itself reduced to nitric oxide and water, with the elimination of oxygen 2HNO=2NO+H2O+O. The salts of nitrous acids, viz., the nitrites, are stable compounds. The nitrites of the alkalies are best prepared by carefully heating the nitrates; thus, when potassium nitrate is fused, it parts with oxygen and is transformed into potassium nitrite KNO2=KNO2+0. At a higher temperature the nitrite is also decomposed. Nitrites are decomposed by dilute acids evolving brown vapours, and in this way are at once distinguished from nitrates. Nitrogen Trioxide.-There is considerable doubt as to the existence of this compound. It has been usually stated that it is formed by the action of nitric acid upon arsenious oxide, according to the equation— A$40+4HNO3=2A$qO5+2H2O+2N2O3. It has, however, been shown by the determination of the vapour density that in the gaseous state the compound N2O, does not exist, but that the gas is a mixture of molecules of NO and NO. It will be seen that a mixture containing equal volumes of these two gases will have a composition represented by the formula NO3, therefore the above reaction may be regarded as taking place thus As406+4HNO3=2As2O5+2H2O+2NO+2NO2. Simultaneously with this reaction the following decomposition also goes forward As406+8HNO3=2AsqO5+4H2O + 8NO2. The result, therefore, of the action of nitric acid upon arsenious oxide is a mixture of nitric oxide and peroxide in varying proportions. When this mixture is strongly cooled it condenses to a blue liquid, believed by some to be the true compound NO3. Others regard it as merely a solution of the difficultly liquefied gas, NO, in liquid nitrogen peroxide, NO. If the two ox des are in a state of combination, it would appear to be at best a feeble union, for it has been shown that at temperatures as low as - 90° the liquid slowly evolves NO, while at this temperature no nitrogen peroxide is given off. The most recent work on the subject, however, based upon minute changes of volume which result when NO and NO, are mixed (Dixon and Peterkin, Proc. Chem. Soc., June 1899), points to the conclusion that the reaction which may be expressed N2O=NO+NO, is to a slight extent a reversible one; and that therefore a mixture of the two gases NO and NO, at ordinary temperatures actually does contain a small percentage of N.Og molecules. NITRIC OXIDE. Formula, NO. Molecular weight=30.04. Density=15.02. History. Nitric oxide was first obtained by Van Helmont. Priestley, however, was the first to investigate this gas, which he termed nitrous air, and which was employed by him in his analysis of air. Modes of Formation.—(1.) This gas is obtained by the action of nitric acid of specific gravity 1.2 upon copper or mercury. In practice copper is always employed.* The action may be represented thus 3Cu+8HNO3=3Cu(NO3)2+4H2O+2NO. The gas obtained by this method is always liable to contain nitrous oxide and even free nitrogen; the amount of these impurities rapidly increasing if the temperature be allowed to rise, and still more so as the amount of copper nitrate in solution increases. (2.) Pure nitric oxide is readily obtained by the action of nitric acid upon ferrous sulphate. The reaction is best applied by generating the nitric acid from potassium nitrate and sulphuric acid in the presence of ferrous sulphate. A mixture of the two salts, in the proportion of about one part of nitre to four of ferrous sulphate, is introduced into a flask, with a small quantity of water. Strong sulphuric acid is dropped upon the mixture by means of a dropping funnel, and the mixture gently warmed, when a steady stream of pure nitric oxide is evolved— 2KNO3+5H2SO4+6FeSO ̧=2HKSO ̧+3Feg(SO4)3+4H2O+2NO. A precisely similar result may be obtained by the reduction of potassium nitrate by means of ferrous chloride in the presence of hydrochloric acid, thus KNO3+3FeCl2+4HCl=3FeCl3+ KCl+2H2O+NO. Properties.-Nitric oxide is a colourless gas, having a specific gravity of 1.039. When brought into the air, it combines with the atmospheric oxygen, forming red-brown vapours, consisting of * Experiment 314, "Chemical Lecture Experiments," new ed. nitrogen peroxide, the combination being attended with a rise of temperature. The formation of these red fumes in contact with oxygen is characteristic of this gas, thereby distinguishing it from all other gases. This property of nitric oxide renders it impossible to ascertain whether this gas has any smell, or is possessed of any toxicological action. Nitric oxide is only very sparingly soluble in water. It is the most stable of all the oxides of nitrogen, being able to stand a dull red heat without decomposition. It is not a supporter of combustion. A lighted taper, or a burning piece of sulphur, when introduced into the gas, are extinguished. If the temperature of the burning substance is sufficiently high to decompose the gas, combustion then continues at the expense of the liberated oxygen: thus, if a piece of phosphorus, which is freely burning in the air, be plunged into this gas, it continues its combustion with great brilliancy; if, however, the phosphorus be only feebly burning when thrust