Part 4

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.

DIVISION II.

_EXPERIMENTS and OBSERVATIONS on the composition of AMMONIAC and on its combinations with WATER and NITRIC ACID._

I. _Analysis of AMMONIAC or VOLATILE ALKALI._

The formation and decomposition of volatile alkali in many processes, was observed by Priestley, Scheele, Bergman, Kirwan, and Higgins; but to Berthollet we owe the discovery of its constituent parts, and their proportions to each other. These proportions this excellent philosopher deduced from an experiment on the decomposition of aëriform ammoniac by the electric spark:[60] a process in which no apparent source of error exists.

[60] Journal de Physique, 1786. Tom. 2, pag. 176.

Since, however, his estimations have been made, the proportions of oxygene and hydrogene in water have been more accurately determined. This circumstance, as well as the conviction of the impossibility of too minutely scrutinizing facts, fundamental to a great mass of reasoning, induced me to make the following experiments.

A porcelain tube was provided, open at both ends, and well glazed inside and outside, its diameter being about,5 inches. To one end of this, a glass tube was affixed, curved for the purpose of communicating with the water apparatus. With the other end a glass retort was accurately connected, containing a mixture of perfectly caustic slacked lime, and muriate of ammoniac.

The water in the apparatus for receiving the gases had been previously boiled, to expel the air it might contain, and during the experiment was yet warm.

When the tube had been reddened in a furnace adapted to the purpose, the flame of a spirit lamp was applied to the bottom of the retort. A great quantity of gas was collected in the water apparatus; of this the first portions were rejected, and the last transferred to the mercurial trough.

A small quantity examined, did not at all diminish with nitrous gas, and burnt with a lambent white flame, in contact with common air.

2¾ of this gas, equal to 110 grain measures, were fired with 2, equal to 80, of oxygene, in a detonating tube, by the electric spark. They were reduced to 2¼, or 90. On introducing to the remainder a solution of strontian, it became slightly clouded on the top, and an absorption of some grain measures took place.

It was evident, then, that in this experiment, charcoal[61] had been somehow present in the tube; which being dissolved by the nascent hydrogene, had rendered it slightly carbonated, and in consequence made the results inconclusive.

[61] Though the tube had never been used, and was apparently clean and dry on the inside, it must have contained something in the form of dust, capable of furnishing either hydrocarbonate, or charcoal.

A tube of thick green glass carefully made clean, was now employed, inclosed in the porcelain tube. Every other precaution was taken to prevent the existence of sources of error, and the experiment conducted as before.

140 grain measures of the gas produced, fired with 120 of oxygene, left, in two experiments, nearly 110. Solution of strontian placed in contact with the residuum, did not become clouded, and no absorption was perceived.

Now 150 measures of gas were destroyed, and if we take Lavoisier’s and Meusnier’s estimation of the composition of water, and suppose the weight of oxygene to be 35 grains, and that of hydrogene 2,6 the hundred cubic inches; the oxygene employed will be to the hydrogene as 243 to 576. Put _x_ for the oxygene, and _y_ for the hydrogene.

Then _x_ + _y_ = 150

_x_ : _y_ :: 243 : 576

243_y_ _x_ = —————— 576

819_y_ = 86400

_y_ = 105 _x_ = 45

And 140 - 105 = 35

Consequently, the nitrogene in ammoniac is to the hydrogene as 35: 105 in volume: and 13,3 grains of ammoniac are composed of 10,6 nitrogene, (supposing that 100 cubic inches weigh 30,45 grains) and 2,7 hydrogene.

According to Berthollet, the weight of the nitrogene in ammoniac is to that of the hydrogene as 121 to 29.[62] The difference between this estimation and mine is so small as to be almost unworthy of notice, and arises most probably from the slight difference between the accounts of Lavoisier and Monge, of the composition of water, and the different weights assigned to the gases employed.

[62] Journal de Physique, 1786, t. 2, 177.

We may then conclude, that 100 grains of ammoniac are composed of about 80 nitrogene, and 20 hydrogene.

