the natural production of Saltpetre, 511 
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 lires that are lighted 
daily ; but this circumstance 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 which, from being inclosed, 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 
deposition. 
I may here mention that the occasional formation of nitre is 
observable in many other buildings and parts of Oxford, be- 
sides the laboratory of the Ashmole Museum ; as on the wall, 
called Long Wall, which bounds the park of Magdalene Col- 
lege to the west — 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 Exa- 
mination School, looking to the north — and also very abun- 
dantly 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 efflo- 
rescence takes place is the common limestone of Oxfordshire. 
I have only once observed its formation on the surface of a 
3 U 2 
