_EXPERIMENTS and OBSERVATIONS on the composition
of_ NITRIC ACID, _and on its combinations
with_ WATER _and_ NITROUS GAS.
I. Though since the commencement of Pneumatic Chemistry, no substance
has been more the subject of experiment than Nitrous Acid; yet still
the greatest uncertainty exists with regard to the quantities of the
principles entering into its composition.
In comparing the experiments of the illustrious Cavendish on the
synthesis of nitrous acid, with those of Lavoisier on the decomposition
of nitre by charcoal, we find a much greater difference in the results
than can be accounted for by supposing the acid formed, and that
decomposed, of different degrees of oxygenation.
In the most accurate experiment of Cavendish, when the nitrous acid
appeared to be in a state of deoxygenation, 1 of nitrogene combined
with about 2,346 of oxygene.[3] In an earlier experiment, when the
acid was probably fully oxygenated, the nitrogene employed was to the
oxygene nearly as 1 to 2,92.[4]
Lavoisier, from his experiments on the decomposition of nitre, and
combination of nitrous gas and oxygene, concludes, that the perfectly
oxygenated, or what he calls nitric acid, is composed of nearly 1
nitrogene, with 3,9 of oxygene; and the acid in the last state of
deoxygenation, or nitrous acid, of about 3 oxygene with 1 nitrogene.[5]
[3] Phil. Trans. v. 78, p. 270.
[4] Phil. Trans. v. 75 p. 381.
[5] Elem. Kerr’s Trans. page 76, and 216, and Mem. des Sav. Etrang.
tom. 7, page 629.
Great as the difference is between the estimations of these
philosophers, we find differences still greater in the accounts of the
quantities of nitrous gas necessary to saturate a given quantity of
oxygene, as laid down by very accurate experimentalists. On the one
hand, Priestley found 1 of oxygene condensed by 2 of nitrous gas, and
Lavoisier by 1⅞. On the other, Ingenhouz, Scherer, and De la Metherie,
state the quantity necessary to be from 3 to 5.[6] Humbolt, who has
lately investigated Eudiometry with great ingenuity, considers the mean
quantity of nitrous gas necessary to saturate 1 of oxygene, as about
2,55.[7]
[6] Ingenhouz sur les Vegetaux, pag. 205. De la Metherie. Essai sur
differens Airs, pag. 252.
[7] Annales de Chimie, tome 28, p. 168.
II. To reconcile these different results is impossible, and the
immediate connection of the subject with the production of nitrous
oxide, as well as its general importance, obliged me to search for
means of accurately determining the composition of nitrous acid in its
different degrees of oxygenation.
The first desideratum was to ascertain the nature and composition of a
fluid acid, which by being deprived of, or combined with nitrous gas,
might become a standard of comparison for all other acids.
To obtain this acid I should have preferred the immediate combination
of oxygene and nitrogene over water by the electric spark, had it
been possible to obtain in this way by a common apparatus sufficient
for extensive examination; but on carefully perusing the laborious
experiments of Cavendish, I gave up all thoughts of attempting it.
My first experiments were made on the decomposition of nitre,
formed from a known quantity of pale nitrous acid of known specific
gravity, by phosphorus, tin, and charcoal: but in those processes,
unascertainable quantities of nitrous acid, with excess of nitrous
gas, always escaped undecompounded, and from the non-coincidence
of results, where different quantities of combustible substances
were employed, I had reasons for believing that water was generally
decomposed.
Before these experiments were attempted, I had analized nitrous gas
and nitrous oxide, in a manner to be particularly described hereafter;
so that a knowledge of the quantities of nitrous gas and oxygene
entering into the composition of any acid, enabled me to determine the
proportions of nitrogene and oxygene it contained. In consequence of
which I attempted to combine together oxygene and nitrous gas, in such
a manner as to absorb the nitrous acid formed by water, in an apparatus
by which the quantities of the gases employed, and the increase of
weight of the water, might be ascertained; but this process likewise
failed. It was impossible to procure the gases perfectly free from
nitrogene, and during their combination, this nitrogene made to pass
into a pneumatic apparatus communicating with a vessel containing the
water carried over with it, much nitrous acid vapor, of different
composition from the acid absorbed.
