NITRE. 
449 
The second circumstance favourable to the pro- 
duction of saltpetre is the putrefaction of animal and 
vegetable matters. It is well known that stables, 
dunghills, and other places containing decayed ani- 
mal or vegetable substances, afford abundance of 
this salt. Beds for the artificial production of ni- 
tre have always been established upon this prin- 
ciple ; and the manner which is pursued to effect 
the purpose has been thus described : ditches or 
covered sheds are formed, and left exposed on all sides 
to the air ; these are then filled with vegetable sub- 
stances and dung of all kinds, the mass being stirred 
from time to time to expose a fresh surface to the ac- 
tion of the air, and water occasionally poured upon it. 
When the putrefaction is considerably advanced, a 
small portion of the matter is washed in order to 
detect the nitre which it may contain ; and if it is 
found sufficiently abundant, the whole mass is 
treated in the same manner. 
Air is the third agent which seems especially re- 
quisite to the formation of nitre. This is said to be 
a principal cause of the saline efflorescence on the 
surface of walls ; and it is to receive all possible as- 
sistance from this element, that the mixture depo- 
sited for the production of artificial nitre is so fre- 
quently stirred. 
The following method is commonly used to ex- 
tract this salt from the substances in which it is 
formed. It is necessary, however, to premise that 
nitre dissolves very easily in water, which when 
von. hi. 2 G 
