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The photographic news
- Bandzählung
- 27.1883
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- 1883
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- Hochschule für Grafik und Buchkunst Leipzig
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- Bandzählung
- No. 1297, July 13, 1883
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The photographic news
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- Titelblatt Titelblatt I
- Register Index III
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ABERRATION.—See Lenses. ABSORPTION OF LIGHT.—When light is absorbed by any mate rial, it generally heats the substance, or effects a chemical change. That light which is absorbed by the leaves of plants serves to decompose the carbonic acid of the air, and store up the means of producing heat and light at any time. A plate is only sen sitive to those rays which are absorbed by the film, and it has been asserted by Vogel and others that by staining a film it can be made especially sensitive to those radiations which are comple mentary to the colour of the stain. Many substances, as calcium salts, fluor spar, or glass, possess the property of storing up light and gradually evolving it when taken into a dark place. This phenomenon is generally known as phosphorescence ; and M. Niepce de St. Victor found that a photographic copy of an ordinary engraving may often be obtained by the agency of the light stored up in the white paper. See Light. ACCELERATING AGENTS.—A term rather vaguely applied to bodies which serve to expedite photographic exposure or development. This effect of chlorine or bromine in reducing the exposure required for a Daguerreotype plate may be referred to under this head. Another class of reactions may be typified by the effect of a powerful reducing agent in facilitating the action of light on haloid salts of silver, and with respect to this point we may mention that the haloid salts of silver appear to be absolutely insensitive to light provided that neither moisture nor any foreign substance is present. The tendency of light is to liberate the halogen; but if no substance capable of taking up or uniting with this halogen is present, there is no photographic action. Accelerating agents of this kind are often called sensi tizers. In the case of the gelatino-bromide work, the gelatine itself may be regarded as the sensitizer ; while in the Daguerreo type process, the sensitizer is the film of metallic silver which underlies the sensitive film. Nitrate of silver acts as sensitizer in the collodion process and in the ordinary method of printing on albumenized paper; but nitric acid is liberated by the action of haloids on silver nitrate, and this acid, when in a free state, tends to act as a de-sensitizer. ACCUMULATION, Development by.—An old-fashioned term for that which is now known as physical development; the deposition of fresh silver on the lines of a wet plate negative affording a good instance. The mixture of pyro and silver nitrate slowly deposits silver, and the silver tends rather to accumulate on the metallic image than on those parts where no reduction has already taken place. ACETIC ACID {Acetum, Vinegar).— C,H,O.HO. Acetic acid appears to exist already formed in the juice of certain plants; but in ordinary cases, it is a product of the secondary decompo sition of various organic bodies. Until recently, the main supply of acetic acid was derived from alcohol; as dilute alcoholic liquids readily undergo a kind of oxidatory fermentation, acetic acid being formed, and ordinary vinegar resulting- The so-called acetous fermentation appears to depend on a growth which Pasteur calls mycoderwa aceti. But that the presence of thismycodenn is not an essential to the direct acetic oxidation of alcohol, is evi denced by the fact that when mixed alcohol vapour and air are passed over platinum black, acetic acid is produced. Acetic acid is a frequent product of the action of composite oxidising agents on organic bodies. At the present time, enormous quantities of acetic acid are pre pared by the dry distillation of wood ; this material being baked in iron retorts like those used for the preparation of coal gas, and the acetic acid is condensed along with tar and numerous other volatile products. The crude product is either converted into a sodium or a calcium salt, and this, after purification, is decomposed by distillation with sulphuric acid. Sometimes, however, the storing acid is prepared by distilling the acid acetate of potas sium. Acetic acid, when nearly or quite free from water, forms a somewhat corrosive and highly-pungent liquid, boiling at 117" 0., and it is known as glacial acetic acid, as it readily solidifies when exposed to a moderate degree of cold. When quite free from water it may become solid at 16° C., but it often happens that it remains liquid at a much lower temperature, and suddenly solidifies when agitated or brought into contact with a particle of solid acetic acid. An acid very considerably weaker than the glacial acid is occasionally sold, but, from a photographic point of view, it is generally much more convenient to purchase the strong acid. The subjoined table shows the specific gravity ot acetic acid when in conjunction with various proportions of Per cent, of Acetic Acid 0 Sp. gr. at 15° C. 1.0000 1 1.0007 5 1.0057 10 1.0112 15 1.0214 20 1.0284 25 1.0350 30 1.0412 35 1.0470 40 1.0523 45 1.0571 60 1.0615 60 1.0685 70 1.0733 80 1.0748 90 1.0713 100 1-0553 Commercial acetic occasionally contains sulphuric acid, which can be detected by diluting with about three times its volume of water, and adding a few drops of chloride of barium ; a white precipitate indicating the presence of sulphuric acid. Hydro chloric acid can be detected by testing in a similar manner with silver nitrate, a white precipitate of silver chloride being thrown down. It is not necessary to enumerate the special photographic uses of acetic acid, as they will be treated of in other places. Some few acetates are of interest to the photographer. Aluminium acetate is an unstable salt only existing in solution. It is a very effectual agent for hardening gelatine films. Lead acetate, a white, readily soluble salt, known in commerce as sugar of lead. Silver acetate forms thin white flexible lamin®. It is soluble in about a hundred times its weight of cold water. Sodium acetate is a very soluble salt, and crystallises with three molecules of water. It effloresces in dry air. Its addition to the solution of chloride of gold used for toning serves to replace any free hydrochloric acid which may ba present, by the weaker acetic acid. PHOTO-LITHOGRAPHY AND PHOTO ZINCOGRAPHY. BY MAJOR J. WATERHOUSE, B.S.C., Assistant Surveyor-General of India. Chapter XI.— Transferring, Proving, and Printing.* latching Zinc Plates.—The procedure of etching a transfer to zinc is similar to that of transfers to stone, but, instead of using a simple mixture of gum-water and dilute nitric acid, an etching fluid is used consisting of a mixture of gum water and decoction of nut-galls, acidified with phosphoric acid. The decoction of gall-nuts is prepared by soaking half-a- pound of bruised Aleppo gall-nuts in cold water for twenty- four hours, then pouring water and gall-nuts into a sauce pan with about two gallons of water and boiling down to one gallon. The liquor is then strained off into ajar, and one ounce of phosphoric acid is added to it. A mixture of equal quantities of the decoction and of fresh gum-water about as thick as cream forms the etching solution. Too much of it should not be mixed at once, as the gum-water becomes sour. The phosphoric acid is prepared by placing sticks of phosphorus in a bottle of water so as to leave the ends un covered. The mouth of the bottle is closed by a cork with a hole in it, so that, air being admitted, the phosphorus becomes oxidised, forming a mixture of phosphorous and phosphoric acids, which dissolve in the water as they are formed. The solution is ready for use in a few days. The strength of the etching liquid varies according to the nature of the subject ; for fine work it should be used weak, but for strong line work, or if the ink contains a good deal of soap, it may be used stronger, in order to thoroughly neutralise the alkali in the ink. If the etching is not sufficiently strong, the work is apt to run smutty— that is, become dirty; and if etched too strongly, the delicate lines are destroyed. The usual strength is such • Continued from page 429.
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