The decomposition of ammoniac by heat, as well as by the electric spark, was first discovered by Priestley. In an experiment[63] when aëriform ammoniac was sent through a heated tube from a caustic solution of ammoniac in water, this great discoverer observed that an inflammable gas was produced, though in no great quantity, and that a fluid blackened by matter, probably carbonaceous, likewise came over.

[63] Phil. Trans. vol. 79, page 294.

In my experiments the whole of the ammoniac appeared to be decomposed; the quantity of gas generated was immense, and not clouded, as is usually the case with gases generated at high temperatures. It is possible, that the larger quantity of water carried over in his experiment, by its strong attraction for ammoniac in the aëriform state, might have, in some measure, retarded the decomposition. It is however, more probable to suppose, that a fissure existed in the earthen tube he employed, through which a certain quantity of gas escaped, and coaly matter entered.

Priestley found that the metallic oxides when strongly heated, decomposed ammoniac, the metal being revivified and water and nitrogene produced.[64] The estimations of the composition of ammoniac that may be deduced from his experiments on the oxide of lead, differ very little from those already detailed.

[64] Vol. 2, page 398.

II. _Specific gravity of Ammoniac._

From the great solubility of ammoniac in water, it is difficult to ascertain its specific gravity in the same manner as that of a gas combinable to no great extent with that fluid. It is impossible to prevent the existence of a small quantity of solution of ammoniac in the mercurial airholder,[65] or apparatus containing the gas; and during the diminution of the pressure of the atmosphere on this solution,[66] a certain quantity of gas is liberated from it, and hence a source of error.

[65] Ammoniac generated at a temperature above that of the atmosphere, always deposits ammoniacal solution during its reduction to the common temperature.

[66] By the introduction of aëriform ammoniac into the exhausted globe.

To ascertain, then, the weight of ammoniac, I employed an apparatus similar to that used for the absorption of nitrous gas by nitric acid.

50 cubic inches of gas were collected in the mercurial airholder, from the decomposition of muriate of ammoniac by lime; thermometer being 58°, and barometer 29,6.

100 grains of diluted sulphuric acid were introduced into the small graduated cylinder, which after being carefully weighed, was made to communicate with the airholder, the curved tube containing a small quantity of water. The gas was slowly passed into the fluid, and the globules wholly absorbed before they reached the top; much increase of temperature being consequent. When the absorption was compleat, the phial was increased in weight exactly 9 grains.

This experiment was repeated three times. The difference of weight, which was probably connected with alterations of temperature and pressure, never amounted to more than one sixth of a grain.

We may then conclude, that at temperature 58°, and atmospheric pressure 29,6, 100 cubic inches of ammoniac weigh 18 grains.

According to Kirwan, 100 cubic inches of alkaline air[67] weigh 18,16 grains; barometer 30°, thermometer 61. The difference between these estimations, the corrections for temperature and pressure being made, is trifling.

[67] Additional Observations, page 107.

III. _Of the quantities of true Ammoniac in Aqueous Ammoniacal Solutions, of different specific gravities._

To ascertain the quantities of ammoniac, such as exists in the aëriform state, saturated with moisture, in solutions of different specific gravities, I employed the apparatus for absorption so often mentioned. Thermometer being 52°, the mercurial airholder was filled with ammoniacal gas, and the graduated phial, containing 50 grains of pure water, connected with it. During the absorption of the gas, the phial became warm. When about 30 cubic inches had been passed through, it was suffered to cool, and weighed: it had gained 5,25 grains, and the fluid filled a space equal to that occupied by 57[68] grains of water.

[68] It is necessary in these experiments, that the greatest care be observed in the introduction and extraction of the capillary tube. If it is introduced dry, there will be a source of error from the moisture adhering to it when taken out. I therefore always wetted it before its introduction, and took care that no more fluid adhered to it after the experiment, than before.

Consequently, 100 grains of solution of ammoniac in water of specific gravity,9684 contain 9,502 grains of ammoniac.