After many unsuccessful trials, Dr. Priestley’s experiments on nitrous
vapor[8] induced me to suppose that oxygene and nitrous gas, made to
combine out of the contact of bodies having affinity for oxygene, would
remain permanently aëriform, and on throwing them separately into an
exhausted glass balloon, I found that this was actually the case;
increase of temperature was produced, and orange colored nitrous acid
gas formed, which after remaining for many days in the globe, at a
temperature below 56°, did not in the slightest degree condense.
[8] Experiments and Observations, Vol. iii. last edition, page 105, &c.
This fact afforded me the means not only of forming a standard acid,
but likewise of ascertaining the specific gravity of nitrous acid in
its aëriform state.
III. Previous to the experiment, for the purpose of correcting
incidental errors, I was induced to ascertain the specific gravity of
the gases employed, particularly as I was unacquainted with any process
by which the weight of nitrous gas had been accurately determined. Mr.
Kirwan’s estimation, which is generally adopted, being founded upon
the comparison of the loss of weight of a solution of copper in dilute
nitrous acid, with the quantity of gas produced.[9]
The instruments that I made use of for containing and measuring my
gases, were two mercurial airholders graduated to the cubic inch of
Everard, and furnished with stop-cocks.[10]
[9] When copper is dissolved in dilute nitrous acid, certain quantities
of nitrogene are generally produced, likewise the nitrous gas carries
off in solution some nitrous acid.
[10] This airholder, considered as a pneumatic instrument, is of
greater importance, and capable of a more extensive application than
any other. It was invented by Mr. W. CLAYFIELD, and in its form is
analogous to Mr. WATT’S hydraulic bellows, consisting of a glass bell
playing under the pressure of the atmosphere, in a space between two
cylinders filled with mercury. A particular account of it will be given
in the appendix.
They were weighed in a glass globe, of the capacity of 108 cubic
inches, which with the small glass stop-cock affixed to it, was equal,
when filled with atmospheric air, to 1755 grains. The balance that I
employed, when loaded with a pound, turned with less than one eighth of
a grain.
Into a mercurial airholder, of the capacity of 200 cubic inches, 160
cubic inches of nitrous gas were thrown from a solution of mercury in
nitrous acid.
70 measures of this were agitated for some minutes in a solution of
sulphate of iron,[11] till the diminution was complete. The nitrogene
remaining hardly filled a measure; and if we suppose with Humbolt[12]
that a very small portion of it was absorbed with the nitrous gas, the
whole quantity it contained may be estimated at 0,0142, or ¹/₇₀.
[11] This absorption will be hereafter particularly treated of.
[12] Annales de Chimie. Tome xviii. page 139.
75 cubic inches received from the airholder into an exhausted balloon,
increased it in weight 25,5 grains; thermometer being 56°, and
barometer 30,9. And allowing for the small quantity of nitrogene in the
gas, 100 cubic inches of it will weigh 34.3 grains.
One hundred and thirty cubic inches of oxygene were procured from oxide
of manganese and sulphuric acid, by heat, and received in another
mercurial airholder.
10 measures of it, mingled with 26 of the nitrous gas, gave, after
the residuum was exposed to solution of sulphate of iron, rather more
than one measure. Hence we may conclude that it contained about 0,1
nitrogene.
60 cubic inches of it weighed 20,75 grains; and accounting for the
nitrogene contained in these, 100 grains of pure oxygene will weigh
35,09 grains.
Atmospherical air was decomposed by nitrous gas in excess; and the
residuum washed with solution of sulphate of iron till the Nitrogene
remained pure; 87 cubic inches of it weighed 26,5 grains, thermometer
being 48°, barometer 30,1; 100 will consequently weigh 30,45.
90 cubic inches of the air of the laboratory not deprived of its
carbonic acid, weighed 28,75 grains; thermometer 53, barometer 30: 100
cubic inches will consequently weigh 31,9.[13] 16 measures of this air,
with 16 nitrous gas, of known composition, diminished to 19. Hence it
contained about,26 oxygene.[14]
In comparing my results with those of Lavoisier and Kirwan, the
estimation of the weights of nitrogene and oxygene is very little
different, the corrections for temperature and pressure being made,
from that of those celebrated philosophers. The first makes oxygene to
weigh[15] 34,21, and nitrogene 30,064 per cent; and the last, oxygene
34,[16] and nitrogene 30,5.
