A treatise on photography

Preface

The ATM of this book is chiefly to give a rational explanation of some of the different phenomena to be met with in photography ; and with this to give sufficient practical instruction to enable the student to produce a landscape picture which shall be technically good, and at the same time to be of use to him if he make photography an aid to research.

In regard to the theories which the author has enunciated, it is believed that experimental evidence completely justifies their adoption. Though rapid advance has been made of late years in rule of thumb photography, yet the progress has been but slow in the science of it since the days when Herschel, Draper, Becquerel, and others instituted their researches ; and if this book can be but the means of enlisting a few earnest workers in investigating some of the remaining problems which still require solution, another aim of the author will be accomplished.

In a measure supplemental to this treatise, as far vi Preface.

as practical work is concerned, is ' Instruction in Photography,' by the same author. At one time it was hoped that the two might be combined, but after much consideration it appeared impossible to compress the necessary matter into the pages to which each volume of this series is limited.

This will explain why some — what may be called commercial — applications of photography have not been noticed in the present work, as it was thought more advisable that the available space should rather be devoted to the more theoretical aspect of the subject.

The author has to record his thanks to Mr. Dallmeyer, and to Mr. H. P. Robinson, for the criticism they passed on the proof sheets of chapters xxix. and xxxi. respectively ; and also to Mn J. Traill Taylor for the permission to use copies of some of the diagrams of lenses in chapter xxix. which illustrated his article in the 'British Journal Photographic Almanac,' 1870.

Nor can the author omit to acknowledge the

great obligation he is under to Mr. C. W. Merrifield,

F.R.S., the editor of a part of this series, for the

valuable help and advice which was so cordially

given whilst this work was passing through the press.

Chapter I. Historical Sketch Of The Discovery And Progress Of Photography

PAGE

Scheele and Ritter's Experimeats i

Wedgwood and Niepce 2

Daguerre's Discovery 3

Talbot's Photogenic Drawing , , 4

Positive and Negative Pictures 4

The Calotype Process . . . . . . .5

The Collodion Process 5

Processes with Gelatine, &c 6

Chapter II. Experiments With Light

The Prismatic Spectrum .6

Light, Heat, and Actinic Rays 8

Limits of the Spectrum • • 9

Work caused by the Absorption of Rays lo

Chapter III. Theory Of Sensitive Compounds

Arrangement of Molecules i

Chemical Effects not an absolute measure of Work expended . i viii

Contents.

Comparison of Photographic Compounds with Explosives . .13

Oscillations of a Molecule . . . . . . . • 14

The co-existence of Longer Waves with Shorter Waves . .15

The Visible and Invisible Photographic Image . . . .18

Methods of Derelopment _ .19

Fixing the Pfaotographic Image . . . . . . .21

Chapter IV. The Action Of Light On Various Compounds

Action of Light on Silver Chloride . . . . . .21

Action of Light on Silver Iodide 23 Absorbents of the Liberated Atoms ...... 25

Action of Light on Silver Bromide 26

Action »f Light on Organic Compounds of Silver . . .27 Action of Light on Ferric and Uranic Compounds . . .29 Action of Light on Chromium Compounds . . . . .31 Action of Light on Asphaltum, Dyes, and a Mixture of Chlorine

and Hydrogen 33 Light causing a Combination between Sulphur and Antimoniuretted

Hydrogen

Chapter V, On The Support And The Substratum

Essentials of a Support . . . . . , . .35 The Vehicle Holding the Sensitive Compounds on the Support , 36

Chapter VI. The Daguerreotype

Preparation of the Sensitive Surface . Development of the Image Strengthening the Image by Gold Reproductions of Daguerreotypes

Contents. ix

Chapter VII. Collodion

PAGE

Action of Sulphuric Acid on Organic Matter , , . .42 Action of Anhydrous Nitric Acid on Cotton . . . -43 Preparation of Pyroxyline ........ 45

