The expense and inconvenience of metallic tablets rendered it in the highest degree desirable that paper should be employed in their place. A very extensive series of experiments at length led to the pleasing conclusion of being enabled to prepare a paper which answered in every respect as well as the silver plates, and in many much better.
This discovery formed the subject of a communication to the Royal Society, which that learned body did me the honour to print in their Transactions. My memoir is entitled,—" On the Influence of Iodine in rendering several Argentine Compounds, spread on Paper, sensitive to Light; and on a New Method of Producing, with greater distinctness, the Photographic Image." This paper contains the substance of the following remarks ; but since the publication of the Transactions I have been successful in simplifying the process of preparation.
My experiments established, in the most satisfactory manner, that even on the silver tablets a semi-oxidized surface was presented to the iodine. They also proved that perfectly pure untarnished silver was by no means readily acted on by the iodine. From this I was led to prepare oxides of silver in many different ways, which enabled me to spread them over paper, and the result was instructive. Any of the ordinary photographic papers allowed to darken to a full brown, which is a stage of induced oxidation, become, by long exposure to iodine, of a steel-blue or violet colour. If exposed in this state to sunshine for a long period, their colour changes from gray to a clear olive. Now, exposure to sunshine for a minute, or to diffused daylight for five minutes, produces no apparent change ; but mercurial vapour speedily attacks the portions which have been exposed to light, and a faithful picture is given of whatever may have been superposed. There is, however, a want of sufficient contrast between the lights and shadows. By allowing the first darkening to proceed until the paper acquires the olive colour, which indicates the formation of a true oxide of silver, it will be found, although it is not more speedily acted on by the iodine, that it is more sensitive, and that a better picture is formed. The kind of photographic preparations used appears to have but little influence on the results,—a chloride, iodide, or bromide of silver, allowed to darken, answers equally well.
There are many things, unfortunately, which prevent our availing ourselves of this easy method of producing a tolerably sensitive Daguerreotype paper. These are, certain irregular formations of oxides in different states, and the revival of metallic silver in some parts of the surface.
I next spread papers with the pure oxide formed by chemical means, and also the protoxide, and many of its salts. These papers were not very readily affected by iodine, or influenced by light during short exposures.
Silver is revived from its solutions by hydrogen gas ; conse-qently, nothing is more easy than, by washing a paper with nitrate of silver in solution, to procure a fine silver paper, by passing a current of hydrogen gas over it.
A picture of a peculiarly delicate character may be produced on this kind of paper ; but it has not the required sensibility, and there is a great want of contrast in the lights and shadows. It may be interesting to state, that the yellow-brown phosphate of silver is as readily acted on by iodine as the oxides, and is quite as sensitive to luminous influence. Phosphuretted hydrogen gas effects the revival of metallic silver, and the surface produced by means of this gas, used as the hydrogen was in the former case, is of a fine steel-blue, which colour arises from a portion of phosphorus having entered into combination with the silver. These kinds of paper comported themselves in every respect as the metallic tablets—were equally sensitive, and produced pictures as delicately beautiful. Unfortunately, however, owing to the spontaneously inflammable nature of the phosphuretted hydrogen gas, it is not safe to operate with it. After various ineffectual contrivances to overcome this difficulty, I was obliged to abandon the use of this gas entirely—warned of the danger I incured,—by several violent but fortunately harmless explosions. The vapour of phosphorus and of sulphur was also tried, and many very beautiful effects were produced. At length, however, I stopped at sulphuretted hydrogen, which answers in every respect.*
To prepare this, soak a paper of very firm texture, not too much glazed, in a weak solution of the muriate of ammonia. It must then be wiped with clean cloths, and carefully dried. The paper is then dipped into a weak solution of the nitrate of silver, and the small bubbles which form on its surface are carefully removed with a camel's hair pencil. When the paper is nearly, but not quite, dry, it must be exposed in a closed vessel to sulphuretted hydrogen gas, slowly formed from the sulphuret of antimony and hydrochloric acid : in a few minutes it will become of an iron-brown colour, having a fine metallic lustre. It is again to be passed through a solution of silver, somewhat stronger than the first, and dried, taking care that no shadow falls on the paper whilst it is drying. It is then a second time submitted to sulphuration, and, by careful management, the process is now generally completed. If, however, the paper is not considered to be sufficiently dark, it must be once more washed in the solution of silver, and again subjected to the action of sulphuretted hydrogen.