[13] A table of the specific gravities of these gases, and other gases,
hereafter to be mentioned, reduced to a barometrical and thermometrical
standard, will be given in the appendix.
[14] 40 measures, exposed to solution of potash, gave an absorption of
not quite a quarter of a measure: hence it contained an inconsiderable
quantity of carbonic acid.
[15] Traité Elementaire.
[16] Essai sur le phlogistique, page 30.
The specific gravity of nitrous gas, according to Kirwan, is to that of
common air as 1194 to 1000. Hence it should weigh about 37 grains per
cent. This difference from my estimation is not nearly so great as I
expected to have found it.[17]
IV.[18] The thermometer in the laboratory standing at 55°, and the
barometer at 30,1, I now proceeded to my experiment. The oxygene that
I employed was of the same composition as that which I had previously
weighed. The nitrous gas contained,0166 nitrogene.
For the purpose of combining the gases, a glass balloon was procured,
of the capacity of 148 cubic inches, with a glass stop-cock adapted to
it, having its upper orifice tubulated and graduated for the purpose of
containing and measuring a fluid. The whole weight of this globe and
its appendages, when filled with common air, was 2066,5 grains.
[17] The diminution of the specific gravity of the gas from the
quantity of nitrogene evolved in his experiment, probably destroyed, in
some measure, the source of error from the nitrous acid carried over.
[18] Experiment I.
It was partially exhausted by the air-pump, and lost in weight just
32 grains. From whence we may conclude that about 15 grains of air
remained in it.
In this state of exhaustion it was immediately cemented to the
stop-cock of the mercurial airholder, and the communication being made
with great caution, 82 cubic inches of nitrous gas rushed into the
globe, on the outside of which a slight increase of temperature was
perceived, while the gases on the inside appeared of a deep orange.
Before the common temperature was restored, the communication was
stopped, and the globe removed. The increase of weight was 29,25
grains; whence it appeared that 1,14 grains of common air, part of
which had been contained in the stop-cocks, had entered with the
nitrous gas.
Whilst it was cooling, from the accidental loosening of the stopper of
the cock, 3 grains more of common air entered.[19]
[19] That no greater contraction took place depended on the solution
of the nitrous acid formed in the nitrous gas; a phænomenon to be
explained hereafter.
The communication was now made between the globe and the mercurial
airholder containing oxygene. 64 cubic inches were slowly pressed in,
when the outside of the globe became warmer, and the color on the
inside changed to a very dark orange. As it cooled, 6 cubic inches more
slowly entered; but no new increase of temperature, or change of color
took place.
The globe being now completely cold, was stopped, removed, and
weighed; it had gained 24,5 grains, from whence it appears that 0,4
grains of common air contained in the stop-cocks, had entered with the
oxygene.[20]
[20] I judged it expedient always to ascertain the quantity of air in
the stop-cocks by weight, as it was impossible to join them so as to
have always an equal capacity. The upper tubes of the two stop-cocks
not joined, contained nearly an inch and half.
To absorb the nitrous acid gas, 41 grains of water were introduced by
the tube of the stop-cock, which though closed as rapidly as possible,
must have suffered nearly,5 grains of air to enter at the same time, as
the increase of weight was 41,5 grains. The dark orange of the globe
diminished rapidly; it became warm at the bottom, and moist on the
sides. After a few minutes the color had almost wholly disappeared.
To ascertain the quantity of aëriform fluid absorbed, the globe was
again attached to the mercurial air apparatus, containing 140 cubic
inches of common air. When the communication was made, 51 cubic inches
rushed in, and it gained in weight 16,5 grains.
A quantity of fluid equal to 54 grains was now taken out of the globe.
On examination it proved to be slightly tinged with green, and occupied
a space equal to that filled by 41,5 grains of water. Its specific
gravity was consequently 1,301.
