NINTH ORDINARY MEETING.

ROYAL INSTITUTION.-19th February, 1855.

JOSEPH DICKINSON, Esq., M.D., F.R.S., PRESIDENT, in the Chair.

The following gentlemen were balloted for, and duly elected members of the Society:-WILLIAM BYROM, Esq., J. S. TAYLOR, Esq., M.D., and ALFRED KING, Esq.

The PRESIDENT exhibited several medals recently struck by the Crystal Palace Company, of very elegant design and beautiful workmanship, presented to Mr. T. C. Archer.

The SECRETARY read a communication from Mr. James Boardman, on the compass of modern Rome, as estimated by him by walking measurement. His pace was at the rate of 3.5 miles an hour, and he completed the circuit of the walls in three hours forty-five minutes, showing the circumference to be 12.5 miles.

Bearing upon the subject of the paper for the evening, a large number of photographs on glass and paper, positives and negatives, were exhibited; and the following communication was made to the Society, and experimentally illustrated:

ON PHOTOGRAPHY,

WITH SPECIAL REFERENCE TO THE CHEMICAL PRINCIPLES INVOLVED IN THE COLLODION AND PAPER PROCESSES.

By J. BAKER EDWARDS, Esq., PH.D., F.C.S.

ALTHOUGH many ingenious processes have been devised for the production of photographic impressions, none have been equally satisfactory with those which depend for their success on the reduction of silver from its salts. This mainly is the principle involved in the daguerreotype, calotype, and collodion processes, although the metal is precipitated in modified and varied forms by each of these methods. The reduction is first induced by an occult change produced in the chrystalline or chemical structure of the silver salt by the direct action of reflected light; this change is continued and accelerated by the use of chemical reducing agents, and is arrested by the application of a solvent for the undecomposed silver salts. The result is a thin veil of reduced metal, which may be obtained in various conditions or in

combination with other bodies by the adoption of different modes of reduction. In the main, however, and in general terms, these processes have but one object in view, and the chemical principles involved are analogous, although the results vary very much, both in appearance and in actual condition. They resemble, however, more or less the products which would be obtained by the action of the reducing agents under favourable circumstances upon the salts themselves, the portions acted upon by light having only anticipated, in point of time, the result which would follow over the whole surface were not the action arrested at the proper moment.

For example, in the daguerreotype process very finely divided iodide of silver is obtained by submitting a highly polished silver plate to the vapour of iodine. The light affects the structure of these crystals so that when submitted to the vapour of mercury, reduction and amalgamation takes place, which the undecomposed iodide restrains from spreading over the whole surface as would otherwise take place. Then by immersion in an alkaline solvent, such as the hyposulphite of soda, the iodide is removed and the silver beneath it exposed intact. This white amalgam is obtained in a crystalline form when mercury is sus pended in a solution of nitrate or sulphate of silver, and is known as the "silver tree."

In the calotype process, paper is first saturated with iodide of potassium, then with nitrate of silver, washed and exposed to the light: the surface thus presented consists of iodide of silver, free nitrate of silver, and organic matter; the latter materially assisting in the reduction. The chemical agent employed to develope the image in this process is gallic acid, which throws down metallic silver from its solutions in the form of a black powder. In this state the silver is therefore reduced in intimate adhesion to the pores and surface of the paper. The undecomposed iodide is dissolved out as before, and the negative rendered more transparent by being saturated with wax. Among the processes on glass the albumen process most nearly assimilates to the calotype. Plates of glass being coated with albumen containing iodide of potassium, are dried, saturated with nitrate of silver, exposed to the light, then developed by gallic acid, and lastly, fixed by hyposulphite of soda. Thus the re-agents employed are the same as before, and effect the same purpose, but the organic matter is very different; of course we have all the advantage of additional translucency and great brilliancy of effect; but besides this we find the process greatly accelerated, and the tone of the result much modified. How shall we account for this except by referring to the nature of albumen as a highly complex organic com

D

pound, readily decomposed, and containing phosphorus and sulphur, which elements, possessing as they do a high degree of affinity for silver, probably assume an important part in the decompositions which

ensue.

In the collodion process the same salts of silver are employed, but gallic acid is no longer applicable as the developing agent; other substances are therefore resorted to, viz. pyrogallic acid, which reduces silver as a black powder,—or sulphate of iron, which precipitates it in a white crystalline condition; thus by the first agent we obtain the result required for negative impressions, and by the latter positive pictures of great delicacy and beauty. But will the substitution of these red cing agents for the gallic acid account for the extraordinary and disproportionate rapidity of the collodion process? Probably not. We must then seek for the solution of this problem in the nature of the organic matter subjected to these conditions; for the cause of dissimilarity is not apparent in the nature of the chemicals employed. Indeed the decomposition in all the silver processes is capable of a very general expression, as follows:

Salts of silver, as the iodide, bromide, chloride, &c., are partly reduced by light; further reduced, and the structure changed by developing agents, as mercury, gallic acid, pyrogallic acid, sulphate of iron, &c.; while the remaining salts of silver are dissolved in the fixing bath of hyposulphite of soda, or cyanide of potassium.