The Solvents of Pyroxyline ....... 49

Formula for Plain Collodion ....... 50

Order of Sensitiveness of Sensitive Silver Compounds prepared

from different Metallic Compounds . . . . .52

Formulae for Iodized Collodion ....... 53

Testing Plain Collodion ........ 54

Chapter VIII. Preparations Necessary For Working The Wet Process

Cleaning the Plate . . . . . . . . • 55

The Sensitizing Bath . . . . .• . . -5^

Distilling Water ......... 59

Formuloa for the Sensitizing Bath . . , . . . 6i

Keeping the Bath in Order ....... 62

Chapter IX. On The Development Of The Photographic Image

Theoretical Considerations of Development . . . -63 On Different Strengths of Developers ..... -65

On Viscid developing Solutions ....... 66

Formulae for Developers , . . . . . . .67

Chapter X. Giving Intensity To And Fixing The Image

Different Methods of giving Intensity ...... 70

Formulae for Intensifiers . . . . . . . .71

X Contents.

pa(;k

Action of the Solvents in Fixing the Image . . . -74 Formulae for Fixing Solutions ...... -75

Varnishing the Collodion Film ...... -76

Chapter XI. Manipulations In Wet-plate Photography

Cleaning the Plate . . . , . . . . -77

Coating the Plate with Collodion ... ... 78

Intensifying the Image . . . . . . . .81

Fixing the Image -83 Varnishing the Negative 84

Chapter XII. Defects In Negatives

Defects due to the Chemical Processes . . . . -85

Defects due to the Lens ........ 86

Chapter XIII. Positive Pictures By The Wet Process

Formula for Sensitizing Bath ....... 89

Formulae for Developer and Collodion ..... 90

Chapter XIV. Dry Plate Processes With The Bath

Substrata 91

Collodion . 93

Sensitizing . . .... . . . 93

Applying the Preservative 94

Drying the Film ......... 95

Contents. xi

I'AGK

Experiments with Alkaline Developers ..... 98

Comparison of the Methods of Development .... 102

Formulae for Developers . . . . . . . .10

Intensifying the Image . . - , ■ . .105

Chapter XV, Descriptiom Of The Gum Gallic And Albumen-beer Processes

Details of the Gum Gallic Process 106

Albumen-beer Process . , . . . . . -.109

Chapter XVI. Emulsion Processes

Experiments with Silver Bromide Emulsion . , . .112 Canon Beechey's Unwashed-Emulsion Process . . . .115

CHAPTER XVIL

EMULSION PROCESSES.

Washed-Enjulsions . . > . . . . . -119

Preparation of Emulsion . . . . . . . .119

Transferring Films to Gelatinized Paper ; , , . .124

CHAPTER XVIIL

THE GELATINO-BROMIDE PROCESS.

Preparation of Plates 125

CHAPTER XIX.

CALOTYPE PROCESS.

The Preparation of the Iodized Paper . . . . .128 Sensitizing . . . . . . ; . . .130 xii Contents.

PAGE

Developing and Fixing the Image . . . . . . 131

Waxing the Paper ......... 132

Process of Le Gray ........ 132

CHAPTER XX.

SILVER PRINTING.

Experiments with Sensitized Papers ...... 133

Theory of Toning . . . . . . . . . 13S

Fixing the Print ...... ... 142

Eliminating Sodium Hyposulphite ...... 143

CHAPTER XXI.

MANIPULATIONS IN SILVER PRINTING.

Preliminary Preparation of the Paper ..... 144

The Sensitizing Bath . . . . . . . • '45

Printing the Picture . . . . . . . . . 146

Toning the Print ......... 148

Fixing the Print . . . . . . . . -149

Washing the Print . . . . . . . . ■ 1 50

Testing for Sodium Hyposulphite . . . . . • 151

Defects in Prints ......... 152

CHAPTER XXII.

COLLODIO-CHLORIDE PROCESS.