To ascertain if any unabsorbed aëriform nitrous acid remained in the
globe, 13 grains of solution of ammonia were introduced in the same
manner as the water, and after some minutes, when the white vapor
had condensed, the communication was again made with the mercurial
airholder containing common air. A minute quantity entered, which could
not be estimated at more than three fourths of an inch, and the globe
was increased in weight about 13,25 grains.[21]
Common air was now thrown into the globe till the residual gases of
the experiment were judged to be displaced; it weighed 2106,5 grains,
that is, 40 grains more than it had weighed when filled with common air
before the experiment.[22]
[21] That is, by the solution of ammonia, and air.
[22] The following is an account of the increase and diminution of
weight of the globe, as it was noted in the journal.
Globe filled with common air gr. 2066,5
After exhaustion 2034,5
After introduction of nitrous gas, 82 cubic inches 2064,25
After the accidental admission of common air 2067,25
After the admission of oxygene 2091,75
—— —— 41 grains of water 2133,25
—— —— 51 cubic inches of air 2149,75
Taken out 54 grains of solution 2095,75
Introduced 13 grains of ammoniacal solution 2109,25
After introduction of common air 2106,5
And if from those 40 grains we take 13 for the solution of ammonia
introduced, the remainder, 27, will be the quantity of solution of
nitrous acid in water remaining in the globe, which added to 54, equals
81 grains, the whole quantity formed; but if from this be taken 41
grains, the quantity of water, the remainder 40 grains, will be the
quantity of nitrous acid gas absorbed in the solution.
To find the absolute quantity of nitrous acid formed, we must find
the specific gravity of that absorbed; but as during, and after its
absorption, 17 grains of air, equal to 53,2 cubic inches entered, it
evidently filled such a space. 53,2 cubic inches of it consequently
weigh 40 grains, and 100 cubic inches 75,17 grains. Then,75 cubic
inches weigh,56 grains, and this added to 40, makes 40,56 grains, equal
to 53,95 cubic inches, the whole quantity of aëriform nitrous acid
produced.
But the quantity of nitrous gas entering into this, allowing for the
nitrogene it contained, is 27,6 grains, equal to about 80,5 cubic
inches; and the oxygene is 40,56-27,6 = to 12,96 grains, or 36,9 cubic
inches.
V. There could exist in this experiment no circumstance connected with
inaccuracy, except the impossibility of very minutely determining
the quantities of common air which entered with the gases from the
stop-cocks. But if errors have arisen from this source, they must be
very inconsiderable; as will appear from a calculation of the specific
gravity of the nitrous acid gas, founded on the volume of the gases
that entered the globe.
The air that remained in the globe
after exhaustion was 15 grains = 47[23] cub. in.
The nitrous gas introduced was 82
Common air 13
Oxygene 70
Common air 1
——
Whole quantity of air thrown into the globe 213
From which subtract its capacity 148
——
The remainder is 65
[23] Decimals are omitted, because the excess of the two first numbers
is exactly corrected by the deficiency of the last.
And this remainder taken from 80,5 nitrous gas + 36,9 oxygene, leaves
52,4 cubic inches, which is the space occupied by the nitrous acid gas,
and which differs from 53,95 only by 1,55 cubic inches.
I ought to have observed, that before this conclusive experiment, two
similar ones had been made. In comparing the results of one of them,
performed with the assistance of my friend, Mr. JOSEPH PRIESTLEY, Dr.
PRIESTLEY’S eldest son, and chiefly detailed by him in the journal, I
find a coincidence greater than could be even well expected, where the
processes are so complex. According to that experiment, 41,5 grains
of nitrous acid gas fill a space equal to 53 cubic inches, and are
composed of nearly 29 nitrous gas, and 12,5 oxygene.
We may then conclude, First, that 100 cubic inches of nitrous acid,
such as exists in the[24] aëriform state saturated with oxygene, at
temperature 55°, and atmospheric pressure 30,1 weigh 75,17 grains.
[24] As is evident from the superabundant quantity of oxygene thrown
into the globe.
