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The photographic news
- Bandzählung
- 29.1885
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- 1885
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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. 1397, June 12, 1885
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The photographic news
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Band
Band 29.1885
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- Register Index III
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June 12, 1885.J THE PHOTOGRAPHIC NEWS. 379 * Analysis showed that the proportion of Ag to Cl was within one per cent, of the fractional amounts required for AgaCl, to the action of the spectrum, we get a coloured representation of that spectrum. Further, if a similar film be exposed to light, and the silver be reduced to the metallic state by a developer, and it is then immersed till it assumes a grey colour in ferric chloride or cupric chloride, the same colorific action takes place ; or, again, if a metallic silver plate be treated in the same way, we have a coloured spectrum. It is curious that such should be the case if the compounds are different. Guthrie's experiments show that a weighable quantity of the chlorides are converted into the coloured salt, but in any case the amount of chlorine liberated is small compared to what it should be theoretically. This acts against the metallic theory equally as against the sub-chloride theory. To my own mind there is no difficulty, however. It must be recollected that chlorine, as it is liberated from the chloride by light, has always sub-chloride beside it, and it is always more probable that it will combine with the sub-chloride and form chloride Ag,C1+ClAg= Ag.Clj, rather than escape. For this reason the difficulty of finding a large amount of liberated chlorine is not present in my mind ; in fact, I could not conceive well otherwise. The same argument applies to the formation of the sub-bromide from the bromide. Guthrie finds that the substance he gets in every case is insoluble in nitric acid, but that after treatment with ammonia the residue is soluble. This is exactly the behaviour of chemically-produced argentous chloride, and is a striking proof that the light-produced compound and this are identical. It may be of interest to show the members of the Society sub- chloride in bulk. My friend Dr. Hodgkinson. Professor of Chemistry at the Royal Military Academy at Woolwich, has kindly isolated for me the sub-chloride formed by the action of light on the chloride. This he has done by exposing chloride to light under a proper chlorine absorbent, and then treating the mass of chloride and sub-chloride with a solution of sodium chloride, which dissolves the chloride, leaving the sub-chloride behind.* Further, I show a sample of the sub-chloride prepared by direct chemical means. The argentic citrate was reduced in a current of hydrogen at the temperature of 100° C., and then acted upon by a solution of soluble chloride, with the result that we have a sub-chloride. Both the light-produced and chemically- produced Ag 2 U are unacted upon by nitric acid, but by treat ment with ammonia split up into AgCl and metallic silver. It is usually esaid, silver bromide be exposed to light in nitric acid, that no change takes place in it-is that it remains as silversbromide. Now.to test this 1 -PO-d snverbrTmMe:: light in nitric acid—in strong nitric acid-and subsequently treated the acid with silver nitrate. A faint precipitate immediately showed. To make this evidence of value, I asked Dr. Hodgkinson to make an experiment of the same nature, with the result that he obtained the same precipitate. If dilute nitric acid be used, the silver is dissolved from the sub-bromide. I’or the visible image, then, I can conceive of no difficulty in the reduction of the sub-salt. I have here no intention of entering into the theory of development, which has recently been put down to an electrical or voltaic action. I would only remark that development must always be dependent on such action, and that the mere fact of reduction of a compound to an elementary body entails such an action, as is very well known. Chemical action and voltaic action are interdependent one on the other; neither one can take place without the other, as I endeavoured to show in my Royal Institution lectures of last year. I now pass on to the photographic or developable image, and the general arguments apply whether it be assumed that metallic silver or the sub-haloid is produced by the prolonged action of light. In a small book which I published on emulsions some two years ago, I have assumed that the photographic image is formed by a sub-chloride and sub-bromide of silver, and I have recently been taken to task for not having proved the similarity between the visible and the invisible image. What I said was this : — “ Evidence of the most unmistakeablo character points to the chemical theory of the photographic image—evidence so strong and so well known that it would be out of place to record it here.” Now I find that I have made a mistake. It appears that it would not have been out of place, and that the evidence is not strong. That being the case, I will endeavour, in