to the sun's rays for SILVER SOLUTIONS. Ill five or six minutes, we shall ohtain a picture similar to Fig. 2, and this also is the process by which the portrait at the head of this Treatise was obtained by a very able HXY Fig. 1. Fig. 2. experimentalist. The negative was obtained by the usual Talbotype process, and the positive by placing the negative upon the prepared wood. Now it is very obvious that, by removing the nitrate of silver from the white parts of the paper, and thus protecting them from any further action of light, we can, by repeating the experiment and using the negative just obtained, instead of the card- board letters, obtain a perfect copy of the latter. This is the principle of photographic reproduction — Having once produced a negative, whether in the camera or by contact, the number of copies that may be obtained from it are almost without limit. Nitrate of silver, when melted in a crucible, and cast into moulds giving it a shape resembling pieces of pipe stems, becomes the lunar caustic used in surgery ; but this is scarcely pure enough for photographic purposes. It will be always better for the amateur to procure the white re-crystallized nitrate. If the nitrate obtained from the chemist or otherwise exhibit traces of nitric acid, the latter may be got rid of by heating the crystals carefully to some few degrees above boiling water for a short time. This must be done in a glass or porcelain vessel, as almost every metal has the property of decomposing the nitrate. So loosely is it combined with the oxygen that even light, as I have already shown, reduces it. If you take a solution of nitrate of silver, no matter what is the strength, and immerse in it a clean strip of copper, brass, iron, zinc, tin, or any other of the base metals, you will at once see that an action com- mences, the silver being thrown down as a metallic powder, and the other metal becomes dissolved ; in other words, the silver has so slight an affinity (or liking) for oxygen, that the slightest force is able to separate them. This gives us another reason why silver is so very useful in photography. Salts of Silver. — We can very easily obtain a great many salts of silver by double decomposition. I shall explain the meaning of double decomposition by a simple ex- periment, and one also that is essentially photographic : — Take nitrate of silver, 30 grains ; distilled, or boiled rain water, 1 ounce (when the nitrate is dissolved take in another measure) ; common salt (chloride of sodium), 10 grains ; water, not necessary to be distilled, 1 ounce. Now, if we pour the common salt solution into the silver one, we obtain directly a white curdy precipitate, perfectly insoluble even in boiling water or nitric acid. This is chloride of silver, or a mixture of metallic silver and chlorine. The double decomposition takes place thus : — Nitrate of silver Common salt. ( Nitric acid \ Silver ( Chlorine / Sodium The chlorine, having a strong affinity for the silver, joins it ; an d t ie nitric acid being 112 SILVER SOLUTIONS. set free, very good-humouredly goes over to the sodium. We thus obtain one of the most important salts used in photographic printing. Expose the white powder to the light after washing it several times in fresh water, an operation easily performed because of its insolubility ; and you will observe that it darkens all over, and presently becomes black. But do not for a moment suppose that it has blackened all through. No ; the smallest possible quantity of the surface only has been acted on, the thinnest possible layer only blackened ; remove the upper surface in the slightest degree, and you will find the part underneath perfectly white. Bear this fact in mind, as I shall have to refer to it more fully on another occasion. Iodide of Silver. — This is produced in a similar way to the chloride, with the ex- ception that we here use iodide of potassium, which by double decomposition, as before, produces — When the two solutions are mixed in the following proportions: — Nitrate of silver, 20 grains ; distilled water, 1 ounce ; — iodide of potassium, 20 grains ; water, 1 ounce — a copious yellow precipitate is produced, which, like the chloride, is insoluble in water and nitric acid. This salt, when perfectly pure, is not changed by the action of light ; but if any excess of nitrate of silver be present, it becomes decomposed. The decomposition, in tbis case, has the property of proceeding onwards to blackness by the agency of the numerous developing solutions that we use, and that after the decomposition has once commenced. In this salt and the bromide of silver, being nearly alike in their nature, we observe the base of all the solutions for papers used in the camera. & ■'- I Bromide of Silver. — This salt is produced in the same manner as the iodide, using bromide of potassium instead of the iodide. If you use any bromide or iodide, such as the iodide of tin, iron, zinc, mercury, cadmium, sodium, &c, you will obtain the same result, the oxygen going to the metal, the iodide or bromine going to the silver. In photographic experiments, however produced, the iodide of silver and the bromide of silver are alw r ays the same when in the solid or crystalline state. The iodide of potassium, and the bromide also, have the power of dissolving the iodide or bromide of silver when added in sufficient quantities, a property of which we avail ourselves for the formation of iodized and brominized papers, to be explained more fully hereafter. Acetate of Silver. — The acetate of silver will be found very useful in correcting a new nitrate of silver bath for collodion negatives, and may be made thus : — Neutralize one ounce of acetic acid by adding sufficient carbonate of soda, and add it to one ounce of distilled water, containing about 100 grains of nitrate of silver in solution. The action is thus represented : — Nitrate of silver . . j Nitrate of potassium. Iodide of silver* Iodide of potassium j Nitrate of ( Nitric acid silver solution. ( Silver Acetate of silver Nitrate of soda. Neutralized /Acetic acid acetic acid. \ Soda Hyposulphite of Silver. — This salt is formed by the action of hyposulphite of soda 113 on nitrate of chloride of silver, and will be more fully treated under the head of Colouring Baths. It is very soluble in an excess of hyposulphite of soda, and its chief photographic use is in connection with that salt, to impart those rich brown or purple tints so much admired in finished photographs. Ammonia Nitrate of Silver. — This is one of the most useful and important compound salts of silver; it is extensively used in printing, and may be made thus :-Dissolvein one ounce of distilled water fifty grains of nitrate of silver, and then add, drop by drop, liquor ammonia until the brown precipitate first formed is gradually redissolved ; when filtered it should be put away in a dark bottle and kept from the light. On the Means of Recovering Silver from Old Solutions. — This may be done in various ways. One of the most simple of the many is to insert a strip of clean zinc or copper in the solution of silver, and let it remain until the silver is all thrown down ; but old hyposulphite of soda solution, containing silver, must be treated in a different manner. To recover silver from the' latter, it will be necessary to pass a stream of sul- phuretted hydrogen through the solution, or to add a sufficient quantity of hydrosul- phuret of ammonia to precipitate tbe silver, which will be thrown down as a sulphuret in either case. The first is the most troublesome, but by far the cheapest method. To make and pass the sulphuretted hydrogen gas you will proceed thus :— Get a large bottle, to which fit a piece of gutta percha tube bent thus (Fig. 3), which must fit in the neck or mouth of the bottle air tight, then put into the bottle about a quarter of a pound of sulphuret of iron, and about three-quarters of the contents of the bottle of water, to which has been added an eighth part of sul- phuric acid (oil of vitriol), place the gutta-percha tube in its place, and the other end must go in the solution of hyposulphite of soda and silver, touch- ing the bottom of the vessel ; the sul- phuretted hydrogen escaping up through the liquid decomposes its parts, and throws down the silver, the latter, as I have said, becoming a sulphuret of silver. A represents the bottle, B the vessel containing the hyposulphite-solution, C C the gutta-percha tube, D the solution of water and sul- phuric acid, E the solution of hyposulphite of soda and silver, F the bubbles of sulphu- retted hydrogen, and G the sulphuret of iron. The action must be continued until all the silver is thrown down, which may be ascertained by adding a little hydrosulphuret of ammonia, or by agitating the solution and smelling it ; in the former case, if any silver remain there will be a black precipitate, and, in the latter, if the smell of the sulphu- retted hydrogen be very strong from the liquid, it is a proof that all the silver has been thrown down. It may be as well to state here that the fumes, or gas, of the