About 35 cwt. of the grough saltpetre, as it is termed, viz., as it is imported in its impure state, is put into a copper capable of holding 500 gallons, with 270 gallons of water, in the proportion of about 1¹⁄₂lbs. of nitre to 1lb. of water, (which proportion varies with the quality of the saltpetre). This is allowed to boil, and the impurities are skimmed off as they appear on the surface. Cold water is occasionally thrown in to precipitate portions of the chloride, which otherwise would remain on the top by the action of boiling. After being allowed to boil from three and a half to four hours, the furnace doors are thrown open, when the chlorides and salts fall to the bottom. In about two hours, a copper pump is lowered into the liquor, which is pumped out into a wooden trough, having four or five brass cocks, under which are suspended canvas filtering bags in the shape of a V. The solution is then filtered, and run off into pans, containing about 36 gallons, and allowed to remain for twenty-four hours, to crystallize, when they are set up on edge, to drain off the liquor which remains uncrystallized, and which is called mother liquor. The saltpetre thus obtained is called once-refined, and undergoes the same process twice again, the only difference being that there is a greater proportion to the water each time, viz. 1³⁄₄lb. to 1lb. of water the second time, and 2lb. to 1lb. of water the third time: moreover, the third time, a small quantity of ground charcoal is put into the solution, and it passes through double filters, which brings it to a very fine pure white colour when melted. The mother water which remains in the pans after each crystallization is conveyed away by gutters to cisterns under the building; it is then evaporated in iron pots to one quarter of its original bulk, filtered, and allowed to crystallize. The saltpetre obtained from the first mother water is considered one stage inferior to grough; that from the second, equal to grough; that from the treble-refined, equal to once-refined saltpetre. The water left from every stage is treated in the same way, so that actually nothing is lost of the pure material. Saltpetre treble-refined by this process is perfectly pure, and fit for the manufacture of Gunpowder; and in order to free it from moisture, as well as for the convenience of storage and transport, it is melted in iron pots holding about 4 cwt., by raising it to a temperature of 600° Fahrenheit, and cast into gun-metal circular moulds holding about 38lbs. each. It must be observed that it requires about two hours to bring the saltpetre into a liquid state, and that, after this, the furnace doors are thrown open, to lower the heat to the proper temperature for casting into the moulds. When the cakes are cold, they are packed away in barrels containing 1 cwt., 1 qr. each, and put into store. Care must be taken, in melting the saltpetre, not to raise it to too high a temperature, as this would reduce the quantity of