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The photographic news
- Bandzählung
- 35.1891
- Erscheinungsdatum
- 1891
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- Englisch
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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. 1723, September 11, 1891
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The photographic news
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- The photographic news
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638 THE PHOTOGRAPHIC NEWS. [SEPTEMIBER 11, 1891. PHOTOGRAPHIC CHEMISTRY* BY PROFESSOR li. JIELDOLA, F.R.S. The general considerations which have been offered apply to the special subject of photography with full force. A person may become an adept as an operator without knowing anything of physics oi chemistry ; there are thousands of photographers all over the country, who can manipulate a camera and develop and print pictures with admirable dexterity, who are in this position. If we adopt the narrow definition of technical in struction, we should appoint such experts in our colleges, and through them impart the art of taking pictures to thousands of others. But would our position as a photographing nation be improved by this process ? I venture to think not. We might be carrying out the ideas of certain technical educators by adopting this method, but I do not imagine that in the long run the subject itself would be much advanced ; our position in the scale of industry would not be materially raised by the wholesale manufacture of skilful operators. And so with all other branches of applied chemistry ; it is technologists whose knowledge is based on a broad foundation that are wanted for the improvement of our industries. These are the men which are raised in the technical high schools of the Continent, and whose training the continental industries have had the wisdom to avail themselves of. To become a photographic technologist, as distinguished from a photographer, it is desirable, therefore, that the student should have received instruction in the general elementary principles of physics and inorganic and organic chemistry. When thus prepared, he may begin to specialise bis studies, and the real technical training will be commenced. The application of chemistry to photography will naturally divide itself into two branches ; the chemistry of the materials used in the art, and the theory of the chemical changes occurring in photo graphic processes. With respect to the chemistry of photo graphic materials, time will not admit of any attempt to draw up a complete code of instruction. Nor is this necessary on the present occasion, for the requirements will be met by the simple statement that this branch of the subject should be an extension of the ordinary chemical training, with special refer ence to the preparation, properties, and reactions of the com pounds which the student is most likely to have to deal with in photography. 1 Since the compounds of silver are by far the most important of photographic chemicals, it will be advisable to commence with these. After the study of the ordinary qualitative reactions of this metal, the student should be well practised in the quantitative estimation, both gravimetrically and volu- metrically, so that the value of commercial silver nitrate may be ascertained with precision. The reducing action of fused sodium carbonate, of zinc and acid, and of alkaline solutions of glucoses on the silver haloids, can be made the basis of practical exercises in the recovery of silver from residues. It is desira ble also to point out, and to illustrate by experiment, that silver is displaced from the solution of its salts by the more electro-positive elements—hydrogen, copper, mercury, iron, zinc, lead, and so forth. It is important also to demonstrate that silver is more electro-positive than gold and platinum, and displaces these metals from solutions of their salts. In illustrating such points in the chemical history of silver as those referred to, and, in fact, in all practical work leading from ordinary chemistry to photographic chemistry, it will be found advantageous to adopt the general principle of per forming the experiments, whenever practicable, both in glass vessels and on films. This method is admirably adapted to lead the student from the general science to its special applica tion to his subject. Eor example, having shown that the salts of silver are reduced by such reducing agents as alkaline pyrogallol, ferrous sulphate, &c., in test tubes, and having allowed the experimenter to convince himself that the pre cipitate in these cases is really metallic silver, the production of a film of the metal may be shown by taking a sheet of paper coated with silver nitrate, and, when dry, painting stripes of ferrous sulphate solution on the coated surface. On washing * Abs’ract of Cantor Lecture?, continued from page 629. out the excess of nitrate from the unreduced portions, it will then be realised that the dark stripes consist of finely-divided silver on a paper surface. With these silver films many in structive demonstrations can be given. Thus, the fact that silver displaces gold and platinum from solutions of the salts of these metals may be shown by passing the paper through a bath of platinic or auric chloride, when the silver stripe undergoes a change of colour, indicating the replacement of that metal by gold or platinum. A comparison of the strip thus treated with a portion of the original strip—by treating both with nitric acid—clearly proves that the platinised stripe has lost its solu bility in that acid. The application of this principle to toning and intensification will naturally follow when photographic processes are being dealt with. While demonstrating the reducibility of silver salts by such reagents as ferrous salts, &c., it must be strongly urged that reduction of the silver salt is accompanied by a corresponding oxidation of the reducing agent. This fact will be made appar ent by the chemical equations ; but it is important that the student should verify it experimentally. Many ways of doing this will suggest themselves, but it will be sufficient if I give one appropriate example. Every student may be assumed to be familiar with the different behaviour of ferrous and ferric salts towards potassium ferrocyanide. Now, on adding a solu tion of ferrous sulphate to a solution of silver nitrate we get a precipitate of silver, indicating the reduction of a silver salt. That a simultaneous oxidation of the ferrous salt takes place is proved by filtering off the silver and adding ferrocyanide to the solution, when Prussian blue is at once formed. The precipitation of finely-divided silver may be made use of to illustrate some of the more obscure phenomena with which the photographic chemist is frequently confronted. It may be well at this stage to broach the idea that photographic chem istry, like all other branches of applied chemistry, does not begin and end with a series of reactions which can be written down in the form of equations. This mode of treating the subject may be academic, but it is not technical. It is believed by many experimenters that silver is capable of existing in several different conditions of physical aggregation according to the manner in which it is precipitated from its solutions. Thus the deposit obtained by adding ferrous sulphate to a solution of silver nitrate is grey; the product obtained by reducing silver solutions with organic ferrous salts is darker in colour.* It is possible that the difference of colour in these cases may be due, as suggested, to the differences of molecular aggregation. The rate of reduction may have something to do with it, ami the well-known coloured forms of precipitated gold, studied by Earaday, may be referred to in this connection. But, on the other hand, there is another possibility which must be borne in mind, and well emphasised in dealing with photographic chemistry. I allude to the tendency which silver and its compounds possess, in common with many other metals, of bringing down and retaining traces of foreign substances, in whose presence the metal or its compounds may be precipitated. This kind of combination is not sufficiently recognised by ortho dox chemistry, because it does not take place in definite pro portions, but there are many branches of applied chemistry where this so-called molecular combination plays a very import ant part. The practical outcome of these considerations is that the student of photographic chemistry cannot be too early prepared for the occurrence of indefinite combination, and he must not be allowed to suppose that, because a distinct formula cannot be ascribed to such compounds, that they are outside the pale of chemical science. I have thought it necessary to utter this caution, because in the present state of knowledge we must not commit ourselves to dogmatic assertions about allotropic modifications of silver. It may be that the different colours of reduced silver are due to the retention of “ traces ” of some foreign substance. The haloid salts of silver will, of course, demand a consider able share of attention from the photographic chemist. The preparation and properties of these compounds should be ♦ Pyrogallol will reduce silver nitrate in neutral or slightly acid solu tions. It is not necessary to add alkali to show this reduction; the solution may be distinctly acid with acetic acid.
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