sulphu- retted hydrogen is very poisonous when in a concentrated state, and, therefore, the operation should be carried on out of doors. The hyposulphite solution should be frequently stirred. When all the silver is thrown down, it must be collected and washed on a filter by pouring water through it until the latter passes through quite clear, and will not give PRACTICAL CHEMISTRY. — No. IV. 114 RECOVERING SILVER IN SOLUTIONS. a precipitate with a few drops of nitrate of silver solution ; the black mass remaining may now be boiled up with nitric acid one part, water two ; when all the red fumes cease to be evolved, the solution is to be dilated with water and filtered to get rid of any insoluble matter, which principally consists of sulphur with perhaps a small portion of chloride of silver and sulphuret ; if the nitric acid contains any trace of chlorine, or if the insoluble portion be large in quantity, you may heat it pretty strongly on a piece of iron plate to get rid of the sulphur, and dissolve the remaining portion in a strong solution of hyposulphite of soda, and add it to the colouring hypo-bath. The solution that has been passed through the filter will be a solution of silver in nitric acid, or nitrate of silver, but will not be pure enough for photographic use ; it will be better to convert it into chloride of silver by adding a solution of common salt, and washing the precipitate two or three times. The chloride, or sulphuret of silver may be converted to metallic silver by fusing it in a crucible with twice its weight of car- bonate of potash, or a mixture of carbonate of potash and soda. "When the heat has been carried on sufficiently, the whole flux may be poured out of the crucible, or the crucible and its contents may be allowed to cool, when the silver, a beautifully bright button, will be found at the bottom. On the Means of Converting Chloride into Kitrate of Silver. — As we already know that nitric acid, even when boiling, will not act on the chloride of silver, we must go a little round to bring the two together, and the best method of obtaining that important result will be as follows : — After well washing the chloride, pour it out into a flat dish, in which place a bar of metallic zinc in contact with the chloride, a small quantity of oil of vitriol diluted with four parts of water is then added, until a slight effervescence is seen to take place. The dish must then be set aside for two or three days, and must not be disturbed in any manner. The reduction commences with the chloride imme- diately in contact with the zinc, and afterwards radiates in all directions. When the whole mass has become of a gray colour, the bar of zinc is to be carefully removed and the adhering silver washed off with a small stream of water. In order to insure the purity of the silver, a fresh addition of oil of vitriol must be made after the zinc has been removed, in order to dissolve any fragments of metallic zinc which may have become detached by accident, and after the digestion has been continued for a few hours, the gray powder is to be washed several times with water, until the water which runs off will not give a precipitate with carbonate of soda ; it may then be converted into nitrate of silver by boiling with nitric acid one part, water two, and evaporated to crystals. The above formula is not so expensive or troublesome as the fusing with carbonate of potash. Bear in mind, that you must pour the oil of vitriol into the water, and the vessel in which they are mixed must be such as will stand heat. Oxide of Silver is an olive brown powder obtained by adding potash to nitrate of •ilver. It is soluble in hyposulphite of soda, cyanide of potassium, ammonia, and nitrate of ammonia. Sulphuric Acid, or oil of vitriol, is an acid possessing intense chemical powers, and readily displaces the greater number of acids from their salts ; it clears organic sub- stances by depriving them of water, and converts alcohol into ether by the same means, and is one of the elements used for the production of gun-cotton. Its action in the latter case will be more fully explained further on. Tetrathionic Acid. — (See " Colouring Bath.") OPTICS OF PHOTOGRAPHY. 115 ON THE OPTICS OF PHOTOGRAPHY. Action of Light.— Having, for the present, finished the necessary remarks on Photographic Chemistry, I shall proceed to explain the optical and actinical (or chemical decomposing ray power) action of light on surfaces prepared photogra- phically ; and I may remark, en passant, that, hut for our knowledge of the chemical action of light through glass, all our chemical knowledge of the theory of photo- graphy would be perfectly useless ; we could no more obtain a perfect copy of a tree, a house, or a hay-stack, than we could fly— this being another proof, if such be necessary, of how dependent one branch of science is on another. Light, the agent by which we are enabled to depict nature or art with an accuracy that baffles the most experienced artist, is derived from the sun. True it is that there are other sources of light ; but at present we have nothing to do with them— wc must confine our attention to solar light, and the chemical change it produces. This glorious light, which " Was given to quicken slumbering nature, And lead the seasons' slow vicissitudes Over the fertile breast of mother earth," now pours forth its beams, and in a sense not dreamed of by the poet, diipenses " Life and light on every side ; Brightening the mountain cataract, dimly spied." And yet how little do we know of the nature of a sunbeam. A solar beam of light is a bundle of rays, a ray being the smallest portion of light which can emanate from a luminous body. Each of these rays possesses distinctive characters, both as regardt their chemical functions and colours. Sir Isaac Newton proved that the white light emitted from the sun is not so simple as it appears, but is composed of vivid colours and tints which wc may prove to our own satisfaction, by performing the beautiful experiment called " New- ton's Analysis of Light," being a prism (Fig.^ 4), !or triangular mass of glass, which is so contrived t hat it may be adjusted to any angle, or placed in any required position. The shutters of the room being closed, we may admit a ray of light either by boring a hole in the shutters or separating them a little. The ray of light A E (Fig. 4), being admitted into the darkened room by means of a hole A in the shutter. It will be seen that the space between the shutter and the spectator is traversed by the sunbeam or ray of light, which appears to cause little particles of dust to dance in the atmosphere of the room. This appearance, however is owing to the illuminating power of the sunbeam contrasting with the other darkened or non-illuminated space in the room, which renders the email particles of dust floating in the air visible. As soon as the prism B C (Fig. 4) is placed in the path of the sunbeam, so as to allow it to fall on one of its angles B, the ray will be refracted, or bent out of its course, so as to pass towards the back of the prism (as in the line D), and not in the same line A E that it would otherwise have Fig. 4. 116 NATURE OF LIGHT. done, had not the prism been interposed. Another effect also takes place : an elon- gated delicately-coloured image is formed upon the wall D E ; and if you stand at a short distance from the prism you will see' that these colours are spread out in a triangular form, the base of which is on the wall, and the apex, or point of origin, at the back C of the prism. Beniove the prism, and it is seen that the splendid dis- play of colours upon the wall has disappeared, and a round spot of white light E is seen below the place occupied by the solar spectrum. The coloured image upon the wall is called the prismatic or solar spectrum, which, according to Sir Isaac Newton, is composed of seven different colours (Fig. 5). The colour at the lower portion of the image, or that nearest to the round white spot E on the wall when the prism was removed is of a red colour, and the one at the other end is of a violet colour ; the whole intermediate parts being occupied by five other colours, and the whole arranged accord- ing to the table exhibited below, the proportion of each colour having been measured by Fraunhofer with the greatest care, with the results placed opposite to each corresponding with the 360 degrees of a circle, the red ray being the least, and the violet the most refracted of this chromatic image :— Top. Violet Indigo Blue Green Yellow Orange Red Bottom. 