Volltext Seite (XML)
December 18, 1863.] THE PHOTOGRAPHIC NEWS. 605 pared by dissolving pure granulated tin in hydrochloric acid. When no more action takes place, the solution is poured off and evaporated gently until the crystallizing point is arrived at; it is then allowed to cool, and the crystals are separated ; they are known in commerce under the name of “salt of tin.” This is prepared in enormous quantities for dyeing purposes, the solution being effected on the large scale in copper vessels which are not attacked so long as any portion of the tin remains undissolved. Protochloride of tin is a very valuable agent for dyers; it is used in calico printing, both as a mordant and a de oxidizing agent. When exposed to the air, chloride of tin absorbs oxygen, forming binoxide and bichloride of tin. Manufacturers prevent this by the addition of chloride of ammonium, which forms a double salt with tin, not so easily oxidizable in the air. Protochloride of tin is of great use in the laboratory for deoxidizing purposes ; it is usually dis solved in dilute hydrochloric acid, and When added to solu tions of metals reduces them, if possible, to the lowest state of oxidation, or to the metallic state ; thus, per-salts of man ganese and iron are reduced to the proto-salts, arsenic acid is reduced to arsenious acid ; almost all the compounds of mercury are reduced to the metallic state; and silver salts to metallic silver. With gold, protochloride of tin forms a brown or purple precipitate according to circumstances. Bichloride of tin, SnCl 2 , is prepared by the action of dry chlorine gas on tin foil, or by distilling common salt with persulphate of tin, and rectifying in retorts over oil of vitriol. It forms a thin colourless liquid, boiling at 120°C., and giving off white fumes in the air at ordinary temperatures. Water unites with anhydrous bichloride of tin with great avidity : with one-third of its weight of water, the liquid solidifies to a colourless mass of crystals, a great deal of heat being evolved, the first portions of water hissing as if they had been poured upon hot iron. The addition of more water dissolves the bichloride of tin, forming a colourless liquid which, when evaporated and cooled, yields deliquescent crystals. This solution is like that of the protochloride, largely used by dyers and calico printers. Purple of Cassius is a curious compound of tin and gold, the exact composition of it being a matter of some doubt. The best method of forming it is to mix one part of the liquor ferri muriatici of the shops, with three parts of water, and a solution of one part crystallized protochloride of tin in six parts of water, added till the mixture acquires a greenish tint, after which six parts more of water are added. On the other hand, some gold is dissolved in hydrochloric acid with gradual addition of nitric acid, excess of acid, especially the latter, being carefully avoided. The solution is diluted so far that 360 parts of the liquid contain only one part of gold, and the tin solution is then added with constant stirring as long as a precipitate is produced By this process 100 parts of gold yield 360 parts of dried purple In order to ensure success, it is important that the above proportions be closely adhered to. In the moist state, gold purple is of a dark purple red colour, brown after drying. Whilst moist, it dissolves in aqueous ammonia, forming °a deep purple red liquid. When mixed with glass, and melted, the purple imparts a strong ruby colour to it. The analysis of purple of cassius has not yielded good results, owing, no doubt, to the difficulty of obtaining it free from cither metallic gold or binoxide of tin. Some chemists consider it a mixture of binoxide of tin with metallic gold, whilst others look upon it as a compound of binoxide of tin and oxide of gold. Recent researches of Faraday seem to lead to the opinion that purple of cassius is simply binoxide of tin mechanically mixed with metallic gold which has been reduced from solution in an extremely fine state of division. It was formerly largely employed in glass staining, the cele brated old ruby glass being coloured with this compound. Much more is thought of this glass than it deserves, and many persons imagine that the art of producing it has been lost, fabulous sums being occasionally paid for small vessels ° it. This is quite a mistake, the art of glass staining was never in greater perfection than it is at the present day; we can not only make ruby glass equal to the old kind, but we can make it far better, and if the employment of purple of cassius for this purpose is not so general as it was formerly, it is because it is attended with great risk of spoiling the glass, whilst other processes are known by means of which an equally good colour may be obtained with no uncertainty whatever. PHOTOGRAPHY IN ITS APPLICATION TO MILITARY PURPOSES. BY JOHN SIPLLEB, F.C.S.* Assistant Chemist in the War Department. (Continued from page 597.) The cameras, printing frames, and other similar mechanical contrivances used in our department, are in no way different from those in common use. The necessity for providing means of taking pictures at a distance from the photographic labora tory has been met by the construction of a portable dark room, which, at Shoeburyness, is moved as required by two men, who carry it sedan fashion. There is no flooring, but a loose black canvas border on all sides effectually excludes the light. A working bench, with windows appropriately placed, shelves and wooden partitions for the reception of bottles holding chemical solutions, and on the roof a very effective ventilating con trivance, completes the equipment. At Woolwich we have a photographic van, which was constructed and originally used by the Royal Carriage Department. It is four-wheeled, and, being somewhat largo in dimensions, is drawn by two horses, which latter are supplied, when required, from the Military Train Service. Some few years ago, this van accompanied the troops from Woolwich to Dartford, and enabled Mr. Butter to produce a series of photographs illustrative of camp life. Besides these transportable operating rooms, we have success fully used the square tent designed by Mr. Smartt. On several occasions the Artillery Officers have had “ field days,” both in the grounds of the Royal Military Repository, and on Woolwich Common, and with tents pitched have, under photographic canvas, allowed me to assume the command. Many useful sketches have thus been secured, and out-door experience gained, which has since been further extended by my pupils, some of whom, at distant stations, have given proof of the value which attaches to photography as a ready means of recording faith fully the geographical and military features of a country, or of reporting details of construction, whether relating to stockades, forts, or suggested improvements in military equip ment. It will be perceived that our usual course of operating has been to sensitize, expose and develop the plate on the spot, so that the resulting picture may be examined and approved before returning home, or removing the camera with the rest of the apparatus to another subject. Much progress has been made towards the perfection of the so-called dry processes since the early date at which, in conjunction with Mr. Crookes, I had the honour of announcing the first and necessarily imperfect results obtained upon this system. Since the proposals referring to the use of the nitrates of zinc and magnesia, in May, 1854, and the employment of honey, recommended shortly afterwards by Mr. Shadbolt, an immense variety of preservative agents have been successively brought under the criticism of the practical photographer, and the results in many instances have testified to the general efficiency and uniformity of such processes; but it is nevertheless true that at the present moment, wo are without a guide as to the precise interval of exposure required for the perfect rendering of detail in the diversified range of subjects to which the camera has to bo directed. Then, from the inverted position of the image upon the ground glass, we are, at the time of focussing, apt to pass over little inaccuracies which could not escape our observation when examining the finished negative. Under these circumstances we still prefer for general use the wet collodion process, which has, moreover, the advantage of reducing to a minimum the period of exposure, a matter often of vital importance, as we have already seen. For collodion, we have of late been dependent upon com- * Read before the Photographic Society, on the evening of Tuesday, December 1st.