44 6 
NITRE. 
housscige. It is a neutral salt formed by the union 
of nitrous acid and fixed vegetable alkali, or potash, 
and although so abundantly diffused in Nature, is 
very rarely crystallized in the form obtained by 
means of art, and noticed in the specification. 
Most countries produce nitre, but not in consi- 
derable masses; and it may be remarked that, con- 
trary to sal gem, it is very rarely if ever found 
within the earth, being always confined to the 
surface ; and therefore we may reasonably suppose 
that the air is indispensably necessary to its pro- 
duction. 
Chalky plains which are dry and much exposed 
to the sun, as well as sandy situations which con- 
tain chalk, are frequently covered with a crust of 
nitre. Thus it is found in considerable quantities 
in several plains in Spain, as well as in Hungary, 
which appears to be a most productive salt country, 
since it contains the finest mines of sal gem, as well 
as great abundance of nitre. This salt is found on 
the surface of the soil to the north-west of Debret- 
zin ; besides which there are nitrous waters about 
thirty feet deep in several parts of this country. 
But the most remarkable collection of native ni- 
tre is that in Italy, discovered by the Abbe Fortis 
in the year 1783. It is situated at Molfetta in a hol- 
low called the Puio. In this hollow, which is about 
one hundred feet deep, there are several natural 
grottos ; in the interior part of which, between 
strata of compact limestone, nitre is found regularly 
crystallized. The stone itself is so richly impreg- 