The apparatus being adjusted as before, 50 grains of pure water were now perfectly saturated with ammoniac. They gained in weight 17 grains, and when perfectly cool, filled a space equal to 74 of water. Consequently 100 grains of aqueous ammonial solution of specific gravity,9054 contain 25,37 grams of ammoniac.

The two solutions were mingled together; but no alteration of temperature took place. Consequently the resulting specific gravity might have been found by calculation.

On mingling a large quantity of caustic solution of ammoniac with ¼ of its weight of water, of exactly the same temperature, no alteration of it was perceptible by a sensible thermometer.—Hence the two experiments[69] being assumed as data, the intermediate estimations in the following table, were found by calculation.

[69] Previous to those experiments, I had made a number of others on the combination of ammoniac with water.—My design was, to ascertain the diminution of specific gravity for every three grains of ammoniac absorbed; but this I found impossible. The capillary tube, when taken out of the phial, always carried with it a minute portion of the solution, which partially evaporated before it could be again introduced; and thus the sources of error increased in proportion to the number of examinations.

TABLE IV.

_Of approximations to the quantities of AMMONIAC, such as exists in the aëriform state, saturated with water at 52°, in AQUEOUS AMMONIACAL SOLUTIONS of different specific gravities._

+----------+-----+----------+--------+ | 100 | | Ammoniac.| Water. | | Specific | | | | | Gravity. | | | | +----------+ +----------+--------+ | 9054 | | 25,37 | 74,63 | | 9166 | | 22,07 | 77,93 | | 9255 | | 19,54 | 80,46 | | 9326 | c | 17,52 | 82,48 | | 9385 | o | 15,88 | 84,12 | | 9435 | n | 14,53 | 85,47 | | 9476 | t | 13,46 | 86,54 | | 9513 | a | 12,40 | 87,60 | | 9545 | i | 11,56 | 88,44 | | 9573 | n | 10,82 | 89,18 | | 9597 | | 10,17 | 89,83 | | 9619 | | 9,60 | 90,40 | | 9684 | | 9,50 | 90,50 | | 9639 | | 9,09 | 90,91 | | 9713 | | 7,17 | 92,83 | +----------+-----+----------+--------+ As yet no mode has been discovered for obtaining gases in a state of absolute dryness; consequently we are ignorant of the different quantities of water they hold in solution at different temperatures. As far as we are acquainted with the combinations of ammoniac, there is no state in which it exists so free from moisture, as when aëriform, at low temperatures.

That no considerable source of error existed in the two experiments, is evident from the trifling difference between the estimations of the quantities of real ammoniac, in the solution of,9684, as found in the first experiment, and as given by calculation from the last.

The quantity of ammoniac in a solution of specific gravity not in the table, may be thus determined—find the difference between the two specific gravities nearest to it in the table; _d_, and the difference between their quantities of alkali, _b_; likewise the difference between the given specific gravity and that nearest to it, _c_.

then _d_ : _b_ :: _c_ : _x_

_bc_ and _x_ = ————— _d_

Which, added to the quantity of the lower specific gravity, is the alkali sought.

The differences in specific gravity of the solutions of ammoniac at temperatures between 4O° and 65°[70] are so trifling as to be hardly ascertainable, by our imperfect instruments, and consequently are unworthy of notice.

[70] The expansion from increase of temperature is probably great in proportion to the quantity of ammoniac in the solution.

It is possible at very low temperatures to obtain ammoniacal solutions of less specific gravity than,9, but they are incapable of being kept for any length of time under the common pressure of the atmosphere.

IV. _Combinations of Ammoniac with Nitric Acid, Composition of Nitrate of Ammoniac, &c._

200 grains of ammoniacal solution, of specific gravity,9056, were saturated by 385,5 grains of nitric acid, of specific gravity 1,306. The combination was effected in a long phial, the nitrous acid added very slowly, and the phial closed after every addition, to prevent any evaporation in consequence of the great increase of temperature.[71] The specific gravity of the solution, when reduced to the common temperature, was 1,15. Evaporated at a heat of 212°,[72] it gave 254 grains of salt of fibrous crystalization. This salt was dissolved in 331 grains of water; the specific gravity of the solution was 1,148 nearly.