To the chemical constitution of albumen (C40, H31, Ná, O12, PS.), reference has already been made; its composition is highly complex, and it is readily subject to decomposition. Pure paper, which is one of the forms of lignine (C24, H20, O20), is a much more simple and stable compound, and probably undergoes no decomposition during the photographic process; but the porous character of its texture affords a kind of mechanical force, called catalysis, which, as in the case of spongy platinum and burnt charcoal, assists in overcoming the resis tance of chemical affinity, and thus contributes to the reduction of the metal. In this view of the case we might expect a less rapid though similar action from paper as that obtained from albumen. lignine, however, is not to be obtained in the form of paper, and the starch or size employed in its manufacture (as well as extraneous substances introduced), causes considerable variation in the sensitiveness of the paper under different processes. Many modifications of the calotype have therefore been adopted, and their success depends in some measure on the choice of the paper most suitable to the particular process.

Pure

In the collodion process the character of the organic matter is yet more obviously changed. Collodion is a solution of gun cotton in alcoholised ether; and the gun cotton, so dissolved, may be prepared from several substances besides cotton, as flax, paper, sawdust, &c., all having the same chemical composition, and coming under the generic name of lignine. These substances, when acted upon by nitric acid, form a series of substitution compounds, which contain relatively two, three, or five equivalents of nitrous acid in the place of as many equivalents of hydrogen removed.

The decomposition may be thus represented:

From Starch, by strong nitric acid,

C24 H20 020-H2+2NO1=

C24 (H18, 2NO4), O20

Xyloidine.

From Lignine, i.e. cotton, flax, paper, &c., by mixed acids,

C24, (H17, 3NO4) O20

C24 H20 020-H3 + 3NO,=

Collodion Xyloidine.

From Lignine, by strongest nitric acid,

C24, H20, 020-Нs + 5N04=

5

C24, (H1, 5NO,) O
Pyroxyline.

20

C24, H17, 3NO, O (cotton xyloidine.)

3

3NO

Nitric Acid={3N0,= +

30.

зно
(water.)

We thus see that gun paper or gun cotton contains nitrous acid, one of the most readily decomposed substances we know, equally ready to throw off 2 eqs. of oxygen, and be converted into NO2, or on the other hand to absorb oxygen and form NO. We may judge then, a priori, that it is a substance very prone to decomposition. But we need not depend on supposition: we have the direct experiments of Dr. Gladstone to prove, that of these compounds the first is the most ready to decompose, which it does spontaneously, leaving a gummy mass, and yielding NO, as gas. Still further, he finds that the second compound, the one we employ for collodion, also suffers spontaneous decomposition,

but especially if it be kept in the light, the result being similar to the foregoing. The third substance, pyroxyline, (explosive gun cotton), is much more stable, and does not decompose under ordinary conditions. These experiments I can confirm from my own observation; and, coupled with the fact that the sensitiveness in some measure varies with the solvent, we have ample grounds for assuming that the reactions involved depend largely on collodion itself as a reducing agent.

These considerations derive especial interest from the case of Talbot v. Laroche, recently decided in the Court of Common Pleas, in which the plaintiff sought to restrain the defendant from practising the collodion process, in virtue of his patent for the paper process. In this case. as in the celebrated "coal case," the scientific evidence was very conflicting, and high authorities held opposite opinions. None can read the account of this trial without feeling great interest in the course of the enquiry, and perceiving that great legal and scientific principles are involved in the decision; and although in the course of the trial these may appear at issue, the summing-up of Chief Justice Jervis on the case exhibits a remarkably clear, intelligent, and comprehensive view of the question, which in itself must be considered a difficult and obscure one. The claim in the patent of Mr. Fox Talbot, as limited by the Court, amounts only to the use of gallo-nitrate of silver as a sensitive agent, applied to paper previously prepared with iodide of silver, and the use of the same compound applied as a developing agent, and lastly, the use of a soluble bromide as a fixing agent. Under this construction the jury found for the defendant. The claim, however, advanced by the plaintiff under his patent, amounted to the general use of salts of silver, developed by reducing agents and fixed by chemical solvents, which of course would include every known efficient process. This claim is based upon the principle that "chemical equivalents are infringements of patents," and that the employment of any known chemical to effect the same result as that specified in the patent, would infringe the patent, by virtually using the same means, just as a mechanical equivalent of power is protected under a patent right. With this view it was argued that the collodion is a mechanical equivalent for the paper, and simply serves as a surface for the precipitation of the silver iodide; that pyrogallic acid is allied to gallic, and both perform the same office in the reduction of silver; and that the choice of a solvent or fixing agent is comparatively unimportant: hence that the processes were identical in principle, and nearly related in practice. Against this view it may be urged, that considering the nature of collo