HISTORY OF PHOTOGRAPHY.@

By ROBERT HUNT.

I. THE PHOTOGRAPHIC PROCESSES ON PAPER OF SIR JOHN HERSCHEL.

The researches of Sir John Herschel have been principally directed to the investigation of the physical laws which regulate the chemical changes we have been considering. His analyses of the prismatic spectrum have been most complete, and as far as they have been carried out go to prove the operation of forces other than those with which we are acquainted.

At the same time, however, as this philosopher has been engaged in investigations of this high order, he has from the multitude of his experiments been successful in producing several processes of great beauty. There are not any which are to be regarded as peculiarly sensitive they are, indeed, for the most part rather slow-but the manipulation required is of the easiest character, and the results are most curious and instructive.

The philosophy which is forever united with the scientific investigations of Sir John Herschel is too valuable to be omitted from any description of the processes which he recommends. The following quotations are, therefore, taken from his communication to the Royal Society, and linked together by my own remarks in such a manner as it is hoped will be most easily understood by the unscientific amateur.

CYANOTYҮРЕ.

The processes in which cyanogen is employed are so called. Sir John Herschel makes the following remarks on the subject of his experiments with the cyanides:

I shall conclude this part of my subject by remarking on the great number and variety of substances which, now that attention is drawn to the subject, appear to be photographically impressible. It is no longer an insulated and anomalous affection of certain salts of silver or gold, but one which doubtless, in

a Extracts from A Manual of Photography, by Robert Hunt, fourth edition, London and Glasgow, 1854.

a greater or less degree, pervades all nature and connects itself intimately with the mechanism by which chemical combination and decomposition is operated. The general instability of organic combinations might lead us to expect the occurrence of numerous and remarkable cases of this affection among bodies of that class, but among metallic and other elements inorganically arranged instances enough have already appeared and more are daily presenting themselves to justify its extension to all cases in which chemical elements may be supposed combined with a certain degree of laxity and, so to speak, in a tottering equilibrium. There can be no doubt that the process in a great majority, if not in all, cases which have been noticed among inorganic substances is a deoxidizing one so far as the more refrangible rays are concerned. It is obviously so in the cases of gold and silver. In that of the bichromate of potash it is most probable that an atom of oxygen is parted with, and so of many others. A beautiful example of such deoxidizing action on a nonargentine compound has lately occurred to me in the examination of that interesting salt, the ferrosesquicyanuret of potassium, described by Mr. Smee in the Philosophical Magazine, No. 109, September, 1840, and which he has shown how to manufacture in abundance and purity by voltaic action on the common or yellow ferrocyanuret. In this process nascent oxygen is absorbed, hydrogen given off, and the characters of the resulting compound in respect of the oxides of iron forming, as it does, Prussian blue with protosalts of that metal, but producing no precipitate with its persalts, indicate an excess of electro-negative energy, a disposition to part with oxygen, or, which is the same thing, to absorb hydrogen (in the presence of moisture) and thereby to return to its pristine state under circumstances of moderate solicitation, such as the affinity of protoxide of iron, for instance, for an additional dose of oxygen, etc.

Paper simply washed with a solution of this salt is highly sensitive to the action of light. Prussian blue is deposited (the base being necessarily supplied by the destruction of one portion of the acid and the acid by decomposition of another). After half an hour or an hour's exposure to sunshine a very beautiful negative photograph is the result, to fix which all that is necessary is to soak it in water in which a little sulphate of soda is dissolved to insure the fixity of the Prussian blue deposited. While dry the impression is dove color or lavender blue, which has a curious and striking effect on the greenish-yellow ground of the paper, produced by the saline solution. After washing the ground color disappears and the photograph becomes bright blue on a white ground. If too long exposed it gets over sunned" and the tint has a brownish or yellowish tendency, which, however, is removed in fixing, but no increase of intensity beyond a certain point is obtained by continuance of exposure.

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If paper be washed with a solution of ammonio-citrate of iron and dried, and then a wash passed over it of the yellow ferrocyanuret of potassium, there is no immediate formation of true Prussian blue, but the paper rapidly acquires a violet-purple color, which deepens after a few minutes, as it dries, to almost absolute blackness. In this state it is a positive photographic paper of high sensibility and gives pictures of great depth and sharpness, but with this peculiarity, that they darken again spontaneously on exposure to the air in darkness and are soon obliterated. The paper, however, remains susceptible to light and capable of receiving other pictures, which in their turn fade, without any possibility (so far as I can see) of arresting them, which is to be regretted, as they are very beautiful and the paper of such easy preparation. If washed with ammonia or its carbonate, they are for a few moments entirely obliterated, but presently reappear with reversed lights and shades. In this state they are fixed, and the ammonia, with all that it will dissolve, being removed

by washing in water, their color becomes a pure Prussian blue, which deepens much by keeping. If the solution be mixed, there results a very dark violetcolored ink, which may be kept uninjured in an opaque bottle, and will readily furnish by a single wash at a moment's notice the positive paper in question, which is most sensitive when wet.