109 47 48 46 27 27 56 360 Since Newton's time, various experiments have been instituted and other rays detected ; for instance, a crimson or extreme red ray has been discovered below the red ray, by examining the solar spectrum through a deep blue glass ; and Sir J ohn Herschel observed a lavender beyond the violet ray, by throwing the spectrum upon a piece of yellow paper. Mr. Stokes has also proved the existence of an extra spectral ray far beyond the violet ; but, as we have remarked before, our consideration of light does not extend beyond its practical use to photographers. Sir Isaac Newton was of opinion that white light was composed of seven primary rays, each possessed of a certain degree of refrangibility, or capability of being turned out of its natural course ; and he also considered that the colour of a ray indicated its angle of refraction. Sir David Brewster has demonstrated that the seven primary colours, as Sir Isaac Newton called the rays of the solar spectrum, are not primary, but that only three of them are so— viz., blue, yellow, and red ; the rest are compounds of the three primary colours, which form the spectrum by overlapping each other ; and these are explained in the annexed diagram (Fig. 6). Fig. THE SOLAR SPECTRUM. 117 Such are a few of the phenomena relating to light regarded by the philosphers ; its application to photography are as follows : — Of the real nature of the rays, which form the sunbeam, little is known. The theory of Newton consisted in supposing the ray of light was produced by the emission of minute particles of matter travelling at an enormous velocity from a luminous body, and, when these minute particles impinged on any body, they were either thrown back, j reflected, or absorbed, according to the surface on which they fell. These particles entering the human eye, produce the sensation of light on the retina, which sensation is conveyed through the optic nerve to the brain. The theory of the celebrated Huygens pre-supposes that the space beyond our atmosphere, and the interstices between the molecules, or ultimate atoms of all bodieSj are filled with an imponderable ether, and that light is produced by the oscillation or vibration of this ether, which undulation is set up by some self-luminous body — of course, the sun. Another theory may here be mentioned, although but very slenderly supported — namely, that set forth by Oersted, who considered that light was the effect of a rapid succession of minute electrical discharges taking place between a luminous body and the eye. Leaving these theories, however, to the philosopher, let us see how they affect the photographer. The sunbeam — the ray of white light — contains powers within it of which the earlier philosophers had but a faint idea ; besides its accompanying heat, there is a principle associated intimately with it, which has the power of decomposing and of determining the recomposition of chemical compounds. This principle has been already alluded to— it is " Actinism," and is as perfectly distinct in the nature of its properties, from light, as light is from the principle of heat, with which it is also closely connected. Actinism may then be considered [as the fundamental principle on which photo- graphy is based ; and we would wish, before entering on a description of the various methods of obtaining sun pictures, to draw a broad distinction between light and actinism, more especially as many apparent difficulties present themselves, and seem almost insurmountable until tried by the principle we are about to lay down. From what has been said, it will be supposed that what we consider light exerts a decided influence over certain chemical salts having a metallic base ; but it now becomes necessary to show that light docs no such thing — it is not light, but a compo- nant part of light which exerts this influence. In order to explain this seeming anomaly, let us consider the subject a little more carefully. A ray of white light consists of the three primitive colours — blue, yellow, and red ; and their combinations forming, en tablette, the following : — Violet, indigo, blue, green, yellow, orange, red; these colours and shades being produced by the decomposition of white light by means of a prism. Of these shades, the violet has the greatest reducing or decomposing power, y By this, I mean that the violet part of the decomposed portion of light exerts the most powerful influences on the unstable metallic salts, reducing them to their bases. This action is the actinic of the photographers ; and the study of the action itself may be properly designated as actinic-chemistry. Every beam of light which We receive from the sun is composed of the three primary colours ; these blending one with the other, form shades or mixtures of the three ; thus we get four shades independent of the primitive colours, viz., indigo from blue and violet, green from blue and yellow, orange from yellow and red. It may be asked, where does the violet come from ? It is easily accounted for 118 ACTINIC CHEMISTRY. thus— if we decompose a single ray ©f white light, we get the following component parts by means of the prism : — Violet. Fig. 7. Now, if we decompose another ray, just below the above, we get the same parts reproduced, thus — violet comes first, or next the red, and is evidently produced by the mixture of red and blue (the next primary colour), or, more properly speaking, by the mixture of a deep red, which slightly extends lower than the red of the visual spec- trum, with the indigo of the ray immediately above the under one ; now this ray, or portion of a ray, has the power of more perfectly decomposing the unstable salts of silver than any other of the series ; and, therefore, has acquired the term actinic ray. The actinic power, and the light-giving power, may be more fully explained in the following diagram: — b Lavender. Actinism, or chemical ) F radiant power. ) Light. C ( Space of extra spectral blue, ob- < tained by a solution of quinine, I and some of the mineral oils. Fig. 8. In the above diagram, the greatest actinic, or chemical action, is shown opposite REFRACTIVE POWER OF DIFFERENT SUBSTANCES. 119 the violet ray E, and the least opposite the mixture of the yellow and orange C ; below the red at A the actinic power becomes active again, because the extreme or deep red is about to pass into the violet with the indigo of the blue in the ray next below ; at the same time, the part of the ray giving the brightest light is opposite the yellow and orange light C. We observe, also, that the point giving the greatest heat is just below the red D ; but with that we have nothing to do. "We thus ascertain that the chemical, or photographic action is confined, as already stated, to only a portion of the visual ray of light. To speak more plainly, certain colours or shades act more powerfully than others, which can be proved by the following simple experiment: — Prepare a sheet of paper thus — float it on a weak solution of common salt, say ten grains to the ounce, and when dry, float it again on a solution of nitrate of silver, say thirty grains to the ounce- This must be done and the sheet dried while it is protected from white light. When dry, place on it three pieces of coloured glass, viz., red, yellow, and blue ; expose the whole to the sun's rays for a short time, when it will be found that the paper has become rapidly discoloured under the blue glass, but remains unchanged under the red and yellow, although the last is by far the most transparent. This property of red or yellow colours of intercepting the actinic rays of light, we make the greatest use of in photography ; but this subject will be treated of more fully under the head of "The Dark Chamber." A ray of light is always more or less refracted or bent, depending on the density of the medium or substance through which it passes. The refractive power of some substances is immense, while that of others is very trifling, as the following table of some of the most important will show: — Air 1-000294 Plate glass .... 1-542 "Water 1-336 Flint glass .... 1830 Alcohol .... 1-372 Do. containing much lead . . 2-028 Oil of cloves . . . 1535 Diamond 2 439 Crown glass . . . 1534 A ray of light, passing through a vacuum, progresses in a perfectly straight line, and were it possible, under such conditions, to look at a brilliantly illuminated point, we should see it in its true position, viz., the numerous rays coming undisturbed directly to the eye. But all matter, however attenuated it may be, has the pro- perty of refracting or bending the ray of light ; consequently we do not see the stars in their true position, owing to the refractive power of the atmosphere. The law of refraction can be easily and decidedly demonstrated thus — take a basin, in the bottom of iig - 9 * which place half-a-crown, or any other small bright substance, and removing a sufficient distance from it to lose sight of the coin, it will appear as in Fig. 9 ; A representing half-a-crown, and B the eye of the observer. The half-a-crown, of course, is invisible. Then request some person to pour water into the basin, taking care to keep your eye 120 REFRACTIVE POWER OF DIFFERENT LIQUIDS. fixed on the same spot during the operation. The half-a-crown begins to appear, and gradually becomes more visible until it comes entirely into view. This fact is owing to the ray of sight (or light) being refracted, or beaten back, as in Fig. 10 ; C representing the water, and B A the ray of light refracted. The explanation of this pheno- menon is, that the ray of light pro- ducing