What is denominated the _saltpetre rot_, is an efflorescence observed on the walls of old buildings, and on the ground. Dr. C. F. Gren, professor at Halle, in Saxony, (_Principles of Modern Chemistry_, vol. ii, p. 128), very justly remarks, that, among the matters capable of corruption, those are the most convenient in making nitre, which contain the greatest portion of azote, of which animal substances are the first; among which he enumerates flesh, blood, skins, excrements of animals, old woolen stuffs, and urine. He also mentions marsh plants, green herbs, mud from streets trodden by cattle, and the ground from marshes or bogs. As a compost he adds, that the ground from church-yards, where corpses have successively, and during a long series of years, undergone corruption, would be the best for artificial nitre beds. On the subject of nitre beds, the reader may consult the _Recueil de Mémoires et de Pièces sur la formation et la fabrication du saltpetre, à Paris_, 1786, 4to. These remarks on the generation of nitre, although of more ancient date, are confirmed by James and Herman Boerhaave, (_Chemistry, &c._) Hoffman, (_de Salium Medicorum, et de Præstantissima Nitri Virtute_), Stahl, (_de Usu Nitri Medico_), Neuman, (_chemical works_), and Lewis, (_Materia Medica_)--all of whom have written more or less on the formation of saltpetre; to which we may add the observations of Parr, (_London Medical Dictionary_, vol. ii, p. 24.) The process for extracting saltpetre from damaged gunpowder is nothing more than putting it into a boiler, and adding water sufficient to cover it. On applying heat, the nitre will be dissolved. If any scum forms, it must be removed. When the solution is effected, pour it on a sufficient number of filters, and collect the fluid which passes through. The residue may be treated with more water, and the whole again filtered. After boiling the solution, set it aside to crystallize. The sulphur may be recovered, by subliming the residue in a temperature not sufficient to inflame it. The charcoal may be used again for the same purpose.