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shine. The exposed surfaces of the iodide will blacken; remove the vessel into the dark, and after a few hours all the blackness will disappear: we may thus continually restore and remove the blackness at pleasure. The next experiment not only illustrates the phenomenon we are considering, but it further shows in a very marked manner, the influence of sunshine in producing chemical change. In one watch-glass was placed a solution of the nitrate of silver; in another, a solution of the iodide of potassium. The two glasses were connected by a filament of cotton, and a circuit made up with a piece of platina wire. This little arrangement was exposed to Light; and in a very short time iodine was liberated in one glass, and the yellow iodide of silver formed in the other, which blackened as quickly as it formed. A similar arrangement was placed in the dark. Iodine was slowly liberated; no iodide of silver formed, but around the wire, in the glass containing the silver solution, was a beautiful crystalisation of metallic silver. If the watch-glasses in which the processes of decomposition have taken place, be placed in the dark, it will be found in a few hours, that the solution which has become brown from the liberation of iodine, again gets gradually clear, and the darkened precipitate is converted into the yellow iodide of silver. The power of the sun's rays in influencing or disposing chemical affinity, is very marked in these experiments.
(156.) Prismatic Analysis. — The bleaching action on these papers is carried on by the influence of the blue and the more refrangible rays; but at the same time the least refrangible portion of the spectrum acts powerfully on the prepared surface, and induces an extreme degree of blackness. This peculiar and complex action has been much more fully examined by Sir John Herschel, who repeated the analysis with papers prepared by myself.* When a
• Sir John Herschel on the Action of the Rays of the Solar Spectrum (Philosophical Transactions).
paper prepared as above, was exposed to the spectrum, and washed with a solution too weak fully to excite it, two contrary actions were produced by the rays above and below the zero point or mean yellow. By the former the paper was bleached, the action beginning in the least refrangible violet, and extending upwards a considerable distance, and downwards to the circumference of a semicircle, having the point at which the action commenced for a centre. By the latter the paper was darkened, the blackness spreading upwards and downwards ; upwards, till it passed the zero point, and nearly or quite attained the semicircle above-mentioned ; and downwards to a space beyond the extreme red ray. By repeatedly washing the paper with the iodine solution, both actions grew more intense, but the bleaching action the most so. At length, by repeated washings the darkness produced at the lower part of the spectrum, began to give way, and was slowly replaced by a very feeble bleaching, which at length extended very far indeed below the extreme red rays, and upwards to join the semicircle c. Within this semicircle
and its train, a somewhat dark, perfectly circular and well defined solar iinage arose, its diameter being somewhat less than that of the semicircular terminations, so as to leave a perfectly clear and distinct white border all around it, as represented in No. 2. This circle gradually extended EFFECTS OF COLOURED MEDIA.
itself into an oval or tailed form, but preserving its circular shape below, and maintaining the white border inviolate. Finally, after long-continued action, the interior browned oval above-mentioned, was found to have been prolonged into a figure of the annexed form, of which the termina
tion by a narrow neck and circular enlargement, indicates the definite action of a ray much further removed along the axis of the spectrum.
(157.) We shall perceive from this, that four distinct actions are exerted on these papers:- 1st, Bleaching by the most refrangible rays; 2ndly, Blackening by the least refrangible ; 3rdly, Darkening by the most refracted portion of the spectrum, after the first bleaching has been continued for some time; and, 4thly, Bleaching by the rays of least refraction, which in the first case produced an exaltation of the dark colour. The two last actions may be imitated to a considerable extent by radiant heat; and I have but little doubt that the bleaching by the red rays is due to thermic influence. The two first actions, it will be evident, are, first, the formation of the iodide of silver; and, secondly, the oxidation of this iodide. The definite spaces to which the rays are confined, on which these actions depend, are very remarkable, and appear to point to an involved system of solar emanations, which we are not in a situation at present to explain.
(158.) These very peculiar influences may be prettily illustrated by placing a piece of the prepared paper, with an engraving superposed, behind four pieces of coloured glass. Beneath a blue glass the engraving will be very perfectly copied, as a positive picture, with correct light and shadow. If a deep green glass is used, a negative picture will result from the blackening of the paper under those parts which are white admitting the most light, and
the same takes place under a yellow glass; but under a glass stained red with oxide of gold, a negative picture is formed, not by the darkening of the oxide of silver, but by the eating out of strong lights on all the lines which correspond with the dark parts of the engraving, these parts having the most calorific influence.
(159.) Experience has convinced me, that the use of coloured glasses, in any experiment which has for its object the elucidation of any of the phenomena connected with the chemical power of the sun's rays, is to be avoided, as leading to error in many cases, and giving uncertain results in all. No correct conclusions can be arrived at in any way, but by prismatic analysis. The above, and some other experiments, which are to be found in these pages, are only given as an easy way of showing some of the very peculiar and beautiful phenomena with which the art of photography has made us acquainted. I shall, in a future page, make some remarks on the influence of coloured media, when I shall endeavour to show the numerous sources of error to which we are liable, when we trust solely to their use, however carefully they may have been analysed by the prism.
(160.) IODIDE OF SILVER ON METAL PLATES. – If upon a plate of polished silver we place a small piece of iodine, and apply the heat of a spirit-lamp or candle beneath the plate for a moment, a system of rings is speedily formed, which is somewhat remarkable. The first ring formed, which, spreading constantly, forms the exterior of the circle, is of a bright yellow colour; within this there arise successively rings of green, red, and blue colours, and then again a fine yellow circle, centered by a grayish spot on the place occnpied by the iodine. On exposing these to sunshine, it will be found that the outer yellow circle almost instantly changes colour, and that the others slowly change in the order of their positions, whilst the interior yellow circle resists for a long time the solar influence. These rings must be regarded as films of the
iodide of silver, varying, not only in thickness, but in the more or less perfect states of combination in which the iodine and the metal are. The exterior circle is an iodide, in an exceedingly loose state of chemical agregation: the attractive forces increase as we proceed towards the centre; where a well formed iodide is formed, which is acted upon slowly by sunlight. The exterior and most sensitive film constitutes the best surface for the Daguerreotype plates. The changes which these coloured rings undergo are remarkable ; by a few minutes' exposure to sunlight, an inversion of nearly all the colours takes place, the two first rings becoming a deep olive green and a deep blue inclining to black. Is is a very pleasing ex. periment to form those beautiful rings by placing a small piece of iodine on a little silver leaf, and then, covering one half of them with an opaque body, expose them to the solar rays.
(161.) THE DAGUERREOTYPE. — The material used as the tablet upon which the solar radiations are made to impress external nature, in this process is copper, plated with silver. The copper serves principally to support the silver ; but it has been stated, with some degree of correctness, that the combination of the two metals tends to the improvement of the effect. It is essential to success that the silver should be brought up to the most perfect polish. This is best done by polishing with cotton dipped in sweet oil, and finely levigated Tripoli powder or rotten stone. The cotton is to be frequently changed, and great care must be taken that the plate is not touched by the fingers. Dry cotton and very fine Tripoli dust must be used to complete this part of the process. The plate is then subjected to the heat of a spirit-lamp or a charcoal fire, for a few minutes, and cooled as speedily as possible, by placing it on a mass of metal or a stone floor; the object of this is to remove the organic film which the oil has left on the plate. Daguerre has stated it as the result of his experience, that any organic or atmospheric film