vision in the eye is bent, on emerging from the water, and has all the effect of conveying our sight round a corner. The refractive power of water is also observable when we thrust a straight stick or instrument into it, on aiming at any object. "We see that the stick seems to be bent, and fails in reaching the point which we desired it should reach. On this account, the aim by a person not directly over a fish, must be made at a point apparently below it, otherwise the weapon will miss by flying too high. Persons who spear salmon in rivers require to calculate upon this refractive power in taking their aim. Another illustration of refraction is to allow a sunbeam S (Fig. 11), passing through a hole in the window-shutter of a dark room, to fall upon the surface of a fluid contained in a glass vessel, C C ; instead of proceeding onward to D, it will be found to alter its course at the surface of the fluid, and pass along the line to D refractive powers in virtue of its physical constitution ; but a ray of light incident perpendicularly on a refracting medium, as the ray E (Fig. 11), suffers no refraction. Again, if we float, one upon the other, fluids, B, C, D, having different powers cf refraction, we shall then see the relative phenomena exhibited by the bending of the ray B B, as it passes through these different media, as represented in Fig. 12. The mode of the refraction depends on the comparative density or rarity of the respec- tive media. If the medium which the rays enter be denser, they move through El ' D Fig. 11. Every substance has different Fig. 12. VARIOUS-SHAPED LENSES. 121 it in a direction nearer to the perpendicular drawn to its surface. On the contrary, when light passes out of a denser into a rarer medium, it moves in a direction farthci I from the perpendicular. This refraction is greater or less — that is, the rays are more 1 or less Dent, or turned aside from their course — as the second medium through which J they pass is more or less dense than the first. To prove this in a satisfactory manner, j and at the risk of repetition, wc make the following experiment : — Take an upright empty vessel into a darkened room, which admits but a single beam of light obliquely through a hole in a window shutter. Let the empty vessel stand on the floor, a few feet in advance of the window which admits the light, and let it be so arranged that, as the beam of light descends towards the floor, it just passes over the top of the side of' the vessel next the window, and strikes the bottom on the side farthest from the window. Let the spot where it falls be marked. Now, on filling the vessel with water, the ray, instead of striking the original spot, will fall considerably nearer the side towards the window. And if we add a quantity of salt to the vessel of water, so as to form a dense solution, the point where the ray strikes the bottom will move still nearer to the window. In like manner, if wc draw off the salt water, and supply its place with alcohol, the beam of light will be still more highly refracted ; and oil will refract yet more than alcohol. Our next care is to study the practical application of these laws of refraction to the manufacture of "lenses." By lens is meant what is commonly called a magnifying glass, which may be composed of any transparent substance ; but in its application to photography it is generally made of glass as pure and colourless as can be procured, therefore we shall consider that a lens is a glass ground into such a form as to collect or disperse the rays of light which pass through it. These are of different shapes, and thence receive different names. The following figures individually represent sections of the variously-shaped lenses and other glasses used in optics. A is a trian- A B C D E _ F C H I Fig. 13. gular stalk of pure glass, of which we have here a cross sectional or end view, and which is called a prism. Each side of the prism is smooth. B is a section of a piece of plane glass, with sides parallel to each other. C is a sphere or ball of glass, and consequently is convex on all parts of its surface. D is a piece of glass convex or bulging on its two sides, and is called a double convex lens. It is this kind of lens which is used for magnifying objects, in spectacles, telescopes, and other instruments. E is a plano-convex lens, flat on one side and convex on the other. F is a double concave lens, or glass hollowed on each side. G is a plano-concave lens, or planed on one side and concave on the other. II is a meniscus, or lens convex on one side and concave on the other, both surfaces meeting, and of which we have an example in watch-glasses. I is an example of the concavo-convex lens, in which the surfaces disagree, or do not meet when continued. In all these lenses an imaginary line, re- 122 CONSTRUCTION OF LENSES. presented by M G N, and passing through the centres of the surfaces, is called the axis. Thus, the line said to pass through the centre of any lens, in a direction perpen- dicular to its surface, is called its axis. The design in forming lenses is to procure a medium through which the rays of light from any object may pass, and converge to a corresponding point beyond. The manner in which the rays proceed through the glass, and then centre in a focal point, will depend on the form of the lens, its capacity for refraction, and the distance of the object. If we take a piece of glass, flat on one side and cut into different faces on the other, and then look through it from the flat side at any object — for instance, a pea — we shall see as many peas as there are faces receiving rays from the single pea. We may exemplify this principle of multiplication by the annexed figure (Fig. 14), in which A B is a lens flat on one side, and cut into three faces on the other, GH. Y is the eye of the spectator, and P the pea to be looked at. The eye receives a pencil of rays direct through the lens at I, and sees the 14, object without refraction. A pencil also proceeds from P to face Gr A, and another pencil proceeds from C to the face H B, and in both cases the rays are bent and refracted to the eye. This eye, however, does noc recognise the path of either of these oblique rays, but perceives the image of a pea at D and at E ; and thus three peas seem to be seen in place of only one. In smoothly ground lenses, in which there are no distinct faces to multiply the images of an object, the rays bend, as we have said, so as to meet in a corresponding point beyond them. A lens may consist of a perfect globe of glass, or globe filled with pure water, in which case the refractive power will be considerable. A double convex lens, which is the more common kind, may be viewed as a portion cut out of the side of a sphere, as seen in Fig. 15. Here, as in all cases of convexity, the focus of the parallel rays passing through the lens is at F, which is the centre of the sphere, of which the farther, or anterior side, is a portion, or a point at half the diameter of the sphere from it. (Half the diameter is tech- nically called the' radius.) Should we take a plano-convex lens, the focal point would be considerably different. In Fig. 16 we have an example of this kind of lens, which evidently possesses only half the refractive power of the double convex glass. Here the parallel rays, falling on the convex side of the lens, are seen to converge at the distance of the whole diameter of the sphere. Thus, the focal point at which the rays of light fall is always regulated by the degree of curvature of the lens. I shall illustrate this by various diagrams,"and ask the reader's careful attention, for the subject is difficult, and cannot be comprehended by a superficial glance. Fig. 15. Fig. 16. POWER OF LENSES. 