PREPARATION AND PROPERTIES OF NITROUS OXIDE.

97

that the whole of the nitrogen, both of the nitric acid and of the ammonia, is liberated in the form of nitrous oxide; HN NO, becoming 2 H, + N,O. An ounce of the salt furnishes about 500 cubic inches of the gas.

3.-L. Smith adopts a modification of the foregoing process for preparing the gas: he decomposes sal ammoniac by means of nitric acid (of sp. gr. 120) at a gentle heat. The gas which is obtained in this manner is not pure, but is contaminated with small quantities of chlorine and of nitrogen; the chlorine may be removed by allowing the gas to bubble up through a solution of potash. Advantage may sometimes be taken of this action of nitric acid on sal ammoniac to destroy an excess of muriate of ammonia in solution in the course of an analysis.

Properties.-Nitrous oxide is a transparent, colourless gas, with a faint sweetish smell and taste: 100 cubic inches of water at 32° dissolve 130 cubic inches of the gas; at 59°, 77 cubic inches; and at 75°, only 60 cubic inches (Bunsen). Owing to this considerable diminution in solubility with the rise of temperature, it should be collected over warm water. Under a pressure of 50 atmospheres at 45°, it is reducible to a colourless liquid, of sp. gr. at 45°, of 0908. It boils at about —126° (Regnault), and may be frozen into a transparent solid at about —150°. When the liquid nitrous oxide was mixed with bisulphide of carbon and exposed to evaporation in vacuo, Natterer obtained a reduction of temperature which he estimated at-220° F.; this is a lower point than has hitherto been attained by any other means. The gaseous nitrous oxide has a specific gravity of 1527, which coincides with that of carbonic anhydride. This gas possesses the qualities neither of an acid nor of an alkali. It supports the combustion of many bodies with a brilliancy resembling that which they exhibit when burned in oxygen. It is, however, at once distinguished from oxygen by its considerable solubility in water. A glowing match bursts into flame when plunged into nitrous oxide: sulphur when kindled burns in it with a pale rosecoloured flame.

Soon after the discovery of the nitrous oxide, Davy ascer tained that it may be respired for a few minutes: it then produces a singular species of transient intoxication, attended in many instances with an irresistible propensity to muscular exertion, and often to uncontrollable laughter; hence the gas has acquired the popular name of laughing-gas.

Composition. If nitrous oxide be passed repeatedly through a porcelain tube heated to bright redness, the gas is decomposed

98

ANALYSIS OF NITROUS OXIDE.

into a mixture of oxygen and nitrogen, 2 volumes becoming expanded permanently into the space of 3 volumes. An easy method of analysing the nitrous oxide consists in mixing it with hydrogen, and passing an electric spark through the mixture. If 4 measures of nitrous oxide be mixed with an excess of hydrogen gas, say with 6 measures of hydrogen, in the bent eudiometer (fig. 281), 10 measures of mixed gas will be produced; and on transmitting the electric spark, inflammation will occur; steam will be formed by the oxidation of the hydrogen, and will be immediately condensed; the 10 measures will thus be reduced to 6 but the quantity of oxygen contained in the nitrous oxide cannot be at once inferred from this change of bulk: before this can be done, it is needful to ascertain how much hydrogen is left in the mixture. This may be effected by mixing the 6 remaining measures with 2 measures of oxygen, thus making 8 measures, and again transmitting the electric spark. Steam will again be formed, and immediately condensed: the 8 measures of the mixture will now be reduced to 5; 3 measures of the gas will therefore have disappeared, two-thirds of which, or 2 measures, are hydrogen: I measure of the gas now left must consequently be oxygen which was added in excess, and the remaining 4 measures are nitrogen. Of the 6 measures of hydrogen originally added, 4 have therefore combined with oxygen derived from the nitrous oxide; and since 4 measures of hydrogen require 2 measures of oxygen for conversion into water, the 4 measures of the nitrous oxide must have contained 2 measures of oxygen. It appears, also, that nitrous oxide contains its own bulk of nitrogen, since the 4 measures of the gas originally employed furnish 4 measures of nitrogen; this nitrogen is moreover so combined with 2 measures of oxygen, that the 6 measures of the two gases when united are condensed into the space of 4 measures, or into twothirds of the bulk which they occupied when separate. The specific gravity of the gas shows that this conclusion is correct, for 100 cubic inches are found by experiment to weigh between 47 and 48 grains, and by calculation we find that such should be the case, for

The proportion of nitrogen contained in the gas may also be ascertained by means of potassium; for if potassium be heated in

PREPARATION OF NITRIC OXIDE.

99

nitrons oxide, it burns vividly, and is converted into potash, leaving a volume of nitrogen equal to that of the gas employed. The composition of this gas may therefore be thus represented :--

(365) NITRIC OXIDE; Nitrosyl; Deutoxide, or Binoxide, of Nitrogen, (N→ or NO,=30)*; Sp. Gr. 1'039; Atomic Vol. -Preparation.-1. If nitric acid be diluted with twice its bulk of water, so as to reduce it to a specific gravity of about 12, and be poured upon copper clippings or metallic mercury placed in a retort, brisk action speedily occurs; a gentle heat, if necessary, may be applied until it commences; the retort becomes filled with red fumes, and a gas is disengaged, which if collected over water will be found to be colourless; I ounce of copper by solution in about 4 ounces of the diluted acid would yield nearly 420 cubic inches of nitric oxide. If the heat be too high the gas is apt to be contaminated with nitrogen. During this decomposition the metal displaces hydrogen from one portion of the acid and forms a nitrate, which is dissolved, whilst the hydrogen, in the moment of its liberation, decomposes another portion of the acid, water is formed, and nitric oxide liberated. The following equation shows the reaction which occurs between 3 atoms of copper and 8 of nitric acid, resulting in the formation of 2 atoms of nitric oxide and 3 atoms of nitrate of copper :

Copper.

