the natural production of Saltpetre, 51^ 
the phenomena of this natural process previously to the 
commencement of last year: about which time having care- 
fully brushed away the whole of the saline efflorescence from 
a part of the north wall situated below the level of the street, 
and very imperfectly exposed to the light, I was surprised by 
the fact of its quick reappearance. It was brushed away at 
the end of January, but within three days it had again efflo- 
resced in sufficient quantity to present that appearance of hoar 
frost, or down, or mould, which is very characteristic of the 
manner in which naturally formed saltpetre is often accumu- 
lated; and which a person, utterly incapable of judging of the 
real nature of the substance, described at the time by the term 
“ damp."' 
I was still more surprised, however, in observing after a 
few days, that the quantity of the saltpetre was apparently 
very much diminished ; and that at the end of eight or ten 
days there was scarcely any appearance of it remaining : 
though there was no reason to suppose a particle had either 
accidentally fallen to the ground or been intentionally removed. 
The same part of the wall was again brushed perfectly 
clean, and I continued to make daily observations in expecta- 
tion of a renewal of the process ; but no new collection was 
perceptible, excepting in the form of a very few detached and 
minute capillary crystals, till the 16th of March: on which 
day, having been absent during the whole of the 14th and 
15th, I found a more abundant accumulation than in the pre- 
ceding instances. 
It was an obvious supposition that the reproduction of the 
nitre was connected with some change in the state of the at- 
mosphere : and it is to be remarked, that its first mentioned 