Secondly, that 100 grains of it are composed of 68,06 nitrous gas, and
31,94 oxygene. Or assuming what will be hereafter proved, that 100
parts of nitrous gas consist of 55,95 oxygene, and 44,05 nitrogene, of
29,9 nitrogene, and 70,1 oxygene; or taking away decimals, of 30 of the
one to 70 of the other.
Thirdly, that 100 grains of pale green solution of nitrous acid in
water, of specific gravity 1,301, are composed of 50,62 water, and 49,38
acid of the above composition.
VI. Having thus ascertained the composition of a standard acid, my next
object was to obtain it in a more condensed state, as it was otherwise
impossible to saturate it to its full extent with nitrous gas. But this
I could effect in no other way than by comparing mixtures of known
quantities of water, and acids of different specific gravities and
colors, with the acid of 1,301.
For the purpose of combining my acids with water, I made use of a
cylinder about 8 inches long, and,3 inches in diameter, accurately
graduated to grain measures, and furnished with a very tight stopper.
The concentrated acid was first slowly poured into it, and the water
gradually added till the required specific gravity was produced;[25]
the cylinder being closed and agitated after each addition, so as to
produce combination without any liberation of elastic fluid.
[25] The weight of the acid poured into the cylinder being known, its
specific gravity was known from the space it occupied in the phial.
The weight of water being likewise known, the specific gravity of the
solution, when the common temperature was produced, was given by the
condensation.
After making a number of experiments with acids of different colors
in this advantageous way, I at length found that 90 grains of a deep
yellow acid, of specific gravity 1,5, became, when mingled at 40° with
77,5 grains of water, of specific gravity 1,302, and of a light green
tinge, as nearly as possible resembling that of the standard acid.
Supposing, then, that these acids contain nearly the same relative
proportions of oxygene and nitrogene, 100 grains of the deep yellow
acid of 1,5, are composed of 91,9 grains true nitrous acid,[26] and 8,1
grains of water.
[26] That is, such as it exists in the aëriform state at 55°. From the
strong affinity of nitrous acid for water, we may suppose that this
acid gas contains a larger proportion of it than the other gases.
To ascertain the difference between the composition of this acid,
and that of the pale, or nitric acid, of the same specific gravity,
I inserted 150 grains of it into a small cylindrical mattrass of the
capacity of,5 cubic inches, accurately graduated to grain measures,
and connected by a curved tube with the water apparatus. After heat had
been applied to the bottom of the mattrass for a few minutes, the color
of the fluid gradually changed to a deep red, whilst the globules of
gas formed at the bottom of the acid, were almost wholly absorbed in
passing through it. In a short time deep red vapour began to fill the
tube, and being condensed by the water in the apparatus, was converted
into a bright green fluid, at the same time that minute globules of
gas were given out. As the heat applied became more intense, a very
singular phænomenon presented itself; the condensed vapor, increased
in quantity, at length filled the curvature of the tube, and when
expelled, formed itself into dark green spherules, which sunk to the
bottom of the water, rested for a moment, and then resolved themselves
into nitrous gas.[27]
[27] This appearance will be explained hereafter.
When the acid was become completely pale, it was suffered to cool,
and weighed. It had lost near 15 grains, and was of specific gravity
1,491. 2 cubic inches and quarter of nitrous gas only were collected.
From this experiment evidently no conclusions could be drawn, as the
nitrous gas had carried over with it much nitrous acid (in the form of
what Dr. Priestley calls nitrous vapor) and was partially dissolved
with it in the water.[28]
[28] This phænomenon will be particularly explained hereafter.
To ascertain, then, the difference between the pale and yellow acids, I
was obliged to make use of synthesis, compared with analysis, carried
on in a different mode, by means of the following apparatus.
