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604 THE PHOTOGRAPHIC NEWS. [December 18,1863. Sensitizing.—The collodion being poured on to the plate, and a certain amount of evaporation having taken place, it is sensitized in a bath formed of 7 to 10 parts of nitrate of silver, dissolved in 100 parts of water, with the addition of 5 parts of crystallizable acetic acid. Washing.—The object of this operation is to remove the free nitrate of silver remaining on the surface of the collodion. If the washing is not complete, the sensitized plate will not keep very long, and spots appear on the collodion film, especially at the moment of the appearance of the picture, sometimes even sooner. A collodion plate, sensitized, and properly washed, and consequently deprived of free nitrate of silver, should, if kept from light and moisture, keep at least two months. Preserving Coating.—Major Russell employs an aqueous solution of tannin to coat the well-washed collodion film, so as to preserve its sensitiveness after drying. The newer the collodion, the thicker the coating of tannin must be Mr. England proposes to add honey to this tannin solution. After studying the influence of sugary matters upon collodion, M. de Brebisson prefers to replace them by solid substances, the quantities' of which are perfectly known, therefore he employs with much success, barley sugar and jujube paste. He has also recognized, in wishing to recur to gumarabictoincreasethe solidity of the preservating solution, that this substance possesses the faculty of increasing the sensibility of collodion, and securing its keeping. Once this coating becomes dry, to avoid blisters and cracks, when it softens at the time the picture appears, it suffices to fix the borders by means of a solution of white wax in benzole. This precaution is however, unnecessary, if we replace the honey by jujube paste or barley sugar. Among the various formul for preserving solutions, indicated by the author in his treatise, we extract two, into the composition of one of which tannin enters, and not into the other. 1. Distilled water ... ... ... 90 parts Alcohol, at 30° 10 „ Tannin ... ... ... ... 2J „ Barley sugar ... ... ... 2 „ Gum Arabic ... ... ... 0 „ 2. Distilled water ... 90 parts Alcohol, at 36° ... ... ... 10 „ Jujube paste ... ... ... 3 „ Gum Arabic ... 5 „ Exposure in the Camera.—It will be understood that the operation is executed in the usual manner, making use of the ordinary apparatus. Only in the case of an instantaneity, M. Humbert de Molard proposes an apparatus which he calls Stere-obturator, or spring shutter-stop. This stop is a sort of blind, of stuff impervious to light, which is extended by means of a spring upon two cylinders placed in front of the objecture. A round hole is cut in the middle of the blind. When the finger is placed upon the spring which governs the fastening, the blind, pulled by one spring, leaves the other spring upon which it was rolled, and for a moment covers the other cylinder. Although this movement of transfer is so rapid that the eye cannot follow it, the aperture in the blind permits the light, during this instantaneous passage, to act upon the sensitized plate contained in the camera. It is by the assistance of this apparatus that M. de Brebisson has controlled the degree of sensibility in his dry collodion to various spaces of time, and, as he states, to have each time obtained as good pictures as if he had made use of wet collodion. Development of the Picture.—The author asserts, although the contrary has been maintained by eminent practicians, that this operation may be deferred several days without inconvenience. Still, the certainty of a long keeping after exposure, can never be absolutely attained. Many causes may, in fact, contribute to hasten the reduction, and pro duce spots—such, for instance, as a sudden elevation of temperature, imperfect washing especially, and the employ ment of bad chemicals. Almost all the baths with an iron base develop the picture quickly, but it is very rarely, says M. de Brebisson, that they are of equal tone throughout, especially when the development is very rapid. These inequalities are due, we think, to an irregular impregnation of the dry collodion. The developing solution of sulphate of iron, acidulated with tartaric acid, as proposed by M. E. Renet, appears to be the most successful. It is necessary to remark that, in winter, when the tem perature of the operating room is rather low, the pic ture is very slow in making its appearance. It must bo accelerated by the aid of heat. Fixing.—The proofs must be fixed by means of an almost saturated solution of hyposulphite of soda, which must be poured over the surface, until the milky appearance of the collodion film has disappeared, and the cliche has become transparent, without white cloudiness. We next proceed to wash the plate carefully. When the film is dry, apply the protecting varnish.—Cosmos. PHOTOGRAPHIC CHEMICALS : Their Manufacture, Adulteration, and Analysis. Tin.—This is one of the earliest known metals, and is in many of its compounds of considerable interest. The metal is too well known to require special description here. It occurs in commerce under several forms, the purest varieties being Malacca and Banca tin, and English grain tin. The best tin foil is also pretty pure, although some foil is largely adulterated with other metals. The impurities in commer cial tin are arsenic, antimony, bismuth, zinc, lead, iron, and copper. They may be separated by heating tin filings with an excess of nitric acid; this oxidises the metal, and con verts it into a white insoluble binoxide. It is well washed with dilute hydrochloric acid, and then reduced tothe metallic state by being heated in a charcoal crucible at a bright red heat. In softness, the pure metal is intermediate between gold and lead ; it is highly crystalline, and crackles when it is bent. It melts at about 240° C, and boils at a white heat. According to Fridemann, tin may be easily distinguished from other metals to which it is similar, by its behaviour with a solution of gold, containing an excess of hydrochloric acid ; a bright surface of tin immersed in such a solution is blackened without evolution of gas ; zinc turns black, and liberates gas; lead does not blacken. The most characteristic behaviour of tin is its conversion into binoxide, by the action of nitric acid. U nl ike the action of this acid on most other metals nothing is dissolved, but the whole of the tin is converted into a heavy white powder called stannic acid. This is quite in soluble in nitric acid or water, and the reaction is one which is very frequently made use of in the laboratory. If any phos phoric acid is present, it is also retained in the insoluble form by the binoxide of tin, and this fact is frequently used in the estimation of phosphoric acid. Tin and sulphur unite together to form a remarkable golden coloured compound called aurum musivum or mosaic gold. The best method of preparing this, is to mix together five parts of protosulphide of tin and eight parts of corrosive sublimate, or to mix a pulverised amalgam of twelve parts of tin, and six of mercury, with seven of sulphur, and six of sal ammoniac. Either of these mixtures is heated in a loosely closed Florence flask, placed in the sand bath. A gentle heat is first applied for some hours; afterwards the heat is raised, but not quite to redness. The greater part of the mosaic gold is found at the bottom of the vessel; the smaller, but purer and finer portion sub limes. Bisulphide of tin prepared in this manner forms gold coloured translucent delicate scales or six-sided lamin unctuous to the touch. Mosaic gold is used in the arts for a bronze powder for touching the edges of painted plaster busts, it is also used by house painters and paper stainers. Tin forms two compounds with chlorine, the protochloride and the bichloride. The former compound, SnCl, is pre-