marked, and, being allowed to remain upon it for half a minute, it must be dipped into water, and then dried lightly with blotting paper. This operation in particular requires the exclusion of daylight; and although the paper thus prepared has been found to keep for a short time, it is advisable to use it within a few hours, as it is sometimes. rendered useless by spontaneous change in the dark.

Paper thus prepared is exquisitely sensitive to Light, exposure of less than a second to diffused daylight being quite sufficient to set up the process of change. If a piece of this paper is partly covered, and the other part exposed to daylight for the briefest possible period of time, a very decided impression will be made. This impression is latent and invisible. If, however, the paper be placed aside in the dark, it will gradually develope itself; or it may be brought out immediately by being again washed over with the gallo-nitrate of silver, and held at a short distance from the fire, by which the exposed portions become brown, the covered parts remaining of their original colour.

The pictures being thus procured, are to be fixed by washing in clean water, and lightly drying between blotting paper, after which they are to be soaked in a solution. of the hyposulphite of soda for a few minutes; after this they are to be again dipped into water, allowed to remain until all the soluble salt is removed, and then finally dried.

(131.) The discovery of the extraordinary property of the gallic acid, in increasing the sensibility of the iodide of silver, is amongst the numerous claims which Mr. Talbot has made to discoveries in the photographic art. It must however be remembered that Sir J. Herschel used gallic acid, but not successfully, and that previously the infusion of galls had been employed by the Rev. Mr. Read with success. The calotype process, as described and practised by Mr. Fox Talbot, yields pictures of exquisite beauty, which preserve, with the utmost fidelity, not only

THE CALOTYPE PROCESS.

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the bold outline of the object, but its minute and delicate details. The charm of colour alone is wanting, and this is compensated by the harmony of the whole. The gradation of shadow is often given in a really wonderful manner, the lights of the picture decaying in soft and almost invisible tints into the deepest shades; the middle lights being preserved in a manner, which renders these pictures the most truthful studies for the artist who desires to fix the charms of Nature on his canvass, rather than those, so called, artistic effects, which are the bane of modern art and destructive alike to truth and good taste.*

* Mr. Fox Talbot has, in his patent, claimed the use of gallic acid as being his own discovery, and he has enforced his claim by legal proceedings in several cases. Now, Mr. Talbot's patent was sealed on the 8th February, 1841. On the 10th of April, 1839,—that is, nearly two years previously to the date of this patent,-Mr. E. W. Brayley exhibited and explained at one of the soirées of the London Institution pictures obtained by the Rev. J. B. Reade, F. R. S., prepared by the following process, as described by Mr. Reade on the 9th March, 1839, in a letter to Mr. Brayley.

"The more important process, and one probably different from any hitherto employed, consists in washing good writing paper with a strong solution of nitrate of silver, containing not less than 8 grains to every drachm of distilled water. The paper thus prepared is placed in the dark, and allowed to dry gradually. When perfectly dry, and just before it is used, I wash it with an infusion of galls, prepared according to the Pharmacopeia, and immediately, even while it is yet wet, throw upon it the image of microscopic objects by means of the solar microscope.

"It will be unnecessary for me to describe the effect, as I am able to illustrate it by drawings thus produced. I will only add, with respect to the time, that the drawing of the flea was perfected in less than five minutes, and the section of cane, and the spiral vessels of the stalk of common rhubarb, in about eight or ten minutes. These drawings are fixed by hyposulphite of soda. They may also be fixed by immersing them for a few minutes in weak salt and water, and then, for the same time, in a weak solution of hydriodate of potash. The drawing of the Trientalis Europea was fixed by the latter method: it was procured in half a minute, and the difference in the colour of the ground is due to this rapid and powerful action of the solar rays. This paper may be successfully used in the camera obscura.

