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
• Rechtehinweis: Public Domain Mark 1.0
• URN: urn:nbn:de:bsz:14-db-id1780948042-186200003
• PURL: http://digital.slub-dresden.de/id1780948042-18620000
• OAI: oai:de:slub-dresden:db:id-1780948042-18620000
• Sammlungen: LDP: Historische Bestände der Hochschule für Grafik und Buchkunst Leipzig
• Sammlungen: Fotografie
• Bemerkung: Seite 1-72 fehlen in der Vorlage. Vorlagebedingter Textverlust.
• Strukturtyp: Band

• Digitalisat: SLUB Dresden
• Strukturtyp: Ausgabe

May 23,1862.] THE PHOTOGRAPHIC NEWS. 243 any such uses, trays made of other materials, such as gutta percha, ebonite, or wood well coated with rosin cement can be employed. Many of the remarks made concerning the various kinds of baths will also apply respecting trays of the same mate rial. Thus the glass trays are the least well made of all so far as workmanship goes, and from the amount of material used owing to their enormous thickness they are extremely expensive. Some glass trays, however, are much better made than others, having tolerably flat bottoms, parallel sides, and being provided with a lip ; but the greater number are clumsy, inconvenient, and ill-made utensils, by no means up to the mark, or as good as we have a right to expect for the price charged. Porcelain dishes are generally well made and suited to the work for which they are intended. They are constructed with a lip, and can be had of every size and shape. The same remarks apply equally to trays of gutta-percha, which, when made of the pure gum may be safely used for all purposes except developing. They are mostly very well made. These trays are especially useful for washing plates in, for those processes in which such an operation is neces sary : as the soft nature of gutta-percha permits the operator to shake about the plate roughly, which is the most efficient way to wash it, without danger of injuring it. It is indis- pensible to procure these trays as well as all other apparatus constructed of gutta-percha from a house where it is known that only articles manufactured of the pure gum are sup- elied, otherwise disappointment will inevitably ensue. tensils made of adulterated specimens of gutta-percha not only act in an injurious manner upon the various chemicals that the photographer is in the habit of using, but they will stand little wear and tear, and in a short time crack to pieces, proving the false economy of buying cheap trumpery. These observations are necessary, as there is a great deal of rubbish in the market. There are several respectable firms who are known to manufacture all articles for the use of photographers out of perfectly pure gutta-percha, and the purchaser will do well to supply himself from such a shop, even if the prices be a trifle dearer : though it is worthy of remark that the most worthless adulteration of gutta-percha is generally charged for at the same rate as the pure article. Ebonite trays have been lately introduced. Ebonite does tot appear to possess any' advantages over gutta-percha for photographic dishes; and it is nearly three times as costly, its Use, therefore, is not likely to become very general. The articles of this material are generally very well made. Trays for sensitizing positive paper, and washing, can be made of wood, as described for baths. They must be Purely fastened with screws, and properly coated with 1osin cement. With perfect efficiency they'combine the Advantage of extreme cheapness, and the photographer can "ake them himself. These dishes are probably the most Convenient kind to employ for washing paper prints after fixing, as they can be made large at little expense, and are Hot liable to fracture. (To be continued.') PHOTOGRAPHIC CHEMICALS: Their Manufacture, Adulterations, and Analysis. In giving an account of the principal chemical compounds which are of interest to photographers, we have been obliged to follow a certain order, which, whilst it has been very convenient in grouping together bodies of like action, and presenting similar substances to our readers in juxta position, has rendered it necessary for us to separate other compounds from those with which they have many points in common. We have conveniently classified the chemicals Wi th " hich If was our task to deal according to the acids which they contained, and under these headings—hydro c i one acid, nitric acid, sulphuric, &c., have given their mos important saline compounds. We have necessarily een obliged to exclude much valuable matter referring to the equally important group of bases, and it is our intention now to glance at the chemistry of these bodies, so far as they are likely to be of interest to our readers, and have not been before referred to. We have already noticed several salts of potash under the headings of the respective acids, we will now speak of the few remaining compounds of that base which are of photo graphic interest. Hydrate of potash, or caustic potash, as it is more fre quently called, is the oxide of the metal potassium in com bination with water: it is the type of the alkali class of bodies, being a highly caustic and energetic body, uniting with great violence to acids forming very stable salts. It is prepared commercially from the carbonate by mixing slaked lime with its hot solution. Carbonate of lime is formed, and the solution, which must be allowed to become clear by standing, is decanted off and rapily concentrated by boiling down in silver vessels. If the liquid be filtered it is liable to be contaminated with organic matter, as the strong caustic alkali has a powerful solvent action upon organic bodies. Filtering also gives the solution of caustic potash an opportunity of absorbing carbonic acid, which it does very greedily, from the air. Caustic potash is liable to con tain many impurities. The following is a list of the principal ones, with their means of detection :—Carbonate of lime, this is owing to the liquid not having been clear when decanted from the precipitated carbonate of lime. Oxide of iron may also be present from the evaporation being conducted in an iron instead of a silver dish. Both the carbonate of lime and the oxide of iron, together with other insoluble substances accidentally present, remain behind when the potash is dissolved in water. When the evapora tion is conducted in the air, peroxide of potassium is formed in small quantity towards the end of the operation. It is owing to the presence of this substance that the hydrate of potash gives off oxygen gas when dissolved in water. The ordinary caustic potash in sticks, which is used in surgery as a caustic, and which is prepared by evaporation in iron vessels, evolves a considerable quantity of oxygen upon solu tion. Graham and Davy state that the amount of oxygen evolved appears to be in direct ratio with the oxide of iron left behind when the potash is dissolved in water. Accord ing to Watts this connection between the oxide of iron and evolved potash suggests the supposition that ferrate of potash is formed. If the solution of carbonate of potash has not had sufficient lime added to it for the purpose of rendering it caustic, or if the mixture has not been suffi ciently boiled, carbonate of potash will remain in the solu tion ; this may be detected by adding an acid in excess to the diluted solution ; an effervescence will show the presence of carbonic acid. It may be removed by cautious addition of lime-water, and subsequent filtration and decantation. Sulphate of potash is also a very frequent contamination, owing to the presence of this impurity in the crude pearl- ash which is generally used for converting into caustic; it may be detected by supersaturating with hydrochloric acid and then adding chloride of barium; a white insoluble precipitate proves the presence of sulphuric acid. Chloride of potassium may likewise be present; it is readily detected by adding nitric acid in excess and then nitrate of silver ; the well-known white precipitate of chloride of silver is pro duced if the slightest trace of chlorine be present in the solution. Nitrate of potash is an impurity which is some times present; it may be detected by testing the solution for nitric acid according to the plan given in a previous number under the head of nitrates. Besides the above-mentioned impurities certain metallic oxides may be present. Alumina, which is a very common as well as a very annoying impurity in caustic potash, may be detected by adding an excess of hydrochloric acid, and then an excess of ammonia, and boil ing, a translucent white precipitate shows the presence of alumina. Other metallic oxides may be detected by super saturating with acetic acid, and then precipitating them with ammonia and sulphide of ammonium, a white, brown,