A manual of photographic chemistry : theoretical and practical

Front matter

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Preface To The Seventh Edition

In this Edition a different arrangement of the Parts of the Work has been adopted, “ The Outlines of General Chemistry ” having been placed first. This alteration may seem unimportant to those who consult the Work merely for practical details, but it is hoped that the re-arrangement will prove satisfactory to those readers who wish to peruse it continuously, that they may obtain a sound theo¬ retical foundation before proceeding to practice. Although this portion of the Book has been in a great measure re¬ written, old-fashioned definitions of Acids and Salts, and modes of representing chemical changes, have been retained, since later and more exact views could hardly be intro¬ duced without the frequent employment of new terms, which seem out of place in a Manual treating only of what is theoretical for practical ends. Those Atomic Weights are given which accord with the ordinary chemical names familiar to Photographers. The “ Vocabulary of Chemi¬ cals ” has been revised and enlarged ; but, as in former Editions, the processes given in this Part are such as suit the amateur rather than the manufacturer. Some re¬ marks on the Elements which enter into the composition of photographic chemicals have also been added, with the view of enabling the reader to be more independent of other works on Chemistry.

In Part II., Mr. Hardwich’s theories of the Action of Light on Salts of Silver are left much as they stood,

\fi PREFACE TO THE SEVENTH EDITION.

inasmuch as his views on these points accord generally with those of the Editors, and have been further confirmed by the researches of Monckhoven, and more recently by those of Dr. Yogel, of Berlin. The new Lenses which have come prominently into notice during the last three years have been described, and the theory of Binocular Vision and of the Stereoscope has been more fully explained.

In discussing the theory of Collodion Negatives, care has been taken to distinguish between the modes of treat¬ ment best suited for Iodized and Bromo-iodized Collodions, but in such a way as to interfere as little as possible with the original arrangement of the Part.

In Part III., the same distinction between the two kinds of Collodion has been maintained throughout, and separate formulse and instructions for manipulation have been given for each ; the object being to explain the details of the Art as at present practised, so far as they have been thoroughly tested. Some Sections have been re-written, and in others it has been found necessary to condense and to curtail, so as to admit more important matter within the prescribed limits.

In the Appendix some additions have been also made, and the Index has been rendered more complete.

In conclusion, the Editors trust that their efforts will not prove to have been unsuccessful in maintaining for this Manual the position it has hitherto held as “ The Photo¬ grapher’s Guide.” Whether they have succeeded or not, they have endeavoured conscientiously to carry out the expressed wish of Mr. Hardwich, who has been truly called “the Photographer’s best friend,” and who has only re¬ linquished his favourite pursuit because he has found a higher and more important field of labour.

George Dawson.

Edward A. Hadow.

King's College , London , January , 1864.

Preface To The Sixth Edition

In preparing a new Edition of his Work, the Author finds it growing under his hands in a way not originally anticipated. The only apology he can offer is, that he hears the ‘ Manual ’ spoken of as being useful for refer¬ ence rather than for continuous perusal, and if so, each Part, and also each separate Chapter, ought to be as nearly as possible complete in itself, even at the expense of some repetition. In furtherance of this object, the em¬ ployment of type of two sizes has been adopted, which will enable the reader to distinguish the elementary parts of the Work from those which, being more elaborate and minute, may be consulted only on occasion.

The Third Chapter, treating of latent images, has un¬ dergone some alteration in this Edition, and the Author having definitely adopted a particular hypothesis, has given it that prominence which it appears to him to de¬ serve.

With reference to Collodion, he believes that every statement which he makes concerning it may be implicitly received, since his opportunities of fixing correct data have been unusually great. At the same time he wishes it to be understood that he has now relinquished the com¬ mercial manufacture of CoHodion, and that for the future no responsibility will attach to him.

PKEFACE TO THE SIXTH EDITION.

viii

Photographic printing has reached a point beyond which any further advance will be difficult. The Chapters rela¬ ting to this subject have been once more re-arranged, but it is not anticipated that such a proceeding will again be necessary unless our present modes should be superseded.

