This process, which is of remarkable sensibility, was discovered by the author, and published in the Athenœum, under the name of the Energiatype; but from a desire to group all those pictures under a general head into which iron salts enter as an element, the present name is preferred. The preparation of the paper is as follows:—Good letter-paper (Whatman's is the best) is washed over with the following solution, viz.: Five grains of succinic acid (it is important that succinic free from any oil of amber, or adventitious matter, should be obtained) are to be dissolved in one fluid ounce of water, to which are added about five grains of common salt, and half a drachm of mucilage of gum arabic. When dry, the paper is drawn over the surface of a solution of sixty grains of nitrate of silver in one ounce of distilled water. Allowed to dry in the dark, the paper is now fit for use, is of a pure white, retains its colour, and may be preserved for a considerable time in a portfolio, until wanted for use.
The preparation of this paper is by no means difficult, but requires care and attention. The solutions must be applied very equally over the paper, which should be immediately hung upon a frame or clothes' horse to dry. Extreme care must be taken that the paper be not exposed to light, after the nitrate of silver solution has been applied, until required for use. Many of the disappointments experienced by the experimenters on the ener-giatype are occasioned by a neglect of this precaution ; as, although no apparent effect may have been produced by the exposure, the clearness of the subsequent picture will be seriously injured. The succinic acid must also be very pure. We shall now briefly describe the method of applying this process to the different purposes for which it is best adapted, premising that the varying circumstances of time, place, and light, will render necessary such modifications of the following directions as the experience of the operator may suggest. As a general rule, an open situation, sunshine, and, if possible, the morning sun, should be preferred, as the image is sharper, and the colour produced more intense, and less affected by the subsequent fixing process.
In the camera, for a building or statue, an exposure of half a minute in strong sunshine is usually sufficient; for a portrait, taken under ordinary conditions, two or three minutes are required.
When the paper is taken from the camera, nothing is visible upon it; but by attending to the following directions the latent picture will quickly develope itself. Having mixed together about one drachm of a saturated solution of protosulphate of iron and two or three drachms of mucilage of gum arabic, pour a small quantity into a flat dish. Pass the prepared side of the paper taken from the camera rapidly over this mixture, taking care to insure complete contact in every part. If the paper has been sufficiently impressed, the picture will almost immediately appear, and the further action of the iron must be stopped by the application of a soft sponge and plenty of clean water. Should the image not appear immediately, or be imperfect in its details, the iron solution may be allowed to remain upon it a short time ; but it must then be kept disturbed, by rapidly but lightly brushing it up, otherwise numerous black specks will form and destroy the photograph. Great care should be taken that the iron solution does not touch the back of the picture, which it will inevitably stain, and, the picture being a negative one, be rendered useless as a copy. A slight degree of heat will assist the development of the image where the time of exposure has been too short.
The picture should be carefully washed to take off any superficial blackness, and may then be permanently fixed by being soaked in water to which a small quantity of ammonia, or, better still, hyposulphite of soda, has been added. The paper must again be well soaked in clean water, to clear it from the soluble salts, and may then be dried and pressed.
Exact copies of prints, feathers, leaves, etc., may be taken on the succinated paper by exposing them to the light in the copying-frame, until the margin of the prepared paper, which should be left uncovered, begins to change colour very slightly. If the object to be copied is thick, the surface must be allowed to assume a darker tint, or the light will not have penetrated to the paper.
Positive copies of the camera negatives are procured in the same manner as the copies of the prints, etc., just described. Instead, however, of using the iron solution, the paper must be exposed to the light, in the frame, a sufficient time to obtain perfect copies. The progress of the picture may be observed by turning up the corner of the paper, and, if not sufficiently done, replacing it exactly in the same position. They should be fixed with hyposulphite, as before directed.
At the meeting of the British Association at York in 1844.I showed, by a series of photographs, that the protosulphate of iron was most effective in developing any photographic images, on whatever argentiferous preparation they may have been received. Every subsequent result has shown that with proper care it is the most energetic agent for developing with which we are acquainted. The difficulty of obtaining, and of preserving, the salt free of any peroxide, or a basic salt which falls as a brownish-yellow powder, has been the principal cause why it has not been so generally employed as the gallic acid : this can be insured by adding a few drops of sulphuric acid and some iron filings to the solution of the protosulphate of iron.