The photographic news

• Bandzählung: 12.1868
• Erscheinungsdatum: 1868
• 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-id1780948042-186800009
• PURL: http://digital.slub-dresden.de/id1780948042-18680000
• OAI: oai:de:slub-dresden:db:id-1780948042-18680000
• Sammlungen: LDP: Historische Bestände der Hochschule für Grafik und Buchkunst Leipzig
• Sammlungen: Fotografie
• Strukturtyp: Band

• Digitalisat: SLUB Dresden
• Strukturtyp: Ausgabe

April 24, 1868.] THE PHOTOGRAPHIC NEWS. 197 noyance, and to the detriment of the wash-stand, towels, &c. However, she survived it, and the next year saw me with a good dark room, which also was the “ locus in quo ” for another series of similar extent. Nothing need, I think, bo added to show the feasibility of the process ; and when its extreme sensitiveness is taken into consideration, I think if a dark room can bo got at within a few hours, it is the plan wheresoever it is im portant to secure rapidity of working. To him who, alas! is gone from amongst us in the prime of a useful life, and not to me, is due this practical application of Mr. Lyte’s process; but having had in my own working so large an experience of its value, both in the years I mention and many times since, I have thought it well to remind your readers of another aid to manipulation which gives with ordinary care results which will in every respect satisfy the most fastidious. I shall be happy to answer any questions as to the matter through the medium of your journal. at once form, with the oxide of silver, a little more nitrate, and leave the solution either 'perfectly neutral or very slightly acid. After iodising, then, Mr. McLachlan dilutes the bath. And here comes a point which requires both more elucida tion from the gentleman himself, and also, I think, more careful observation by others as well. We are told that after a certain time all the iodide will be thrown down, and that then it has no more power to dissolve iodide, and no further dilution can precipitate any more. I do not for a moment doubt these statements, but it is not clear yet to my mind quite how the matter comes about, whether' the bath is used before it throws down the iodide, or whether the action is quite spontaneous and quite independent upon any external circumstances. As we have not yet had this part of the subject fully explained, I would leave it now, as it is more my object to deal at the present moment with the question of the alkalinity than with the keeping properties of the bath. Should what I have stated prove correct, as to the bath being not alkaline, but having a trace of acid, or being per fectly neutral, it will be seen at once that the result, though gained in a very different way, will be much the same as that obtained by Mr. Thomas in his “ normal ” bath. He recommends the addition of oxide of silver to the bath— enough to render it turbid; then, after filtration, he adds just enough nitric acid to balance the alkaline reaction. Mr. McLachlan appears to me, on the other hand, to render the solution acid first, and then to balance this acid by a trace of oxide of silver. It may be—and I would be the last to say it is not so—a matter of great importance how a result is obtained. Many have, I think, noticed that a bath, espe cially under some conditions, works better after sunning. Thus it may prove that Mr. McLachlan’s mode of preparing a bath, albeit the result is the same as we have arrived at before, is much the best that has ever been found out. Now as to the collodion: of course, as Mr. McLachlan says, the collodion must assimilate with the bath, and it seems to me the result which is produced by adding caustic potassa to collodion in a very ripe red condition is merely to eliminate some of the iodine, and reduce the col lodion more nearly to the condition of newly mixed collodion, a condition which was some time ago considered as essential to rapid work, especially when the collodion contained rather more iodides than bromides, as, from my experience with Mawson’s, I believe is the case with that solution. Caustic potassa, when mixed with iodine, forms iodide of potassium, &c.; thus itseems tome that theadditionsuggested to be made to the collodion by Mr. McLachlan simply adds a little to the quantity of iodide of potassium, and eliminates the free iodine. Thus we seem to have a neutral collodion and neutral bath. These conditions would give naturally । the “ fog ” and other mess described by Mr. McLachlan . when using a developer made with new good iron, but when i old, the iron developer has more restraining power, and is 1 well known to work with more cleanness and certainty, ; especially under such conditions as I apprehend Mr. McLachlan has recommended. Now let us see the result of an addition of a solution of caustic potash. Caustic potash, when added to a neutral solution of nitrate of silver, forms at once a precipitate of oxide of silver, and the solution becomes faintly alkaline, not with caustic potassa, because the whole of the potassa is decomposed, but because the oxide of silver is to a slight extent soluble in water, and has, when so dissolved, a faintly- alkaline reaction. Mr. McLachlan, when his silver comes in again from the two-months’ sunning, finds that a trace of caustic potassa added to it does not produce any precipitate of oxide of silver, or that, if produced, that precipitate is at once redis solved. This seems to me to be easily accounted for. The trace of nitric acid I have mentioned as being probably formed in the solution when out in the sun would, of course, MR. McLACHLAN’S DISCOVERY. BY NELSON K. CIERRILL, Mx. McLachlan has fairly taken the photographic public by surprise. His discovery and the manner of its announce ment are alike so novel and unprecedented that societies, journalists, and reader's all seem quite taken aback; every one, of course, promises to experiment and report progress, but no one seems either fully to understand what has been already advanced, or to care to venture their opinion on the merits or demerits of Mr. McLachlan’s proposition. It seems even more difficult for the photographic public to take in the idea of an alkaline bath than that of a bath made with common water in the place of distilled. Of course, when an experi ment takes from one to three months to perform properly, according to the inventor’s own instructions, no definite answer can be given of its results before the lapse of that time. However this may be, a few words on what has been already advanced may not be out of place. Mr. McLachlan is going to read a further paper - on his discovery, and then, no doubt, he will throw more light upon the matter ; but as it will be nearly a month before this paper will be in the hands of the public, it may be as well to make a few re marks on what has been already advanced. In the first place, I do not think it is by any means so certain—at least, Mr. McLachlan has not, it seems to me, set it forth as certain—that he really does recommend the use of an alkaline bath at all. I will explain. It is generally supposed that nitrate of silver is not affected by light; we will let that pass, as it is enough for my purpose that Mr. Mr. McLachlan says it is affected. Now, the first and most obvious effect of light upon silver salts is to reduce them to the metallic state. All must have noticed how, if the slightest organic impurity gets into the bottle in which nitrate of silver is kept, in course of time the small particles adhering to the sides will become black by decomposition and reduction. Nitrate of silver, when reduced, liberates nitric acid and oxygen— Ag 0 NO S = Aq -f- 0 + NO S . The reason why nitrate of silver is not supposed to be reduced unless it is in the presence of some organic matter or other suitable substance is, that the equivalent of oxygen will not leave the silver unless some object be presented to it for which it has more affinity than for silver. It seems to me, however, very probable that in performing the McLachlan experiment some organic matter may be present, or, if not some organic matter, some trace of an inferior metal in the nitrate of silver, which, I think, would probably answer the same purpose. If, then, such is the case, an exposure to the sun for two or three months would certainly effect a slight reduction of silver, so that the solution which was neutral when put out in the sun may very probably be slightly acid when brought in again, from nitric acid liberated in the trifling reduction of silver mentioned above.