[71] From the combination.

[72] I had before proved that at this temperature the salt neither decomposed nor sublimed.

Hence it was evident that some of the salt had been lost during the evaporation.

To find the quantity lost, fibrous nitrate of ammoniac was dissolved in small quantities in the solution, the specific gravity of which was examined after every addition of 3 grains. When 16 grains had been added to it, it became of 1,15.

Consequently, the solution composed of 200 grains of ammoniacal, and of 385,5 of nitric acid solution, contained 262 grains of salt of fibrous crystalization, and of this salt 8 grains were lost during the evaporation.

But the alkali in 200 grains of ammoniacal solution of,9056 = 50,5 grains. And the true nitric acid in 385,5 grains of solution of 1,306 = 190 grains.

Then 262-240,5 = 21,5, the quantity of water.

And 262 grains of fibrous crystalized nitrate of ammoniac, contain 190 grains true acid, 50,5 ammoniac, and 21,5 water. And 100 parts contain 72,5 acid. 19,3 ammoniac, and 8,2 water.

In proportion as the temperature employed for the evaporation of nitro-ammoniacal solutions, is above or below 212°, so in proportion does the salt produced contain more or less water than the fibrous nitrate. But whatever may have been the temperature of evaporation, the acid and alkali appear always to be in the same proportions to each other.

Of the salts containing different quantities of water, two varieties must be particularly noticed. The prismatic nitrate of ammoniac, produced at the common temperatures of the atmosphere, and containing its full quantity of water of crystalisation; and the compact nitrate of ammoniac, either amorphous, or composed of delicately needled crystals, formed at 300°, and containing but little more water than exists in nitric acid and ammoniac.

To discover the composition of the prismatic nitrate of ammoniac, 200 grains of fibrous salt were dissolved in the smallest possible quantity of water, and evaporated in a temperature not exceeding 70°. The greater part of the salt was composed of perfectly formed tetrahædral prisms, terminated by tetrahædral pyramids. It had gained in weight about 8,5 grains.

Consequently 100 grains of prismatic nitrate of ammoniac may be supposed to contain 69,5 acid, 18,4 ammoniac, and 12,1 water.

To ascertain the composition of the compact nitrate of ammoniac, I exposed in a deep porcelain cup, 400 grains of the fibrous salt, in a temperature below 300°. It quickly became fluid, and slowly gave out its water without any ebullition, or liberation of gas. When it was become perfectly dry, it had lost 33 grains. I suspected, that in this experiment some of the salt had been carried off with the water; to determine this, I introduced into a small glass retort, 460 grains of fibrous salt; it was kept at a heat below 320°, in communication with a mercurial apparatus, in a regulated air-furnace, till it was perfectly dry: it had lost 23 grains. No gas, except the common air of the retort came over, and the fluid collected had but a faint taste of nitrate of ammoniac.

Though in this experiment I had removed all the fluid retained in the neck of the retort, still a few drops remained in the head, and on the sides, which I could not obtain. It was of importance to me to be accurately acquainted with the composition of the compact salt, and for that reason I compared these analytical experiments with a synthetical one.

I saturated 200 grains of solution of ammoniac, of,9056 with acid, ascertained the specific gravity of the solution, evaporated it at 212°, and fused and dried it at about 300°-260°. It gave 246 grains of salt, and a solution made of the same specific gravity as that evaporated, indicated a loss of 9 grains. Consequently, 255 grains of this salt contain 50,5 grains alkali, 100 grains acid, and 14,5 grains water.

We may then conclude, that 100 parts of compact nitrate of ammoniac contain 74,5 acid, 19,8 alkali, and 5,7 water.

V. _Decomposition of Carbonate of Ammoniac by Nitric Acid._