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shores, particularly in Scotland, from which is produced a substance called kelp.
But the demand for a pure carbonate of soda having become very considerable of late years, from its great utility in many arts and processes, various means have been tried for procuring it by decomposing the salts, in which it exists, combined with acids. Muriate of soda has been decomposed for
The following method is described in Nicholson's Journal.
Solutions of 500lbs. of sulphate of soda", and 560lbs. of American potash, are made to boil, and are then mixed. As soon as the mixture boils, it is conveyed into a cistern of wood, lined with lead, half an inch thick, which is fixed in a cool place. Sticks of wood are then placed across the cistern, from which slips of sheet lead, two or three inches wide, are hung into the fluid, at four inches distance from each other. When all is cool, the fluid is let off, and the chrystallized salt is detached from the slips of lead, and the bottom of the trough. The salt is then washed, to free it from impurities, after which it is transferred again into the boiler, dissolved in clear water, and evaporated by heat. As soon as a strong pellicle is formed, it is suffered to cool so far that the hand may be dipped into it without injury, and the heat is kept at that temperature as long as effectual pellicles continue to be formed over the whole surface of the boiler, and then fall to the bottom. When no more are formed, the fire is withdrawn, and the fluid ladled out into the cistern to crystallize. The sulphate of potash, &c. which had been deposited, is then taken out of
* Sulphate of soda is sold cheap by the bleachers, who save it as the residue in decomposing common salt by sulphuric acid with manganese,
the boiler, and put aside. By this process from 136 to 139lbs. of soda may be obtained from 100lbs. of sulphate of soda.
MANUFACTURE OF POTASH.
Potash, or the fixed vegetable alcali; exists as an ingredient, in very small quantity, in many minerals. It is also obtained from the tartär, or from lees of wine, in which it is called salt of tartar.
But the great supply of this substance is procured from the ashes of burnt vegetables.
In many districts of England and Ireland they burn the common fern to ashes, which they mould up with a little water, into balls of about three or four inches in diameter; these are called ash balls, and are the radest preparation of this alcali.
The potash of commerce, or black potash; is always procured from the combustion of wood; and can, therefore, only be made in those countries where wood is very plentiful, as Poland, Russia, and Germany. This country is chiefly supplied from America. Thë, ashểs of burnt wood are put into a cistern with water, and a strong lixivium is made. After a time, the water, holding the alcali in solütion, is drawn off, leaving the impurities behind.
Potash is converted into a purer state by calcining it in a reverberatory furnace. It becomes theh dry, porous, considerably caustic, extremely deliquescent, and of a beautiful bluish colour, from which it is called pearl ash.
All these are carbonates of potash.
To obtain potash in a state of perfect purity, or üncombined with carbonic acid, the carbonate must be boiled with twice its weight of quicklime to de
prive it of the carbonic acid; then to free it from other impurities, it must be dissolved in spirits of wine, (which dissolves alcalis and no other salt) and the solution evaporated to dryness. It is then pure and powerfully caustic.
The term refining signifies the purification of some substance: but we mean to confine it at present to the separation of gold, silver, and copper from each other, and obtaining each of them in a
Cupellation. Gold and silver being the only metals capable of withstanding the action of very strong heat, are therefore called perfect metals. All other metals are reduced to the state of oxides when exposed to a violent fire with access of air. Gold and silver may, therefore, be purified from all the baser metals by keeping them fused till the alloy be destroyed : but this process would be very expensive, from the great consumption of fuel, and would be exceedingly tedious. A shorter and more advantageous method of performing this operation has been discovered.
A certain quantity of lead is added to the alloy of gold and silver, and the whole is exposed to the action of the fire.
Lead is one of the metals which is most quickly converted by heat into an oxide, which is easily melted into a semi-vitrified, and powerful vitrifying matter, called litharge. By.increasing the proportion of imperfect metals, it prevents them from being so
well covered and protected by the perfect metals; and by uniting with these imperfect metals, it communicates to them its property of being very easily oxidated. By its vitrifying and fusing property, which it exercises with all its force upon the calcined and naturally refractory parts of the other metals, it facilitates and accelerates the fusion, scorification, and separation of these metals. The lead, which in this operation is scorified, and scorifies along with it the imperfect metals, separates from the metallic mass with which it is then incapable of remaining united. It floats upon the surface of the melted mass, and becomes semi-vitrified. But the litharge so produced would soon cover the melted metal, and by preventing the access of air would prevent the oxidation of the remaining imperfect metals. To remedy this, such vessels are employed as are capable of imbibing and absorbing in their pores, the melted litharge, and thus remove it out of the way; or, for large quantities, vessels are so constructed, that the fused litharge, besides being soaked in, may also drain off, through a channel made in the corner of the vessel.
Experience has shown that for this purpose, vessels made of lixiviated wood or bone-ashes are most proper. These vessels are called cupels, and this process is called cupellation. The cupels are flat and shallow. The furnace ought to be vaulted, that the heat may be reverberated upon the sur. face of the metal during the whole time of the operation. Upon this surface a crust or dark coloured pellicle is continually forming. In the instant when all the imperfect metal is destroyed, and, consequently, the scorification ceases, the surface of the perfect metal is seen, and appears clean and brilliant. This forms a kind of fulguration, or
corruscation, called lightning. By this mark the metal is known to be refined.
Purification of gold by antimony. When gold contains only a small quantity of alloy, it may be separated from them by melting it in a crucible that will hold twice its quantity at least, and throwing upon it, whilst in fusion, twice its weight of crude antimony (sulphuret of antimony). The crucible is then to be covered, and the whole is to be kept in a melting state for some minutes; and when the surface sparkles, it is quickly to be poured into an inverted cone, which has been
previously heated and greased. By striking the cone on the ground, the metal will come out when cold. The compact mass consists of two substances; the upper part is the sulphur of the crude antimony, united with the impure alloy; and the lower part is the gold, united to some of the regulus of antimony, proportionable to the quantities of metals which have been separated from the gold, which are now united with the sulphur of the antimony. This regulus of gold may be separated from the regulus of antimony by simple exposure to less heat than will melt the gold, because antimony is volatile in such a heat, and is then dissipated. If the gold is not sufficiently purified by this first process (which is often the case,) it must be repeated a second, and even a third time. When a part is dissipated, more heat is required to keep the gold in fusion; therefore, the fire must be increased towards the end of the operation. The purification is completed by means of a little nitre thrown into the crucible, which effectually calcines the remaining regulus of antimony. Sometimes, after these