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

684 THE PHOTOGRAPHIC NEWS. [October 2, 1891. ELECTRO-CHEMICAL REVERSALS WITH THIO- CARBAMIDES* BY COLONEL WATERHOUSE. At the meeting of the society in August last, I exhibited some specimens of a curious reversal of the photographic image, produced by adding small quantities of thio-carba mides or sulpho-ureas to the ordinary eikonogen developer, and showed that, although reversal of the image was by no means uncommon, it was usually caused by over-exposure or some other abnormal action of light, whereas, to produce these new reversals, even less than the ordinary exposure was sufficient; and they appeared to be entirely due to some peculiar action of the thio-carbamide, added in very minute quantities to an alkaline eikonogen developer. At that time I was quite unable to offer any opinion as to the probable cause of these reversals, or as to how they were produced, beyond stating the probability that, although there were many points of difference, they would be found to be in accordance with the generally-accepted theory worked out by Capt. Abney, and were due more or less to oxidation or re-halogenisation of the exposed parts of the film, and that, owing to the peculiar reducing action of the alkaline thio-carbamides, the film, during development, was practically in the same state as if it were over-exposed. It seemed also probable that sulphur was the active agent in producing the reversals. Further work with these curious salts, and especially with a compound salt of thio-carbamide and ammonium bromide, discovered by Prof. J. E. Reynolds in 1868, and called by him tetrathiocarbamidammonium bromide, which was found to be exceedingly active in producing perfect reversals of the image with very short exposures, led me to the belief that this complete change of deposit from the lights to the shadows of the photographic image must be more or less due to electro-chemical action. The subject of electro-chemistry is one of which I have little knowledge, but I have been able to make some simple experiments, from which, though not conclusive, it seems probable that my surmise is correct, and, so far as they go, they seem to establish that not only, asformerobservationsbyLermontoff, Eder, and Abney, had shown to be probable, is the ordinary process of photographic development of sensitive surfaces containing silver haloids accompanied by electrical action, but that the addition of these minute quantities of thio carbamides to the developer greatly increases the inten sity of the electrical action, and produces a reversal of the current, which should also account for the reversal of deposit. With the aid of a very sensitive galvanometer, which has been kindly lent me by the Rev. Fr. Lafont, S. J., who also assisted me in the experiment, it was found that, when a pair of pure silver plates coated with finely precipitated silver bromide, one of which had been exposed to light and the other not, were connected to the galvanometer, so as to form a galvanic couple, and immersed in the ordinary eikonogen developer, the exposed plate formed the negative pole, and the needle was deflected to the left; whilst in the developer containing a little thio-sinamine, the exposed plate formed the positive pole, and the needle was deflected to the right. This experiment has been successfully repeated several times with silver plates prepared in the same way, and with other thio-carbamides, also with silver plates bromised by dipping them in bromine water; and so far the occur- * Paper read before the Asjatic Society of Bengal. rence of the reversal is well-established. I propose to repeat the experiment before you, though I cannot be certain of success. [The reversal was successfully shown with bromised plates, the image of the needle and scale being projected on the wall.] I have also tried the same experiment with ordinary dry plates rendered conducive in various ways, the best of which appears to be gold leaf applied either on the face of the film or behind it. Gelatine offers very great resistance to the current, and, though I have obtained distinct evidence of currents in both directions, they are not always observable, nor is it yet quite certain that they are caused by electrolytic action within the gelatine film, and further investigation as to this is necessary. I have also found that reversals of the reduction products, somewhat similar to those obtained by photographic methods, may be obtained entirely without the agency of light by passing a current from a single bichromate cell through a pair of silver plates coated with silver bromide, and immersed in eikonogen developers prepared with or without thio-carbamides. In this case the plate attached to the carbon pole in the plain developer showed only a very little black deposit, whilst the plate attached to the zinc pole showed a very strong, dark deposit all over. A pair of similar plates immersed in some of the same deve loper, to which a few drops of a solution of thio-sinamine had been added, showed quite different results, the plate attached to the carbon pole showing a strong black deposit, while the plate attached to the zinc pole was almost clear on the face and free from deposit, showing only a slight tarnish, caused by sulphur. I have some plates of this kind here, though the reversed effect is not quite so strong as it was on my first plates. I have found it difficult to obtain such marked reversals again, though I quite believe they are obtainable, and the best conditions or securing successful results have yet to be ascertained. Some similar effects were produced on Eastman’s bro mide paper, and on ordinary dry plate films attached to the silver plates. Although results obtained with silver bromide on silver plates are not quite comparable with those obtained with ordinary gelatine plates, these experiments show that, under favourable circumstances, the action of developing solutions on silver bromide is accompanied by distinct electric action, and that these thio-carbamide reversals may be produced by electrical methods, and are attended by a reversal of current. How this reversal of current is brought about is not yet quite clear, but seems to be ex plained by some observations on metallic sulphides by W. Skey, recorded in vol. xxiii. of the Chemical News. He found that sulphides which have the power of conducting can also generate electricity, and that silver sulphide is positive to metallic silver. In a battery consisting of a sulphide and a metal in acidulated water, the gas liberated is sulphuretted hydrogen, the nascent hydrogen exerting a desulphurising action upon the metallic sulphide, the ultimate effect of which is, in some cases, to completely reduce the mineral to the metallic state. He shows also that these sulphides are capable of performing the functions of the negative element of a galvanic couple. It seems probable, therefore, if electrolytic action does take place in gelatine films during the process of photo graphic development, that, according to the laws of electrolysis, with the ordinary developers the exposed parts of the plate form the negative pole, and attract the metallic elements and hydrogen, while the bromine, or