t}ic naUiral Production of Saltpetre. 4 1 7 



quadrangular projecting part of the whole building of the lyin- 

 seum already mentioned. 



Tiie saline efflorescence takes place most copiously on the 

 north wall, and it occurs on various parts of it from nearly the 

 level of the pavement to within three or four feet of the centre 

 of the arched ceiling. It takes place also, though not so abun- 

 dantly, on the east and west walls ; and also at the eastern and 

 western extremity of the south wall ; but it is worth noticing, 

 that I have never seen it on that part of the south wall which is 

 common to the laboratory and the attached projecting building 

 of the Museum. It is true that there are chimneys in this wall 

 connected with fires that are lighted daily ; but this circum- 

 stance does not seem sufficient to account for the absence of the 

 nitre, because its formation takes place in another part of the 

 laboratory equally near a chimney, and in w^hich, from being in- 

 closed, the temperature of the air is always considerably higher. 



It is also worth noticing, that between the highest and lowest 

 points of its appearance in every part of the building there are 

 intercepted spaces entirely and always free from the least depo- 

 sition. 



I may here mention that the occasional formation of nitre is 

 observable in many other buildings and parts of Oxford, besides 

 the laboratory of the Ashmole Museum ; as on the wall, called 

 Long Wall, which bounds the park of Magdalene College to the 

 •w'est — on the exterior surface of the south wall of the Theatre — 

 on the exterior surface of the three walls of the quadrangular 

 projecting part of the Ashmole Museum — very abundantly on 

 the inclined base of the windows of the Examination School, 

 looking to the north — and also very abundantly on the west side 

 of the wall, which separates the square of the Schools from the 

 arched way leading from thence to the Theatre and Convocation 

 House. 



It has been observed repeatedly, that the presence of lime is 

 necessary to the natural production of saltpetre ; and in all the 

 foregoing instances the stone on which the saline efflorescence 

 takes place is the common limestone of Oxfordshire. I have 

 only once observed its formation on the surface of a brick wall : 

 but in that instance the substance of those bricks on whicli the 

 nitre appeared had cnunbled away to some depth ; and if this 

 destruction of their texture be owing to the presence of an un- 

 usual proportion of lime in the clay of which they are made, (a 

 supposition not improbable, since many parts of the stratum of 

 clay from which bricks in this neighbourhood are made do con- 

 tain an unusual proportion of lime), the reason of the exception 

 in the case of this brick wall will correspond with tlie truth ot 

 the general observation above stated, 



Vol.44. No. 200. Dec. 1814. D d The 



