238 A A
X = —————— and Y = ————
239 239
XII. The blue green spherules mentioned in section V. produced by
the condensation of nitrous vapor, and by the combination of nitric
acid with nitrous gas, may be considered as saturated solutions of
nitrous gas in nitric acid. The combinations of nitric acid and nitrous
gas containing a larger proportion of nitrous gas, are incapable of
existing in the fluid state at common temperatures; and, as appears
from the first experiment, an increase of volume takes place during
their formation. They consequently ought to be looked upon as solutions
of nitric acid in nitrous gas, identical with the nitrous vapor of
Priestley.
From the researches of this great discoverer, we learn that nitrous
vapor is decomposable, both by water and mercury. Hence it is almost
impossible accurately to ascertain its composition. In one of his
experiments,[45] when more than 130 grains of strong nitrous acid were
exposed for two days to nearly 247 cubic inches of nitrous gas, over
water: about half of the acid was dissolved, and deposited with the gas
in the water.[46]
[45] Experiments and Observations; last edition, vol. 1, page 384.
[46] Nitrous gas, holding in solution nitrous acid, is more readily
absorbed by water than when in its pure form, from being presented to
it in a more condensed state in the green acid, formed by the contact
of water and nitrous vapor.
XIII. In comparing the results of my fundamental experiment on the
composition of nitrous acid, with those of Cavendish, the great
coincidence between them gave me very high satisfaction, as affording
additional proofs of accuracy. If the acid formed in the last
experiment of this illustrious philosopher be supposed analogous to the
light green acid formed in my first experiment, our estimations will be
almost identical.
Lavoisier’s account of the composition of the nitric and nitrous
acids, has been generally adopted. According to his estimation, these
substances contain a much larger quantity of oxygene than I have
assigned to them.
The fundamental experiments of this great philosopher were made at an
early period of pneumatic chemistry,[47] on the decomposition of nitre
by charcoal; and he considered the nitrogene evolved, and the oxygene
of the carbonic acid produced in this process, as the component parts
of the nitric acid contained in the nitre.
[47] Mem. des Savans Etrangers, v. xi. 226. Vide Kirwan sur le
phlogistique pag. 110.
I have before mentioned the liberation of nitrous acid, in the
decomposition of nitre by combustible bodies; and I had reasons for
suspecting that this circumstance was not the only source of inaccuracy.
That my suspicions were well founded, will appear from the following
experiments:
EXPERIMENT _a_. I introduced into a strong glass tube, 3 inches long,
and nearly,3 wide, a mixture of 10 grains of pulverised, well burnt
charcoal, and 60 grains of nitre. It was fired by means of touch-paper,
and the tube instantly plunged under a jar filled with dry mercury.
A quantity of gas, clouded with dense white vapor was collected.
When this vapor was precipitated, so that the surface of the mercury
could be seen, it appeared white, as if acted on by nitrous acid. On
introducing a little oxygene into the jar, copious red fumes appeared.
EXP. _b_. A similar mixture was fired[48] under the jar, the top of
the mercury being covered with a small quantity of red cabbage juice,
rendered green by an alkali. This juice, examined when the vapor
was precipitated, was become red, and on introducing to it a little
carbonate of potash, a slight effervescence took place.
[48] In this experiment, as well as in the last, some of the mixture
was thrown into the jar undecompounded.
EXP. _c_. Five grains of charcoal, and 20 of nitre, were now fired in
the same manner as before, the mercury being covered with a stratum
of water. After the precipitation of the vapor on the introduction of
oxygene, no red fumes were perceived.
EXP. _d_. 30 grains of nitre, 5 of charcoal, and five of silicious
earth,[49] were now mingled and fired. The gas received under mercury
was composed of 18 carbonic acid, and nearly 12 nitrogene.[50] A
little muriatic acid was poured on the residuum in the tube; a slight
effervescence took place.
[49] To detach the potash from the carbonic acid.
[50] This nitrogene contained a little nitrous gas, as it gave red
fumes when exposed to the air. The free nitrous acid was decomposed by
the mercury, as it was not covered with water.