Formula for Collodio-Chloride 152

Paper for Collodio-Chloride . . . , . . .153

CHAPTER XXIH.

PRINTING WITH IRON AND URANJUM COMPOUNDS. ' '

Printing Processes with Salts of Iron . . . . -155

Chrysotype 1 56

The Platinum Printing Process . . . . . , ,157

Salmon and Garniers Process . . . . . . .158

Poitevin's Process with Ferric Chloride and Tartaric Acid . . 158 Printing Process with Uranium Salts 159

Contents. xiii

CHAPTER XXIV, PRINTING WITH CHROMIUM SALTS.

PAGE

Swan's Process ... ..... . i6i

J. R. Johnson's and Sawyer's Improvements . . . .163

Preparation of Tissue . . . . . . . .166

Sensitizing the Tissue . , . , . . . .168

Developing the Print . . . . . . , .170

CHAPTER XXV.

MISCELLANEOUS PRINTING PROCESSES WITH CHROMIUM SALTS.

Willis's Aniline Process . . . . . . . .172

The Powder Process . . . . . . . .173

Woodburytype . . , , . . . . .174

CHAPTER XXVI.

PHOTO-LITHOGRAPHIC TRANSFERS.

Requisites in the Paper and Ink 176

Formula for Preparing the Paper . . . . . .177

Inking the Printed Picture and Developing the Transfer . .178

CHAPTER XXVII.

PHOTO-ENGRAVING AND RELIEF PROCESSES.

Niepce's Process jgj

Ehrard's Process ........ ig-j

Woodbury's Photo-Engraving Process 18^

CHAPTER XXVIII.

PHOTO-COLLOTYPE PROCESS.

General Principles of the Photo-collotype Processes . . .186

Albert's Process . . . . . , _ _

Process by Capt. Waterhouse .189 xiv

Contents.

CHAPTER XXIX.

THE LENS.

PAGE

Laws of Refraction . . . . . . . . .191

Dispersion and its Correction ....... 193

Spherical Aberration . . ... . , . 195

Use of the Diaphragm or Stop ....... 196

Use of the Swing"back ........ 197

Distortion of the Image ........ 198

The Optical Centre ......... 199

Portrait Lenses ......... 202

Landscape Lenses ......... 203

F"urther Uses of the Stop ......... 206

Illuminating Power of the Lens ...... 207

Definition of the Focus of an Object ...... 208

CHAPTER XXX.

APPARATUS.

Hare's Changing Box . . . . . . . .216

Warnerke's Roller Slide . . . . . . . .217

The Jonte Camera . . . . . . . . .218

The Camera Stand ......... 220

Howard's Dark Tent ........ 222

Rouch's Dark Tent ......... 224

CHAPTER XXXI.

ON THE PICTURE.

Principles to be followed in Choosing a Point of View , . 227

Examples illustrating Landscape Photography .... 229

Examples illustrating Groups ....... 240

Epitome of Simple Rules to be followed in Landscape Composition .......... 242

Placing the Camera ......... 243

Judicious Use of the Swingback ...... 244

Architectural Pictures ........ 245

Contents.

XV

CHAPTER XXXII.

ACTINOMETRY.

PAGE

Draper's Experiments ........ 247

Bunsen and Roscoe's Silver Chloride Actinometer . . . 249

Roscoe's Actinometer

Reading the Tints of the Actinometer 255

Discussion of the Truth of Light and Shade in Photographs . 256 Ordinary Actinometers ........ 262

CHAPTER XXXIII.

PHOTO-SPECTROSCOPY.

Herschell's Experiments

Arrangement for a Photo-Spectroscopic Apparatus Heliostat ....

CHAPTER XXXIV.

SENSITIVENESS OF DIFFERENT SALTS.

Discussion of the Influence of the Spectrum on Sensitive Silver Salts .........

Reversing Action of the Rays of Low Refrangibility . Photographs in Natural Colours ....