VII. To the stop-cock of the upper cylinder of the mercurial airholder,
a capillary tube was adapted, bent so as to be capable of introduction
into an orifice in the stopper of a graduated phial similar to that
employed for mingling acids with water, and sufficiently long to reach
the bottom. With another orifice in the stopper of the phial was
connected a similar tube curved, for the purpose of containing a fluid,
and of increased diameter at the extremity.[29]
50 cubic inches of pure nitrous gas[30] were thrown into the mercurial
apparatus. The graduated phial, containing 90 grains of nitric acid, of
specific gravity 1,5, was placed on the top of the airholding cylinder,
and made to communicate with it by means of the stop-cock and first
tube. Into the second tube a small quantity of solution of potash was
placed. When all the junctures were carefully cemented, by pressing on
the airholder, the nitrous gas was slowly passed into the phial, and
absorbed by the nitrous acid it contained; whilst the small quantities
of nitrogene evolved, slowly drove forward the solution in the curved
tube; from the height of which, as compared with that of the mercury in
the conducing tube, the pressure on the air in the cylinder was known.
In proportion as the nitrous gas was absorbed, the phial became warm,
and the acid changed color; it first became straw-colored, then pale
yellow, and when about 7½ cubic inches had been combined with it,
bright yellow. It had gained in weight nearly 3 grains, and was become
of specific gravity 1,496.
[29] The outline only of this apparatus is given here, as far as was
necessary to make the experiment intelligible; a detailed account of
it, and of its general application, will be given in the appendix.
[30] That is, from nitrous acid and mercury.
This experiment afforded me an approximation to the real difference
between nitric and yellow nitrous acid; and learning from it that
nitric acid was diminished in specific gravity by combination with
nitrous gas, I procured a pale acid of specific gravity 1,504.[31]
After this acid had been combined in the same manner as before, with
about 8 cubic inches of nitrous gas,[32] it became nearly of specific
gravity 1,5, and had gained in weight about 3 grains.
[31] A pale acid of 1.52, by being converted into yellow acid, became
nearly of specific gravity 15,1.
[32] It is impossible to ascertain the quantity of gas absorbed to more
than a quarter of a cubic inch, as the first portions of nitrous gas
thrown into the graduated cylinder are combined with the oxygene of the
common air in it, to form nitrous acid, and hence the slight excess of
weight.
Assuming the accuracy of this experiment as a foundation for
calculation, I endeavoured in the same manner to ascertain the
differences in the composition of the orange colored acids, and the
acids containing still larger proportions of nitrous gas.
93 grains of the bright yellow acid of 1,5 became, when 6 cubic inches
of gas had been passed through it, orange colored and fuming, whilst
the undissolved gas increased in quantity so much as to render it
impossible to confine it by the solution of potash. When 9 cubic inches
had passed through, it became dark orange. It had gained in weight
2,75 grains, and was become of specific gravity 1,48 nearly. Hence it
was evident that much nitrous gas had passed through it undissolved.
25 cubic inches more of nitrous gas were now slowly sent through it:
it first became of a light olive, then of a dark olive, then of a
muddy green, then of a bright green, and lastly of a blue green. After
its assumption of this color, the gas appeared to pass through it
unaltered, and large globules of fluid, of a darker green than the
rest, remained at the bottom of the cylinder, and when agitated, did
not combine with it. The increase of weight was only 1 grain, and the
acid was of specific gravity 1,474 nearly.
In this experiment it was evident that the unabsorbed nitrous gas
had carried over with it a considerable quantity of nitrous acid. I
endeavoured to correct the errors resulting from this circumstance,
by connecting the curved tube first with a small water apparatus, and
afterwards with a mercurial apparatus; but when the water apparatus
was used, the greater part of the unabsorbed gas was dissolved with
the nitrous acid it held in solution, by the water; and when mercury
was employed, the nitrous acid that came over was decomposed, and the
quantity of nitrous gas evolved, in consequence increased.
As it was possible that a small deficiency of weight might arise from
the red vapor given out during the processes of weighing and examining
the acid in the last experiment, 35 cubic inches of nitrous gas were
very slowly passed through 90 grains of pale nitrous acid, of specific
gravity 1,5: it became of similar appearance to that just described,
had gained in weight 6,75 grains, and was become of specific gravity
1,475.