EXP. _e_. The top of the mercury in the jar was now covered with a
little diluted muriatic acid, and a small glass tube filled with a
mixture of 3 grains of charcoal, and 20 nitre. After the deflagration,
the tube itself with the residuum it contained, were thrown into the
jar. The carbonic acid was quickly detached from them by the muriatic
acid, and the whole quantity of gas generated in the process, obtained;
it measured 15 cubic inches.
4 cubic inches of it exposed to solution of potash, diminished to
1⁴/₁₀; 7 of the remainder, with 8 of oxygene, gave only 12.
EXP. _f_. 60 grains of nitre, and 9 of charcoal were fired, the
top of the mercury in the jar being covered with water. After the
deflagration, the tube that had contained them was introduced, and
the carbonic acid contained by the carbonate of potash, disengaged by
muriatic acid. 30 measures of the gases evolved were exposed to caustic
potash; 20 exactly were absorbed, the 10 remaining, with 10 of oxygene,
diminished to 17.
EXP. _g_. A mixture of nitre and charcoal were deflagrated over a
little water in the mercurial jar: after the precipitation of the
vapor, the water was absorbed by filtrating paper. This filtrating
paper, heated in a solution of potash, gave a faint smell of ammoniac.
EXP. _h_. Water impregnated with the vapor produced in the
deflagration, was heated with quicklime, and presented separately to
three persons accustomed to chemical odors. Two of them instantly
recognised the ammoniacal smell, the other could not ascertain it.
Paper reddened with cabbage juice was quickly turned green by the vapor.
These experiments are sufficient to shew that the decomposition of
nitre by charcoal is a very complex process, and that the intense
degree of heat produced may effect changes in the substances employed,
which we are unable to estimate.
The products, instead of being simply carbonic acid, and nitrogene, are
carbonic acid, nitrogene, nitrous acid, probably ammonia, and sometimes
nitrous gas. The nitrous acid is disengaged from the base by the
intense heat. Concerning the formation of the ammonia, it is useless to
reason till we have obtained unequivocal testimonies of its existence;
it may be produced either by the decomposition of the water contained
in the nitre, by the combination of its oxygene with the charcoal, and
of its nascent hydrogene with the nitrogene of the nitric acid; or from
some unknown decomposition of the potash.
As neither Lavoisier nor Berthollet found nitrous gas produced in
the decomposition of nitre by charcoal, when a water apparatus was
employed; and as it was not uniformly evolved in my experiments, the
most probable supposition is, that it arises from the decomposition of
a portion of the free nitrous acid intensely heated, by the mercury.
In none of my experiments was the whole of the nitre and charcoal
decomposed, some of it was uniformly thrown with the gases into the
mercurial apparatus. The nitrogene evolved, as far as I could ascertain
by the common tests, was mingled with no inflammable gas.
If we consider experiment _f_ as accurate, with regard to the relative
quantities of carbonic acid and nitrogene produced, they are to each
other nearly as 20 to 8; that is, allowing 2 for the nitrous gas, and
consequently, reasoning in the same manner as Lavoisier, concerning the
composition of nitric acid, it should be composed of 1 nitrogene to
3,38 oxygene. But though the quantity of oxygene in this estimation is
far short of that given in his, yet still it is too much. From whatever
source the errors arise, whether from the evolution of phlogisticated
nitrous acid, or the decomposition of water, or the production of
nitrous gas, they all tend to increase the proportion of the carbonic
acid to the nitrogene.
I am unacquainted with any experiment from which accurate opinions
concerning the different relative proportions of oxygene and nitrogene
in the nitric and nitrous acids could be deduced. Lavoisier’s
calculation is founded on his fundamental experiment, and on the
combination of nitrous gas and oxygene.
Dr. Priestley’s experiment mentioned in section 12, on the absorption
of nitrous gas by nitrous acid, from which Kirwan[51] deduces the
composition of the differently colored nitrous acids, was made over
water, by which, as is evident from a minute examination of the
facts[52], the greater portion of the nitrous gas employed was absorbed.
[51] Essay on phlogiston.
