BROMIDE OF SILVER. 119 by placing it in a tolerably strong solution of iodine in water. I have often found that the parts over which the mercury has not been deposited are etched by the solution of iodine, and it does appear that, with some attention, a very important practical advantage may be taken of this remarkable peculiarity. The great difficulty lies in overcoming the spots which are formed by definite spaces in the metal, which are in different electrical conditions from the other parts. These form so many points of action, and give rise to circles, which rapidly spread and obliterate the design. (186.) After exposure to Light, a well-prepared Daguerreotype plate was placed in a vessel in which chlorine was very slowly forming, from manganese and hydrochloric acid, and in a short time the iodized surface became perfectly black. In this state it was subjected to sunshine, the effect of which was to whiten the plate with much rapidity. My engagements did not allow of my following out this discovery; but in two or three trials which I made, very tolerable positive pictures were produced by the first process, without mercurialization. (187.) BROMIDE OF SILVER.-This salt, like the iodide, does not appear to be very readily changed by the solar rays, when it is perfectly pure. The slightest admixture of the nitrate of silver renders it very susceptible of change, and under certain conditions it becomes the most sensitive of the photographic preparations. M. Biot has expressed it as his opinion that it is not possible to find any substance more sensitive to Light than the bromide of silver. This opinion must, however, be qualified by the above conditions. Sir John Herschel has used films. of this salt precipitated upon glass plates, which, when dry, are washed over with a solution of the nitrate, with the greatest advantage, in the camera obscura. It must, however, be observed, with regard to this and other salts which are stated to be unchanged in their pure state, that this applies only to a visible change. We have distinct evidence that a moment's exposure of the pure bromide of silver to the sun's rays is quite sufficient to produce a change, which may be rendered visible by the subsequent applications of agents having a powerful affinity for oxygen. (188.) Prismatic Analysis. As soon as the prismatic spectrum falls upon paper prepared with this salt, it blackens over the whole extent of action with nearly equal intensity. As far as I have been able to detect it, which it is difficult to do, from the rapidity of the action, the maximum effect is produced somewhere about the indigo ray. My own results correspond exactly with those obtained by Sir John Herschel, who says, "But the most characteristic peculiarity of the spectrum is its extravagant length. Instead of terminating at the fiducial point (the mean yellow ray) or thereabouts, the darkened portion extends down to the very extremity of the visible red rays. In tint it is pretty uniform (a grey black, not by any means intense) over the whole length, except that a slight fringe of redness (but no green or blue) is perceptible at the least refracted end." The author has, however, found that the grey-black may be very much darkened by allowing the nitrate of silver to be pretty much in excess; and by using a faint spectrum he has sometimes got decided evidences of natural colouration. Below the red ray an extended space is protected from the agency of the dispersed light, and its whiteness maintained; thus confirming the evidence of some chemical power in action, over a space beyond the luminous spectrum, which corresponds with the rays of the least refrangibility. In a subsequent chapter, devoted to the general consideration of all the conditions-particularly after the rays have been subjected to absorption a still further examination of these salts of silver will be found. (189.) Photographic Application. If paper is first washed with a solution of the nitrate of silver (100 grains to an ounce of distilled water), and when dry with a MERCURY AND BROMIDE OF SILVER. 121 solution of 20 grains of the bromide of potassium in four drachms of water, a good bromine paper is prepared; to render this sensitive, the silver solution is again applied. If used while yet wet, the paper is of exquisite sensibility, and may be used with advantage in the camera obscura. The advantages of the bromide of silver over every other salt which is usually employed, should lead to its general adoption, since all the least refrangible rays act on it with much greater energy than on either the iodide or chloride. (190.) In 1841, I published a process, which I then considered as new, by which pictures were produced on bromidized papers, after an exposure of an exceedingly short duration. This process had, I afterwards found, been previously pointed out by M. Bayard: of this I was, however, quite ignorant. Although there are some objections to the process, it is so very interesting, as giving evidence of the rapidity with which faint solar radiations