The History and Practice of the Art of Photography [12]
the metalic silver is seen to be revived on the surface of the salt. Great differences of color are produced on chlorides of silver precipitated by different muriates. Nearly every variety in combination with the nitrate, becomes at last of the same olive color, the following examples, therefore, have reference to a few minutes exposure, only, to good sunshine; it must also be recollected that the chloride of silver in these cases is contaminated with the precipitant.
Muriate of ammonia precipitates chloride to darken to a fine chocolate brown, whilst muriate of lime produces a brick-red color. Muriates of potash and soda afford a precipitate, which darkens speedly to a pure dark brown, and muriatic acid, or aqueous chlorine, do not appear to increase the darkening power beyond the lilac to which the pure chloride of silver changes by exposure. This difference of color appears to be owing to the admixture of the earth or alkali used with the silver salt.
The prismatic impression on paper spread with the chloride of silver is often very beautifully tinted, the intensity of color varying with the kind of muriate used. Spread paper with muriate of ammonia or baryta and you obtain a range of colors nearly corresponding with the natural hues of the prismatic spectrum. Under favorable circumstances the mean red ray, leaves a red impression, which passes into a green over the space occupied by the yellow rays. Above this a leaden hue is observed, and about the mean blue ray, where the action is greatest, it rapidly passes through brown into black, and through the most refrangible rays it gradually declines into a bluish brown, which tint is continued throughout the invisible rays. At the least refrangible end of the spectrum, the very remarkable phenomenon has been observed, of the extreme red rays exerting a protecting influence, and preserving the paper from that change, which it would otherwise undergo, under the influence of the dispersed light which always surrounds the spectrum. Not only the extreme red ray exerts this very peculiar property, but the ordinary red ray through nearly its whole length.
In photographic drawing this salt is of the utmost importance. Mr. Talbot's application of it will be given hereafter in another portion of this work.
IODIDE OF SILVER--Perfectly pure, undergoes very little change under the influence of light or heat; but if a very slight excess of the nitrate of silver be added it becomes infinitely more senitive than the chloride
The spectrum impressed upon paper prepared with a weak solution of the hydriodate of potash presents some very remarkable peculiarities. The maximum of intensity is found at the edge of the most refrangible violet rays, or a little beyond it, varying slightly according to the kind of paper used, and the quantity of free nitrate of silver present. The action commences at a point nearly coincident with the mean red of the luminous spectrum, where it gives a dull ash or lead color, while the most refrangible rays impress a ruddy snuff-brown, the change of tint coming on rather suddenly about the end of the blue or beginning of the violet rays of the luminous spectrum. Beyond the extreme violet rays, the action rapidly diminishes, but the darkening produced by these invisible rays, extends a very small space beyond the point at which they cease to act on the chloride of silver.
In its photographic application, it is, alone, of very little use; but in combination with other reagents it becomes exquisitely sensitive. With gallic acid and the ferrocyanate of potash it forms two of the most sensitive photographic solutions with which we are acquainted. These are used in the calotype process.
IODURET OF SILVER.--If upon a plate of polished silver we place a small piece of iodine, and apply the heat of a lamp beneath the plate for a moment, a system of rings is speedily formed. The first ring, which spreading constantly forms the exterior of the circle, is of a bright yellow color; within this, there arises, sucessively, rings of green, red and blue colors,
Muriate of ammonia precipitates chloride to darken to a fine chocolate brown, whilst muriate of lime produces a brick-red color. Muriates of potash and soda afford a precipitate, which darkens speedly to a pure dark brown, and muriatic acid, or aqueous chlorine, do not appear to increase the darkening power beyond the lilac to which the pure chloride of silver changes by exposure. This difference of color appears to be owing to the admixture of the earth or alkali used with the silver salt.
The prismatic impression on paper spread with the chloride of silver is often very beautifully tinted, the intensity of color varying with the kind of muriate used. Spread paper with muriate of ammonia or baryta and you obtain a range of colors nearly corresponding with the natural hues of the prismatic spectrum. Under favorable circumstances the mean red ray, leaves a red impression, which passes into a green over the space occupied by the yellow rays. Above this a leaden hue is observed, and about the mean blue ray, where the action is greatest, it rapidly passes through brown into black, and through the most refrangible rays it gradually declines into a bluish brown, which tint is continued throughout the invisible rays. At the least refrangible end of the spectrum, the very remarkable phenomenon has been observed, of the extreme red rays exerting a protecting influence, and preserving the paper from that change, which it would otherwise undergo, under the influence of the dispersed light which always surrounds the spectrum. Not only the extreme red ray exerts this very peculiar property, but the ordinary red ray through nearly its whole length.
In photographic drawing this salt is of the utmost importance. Mr. Talbot's application of it will be given hereafter in another portion of this work.
IODIDE OF SILVER--Perfectly pure, undergoes very little change under the influence of light or heat; but if a very slight excess of the nitrate of silver be added it becomes infinitely more senitive than the chloride
The spectrum impressed upon paper prepared with a weak solution of the hydriodate of potash presents some very remarkable peculiarities. The maximum of intensity is found at the edge of the most refrangible violet rays, or a little beyond it, varying slightly according to the kind of paper used, and the quantity of free nitrate of silver present. The action commences at a point nearly coincident with the mean red of the luminous spectrum, where it gives a dull ash or lead color, while the most refrangible rays impress a ruddy snuff-brown, the change of tint coming on rather suddenly about the end of the blue or beginning of the violet rays of the luminous spectrum. Beyond the extreme violet rays, the action rapidly diminishes, but the darkening produced by these invisible rays, extends a very small space beyond the point at which they cease to act on the chloride of silver.
In its photographic application, it is, alone, of very little use; but in combination with other reagents it becomes exquisitely sensitive. With gallic acid and the ferrocyanate of potash it forms two of the most sensitive photographic solutions with which we are acquainted. These are used in the calotype process.
IODURET OF SILVER.--If upon a plate of polished silver we place a small piece of iodine, and apply the heat of a lamp beneath the plate for a moment, a system of rings is speedily formed. The first ring, which spreading constantly forms the exterior of the circle, is of a bright yellow color; within this, there arises, sucessively, rings of green, red and blue colors,