Photographs of the Thermal end of the Spectrum

274 276 278

CHAPTER XXXV.

SOLAR, STELLAR, AND METALLIC SPECTRA.

Lockyer's Photo-Spectroscopic Apparatus . Method of Examining Metallic Vapours . Star Spectra ......

Miller's Researches in Absorption Spectra The Thermal Prismatic Spectrum

282 285 286 287 xvi

Contents.

CHAPTER XXXVI.

CELESTIAL PHOTOGRAPHY.

?AGE

Solar Photography with a Fixed Telescope .... 288

The Photo-Heliograph 292

Apparatus for Solar Photography ...... 296

Solar Eclipse Photographs . • 299

Lunar Photography 300

Stellar Photography . . . . . . ... . 304

CHAPTER XXXVII.

PHOTOGRAPHY WITH THE MICROSCOPE.

Apparatus required ......... 305

Monochromatic Light ........ 308

CHAPTER XXXVIII.

MISCELLANEOUS APPLICATIONS OF PHOTOGRAPHY.

Photographic Registration of Meteorological Instruments . .312 Photography applied to Military Service . , . . -313 Photography and Crime . . . . . . . '314

Appendix

Table of Strength of Alcohol . . . . . . - SIS

Comparison of the Metrical with the Common Measures . . 319

A TREATISE

ON

PHOTOGRAPHY.

Chapter I. Historical Sketch Of The Discovery And Progress Of Photography

To the alchemists of the sixteenth century belongs the honour of having first noticed the change that took place in silver chloride (known to them as 'Luna cornua') by exposure to light, but they regarded the darkening as a species of transmutation of metals, and it remained for Scheele, the Swedish chemist, in 1777, to investigate the properties of this compound, though his researches led at the time to no practical end. Scheele found, when he exposed silver chloride to the action of light beneath water, that in the fluid was dissolved a substance which, on the application of silver nitrate, gave once more silver chloride, and that, after applying ammonia to the blackened body, an insoluble residue of metallic silver remained behind. These were the only facts elicited at the time, and a delay of more than half a century occurred before they were put to really good purpose. In 1 80 1 Ritter, of Jena, repeated the experiments of Scheele, and discovered that the chloride darkened rapidly in those rays of the spectrum which lie beyond the extreme violet. To him also is due the announcement that the red

Historical Sketch.

rays have the property of undoing the work effected by the violet, though he attributed the effect to the wrong cause.

In 1802 Thomas Wedgwood read a paper before the Royal Institution, entitled 'An account of a Method of Copying Paintings on Glass ' and of making Profiles by the agency of Light upon Nitrate of Silver.'

With these experiments of Wedgwood's Sir Humphry Davy was associated, and in their record we find it stated that muriate of silver was more readily acted upon by light than the nitrate, and that white leather used as a basis gave better images than paper. Images obtained by the solar microscope were impressed without any serious difficulty, but no means was discovered of rendering them anything but transitory when exposed to daylight. For Charles, a Frenchman, has been claimed the credit of employing at an earlier date the same method of obtaining black profiles by the action of light, but there seems to be no authentic proof extant that this claim should be allowed. Dr. Wollaston, in 1803, discovered that gum guaiacum, when exposed to the action of the blue rays of light, became changed in colour, and that on exposing those altered portions to the red rays, the original tint was restored.

In 1 8 14 Photography was to receive a new votary in the person of Joseph Nicephore de Niepce. Leaving the salts of silver, he devoted himself to the study of the action of light on resins. After several years of research, he at length completed the process known as heliography, which consisted in the production of a picture in bitumen on a polished metal plate. The discovery he made in regard to this resin was that, after insolation, it became insoluble in its ordinary solvents. An exposure of many hours in a camera obscura was necessary to produce the required effect; hence, as may be imagined, the views taken by this m eans

' A mistake often occurs in the reading of this sentence. Wedgwood did not make the copies on glass, but copied paintings which were drawn on glass.

