The photographic news

• Bandzählung: 29.1885
• Erscheinungsdatum: 1885
• 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-id1780948042-188500006
• PURL: http://digital.slub-dresden.de/id1780948042-18850000
• OAI: oai:de:slub-dresden:db:id-1780948042-18850000
• Sammlungen: LDP: Historische Bestände der Hochschule für Grafik und Buchkunst Leipzig
• Sammlungen: Fotografie
• Bemerkung: Seite I-II fehlen in der Vorlage. Paginierfehler: Seite 160 als Seite 144 gezählt.
• Strukturtyp: Band

• Digitalisat: SLUB Dresden
• Strukturtyp: Ausgabe

October 16, 1885.] after lengthened drying at 100° C., with dry carbonic acid, and the third with nitrogen, and sealed them. All three quickl!/ blackened in sunlight without the action being hindered by the absence of moisture or oxygen. The presence of iodine or hydriodic acid I could not detect, even after a week’s exposure to light, and blacken ing through and through of the mercurous iodide, not even by heating to 100° C., and examination of the vapour. Alcohol did not extract any mercuric iodide from the blackened powder. On the other hand, a goldleaf sus pended over the powder heated to 150° C. became whitened, so that the presence of free mercury may be deduced. If, then, in the decomposition of mercurous iodide by light—1. Oxygen and moisture play no part; 2. No iodide or hydriodic acid is set free (contrary to the statement of Artus); 3. A trace of metallic mercury is formed ; there can only be a separation into mercury and mercuric iodide. But since spirit of wine does not dissolve any mercuric iodide from the blackened mercurous compound, then mercurous-mercuric iodide (HgIg according to Boullay) must be formed. The chemical decomposition of insolated mercurous iodide will be represented thus:—- 8Hg 2 I 1 =2Hg+Hg 4 I 6 that is to say, it is changed into finely divided metallic mercury which produces the blackening and mercurous mercuric iodide. Blue and violet light are especially con cerned in producing this change as Chastaing has shown. PRACTICAL CHEMISTRY FOR PHOTOGRAPHERS BY RALPH W. ROBINSON. I.—Soda, and How to Test It. Soda—or, as it is more scientifically named, sodic carbonate —isasalt which is frequently used by photographers, and it may therefore be interesting to know some of the properties of this body, and how to test it. Carbonate of soda, as it is generally met with, is a white crystalline salt consisting of carbonate of soda and water in the proportion of 106 parts of actual carbonate of soda to 180 parts of water, or about 37 per cent, of actual carbonate of soda. This salt is what chemists call the neutral or disodic carbonate, and is represented by the formula Na:CO,,10H,O. The substance usually sold by druggists as carbonate of soda is of rather different composition. It is, in fact, the mono-sodic carbonate, commonly called the bi-carbonate, and is represented ths,NaHCOa In many cases it is only necessary to test the substance for the impurities which, from the process of its manufac ture, or other cause, we know are likely to be present, and which, if they be present, are known to be injurious for the purpose to which it is applied. The most probable impurities are chloride and sulphate of soda. To test for the chloride, put as much of the carbonate to he operated on into a large test tube as could be raised on a threepenny-piece. To this, add about one ounce of dis tilled water and pure nitric acid drop by drop till efferves cence ceases. Boil by holding the test-tube in the flame of a spirit lamp or Bunsen burner, taking care to move it continuously. Now add another drop or two of nitric acid and boil again to make sure of driving off all carbonic acid. Finally add a few drops of a weak solution of nitrate of silver. The presence of much chloride is marked by a flakey precipitate, but if the solution only turns slightly milky, the quantity of chloride present is very small indeed. To test for the sulphate, proceed, to commence with, in exactly the same way as in testing for the chloride ; but after the last boiling, instead of the addition of nitrate of silver, a few drops of a weak solution of chloride of barium should be added. This, in the event of the presence of a sulphate, will produce a white, finely-divided precipitate. In testing for the sulphate, care must be taken not to add too great an excess of nitric acid, but sufficient, with the help of heat, to drive off all the carbonic acid, is absolutely necessary in both cases. If, however, it is desired to estimate how much actual carbonate of soda is present in a given sample, more elaborate preparations must be made. The apparatus required consists of a fairly accurate chemical balance, a burette, tripod stand, Bunsen burner or spirit lamp, a porcelain evaporating basin about four inches in diameter, a small porcelain crucible, and a litre measuring flask ; also a glass vessel of about one quart capacity. In carrying out all the operations, great care must be taken that everything is perfectly clean. The first thing to be done is to prepare what is called a standard solution of sulphuric acid. For this purpose, we must have some alkali, the exact composition of which we know, and a given weight of which will neutralise a certain quantity of sulphuric acid. The best way to obtain this is to heat pure bi carbonate of soda (a salt which is easily obtained in a pure state) to redness in the crucible for a few hours. We shall thus obtain pure dry carbonate of soda, Na:CO3 Allow to cool, and weigh out 106 grains accurately in the balance. Transfer to the evaporating basin, and dissolve in two or three ounces of distilled water. Now place in a litre flask about three ounces (measured) of strong sulphuric acid, and add water to it till the flask is about three-quarters full. Mix thoroughly. A burette is now required. This consists of a glass tube about two feet long, drawn to a point at the bottom. To this is attached a short length of india- rubber tubing, ending in a glass tube drawn to a small point. A small brass spring clamp is placed on the rubber tube, so that the flow of the liquid from the small opening in the glass tube may be adjusted to a nicety. The long tube is divided throughout the greater part of its length into equal divisions, each of which is usually made of the capacity of one cubic centimeter. The burette is held in position over the basin, standing on the tripod, by a wooden clamp. It is now filled up to the top mark, and the solution of carbonate of soda brought to the boiling point, enough solution of litmus having been added to colour the liquid a pale blue. Acid is now carefully run from the burette until the litmus assumes a red colour, the liquid being thoroughly mixed all the time by means of stirring with a glass rod. Heat is again applied, which will drive off more carbonic acid, and the solution again will become alkaline, which is marked by the litmus becoming blue. Addition of more acid and boiling is repeated again and again, until a stage is reached when the litmus remains, after boiling, a shade between blue and red, the standard acid being added, drop by drop, towards the end. We must now ascertain, by reading off from the burette, how much acid has been used. Let us say, for example, that 106 grains ot soda, which is 1 grain-equivalent of carbonate of soda, has required 60 c. c. of the acid. W e must then take 600 c. c. of the solution, and make it up to 1,000 c. c. (1 litre) with water, and mix thoroughly. The standard solution is now finished, but must once more be tested with 106 grains of the pure sodic carbonate, which should require exactly 100 c. c. for neutralization. To test our sample of carbonate of soda, we weigh out 106 grains of the substance, previously powdered, and proceed as in