The photographic news

• Bandzählung: 35.1891
• Erscheinungsdatum: 1891
• Sprache: Englisch
• Signatur: F 135
• Vorlage: Hochschule für Grafik und Buchkunst Leipzig
• Digitalisat: Hochschule für Grafik und Buchkunst Leipzig
• Digitalisat: SLUB Dresden
• Rechtehinweis: Public Domain Mark 1.0
• URN: urn:nbn:de:bsz:14-db-id1780948042-189100009
• PURL: http://digital.slub-dresden.de/id1780948042-18910000
• OAI: oai:de:slub-dresden:db:id-1780948042-18910000
• Sammlungen: LDP: Historische Bestände der Hochschule für Grafik und Buchkunst Leipzig
• Sammlungen: Fotografie
• Strukturtyp: Band

• Digitalisat: SLUB Dresden
• Strukturtyp: Ausgabe

PHOTOGRAPHY IN ANILINE COLOURS * BY A. G. GREEN, 0. F. CROSS, AND E. J. BEVAN. In the diazotype process, which is the main subject of this paper, we have as the actual photographic agent not only a highly-sensitive compound—highly sensitive, that is, to the action of light—but one which can be readily converted into a variety of colouring matters. The process consists of three stages :—(1) Drying or coating the surface upon which it is required to photograph with a particular compound, which is then (2) converted into a photo-sensitive derivative ; and (3) exposed to the light under the usual conditions for giving the picture ; (4) converting the sensitive compound, wherever it survives, through having been protected by the shadows of the object photographed, into colouring matters, and thus the pic ture is developed from the weakly coloured sensitive compound into well-marked shades of red, orange, brown, purple, or blue; and these shades being formed in stable colouring matters, the picture is at the same time fixed. For the better understanding of the process, it is necessary that we should acquaint ourselves with the properties of these substances, apart from any reference to photographic action. The compound we start with is a yellow-coloured body, to which the “ trivial ” name of primuline has been given. This compound is obtained by the action of sulphur upon toluidine, a coal-tar base closely allied to aniline. In this action not only is there combination with sulphur, but several molecules of the toluidine are built up into one complex molecule. Then, to express the matter in our modern formula), westart with CeH, Cu 8 NH 2 toluidine, and obtain— CH,C,E,<S>c. C,H,<S>c. C,H,<S>c. C,H,Nu. primuline base—a very formidable looking compound. Now, the important part of this formula to us is not the unwieldy | body, but the head, the small group NH,. We can consider primuline in its simplified form X.NH 2 , and we see that this differs from ammonia, H.NH 2 , in having one H replaced by a complete group. Corresponding to this general resemblance, there is a similarity in their distinguishing property ; both are basic or alkaline substances, and combine with all acids to form salts. One only of these we have to consider, that very combination with which the stable primuline—highly resistant to all agencies which destroy the more fugitive colouring matters—is converted into a photo-sensitive derivative. This acid is nitrous acid, HN0 2 . Nitrous acid and ammonia com bine to form ammonium tritrite. When this tritrite is heated, it is resolved spontaneously into nitrogen gas—the inert con stituent of atmospheric air — and water. Primuline, in common with all coal-tar bases, combines with nitrous acid to form a species of nitrite, which is termed a diazo compound. All that we need note about these diazo compounds is that they correspond to the intermediate stage in the transi tion of the ammonium nitrate to the condition of nitrogen (and water) thus :— NH 3 . HN0 2 H. N N OH N, Ammonium nitrite. ( — H, 0) (— 2HsO) X. N = N.OH Diazo-primuline. These diazo derivatives are, as a class, sensitive to light undergoing decomposition with evolution of nitrogen gas. But what concerns us, for the moment, is their avidity for construc tion or synthetic reaction with two large groups of coal for compounds—the bases and phenols. With these they combine very much as the original bases combine with nitrous acid, to form these active diazo compounds, and the products are the azo-colouring matters. We cite the more important of these, obtained from primuline. In the case of the several phenols, their alkaline solutions are employed, while in the case of the bases and solutions are used— B Naphthol. Red. I Phenol. Yellow. Primuline and nitrous acid gives Diazo-primuline. Resorcin. Pyrogallol. Orange. Brown. I a Naphthylamine. Hydrochlorinde. Purple. I Amido. B Naphthol. Sulphuric Acid. Blue. These relations we demonstrate with diazo-primuline and the several reagents above-mentioned. We now come to the photographic application of these reactions. The essential condition of primuline photography is—(1) that these reactions take place with primuline after its application to any surface or material, such as wool or silk, as a dye, without in the least affecting its union with the material ; and (2) that the diazo-derivative produced, in combination with these materials, by treating the primuline-dyed surface with nitrous acid, is photo-sensitive in the highest degree, far more so, in fact, than the derivative itself, when free or un combined. Having thus prepared the way, we proceed to a complete experimental demonstration of the process. We first show the building up of the colouring matters upon the cotton and silk fabrics, showing that the reactions take place in silk with out disturbing the union between the colouring matter and fabric ; and that the azo colouring matters so built up with stand the action of boiling soap. We have now to demonstrate the sensitiveness to light of a surface dyed with primuline, and sensitised by treatment with nitrous acid, and we shall show that the action of the light is to destroy the diazo-derivative, that this destruction is due to the splitting off of its two characteristic nitrogen atoms in the gaseous form. We are enabled to show this to an audience owing to the curious fact that, in a gelatine medium, the nitrogen, though liberated from combination, is not evolved until the gelatine is softened by contact with water. We expose a primuline-gelatine plate to photographic action, and on plunging into a transparent bath of water, an image of which is thrown on the screen, we see the form of the print * A paper read last Wednesday night before the Society of Arts, with Capt. W. de W. Abney in the chair. We omit the preamble. instantly developed in minute bubbles of gas. On then treating the plate with one of our developers, we see that when the gas has formed—-the evidence, i.e., of the destruction of the diazo-primuline—then no development of colour takes place. We now develop a number of prints which have been exposed in the daylight, and brought here in the condition in which they left the printing frame. In these developed pic tures the high lights are represented by the neutral tint— varying from buff to cream—of the product of destruction of the diazo derivative, the absence of development in these portions being due to the chemical mixtures of this product. The development of colours corresponding to the shadows and half tones of the object is exactly proportionate to their depth, to the degree of protective action which they have exerted on the sensitive surface beneath—in other words, to the quantity of diazo-primuline which survives. Reverting to the considerations which we advanced in the earlier portion of this paper, we should explain the action of the light on an addition of energy or atomic motion to the molecule of the diazo-primuline, whereby the split or cleavage of the molecule at the point occupied by the two nitrogen atoms is determined, just as by adding heat to ammonium nitrite its molecule is resolved into nitrogen, and a residue, which in this case is not a complex compound, but water. We have already alluded to the fact that the rays which attack the diazo-primuline are by no means confined to the blue and violet constituents of the spectrum, but extend well through the yellow to the orange red. In other words, a primuline print is more nearly a measure of the visual intensity of the sun’s rays than the sensitive substances used in the ordinary methods of photography, in relation to which it is well known that the photographic intensity of sunlight is a very different thing from visual intensity, owing to the enor-