Dry Collodion photography is in this Edition placed in the first or theoretical, as well as in the second or practical part of the Work. Major Russell has communicated an improved method, of the utility of which high expectations are entertained.

In conclusion, the Writer desires to thank those kind friends who have assisted him in collecting materials for this Edition, and to acknowledge his obligations to the authors of papers published during the past two years. His aim throughout has been to show strict impartiality in recording the results of his fellow-labourers, and if he has failed in doing so, the fault was not in the intention.

London , January 1st, 1861.

Chapter I. The Chemical Elements And Theie Combinations

The more important Elementary Bodies, with their Sym¬ bols and Atomic Weights. — Compounds and Mixtures.

— Bases. — Acids. — Neutral Oxides. — Salts. — Laws of combination by weight. — The Atomic theory. — Sym¬ bols and formulae. — Chemical nomenclature. — Che¬ mical changes. — Chemistry of organic bodies ... 9

Chapter II. Description Oe Photographic Chemicals And Their

ELEMENTS, ALPHABETICALLY ARRANGED .... 29

X

CONTENTS.

Part II. The Theory Oe Photographic Processes

CHAPTER I.

HISTORICAL SKETCH OE PHOTOGRAPHY

CHAPTER II.

Page

THE SALTS OE SILVER EMPLOYED IN PHOTOGRAPHY.

Section I. — The Chemistry of the Salts of Silver. — The pre¬ paration and properties of the Nitrate of Silver — of the Chloride, Bromide, and Iodide of Silver. — The Chemis¬ try of the Oxides of Silver . 123

Section II. — The Photographic Properties of the Salts of Silver. — The action of Light upon Nitrate of Silver — upon Chloride, Bromide, and Iodide of Silver — upon organic Salts of Silver. — Simple experiments with sen¬ sitive Photographic Paper. — Nature of the chemical changes produced by light upon the Compounds of Silver . 129

Chapter III. On The Development Oe An Invisible Image

Simple experiments illustrating the process . 137

Section I. — The Substances employed as Developers. — The chemistry of the principal reducing agents, Gallic Acid, Pyrogallic Acid, and the Protosalts of Iron. — Reduc¬ tion of Salts of Silver by Developing Agents. — Con¬ ditions which retard or otherwise modify the process

of reduction . 138

Section II. — The Formation and Development of the Latent Image. — Hypothesis on the nature of the image. —

The mode of its development. — Causes which affect

CONTENTS.

XI

Page

the sensitiveness of the plates. — Fogging and other irregularities of development. — Moser’s researches. — Experiments of Norris, Busk, and others. — The Che¬ mical nature of developed images . . 145

CHAPTER IY.

ON FIXING THE PHOTO GEAPHIC IMAGE.

Chemistry of the various substances which may be em¬ ployed as Fixing Agents. — Ammonia, Alkaline Chlo¬ rides, Bromides, and Iodides. — Hyposulphite of Soda.

— Cyanide of Potassium. — Sulphocyanides. — Relative affinities of Chlorine, Iodine, and Hyposulphurous Acid for Silver . 166

CHAPTER Y.

ON THE NATUKE AND PEOPEETIES OP LIGHT.

Section I. — The compound Nature of Light , with the laws of refraction. — Decomposition of Light into elemen¬ tary coloured rays. — Phenomena of simple refraction by parallel and inclined surfaces. — The various forms of Lenses. — The Foci of Lenses. — Formation of a Lu¬ minous Image by a Lens. — Enlarged and reduced

images. — Lenses of long and short focus . 172

Section II. — The Photographic Camera. — Its simplest form

— Chromatic aberration : visual and actinic focus. — Spherical aberration. — Curvature of the field. — Varia¬ tion of focus for near and distant objects. — The single Actinic Yiew Lens. — The Aplanatic Lens. — The Orthoscopic or Orthographic Lens. — The Triplet Lens.

— Sutton’s Panoramic Lens. — The Globe Lens. — The Petzval Orthoscopic Lens. — The double, or Portrait combination Lens. — On distortion in Lenses . . . 180

Section III. — On Actinism. — Diagram of Chemical Spec¬ trum. — Illustrative experiments. — Superior sensibility

Xll

CONTENTS.

