450 
NITRE. 
boiling takes up a quantity nearly equal to twice its 
own weight ; and upon this principle alone, the 
salt is extracted from the rubbish that contains it. 
The saltpetre-makers, after having pounded the 
plaster or rubbish, put it into a vessel with a hole 
at the bottom, and cover it with ashes. Through 
this they pour water already impregnated with ni- 
tre, in order that it may be completely saturated ; 
after which the lixivium is evaporated in copper 
vessels. The first pellicles which they skim off 
during the evaporation, consist only of the marine 
salt contained in the rubbish, called grain-, and 
this, it appears by their regulations, they are obliged 
to carry to the refining-houses. When the water 
is evaporated to such a degree that the residue 
when cool must become solid, they put it into other 
vessels in which the nitre is crystallized. This salt, 
which is very impure and dirty, is called nitre of 
the first boiling, and contains several other salts be- 
sides saltpetre. The mixture is separated from 
these extraneous substances by dissolving it in the 
smallest possible quantity of water, and clarifying 
the boiling liquor with bullocks’ blood, which col- 
lects all the impurities on the surface in the form of 
scum. The lixivium is then evaporated, and a much 
purer nitre obtained from it, called nitre of the se- 
cond boiling. This, however, is yet vitiated by a 
certain quantity of other salts, from which it is 
purified by a third boiling; when it is made to 
crystallize very rapidly, and forms large masses at 
