The photographic news

• Bandzählung: 35.1891
• Erscheinungsdatum: 1891
• 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-id1780948042-189100009
• PURL: http://digital.slub-dresden.de/id1780948042-18910000
• OAI: oai:de:slub-dresden:db:id-1780948042-18910000
• Sammlungen: LDP: Historische Bestände der Hochschule für Grafik und Buchkunst Leipzig
• Sammlungen: Fotografie
• Strukturtyp: Band

• Digitalisat: SLUB Dresden
• Strukturtyp: Ausgabe

September 11, 1891.] THE PHOTOGRAPHIC NEWS. 639 studied in detail ; their solubility in well-known reagents, such as ammonia, potassium cyanide, and sodium thiosulphate, should be made the subject of practical exercises, and the chemical changes undergone can be made readily intelligible to the student whose elementary training has reached the necessary stage of efficiency. It will add to the thoroughness of the instruction if the student is made to realise that the statement that the silver haloid is soluble or insoluble in such or such a reagent is by itself inadequate ; he must understand that solubility means the formation of a new compound which is more soluble than the original haloid. Thus, the absorption of ammonia by silver chloride can readily be shown by putting some of the dry haloid into a tube, weighing, and then passing dry ammonia gas till there is no further increase in weight. Then, again, silver chloride may be dissolved in strong am monia, and the solution allowed to stand till the crystals of the ammonio-silver chloride separate. The preparation of the soluble silver sodium thiosulphate, by the method of Lenz,* is a good practical exercise, apd the study of the salt will help to make clear why the silver haloids are dissolved by the thio sulphate. It is necessary to point out that by the same reagent threet distinct products may be obtained :— 1. Silver thiosulphate, Ag 2 S 2 O 3 , by adding a solution of sodium thiosulphate to a solution of a silver salt, keeping the latter in excess. This is a white insoluble salt, which soon darkens by the formation of the sulphide :—Ag:S,0, + H,O = Ag:S + H,$O4. 2. The insoluble double salt Ag2Na-(S,03),, formed by adding silver nitrate solution to a solution of sodium thiosulphate till a permanent precipitate is formed. The product is dark coloured, and probably contains sulphide ; it gradually becomes darker on standing, owing to decomposition with the production of sulphide. 3. The soluble double salt Ag2Na,(S,0.)a, formed by the action of excess of sodium thiosulphate upon the last salt, or by adding a silver salt to a strong solution of the thiosulphate, keeping the latter in excess. In the solid condition this is a white crystalline salt, readily soluble in water, and much less prone to decompose into sulphide than the preceding salt. Among other properties of the silver haloids to which atten tion may be directed are their decompositions by various haloid acids and salts. This information cannot but be of the great est use in practical photography. I have summarised the facts in the form of a table. + The haloid salts of the alkaline metals and of ammonia, especially when in concentrated solutions, dissolve more or less of the silver haloid, with or without decomposition. The silver haloid is thrown out again either in an unaltered state, or transformed into another haloid by decomposition on diluting the solution with water. The solubility of the silver haloids in solutions of other salts is a feature in their chemical history which the photographic chemist will find it useful to be put in possession of. Thus, these haloids are to some extent soluble, and especially the iodide, in strong solutions of silver nitrate. It must be pointed out that in all cases where a silver haloid is dissolved by another salt, a double salt is probably formed. The study of the forms of reduced silver will have prepared the way far taking into consideration the state of molecular aggregation of a substance as influencing its characters. The silver haloids should be dealt with from this point of view, both on account of the importance of bringing into prominence the factor of physical condition, and because of the possible practical bearing of the subject in connection with the pre paration of sensitive emulsions. According to the mode of preparation of the haloid, such important characters as solu- ■ A solution of silver nitrate is added, drop by drop, to a strong solution of sodium thiosulphate till a permanent precipitate (the insoluble double salt) just begins to appear. The solution is Altered, and alcohol added till the wnite crystalline (soluble) double salt separates out. t There are probably many more double thiosulphates of silver and sodium, and even this series of salts requires further