Dr. Kidd's Observations , &c. 509 
Hie usual and almost exclusive occurrence of saltpetre on 
walls constructed of limestone, and in situations exposed to 
animal and vegetable effluvia, in all probability led to the em- 
pirical practice of heaping together the mortar and refuse of 
old buildings with putrescent animal and vegetable matter; 
from a mixture of which kind, after exposure for a sufficient 
length of time to the action of the air, a quantity of nitre may 
usually be obtained by lixiviation : but it would be a question 
of mere curiosity, on this occasion at least, to investigate the 
origin of the practice. The intention of the present Paper is 
to state the result of a series of observations made during the 
last year, on the connexion that exists between the natural 
production of nitre and the state of the atmosphere. In de- 
tailing these observations, it will be convenient to give previ- 
ously a description of the laboratory of the Ashmole Museum, 
in which building they were principally made: nor shall I be 
afraid of being thought too minute in this description, or in 
any other part of the following detail, by those at least who 
know the precision that is requisite in every induction, which 
like the present rests on phenomena of an obscure and equi- 
vocal nature. 
The Ashmole Museum, which was built by Sir C. Wren 
in the reign of Charles the Second, is an insulated building, 
constructed entirely of calcareous freestone, and consisting of 
three stories. The lowermost of these stories was originally 
designed for, and has constantly been used as, a chemical 
laboratory. 
The pavement of the laboratory, on its eastern, northern, 
and western sides, is about nine feet below the level of the 
street in which the Museum stands : on its southern side it is 
mdcccxiv. 3 U 