It seems at first sight natural to refer these curious and complex changes to the instability of the cyanic compounds, and that this opinion is to a certain extent correct is proved by the photographic impressions obtained on papers to which no iron has been added beyond what exists in the ferrocyanic salts themselves. Nevertheless, the following experiments abundantly prove that in several of the changes above described the immediate action of the solar rays is not exerted on these salts, but on the iron contained in the ferruginous solution added to them, which it deoxidizes or otherwise alters, thereby presenting it to the ferrocyanic salts in such a form as to precipitate the acids in combination with the peroxide or protoxide of iron, as the case may be. To make this evident, all that is necessary is simply to leave out the ferrocyanate in the preparation of the paper, which thus becomes reduced to a simple washing over with the ammonia-citric solution. Paper so washed is of a bright yellow color and is apparently little, but in reality highly, sensitive to photographic action. Exposed to strong sunshine, for some time indeed, its bright yellow tint is dulled into an ochery hue or even to gray, but the change altogether amounts to a moderate percentage of the total light reflected and in short exposures is such as would easily escape notice. Nevertheless, if a slip of this paper be held for only four or five seconds in the sun (the effect of which is quite imperceptible to the eye, and, when withdrawn into the shade, be washed over with the ferrosesquicyanate of potash, a considerable deposit of Prussian blue takes place on the part sunned and none whatever on the rest, so that on washing the whole with water a pretty strong blue impression is left, demonstrating the reduction of iron in that portion of the paper to the state of protoxide. The effect in question is not, it should be observed, peculiar to the ammonio-citrate of iron. The ammonio and potassia-tartrate fully possess and the perchloride, exactly neutralized, partakes of the same property, but the experiment is far more neatly made and succeeds better with the other salts.

In further development of these most interesting processes Sir John Herschel says:

The varieties of cyanotype processes seem to be innumerable, but that which I shall now describe deserves particular notice, not only for its preeminent beauty while in progress, but as illustrating the peculiar power of the ammoniacal and other persalts of iron above mentioned to receive a latent picture, susceptible of development by a great variety of stimuli. This process consists in simply passing over the ammonia-citrated paper on which such a latent picture has been impressed, very sparingly and evenly, a wash of the solution of the common yellow ferrocyanate (prussiate) of potash. The latent picture, if not so faint as to be quite invisible (and for this purpose should not be so), is negative. As soon as the liquid is applied, which can not be in too thin a film, the negative picture vanishes, and by very slow degrees is replaced by a positive one of a violet-blue color on a greenish-yellow ground, which at a certain moment possesses a high degree of sharpness and singular beauty and delicacy of tint. If at this instant it be thrown into water, it passes immediately to Prussian blue, losing at the same time, however, much of its sharpness, and sometimes, indeed, becoming quite blotty and confused. But if this be delayed, the picture, after attaining a certain maximum of distinctness,

grows rapidly confused, especially if the quantity of liquid applied be more than the paper can easily and completely absorb, or if the brush in applying it be allowed to rest on or to be passed twice over any part. The effect then becomes that of a coarse and ill-printed wood cut, all the strong shades being run together, and a total absence prevailing of half lights.

To prevent this confusion, gum arabic may be added to the prussiated solution, by which it is hindered from spreading unmanageably within the pores of the paper, and the precipitated Prussian blue allowed time to agglomerate and fix itself on the fibers. By the use of this ingredient also a much thinner and more equable film may be spread over the surface; and when perfectly dry, if not sufficiently developed, the application may be repeated. By operating thus I have occasionally (though rarely) succeeded in producing pictures of great beauty and richness of effect, which they retain (if not thrown into water) between the leaves of a portfolio, and have even a certain degree of fixity— fading in a strong light, and recovering their tone in the dark. The manipulations of this process are, however, delicate, and complete success is comparatively rare.

If sulphocyanate of potash be added to the ammonio-citrate or ammoniotartrate of iron, the peculiar red color which that test induces on persalts of the metal is not produced, but it appears at once on adding a drop or two of dilute sulphuric or nitric acid. This circumstance, joined to the perfect neutrality of these salts, and their power, in such neutral solution, of enduring, undecomposed, a boiling heat, contrary to the usual habitudes of the peroxide of iron, together with their singular transformation by the action of light to proto-salts, in apparent opposition to a very strong affinity, has, I confess, inclined me to speculate on the possibility of their ferruginous base existing in them, not in the ordinary form of peroxide, but in one isomeric with it. The nonformation of Prussian blue, when their solutions are mixed with prussiate of potash, and the formation in its place of a deep violet-colored liquid of singular instability under the action of light, seem to favor this idea. Nor is it altogether impossible that the peculiar "prepared" state superficially assumed by iron under the influence of nitric acid, first noticed by Keir, and since made the subject of experiment by M. Scöhnbein and myself, may depend on a change superficially operated on the iron itself into a new metallic body isomeric with iron, unoxidable by nitric acid, and which may be considered as the radical of that peroxide which exists in the salts in question, and possibly also of an isomeric protoxide. A combination of the common protoxide with the isomeric peroxide, rather than with the same metal in a simply higher stage of oxidation, would afford a not unplausible notion of the chemical nature of that peculiar intermediate oxide to which the name of "Ferroso ferric" has been given by Berzelius. If (to render my meaning more clear) we for a moment consent to designate such an isomeric form of iron by the name siderium, the oxide in question might be regarded as a sideriate of iron. Both phosphorus and arsenic (bodies remarkable for sequi combinations) admit isomeric forms in their oxides and acids. But to return from this digression.

If to a mixture of ammonio-citrate of iron and sulphocyanate of potash a small dose of nitric acid be added, the resulting red liquid, spread on paper, spontaneously whitens in the dark. If more acid be added till the point is attained when the discoloration begins to relax, and the paper when dry retains a considerable degree of color, it is powerfully affected by light, and receives a positive picture with great rapidity, which appears at the back of the paper with even more distinctness than on its face. The impression, however, is pallid, fades on keeping, nor am I acquainted at present with any mode of fixing it..