123 We take a double convex lens, represented by A B C (Fig. 17), the axis of which is the line G' C D\ The ray D' G', being straight through the centre, suffers no re- fraction ; but the rays D A and D" B are refracted, so as to meet at the focal point G'. We now observe that the parallel rays c' E A, E' C, and E" B, and also F A, F' C, and F" B, falling obliquely on the lens, /f will, in a similar manner, be refracted, and 0 have their foci at G and G'', at the same distance from the lens. Those lines which Fig. 17. pass through the centre, as £' C G" and F' C G, do not alter their direction, not being refracted. Thus, in whatever way parallel rays pass tb rough a lens, we have a focal point beyond it, be it straight forward or in an oblique direction. The distance at which the rays meet beyond the len3 is exemplified in the next diagram (Fig. 18). Dr. Arnott, in his Treatise on Physics, says — " Bays falling from A Fig.ia. on a comparatively flat or weak lens at L, might meet only at D, or even farther off, while, with a stronger or more convex lens, they might meet at C or at B. A lens weaker still might only destroy the divergence of the rays, without being able to give them any convergence, or to bend them enough to bring them to a point at all, and then they would proceed all parallel to each other, as seen at E and F ; and if the lens were yet weaker, it might only destroy a part of the divergence, causing the rays from A to go to G and H, after passing through, instead of to, I and II, in their original direction. " In an analogous manner, light coming to the lens in the contrary direction from BCD, &c, might, according to the strength of the lens, be all made to come to a focus at A or at L, or in some more distant point ; or the rays might become parallel, as M and N, and therefore never come to a focus, or they might remain divergent. " It may be observed in the annexed figure, that the farther an object is from the lens, the less divergent are the rays darting from it towards the lens, or the more nearly do they approach to being parallel. If the distance of the radiant point be very great, they really are so nearly parallel that a very nice test is required to detect the non-accordance. Bays, for instance, coming to the earth from the sun, do not diverge the millionth of an inch in a thousand miles. Hence, when we wish to make experiments with parallel rays, we take those of the sun. 124 QUALITIES OF A CONVEX LENS. " Any two points so situated on the opposite sides of a lens, as that when either becomes the radiant point of light, the other is the focus of such light, are called con- jugate foci. An object and its image formed by a lens, must always be in conjugate foci ; and when the one is nearer the lens, the other will be in a certain proportion more distant. " "What is called the principal focus of a lens, and by the distance of which from the glass we compare or classify lenses among themselves, is the point at which the sun's rays — that is, parallel rays — are made to meet ; and thus, by holding the glass in the sun, and noting at what distance behind it the little luminous spot or image of the sun is formed, we can ascertain the solar focus of a glass, as at A for the rays E and F." From the preceding explanations it will be understood, that when an object is placed at any distance from a lens, an image of it will be formed in the corresponding- conjugate focus ; but to see this image distinctly, the eye must generally be placed at least six inches behind it, that is, farther from the lens. When, however, the object is placed in the principal focus, the rays are refracted parallel, and the image in this case is distinct when seen at any distance. But the most remarkable quality of a double convex lens remains to be noticed ; we allude to its magnifying power. This quality is entirely a result of the refractive power of u the glass ; embraced within the be its apparent size when seen by the unaided eye. If a convex lens A B is now interposed between the eye and the object, so that the object B W shall be in the principal focus of the lens, an enlarged image K' W of the arrow will then be seen, its extremities R' W lying in the directions E A, E B. The directions of these rays are determined thus :— From B and W draw the central rays B C P, W C Q, through the centre C of the lens ; then the rays of the conical pencil, proceeding from the point B to every point of the nearer surface of the lens, are refracted in such a manner by the lens, that they all emerge in directions parallel to the central ray RCP; but of the whole refracted pencil only a small portion enters the eye, namely, the pencil A M N A, limited by the size of the pupil M N ; and the head A of the arrow, whence this pencil proceeds, appears to lie in the direction of the pencil E A B' at B'. It is shown exactly in the same manner, that the point "W will appear in the direction EBW'at W\ The enlarged image of the small arrow B W is therefore B' W\ The proportion in which the image is enlarged will be easily ascertained thus : — The triangles E B' W, C B W, are similar, aud therefore the ratio of B' W to B W, is that of E B' to C B, or of EMtoCM; that is, as the least distance E M of distinct vision, to the focal length C M of the lens. If, therefore, the least distance of distinct vision sphere of the rays from the lens, the object is apparently expanded in size, and seems brought nearer to the eye. This may be elucidated, for small objects seen near, by a refer- ence to the diagram (Fig. 19.) Fig. 19. Let E be the eye, and M N the diameter of its pupil, B W a small object placed at the least distance of distinct vision (about six inches from the eye for small objects), and let B W MAGNIFYING POWER EXPLAINED. 125 be divided by the focal length of the lens, the quotient will be its magnifying power. If E M be reckoned 6 inches for small objects, and if the focal length C M be 2 inches ; then, since 6, divided by 2, gives 3 for a quotient, the magnifying power is 3 times. If C M were one quarter of an inch, then 6, divided by J, gives 24 for a quotient, and the magnifying power would in this case be 24 times. A more simple explanation may be attempted as follows : — Turn to Fig 14, repre- senting the lens with three faces on one side and flat on the other. There it is observed that the vision travels in the direction of the ray from the object, as it passes through the glass, and therefore sees an appearance of three objects. Now, in the above case of a magnifying lens, the vision in the same manner travels from the eye at E in the direction of the angle of refraction ; it goes on to E.' and W, and thus the actual object being drawn out, as it were, to meet these points of vision, or seemingly expanded by the bent rays, we of necessity see an apparently larger object. If the glass were cut in faces, instead of being smooth, the object would not appear drawn out, but would be multiplied in as many points as there are faces. The inversion of the image by a lens may be illustrated by the diagram, (Fig. 20.) A B C is an arrow, with the point uppermost, placed beyond the focus at F, of a double convex glass d e f. In virtue of the refractive power of the lens, the rays which proceed at A meet at Z, and form an imoge of the arrow-point inverted ; while the rays from C meet at X, and form a similarly inverted image of the feather part of the arrow. The rays proceeding from B unite at b. Here, only rays from A, B, and C are represented, for the sake of clearness ; but, in point of fact, rays from all parts of the object proceed through the lens, and hence an entire image is formed in an Should the object A B C be brought nearer the lens, the image to a greater distance, because then the rays are rendered more To Fig. 20. inverted position, will be removed divergent, and cannot so soon be collected into corresponding points beyond, procure a distinct image, the object must be removed farther than the focal point F from the glass. In this exemplification, the object seems to be diminished ; but if we make the small arrow the object, the larger one will be the image of it magnified. In order to explain the power of lenses in magnifying distant objects, and bringing them near us, let us suppose an object placed at one hundred feet distance from the eye of a spectator. Let us place a convex glass of twenty-five feet focal distance half way between the object and the eye ; then, as has been previously observed, an in- verted image of the object, and of the same size, will be formed fifty feet behind the lens. If this picture is looked at six or eight inches behind it, it will be very distinctly seen, and nearly as well as if the object itself had been brought to within six or eight inches of the eye of the spectator. If, however, instead of a lens of twenty-five feet focal length, a lens of a shorter focus is made use of, and so situated with respect to the eye and the object that its conjugate foci are at the distance of twenty and eighty feet from the lens — that is, the object is twenty feet before the lens, and its image eighty feet behind it — then the size of the image will be four times that of the object. If the eye, therefore, looks at this magnified image six inches behind it, it will be seen 128 CHROMATIC ABERRATION. with great distinctness. In this case the image is magnified four times directly by the lens, and 200 times by being brought 200 times nearer the eye ; so that its apparent magnitude is 800 times larger than before. At distances less than the preceding, the rule for finding the magnifying power of a lens, when the eye views the image which it forms at six inches distance, is, according to Sir David Brewster, as follows :— " From the distance between the image and object in feet, subtract the focal distance of the lens in feet, and divide the remainder by the same focal distance. By this quotient divide twice the distance of the object in feet, and the new quotient will be the magni- fying power, or the number of times that the apparent magnitude of the object is increased. "When the focal length of the lens is quite inconsiderable, compared with the distance of the object, as it is in most cases, the rule becomes this :— Divide the focal length of the lens by the distance at which the eye looks at the image; or, as the eye will generally look at it at the "distance of six inches, in