These experiments did not afford approximations sufficiently accurate
towards the composition of deoxygenated acids, containing more nitrous
gas than the dark orange colored. To obtain them, a solution consisting
of 94,25 grains of blue green, or perfectly nitrated acid, (if we
may be allowed to employ the term), of specific gravity 1,475, was
inserted into a graduated phial, and connected by a curved tube, with
the mercurial airholder; in the conductor of which a small quantity of
water was inserted to absorb the nitrous acid which might be carried
over by the gas. Heat was slowly applied to the phial, and nitrous gas
given out with great rapidity. When 4 cubic inches were collected,
the acid became dark olive, when 9 dark red, when 13 bright orange,
and when 18 pale. It had lost 31 grains, and when completely cool,
was of specific gravity 1,502 nearly. The water in the apparatus was
tinged of a light blue; from whence we may conclude that some of the
nitrous gas was absorbed by it with the nitrous acid: but it will be
hereafter proved that the orange colored acid is the most nitrated acid
capable of combining undecompounded with water, and that the color it
communicates to a large quantity of water, is light blue. If then we
take 6,1 grains, the quantity of gas collected, from 31 the loss, the
remainder is 24,9, which reasoning from the synthetical experiment,
may be supposed to contain nearly 3 cubic inches of nitrous gas.
Consequently, 94,25 grains of dark green acid, of specific gravity
1,475, are composed of nearly 21 cubic inches, or 7,2 grains of nitrous
gas, and 87,05 grains of pale nitrous acid, of 1,504.
VIII. Comparing the different synthetical and analytical experiments,
we may conclude with tolerable accuracy, that 92,75 grains of bright
yellow, or standard acid of 1,5, are composed of 2,75 grains of nitrous
gas, and 90 grains of nitric acid of 1,504; but 92,75 grains of
standard acid contain 85,23 grains of nitrous acid, composed of about
27,23 of oxygene, and 58, nitrous gas: now from 58, take 2,75, and the
remainder 55,25, is the quantity of nitrous gas contained in 90 grains
of nitric acid of 1,504; consequently, 100 grains of it are composed
of 8,45 water, and 91,55 true acid, containing 61,32 nitrous gas, and
30,23 oxygene; or 27,01 nitrogene, and 64,54 oxygene: and the nitrogene
in nitric acid, is to the oxygene as 1 to 2,389.
IX. My ingenious friend, Mr. JAMES THOMSON, has communicated to me some
observations relating to the composition of nitrous acid (that is, the
orange colored acid), from which he draws a conclusion which is, in my
opinion, countenanced by all the facts we are in possession of, namely,
“that it ought not to be considered as a distinct and less oxygenated
state of acid, but simply as nitric or pale acid, holding in solution,
that is, loosely combined with, nitrous gas.”[33]
[33] In a letter to me, dated Oct. 28, 1799, after giving an account
of some experiments on the phlogistication of nitric acid by heat and
light, he says, “It was from an attentive examination of the manner in
which the nitric acid was phlogisticated in these experiments, that
I was confirmed in the suspicion I had long before entertained, of
the real difference between the _nitrous_ and _nitric_ acids. It is
not enough to shew that in the _nitrous_ acid, (that is, the nitric
holding nitrous gas in solution), the proportion of oxygene in the
whole compound is less than that entering into the composition of the
nitric acid, and that it is therefore less oxygenated. By the same
mode of reasoning we might prove that water, by absorbing carbonic
acid gas, became less oxygenated, which is absurd. Should any one
attempt to prove (which will be necessary to substantiate the generally
received doctrine) that the oxygene of the nitrous gas combines with
the oxygene of the acid, and the nitrogene, in like manner, so that
the resulting acid, when nitrous gas is absorbed by nitric acid, is a
binary combination of oxygene and nitrogene, he would find it somewhat
more difficult than he at first imagined; it appears to me impossible.
It is much more consonant with experiment to suppose that nitrous acid
is nothing more than nitric acid holding nitrous gas in solution, which
might in conformity to the principles of the French nomenclature, be
called nitrate of nitrogene. The difficulty, and in some cases the
impossibility, of forming nitrites, arises from the weak affinity
which nitrous gas has for nitric acid, compared with that of other
substances; and the decomposition of nitrous acid (that is, nitrate of
nitrogene) by an alkaline or metallic substance, is perfectly analogous
to the decomposition of any other nitrate, the nitrous gas being
displaced by the superior affinity of the alkali for the acid.
