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The photographic news
- Bandzählung
- 24.1880
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- 1880
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- Bandzählung
- No. 1160, November 26, 1880
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Zeitschrift
The photographic news
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Band 24.1880
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- Register Index 631
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Band 24.1880
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NOvENBER 26, 1880."] THE PHOTOGRAPHIC NEWS. 571 PURITY OP CHEMICALS. As a rule, when photographic chemicals are obtained from reputable dealers, there is not much to complain of regarding their purity ; yet sometimes it happens that photographic failures are met with which may be due to impurity, and the difficulty is to find out where they are in fault. Lately, when preparing gelatine and other emul sions in an experimental way, we have experienced some such failures. In one gelatine emulsion we found that a distinct disinte gration of the gelatine and frilling of the film took place after boiling, though the images developed on it were beautifully bright and free from the slightest trace of veil. Another emulsion gave a fogged image, which apparently nothing could remedy. In both these cases we were able to track down the cause of our failure, and since some of our readers may meet with like misfortunes, we give them the benefit of our experience. In the first case, we examined the nitrate of silver, and found on testing it that it was distinctly acid. It was not a recrystallized sample, but sold as ordinary commercial nitrate of silver. By applying further tests, we found, as might have been expected, that the free acid was nitric acid which had not been entirely eliminated when the first crystallization had taken place. We may here mention that the effects of acids in gelatine are somewhat anoma lous. In very dilute solutions, when applied cold to solid gelatine, the first effect seems to be to render the gela tine insoluble, whilst heating a solution of gelatine with the same acid tends to cause a speedy disintegration of the substance. The cause of our first failure was now appa rent, so we effected a cure in our next batch of emulsion by adding a few drops of a solution of carbonate of soda to the nitrate of silver solution, till a very slight precipitate remained. The emulsion made with this rectified silver be haved in its normal and proper fashion, the small trace of carbonate having no apparent effect on the emulsion. The fog in our next emulsion was evidently not due to defective nitrate of silver, so we were forced to look to the other ingredient with which we formed the emulsion. This was bromide of potassium, and we found that it was slightly alkaline in reaction. It was a matter of some difficulty to decide in which way to treat this salt, since there was danger in mixing acid with it. We finally hit upon the following plan, which seems to answer perfectly. The solution of bro mide of potassium we treated with dilute nitric acid till there was no alkaline reaction, and to such a point that litmus paper was slightly reddened. To the solution of nitrate of silver with which wo were going to prepare our emulsion we added a trace of carbonate of soda, as in the first case, leaving a small precipitate, and this we added to the bulk of emulsion after the emulsification had taken place. By this means we had a perfectly neutral solution to boil, and hence no disintegration of the gelatine took place. We have also treated gelatine which had an alkaline reaction in a similar manner, with perfect success. Thus, after an emulsion was formed, a few drops of very dilute nitric acid were added to it. This rapidly neutralized all alkalinity, and the excess was taken up by the carbonate of silver, which, of course, we purposely added. We have not the slightest doubt that many failures occur either through the acidity of the silver nitrate or alkalinity of the bromide or gelatine employed. An im- provenent may, perhaps, be made on the above method, but at any rate it has proved satisfactory in our hands. For our own part, we should say that where there is no great chemical knowledge, it is always safe to add five drops of a 1 percent, solution of nitric acid in water to every 100 grains of bromide of potassium employed, and to add half a grain of carbonate of soda to every 100 grains of nitrate of silver used. The carbonate does no harm to the emulsion, and perfect neutrality is secured. In fused nitrate of silver, and in recrystallised samples as well, there is often a trace of nitrite ; where the bromide is in excess such is not hurtful, in fact, as has been pointed out, nitrate of potash, which would be formed by the double decomposition of the bromide of potassium and the nitrate of silver, is a sensitizer, and we may add, at all events, harmless. When ammonium bromide is used , alkalinity need not be feared, but rather acidity ; hence in this case', too, the addition of a trace of carbonate of silver would not be objectionable. o Hlotes. Last month there were but two photographs of Daguerre in the United Kingdom ; to-day, there are many thousands. It is our good fortune to announce that they will consti tute the frontispiece to the Year-Book of Photography for 1881. Photographers will be glad to learn of the safe return of Mr. W. J. A. Grant from his fourth voyage to the Polar regions. Mr. Grant’s arctic pictures, taken during former voyages, are well known at Pall Mall, and this time he brings back with him photographic sketches taken on board Mr. Leigh Smith’s yatch Eira, which, as our readers may remember, paid a visit to Franz Josef land during the summer. Mr. Grant says, “ fortunately, my troubles were only those of an ordinary character, for my dark room was generally warm. We never had it colder than 10 or 11 degrees of frost, and our chief difficulties were caused by repeated mists, and the ordinary attendant troubles of working on board ship. The first two years I used the collodio-bromide and the wet process, while during 1879 and 1880 I employed only gelatine.” Arctic photography, through the medium of Lieut. Chermside, R.E., Mr. Mitchell, R.N., and particularly Mr. Grant, has been more instrumental in showing us the true character of the Polar regions than all the volumes that have ever been written on the subject. Instead of calling up false sentiments of romance, the photographs show us cold inhospitable shores, black seas, and storm-swept reaches, that bring home most vividly a sense of weird desolation and of ice-bound solitude. Instantaneity in photographic processes has always been the goal of every pioneer in our art, and even the gelatine plates of to-day are considered capable of much improve ment; nay, we have had promises on several sides of most extraordinarily rapid films being forthcoming. It is but another instance of history repeating itself, for in 1840 we find Daguerre writing to Mr. Hunt upon the subject of an improved process by means of which “it shall be possible to fix the images of objects in motion—such as public ceremonies, market-places covered with people, cattle, &c.—the effect being instantaneous.” If Daguerre had had Mr. Mayland’s sketches of the busy Thames, or Mr. Williams’ animated pictures of Margate Sands, before him, he could not have written more enthusiastically.
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