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June 19, 1885.j THE PHOTOGRAPHIC NEWS. 391 • Continued from page 380, before, then the plate be allowed to cool, and after such cooling the plate be exposed to light, no trace of the iron shape is visible ; the plate returns to its normal condition. In the one case we have the amplitude of the vibrations of the atoms of the molecules (which, when at any temperature pre sumably above the absolute zero, are always vibrating) increased by the application of the hot iron, but not sufficiently to throw off the atoms. Where light of proper colour impinges on these atoms, which are already swinging with increased amplitude, they are more readily swung off than when it has the whole of the work to perform upon them, and as a result we have the image of the iron shown by the increased number of molecules which have been de-atomized. In the other case the plate is heated, and the increased amplitude of vibration must still be there, as in the other, case, and, accoiding to the vibration theory, should continue ; but the fact is that it does nothing of the kind. The atoms resume their normal swing when the plate is cold. It is for those who hold the physical theory of the formation of the photographic image to explain why the in creased amplitude due to the heating dies away, whilst that due to light does not. The chemical theory, then, fully accounts for these experiments, which, as far as. I am aware, no other .theory does. Let us take another experiment. Take a bromide collodion film, and expose it in a solution of peroxide of hydrogen, a solu tion of permanganate of potash, or hydrochloric acid, and attempt to develop an image. The result will be failure. Expose another plate in any substance that will absorb bromine, such as sodium sulphite, potassium nitrite, and an image is at once obtained. There is no reason for these opposite results accord ing to the physical theory of the photographic image, but there is according to the chemical theory. Ln the one case the deve loping power of the image is destroyed by oxidizing agents as fast as formed, in the other the liberated bromine is taken up at once, and the image is preserved. A certain experiment by Mr. Carey Lea has been quoted against the chemical theory. Ho took pure iodide of silver on a clean plate, and on it he could at first develop an image after exposure to light, and that a similarly prepared iodide plate which had received also an impression of light faded away and refused to develop. The paper already quoted on the destruction of the photographic image disposes of this, and is certainly no crucial experiment as to the vibration theory. I hardly know that I need continue these ordinary every-day proofs of the truth of the chemical theory. A simple molecular change of form will not do it. That a change in molecular form is possible we know. In a gelatine plate the range of spectrum sensitiveness is far greater than in a collodion emulsion of the ordinary type ; whilst in the special green form of collodion emulsion we have a still different molecular arrange ment, in which the atoms are so placed that they answer to the swing of the dark rays below the red. Yet these molecules are still bromide of silver, and not capable of development till they have received the well-timed shocks of light. Sensitiveness means placing the atoms which compose the molecule in the state of most indifferent equilibrium possible, so that a small shock will set free the halogen. One word more. When a substance which is in a state of tottering equilibrium is made to take up a position of stable equilibrium, energy is given out (thus a gun-cotton molecule when detonated gives out energy) ; but the energy it gives out is exactly that which it would take to set the atoms from the position of stable to tottering equilibrium. So in preparing an excessively sensitive salt of silver, energy has to be applied to set it in this equilibrium, hence we arrive at the measures tiken in a gelatine emulsion to change the molecular form by applying heat and so on. I should be glad of hearing of any single experiment which : can be explained by the vibration theory, and which cannot be as ■ easily, and in all probability be better, explained by the chemical j theory. On the other hand, I think I have given experiments i which can alone be explained on the latter theory. Let me say I that the chemical theory is more than a mere working hypo- , thesis. Experiment after experiment proves its truth, and there is not, so far, a single experiment, to ray knowledge, which dis proves it. that such vibrations subsidle rapidly. If we take a hot iron and press it to the back ot a gelatine plate, and expose it to light whilst hot, and then, after cooling, develop the plate, we get an image of the iron shown by increased blackening of those parts which were in contact with the glass heated by the iron, If, however, the hot iron be applied to the back of the plate as THE PHOTOGRAPHIC IMAGE. "I captain Abney, R.E.,F.R,S.* ONEformf physicaltheory of the Photographic image is that the light sets uP.yibrations, and that whilst no chemical change is wrought, yet that it is the increased vibrations which give the developing power, and that when the vibrations cease the image is non-existent. This is a theory which to me seems like one to be found in a story by Jules Verne, and to be one of those ideas which have to be relegated to the same limbo as perpetual motion. We hear of a gelatine plate being exposed, and developed with unimpaired vigour after a lapse of twelve or twenty months. I myself have kept gelatine plates a year, and developed some of them every three months. These plates were exposed behind a sensitometer, and kept to ascertain if there were any fading of the image. Plates which gave 22 on the sensitometer at first, after a lapse of a year from the time of exposure gave an image showing 21. Hence we have to conclude that the vibrations commenced at a certain time kept of the same amplitude, or nearly so, for the space of a year. The question is, can that be so ? If a collodion plate is treated in the same way, the image ■will die out much more rapidly. A couple of months is sufficient in most cases to obliterate nearly every trace of an image, and yet it is the same material acted upon in both cases. Why should it act differently in the two cases ? The answer to this is somewhat hard, I imagine, on the vibration hypothesis ; but it is perfectly easy on the chemical theory of the photographic image. I scarcely need repeat papers which have appeared in the Journal on the destruction of the photographic image, in which I have shown the cause of such destruction on the chemical theory. Exposure to the atmosphere, or to a substance which can oxidize the image, has been shown to destroy the developing capacity of the image, and the difference of the exposures of the bromide in the gelatine and collodion plates fully explains the reason why the destruction is more rapid in the one case than the other. Again, we can absolutely show that if increased amplitude of vibration be given to the atoms of a bromide plate, the old process, and herein appears to lie a strong element of permanence in the prints. At any rate, there is a marked absence of a tendency to the brown staining and discolouration of the ordinary silver prints. The early disappearance from the prints of the sweet but strongly-metallic taste which albumen prints retain, even after comparatively long washing, is also very marked. It is necessary to evenly and thoroughly wash the prints from the alum before fixing, otherwise the toning will be patchy. The toning should not be carried too far, as the prints dry off much bluer than they appear while wet. The use of a second alum bath, after fixing and washing in several changes of water, appears to slightly improve the tones, and may be useful in getting rid of the last traces of hypo more quickly. After a few more changes of water, the prints may be taken out and dried. It is essential to give the prints a strongly glazed surface, otherwise they dry very dull, and lose all the beauty they have while wet. As 1 did not succeed in stripping the prints from glasses rubbed with French chalk, I oiled the glasses well, and then rubbed them over with a cloth so as to leave a thin even coating of oil. This answered per fectly. I have found no difficulty in keeping the whites clear, if the prints are well washed before toning and fixing, and are not over-developed. A great advantage the paper will have for Indian amateurs is that it will keep good and fit for use at any time, whereas the ordinary ready- sensitized albumenized paper cannot be kept much more than two or three months without spoiling. I have not found the nse of ice necessary, though the gelatine surface gets very tender in this hot weather, and is liable to be rubbed and damaged. The prints make excellent transparencies, and show far more detail in the shadows than is visible by reflected light.