"Farther experiments must determine the nature of this very sensitive argentine preparation. I presume that it is a gallate or tannate of

(132.) The calotype picture is a negative one,— that is, the shadows are represented by lights, and lights by shadows; also reverse, as it regards right and left; but when fixed by the above process, a great number of positive copies, correct in all respects, may be taken from it. To do this, it is only necessary that the drawing be placed with its face against the sensitive side of a piece of ordinary photographic paper, and being pressed into close contact by a board below, and a glass above, exposed for a short time to good sunshine. This period, of course, varies with the transparency of the original calotype, and the brilliancy of the sunshine. It must be remembered that the Light has to permeate a piece of paper, the yellow tint of which offers considerable interruption to the free passage of those rays which are active in producing chemical change; we must therefore be exceedingly careful to preserve the prepared sheet of as pale and uniform a tint as possible. All processes on paper, which require the production of the positive from the negative drawing, are, in one particular, alike defective. The irregularities of the paper itself are copied with the picture. This is only to be remedied by substituting some more transparent material, or getting a paper manufactured superior to any which is at present in the market, uniting transparency with firmness of texture and evenness of surface.

(133.) With a view to overcome these objections, the "WAX-PAPER PROCESS" has been introduced. This consists in saturating fine writing-paper with white wax, by placing the sheet on a hot plate, and rubbing it with wax. This waxed paper is placed in a solution of iodide of potassium, and in a short time the salt enters into combination with the wax. When this union is complete, the paper is readily covered with the iodide of silver, by

silver, and, if so, it will be interesting to you to know that what has hitherto been looked upon as a common chemical compound is produced or suspended at pleasure by our command over the rays of light."

THE COLLODION PROCESS.

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floating it upon a bath of nitrate of silver. It is then excited in the ordinary way with the combined gallic acid and nitrate of silver. (There are many manipulatory details which do not properly belong to this volume, which will be found fully described in my Treatise on Photography.)

(134.) Glass plates covered with albumen, in which some iodide of potassium has been dissolved, have been most successfully employed for obtaining the negative picture. The glass having been carefully coated with the strained white of egg and iodine salt, is dried by a gentle heat it is then immersed in a solution of nitrate of silver, and thus coated with iodide of silver. When excited by the gallic acid, this preparation receives beautiful images in the camera obscura, and the positive copies, obtained from these negatives, are free of the defects of those obtained from the paper. The COLLODION PROCESS naturally forms a section of this division; but as some of the peculiarities require very exact attention, it is deferred to a future page for separate consideration.

(135.) The part which the gallic acid plays in these processes is sufficiently obvious. The chemical action of this acid on most of the metallic salts is well known. It seizes readily upon the oxygen of the metallic oxides, and precipitates the metal in a pure state from its combination. with even the powerful acids. Since the attention of chemists has been more forcibly directed to the several phenomena connected with alterations produced in chemical compounds by luminous agency, numerous very curious instances have been discovered of the continuation in the dark of that change which Light has begun to produce. If we unite a solution of gallic acid and nitrate of silver, even in weak diffused Light, it will be found that a precipitate is almost immediately formed, whereas the same solution will often remain clear for many hours in the dark. If we apply this mixture whilst clear to any preparation of silver on paper, which is sufficiently sensitive, or has

been exposed to Light for a sufficient time for a change to have co:nmenced, the formation of metallic silver in a state of fine division, is carried on over those parts on which the solar rays have exerted their influence, with an energy equal to the intensity of the radiations which have acted on the several parts, or, in other words, to the degree of change which the preparation has undergone; while, for some time, the parts in shadow, exerting no extraordinary power, remain clear and unchanged. To preserve these parts quite transparent, it is therefore advantageous to accelerate the decomposition over the other parts by the aid of warmth; and hence it is advised that the drawing be held, for a few seconds, at a short distance from the fire. The gradual development of the calotype photographic picture, in any of its varieties, is, to a person who witnesses it for the first time, a phenomenon of a remarkable, indeed almost magical, character. Some experience is required to check the action of the gallo-nitrate of silver at the proper time: if it has not remained on the paper long enough, the opacity of the dark parts is not sufficient to insure good positive copies; and if it remains too long, the light portions begin to darken, and, as this darkening proceeds with rapidity, the picture is soon rendered useless as an original from which copies can be taken.

(136.) The calotype process may, for many purposes, be most advantageously simplified; but before we proceed to this, it will be interesting to know the particular action of pure gallic acid upon several varieties of argentiferous preparations which have been acted on by Light. Paper simply washed over with the nitrate of silver, and exposed for two minutes in the camera obscura, which was the time allowed in all cases, unless it is stated to be otherwise, gave, when washed over in the dark, a very faint image. The chloride of silver on paper gave, under the same treatment, a good picture; but it wanted clearness and depth of colour. The oxide and also the bromide