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moisture in the atmosphere produces an injurious effect. To this is due the annoying appearance in lenses, occasion ally even by our best makers, known as sweating, caused by the atmospheric moisture separating out the potash and soda, and standing on the surface in a fine dew consisting of globules of alkaline solution. The silica and lime are left behind on the surface of the glass, and in time this decomposition causes the surface to exhibit prismatic colours. Sometimes this superficial decomposition is scarcely visible, but on warming the glass, numerous fine scales peel off, and leave the surface dull and opaque. The pearly stratum with which specimens of antique glass found buried in the earth are covered consists almost wholly of silica. Most acids attack glass, the action varying with the strength of the acid, and also with the quality of the glass— alkali being dissolved from it. Solutions of potash and soda also decompose glass, dissolving out the silica with greater ease in proportion to their temperature and degree of concentration. In some common varieties of glass, even at ordinary temperatures, the action is so strong that the glass sometimes cracks to pieces. When lead is present in glass, it tarnishes very readily by exposure to sulphuretted- hydrogen. The dense glass used for prisms and lenses requires preserving away from this gas with great care, as it is acted on and darkened on the surface even by the ordinary atmosphere of a dwelling-house. ON THE INFLUENCE OF SULPHATE OF SILVER IN THE SENSITIZING NEGATIVE BATH. Sulphate of silver is often contained in nitrate of silver made with nitric acid containing sulphuric acid. The solution of such nitrate, therefore, contains a small quantity of sulphate of silver which imparts to the bath the property of furnishing films more sensitive than if the nitrate em ployer! had been purer ; it gives quicker and more sensitive negatives. If the quantity of sulphuric acid be too great, the sensibility remains, but the negative is spoiled by short lines resembling the letters of the Turkish alphabet and the characters used in stenography. These lines become fully visible only upon developing, and are caused by the absence of iodide of silver upon their path. If the nitrate contains a greater quantity, it remains on the filter, for the sulphate of silver is but slightly soluble. Failures due to the presence of sulphate in the nitrate are very frequent with the nitrate obtained from the residues of paper, filters, &c., treated as they are too frequently directed in books on photography. The mode of treating then indicated by many writers must necessarily furnish a nitrate as rich as possible in sulphate of silver ; to be convinced of this, it is only necessary to examine the prescriptions. The papers are burned and the ashes placed in contact with liver of sulphur; sulphide and sulphate of silver are formed. The residue washed, decanted, then dried, is roasted to transform the sulphide into the metal silver To the last residuum nitric acid is added, then diluted with water, to be filtered. What passes through the filter is the nitrate in solution accompanied with sulphate which is found in exact propor tion to the water added. This water evaporated, generally gives a nitrate contain ing from 5 to 10 per cent, of sulphate of silver; a nitrate quite unsuitable for negative sensitizing baths. It is supposed that the sulphate may be separated by crystallizing the nitrate, but it is not; the sulphate crystal lizes also, or is so completely in contact with the crystals of nitrate, that there always remains sufficient to cause the lines described above. Method of treating Spoiled Nitrate Baths. Baths are often put aside upon pretexts more or less well- grounded, and every one knows that a new bath is always better than a doubtful one. In order to obtain new baths without great loss, the fol lowing process will furnish excellent nitrate without much difficulty: — The spoiled bath is filtered into a flat dish, in which a piece of pure copper is placed. After a contact of two or three hours (according to the quantity of the bath and the size of the piece of copper), all the nitrate of silver con tained in the bath is converted into silver in the form of powder. The contents of the dish are filtered, then the silver powder is washed with several quarts of water poured upon the filter; this powder being dried, it will serve to make nitrate, taking care to keep it in a state of fusion for several minutes, in order to convert any particles of copper present into insoluble oxide. The silver is in a very finely divided state; it is best to pour the nitric acid upon it in small quantities only, so that its action, which is sudden, may not cause particles of silver to be thrown out of the crucible.—Bulletin Beige. —• GENERAL STUDY OF PHOTOGRAPHIC POSITIVES. BY MM. DAVANXE AND GIRARD. CHAPTER V., ON FIXING (CONTINUED). 2 vi.—On the Employment of Sulpho-cyanide ofAmmonwm. Beside the three substances previously proposed for fixing positives, cyanide of potassium, caustic ammonia, and hypo sulphite of soda, another has been recently introduced—a salt hitherto rare but of which many circumstances combine to render the manufacture abundant and cheap. This salt, which M. Meynier, chemist, of St. Barnabas, near Marseilles, has proposed and described the qualities, is the sulpho cyanide of ammonium, or sulpho-cyanhydrate of ammonium. The examination of the properties which can recommend this substance in an economical point of view, necessarily find a place in our “ General Study of Photographic Posi tives.” In order to render this study profitable, and to allow a comparison to be made of the qualities and defects of the salt in question with those of other fixing agents, we follow in this part of our task, the same method as with hyposulphite of soda, ammonia, &c. Properties of the Sulpho-cyanide of Ammonium.—This is a white salt, crystallizable, decomposable at high temperatures, and extremely soluble in water and in alcohol. Considered in reference to the decompositions it can undergo by the moist way, this salt exhibits very remarkable conditions of stabi lity in a fixing point of view. For the substances which photographers have most to fear, with regard to the fading of their pictures, are, as we have frequently observed, free sulphur, sulphuretted hydrogen, or the compounds susceptible of producing them. Now, chemistry informs us, and we have verified it by new researches, that the reducing agents alone—nascent hydro gen, acidulated protosulphate of iron, &c., are susceptible of disengaging, in the state of hydrosulphuric acid, the sulphur of the sulpho-cyanides, but none of these agents intervene in positive photography. Doubtless, the sulpho-cyanides are not stable ; left for a considerable time in contact with the atmosphere, they deposit a yellow powder, sulpho-cyanogen, the formula of which is not yet fully established, but which we may con sider as a sulphide of cyanogen with sulphur in excess. On the other hand, the alkaline sulpho-cyanides, treated with the concentrated mineral acids, or by chlorine, leave a de posit, either of sulpho-cyanogen in powder, or persulpho- cyanhydric acid in fine yellow needles. But the compounds thus obtained are themselves exceedingly stable, and do not, under the conditions in which photographic fixing is effected, either separate sulphur in a free state, or in the state ot sulphuretted hydrogen. As to the sulpho-cyanides, such
