C 508 3 
XXV. Ohsewations respecting the natural production of Saltpetre 
on the walls of subterraneous and other Buildings. By John 
Kidd, M. D. Professor of Chemistry at Oxford. Communicated 
by William Hyde Wollaston, M, D. Sec. R. S. 
Read June 1^, 1814. 
Although the following observations afford no positive evi- 
dence of the source of that saline efflorescence, which is so 
frequently seen on the walls of subterraneous and other build- 
ings, 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 ingre- 
dient 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 
* Saltpetre (Sal Petrae). 
means. 