order to see it most dis- tinctly, divide the focal length by six inches, or, what is the same thing, double the focal length in feet, and the result will be the magnifying power." Having given the laws of optics sufficient notice, we shall next consider that portion which is more intimately connected with photography. One of the first objects to be \ Fig. 21. considered in the manu- facture of a lens for pho- tographic purposes, is to produce one with the least spherical aberration. Now, if we take a double convex lens and produce the im- age of a figure (Fig. 21), we observe that the pro- duced image is curved; and a little consideration will show that it is not possible that such a curv- ed surface as that represented could produce an image of equal distinctness over every part of a plane surface : the rays cannot meet, as they are refracted from curved surfaces along any straight line ; and supposing we receive on the surface of a lens a bright cir- cular image, it will be brilliant and well defined around the centre, the light becoming fainter towards the edge, and at length passing into a cloudy halo, exhibiting the prismatic colours. This is called spherical aberration, and to it is due that want of dis- tinctness which commonly is found around the edges of pictures taken in the camera obscura. It is, therefore, important, in the selection of lenses, that we look for sharpness of definition over the whole of a perfectly flat field. But by attention to the two facts, that a Fig. 22. lens, one surface of which is a section of an ellipse, and the other of a circle struck SPHERICAL ABERRATION. 127 from the farthest of the two foci of that ellipse, as in Fig. 22, produces no aberration, much may he effected. A lens of this form, therefore, with a convex surface, part of an ellipsoid, the focal distance of which coincides with its farther focus, and a concave surface, part of a sphere, whose centre is that focus, will meet all our require- ments. The mechanical difficulties of producing such lenses are great, hut they may, hy cautious manipulation, be to a great extent overcome. If we take such a lens as we have been describing, and stop its centre with a blackened disc, leaving only a small portion of the edge for the light to pass through, and throw its image on a screen, we shall find it bordered with fringes of colour. At one distance red will prevail, at another violet. This is the result of chromatic aber - ration, and arises from the unequal rcfrangibility of the dissimilar rays. The red ray V K Fig. 23. is less bent than the violet ; consequently, supposing the rays E l"l (Fig. 23) to fall on the edge of a lens, they will converge to a point at F, whereas if the rays V V fall along the same circular line, they will, being more refracted, meet at F. Now if we place a disc at E, just the size of the cone of light, it will be edged with violet; but if we move it to A, the coloured border will be red. By the table of the refractive powers of transparent bodies (page 119), it will be seen that, for a beam of white light, the difference between the most refractory flint glass and crown glass, in their refracting powers, is as 2-028 is to lo34 ; and this proportion is maintained nearly, but not exactly, for all the coloured rays. If, there- fore, we have a crown glass lens, the refractive power of which will place the focus at a for the violet rays, and at b for the red rays, and we grind to fit it a flint-glass lens/the refracting power of which would place the foci of the rays at c, d (Fig. 24), Fig. 24. it will be seen that the result of such a combination would be the formation of a colourless image at a mean point between them, by re-combining the rays into white light ; and such becomes the achromatic lens of the camera. In fact, to combine the violet and blue rays with the less refrangible red is all that is required ; for this reason : — Suppose there be two prisms B F C and CDF, placed in juxtaposition and 128 DISPERSIVE POWERS OF PRISMS. Fio turned in contrary directions, as in Fig. 25. If we first assume these prisms to be of the same substance, the refracting angle C F D of the second being smaller than the refracting angle B C F of the first, the two prisms will pro- duce the same effect as one prism B A F ; that is, the white light which passes through them will not only be bent, but decomposed. But if the first prism B C F bo made of crown glass, and the second of flint, we can destroy the dis- persion, while preserving the refraction. The flint being more dispersive than the crown, and the dispersion produced by a prism dimin- ishing with the angle of refraction in the prism, it follows that in suitably diminishing the angle of refraction C F D in the flint prism, with relation to the angle of refraction B C F in the crown prism, we can render the dispersive power of these two prisms equal ; and as from their position the dispersion occurs in opposite directions, it is compensated ; that is, the emergent rays E 0 are obviously reduced to a parallelism, and consequently give white light. The relation of the angles B C F and C F D, however, which bring to a paral- lelism red and violet rays, not having the same effect on the intermediate colours, it follows that with two prisms we can in reality achromatize only two rays of the spectrum ; so that, in order to obtain perfect achromatism, it would be necessary to have seven prisms, of substances unequally dispersive, and whose angles of refraction should be suitably determined. So that one cause of rapidity in a lens is the perfection of the coincidence "of the chemical and visual foci. Another cause, is the shortness of the focus. The greater length of focus possessed by a lens, the larger the picture produced, as a lens is generally calculated to cover, that is, have an uniform action over two- thirds its length of focus ; or, to explain more fully, a lens of twelve inches focus will cover eight inches square, or nine by seven. It may seem strange that a lens that will cover nine by seven, could not cover nine by nine, but a little reflection will prove the contrary. Thus, if we draw a circle of the size properly covered by a twelve inch focused lens, and make a square, as represented by the solid lines (Fig. 26), we can observe that by taking an inch off one side we may add it to the other, or nearly so— the change being represented by the dotted lines — and that without going out of the circle ; so that a lens of twelve inches focus covering eight square inches, would not be half as rapid as a lens of six inches focus, covering four square inches. The amount of light reflected from the same object being four times as much in one case as in the other. To copy an object requiring to be done quickly, we must therefore use two large lenses, placed some distance asunder, by which the length of focus is diminished and the rapidity is increased ; the back lens catching the refracted rays of the front one, and refracting them still more. We thus obtain what is called a double lens, or more properly a double combination of lenses,'as shown in Fig. 27. Fig. 26. COMBINATIONS OF LENSES. 129 Combinations of tenses.— These combinations can be obtained so as to take both portraits and views. The lenses for portraiture are arranged as represented in Fig. 27. If the lenses are removed from the cells, especial care must be taken to replace them in their former position, thus : — The flat- test side of the lens B, the concave side of the inner lens A, and the least convex side of the outer len s A, must be turned towards the interior, of the camera, and the ring of brass must be placed between the two lenses A so as to separate them. If views, pictures statuary, &c, are to be taken, the cell containing the lens A must be unscrewed and removed ; the hood E Fiff- and the cell containing the lens B must be unscrewed ; the sliding tube holding the lens is now to be pulled out of the cell, and one of the circular plates of metal with a central aperture (called a stop) dropped into its place ; the tube holding the lens is now reversed and pushed in so that the convex side of the lens is towards the interior of the camera, and the whole arrangement as represented by Fig. 28, where C is the sliding tube, B the E lens, and D the stop. Three stops with different sized apertures belong to each set of lenses ; but which is to be selected for use in any particular case, must depend on the judgment of the operator. In dull weather, and in Fig. 28. copying objects indifferently illuminated, the largest size aperture stop is used ; the middle size stop is for general use in moderate light, and the smallest size where the object to be copied is exposed to full sunshine or where great sharpness is required; it may be taken as a general rule, within certain limits, that the smaller the aperture ■which admits the light, the greater is the sharpness of the picture produced, but the time of exposure must be increased where such small apertures are employed. Ciaudet on Lenses. — The following observations on lenses by M. Claudet may not be out of place here : — " The question of the actinic focus is involved in another kind of mystery, which requires some attention. I have found that, with the same lenses, there exists a constant variation in the distance between the two foci. They are never in the same relation to each other; they are sometimes more or less separate ; in some lights they are very distant, and in some others they are very near, and even coincide. For this reason I constantly try their position before I operate. 