A very interesting account of the revival of gold and silver from their solutions by these gases, will be found in a tract on Combustion, published by Mrs. Fulhame.
If the above paper be allowed to remain in the sulphuretted hydrogen gas after the maximum blackness is produced, it is again whitened with some quickness. This may be accounted for in two ways : the gas may be mixed with a portion of muriatic acid vapour, or a quantity of chlorine sufficient to produce this effect may be liberated from the preparation on the paper to react on the sulphuret of silver.
The perfection of these papers consists in having a deep black ground to contrast with the mercurial deposit, by which means the pictures have the advantage of being seen equally well in all positions, whereas Daguerre's pictures on the metal plates can only be seen to advantage at certain angles.
The sulphuretted paper may be rendered sensitive in the same manner as the plates by exposure to the vapour of iodine. I, however, prefer drawing the paper over a solution thus formed :—A saturated solution of any salt of iodine is made to dissolve as much pure iodine as possible, and of this liquid two drachms are mingled with four ounces of water. Care is required that one side only of the paper is wetted, which is by no means difficult to effect, the fluid is so greedily absorbed by it ; all that is necessary being a broad shallow vessel to allow of the paper touching the fluid to its full width, and that it be drawn over it with a slow steady movement. When thus wetted, it is to be quickly dried by a warm, but not too bright fire ; of course daylight must be carefully excluded. Papers thus iodidated do not lose their sensitiveness for many days if carefully kept from light.
On examining the sheet after the Daguerreotype processes in the camera, and of mercurialization, have been completed, a very perfect picture is found upon it ; but it is still capable of vast improvement, which is, by the following simple plan, accomplished in a way which is at once magical and beautiful.
Dip one of the Daguerreotype pictures, formed on the sulphuretted paper, into a solution of corrosive sublimate : the drawing instantly disappears, but, after a few minutes, it is seen unfolding itself, and gradually becoming far more distinct than it was before; delicate lines, before invisible, or barely seen, are now distinctly marked, and a rare and singular perfection of detail given to the drawing. It may appear, at first sight, that the bichloride of mercury dissolves off the metal, and again deposits it in the form of chloride (calomel). But this does not account for the fact, that if the paper has been prepared with the nitrate of silver, the mercury disappears, and the drawing vanishes, the deposit taking place only on those parts upon which light has acted but feebly; as, for instance, on the venations of leaves, leaving those portions of surface which were exposed to full luminous influence without a particle of quicksilver. When the paper has been either a chloride or iodide, the effect is as above, and the thickness of the deposit is as the intensity of the light has been; consequently, the semi-tints are beautifully preserved. If the drawing remains too long in the solution, the precipitate adheres to the dark parts and destroys the effect. The singularity of this operation will be more striking if the picture has been soaked some time in the solution of the hyposulphite of soda, and then dipped into the bichloride of mercury. As the drawing disappears, a series of circles, formed of a white powder, appear to arise from the paper, generally commencing at the centre, and slowly extending over the whole surface: the powder is afterwards deposited, and the sheet is buried in the precipitate ; but on taking the paper from the liquid, and passing a stream of water over it, the precipitate is entirely removed from all the parts except the lights of the picture. I have also found the invisible photographic image become evident, without the aid of mercurial vapour, by simply soaking for some time in a solution of corrosive sublimate.
When these papers are prepared with due care, they are extremely sensitive, and if used for copying engravings during bright sunshine, the effect is instantaneous. The great difficulty is to present the paper to the sun, and withdraw it with sufficient celerity. In the weak light of the camera a few minutes during sunshine is quite sufficient for the production of the best effects. One great advantage of these pictures over those procured on the plated copper is, that the mercury does not lie loosely as on the tablets, but is firmly fixed, being absorbed by the paper; therefore these pictures may be kept without injury in a portfolio.
If, instead of immersing the paper in a vessel full of sulphuretted hydrogen gas, a stream of the gas is made to play upon it, it assumes a most richly iridescent surface; the various colours are of different degrees of sensibility, but for surface drawings they may be used ; and in copying of leaves or flowers, beautiful pictures, which appear to glow with the natural colours, are procured.