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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174 THE PHOTOGRAPHIC NEWS. [April 11, 1862. and to increase its sensitiveness, I add a little nitrate of potassa to it. I have always filtered the alcoholic solution of this iodide before adding it to the collodion—not to separate any oxy-iodide from it, the presence of which I did not suspect prior to the recent communications of M. Martin —but because it is always a good precaution. It is true that by not employing absolute alcohol, I should have dis solved in part this oxy-iodide, and put iu action the causes noted by M. Girard, as to verify them upon proofs was not an easy thing to do. I thought of nitrate of cadmium, which is found in large proportion in old silver baths, as every proof leaves a proportion equivalent to the iodide of silver it removes from it. To distinguish its special action I have dissolved it iu a solution of silver, saturated with iodide, the acidity of which quickly manifested itself under the influence of the iodine ; the presence of the nitrate of cadmium had retarded the acid reaction to such a degree that many days elapsed before it became very evident. I have obtained in a lesser degree the same effect from the acetate of silver dissolved in the bath. I made this second experiment because the ether introduced into the silver bath may acidify it in contact with the air, and transform the carbonate of silver into acetate. I believe therefore that I am able to attribute to the car bonate of silver and to the nitrate of cadmium the neutrality which appears to persist in the argentiferous solution, not withstanding the presence of iodine. Iodide of potassium and iodide of ammonium do not communicate to the silver bath the same properties, for the nitrates are inactive. The nitrite of silver also has no action in this direction. I have been able to satisfy myself of it, as the collodion contained a little nitrite of potassa. In these experiments we must take into account the secondary actions which I pointed out in my first note : thus, when fragments of iodine float about for a long time, or form bubbles of air in the bath, we perceive them quickly surrounded with the yellow iodide and produce a sensible acidity. To remove these causes foreign to the principal fact, we may at first throw very little of the silver solution upon the iodine, moisten it completely by agitating it, and replace this liquid by that we desire to test. We must also beware of a fact which I believe it will be useful to note :—When the acidity of the silver bath is very feeble, and we proceed to test it with litmus paper, we must note its action at the very first moments; for after a short time the nitrate of silver forms with the colouring matter of the litmus a greenish-blue lake, which entirely masks the acid reaction; and if, as often happens under other circum stances, we wait, in order to give the acidity time to manifest itself more clearly, this blue tint acts upon the colour of the paper, and tends us to believe in an alkaline reaction, when in reality it is acid. If it be desired to explain the apparent inaction of the iodine amid the circumstances above mentioned, we may say that it envelopes itself in an invisible film of iodide, which the nitrate of silver saturated in advance, can no longer dissolve, or even penetrate, except with extreme slowness in in presence of nitrate of cadmium: resembling in this respect, nearly, the passivity of iron in nitric acid when covered with an invisible film of oxide, which protects it from all ulterior action. We may comprehend more easily the impotence of nitrate of silver, if we consider that it is in some measure saturated with iodide in a bath often used, because every operation weakens the nitrate of silver, without however, precipitating the iodide. It is true that it is replaced by nitrate of cadmium, but we may satisfy ourselves by direct experiment that the nitrate of cadmium, does not dissolve iodide of silver. It is recognised now, that, that to obtain a negative, the impressioned film must necessarily present itself to the to the developing agent with a tendency to acidity; a tendency which it will have derived either from the free iodide of the collodion, or from the silver bath. This acidity may be feeble enough to escape chemical tests, and still suffice to consolidate the purity of the whites, with I the maximum of sensitiveness. This is not the only® in which extremely delicate causes produce a new order things in photography. We know, for example, the i rious action of hydrocarbon vapours : the quantity of t vapours acting at a given moment upon the sensitized i is so minute, that no reagent in chemistry can manifest presence, while in photography, in the eyes of the ent siastic photographer, it reveals itself by a failure. 1 slow and progressive action of iodine, in the conditi under which I employed it, has appeared to me excell 1 to produce this simple tendency to acidity, which I h often called apparent neutrality. We may perhaps, apj it to other baths, by diminishing the dose, and suspel ing its action when it is sufficient. In all this we clearly perceive the nitric acid at wot but, the thing I cannot now explain, if the nitric acid added directly, in homeopathic dose, it does not produce' same effects. Since the efficacy of the iodine depends upon the na of the silver bath, I ought to have spoken of it in my% note. I candidly confess that the method of preparing solution, has become so much a habit with me, that it n occurred to me to mention it even to myself. I should t probably have tried the ordinary solution, and this wo have led me to the completion of my communication. J I thank M. M. Girard and Thouret for making kn the result of their experiments, and thus warned tl whom I had temporarily set upon a wrong road. ON WASHING NEGATIVES TAKEN BY 71 LINSEED OIL PROCESS. BY M. x’ADBE LABORDE. Having the intention of resuming my experiments up linseed oil employed as a sensitive varnish, I beg refer to a note I addressed to the French Photograp Society on this subject, in August 1858. I concluded • by asking the co-operation of all those photographers * are stimulated by a new agent, to make researches; and order to put them on the way which alone I believe* lead to success, I was careful to fully explain, in what 1 real obstacle consists. This very obstacle has been f mounted in a very ingenious manner by M. Farg in another process. I congratulate him upon it, as success seems to justify my observations, and if I take liberty of introducing them here, it is in order to be < to apply them to the linseed oil process, as I understand without being accused of plagiarism. I copy them verbatim, as follows: “ I must point out the principal defect in this linseed’ process; the light tones of the negative disappear in positive, when we pour upon the former ether, in ord 1 * remove the non-impressed parts ; but yet they exist b this part of the operation; for upon breathing on proof, after removing it from the negative, we may perd all the details of the picture faintly. Upon this subje shall make a remark equally important for the linseed process, as for the Bitumen of Judea process ; in the sens" film, however thin it may be, we must distinguish I surfaces ; the one external, the other internal, appliegi the very substance that supports it. The insolub roduced by the light, commences upon the external st" ecause it is in contact with the air, which favours10 action ; in the parts most lighted, it penetrates litti0 little, until the inner surface becomes equally ‘j luble; but under the semi-transparent portions « negative, it stops at the external surface, or penetryi more or less, so that the interior surface retains nea original solubility. The ether afterwards spread , a film thus modified, first dissolves the parts which entirely sheltered from light; then, without dissolving