Page

of Bromide of Silver to coloured light.— Mode in which dark-coloured objects impress the film. — Actinic

transparency . 198

Section IY. — On Binocular Vision and the Stereoscope. — Phenomena of Binocular Vision. — Theory of the Ste¬ reoscope. — Wheatstone’s reflecting Stereoscope. — Brewster’s Stereoscope. — Buies for taking Stereoscopic pictures . . 205

CHAPTEB YI.

COLLODION AND THE NITEATE BATH.

Section I. — Collodion. — Pyroxyline — its chemical composi¬ tion. — Parchmentizing action of Oil of Vitriol. — The solution of the Pyroxyline in Ether and Alcohol. —

The changes which Iodized Collodion undergoes by keeping. — Observations on some points of importance in the manufacture of Photographic Collodion . . . 216

Section II. — The Chemistry of the Nitrate Bath. — Its sol¬ vent action on Iodide of Silver. — Acidity and Alka¬ linity of the Nitrate Bath. — Formation of Acetate of Silver in the Bath. — Contamination by organic mat¬ ters. — Use of fused Nitrate of Silver for the Bath . . 247

CHAPTEB VII.

ON POSITIVE AND NEGATIVE COLLODION PHOTOGEAPHS.

Definition of the terms Positive and Negative. — The same

Photograph often capable of representing both varieties 255 Section I. — Collodion Positives. — Process with attenuated films. — Improved mode with Bromide in the Collo¬ dion. — Proper condition of the Nitrate Bath. — Deve¬ lopers for Collodion Positives. — Fixing Agents. — A process for whitening Glass Positives by means of

Chloride of Mercury . 257

Section II. — Collodion Negatives. — Conditions which affect

CONTENTS.

mi

Page

the formation and . development of the Negative. — Iodized and Bromo-iodized Negative Collodion. — The Nitrate Bath and developers suitable for Iodized Col¬ lodion. — Ditto for Bromo-iodized Collodion. — Fixing liquids. — Artificial modes of strengthening Negatives . 267

CHAPTER VIII.

ON THE THEORY OP POSITIVE PRINTING.

General observations . 287

Section I. — Printing the Proof. — The stages of its darkening by light. The function of each constituent of the sen¬ sitive paper. — Observations on the various conditions which affect the sensitiveness of the paper, and the

vigour of the image . . 288

Section II. — Toning the Proof. — Toning by Sulphur. — The Salts of Gold as toning agents. — The alkaline Gold Toning Bath with the more important of the condi¬ tions which affect its action . - . . . 305

Section III. — Fixing the Proof. — The properties of Hypo¬ sulphite of Silver. — Conditions of a proper Fixing. —

Injurious states of the Fixing Bath . 313

Section IV. — The Fading of Photographic Prints. — The causes which produce it. — The comparative perma¬ nency of Prints. — The mode of testing permanency . 319

CHAPTER IX.

COLLODION PRESERVATIVE AND DRY COLLODION PROCESSES.

The Method of Messrs. Spiller and Crookes. — Shadbolt’s Honey Process. — The Oxymel Process. — Dry Process of the Abbe Despratz. — Norris’s Researches. — Taupenot’s Collodio-Albumen Process. — Fothergill’s Pro¬ cess. — Russell’s Tannin Process. — Collodion for theDry Processes. — General Observations . 327

XIV

CONTENTS.

CHAPTER X.

ON THE DAGUERREOTYPE AND OTHER PROCESSES.

Section I. — The Daguerreotype. — The nature of the sensi¬ tive film. — Development of the Latent Image. — The strengthening of the Image by means of Hyposulphite

of Gold .

Section II. — The Processes of Talbot , etc. — The Calotype. — The waxed Paper Process of Le Grey. — The Albumen

Negative Process . .

Section III. — Photographic Processes by Sir John Herschel and others. — The Cyanotype. — The Chrysotype. — "Ura¬ nium Printing Process. — Processes with Bichromate of Potash. — Carbon Printing. — Photo-lithography. — Photo-zincography. — Photo-galvanography, and Photo-glyphy .