Dagiierre's Discovery.

were wanting [in vigour, owing to the shifting direction of the sunlight, and, as we shall see later on in this work, from other causes, were of necessity deficient in delicate lights and shades. In 1827 Nie'pce came over to England, with the intention of drawing the attention of the Royal Society to his discovery, but his process being secret, his communication was not received, and he returned to France. In 1824 Daguerre, a French painter, began a series of experiments in the same direction, and in 1829 he and Niepce entered into a partnership, and presumably it was the knowledge of the latter's method of working which gave the former the idea of the daguerreotype. Nie'pce had employed silver plates covered with asphaltum, which, after exposure and appHcation of the solvent, left the metal bare in parts. The image thus formed was brown ; the shadows being represented by the metallic surface. In' order to produce a proper effect, it was necessary that the parts covered by the bitumen should be whitened and 'the bare parts darkened. After various experiments, he applied iodine to the picture, subsequently removing the bitumen. It is to be presumed that Daguerre noticed the action that the light produced on those portions of the plate which had been converted into iodide. At any rate to Daguerre belongs the glory of the discovery that an image could be produced on a silvered plate which had been exposed to the vapour of iodine, though it was by fortuitous circumstances that he hit on the method of developing an invisible image.

In January 1839 the discovery of the daguerreotype process was first announced, and in August of the same year the details of production were given to the world, Daguerre and Niepce the younger (the successor of Nicephore), obtaining a pension from the Government of France. Whilst Daguerre was working in France, we find that one of our own countrymen, Fox Talbot, had been experimenting in another direction. Bearing in mind the work of Scheele and Wedgwood, he devoted himself to the production of

B 2

Historical Sketch.

drawings, &c., on silver chloride, and in January 1839 he read a paper before the Royal Society on ' Photogenic Drawings.' His method of procedure was somewhat as follows : Writing paper was coated with a solution of common salt, and after drying was brushed over with silver nitrate ; by this means silver chloride was obtained, vith a slight excess of the nitrate, in which condition it proved excessively sensitive to light. Various bodies, such as lace and ferns, were laid on this paper, and a reversed facsimile of them in black and white was produced, and he fixed the

Fig. r. Fig. 2.

impressions by solutions of bromides and chlorides. When such a reversed facsimile was placed over similarly-prepared paper, and the light allowed to act through it, the result was the formation of a facsimile, only this time not reversed in shades.

These two prints were respectively named the negative and the positive (fig. i and fig. 2.)

Comparing this process with the former, we see what an immense advantage Talbot's process had over the daguerreotype. With Talbot's any number of copies of a subject could be cheaply produced, whilst with the latter

Calotype.

one positive was the sole result, unless expensive electrochemical means were resorted to.

The Rev. J. B. Reade was also an ardent experimentalist in this process, and to him is to be ascribed the discovery of the accelerating power of gallic acid, in the presence of silver nitrate, for the production of an image, and also for the development of the invisible image by the same agency.

From this discovery, together with that of Daguerre's, Fox Talbot reasoned out the calotype process, which he patented in 1841. By it an invisible image is formed on silver iodide on paper, and developed by gallic acid. In this process of Talbot's a negative image was formed, while by the first process the positive pictures were produced ; and it should be remarked that the same method of producing silver prints obtains to the present day with scarcely any alteration.

Sir John Kerschell had drawn attention to the possibility of producing photographic pictures on glass, and in 1843 had actually printed, in a camera obscura on silver chloride, deposited on such a plate, a picture of his 40-foot telescope. Niepce de St. Victor made a further great advance when he succeeded in holding the sensitive salts of silver on glass by using albumen as a vehicle, but to Le Gray must be accorded the credit of suggesting collodion as suitable for retaining them in situ. Scott Archer, with whom was associated Dr. Hugh Diamond, in 185 1, however, introduced the collodion process in the practical form in which it exists to-day, and it may safely be said, that, with the exception of the daguerreotype process, no more important discovery in photography has been made.

