the vn'ural Prod/iclion of Saltpetre. 423 



which took place within the irlass on Octohcr 20 and SO, Still 

 the whole quantity was not nearly ecjiial to that which was usually 

 formed on the bame surface when expo-ed to the free action of 

 the atmosphere. The increased quantity however formed in 

 January, compared with that fonned in Octoher, corresponds 

 with the observation that the natural production of nitre k much 

 influenced hy temperature: hut it is worth mentioning, that 

 while the nitre formed within the glass in January occurred either 

 in distinctly prismatic crystals or in flocculeut tufts, that which 

 was formed on the adjacent parts of the wall was of the granular 

 character above described in page 420. 



Having by the foregoing experiment satisfied myself with re- 

 spect to the necessity of the free presence of atmospherical air 

 for the full eflfect in the spontaneous juoduction of nitre, I wished 

 to ascertain the nature of the coimexion between the nitre 

 formed and the stone on which it is formed ; and particularly 

 Avhether carbonate of lime simply would l)e suilicient for its pro- 

 iluction. I therefore made a nearly liijuid paste with distilled 

 water and prepared chalk *, with which I covered the ext/'rior 

 surface of the glass that had been attached to the wall for the 

 purpose already mentioned: but though nitre was repeatedly 

 formed in the neighbouriiood of the glass, not a particle ap- 

 peared on the whitewash during the space of nearly six weeks, 

 from November 29 to January 8. 



The foregoing statement contains the general results of all the 

 observations I have hitherto been able to make on the present 

 subject. I subjoin an imperfect analysis of the stone of which 

 the laboratory of the Ashmole Museum is built, and of the salt- 

 petre periodically formed on it ; concluding with a few remarks 

 on the local differences connected with a more or less extensive 

 formation of that substance, and on its probable or rather possi- 

 ble source. 



The stone of which the laboratory is constructed belongs to 

 one of those alternating series of clay, and sand, and calcareous 

 freestone, of which the varieties of Bath stone form nearly the 

 lowermost strata; and the Headington stone, of which the 

 greater part of Oxford is built, the uppermost. It is principally 

 made up of the debris of small sea shells, but frequently con- 

 tains organic vegetable remains, and always some proportion of - 

 sand and ochry clav. 



Six portions of this stone taken from different parts of the labo- 



* As clialk iometiinrs contains njagncsia, and almost always a small 

 proportion of silcx and aliiinint, I intend on a future occasion to repeat ttie 

 «Npcritncnt, usin^j very finely pulverized crystallized calcareous spar, in- 

 stead of clialk. 



D d 1 ratory. 



