XXV. Observations respecting the natural production of saltpetre on the walls of subterraneous and other buildings

Abstract

Although the following observations afford no positive evi­dence of the source of that saline efflorescence, which is so frequently seen on the walls of subterraneous and other build­ings, and which, as consisting principally if not entirely of common nitre, long since gave rise to the name by which that salt is most commonly known; yet as tending to throw some light on a very obscure part of natural history, they will not, perhaps, be unacceptable to this honourable and learned Society. There can be no doubt that the production of saltpetre or nitre, in the situations above alluded to, had been observed long before there existed any general inducement to collect it from those sources; but after the invention and subsequent extensive employment of gunpowder, it became an object not only to search out every natural source of the principal ingre­dient of that important compound, but also to investigate the circumstances of its production; for the purpose either of accelerating the natural process, or of imitating it by artificial means.