In 1839 Mungo Ponton published the fact that potassium dichromate, when applied to paper and dried, altered in composition when exposed to the influence of light. This announcement caused much investigation into the subject, and it was subsequently discovered that not only was the chromate altered in composition, but that the sizing

Experiments with Light.

of the paper was oxidised. Gelatine, gum, starch, albumen, were all found to become insoluble when exposed in contact with it; and Poitevin utilised this fact in the production of pictures in powdered carbon by a process analogous to that subsequently to be described in these pages. Swan, Johnson, Woodbury, and others, have more recently extended its application by the production of images formed in gelatine, coloured with pigments; whilst a still wider field has been opened by Albert, Edwardes, and others, in the production by mechanical means of prints in printers' ink from a gelatine image, founded on the fact that oxidised gelatine is incapable of absorbing water.

Chapter II. Experiments With Light

Before entering into the theory of photography, it will be convenient to enter briefly into some of the phenomena of light; for it is with this form of energy that the photographer has to deal. It will be as well first to try one practical experiment with light, in order to clear up certain difficulties which may present themselves.

Darken a room of some 12 or 14 feet in length by means of shutters (light wooden frames covered with opaque paper will answer), and in one fix a small plate of glass, a, which has previously been covered with tinfoil, and on which, with a sharp knife, has been cut a straight line laying bare the glass ; on a table place a glass prism, b, the centre of which is at the same height as that of the slit, and have a screen, covered with white paper, c, and a lens, e, of about 1 2 inches focus, ready at hand; outside the window arrange a mirror, d — an ordinary looking-glass will answer — in such a position as to reflect the sunlight on to the slit: interpose the lens, e,

Experiments with Light.

about 6 inches from a, in such a manner as to cause the beam to fall upon the prism, b. The floating dust in the room will immediately show that the original beam of white light has been split up into a series of coloured rays, and a position for the screen may then be found which will cause the top and bottom edge of the spectrum (as this glorious band of colour is termed) to be sharply defined; and if the cut in the tinfoil be fine enough, a series of dark lines will traverse it vertically. With these, however, we have nothing to do at present. Now experiments, the results of which have formed the groundwork of mathematical reasoning on the theory of light, have conclusively proved that light as light is merely a sensation. Permeating all known space is assumed to be an imponderable and elastic fluid known as ether, and in it a luminous or heat source is able to generate a series of ripples or waves, flowing unbrokenly and continuously from it. What the prime form of these undulations may be we cannot tell. They may be, and most probably are, compounded of an almost infinite number of different undulations, when ordinary • white light is the impression given to the eye, and each of these series of undulations vary in length from crest to crest. Those of certain lengths are able to affect the nerves which line the retina of the eye; whilst some of these are able to affect other nerves lying in our bodies, producing the sensation of heat ; others again, though incapable of producing the sensation of light or heat, exhibit themselves by their effect on certain compounds, causing chemical combination or decomposition. Of those waves whose impact on the eye produces the sensation of light the shortest is about 600 millionths of a millimetre, whilst the longest is about 350 millimetres. The former give the sensation of a violet colour, the latter of a brilliant red. Examining the spectrum thrown on the screen, the intermediate colours of blue, green, yellow, and orange are seen, and the wave-lengths producing their effects * i.e. not polarised.

Waves.

on the eye lie intermediate between the limits given above. There is uncertainty as to the lower limit to which the heatproducing rays are refracted, but probably to a length equal to that of the visible spectrum, whilst the range in length ot the chemically active waves other than those situated in the visible portion of the spectrum, and which lie beyond Ihe violet (being called the ultra-violet or fluorescent rays), is, if anything, still more uncertain. It will be evident on reflection that it must be accidental that, between certain limits, the waves should be capable of producing a sensation of light or of heat. The exact upper limit of the thermal spectrum is unknown, but from theory it must be co-terminous with the chemically active rays, as will be seen further on, the inferior limit of the capacity of any waves to produce decomposition is as yet unascertained. All those series of waves which effect decomposition in any compound are called actinic rays, and, as will be seen, the range of these vary for every ordinary photographic compound.