Chapter I. Photographic Chemicals

Section I. — Preparation of plain Collodion. — Mode of preparing soluble Pyroxyline by the Nitre process. — Commercial process by the mixed Acids. — Manipula¬ tions and Precautions to be observed. — Positive and

Negative Collodion .

Section II. — Formulae for Solutions for direct Positives. — The iodized Collodion. — The Nitrate Bath. — Develop¬ ing fluids. — Fixing liquid. — Tarnishes .

Section III. — Formulae for Negative Solutions. — Iodized Collodion and Nitrate Bath. — Bromo-iodized Collo¬ dion and Nitrate Bath. — Developing fluids. — Fixing liquid. — Tarnish . .

CONTENTS.

CHAPTEE II.

MANIPULATIONS AND PROCESSES.

Section I. — Manipulations of the Collodion process. — Cleaning the Plates. — Coating with Collodion. — Exci¬ ting. — Exposing. — Developing. — Fixing. — Tarnishing Section II. — Positive and Negative Portraiture. — Con¬ struction of Glass Houses. — Working in the open Air. — Arrangement of the apparatus. — Management of Portrait Lenses. — Manipulations in taking Glass Posi¬ tives. — Observations on the Collodion, etc., for Nega¬ tive Portraits. — Carte-de-Yisite Portraits ....

Section III. — Landscape Photography. — The apparatus. — The Chemicals. — The manipulations

Section IT. — On copying Engravings and Maps. — Hints in selecting the Lenses. — The difficulties to be antici¬ pated. — The proper state of the chemicals .... Section V. — Rules for talcing Stereoscopic and instantaneous Photographs. — Latimer Clark’s arrangement. — Instan¬ taneous Photography .

Section YI. — Photographic delineation of Microscopic Ob¬ jects. — Arrangement of apparatus. — Mode of finding the chemical Eocus. — Experience of various operators Section YII. — Photography in hot climates. — Eemarks on a stable Pyroxyline. — Use' of Alcohol to retard Evapo¬ ration of Collodion. — Apparatus and package . . .

CHAPTEE III.

THE PRACTICAL DETAILS OE PHOTOGRAPHIC PRINTING

lection I. — The ordinary process of Positive Printing. — Se¬ lection of the paper. — Preparation of Albuminized paper — of plain paper— of Ammonio-Nitrate paper. — Preparation of the Alkaline Gold Toning Bath. — Pre¬ paration of the Fixing Bath. — Manipulatory details of Photographic Printing. — Mode of washing and mount¬ ing the Proofs. — Yignetting .

ection II. — Positive Printing by Development. — A process on Iodide and Bromide of Silver. — Process on Chloride and Citrate of Silver. — On Printing Transparencies .

xv

Page t.

XVI

CONTENTS.

CHAPTER IV.

CLASSIFICATION OF CAUSES OF FAILUEE IN THE COLLOD PEOCESS.

Section I. — Imperfections in Negative and Positive Col¬ lodion Photographs . — Fogging. — Transparent and opaque spots. — Markings of all kinds. — Under- and over-exposure, etc. — Defects in Collodion Positives . Section II. — Imperfections in Paper Positives ....

CHAPTER V.

DET COLLODION AND COLLODIO-ALBUMEN PEOCESSES.

Practical details of Russell’s Tannin Process — of Fothergill’s Process — of Taupenot’s Collodio-Albumen Process. — Modification of the latter by Messrs. Petschler and Mann .

Appendix

To find the equivalent focus of a lens and the angle of view included. — Quantitative Testing of Nitrate Baths. — Table of Specific Gravity of Nitrate of Silver solutions. — Recovery of Silver from waste solutions and resi¬ dues. — Reduction of Chloride of Silver. — Reduction of Sulphide of Silver. — Separation of Silver and Glold. — Removal of Iodide of Silver from Nitrate Baths. — Mode of taking the Specific Gravity of Liquids. — On Filtration. — The use of Test-papers. — The removal of Silver stains from the hands, etc. — Cleansing of Bot¬ tles. — Removal of Varnish from finished pictures. — Lacquer for blacking brass-work. — Dr. Ure’s tables of the strength of Sulphuric and Nitric Acid of dif¬ ferent densities. — Proportion of Absolute Alcohol in Spirit of different densities. — Baume’s Hydrometer Tables for liquids heavier and lighter than water. — Fahrenheit and Centigrade Thermometer scales. — Table of Weights and Measures. — Twaddell’s Hydro¬ meter . .