a sub sequent edition of the work in question, to supply the want, and meanwhile will give some of the evidence which is sufficiently strong to prove my assertion as to the truth of this theory. In continuation of the above quotation, I say, “ Briefly, we may say that the action of light seems to be to reduce it to a simpler type, which we may call the sub-haloid. Thus— Silver bromide gives Silver sub-bromide Bromine. Ag:Br, = Ag:Br + Br If any reader will not admit it, he should read all the various evidence that has been adduced since the time when Scheele first made his experiments with silver chloride, and we doubt if be can remain unconvinced—more particularly in regard to the visible image formed on the bromide and chloride.” Two writers have seized this last sentence, and with an ingenuity which does credit to their labours at the desk, have endeavoured to make it appear ridiculous—not to strengthen their case, which I hold to be bad, but to weaken mine. I would seriously re peat my invitation to them as to their course of reading, allow ing them to include the earlier literature from the time of Bacon, which one of them recommends for my perusal, and would still hope that they may profit by it. Continuing the same paragraph, I say, “ It is a pertinent question to put as to whether the visible and the invisible (or developable) image are of the same nature; which may be answered by another question—Can the line be drawn where the image is invisible ? If so, what is the boundary between the two ? If we admit the theory of the formation of the visible image, it seems hardly logical to deny a similar formation for the invisible or photo graphic image. It is quite possible that beings with more acute sight than ourselves might be able to see the image which we cannot. As we know, certain insects can hear sounds which do not affect our auditory nerves. Coloured particles are visible when put together en masse ; but if only a few coloured particles are present in amass of colourless particles,itis quite certain that they may remain undetected.” Now I have nothing to modify in these last sentences, and I invite the attention of the members present to the argument—Can it really be asserted that there is a boundary between the visible and invisible image ? Take a couple of plates and coat them with emulsion, and place them in a conducting solution, and connect the films with a very sensitive galvanometer. Allow light for a second to fall on one plate; there will be a deflection of the needle, showing that chemical action is taking place on it; expose for a time sufficiently long to discolour the film, and the same result occurs. There is chemical action, then, in both cases : what is the differ ence between the two ? The chemical theory of the photographic image is based upon the fact that where light of a proper kind acts on a molecule, the atoms are made to swing, however short be that exposure. If it be for a second, the average number of vibrations which light of a mean wave length makes is somewhere about 700 million millions (a goodly number), and which, if the blows by the ether be well timed, is sufficient to make an atom of chlorine or bromine fly off from the molecule ; or, in other words, sufficient to make it swing out of the sphere of molecular attraction, particularly if some other molecule is near which is ready to abstract it and incorporate it with its atoms, and so to form a new body. Let us stop and see whether such is the practical and not theoretical action. Take bromide of silver emulsion in collodion, which, when pure, is almost an absolute neutral substance, and what occurs where it is used dry ? It is sensitive to a certain extent, but add some bromide-loving pre servative to the film, and the sensitiveness is much increased. If the condition of the bromide under the action of light were merely change in the arrangement of the atoms, but not a chemical change, what would be the use of the bromine absorb ent? It ought to be positively detrimental. Let us take still another example of this. If a washed iodide of silver emulsion is prepared with excess of iodide, ami a plate be coated with it, and exposed, no image is developable. Dip half of such a plate into a solution of tannic acid or beer, again expose, and develop. The half that has an iodine absorbent will develop, the other half will not. Surely the beer or tannic acid is not capable of allowing the physical molecular attraction of the molecule to be altered, which must be the case, adopting the physical hypo thesis of the latent image. The chemical theory explains it immediately. One of the most remarkable proofs of the truth of the chemical theory of the formation of the photographic image is found in the explanation of some experiments which I made on the effect of the spectrum on mixtures of the haloid salts of silver, an account of which is published in the “ Proceed ings of the Royal Society.” It is there shown that if you have a mixture of pure iodide and bromide of silver and expose it to the spectrum, that at the place where the iodide alone would show the greatest action or development, the image has been destroyed by liberation of bromine from the bromide which is
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