Schinz's improved apparatus for effecting the decomposition of the materials and formation of ferrucyanide, without contact with the air, is shown in Figs. 210 to 213. At the upper part is a feeding cylinder a, of iron, fitted with a close cover b, and supported on an iron base-plate c. This plate has a circular hole in it corresponding with the interior of the cylinder. Beneath is placed an iron frame d, in which moves a slide e, Fig. 211. This slide also has a circular opening e', which may be brought under, or withdrawn from the hole in the base-plate by means of a rod f, worked by a rack f', a pinion f", and wheel fm. Beneath the iron frame is a flue g, which communicates, by means of a circular grate g', with a vertical retort 7x, placed immediately below. This grate g' is movable, in order that it may be cleaned when requisite. The furnace is placed below the flue g, from whence issue two gas-pipes 11 beyond the walls i i of the furnace. The retort h is a sheet-iron tube, and it is surrounded by sand, &c., placed in the space j, to allow of contraction and expansion. The fireplace, at the lower part of the retort, has a cylinder of fire clay lc interposed between it and the sand. The retort has a second flue /, with a circular grate P, also furnished with two gas-pipes. This flue communicates, through the grate /', with a vertical cylinder m, placed immediately beneath the retort 7x, but larger in diameter. Into this descend the materials when cooled, and as it is air-tight they are preserved from the action of the atmosphere. The cylinder is supported by a rectangular box n, in which moves a flanged cylindrical slide o, furnished with a plate o', and so worked by a rod, rack, pinion and handwheel, p, p', p," p"1, that it can open or close a passage into the receiver g. This receiver runs upon wheels gi g", and has a flanged top which fits tightly against the slide box n. In it is a sieve of iron wire r, which can be removed when full and replaced when empty.
The feeder is filled with small pieces of coke, or charcoal, mixed with dry pearl-ash and iron borings or nails. The cover being replaced, the first slide e is removed, and the contents of the feeder drop into the retort h. Here they meet with the nitrogenous gases which are introduced by the gas-pipes, and steady decomposition takes place. After passing through and acting upon the alkaline materials, the waste gases issue through the lower grate and flue into any suitable exit. When the operation has proceeded long enough to saturate the volatilized potassium with cyanogen, the slide o is moved under the retort, and a portion of the produce falls into it. The slide is then forced forward and the contents fall into the receiver g, and an equal quantity of fresh material is introduced from the feeder. The process is thus continuous.
These methods present many advantages. A considerable saviog of fuel and manual labour is effected ; volatilization of potassium and potash com pounds is prevented, thereby largely increasing the yield, and economizing the nitrogenous material.
Finally, by cooling in an air-tight compartment the combustion of cyanogen into cyanic acid is rendered impossible. A modified form of Schinz's apparatus may be used for distilling or carbonizing the animal matter.
Commercial prussiate may be rendered chemically pure by causing the crystals to effloresce in a stove, fusing them at a gentle heat in a glass retort, dissolving the fused mass in water, adding a little acetic acid, then precipitating the ferro-prussiate with alcohol, and twice crystallizing.
Ferrocyanide of potassium is used to a considerable extent in the production of the ferricyanide (" red prussia te ") and cyanide. It also enters into the manu facture of Berlin blue and other pigments. Its principal uses arc, however, in dyeing and calico printing, in the production of various shades of blue and to form prussiate of tin (" tin pulp ") for steam blues. It is the production of the ferrocyanide that is chiefly valuable in these dyeing and printing processes ; the salt is decomposed by an acid and the iron turned into prussian blue by com bination with another portion of the same salt. Tin pulp, used largely in steam blueing, is made by mixing murlate of tin and yellow prussiate together and allowing the "pulp" to settle out. The evolution of prussic acid in manipulation of all steam blues should be guarded against.
Iodide of Potassium. Formula, KL—Beyond some use as a drug, this substance is of slight importance, though exceedingly interesting in its reactions and characteristics. It is prepared (1) by adding iodine to a strong solution of caustic potash, free from all traces of carbonate. Tho iodine dissolves freely, the solution containing both iodide and iodate of potassium. Upon evapo ration and ignition at a low red heat, the iodate is decomposed, yielding iodide and free oxygen. The mass is then dissolved in water, filtered, and crystallized. (2) Iodine (2 parts), water (10 parts), iron filings (1 part), or scraps of zinc, are digested together in a warm place. The resulting iodide of iron or zinc is filtered or decanted off, and boiled. A solution of pure carbonate of potash ie added until all effervescence ceases, and a slight precipitate makes its appearance, iodide of potassium and carbonate of protoxide of iron being formed. The iodide is separated by filtration and evaporated. Upon cooling, the iodide crystallizes out. (3) Iodide of lime or barium is decomposed by sulphate of potassium, yielding insoluble sulphate of baryta and iodide of potassium in solution. To prepare the iodide of barium, 1 part of amorphous phosphorus is added to 40 parts of warm water, and 20 parts of dry iodine are gradually stirred in. If the mixture be coloured, it is heated upon a water-bath until perfectly clear, and allowed to settle. The clear supernatant liquid is then decanted off, and neutralized with a slight excess, first, of carbonate of barium, and then with baryta water. Insoluble phosphate of barium forms, and is filtered off, the filtrate consisting of pure iodide of barium.