It may help us in a right comprehension of our subject if reference is made here to one quality of these undulations. The interstellar ether in which these waves ripple is assumed to permeate every body, solid, liquid, and gaseous ; and it depends upon the disposition of the ultimate molecules of the body whether it is opaque or transparent to any of the visible or dark rays of light. It must be borne in mind that the molecules of every substance are presumably in a state of vibration, the extent and velocity of which depend partly upon the temperature, and partly upon the nature, of the substance, and that this must ever be so unless the purely theoretical condition of absolute cold be arrived at. Supposing, then, we have a glass, which with white light falling on it allows only the transmission of red light, and we look through it at the spectrum formed by white light, we should find that it cuts off the whole of the colours excepting the red, obliterating them more or less perfectly, that is, in technical language, it absorbs them. Now, according to all

ExpeiHments with Light.

ideas of the conservation of energy, this absorption must indicate the performance of some kind of work. It may be that it causes the already vibrating molecules of the glass to take up and swing in some compHcated manner with those rays particularly absorbed, and thus to cause a rise in temperature in the body, so small indeed, perhaps, as to be indistinguishable, owing to the rapid cooling due to radiation; or it may be that work is performed in effecting chemical decomposition, for even glass is thus affected by light. The rays which simply pass through the glass produce no effect on it — their energy is unimpaired.

It should also be noted that where light is not entirely absorbed, but is only reduced in intensity, even then also work must be performed by it ; for the intensity of any coloured or white light is dependent on the extent, or ampUtude, as it is termed, of the wave or waves ; and any diminution of the amplitude indicates that a portion of its available energy has been exhausted, and that therefore a transference of the portion so expended must have been made to the body through which it passed. This exchange or transference of energy is an important subject in all photographic matters ; it explains many of the phenomena in photography which often present a great difficulty to the beginner or to the rule-of-thumb photographer, whilst it is all-important in the right understanding of the revelations which are made by the spectroscope.

It may then be laid down as an unalterable law, that where thej-e is absorption of light {whether of dark or visible rays) by any body, work of some description must have been performed in that body. An account of the valuable experimental research of Joule on the mechanical value of light is given at length in the 'Philosophic Magazine' for 1843, and is deserving of special study.

Theory of Sensitive Compotmds.

Chapter III. Theory Of Sensitive Compounds

Every particle of matter may be considered to be made up of molecules, each molecule consisting of constituent atoms. Thus a particle (and when we say particle we mean to convey the idea of the smallest visible quantity of matter) of silver iodide is composed of molecules of a like definite composition, the components being— two atoms of iodine and two of silver, or multiples of these numbers. The physical aspect of matter often conveys to the mind an idea of a certain kind of arrangement in the molecules ; as does the analysis of a compound, if not of the absolute arrangement of the atoms, at all events of the arrangements which they cannot take. Oxalic acid, for instance, we know is composed of carbon, hydrogen, and oxygen, having the formula C2 H2 O4, or the exact equivalents of water, H2 O, carbon monoxide C O, and carbon dioxide C O2, yet the compound is totally different in its physical characters and chemical reactions from any of these. From this we can argue that the atoms of its molecules must be separated in such a manner that the oxygen molecules cannot seize I'^'g- 4-

upon the hydrogen to form water, or on carbon to form carbon monoxide or dioxide. When the atoms are so arranged as to be incapable of forming a molecule of a simpler type, they occupy a position of excessively stable equilibrium, and it would be necessary to expend a large amount of work to separate them. On the other hand, where the atoms of the molecule are so grouped that by rearrangement they may form perhaps more than one molecule, each of which may be