521 
the natural production of Saltpetre. 
repeatedly formed in the neighbourhood of the glass, not a 
particle appeared on the whitewash during the space uf 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 saltpetre 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 possible source. 
The stone, of which the laboratory is constructed, belongs 
to one of those alternating series of clay, and sand, and cal- 
careous 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 contains organic vegetable remains, and always 
some proportion of sand and ochry day. 
Six portions of this stone taken from different parts of the 
laboratory, each weighing at least two hundred grains, were 
separately submitted to the action of a sufficient quantity of 
diluted muriatic acid. The undissolved residuum, which 
amounted in no instance to as much as four parts in a hun- 
dred, consisted of a yellowish brown ochry clay, mixed with 
a few particles of white sand, and small laminar fragments of 
a dirty white colour, very much resembling portions of the 
siliceous incrustation of the Geyser spring. Small flocculent 
