Old walls, and the vicinity of stables, frequently exhibit saltpetre in the state of efflorescence. It was the ancient _scrophula contra lapides_, represented as a kind of leprosy. For the spontaneous production of nitre, animal and vegetable substances, in a state of decomposition, and the presence of dry atmospheric air are necessary. That lime, and the calcareous carbonates also promote its formation, there can be no doubt. Notwithstanding the large quantity of saltpetre collected in the East Indies, we are told, that two-thirds of the whole are annually sent into China, and other parts of Asia, to make artificial fire-works. The pyrotechny of the Chinese is said to be very perfect; in variety and beauty, some writers assert they exceed all other nations. There is a natural nitre bed at Apulia, near Naples, which affords 40 per centum of nitre. Pelletier, (_Ann. de Chim._ tome xxii.) has published an analysis. The cavity of Molfetta is one hundred feet deep, containing grottos or caverns. Nitrate of potassa is found in the interior, in efflorescence or crusts, attached to compact limestone. On removing these efflorescences, others appear. The soil in this cavity is richly impregnated with nitre. In Switzerland, the farmers extract an abundance of saltpetre from the stalls under their cattle. During the American revolution, when every expedient was resorted to, to obtain a supply of this article, the floors of tobacco houses, &c. were dug up and lixiviated. In the reign of Charles the First, certain patentees were authorised to dig up the floors of all dove-houses, stables, &c. the floors being again laid with mellow earth. The Ukraine, Podolia, Hungary, Spain, Italy, Peru, and India, furnish more or less of this salt, which is extracted by lixiviating the earths that compose the soil. The springs, in particular districts of Hungary, contain it. We are informed, (_Ann. de Chim._ xx. 298,) that, during the second and third years of the French Republic, the government required every district to send two intelligent young persons to Paris. This convocation, consisting of nearly eleven hundred persons, received regular instruction from their first chemists, partly concerning the manufacture of cannon, and partly respecting the manufacture of saltpetre and gunpowder. This body of pupils was afterwards distributed among the different establishments in proportion to their abilities, and saltpetre was soon furnished in abundance.