[52] Dr. Priestley says, “Having filled a phial containing exactly
the quantity of four pennyweights of water, with strong, pale, yellow
spirit of nitre, with its mouth quite close to the top of a large
receiver standing in water, I carefully drew out almost all the common
air, and then filled it with nitrous air; and as this was absorbed,
I kept putting in more and more, till in less than two days it had
completely absorbed 130 ounce measures. Presently after this process
began, the surface of the acid assumed a deep orange color, and when 20
or 30 ounce measures of air were absorbed, it became green at the top:
this green descended lower and lower, till it reached the bottom of the
phial. Towards the end of the process, the evaporation was perceived to
be very great, and when I took it out, the quantity was found to have
diminished to one half. Also it had become, by means of this process,
and the evaporation together, exceeding weak, and was rather blue than
green.”
_Experiments and Observations_, vol. 1, p. 384. Last edition. XIV.
The opinions heretofore adopted respecting the quantities of real or
true acid in solutions of nitrous acid of different specific gravities,
have been founded on experiments made on the nitro-neutral salts,
the most accurate of which are those of Kirwan, Bergman, and Wenzel.
The great difference in the results of these celebrated men, proves
the difficulty of the investigation, and the existence of sources of
error.[53] Kirwan deduces the composition of the solutions of nitrous
acid in water, from an experiment on the formation of nitrated soda.
In this experiment, 36,05 grains of soda were saturated by 145 grains
of nitrous acid, of specific gravity 1,2754. By a test experiment, he
found the quantity of salt formed to be 85,142 grains.[54] Hence he
concludes that 100 parts of nitrous acid, of specific gravity 1,5543,
contain 73,54 of the strongest, or most concentrated acid.
[53] See Mr. Keir’s excellent observations on this subject. Chem. Dict.
Art. Acid.
[54] Irish Transactions, vol. 4, p. 34.
Supposing his estimation perfectly true, 100 parts of the aëriform acid
of 55° would be composed of 74,54 of his real acid, and 25,46 water.
In examining, however, one of his later experiments,[55] we shall find
reasons for concluding, that the acid in nitrated soda cannot contain
much less water than the aëriform acid. A solution of carbonated soda,
containing 125 grains of real alkali, was saturated by 306,2 grains of
nitrous acid, of specific gravity 1,416. The evaporation was carried
on in a temperature not exceeding 120°, and the residuum exposed to a
heat of 400° for six hours, at the end of which time it weighed 308
grains. Now according to my estimation, 306 grains of nitric acid, of
1,416, should contain 215 true acid; and we can hardly suppose, but
that during the evaporation and consequent long exposure to heat, some
of the nitrated soda was lost with the water.
[55] Addit. Obs. pag. 74.
Bergman estimates the quantity of water in this salt at 25, and the
acid at 43 per cent; but his real acid was not so concentrated as
Kirwan’s, consequently the nitric acid in nitrated soda should contain
more water than my true acid.
Wenzel, from an experiment on the composition of nitrated soda,
concludes that it contains 37,48 of alkali, and 62,52 of nitrous acid;
and 1000 of this acid, from Kirwan’s calculation, contain 812,6 of his
real acid; consequently, 100 parts of my aëriform acid should contain
93,28 of Wenzel’s acid, and 6,72 of water.
I saturated with potash 54 grains of solution of nitric acid, of
specific gravity 1,301. Evaporated at about 212°, it produced 66 grains
of nitre. This nitre exposed to a higher temperature, and kept in
fusion for some time, was reduced to 60 grains.
Now from the table, 54 of 1,301, should contain 26,5 of true acid. But
according to Kirwan’s estimation, 100 parts of dry nitre contain 44[56]
of his real acid, with 4 water; consequently 60 should contain 26,4.
[56] Additional Observations, page 70.
Again, 90 grains of acid, of specific gravity 1,504, saturated with
potash, and treated in the same manner, gave 173 grains of dry nitre.
Consequently, 100 parts of it should contain 47,3 grains of true acid.
Now Lavoisier[57] allows about 51 of dry acid to 100 grains of nitre,
and Wenzel 52.
From Berthollet’s[58] experiments, 100 grains of nitre, in their
decomposition by heat, give out nearly 49 grains of gas.[59]
Hence it appears that the aëriform acid, that is, the true acid of my
table, contains rather less water than the acid supposed to exist in
nitre.
[57] Elements, pag. 103, Kerr’s Translation.
[58] Mem. Acad. 1787.
[59] As well as oxygene and nitrogene, Mr. Watt’s experiments prove
that much phlogisticated nitrous acid is produced.