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The photographic news
- Bandzählung
- 27.1883
- Erscheinungsdatum
- 1883
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- Englisch
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- Hochschule für Grafik und Buchkunst Leipzig
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- Hochschule für Grafik und Buchkunst Leipzig
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- Bandzählung
- No. 1304, August 31, 1883
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Zeitschrift
The photographic news
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Band
Band 27.1883
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- Titelblatt Titelblatt I
- Register Index III
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Band
Band 27.1883
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- Titel
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558 THE PHOTOGRAPHIC NEWS. [August 31, 1883. these reasons, the spermaceti candle, burning 120 grains per hour, has been chosen as the standard measure of light over all the scientific world. The “ bottle-nose ” whale has been a subject of much doubt, and great, though surreptitious utility. It is smaller than thebalaena and cachalot, seldom exceeding 30 feet. Although, no doubt, thousands of these fish have been caught, and their oil sold as sperm oil, it is only quite recently that the oil has been made a distinct commercial article. Messrs. Bicknell have taken the matter in hand, and extracted, after much experiment,real sperm aceti therefrom, of a slightly higher melting point than the cachalot sperm. Mr. Allen has published several communications concerniny this oil, on which I may have more to say when we speak of oils proper. (To bo continued.) Uorxesgondente. THE BRUSSELS EXHIBITION. (A Monsieur le Directeur du Photographic News, Londr'es.) Monsieur,—Nous avons le plaisir de porter ii votre con- naissance que le jury international de notre Exposition vous a dcern un Diplome d’Honneur. Permettez-nous, Monsieur, de vous adresser toutes nos felicitations.—Pour le Comity, Alex, de Blockhouse, Le President. Alf. Geruzet, Le Secretaire. CAUSE OF THE INSENSITIVENESS OF COLLO DION EMULSIONS. Dear Sir,—Dr. Vogel, in a recent communication,* having referred to some of my experiments bearing on this subject, I venture to bring before the readers of the News the theory—or, rather, explanation—which these experi ments have suggested to me as being the principal cause of the great differences observed in the sensitiveness of collodion and gelatine emulsions. It may briefly be stated as follows:—That the differences in sensitiveness are due in the main to the different chemical characters of the two vehicles, gelatine and pyroxyline; that whereas gelatine is a substance having, if anything, a slight reducing action, pyroxyline is a powerful oxydizing agent, and behaves as such in preventing the formation of a developable image. It may appear to some that an attempt to saddle our old friend pyroxyline with the property of being a power ful restrainer is against all previous teaching and experi ence. For instance, it is stated in Hardwich’s Photo chemistry that pyroxyline is a strictly neutral substance, and has no chemical action on the sensitive salt, and other writers have expressed similar views. In all these cases, however, films prepared with excess of silver nitrate are referred to, and this is where a qualification should, I think, come in. It is well known that the darkening of silver chloride in presence of excess of silver nitrate is but little retarded even by the presence of such a powerful restrainer as free nitric acid; and the same may be said of pyroxyline. In presence of free silver nitrate its re straining power is masked or compensated for to some extent; but let a little nitric acid be added to an emulsion without excess of silver nitrate, gelatine or otherwise, and a very great reduction of sensitiveness at once takes place. The same with pyroxyline. If present in emulsions without free silver nitrate, it exerts the normal restraining effect which might be expected from its composition. There are one or two facts bearing on the subject which I will also mention here. It is known, and has been pro minently brought forward by Mr. Binks, that the sensi tiveness of collodion emulsions or plates, especially if they contain gelatine, increases as the percentage of pyroxyline decreases—i e., the sensitiveness is inversely proportional to the quantity of pyroxyline present; and again, of various samples of pyroxyline, that which increased least in weight • PKOrOGAAPMIC NEws, p. 426. during nitrification—or, in other words, contains the least percentage of nitric acid—will yield the most sensitive emulsion. How this rythmic action between the sensitive ness and the quantity of nitro-cellulose, the one increasing as the other diminishes, either by reducing the quantity of pyroxyline, or using one with a smaller percentage ot the substitution compound, is explained on the theory of two modifications of silver bromide, it is difficult to see. I will also offer a few remarks on the points of difference pointed out by Dr. Vogel. 1. Spectroscopic differences.—As regards the different points of maximum sensitiveness observed by Dr. Vogel in silver bromide precipitated in collodion and gelatine re spectively, it becomes a question as to whether the differ ence invariably goes hand in hand with the difference in sensitiveness. I have found that silver bromide precipitated in gelatine, and then emulsified in collodion, is even slower than if it had been originally precipitated in collodion ; and Dr. Vogel states that under such circumstances he found no change in the spectroscopic behaviour of the silver bro mide ; it follows, therefore, that silver bromide, which by spectroscopic examination has the sensitive form, loses this sensitiveness when emulsified in collodion. 2. The unequal capability of diffusion of the two bromides. —In the case of silver bromide precipitated from alcoholic solutions not diffusing itself in gelatinous solutions, and silver bromide precipitated from aqueous solutions not diffusing in collodion, it would appear to be due to the liquid from which the bromide is precipitated not being readily miscible with the other. In either case a little glycerine will abolish the difference. There must also ba a certain proportion of mixed alcohol and water, from which it must be a nice point as to which modification will be precipitated. 3. Unequal reducibility of the two bromides.—Dr. Vogel finds that silver bromide prepared in gelatine is reduced with more difficulty than when prepared in collodion, and cites, as evidence of the difference not being due to the greater porosity of the collodion film, the fact that a collodion plate coated with gelatine will stand a much more powerful developer. It seems to me that since the silver bromide is not altered in any way by the coating of gelatine, the difference observed shows that the vehicle, and not the silver bromide, is the agent which allows a stronger developer to be employed. I imagine Dr. Vogel, in these experiments, is referring to collodion emulsions prepared with excess of silver on the one hand, and gela tine emulsion prepared with excess of bromide on the other ; for with a number of collodion emulsions prepared with excess of bromide, I have not found them fog any more than the corresponding gelatine ones with the same strength of developer, and I need hardly say that gelatine emulsions prepared with excess of silver fog just as readily as collodion ones. There is, it is true, a thin fog frequently seen on collodion plates, even if prepared with excess of bromide, when developed with strong alkaline developers ; but this appears to be due to the exposed condition in which the silver bromide is on a dry collodion plate, for with collodion emulsions containing small quantities of camphor and shellac respectively, to render the films more homo geneous, and with collodion plates coated with gelatine, not a trace of fog appeared. 4. The unlike behaviour with chemical and optical sensitizers. —This is readily explained on the supposition that pyroxy- line is a restrainer. In all sensitive preparations, if there is no restraining substance present, the action of sensitizers is very small; for instance, in gelatine emulsions, the addition of silver nitrate or other powerful sensitzer—such as the hydrates of barium, strontium, or calcium, &c—has very little sensitizing effect. On the other hand, when a restrainer is present, the action of sensitizersis very marked, compensating or neutralizing, as it were, the restrainer, and restoring the sensitiveness to a greater or less extent.
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