1 have not been able to dis- cover the cause of that singular phenomenon, but I can state positively that it exists. An optician, according to M. Lerebours' calculation, can at will, in the com- bination of the two glasses composing an achromatic lens, adapt such curvatures or angles in both that the visual focus shall coincide with the actinic focus ; but he can obtain this result only for one length of focus. The [moment the distance is altered the two foci separate, because the visual and actinic rays must be refracted at different angles in coming out of the lens, in order to meet at the focus given or one distance of the object. If the distance is altered, the focus becomes longer or shorter ; and as the angle at which different rays are refracted remains nearly the same, they cannot meet at the new focus, and they form two images. If the visual and actinic rays were refracted parallel to each other, in coming out of the lens they would always coincide for every focus ; but this is not the case. It seems, therefore, imp ssible that lenses can be constructed in whicn the two foci will agree for all the various distances, PRACTICAL CHEMISTRY— No. V. K 130 until we have discovered two kinds of glasses in which the densities or the refractive power will be in the same ratio as the dispersive power." Before concluding my present remarks on lenses, let me tell the reader that without a good lens he need not expect good pictures ; and that 'economy in a lens produces twice the outlay in other ways. Let him not imagine, as many have — u Oh, I only want something to try with.'' He cannot come to a more false conclusion, as had materials — had lens especially — have been the cause of many a beginner never being anything else. I shall have to say something more about lenses hereafter, which will be more fully understood by the reader then than now. Focimeters. — There is a neat little instru- ment made use of by most photographers for testing the lens they are about to use, and determining whether it works to focus or not ; it is called the focimeter, and is the invention of M. Claudet. It is composed of fans placed at aome little distance from each other, and numbered from 1 to 8. Supposing it is wished to try a lens, let the focus be tried upon say No. 4, and if that number prove to be the sharpest on the prepared plate or paper, the lens works to focus. If 2 or 3 should be sharper, then the lens must be pushed nearer to the ground glass, and the lens is not enotfgh corrected. If, on the other hand, 5 or 6 should be sharpest, then the lens is over corrected, and must be drawn out a little more from the ground glass. There is one other quality to be looked for in a lens, and that is flatness of field. This can be easily ascertained at once by focusing on a window, when if you are using a 12-inch lens, and it gives an image of a window sash about 8 inches on the ground glass, you'may be certain, if it show the bars perfectly straight, that it has a flat field, "'a property of the greatest importance in a good lens. Some amateurs reject a really good lens on ac- count of air bubbles, but these are not in the slightest degree hurtful ; one of the best lenses I ever saw had a dozen of them. This focimeter was invented by the author for ascertaining the best condition for the camera, without the trouble of putting it up. It folds up so smalFas to go in the waistcoat pocket. The square A (Fig. 31) is cut out, and bears a proportion to the ground glass of the camera ; by looking through the small hole B we see the view, as if it were framed (Fig. 32) the same in the act of being folded. Fig. 32. Fig. 31. THE CAMERA DESCRIBED. 131 THE PHOTOGRAPHIC APPARATUS. The first subject coming under our consideration, agreeable with the method I intend pursuing, that of making the reader acquainted with all the details and acces- sories before attempting to combine them, will be The Camera Obscura, or Darkened Chamber. — This instrument was the invention of Baptista Porta, of Padua. Its principle will be best understood by the very simple experiment of darkening a room by closing the window-shutters, and admitting a pencil of light through a small hole in them. If a piece of paper is held at a little distance from this hole, the figures of external objects will be seen delineated upon it ; and, by putting a small lens over the hole, they are rendered much more evident from the condensation of the rays by the spherical glass. This will be best understood by the following diagram (Fig. 33). Lei C D be a window-shutter having Fig. 33. a small aperture A, and E F a piece of paper placed in a dark chamber. Then, if an illuminated object, R G B, is placed on the outside of the shutter, we shall observe an inverted image of this object painted on the paper at r g b. In order to understand how this takes place, let us suppose the object R G B to have three distinct colours — red at R, green at G, and blue at B ; then it is plain that the red light from R will pass in straight lines through the aperture A, and fall upon the paper E F at r. In like manner, the green from G, and the blue light from B, will severally full upon the paper at g and 6, and an inverted image r g b of the object RGB will be painted upon it, every coloured point in the object RGB having a coloured point corres- . ponding with it on the piece of paper E F. If, instead of a dark- ened room, we substitute a darkened box (Fig. 34), the same effect will be seen. Suppose, in the first place, the box to be without the lens, the rays would pass from the external arrow in nearly right lines through the opening, Fig. 34. 132 CONSTRUCTION FOR DIFFERENT PURPOSES. refracted only in passing the solid edges of the hole, and form an image on the back of the dark box. The lens refracts the'rays, and a smaller but a more perfectly-defined picture is the result. This is the camera obscura. Although highly appreciated for the magical pictures it produced, this instrument Fig. 35. Fig. 36. remained little more than a scientific toy until the discovery of MM. Daguerre and Niepce developed its powers. It is now so well known as scarcely to require description. The camera is a dark box with doors attached, having a tube for containing the lenses in one of its ends, tbrough which the radiations from external objects pass, and form a diminished and reversed image upon the ground glass at the other extremity. The dis- position of the various parts of this apparatus will be understood by reference to Figs. 35 and 36, where A represents the body of the camera ; B, the lens ; C, the ground glass focusing plate; and D, the dark slide, or back, for holding the prepared plate. There are four grand distinctions in cameras, as to their structure, each being adapted to some peculiar branch of the photographic art ; they have been named, from the nature of their configuration, Rigid, Sliding-body, Folding, and Semi-folding. The sliding-body camera will be found of most service in the glass operating room, from the capability it has of admitting a vast range of adjustment, which enables it to be used for almost every purpose. The peculiarities of this form of camera will at once become apparent by referring to Fig. 37, in which A represents the fixed body of the instrument, to which, at the front part, is fixed the lens ; B is the second, or inner body, which slides along the board C, fastened to the fixed, body ; the groove for holding the focusing glass and the dark slide (Fig. 38) is in Fig. : the hinder part of the sliding box, and is represented by the letter D. There is a slit in the bottom board, in which works the screw and button fastened to the moveable body, which allows of the latter being fixed after its proper position has been deter- mined. From this description, it will be quite evident that a very long range of THE FOLDING CAMERA. 