t)i6 Dr. Kidd's Observations respecting 
in groups, which resemble recently fallen flakes of snow, or 
investin<y the wall like a fine down. 
During the severely cold weather of January, 1814, it 
appeared in some places in the form of minute dense grains 
closely aggregated ; while in others it still continued to wear 
the appearance of down or wool : and the local circumstances 
most obviously connected with this difference in the manner 
of crystallization, were the presence of a greater degree of 
light, where the granular deposition took place, and a less 
degree of shelter from the influence of cold air. 
In some instances the production of the saltpetre is accom- 
panied with a disintegration of the substance of the stone on 
which it is formed : but this circumstance is only observable 
on stones of a loose texture. 
The shortest interval I have observed between the time of 
its having been brushed away and its reappearance, is four 
hours: but it was then in full efflorescence, and would pro- 
bably have been visible much earlier. The observation was 
made on November the 17th, about midnight : there had been 
snow in the middle of the day, and the night was frosty. 
The spontaneous formation of nitre takes place indifferently 
on the surface of the stones composing a wall, and of the 
mortar by which those stones are cemented : and near the 
close of the late frost, I observed it for the first time on the 
surface of a partition, consisting entirely of laths plastered over 
with the mortar or stucco commonly used for that purpose. 
It accumulates in greater quantity on some parts of a given 
surface than on others ; and this difference in the degree of its 
accumulation, is probably connected with some slight difference 
in the texture or composition of the stone on which it is formed : 