A MANUAL

OF

PHOTOGRAPHIC CHEMISTRY.

INTRODUCTION.

In attempting to impart knowledge on any subject, it is not sufficient that the writer should himself be acquainted with that which he professes to teach. EATen supposing such to be the case, yet much of the success of his effort must depend upon the manner in which the information is conveyed ; for as, on the one hand, a system of extreme brevity always fails of its object, so, on the other, a mere compilation of facts imperfectly explained tends only to confuse the reader.

A middle course between these extremes is perhaps the best to adopt ; that is, to make selection of certain funda¬ mental points, and to explain them with some minuteness, leaving others of less importance to be dealt with in a more summary manner, or to be altogether omitted.

But independently of observations of this kind, applying to educational instruction in general, it may be remarked,

I that there are sometimes difficulties of a more formidable | description to be overcome. Eor instance, in treating of a science, like Photography, which involves numerous and | complex reactions, there is great danger of attributing effects to wrong causes. Perhaps none but he who has

B

INTRODUCTION.

himself worked in the laboratory can estimate this point in its proper light. In an experiment where the quantities of material acted upon are infinitesimally small, and the che¬ mical changes involved of a most refined and subtle de¬ scription, it is soon discovered that the slightest variation in the usual conditions suffices to alter the result.

Nevertheless Photography is truly a science , governed by fixed laws ; and hence, as our knowledge increases, we may fairly hope that uncertainty will cease, and the same precision be attained as that with which chemical opera¬ tions are usually performed.

The intention of the author in writing this work, is to impart a thorough knowledge of what may be termed the “ Pirst Principles of Photography,” that the amateur may arm himself with a theoretical acquaintance with the sub¬ ject before proceeding to its practice. To assist this ob¬ ject, care will be taken to avoid needless complexity in the formulae, and all ingredients will be omitted which are not believed to be useful.

The impurities of chemicals will be pointed out as far as possible, and special directions given for their removal.

Amongst the great variety of Photographic processes which have been devised, those only will be selected which are correct on theoretical grounds, and are found in prac¬ tice to succeed.

As the work is addressed to one supposed to be unac¬ quainted both with Chemistry and Photography, pains will be taken to avoid the employment of all technical terms of which an explanation has not previously been given.

A SKETCH OF THE MAIN DIVISIONS TO BE ADOPTED, WITH THE PRINCIPAL SUBJECT-MATTER OF EACH.

The title given to the Work is “A Manual of Photo¬ graphic Chemistry,” and it is proposed to include in it a familiar explanation of the nature of the various chemical agents employed in the Art of Photography, with the ra¬ tionale of the manner in which they are thought to act.

INTRODUCTION.

The division adopted is threefold.

Part I. contains a statement of the Laws of Chemical combination, an explanation of Chemical terms, and of Symbolic notation (or short-hand) ; and also a list of Photo¬ graphic chemicals alphabetically arranged, including their preparation and properties as far as required for their employment in the Art.

Although Photography may certainly be practised suc¬ cessfully without a knowledge of Chemistry, yet a slight acquaintance with this science is so material an assistance in enabling a Photographer to explain, avoid, and escape from his difficulties, that it is recommended to the reader to take the trouble to master the first principles contained in Chapter I., and also to refer to the Vocabulary when a chemical is employed for the first time.

Part II., or “the Science of Photography,” includes a full description of the chemical action of Light upon the Salts of Silver, with its application to artistic purposes ; all mention of manipulatory details, and of quantities of ingredients, being, as a rule, omitted.

In this division of the Work will be found ten Chap¬ ters, the contents of which are as follows : —

Chapter I. is a sketch of the history of Photography, intended to convey a general notion of the origin and pro¬ gress of the Art, without dwelling on minute particulars.