into the gas, it is at once extinguished. A mixture of carbon disulphide vapour and nitric oxide, obtained by allowing a few drops of the liquid to fall into a cylinder of the gas, burns, when inflamed, with an intensely vivid bluish flame, which is especially rich in the violet or actinic rays, and has on this account been sometimes employed by photographers to illuminate dark interiors. Nitric oxide is soluble in a solution of ferrous sulphate, forming a darkbrown solution, containing an unstable compound of ferrous sulphate and nitric oxide, 2FeSO4, NO. This compound is readily decomposed by heat, nitric oxide being evolved. By means of this reaction, nitric oxide may be separated from other gases. Nitric oxide is a difficultly liquefiable gas, its critical temperature being -93.5° at this temperature a pressure of 71.2 atmospheres is required to liquefy it. FIG. 52. The composition of nitric oxide may be proved by heating a spiral of iron wire by means of an electric current in a measured volume of the gas (as shown in Fig. 52).* As the metal becomes red hot the gas is gradually decomposed and the oxygen combines *No. 321, "Chemical Lecture Experiments." with the iron to form ferric oxide. The residual nitrogen will be found to occupy one-half the original volume. Two vols. of nitric oxide, weighing 30.04 16.00 weight of 1 vol. of oxygen. Therefore we learn that two volumes of nitric oxide consist of one volume of nitrogen and one volume of oxygen united without condensation. NITROUS OXIDE (Hyponitrous anhydride, Laughing gas). Formula, NO. Molecular weight 44.08. Density = 22.04. History. This gas was discovered by Priestley, and called by him dephlogisticated nitrous air. Modes of Formation.—(1.) Nitrous oxide is formed by the reduction of nitric acid by certain metals, as zinc or copper, under special conditions (see Nitric Acid). These reactions, however, are never made use of for the preparation of the gas for experimental purposes. (2.) The most convenient method for obtaining this compound is by the decomposition of ammonium nitrate. A quantity of the dry salt is gently heated in a flask fitted with a cork and deliverytube. The salt rapidly melts and splits up into nitrous oxide and water NH,NO, 2H,O+N2O. The heat should be carefully regulated, or the decomposition is liable to become violent, in which case nitric oxide is also evolved. Nitrous oxide being rather soluble in cold water, the gas should be collected either over mercury or over hot water. When the gas is to be used for anæsthetic purposes, it should be purified by being passed first through a solution of ferrous sulphate to absorb any nitric oxide, and afterwards through caustic soda, to remove any chlorine which may have been derived from the presence of ammonium chloride in the nitrate. Properties.-Nitrous oxide is a colourless gas, having a faint and not unpleasant smell, and a peculiar sweetish taste. Its specific gravity is 1.52. The gas is somewhat soluble in water, its coefficient of absorption at o° being 1.3052. The solubility rapidly decreases as the temperature rises, as will be seen by the follow The loss of gas during its collection over water in the pneumatic trough, arising from its solubility in that liquid, is therefore greatly lessened by using warm water. Nitrous oxide is much more readily decomposed than nitric oxide; a red-hot splint of wood is instantly rekindled, and bursts into flame when plunged into the gas. Phosphorus burns in it with a brilliancy scarcely perceptibly less dazzling than in pure oxygen. If a piece of sulphur which is only feebly burning be thrust into a jar of this gas, the sulphur is extinguished, the temperature of the flame not being sufficiently high to decompose the gas. When, however, the sulphur is allowed to get into active combustion before being placed in the gas, the combustion continues with greatly increased brilliancy. In all cases of combustion in nitrous oxide, the combustion is simply the union of the burning body with oxygen, the nitrogen being eliminated. From its behaviour towards combustibles, nitrous oxide might readily be mistaken for oxygen; it can, however, be easily distinguished from that gas by the fact that when added to nitric oxide it does not produce red vapours, whereas when oxygen is mixed with nitric oxide these coloured fumes are instantly formed. When equal volumes of nitrous oxide and hydrogen are mixed in a eudiometer, and an electric spark passed through the mixture, the gases combine with explosion, water being produced and nitrogen set free; the volume of nitrogen so resulting being equal to that of the nitrous oxide employed. This compound, therefore, contains its own volume of nitrogen, and half its own volume of oxygen. Nitrous oxide, when inhaled, exerts a remarkable action upon the animal organism. This fact was first observed by Davy. If breathed for a short time, the gas induces a condition of hysterical excitement, often accompanied by boisterous laughter, hence the name laughing gas. If the inhalation be continued, this is followed by a condition of complete insensibility, and ultimately by death. |