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July 20, 1883.] THE PHOTOGRAPHIC NEWS. 453 What that explanation is I dare not put forward with confidence, since there is much to be taken into account. The matter is one of importance, in more ways than one. Theoretically there can be no doubt of its significance, and practically it may prove to be of value in emulsion-making. At present I am inclined to the view that the shearing force, which often reaches half a ton to the inch, is so great, that it breaks up a particle of silver salt much in the same way light does, and that mere pressure makes the particles more compact, and 1herefore less sensitive. When more than this pressure is applied, we have a further breaking down, which causes the broken-down particle to be capable of development. I have not been able to pursue these experiments further, but I think they show that a variety of phenomena may be explained by the effect of pressure. I have not finished my investigations, and must reserve any definite opinion. As this is the last meeting of the session, I thought it well not to with hold what I had already obtained. GUN-COTTON AND PYROXYLINE : THE QUESTION OF PERMANENCE. BY JOHN SPILLER, F.C.s.* In overhauling my collection of chemical preparations lately, I came upen a small box containing a number of specimens of gun cotton separately wrapped in paper, and labelled both as to quality and date. As their history was known to me, and mode of preparation recorded, I naturally anticipated much interest in their examination after a lapse of twenty-one years ; for although Sir Frederick Abel has treated of this subject almost exhaustively in his several communications to the Royal Society, it is seldom that a chemist gets the chance of experimenting for himself upon such ancient samples as those herein referred to ; in fact, it is not usual to keep these articles in store for so long a period. The specimens in question, six in number, were labelled as follows:— I. Gun-cotton, highly explosive and insoluble. April 10th, 1862. II. Gun-cotton treated with chlorate of potash. (Same date). III. Collodion cotton, perfectly soluble. 1862. Hadow’s formula. IV. Gun-cotton and paper. (No date, but certainly very old). V. Dr. Liesegang's papyroxyline. April, 1869. VI- Mr. Daniel Spill’s low-nitrated cotton pulp, as used for the manufacture of xylonite. 1870. Besides these, I found another sample of highly explosive gun cotton, withoutlabel or date, contained in a wide-mouthed bottle, which I remember as having been in my possession for a great many years. This bottle was closed with a cork of rather coarse texture, and therefore not hermetically sealed from contact with air. And, lastly, I have still by me some specimens of Abel’s gun-cotton yarn and pulp, which Mr. E. 0. Brown, of Wool wich Arsenal, gave me in April, 1869, after delivering his lecture (at the Conduit Street Gallery), “ On the manufacture, pro perties, and military applications of gun-cotton.” In regard to these last-named specimens, I may remark that they are not photographic qualities, but more highly nitrated compounds, suitable for blasting and military purposes, and, beyond becoming very faintly acid by keeping, have not sensibly changed : they are all still very highly explosive. Speaking now of the contents of the box, I found, on opening it, that the wrapping-papers and string were very tender, and that the ultramarine in those papers, which had once been blue, was almost entirely discharged or bleached, as though by acid vapours, and, on testing with blue litmus, the wrappers and their contents were now in most cases distinctly acid. Dr. Liesegang’s papyroxyline and Hadow’s soluble cotton were, how ever, very well preserved, and the gun-cotton impregnated with chlorate of potash had not appreciably suffered, being still very highly explosive. Spill’s preparations, perhaps from not having been so perfectly washed, had completely rotted the papers, so that they fell to pieces on the slightest touch. On firing portions of the two specimens, they burn now (as originally) with a carbonaceous residue; one, the finer sort, burning off much more quickly than the other. Both of these were found to con tain free sulphuric acid, but no oxalic acid, as a product of their decomposition. The mode of manufacture was described in vol. . * Read before the Photographic Society of Great Britain. xv., page 42, of the Photographic Journal-, and these specimens were shown at the meeting of the Society in December, 1870. With respect to No. 1 specimen—“gun-cotton highly explo sive and insoluble’’—which was contained in a blue-wove official envelope, the paper was bleached in parts, denoting some slight escape of acid fumes ; but the material remains apparently as explosive as ever, is still quite insoluble in ether and alcohol, and in all respects seems practically unaffected at the end of a period of twenty-one years. This result 1 believe to be partly attributable to its position at the top of the box, where it found enough air to allow of the easy escape of acid vapours by diffusion. No oxalic acid could be detected on drenching this sample of gun-cotton with pure water J only a minute trace of soluble sulphate being found in the aqueous solution. Specimen No. 2—“gun-cotton treated with chlorate of potash”—was not acid, and, as already stated, was apparently as explosive as ever. It likewise had withstood the action of time for twenty-one years. Soluble cotton, No. 3, made by Hadow’s formula, is still perfectly soluble in a mixture of ether and alcohol, giving a good tough film, and making excellent photographic collodion. Neither in this nor in Dr. Liesegang’s papyroxyline can I find the slightest evidence of change, although prepared as long ago as twenty-one and fourteen years respectively. From these observations I think it may be fairly inferred, that well-washed gun-cotton or pyroxyline can be preserved indefinitely, if screened from sunshine, and left open to moder ate access of air; but, on the other hand, we know, from Prof. Abel’s experiments, that if heated beyond the normal tempera ture of the air, or enclosed in perfectly air-tight vessels with ever so small a trace of free acid, there is danger of a destructive decomposition being set up, with the chance, in extreme cases, of a spontaneous explosion. No one doubts the permanence of collodion films (apart from the question of cracked varnish) when once properly fixed and washed as in photographic negatives. A NEW UNIT OF LIGHT IN CONNECIION WITH SENSITOMETRY. BY JAMES B. SPURGE.* With the introduction of prepared sensitive films, means for ascertaining their photographic value has become an absolute necessity, as it is of the highest importance for an operator, wishing to secure the series of tints focussed upon his screen, to know the capability of the films he intends to employ, both as regards the time necessary to register the same, and whether their relationship is likely to ba increased or diminished by the process. With the object of finding out means of obtaining this much-desired result, 1 commenced a series of experiments, the outcome of which I intend, with your indulgence, to place before you this evening. Now the first thing I had to decide was the best source of illumination to employ. A standard candle appeared at first sight to be all that could be desired, until experience showed it required great care and time to adjust itself, and, after all, did not give satisfactory results. M. Giroud, of Paris, in an investigation for the purpose of finding a standard of fixed intensity, gives the following experiments upon two standard candles, marked A and B, and one paraffin ditto :— Standard Candles. Paraffin A B Candle. Maximum 1-113 ... 1-027 ... 1’288 Minimum 0’716 ... 0-765 ... 0'795 Rise of value per cent, above minimum 55 ... 34 ... 62 Mean of sixty minutes’ observations 0-875 ... 0’887 ... 1’041 Number of times the mean was ob ¬ served 4 ... 2 ... 6 You observe they show a variation above their minimum value of 55, 34, and 62 per cent, respectively, which is quite in accordance with my experience. The result of his investigations led him to adopt as a standard the flame produced by gas issuing from a vertical hole 1 mm. in diameter, and regulated to a con stant length of 67.5 mm., for he found that small differences in the diameter of the hole did not appreciably influence length of flame, the intensity being affected after the rate of 0’015 of this intensity per 0’05 variation in the size of the hole, whilst the quality of the gas may vary as much as 30 per cent. The illuminating power of this standard is 1o of a mean Carcel