133 Fig. 39. thus throwing the features out of focus is obtained by this arrangement ; and this will be found of the utmost convenience where we wish to obtain large portraits, or pictures small enough to mount as miniatures in a brooch. The great desideratum in a camera is perfect lenses. They should be achromatic, and the utmost transparency should be obtained ; and, under the closest inspection of the glass, not the slightest wavy appearance should present itself, or dark spot be detected; at the same time, a curvature should be secured which prevents, as much as pos- sible, all spherical aberration. The effect pro- duced by this last defect is a convergence of per- pendicularity : as, for instance, two towers of any building would be represented as leaning towards each other, or in a portrait the features would seem contracted, distorted, and mingled together, drawing. A variety of moveable diaphragms or caps to cover the front aperture are useful, as the intensity of the light requires to be modified by them, and they should always accompany an instrument. A handy operator can always supply himself with these diaphragms. The engraving (Fig. 39) represents a section of a single lens ; A, the lens ; B, rack and pinion ; C, the stop or diaphragm ; D, sections of the camera. As in the phenomena of vision, so in the camera obscura, the image is produced by the radiations proceeding from the external object ; and as these radiations progress from various parts, more or less illuminated, so are the high lights, the middle tints and shadows, most beauti- fully preserved in the spectral Fig. 40. appearance. The colours also, being in the first instance the effect of some physical modification of the primary cause, are repeated under the same influence; and the definition, the colour, and soft gradation of light and shadow, are so perfect, that few more beautiful optical effects can be produced than those presented by the camera obscura. By a slight modification of the above simple box, we can form a camera in which we may expose a prepared sensitive plate or sheet of paper to the action of the rays which pass through the lens, the plate or paper being at the same time perfectly protected from the action of any other ray. Some cameras are very simple in construction, merely consisting of a single box, with the lens so mounted or fixed that it can be moved in or out to get the focus, which may be done by means of one tube sliding into another, or of one box having another sliding into it, the lens being fixed as in Fig. 40. The next form is the folding camera, invented MAJOR HALKETT'S PORTABLE CAMERA. by Mr. Ottewill, represented in Figs. 41, 42, and 43. In 41 it is represented packed as a knapsack, in 42 it is fully -fixed, and in 43 half open. This is a very portable camera for travelling, and is kept steady and firm by the front board, which holds the lens, sliding into a groove made to hold it. I shall next, with the reader's permission, introduce a camera which I invented and made for the late Major Halkett, of the 4th Light Dra- goons, who was subsequently killed in the glorious charge at Balaklava. I shall only re- mark, that to the amateur who practices the paper processes it will prove very portable indeed. The form of this Fig. 43. open the air-hole Z, and shut H back into A Fig. 42. camera is that of a box when closed, in size 13 inches long, 11 inches deep, and G inches wide, with a brass handle on the top by which to carry it (Fig. 44). A camera of the above dimen- sions will take pictures 11 X 8| inches. To shut up the camera, from Fig. 45, you first undo the supports R, which will let down the dia- phragm, take out the screws G, G, G, about Fig. 44. put the screws G, G, G in also, and place the two lids, L' and L", side by side on the front, and fasten them there by the two hooks V, V ; then turn F up against D, and fasten it there by its i own hook V. When you get home you take out the pressure-board, and inverting the paper frame, the paper holders will all fall back again out of the box F. When the camera is opened for use, as in Fig. 45, take out the pressure- board and put in eight or ten slides with prepared paper, securing each paper-slide with a screw-pin K, through its eye T ; you then replace the pressure-board, and the frame is charged with prepared paper. To place the focusing glass in its position, you lift the frame D about half an inch, and draw it back, when it will separate from A, as shown at Fig. 46, and make room for the focusing glass ; having obtained a proper focus, you replace the frame D, pull up the sliding-shutter M, and the first sheet of paper is exposed ; you shut down the shutter M, pull back the pressure-board a little, turn the first screw pin back until you hear the paper-holder drop into the box F ; you push in the pressure- board again, DETACHED PARTS OF THE CAMERA. 135 and paper No. 2 is ready to undergo the same process. The first paper-holder may be without an eye, as the pressure-board will keep it in its place until after exposure, and the pressure- board itself can hold another paper on its inside surface, thus increasing the number of pictures which may be taken during one excursion. \ Explanation of Figures. — The light tint indicates brass, the middle tint wood, and the dark shading india-rubber material. Fig. 44.' Side view, camera when closed. Fig. 45. Side view, camera when open. A, frame of camera ; B, body of ditto ; C, cone also of india-rubber material, extended in front of the lens by three sup- ports R ; D, frame for prepared paper, each paper held in a separate holder (S, Fig. 49) ; E, communication between D and F, made of india-rubber material, through which the prepared paper in its holder passes into F, a box made to receive it after being exposed in the camera . Brass. Wood. I India- rubber Back Lateral view. view. Fig. 46. Fig. 47.— Major Halkett's Camera. G, G, G, nuts and screws used when the camera is open ; II, upright frame for front of camera ; I, diaphragm in front of lens ; K, screw-pins to retain paper in frame until after exposure ; L' and L", lids of camera; M, shutter in front of prepared paper; 136 DETACHED PARTS OF THE CAMERA. Ijustment for focusing; Fig. 48. Fig. 49. N, lens screwed into W from inside ; P, rack and pinion V, V, V, V, hooks and eyes. Fig. 46. Diagram showing the way in which D is fastened to A. Fig. 48. Front of camera. I, diaphragm ; C, cone ; H, frame ; W, W, front plate of camera holding the lens ; E, R, R, rests to support the diaphragm ; Y, Y, plates to retain the front in its frame ; X, graduated support to alter the horizontal line ; Z, opening to allow passage to the air in opening and shutting the camera. Fig. 49. Holder in which the paper is retained, made of mill-board ; frame S, S, S, made of veneer, joins S at W, and doubles down on the dotted lines when the paper is in its place ; T, an eye through which the screw-pin K passes. Fig. 50. Lid No. 1. A, A, the lid ; B, plate by which it is screwed on the stand ; C, guiding slit in plate ; D*, hole by which that end is screwed to A (Fig. 45) ; F, F, guiding pins ; X, X, focusing rack. Fig. 51. Lid No. 2. A, A, the lid; B, slit for H (Fig. 45) ; C, C, guiding slits in plates ; D, pinion ; P, handle to ditto ; E, groove for X, X, to work in ; F, guiding pin ; G, nut for screw in screwing the lids together. Fig. 52. Section of prepared paper frame. D, D, space occupied by paper slides ; 0, pressure-board to keep the paper against the sliding shutter. Fig. 53. End view of camera. 0, pressure-board the paper until after exposure. In fact, the forms of the camera are innumerable, and it matters little how they are made, provided that they are solid when working, and have a means of substituting the prepared plate or paper for the ground (or focusing) glass, and impervious to all light, except that which passes through the lens. When speaking of the collodion process, I intend to mention one or two other cameras, more particularly adapted to that pro- cess. A very portable camera is constructed by Vogtlander, the German optician, described as entirely made of brass, so that variations of climate do not affect it, and it occupies a very small space, when packed, even with all the materials for operating. The instrument known as the copying camera-box has an extra slide in the back end, by which it may be considerably lengthened at pleasure. We must not omit to men- tion that of Mr. Martin's, which is known as the Panoramic Camera. The object of this invention is to reproduce, with an objective of medium dimensions, landscapes of great length, analogous to the panoramic feature, to Avhich we shall return. Fig. 50. Fig. 51. K, K, K, screw -pins to retain Fig. 53. PHOTOGRAPHIC APPARATUS. 137 Camera Stands. — The best constructed stands are made of maple or walnut wood, having a cast-iron or brass socket for receiving the camera, and having screws for elevating or depress- ing the instrument (Fig. 54). I shall next present my readers with a drawing of a dark frame for two pieces of paper and a glass, against which paper is pressed with a sheet of blotting- paper between. B B (Fig. 55), clips to fasten the frame when shut ; a a, the slides or shutters. Figs. 56 and 57 represent two useful articles, namely, the nitrate and dipper, and the frame and rod for spreading solu- tions on paper. This consists of a piece of wood covered with soft flannel or blotting paper, on which the paper is laid, and B a glass rod, by means of which the solution is spread evenly over the surface of the paper. Our next figures re- present as follows : — Fig. 58, a precipitating glass used for the purpose l ig. 56. of mating dcmbk iodide of silver. Fig. 59, a glass rod for spreading solutions on the paper ; and Fig. 60, nip- Fig. Pig. 57. Pig. 5