The best known and esteemed crude saltpetre is of Indian manufacture. In many parts of the country the gathering and treatment of the saltpetre earths by " sera wallahs " (sera -= nitre) is a considerable industry. From all possible sources—natural beds, or the products of drains, stables, walls, &c., wherever the process of nitrification has gone on—the earth is gathered up and piled in wooden boxes, or "kieves." Here it is lixiviated with successive washings of water, the resulting liquors being drawn off into rude earthenware or stoneware vessels, and allowed to concentrate by the action of the sun and air, or, after a more civilized fashion, run into iron pans, concentrated by an underneath fire and drawn off into crystallizing cones. Dnring washing, the mass is kept es open as possible. The liquors usually contain about 14 per cent. of nitrate of potassium. The " earn wallah" has his regular round, visiting the same deposits year after year. The first crop of crystals that is obtained is exceedingly impure, containing sulphate of potassium and chlorides of potassium and sodium. They are dissolved in the smallest possible amount of hot water, and the solution is cooled and allowed to crystallize. The well-known " Indian " or " Bengal " saltpetre then separates out, an article of very fair purity.
The Indian saltpetre earth is, as a rule, rich in nitrate of potassium. When this is not the case, or when the solution after lixiviation contains large quantities of the nitrates of calcium, magnesium, sodium, and alkaline chlorides, treatment with carbonate of potassium is adopted, in order to convert all the nitrates present into nitrate of potassium. By this operation, the earthy nitrates yield their nitric acid to the potash of the wood-ashes ; carbonates are precipitated, and the clear lye, now rich in nitrate of potassium, is drawn off, evaporated, and crystallized. Treat ment with wood-ashes has always to be resorted to when manipulating the rocky nitrate deposits from caves.
It has been noted that " saltpetre earth " may be produced by artificial means—grown in fact. Owing its origin to the abnormal demand for saltpetre consequent upon the discovery of gun powder, this industry has now become, in several countries of Europe, an important one, more particularly in Franca, Germany, Switzerland, and Sweden. In the last-named country, saltpetre forms one of the revenue taxes, and its preparation is therefore all but obligatory. A mass of earthy matter, with certain bases, lime, &e., is heaped up and exposed to the action of the air, being kept as open as possible by loose twigs or stones, &c. The heaps are moisten, d from time to time, all descriptions of animal matter and organic refuse being added (urine is especially rich in nitrogen), and a periodical turning over of the whole is carefully practised. This process usually goes on for about three years, long before which time a white efflorescence makes its appearance. It is usually arranged that certain portions of the heaps become " ripe " every year, so that the process may be continuous. Tho saltpetre earth is considered ready for ]ixiviation when 1000 eithic Welles yield about 5 oz. of salt. To bring the nitrate to tho surface as much as possible before removing it, the heaps that are considered ripe are left to themselves for some time beforo it is intended to lixiviate. In this way, tho nitrate formation, undisturbed by fresh additions of liquid matter, rise and form a coat two or three inches thick. This is removed and treated in the same manner as the natural saltpetre earths already described. Sometimes the earth is massed in the form of walls, with nearly perpen dicular sides, the liquid manure being poured down one side, and the saltpetre drawn by capillarity to the other, whence it is readily removed from time to time. By this means a certain amount of
labour is saved, the walls being loft almost intact, but a smaller result is obtained tis the liquors have to be constantly reapplied. The heaps are generally formed about 6 ft. in height and 15 ft. in length. Upon the best " plantations," rude sheds are erected over them, that the amount of moisture may be carefully regulated. The sides of the sheds should be open, but protected from the wind and weather by rough palisading, or hurdles.
Although tho treatment of artificial saltpetre earths is similar to the Indian process, the former is of course carried on in Europe with much more care and judgment as a rule. The most important point is to separate the nitrate of potash as promptly as possible from the chloride of sodium and other salts. For this purpose, the amount of water should be carefully regulated. In concentrating the lye, the different degrees of solubility of the various salts contained must be taken in consideration.
In its crude state, saltpetre is unfit for the manufacture of gunpowder and nitric acid, the presence of the chlorides of potassium and sodium being particularly objectionable. It has there fore to be subjected to a further refining process, which depends partly upon the different rates of solubility of the various salts at different temperatures, partly upon the mechanical action of animal gelatin upon the extractive matters contained, and partly upon the fact that crystals of saltpetre being homogeneous (that is, consisting of one salt alone), separate out without contamination from the solution containing the chlorides of potassium and sodium. The crude article is dissolved in boiling water, the salt being added to saturation and the heat gradually increased. A density of 1.5 or 1.6 should be attained. Small quantities of dissolved glue are introduced into the boiling solution, which separate out the various extractive matters. These partly rise to tho surface, and form a scum which is removed from time to time, and possibly sink to the bottom of the pan. Sometimes the hot solution is further diluted with water to prevent the depositing of crystals of saltpetre, and allow time for the insoluble matters to separate out. Tho liquors are then run off into flat copper crystallizing pans, and while cooling are kept thoroughly stirred up with wooden rakes to prevent the formation of large crystals, which are apt to contain appreciable quantities of tho mother liquors in their interstices, and yield when pulverized a damp powder. The fine needles which are obtained, having the appearance of a white powder, are termed "saltpetre flour." This is fished out and thrown upon a wire-gauze strainer placed across the crystallizing pan, to drain, the mother liquor falling back into the pan. The saltpetre flour is T 2 almost pure, the mother liquors containing the chlorides and returning tliem into the pan. The flour is then removed to the wash-pans and tinated with cold water, or a saturated solution of pure saltpetre. The wash-pans are usually about 10 ft. long, 4 ft. wide, and 3 ft. deep, fitted with a false bottom upon which the flour is placed. When thoroughly washed, and freed from all adhering mother liquor, it is dried at a gentle heat and sifted to separate out the lumps. The mother liquors are evaporated, a sufficient quantity of potash salt is added to decompose the nitrates of the earths contained, and worked over again as crude lye from the saltpetre earths.