Chapter II. describes the Chemistry of the Salts of Silver employed by Photographers ; their preparation and properties ; the phenomena of the action of Light upon them, with experiments illustrating it.

Chapter III. leads us on to the formation of an invisible' image upon a sensitive surface, with the development or bringing out to view of the same by means of chemical reagents. This point, being of fundamental importance, is described carefully ; the reduction of metallic oxides, the properties of the bodies employed to reduce, and the hypotheses which have been entertained on the nature of (he Light’s action, are all minutely explained.

Chapter IV. treats of the fixing of Photographic im-

I

INTEODUCTION.

pressions, in order to render them indestructible by dif¬ fused light.

Chapter V. contains a sketch of the Optics of Photo¬ graphy — the decomposition of white Light into its elemen¬ tary rays, the Photographic properties of the different co¬ lours, the refraction of Light, and construction of Lenses. In the last Section of this Chapter will be found a short sketch of the history and use of the Stereoscope.

Chapter VI. explains the chemistry of Pyroxyline, with its solution in Alcoholized Ether, or Collodion ; also of the Nitrate Bath, in which the photographic plates are ren¬ dered sensitive to light.

Chapter VII. describes the classification of Collodion Photographs as Positives and Negatives, with the distinc¬ tive peculiarities of each, and the most approved modes of obtaining them.

Chapter VIII. contains the theory of the production of Positive Photographs upon paper. In this Chapter will be found an explanation of the complex chemical changes involved in printing Positives, with the precautions which are required to ensure the permanency of the finished proofs.

Chapter IX. is an epitome of all that is known of the dry Collodion and Collodion preservative processes ; their history, and the scientific principles upon which they are founded.

Chapter X. is supplementary to the others, and a brief notice of it will suffice. It explains the theory of the Pho¬ tographic processes of Daguerre and Talbot; especially noticing those points in which they may be contrasted with Photography upon Collodion, but omitting all description of manipulatory details, which if included would extend the Work beyond its proposed limits. Photo-lithography, lately improved and perfected, is also alluded to in this Chapter.

The title of the third principal division of the Work, viz. “ The practice of Photography upon Collodion,” ex¬ plains itself. Attention however may be invited to the

INTEODUCTION.

fourth. Chapter, in which a classification is given of the principal imperfections in Photographs, with short direc¬ tions for their removal.

The reader will at once gather from this sketch of the contents of the volume before him, that whilst the general theory of every Photographic process is described, with the preparation and properties of the chemicals employed, minute directions in the minor points of manipulation are restricted to Photography upon Collodion, that branch of the Art being the one to which the time and attention of the author have been especially directed. Collodion is al¬ lowed by all to be a good vehicle for the sensitive Silver Salts, and successful results can be obtained with a very small expenditure of time and trouble, if the solutions em¬ ployed in the process are prepared in a state of purity.

Part I. Outlines Of General Chemistry

OUTLINES OE GENERAL CHEMISTRY.

CHAPTER I.

All substances with which we are acquainted, whether solid, liquid, or gaseous, are either elements, or compounds of elements, or mixtures of elements, or mixtures of com¬ pounds. It is necessary, in the first place, to understand these terms clearly.

ELEMENTS.

An Element is a substance which consists of one kind of matter only, or which has never yet been decomposed or separated into two or more kinds of matter. The number of such simple substances at present known is 64 ; of these however only the following 34 need be mentioned, as being the most common, and as including all that are alluded to in this Work. The meaning of the symbols and atomic weights attached to each element will be explained after¬ wards.

Symbol. Atomic Wt.

' Oxygen . O 8

Hydrogen . H 1

Non-Metallic J Nitrogen . N 14

Elements. ] Chlorine . Cl 35*5

Iodine . I 127

L Bromine . Br 80

Non-Metallic Elements — continued.

Metals.

Symbol.

Atomic Wt.

f Fluorine . .

Carbon . .

Boron . . .

' Phosphorus .

, Sulphur . .

/ Potassium

Sodium . .

Lithium . .

Barium . . .

68-5

Calcium . .

