POTASH, PRUSSIATE OF 599 



that the workmen may not be incommoded by the heat. The smoke-vent o, issues 

 through the arched top, h, of the. furnace, towards the front, and is thence led back- 

 wards by a flue to the main chimney of the factory, d is an iron or stone shelf, 

 inserted before the mouth of the pot, to prevent loss in shovelling out the semi-liquid 

 paste. The pot may bo half filled with the materials. 



The calcining process is different, according as the animal substances are fresh or 

 carbonised. In the first case, the pot must remain open, to allow of diligent stirring 

 of its contents, with a slightly bent flat iron bar or scoop, and of introducing more of 

 the mixture as the intumescence subsides, during a period of five or six hours, till the 

 nauseous vapours cease to rise, till the flame becomes smaller and brighter, and till a 

 smell of ammonia be perceived. At this time the heat should be increased, the mouth 

 of the pot should be shut, and opened only once every half-hour, for the purpose of 

 working the mass with the iron paddle. When on opening the mouth cf the pot, and 

 stirring the pasty mixture, no more flame rises, the process is finished. 



If the animal ingredients are employed in a carbonised state, the pot must be shut 

 as soon as its contents are brought to ignition by a briskly-urged fire, and opened for 

 a few seconds only every quarter of an hour, during the action of stirring. At first, a 

 body of flame bursts forth every time that the lid is removed ; but by degrees this 

 ceases, and the mixture soon agglomerates, and then softens into a paste. Though the 

 fire be steadily kept up, the flu'me becomes less and less each time that the pot is 

 opened ; and when it ceases, the process is at an end. The operation, with a mass of 

 50 pounds of charcoal and 50 pounds of purified pearlash, lasts about 12 hours the 

 first time that the furnace is kindled ; but when the pot has been previously brought 

 to a state of ignition, it takes only 7 or 8 hours. In a well-appointed factory the 

 fire should be invariably maintained at the proper pitch, and the pots should be worked 

 with relays of operatives. 



The molten mass is now to be scooped out with an appropriate iron shovel, having 

 a long shank, and caused to cool in small portions, as quickly as possible ; but not 

 by throwing it into water, as has sometimes been prescribed, for in this way a good 

 deal of the cyanogen is converted into ammonia. If it be heaped up and kept hot in 

 contact with air, some of the ferrocyanide is also decomposed, with diminution of 

 the product. The crude mass is to be then put into a pan with cold water, dissolved 

 by the application of a moderate heat, and filtered through cloths. The charcoal 

 which remains upon the filter possesses the properties of decolouring syrups, vinegars, 

 &c., and of destroying smells in a pre-eminent degree. It may also serve, when mixed 

 with fresh animal-charcoal, for another calcining operation. 



As the iron requisite for the formation of the ferrocyanide is in general derived 

 from the sides of the pot, this is apt to wear out into holes, especially at its under side, 

 where the heat is greatest. In this event it may be taken out of the furnace, 

 patched up with iron-rust cement, and reinserted with the sound side undermost. 

 The erosion of the pot may be obviated in some measure by mixing iron borings or 

 cinder with the other materials, to the amount of one or two hundredths of the potash. 



The above lixivium is not a solution of pure ferroprussiate ; it contains not a little 

 cyanide of potassium, which in the course of the process had not absorbed the proper 

 dose of iron to form a ferrocj^anide ; it contains also more or less carbonate of potash, 

 with phosphate, sulphate^ hydrogenated sulphuret, muriate, and sulpho-cyanide of the 

 same base, as well as phosphate of lime ; substances derived partly from the impure 

 potash, and partly from the incinerated animal-matters. Formerly that very complex 

 impure solution was employed directly for the precipitation of prussian blue ; but 

 now, in all well-regulated works, it is converted by evaporation and cooling into 

 crystallised ferroprussiato of potash. The mother-water is again evaporated and 

 crystallised, whereby a somewhat inferior ferroprussiato is obtained. Before evapo- 

 rating the lye, however, it is advisable to add as much solution of green sulphate of 

 iron to it as will re-dissolve the white precipitate of cyanide of iron which first falls, 

 and thereby convert the cyanide of potassium, which is present in the liquor, into 

 ferrocyanide of potassium. The commercial prussiate of potash may be rendered 

 chemically pure by making its crystals effloresce in a stove, fusing them with a gentle 

 heat in a glass retort, dissolving the mass in water, neutralising any carbonate and 

 cyanide of potassium that may be present, with acetic acid, then precipitating the ferro- 

 prussiate of potash by the addition of a sufficient quantity of alcohol, and finally 

 crystallising the precipitated salt twice over in water. The sulphate of potash may 

 be decomposed by acetate of baryta, and the resulting acetate of potash removed by 

 alcohol. - 



Berry's Patent Process. Eecluce charcoal into bits of the size of a walnut ; soak them 

 with a solution of carbonate of potash in urine, and then pour over them a solution of 

 nitrate or acetate of iron ; dry the whole by n moderate heat, and introduce them into 

 the cast-iron tubes, presently to be described. The following proportions of consti- 