“Agreeable to this theory, the salts denominated _nitrites_ are in fact
triple salts, or ternary combinations of nitric acid, nitrous gas, and
salifiable bases.”
This theory is perfectly new to me. Other Chemists to whom I have
mentioned it, have likewise considered it as new. Yet in a subsequent
letter Mr. Thomson mentions that he had been told of the belief of a
similar opinion among the French Chemists.
It is impossible to call any substance a simple acid that is incapable
of entering undecompounded into combination with the alkalies, &c; but
it will appear hereafter that the salts called in the new nomenclature
_nitrites_, cannot be directly formed. If, indeed, it could be proved,
that the heat produced by the combination of nitrous acid with
salifiable bases, was the only cause of the partial decomposition of
it, and that when this process was effected in such a way as to prevent
increase of temperature, no nitrous gas was liberated, the common
theory might have some foundation; but though dilute phlogisticated
nitrous acid combines[34] with alkaline solutions without
decomposition, yet no excess of nitrous gas is found in the solid salt:
it is either disengaged in proportion as the water is evaporated, or it
absorbs oxygene from the atmosphere, and becomes nitric acid.
[34] In some experiments made on the nitrites of potash, and of
ammoniac, before I was well acquainted with the composition of nitric
acid, I found that a light olive-colored acid of 1,28, was capable
of being saturated by weak solutions of potash and ammoniac, without
losing any nitrous gas; but after the evaporation of the neutralised
solution, at very low temperatures, the salts in all their properties
resembled _nitrates_.
In proportion as the nitrous acids contain more nitrous gas, so in
proportion do they more readily give it out. From the blue green acid
it is liberated slowly at the temperature of 50°, and from the green
likewise on agitation. The orange coloured and yellow acids do not
require a heat above 200° to free them of their nitrous gas; and all
the colored acids, when exposed to the atmosphere absorb oxygene, and
become by degrees pale.
If the nitrous vapour, i. e. such as is disengaged during the
_denitration_ of the colored acids, was capable of combining with the
alkalies, it might be supposed a distinct acid, and called nitrous
acid; and the acids of different colors might be considered simply as
compounds of this acid with nitric acid; but it appears to be nothing
more than a solution of nitric acid in nitrous gas, incapable of
condensation, undecompounded, and when decompounded and condensed,
constituting the dark green acid, which is immiscible with water,[35]
and uncombinable with the alkalies.[36]
[35] As is evident from the curious appearance of the dark green
spherules, repulsive both to water, and light green acid.
[36] That is, undecompounded.
It seems therefore reasonable, till we are in possession of new lights
on the subject, to consider, with Mr. Thomson, the deoxygenated or
nitrous acids simply as solutions of nitrous gas composed of sulphuric
acid, metallic oxides, and nitrous gas.[37]
[37] The existence of these bodies will be hereafter proved.
Supposing the truth of these principles according to the logic of the
French nomenclature, there is no acid to which the term nitrous acid
_ought_ to be applied; but as it has been used to signify the acids
holding in solution nitrous gas, it is perhaps better still to apply it
to those substances, than to invent for them new names. A nomenclature,
accurately expressing their constituent parts, would be too complex,
and like all other nomenclatures founded upon theory, liable to
perpetual alterations. Their composition is known from their specific
gravity and their colors; hence it is better to denote it by those
physical properties: thus orange nitrous acid, of specific gravity
1,480, will signify a solution of nitrous gas in nitric acid, in which
the nitric acid is to the nitrous gas, nearly as 87 to 5, and to the
water as 11 to 1.
X. The estimation of the composition of the yellow and orange colored
nitrous acids given in the following table, may be considered as
tolerably accurate, being deduced from the synthetical experiments
in the sixth section, compared with the analytical ones. But as
in the synthetical experiment, when the acid became green, it was
impossible to ascertain the quantity of nitrous gas that passed through
it unabsorbed, and as in the analysis the quantity of nitrous gas
dissolved by the water at different periods of the experiment could not
be ascertained, the accounts of the composition of the green acids must
be considered only as very imperfect approximations to truth.