In 1792, M. Pickel announced the discovery of native saltpetre, in a quarry in the neighbourhood of Wurtzburgh. M. de la Rochefoucald discovered nitre in the neighbourhood of chalk in France, in the departments of Seine and Oise. MM. Lavoisier and Clouet, made a number of researches with the same view. Since that time, saltpetre, or nitrous earth has been found in several of the departments of France; and it appears reasonable to conclude, that in all situations favourable to the generation of nitre, where the same causes operate, nitre must occur in more or less abundance. From the rubbish of old buildings, saltpetre is obtained in some quantity. Old plaster is said to give five per cent. The soluble salts it contains, are six in number, viz: nitrate and muriate of lime, nitrate and muriate of magnesia, and nitrate of potassa, and muriate of soda. Now it is obvious, that besides the decomposition of the earthy nitrates, the earthy muriates also are decomposed by the potash, leaving in solution, besides muriate of soda, if it is not decomposed, by the potash, (which has this effect,) muriate, as well as the nitrate of potassa. To refine the saltpetre prepared in this manner, consists in separating the muriates. The proportions, in which these salts are to each other in a hundred parts, are stated by Thenard, (_Traité de Chimie_, Tome ii, p. 485,) to be ten, nitrate of potassa, seventy, nitrates of lime and magnesia, fifteen, marine salt, and five, muriates of lime and magnesia. The mode of extracting saltpetre, and the various processes which have been adopted for refining it, in France, and on the continent generally, have but one object,--that of lixiviating the substances which afford it, and subsequently, separating all foreign salts. The best memoir was written by count Chaptal, occupying forty-seven pages in the _Annales de Chimie_, tome xx. In this he explains the theory at large. In the same work, tome xxiii, there is also a paper by Guyton, and many other memoirs of the same character. In Chaptal's _Chimie Appliqué aux Arts_, tome iv, p. 119, in Thenard's _Traité de Chimie_, tome ii, p. 485, and in the _Annales de Chimie et de Physique_, tome v, p. 173, the subject is ably treated.