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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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- Hochschule für Grafik und Buchkunst Leipzig
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- Bandzählung
- No. 1292, June 8, 1883
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The photographic news
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- Titelblatt Titelblatt I
- Register Index III
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JUXE 8, 1883. ] THE PHOTOGRAPHIC NEWS. 363 has been used, some of the silver will remain unconverted) and will be poured off with the supernatant liquid and lost. Hydrochloric acid is a much better agent to employ, because it it not open to the same objections that sodium chloride is. One fluid ounce per two gallons of wash water will deposit the whole of the silver as chloride in twenty-four hours, or in much less time if the operation is carried on in daylight. The solution will not become perfectly bright; when it is moderately clear, it may be drawn off so as not to disturb the sediment, after which more washing waters may be added, the same operation being repeated each day until a sufficient quantity of chlo ride has accumulated to warrant the trouble of reducing. The simplest plan of treating the chloride is to collect it on a filter (an old soft felt hat answers perfectly), and dry it in an oven. The substance will look more like mud than the beautiful white flocculent precipitate familiar to students of chemistry. It is now ready for the refiner, or may be put into the crucible with one-half its weight of a mixture of potassium and sodium carbonates, together with borax as previously described, and subjected to the furnace for the space of twenty to thirty minutes to obtain a button of pure metallic silver; or the dry chloride may be raised to red heat, and when cool, a small quantity of dilute sul phuric acid (1-6) may be added. Cover the top with granulated zinc, and leave it for half-an-hour ; at the end of that time the metallic silver will be found at the bottom. Wash well with warm hydrochloric acid and water, for the purpose of dissolving any fine particles of metallic zinc which may have become attached to the button ot silver; then convert into silver nitrate crystals by means of strong nitric acid. Gold.—Old toning baths should be kept separately when they are to be operated upon at home. The gold may be recovered by adding five ounces of a 20-grain solution of ferrous sulphate (sulphate of iron) per quart of waste solution. Gold will be deposited in the metallic state, and only requires washing on a filter with hot water until all traces of iron are washed away, which can be ascertained by testing the water for iron with barium chloride solu tion ; pour on strong nitric acid, to dissolve any silver that may be present; finally, wash with hydrochloric acid, and dissolve in the usual solvent—nitro-hydrochloric acid— viz one part of nitric acid to three parts of hydrochloric. When the residues are to be sent to the refiner, all waste gold solutions may be added to the print washings, taking the precaution to add some ferrous sulphate after each addition. It does not interfere with the recovery of the silver chloride ; the refiner separates the gold from the silver, and allows the value for each. Hypo The fixing bath usually contains enough silver to make it desirable to extract the same before throwing it away. Two methods are most frequently employed: one of these is to suspend sheets of zinc in a large tub or other vessel containing the old baths, leaving them until all the silver is deposited on the zinc; and the other is to add so-called liver of sulphur. A saturated solution of sulphuret of potassium should be poured into the old hypo, fixing baths, keeping the same as far away from the dwell ings as possible, on account of the disagreeable smell of hydrogen sulphide evolved. Gold or silver residues of any kind may be added to the vessel containing the sulphuret. It will all go down as a sulphide, and may be forwarded to the refiner. Test for Silver.—Procure two graduated tubes and a suitable stand (see fig. 2). These may be obtained of any dealers in chemical apparatus, graduated for grains or grammes; but there does not appear to be sufficient demand to induce the manufacturers to graduate them for ounces and drachms; the calculation is so simple when we consider 100 cubic centimetres represent 1,700 minims, that no inconvenience is felt. Make up a stock silver solution for standardizing, also a stock solution of the re-agent to be employed. The most perfect re-agent, probably, is thio-cyanate of ammonium (NH,CNS) used at decinormal strength. Given two standard solutions composed of 17 grammes of silver nitrate per litre of distilled water, and another solution composed of ammonium thio-cyanate 7'6 grammes (dry crystals) per litre of water, we have two solutions capable, when mixed, of bringing about a perfect reaction; or, using volume for volume, they are of the same value. Acid solutions of silver may be tested, and the result of the determination shows no apparent difference than when they are tested neutral. Potassium thio-cyanate (KCNS) 9-7 grammes dis solved in one litre of water has the same value as the fore going solution. An easy way to calculate how many grains per fluid ounce are contained in any solution known to consist of so many grammes per litre, is to multiply the number of grammes by 70, the result being so many grains per gallon ; dividing by 160 gives the number of grains per ounce, thus : AgNOa 17 grammes per litre. 17 70 160) 1120 (716 8rains per ounce. 70 For all practical purposes we may call it 7} grains per ounce. The practical application is as follows Fill one burette to the top gradation marked 0, with the sensitizing bath. Fill the second burette also to 0, with the re-agent, which we will term “ thio,” by means of a pipette; deliver a quantity equal to 5 cubic centimetres of the colour indi cator (described in detail below) into a clean beaker; also a similar quantity of dilute nitric acid (1-5). Into this run the silver until the bead reaches 5. Add “ thio ” from the other burette cautiously, shaking gently after each addition until the red colour dis appears. More “thio” is now added, and the operation of gently shaking repeated. When the precipitate is no longer grey, but the liquid retains a pale reddish tinge, the reaction is complete. Note carefully bow much has been used, because the number of cubic centimetres required to bring about the reaction will indicate the amount of silver present. Thus: 1 cubic centimetre of the re-agent = -017 of silver nitrate, or -0108 of actual silver; therefore, if 20 c.c.’s have been required, the sensitizing solution is four times stronger than the standard silver—i.e., each ounce would contain 30 grains of silver nitrate. Although the metric system is undoubtedly the best, it does not follow that every one cares to adopt it; for the use of those who prefer English weights, we will give the
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