The photographic news

• Bandzählung: 6.1862
• Erscheinungsdatum: 1862
• Sprache: Englisch
• Signatur: F 135
• Vorlage: Hochschule für Grafik und Buchkunst Leipzig
• Digitalisat: Hochschule für Grafik und Buchkunst Leipzig
• Digitalisat: SLUB Dresden
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• Sammlungen: Fotografie
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[June 27, 1862. 306 THE PHOTOGRAPHIC NEWS. * From the Photographic Notes. neutralization of the acid substances which are developed, and which possess the faculty of thickening the collodion by acting upon the pyroxyline in the same manner as the mineral acids do. When the collodion bleaches spontaneously, some alkaline matters are formed, as has been shown, and this reaction is always accompanied by oxidation of the ether, which gives rise to compounds of aldehyde : now these compounds enjoy the peculiar property of reducing the salts of silver with that brilliant metallic aspect remarked upon plates prepared with old collodion. As a concluding remark upon the spontaneous changes of colour in iodized collodion, wc have shown that when this product, colourless at first, or bleached ultimately in the dark, acquires a red hue more or less deep, this effect is due to the decomposition of the organic oxides, the principal of which is the periodide of formyle, which is decomposed with great facility under the influence of diffused light, although feeble. (To be continued.) BROMIDES IN DRY PLATES* The further we go on in experimenting on the action of a bromide in collodion, the more interesting and instructive our experiments become. In the rapid dry processes a bro mide seems destined to play an important part, and we will now discuss this branch of the subject. On examining, by daylight, the colours of different sensi tive films of iodide, or bromo-iodide of silver, they will bo found to vary greatly—some being of a whitish-yellow, and others of a deep greenish-yellow tint. Films of the latter colour appear to be the most sensitive in the dry processes, and it therefore becomes important to ascertain upon what the colour of the film depends, and to know how to produce a film of a pale or deep colour at will. For this purpose it is necessary first to examine separately the films of iodide and bromide of silver—and afterwards to examine them in a state of mixture or combination. Letus first confine our attention to the iodide of silver alone. If you add iodine to plain collodion, and then coat a plate with it and immerse it in the nitrate bath, an intensely yellow film is very quickly formed; and you may male this as creamy as you choose, by adding a sufficient quantity' of iodine to the collodion. This, however, gives a very in sensitive film in the wet process, chiefly on account of the large quantity of nitric acid which is set free in the film But if, instead of adding iodine to the collodion, you add iodide of cadmium or potassium, the colour of the filu becomes much whiter. Again, if you add a few drops of 1 solution of nitrate of silver to a solution of iodide of potaf sium you produce a white turbidity in the liquid, and the iodide of silver thus formed is not entirely precipitated, but remains suspended. If, however, you add a few drops of 8 solution of iodide of potassium to a solution of nitrate of silver, the iodide of silver formed is at once thrown down in flakes, and is of a much deeper yellow colour. Our readers are of course aware that iodide of silver 8 entirely soluble in a strong solution of iodide of potassium: but when that solution is not strong enough entirely to dis solve the iodide of silver it seems to form with it a sort w double iodide of silver and potassium, of a whitish colour. It will readily be believed that this complex salt is not ceeteris paribus, so sensitive as the pure, deep, yellow-colourcd iodide of silver. If we perform similar experiments with bromide of silver, we do not arrive at similar results. The bromide of silver obtained under any circumstances appears to have a whitish colour, and never a deep yellow. Whether the collodion contains bromine, or bromide of cadmium or ammonium, the film is still white, like a film of chloride of silver, and not yellow like iodide of silver; and it turns grey, and eventually dark blue, by exposure to light, so that you C21 if you like, obtain vigorous sun prints upon bromide, in tle the same time, becoming acidified, are soon decomposed. The result is, that at the time of preparing the collodion the iodides are attacked, the iodine is liberated and colours the liquid, while the equivalent quantity of the bases of the latter combine with the formic, acetic, oxalic, and other acids. But while these combinations are slowly effected, the iodine which at first colours the solution, acts upon the formyle and the oxide of ethyl, producing organic iodides (compound ethers), which are nearly colourless. These facts clearly explain to us the spontaneous changes iu colour that collodion generally undergoes, and even while admitting that the pyroxyline exercises no influence in causing the primary acidification of the collodion, and consequently, the decomposition of the salts it contains, we have seen that this condition is established, even by the effect of the solution of the iodides solely. Colourless collodions possess an alkaline reaction in pro portion as the absence of colour is more complete. This alkalinity is principally due to the presence of free me tallic oxides, the formation of which is probably favoured by the affinity of the iodine for hydrogenized substances, with which it produces organic iodides, while the major part of the base set at liberty combines with the formic, acetic, and oxalic acids. 1 , We do not yet clearly understand in what manner the reactions are effected, so that there can remain a quantity, whatever it may be, of free metallic oxide in the liquid ; for while the elimination of the iodine of the iodides is produced by the acids, it must make quantities exactly equivalent to those of the metal which was as first united, and which the acids have removed. It must then be admitted that the iodine is not expelled from its first combination, but that it forms, with the com pounds of ethyl and of formyle, new bodies, probably more stable under these circumstances, and thus abandoning the metal, which then oxidizes, and uniting in part to the acids which the liquid may contain, which will naturally be only in equivalent relation. Proceeding now to the change of properties in collodion, we commence by remarking, that immediately after its pre paration, it usually furnishes two distinct results. For one proof obtained with this product, under given circum stances, developes only feebly under the action of the re ducing agent, the shadows presenting no detail, while the lights are strongly marked; and if, to avoid this effect, we prolong the exposure to light, the details come out well in the portions feebly lighted, but they present no harmo nious relation with the shadows in the negative. In the other case, the image is sometimes formed very rapidly, sometimes with excessive slowness, and is always extremely feeble and superficial; every part comes equally grey, even those which have not been submitted to the influence of light, and the negative acquires no greater intensity by the ordinary means of intensifying. The first of these two results proceeds, not from the free iodine which may be disengaged, but from the pre sence of the acids we have shown to be formed in this case, and the neutralization of which cannot be effected at the time of preparing the collodion, for it often requires many days to complete these transformations. The second result can manifest itself iu so short a time only when alkaline substances are employed, but we may always remark during the bleaching of the collodion, either that it takes place spontaneously, or is determined by some cause or other. When the collodion is completely bleached, it is neutral or alkaline ; “ then it will be remarked that it becomes more fluid, that the film it forms is thinner, generally iridescent after- dessication, more permeable in those parts which cor respond to the shadows of the model, and the silver is reduced with a metallic aspect upon the glass plate ; the slightest defect in cleaning, therefore, becomes a spot.”* The fluidity bleached collodion acquires arises from the * Barreswil et Davanne,— Chimin PlLOtodfaphi/iue, p. 117.