investigation. Thus, Mr. 0. H. B athamley informs me that by mixing 4 AgNO, with 5 Na,S,0, and adding alcohol, he obtained a white precipitate in which the’ratio Ag: 8 was 1: 1961, The ratio calculated for AgNas,0, is 1:1 681. This salt is soluble and stable. By the action of 3 Na,s,0, on Ag Br and fractional crystallisation, ho has isolated two crops of' crystals correspond ing with the formula Ag,S,Oa, 3 Na,,,,,,). * See PuoToanAPMIc Nxws, p. 213. bility, reducibility, optical absorption and colour, and photo graphic sensitiveness, are capable of being influenced. Thus, the state of concentration of a solution of silver nitrate, from which the chloride is precipitated by hydrochloric acid, appears to influence the solubility of the chloride in the acid. It is possible that this is due to the different forms of the chloride under these conditions. By dropping a solution of silver nitrate into strong hydrochloric acid, a considerable quantity of the chloride is dissolved. The chloride, prepared in the ordi nary way, by precipitation from silver nitrate and a soluble chloride, after being washed and dried, is certainly not soluble in hydrochloric acid to the same extent. Here, again, it is possible that precipitation in the presence of strong hydrochloric acid gives no time for the molecular condensation of (AgCl) to (AgCI)nx, and that the former of these aggregates is more soluble than the latter. I have called attention to this feature in the chemical his tory of the silver haloids because, in the present condition of practical photography, no student should be allowed to neglect this all-important subject. How far Stas’s classification of the forms of silver bromide and chloride will stand the test of further investigation is at present doubtful. Some experi menters recognise only two modifications, and others three ; while Stas himself recognises four, viz. :— 1. Flocculent, white or yellow. Produced by the addition of a solution of a soluble bromide or hydrobromic acid to a solution of silver nitrate in the cold. Both solutions must be dilute (0'5 to 1 per cent.) ; if the silver is in excess, the bro mide is white ; if the soluble bromide is in excess, the precipi tate is yellow. 2. Pulverulent ; obtained from the preceding modification by brisk agitation with water. This form is produced more rapidly in neutral than in alkaline solutions. It is described as a yellowish-white powder, which, when dry, becomes intensely yellow on heating. 8. Granular ; produced by adding a very dilute boiling solu tion of ammonium bromide to a boiling solution of silver nitrate containing d per cent, of this salt. Obtained also by the action of boiling water on the preceding modifications, the first (flocculent) giving a dull yellowish white, and the second (pulverulent) giving a bright, yellowish-white powder. By prolonged boiling with water, the granular modification gradu ally becomes sub-divided, and, after several days’ boiling, forms a kind of milky emulsion, from which the bromide settles out very slowly. The precipitate which then subsides is pearly white, becoming intensely yellow on agitation with a strong solution of ammonium bromide. 4. Crystalline, or fused; obtained by fusing any of the other forms. This modification is never employed in photo graphic operations. Among other points in the chemical history of the silver haloids which are of photographic importance, the relative reducibility claims special notice. In the earlier part of his practical work, the student will have obtained metallic silver from the haloids by reduction, but he must now be made to realise that this reduction is more readily effected in the case of the chloride than the bromide, and more readily in the case of the latter than with the iodide. And first of all, in order that the true chemical significance of reduction may be made intelligible, let a simple demonstration be given showing that, by the action of reducing agents, such as ammonium pyro gallate, potassio-ferrous oxalate, &c., the halogen is actually withdrawn from the silver, and is to be found in the solution by the usual tests. Then, in order to show that the chloride is more reducible than the bromide or iodide, a solution of potassio-ferrous oxalate may be diluted till it becomes just too feeble to reduce the bromide. Some of the same solution will be found to reduce the chloride readily. Adopting the usual course, and passing from test-tubes to films, sheets of paper coated with the three silver haloids may be streaked with the same solution of ferrous oxalate or ammonium pyrogallate, when the order of reducibility will be shown by the fact that the chloride gives a darker stripe than the bromide, and the latter a darker stripe than the iodide. {To be continued.)