Ol'servalions respecl'mg the natural Production of Saltpetre. 4\:i 



the same method of analysis, I found in the very violet preci- 

 pitate obtained by using an excess of muriate of tin, oxide of 

 tin (i0*18, and gold 39'S2 ; and in precipitate of a fine purple, 

 made with excess of muriate of gold, I found oxide of tin 20*58, 

 and gold 79'42. 



From these experiments it results that precipitates by muriate 

 of tin are very variable, since, vvlien the solutions are concen- 

 trated, the gold is always thrown down in the metallic state ; and 

 that, when the solutions are diluted, the difference in their pro- 

 portions occasions precipitates equally variable in their composi- 

 tion and properties. In wliat state is the gold found in them ? 

 It is difficult to answer this question ; but every thing tends to 

 show, that if it is not entirely in a metallic state, as Proust thinks, 

 there can remain in it only a minute proportion of oxygen. 



LXV. Olservations respecting the natural Production of Salt- 

 petre on the JValls of sid> terraneous and other Buildings. 

 By John Kidd, M.D. Professor of Chemistry at Oxford. 

 Communicated hy Wiljliam Hyde WollastoxX, M.D. Sec. 

 R.S.^ 



XiLTHOUGH the following observations afford no positive evi- 

 dence of the source of that saline efflorescence which is so fre- 

 quently 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 f by which that salt is 

 most commonly known ; yet as tending to throw some light on 

 a very obscure part of natural histoiy, 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 

 emplo)'ment 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 circum- 

 stances of its production ; for the purpose either of accelerating 

 the natural process, or of imitating it by artificial means. 



The usual and almost exclusive occurrence of saltpetre on 

 walls constructed of limestone, and in situations exposed to ani- 

 mal and vegetable effluvia, in all probability led to the empirical 

 practice of heaping together the mortar and refuse of old build- 



♦ From the Philosophical Transactioiu for 1314, imrt ii. 

 i Saltpetre (Sal Petra-.) 