Nitric acid.

Nitrate of copper.

Nitric oxide.

Water.

3 Єu + 8 HNO, = 3 (Eu 2 NO3) + 2 NO + 4 H2O.

3

2

2. The nitric oxide may also be obtained perfectly pure by digesting hydrochloric acid with iron filings till it will dissolve no more, decanting the clear liquid, and adding to it its own bulk of hydrochloric acid: on placing the solution in a retort, and adding nitrate of potassium, the nitric oxide is immediately evolved in large quantity (Pelouze). The reaction is not so simple as in the preceding case; it is represented as follows:

6 FeCl2 + 8 HCI + 2 KNO, give 3 FeCl + 4 H2+ 2 KCl + 2 No.

The vapour density of this compound is anomalous, No yielding 2 volumes (H: I vol.) instead of N,, as would be requisite if the compound followed the usual law.

100

COMPOSITION AND PROPERTIES OF NITRIC OXIDE.

A simple modification of this method consists in placing in a retort 1 ounce of commercial nitre, 8 ounces of ferrous sulphate, and pouring upon them half a pint of diluted sulphuric acid (1 ounce of acid to 3 ounces of water). Such a mixture will give nearly 400 cubic inches of pure nitric oxide.

Composition. The composition of nitric oxide cannot be ascertained by detonation with hydrogen; for equal volumes of hydrogen and nitric oxide burn quietly with a green flame on the approach of a burning body. Davy analysed nitric oxide by heating charcoal strongly in it; 2 volumes of the gas by this treatment furnish I volume of nitrogen, and I volume of carbonic anhydride but carbonic anhydride contains its own volume of oxygen; nitric oxide must therefore have consisted of 1 volume of nitrogen united without condensation with 1 volume of oxygen. The density of the gas confirms the correctness of this result, for by experiment 100 cubic inches weigh rather more than 32 grains, and by calculation,

Sp. gr. 15:06 or 0'50'486

By vol.

17.10

0'5 = 0'552

32.16

1'0 = 1'038

Potassium burns when heated in the gas, potash being produced. If the experiment be conducted in such a manner as to allow the residual gas to be measured after the combustion is over, 2 volumes of the nitric oxide will be found to leave 1 volume of nitrogen. A similar result is obtained when tin is heated in the gas; consequently its composition may be thus represented :

Properties.-Nitric oxide has a strong disagreeable odour, and cannot be respired. It has hitherto resisted all attempts to liquefy it. Water does not dissolve more than of its bulk of this gas. Nitric oxide is the most stable of the oxides of nitrogen; it may even be exposed to a red heat without undergoing decomposition, but a succession of electric sparks converts it, if it be moist, into a mixture of nitrogen and nitric acid: and if digested upon moistened iron filings, or a moist sulphide of one of the alkaline metals, it is slowly converted into nitrous oxide. Many burning bodies, such, for instance, as a lighted taper, or phosphorus just kindled, are extinguished when plunged into the gas; but a decomposition of the gas will be effected if the phosphorus be

OF

PROPERTIES OF NITRIC OXIDE.'

burning vigorously, and it will deflagrate with a brilliancy equal to that produced by its combustion in oxygen.

Nitric oxide is completely absorbed by a solution of ferrous sulphate, with which it forms a deep reddish-brown liquid. All the ferrous salts exert a similar action, and, according to Peligot, 2 atoms of the salt of iron absorb 1 atom of nitric oxide, the solution in the case of the sulphate containing (2 FeSO,, NO.) The deep colour of the liquid thus formed is employed, as has already been mentioned (365), for the purpose of ascertaining the presence of nitric acid in solution. This liquid absorbs oxygen rapidly from the air or from gaseous mixtures; when heated, most of the nitric oxide is expelled from it unchanged. Solutions of stannous and mercurous salts also absorb nitric oxide, but they undergo change, and the gas cannot again be expelled from them by heat. Nitric acid likewise absorbs the gas rapidly: if the acid be concentrated, the solution becomes reddish brown; if more diluted, it is green; if still weaker, the solution is blue, but if diluted below a specific gravity of 115, little of the gas is absorbed, and the acid remains colourless.

The nitric oxide is neither acid nor alkaline in its characters. It has, however, a very powerful attraction for oxygen, and to this circumstance is owing one of the most characteristic properties of the gas. When mixed with oxygen, or with any gas containing uncombined oxygen, dense red fumes are produced. These red fumes are freely soluble in water, and furnish an acid liquid. Formerly this circumstance was employed to determine the quantity of oxygen in mixture with other gases; but the method is now abandoned, as the absorption is not uniform, owing to the formation in uncertain quantity of a mixture of the soluble oxides of nitrogen. It may, however, be used with advantage as a qualitative test to demonstrate the existence of uncombined oxygen in a gaseous mixture. Nitric oxide also unites with half its volume of chlorine when the two gases are mixed, and chloronitrous gas NOCI is formed (377).

3

(366) NITROUS ANHYDRIDE (N ̧Ð ̧=76, or NO,=38; formerly known as Hyponitrous Acid).—Preparation.-1. By mixing in an exhausted flask 4 volumes of nitric oxide with 1 volume of oxygen, both in a perfectly dry state, brownish-red fumes of nitrous anhydride are formed, which at a temperature of o° F. become condensed into a blue very volatile liquid, which emits a red vapour.

2.-Nitrous anhydride may also be obtained nearly in a state of purity, by heating in a capacious retort I part of starch with 8 parts of nitric acid of sp. gr. 1'25: it may be dried by passing