will effect a change of state, that it cannot be well omitted in this place. (191.) Paper is prepared with bromide of potassium and nitrate of silver, in such proportions that the nitrate is in very slight excess. When used, it is washed over with a solution of 120 grains of nitrate of silver, and placed wet in the camera. After being exposed for a second or two, the screen must be closed, the camera carried into a dark room, and the paper allowed to dry in the dark. When dry, it is placed in the mercurial vapour box; and heat being applied, the mercury is slowly vaporised. The picture now begins to develop itself, and gradually a most intense negative photograph results. It often happens that the picture appears at first clouded; but if the paper is carefully placed in the dark, it generally, in the course of a few hours, gets clear. I have often procured most beautiful pictures by this method, after an exposure to solar influence for less than a second; and even moving objects have been well defined, showing the action to be almost instantaneous. Photographs thus obtained are best fixed by soaking the paper in a weak solution of salt in water, and then by brushing them over with the hyposulphite of soda. The great difficulty to be overcome in this process, is, the annoyance continually arising from the blackness produced over every part of the paper by the mercurial vapour. Often, when the best result appears to have been attained, in an instant the delightful picture vanishes away, and a sheet of blank blackness takes its place. It is not quite clear, to what this can be attributed; some kinds of paper are more liable to it than others, from which it would appear that it arises from the condition of the surface. (192.) Bromide of silver may be used very advantageously with the gallic acid or the proto-sulphate of iron for the production of pictures. Since this salt is effected by those rays which exist in the least refrangible portion of the spectrum, I am inclined, under all circumstances, to regard the bromide as preferable to the iodide of silver. Certain it is that if we use either of these salts alone, or in a pure state, the bromide has an advantage over the iodide in sensibility, and all green or yellow objects are more perfectly copied on a paper prepared with bromine than on one in which iodine has been employed. Papers washed once with each of the following solutions— Bromide of potassium 50 grains, water 1 oz. may be used most advantageously for copying any fixed objects. They require but a few minutes' exposure, and at any time, the picture may be brought out, by washing with the solution of proto-sulphate of iron or gallic acid. These photographs, which possess all the requisites of good negative ones, may be well fixed, by washing with a weak solution of the bromide of potassium, or with the hyposulphite of soda. (193.) If a paper is covered with a perfectly pure bromide of silver, it will, when washed over with a solu FLUORIDE OF SILVER. 123 tion of the ferrocyanide of potassium, exhibit an increased degree of sensitiveness to solar agency; but the resulting picture, falls very short of that blackness which we get by using the iodide of silver, the darkest parts never becoming deeper, than a full grey or lead colour. (194.) Prismatic Analysis. -If a bromidized paper, whilst under the influence of the spectrum, is washed with the ferrocyanide of potassium, it begins to darken instantly over the violet rays, which darkening action extends down to the edge of the red ray, some slight interference being observable about the region occupied by the yellow and orange rays. Then a bleaching action begins, over the space on which the red rays fall, which slowly extends up to the green by a long-continued action, an oval spot begins again to darken, about the centre of the bleached space. In this case, as in that with the iodide of silver, if a second wash of the bromide of potassium is applied, the bleaching action is extended over the region of the most refrangible rays, and considerably beyond them. We observe again in this instance, the negative action of the extreme red rays, which has been previously described. (195.) FLUORIDE OF SILVER. The combinations of fluoric acid and silver were first examined by me in 1843, and particulars published in the first edition of this work in 1844. Sir John Herschel, indeed, suggested some experiments on glass plates of a very interesting description, but they do not appear to have been made, the object of which was to obtain at once etchings on the glass by the action of the liberated fluoric acid. Paper washed with fluoride of sodium, and then with nitrate of silver, is not more sensitive to Light than the nitrate itself, but it eventually becomes darker. (196.) Prismatic Analysis. A paper was washed first with nitrate of silver, and then with the fluoride of sodium. Under the spectrum the action commenced at the centre of the yellow ray, and rapidly proceeded upwards, arriving at its maximum in the blue ray. To the end of the |