Strontium

43-8

Magnesium .

12-16

Aluminium . .

13-75

Chromium . ,

26-3

Iron . . . ,

Zinc . . . ,

32 -7

Manganese . ,

27-5

Uranium . .

Cadmium . .

Copper . . .

31-7

Lead . . . .

103-5

Antimony . .

Arsenic . . .

Gold . . . .

196-6

Mercury . . .

Silver . . . .

, Platinum . ,

98-6

COMPOUNDS AND MIXTUEES.

These elementary substances are capable of uniting with each other to form Compounds, which differ from their elements in appearance and properties, the more com¬ pletely that the latter were opposed to each other in their properties. Thus the gaseous element Chlor ine is capable of combining with the metal Sodium, to form with it the well-known solid, common Salt, in which neither a gas nor

AND THEIR COMBINATIONS.

a metal can be detected by the senses. The black solid, Iodine, unites with the metal Potassium, to form the white compound Iodide of Potassium. The two gases Oxygen and Hydrogen form by their union, the liquid, Water. Chemical combination is often attended with heat and light, the union of bodies with the Oxygen of the air, when ac¬ companied with these phenomena, being termed combustion.

Mixture differs entirely from Chemical combination , in that it leaves the properties of the ingredients compara¬ tively unchanged and perceptible to the senses, it is not attended with heat, excepting that which may be due to the friction used, and very simple means usually serve to separate the ingredients again. Thus gunpowder is an example of the most perfect mixture of the three ingre¬ dients, Charcoal, Nitrate of Potash, and Sulphur; yet the sense of taste will detect the Nitre, the Sulphur may be discovered by the smell when rubbed, and the eye perceives the colour of the Charcoal. Sulphur and finely-divided Iron might be so intimately mixed, that the two ingredients could not be separately distinguished by the eye, but the Iron might still be separated by a magnet from the Sulphur; if however true combination were brought about by the application of a gentle heat, a great increase of heat is suddenly perceived, and if the materials were in the right proportions, the magnet would no longer discover any Iron in the resulting compound. Hydrogen and Oxygen gases may be mixed , and they will remain gases still, but when combined the gaseous character is lost, and the liquid, water, results.

Of the compounds produced by the union of two ele¬ ments, the class of Oxides, formed by the combination of Oxygen with another element, is the most important. These Oxides form three important classes, namely Bases, Acids, and neutral Oxides, each of which must be sepa¬ rately considered.

BASES.

The Basic Oxides are found only among the metallic

Oxides, although the whole of the metals do not furnish basic Oxides. The most perfect examples of Bases are the Oxides of the first seven metals given in the list of elements ; the Oxides of the first three, Potassium, Sodium, and Lithium, known as Potash, Soda, and Lithia, are dis¬ tinguished from other bases by the term “Alkali;” the Oxides of the other four are known as “ Alkaline ” Earths. The Alkalies and Alkaline Earths exhibit all the true characteristics of bases in perfection, viz. causticity and corrosiveness, and the power of uniting with another class of compounds, the Acids, equally caustic and corrosive, and of producing compounds comparatively inactive and harm¬ less, they possess also the property of changing the red of Litmus to blue, and the yellow of Turmeric to brown. The property of Litmus and Turmeric in changing colour with bases and acids has caused them to be used for “test-papers,” which serve to detect an exceedingly minute amount of one of the stronger bases, or acids in a free state. The other metallic oxides termed bases exhibit the basic character in very variable degrees, some, such as the Oxides of Silver and Lead, “ neutralize ” the strongest acids perfectly, so that the product no longer affects test-paper ; others, such as the Oxides of Zinc and Iron, neutralize acids, but imperfectly, so that the products of the combina¬ tion always show an Acid reaction {see Acids) with test-paper. (With regard to Ammonia, see the Vocabulary.)

ACIDS.

The most important of the Acid Oxides are those of non-metallic elements ; only a few of the metals, as Arsenic, Chromium, and Manganese, furnishing acid oxides. Some metals, as Chromium and Manganese, have both basic and acid oxides ; when this is the case, the acid oxide always contains the most Oxygen.