Part 3

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.

TABLE I.

_Containing Approximations to the quantities of NITRIC ACID, NITROUS GAS, and WATER in NITROUS ACIDS, of different colors and specific gravities._ +------------------+---+----------+---+--------+-------+---------+ | 100 | | Specific | | Nitric | Water | Nitrous | | Parts | | Gravity | | Acid | | gas | +------------------+---+----------+---+--------+-------+---------+ |Sol. Nitric Acid | | 1,504 | c | 91,55 | 8,45 | — — | |Yellow Nitrous[38]| | 1,502 | o | 90,5 | 8,3 | 1,2 | |Bright Yellow | o | 1,500 | n | 88,94 | 8,10 | 2,96 | |Dark Orange | f | 1,480 | t | 86,84 | 7,6 | 5,56 | |Light Olive‡ | | 1,479 | a | 86,00 | 7,55 | 6,45 | |Dark Olive‡ | | 1,478 | i | 85,4 | 7,5 | 7,1 | |Bright Green‡ | | 1,476 | n | 84,8 | 7,44 | 7,76 | |Blue Green[39] | | 1,475 | | 84,6 | 7,4 | 8,00 | +------------------+---+----------+---+--------+-------+---------+ ‡ = “FOOTNOTE {38}”

[38] The blue green acid is not homogeneal in its composition, it is composed of the blue green spherules and the bright green acid. The blue green spherules are of greater specific gravity than the dark green acid, probably because they contain little or no water.

[39] The composition of the acids thus marked, is given from calculations.

TABLE II.

_Binary Proportions of OXYGENE and NITROGENE in NITRIC and NITROUS ACIDS._[40]

+---------------+---+-------+--------+-------------+-------+-------+ | 100 Parts | |Oxygene| Nitro- | | Nitro-|Oxygene| | | | | gene | | gene | | +---------------+---+-------+--------+-------------+-------+-------+ |Nitric Acid | c | 70,50 | 29,50 | | 1 | 2,389 | +---------------+ o +-------+--------+ +-------+-------+ |Bright yellow | n | 70,10 | 29,90 | | 1 | 2,344 | | Nitrous | t | | |Proportions. | | | +---------------+ a +-------+--------+ Nitrogene. +-------+-------+ |Orange coloured| i | 69,63 | 30,37 | Unity. | 1 | 2,292 | +---------------+ n +-------+--------+ +-------+-------+ |Dark Green | | 69,08 | 30,92 | | 1 | 2,230 | +---------------+---+-------+--------+-------------+-------+-------+

[40] Nitrous gas contains 44,05 Nitrogene, and 55,95 Oxygene, as has been said before.

XI. I have before mentioned that dilute nitric acids are incapable of dissolving so much nitrous gas in proportion to their quantities of true acid, as concentrated ones. During their absorption of it, they go through similar changes of color; 330 grains of nitric acid, of specific gravity 1,36, after 50 cubic inches of gas had been passed through it, became blue green, and of specific gravity 1,351. It had gained in weight but 3 grains; and when the nitrous gas was driven from it by heat into a water apparatus, but 7 cubic inches were collected.[41]

From the diminution of specific gravity of nitric acid by combination with nitrous gas, and from the smaller attraction of nitric acid for nitrous gas, in proportion as it is diluted, it is probable that the nitrated acids, in their combinations with water, do not contract so much as[42] nitric acids of the same specific gravities. The affinities resulting from the small attraction of nitrous gas for water, and its greater attraction for nitric acid, must be such as to lessen the affinity of nitric acid and water for each other.

[41] A great portion of it, of course, dissolved in the water with the nitrous acid carried over.

[42] Their changes of volume, corresponding to changes of temperature, most probably, are likewise different.

Hence it would require an infinite number of experiments to ascertain the real quantities of acid, nitrous gas, and water, contained in the different diluted nitrous acids; and after these quantities were determined, they would probably have no important connection with the chemical arrangement. As yet, our instruments of experiment are not sufficiently exact to afford us the means of ascertaining the ratio in which the attraction of nitric acid[43] for water diminishes in its progress towards saturation.

[43] Probably in the ratio of the square of the quantity of water united to it.

The estimations in the following table, of the real quantities of nitric acid in solutions of different specific gravities, were deduced from experiments made in the manner described in section VI, except that the phial employed was longer, narrower, and graduated to half grains. The temperature, at the time of combination, was from 40° to 46°.

TABLE III.

_Of the Quantities of True NITRIC ACID in solutions of different SPECIFIC GRAVITIES._

+----------------+-----+-------------+-------+ |100 Parts Nitric| |True Acid[44]| Water | |Acid of specific| | | | | gravity | | | | +----------------+ +-------------+-------+ | 1,5040 | c | 91,55 | 8,45 | | 1,4475 | o | 80,39 | 19,61 | | 1,4285 | n | 71,65 | 28,35 | | 1,3906 | t | 62,96 | 37,04 | | 1,3551 | a | 56,88 | 43,12 | | 1,3186 | i | 52,03 | 47,97 | | 1,3042 | n | 49,04 | 50,96 | | 1,2831 | | 46,03 | 53,97 | | 1,2090 | | 45,27 | 54,73 | +----------------+-----+-------------+-------+

[44] The quantities of Oxygene and Nitrogene in any solution, may be thus found—— Let A = the true acid, X the oxygene, and Y the nitrogene.

Then

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.