SALTPETRE, is a natural compound of potash and nitric acid. It abounds in soils where animal substances and limo are in contact, and these being lixiviated, decanted, and evaporated to dryness, crystals of saltpetre are formed. Some-. times they are redissolved, and evapo rated a second time.
The chief of the saltpetre used in this country comes from the East Indies, where at certain seasons of the year, it is found deposited on the surface of the soil, and though swept off once or twice a week, it is as often renewed. At Apulia, near Naples, there is a bed containing 40 per cent. ; and in Switzerland the farmers extract it in abundance from the earth, under the stalls of their cattle for the urine of cattle contains potash in abun dance. In Spain there is enough to sup ply all It appears, in every case, to be a con solidation of the nitrogen of the atmo sphere, which consolidates, in this way, just as oxygen consolidates in the pro duction of rust and oxides of all kinds. Thus considered, the consolidation of one promotes the consolidation of the other, since they are always present in the air as 1 oxygen and 3.5 nitrogen. Saltpetre-earth absorbs a little mois ture at night, and appears like a black foot-dust at the bottom of old walls, or on the streets of populous or old villages. It does not differ in appearance from that which yields salt or soda ; and in deed, one village, or one street, frequently contains the three salts.
The most profitable way of preparing it is, to evaporate it in shallow basins of mortar. The earth is swept up every other day and contains about one-fifth of crude saltpetre. After the saltpetre is extracted, the earth is heaped up till the rains are over, and then spread out, and in a year or two it yields again.
About two gallons of saltpetre-earth is collected at the foot of each yard of wall. The saltpetre gained from black cotton ground contains more common salt than th,at from common earth. The pans of mortar are filled about four in ches deep, about half is evaporated in four or six days, and the saltpetre begins to crystallize. The first day's product is the purest ; the second day's contains about half common salt : the third day's contains scarcely a quarter of saltpetre.
Saltpetre is refined by boiling; adding soap, milk, eggs, and twigs of euphorbia tirucalli : and single refined saltpetre still contains about a quarter of common salt. Bengal saltpetre is browner than that of the coast.
If saltpetre is kept or _prepared in any apartment, it is difficult, in India, to prevent the destruction of the walls, by the continual production of the salt.
Calcareous earths, impregnated with saltpetre, are found in caverns in lime stone, in various places. The saltpetre earth of Georgia, contains both the ni trate of potash and that of lime ; but the latter is changed into saltpetre, by ad ding wood-ashes ; one bushel of earth yields *tin three to ten pounds of salt petre. Kentucky saltpetre-earth is simi ; it is washed, and the ley passed through wood-ashes, when a bushel yields from one to two pounds of salt petre.
Similar earths are found at Molfetta, Naples, Hungary, and various places.
Kentucky rock-ore is a sand-stone, which, when broken to fragments, and thrown into boiling-water, soon falls into sand, and the liquor strained from it by crystallization, from ten to twenty pounds of nitre from each bushel of stone. This nitre contains little or no nitrate of lime, and is considered better for gunpowder than that obtained from Kentucky nitre-earth. Masses of salt petre, of several pounds weight, are sometimes found in the fissures of this saud-stone, accompanied by masses of a black bituminous substance. Similar sand-stones are found in South Africa.
The saltpetre formerly used in Eng land was extracted from the mortar of old buildings, as it still is in France and Prussia. The mark, by which saltpetre workers know good mortar for their pur pose is, that it tastes acrid and salt, when applied to the tongue ; but to this it may be also added, that it ought to be of a grayish colour, and such as, when pow dered and sprinkled upon burning char coal, yields sparks ; and, the more sparks it gives, the better it is for the purpose. Another oharacteristic is, that these well impregnated mortars have a certain unctuosity or fattiness to the touch, which other kinds have not.
The best of all kinds of mortar, for saltpetre work, is such as is had from the ruins of old buildings in a low situa tion, and out of the way of much sun shine, where there has been no great quantity of fire kept, and especially such as has served for the mortar of the walls of stables. In Prussia, the rubbish of old buildings is built up in thin long walls, sheltered from the weather by straw coverings, and sprinkled with urine of all kinds, for the purpose of generating this salt.
A clear dry frosty air is particularly favorable to the production of saltpetre, and it disappears in snow-storms.
Napier detected impurities in nitre, by dropping a strong solution of sugar of lead into a phial of distilled water, saturated with saltpetre ; if it retained any considerable portion of marine-salt or magnesia, it assumed a turbid milky appearance. The best is Russian; yet the manufacturers seldom refine their nitre more than twice ; and it has been found that their saltpetre contains a con siderable portion of marine-salt and mag nesia. There is reason to believe, that powder, made with saltpetre oftener than four times refined, is of inferior strength.
The goodness of saltpetre is'measured by the angle at which light is refracted in passing through it. As the angle is less, the quality is better. This angle varies very considerably. An angle of 5° is called par, and, the variations from it are made up by increasing or diminish ing, not the price, but the quantity : for every degree by which its angle of re fraction exceeds 5, 1 per cent. in weight is allowed, and the contrary.