the acid condenses in the recipient in form of a dark orange red liquid,
while the rest combines with the water in the bottles. During the
distillation, a large quantity of oxygen gas escapes, owing to the
greater affinity of oxygen to caloric, in a high temperature, than to
nitrous acid, though in the usual temperature of the atmosphere this
affinity is reversed. It is from the disengagement of oxygen that the
nitric acid of the neutral salt is in this operation converted into
nitrous acid. It is brought back to the state of nitric acid by heating
over a gentle fire, which drives off the superabundant nitrous gas, and
leaves the nitric acid much diluted with water.
Nitric acid is procurable in a more concentrated state, and with much
less loss, by mixing very dry clay with saltpetre. This mixture is put
into an earthern retort, and distilled with a strong fire. The clay
combines with the potash, for which it has great affinity, and the
nitric acid passes over, slightly impregnated with nitrous gas. This is
easily disengaged by heating the acid gently in a retort, a small
quantity of nitrous gas passes over into the recipient, and very pure
concentrated nitric acid remains in the retort.
We have already seen that azote is the nitric radical. If to 20-1/2
parts, by weight, of azote 43-1/2 parts of oxygen be added, 64 parts of
nitrous gas are formed; and, if to this we join 36 additional parts of
oxygen, 100 parts of nitric acid result from the combination.
Intermediate quantities of oxygen between these two extremes of
oxygenation produce different species of nitrous acid, or, in other
words, nitric acid less or more impregnated with nitrous gas. I
ascertained the above proportions by means of decomposition; and, though
I cannot answer for their absolute accuracy, they cannot be far removed
from truth. Mr Cavendish, who first showed by synthetic experiments that
azote is the base of nitric acid, gives the proportions of azote a
little larger than I have done; but, as it is not improbable that he
produced the nitrous acid and not the nitric, that circumstance explains
in some degree the difference in the results of our experiments.
As, in all experiments of a philosophical nature, the utmost possible
degree of accuracy is required, we must procure the nitric acid for
experimental purposes, from nitre which has been previously purified
from all foreign matter. If, after distillation, any sulphuric acid is
suspected in the nitric acid, it is easily separated by dropping in a
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