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The photographic news
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- 27.1883
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- No. 1270, January 5, 1883
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JANUARY 5, 1883.1 THE PHOTOGRAPHIC NEWS. 3 change the colour of the metal from a dirty green into a very pale lemon—almost invisible. The like effect occur* by the use of arsenic (arsenic trioxide), only that it is brought about in another way—viz , that when th* batch is heated the metallic arsenic is driven off and goes up the slightest variation in the gelatine is enough to produce a great difference in the times taken for decomposition to commence, and even the weather exerts a great influence. We may put the whole affair in a nutshell as follows. There are two causes of fog in ordinary gelatine emul sion methods; one is due to modification of the silver haloid, the other to modification of the gelatine. The first is under control, the latter is not. If we eliminate the latter we overcome the greatest factor of uncertainty in gelatine work, and may concentrate our energies on the bromide of silver alone, pushing our sensitizing processes as far as we like, totally regardless of what may be the re sult on the gelatine. That this is not merely a theoretical consideration, but is borne out by practice, there is considerable evidence to show. We hear of applications of heat or of ammonia for lengths of time which recently would have been con sidered absurd, and yet the results appear to be satisfac tory. In the experiments on a precipitation process which we recently detailed, we gained a sensitiveness which was much above what we have usually been able to gain. We have, as we said, just received an example of precipitated bromide of silver which, on mixing with gelatine, gave a very rapid plate. We gather from Dr. Eder’s paper that by the Plener even after an emulsion is boiled for hours, and treated for days with ammonia, and after the silver bromide has for the most part subsided, there is a certain small quantity kept in suspension in the water, and that this is red by transmitted light. We recently expressed an opinion that in every emulsion there is probably a certain quantity of the silver bromide which might with advantage be sepa rated from an emulsion, and thrown amongst residues. Dr. Eder's remarks confirm this idea. He shows that, in an emulsion which approached the maximum of sensi tiveness, the coarse-grained bromide separated and gave , fog, while the fine-grained bromide was perfectly free from this defect, and was very sensitive. It is quite evident that the coarse-grained bromide would be better in the residue tub than in the emulsion. Even if, when mixed with the fine-grained bromide in an emulsion, the effect is not actual fog, an unnecessarily weak developer must be used, and this is equivalent to a reduction of sensi tiveness. Plener’s process is undoubtedly the one by which the separation may most expeditiously be performed ; and, indeed, the fact that the same result maybe brought about in the ordinary precipitation processes did not at first strike us ; but we now perceive that there need be no difficulty in allowing a certain time to elapse for the precipitation of the coarser bromide, when the supernatant fluid containing the finer bromide may be poured into another vessel, there to precipitate the latter. GLASS. Third Article. In our first and second articles we have shown that sand and alkali (which are the chief ingredients in glass) should be very pure in order to produce a colourless glass, termed metal. As a matter of fact, sand and alkali, if thrown into a crucible and submitted to sufficient heat, will melt or fuse together, such action being termed vitrification, and the result is said to be a silicate. The temperature at which fusion takes place varies very widely, according to the composition of the mixture ; and sand and an alkali alone require a very high temperature to effect their vitrification. If, however, we add chalk, limestone, or oxides of certain metals, the fusing point is much lowered ; there fore glass, as made at the present day, contains various silicates, and may consist of silicates' of soda, potash, calcium, aluminium, lead, &c., such mixtures having been determined by repeated trials resulting in successful productions. Glass made from sand and an alkali alone is extremely durable—very hard ; but requires such a heat, with corres ponding consumption of fuel, that the discovery that the formation of a mixture of silicates not only produces a better glass, but was more easily worked, and with the expenditure of less fuel, may be said to have been the real starting-point of glass manufacture as carried on now. As already pointed out, the elimination of the green colour due to iron and other impurities in the sand and alkali is of the utmost importance, but commercially is impossible, so that recourse has to be sought in means to counteract or neutralize the evil effects of the presence of iron or other impurities in the component of glass-making materials. Such counteracters or neutralizers are found in manganese and arsenic. When dioxide of manganese becomes an ingredient in the batch the action of the heat is to convert the ferrous oxide into ferric oxide, and to chimney, leaving its triple dose of oxygen to combine with the iron present in the other ingredients to form forrie process be used, if the very highest sensitiveness be desired, it becomes a nice question whether the desired end be gained, or, on the other hand, fog be produced. Now let us examine how this condition of fog is brought about. I We know that, whatever means we use to gain sensitive ness, the increase in sensitiveness is accompanied by an increase in the size of the particles of bromide of silver ; and it would appear that when these particles exceed a certain size, then flat images are the result ; when they get somewhat larger still, fog is the result. Here we have one cause of fog, which results, apparently, entirely from a modification of the silver haloid. Another fact, equally well known, is, that any of the ordinary methods used to increase sensitiveness tend to the decomposition of the gelatine, and here, as Dr. Eder points out, is a cause of fog entirely independent of the silver salt. To eliminate this source of fog must of necessity be a benefit, as it leaves us only one cause of the evil to fight against; but, when we consider that the modification caused by the decomposition of the gelatine is most uncertain, and varies with small circumstances over which we have no control, whereas the modification of the bro mide is tolerably uniform if the major conditions be kept uniform, we shall see that, in eliminating the former cause of fog, we gain much more than half the battle. If the same method be used, and the proportions of the different materials be kept the same, it will be found that the increase of the size of the particles of the silver haloid reaches the same point in a given period time after time, and this even if the nature of the gelatine used vary slightly—say not more than is usual in different samples of gelatine of the same make. It is far otherwise with the decomposition of the gelatine. This varies with circum stances so small that we cannot control them. The process a rapidity is gained, apparently with certainty, which even considerably exceeds what Mr. Donald or our selves have reached, and in all these cases the usual ac companiments of excessive sensitiveness—fogs of all colours and thin images—are conspicuous by their absence. 1 here is yet another feature of precipitation processes which is worthy of consideration, and which is strongly brought out in Dr. Eder’s paper. We refer to the possi bility of separating the bromide of silver in emulsions with various degrees of fineness. It is probable that in every emulsion there is a certain quantity of bromide of silver of every degree of fineness, from the finest possible, to that which is so coarse as to be on the margin of fog. Indeed this is proved by the fact cited by Mr. Burton at the last Parent Society’s meeting. He pointed out that, I oxide, which, as just stated, is comparativley eolewless.
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