NITRIC ACID, 
The nitrate of potash is the salt well 
known by the name of nitre, or saltpetre. 
It is found ready formed in the East Indies, 
in Spain, in the kingdom of Naples, and 
elsewhere, in considerable quantities ; but 
nitrate of lime is still more abundant. Far 
the greater part of the nitrate made use of 
is produced by a combination of circum- 
stances which tend to compose and condense 
nitric acid. This acid appears to be pro- 
duced in all situations, where animal matters 
are completely decomposed with access of 
air, and of proper substances with which 
it can readily combine. Grounds frequently 
trodden by cattle, and impregnated with 
their excrements, or the walls of inhabited 
places where putrid animal vapours abound, 
such as slaughter-houses, drains, or the like, 
afford nitre by long exposure to the air. 
Artificial nitre beds are made by an atten- 
tion to the circumstances in which this salt 
is produced by nature. Dry ditches are 
dug, and covered with sheds, open at the 
sides, to keep off the rain ; these are filled 
with animal substances, such as dung, or 
other excrements, with the remains of ve- 
getables, and old mortar, or other loose cal- 
careous earth ; this substance being found 
to be the best and most convenient recep- 
tacle for the acid to combine with. Occa- 
sional watering, and turning up from time 
to time, are necessary, to accelerate the 
process, and increase the surfaces to which 
the air may apply; but too much moisture 
is hurtful. Vl^en a certain portion of ni- 
trate is formed, the process appears to go 
on more quickly : but a certain quantity 
stops it altogether, and after this cessation 
the materials will go on to furnish more, if 
what is formed be extracted by lixiviation. 
After a succession of many months, more or 
less, according to the management of the 
operation, in which the action of a regular 
current of fresh air is of the greatest im- 
portance, nitre is found in the mass. If tlie 
beds contained much vegetable matter, a 
considerable portion of the nitrous salt will 
be common saltpetre ; but, if otherwise, the 
acid will, for the most part, be combined 
with the calcareous earth. 
To extract the saltpetre from the mass of 
earthy matter, a number of large casks are 
prepared, with a cock at the bottom of 
each, and a quantity of straw within, to 
prevent its being stopped up. Into these 
the matter is put, together with wood-ashes 
either strewed at top, or added during the 
filling. Boiling water is then poured on, 
and suffered to stand for some time ; after 
which it is drawn off, and other water 
added in the same manner, as long as any 
saline matter can be thus extracted. The 
weak brine is heated, and passed through 
other tubs, until it becomes of consider- 
able strength. It is then carried to the 
boiler, and contains nitre and other salts ; 
the chief of which is common culinary salt, 
and sometimes muriate of magnesia. 
It is the property of nitre to be much 
more soluble in hot than cold water ; but 
common salt is very nearly as soluble in cold 
as in hot water. Whenever, therefore, the 
evaporation is carried by boiling to a cer- 
tain point, much of the common salt will 
fall to the bottom, for want of water to hold 
it in solution, though the nitre will remain 
suspended by virtue of the heat. The com- 
mon salt thus separated is taken out with a 
perforated ladle, and a small quantity of 
the fluid is cooled, from time to time, that 
its concentration may be known by the 
nitre which crystallizes in it. When the 
fluid is sufficiently evaporated, it is taken 
out and cooled, and great part of the nitre 
separates in crystals ; while the remaining 
common salt continues dissolved, because 
equally soluble in cold and in hot water. 
Subsequent evaporation of the residue will 
separate more nitre in the same manner. 
This nitre, which is called nitre of the 
first boiling, contains some common salt ; 
from which it may be purified by solution 
in a small quantity of water, and subse- 
quent evaporation : for the crystals thus 
obtained are much less contaminated with 
common salt than before ; because the pro- 
portion of water is so much larger with 
respect to the small quantity contained by 
the nitre, that very little of it will crystal- 
lize. For nice purposes, the solution 
and crystallization of nitre are repeated 
four times. The crystals of nitre are usu- 
ally of the form of six-sided flattened 
prisms, with diedral summits. Its taste is 
penetrating ; but the cold produced, by 
placing the salt to dissolve in the mouth, is 
such as to predominate over the real taste 
at first. Seven parts of water dissolve two 
of nitre, at the temperature of sixty de- 
grees : but boiling water dissolves its own 
weight. One hundred parts of alcohol, at 
a heat of one hundred and seventy-six de- 
grees, dissolve only 2.9. 
On being exposed to a gentle heat, nitre 
fuses ; and in this state being poured into 
moulds, so as to form little round cakes, or 
balls, it is called sal prunella, or crystal mi- 
neral. This at least is the way in which 
