Nitrogen (Fr. azote; sym. N; atom. wt. 14) is an elementary gas, which in the free state forms nearly four-fifths by volume of our atmosphere. In combination with other elements nitrogen is a necessary constituent of every organised body, and it forms a very large number of most important compounds. For its relation to that gas, see ARGON. Its name was given to it shortly after it had been proved to be an essential constituent of nitre or potassium nitrate. Its presence in the atmosphere was discovered in 1772 by Rutherford, at that time a professor of botany in the university of Edinburgh. It was more particularly investigated soon after by Priestley, Scheele, and Lavoisier. It is a colourless, tasteless, inodorous gas, and was formerly regarded as permanent or incandescable; but it can be reduced to the liquid state by the application of sufficient pressure after it has been cooled to a very low temperature. The Critical Temperature (q.v.) of nitrogen, or the point of temperature above which it cannot be condensed into a liquid by the application of any pressure however great, is C. When cooled to this temperature a pressure of thirty-five atmospheres must be applied in order to liquefy it.
Nitrogen is fourteen times as heavy as hydrogen, and is slightly lighter, bulk for bulk, than atmospheric air, its sp. gr. being 0.9713, compared with air = 1. It is but slightly soluble in water, one hundred volumes of water at ordinary temperatures dissolving only about one and a half volumes of it.
Whilst nitrogen is a constituent of all plant and animal organisms and of many important compounds, it is, in the free state, rather inert towards other elements, and does not readily enter into direct combination with them. It is not combustible, nor does it act in the atmosphere as a supporter of combustion, a lighted taper plunged into a jar containing nitrogen being at once extinguished. It is almost unnecessary to say that nitrogen is not poisonous, since it is breathed freely along with oxygen by all animals; but it cannot support animal life, and an animal placed in it will die from suffocation for want of the oxygen necessary for respiration. Its function in the atmosphere seems to be mainly that of diluting the oxygen with which it is there associated. Although nitrogen forms about 79.1 per cent. of the total volume and 77 per cent. of the total weight of the atmosphere, the free gas does not appear to play any important part in supplying nitrogen for the construction of the tissues of plants. Vegetable physiologists are at present actively inquiring into this question; and it appears to be definitely established that plants cannot directly absorb from the atmosphere the nitrogen which they require. It is found, however, that certain classes of plants when they are grown in a soil which is practically free from nitrogen compounds to start with do take up nitrogen. One explanation of this fact is that free atmospheric nitrogen becomes converted in small quantity into nitrogen compounds in the soil through the influence of micro-organisms present there. Another explanation attributes the fixation of nitrogen to micro-organisms existing in, or on the surface of, the plant. On the other hand, it has been conclusively shown that certain plants are unable to exist unless appropriate compounds of nitrogen are supplied to them in the soil in which they grow. The main supply of the nitrogen of plants is certainly drawn from nitrogen compounds existing in or artificially supplied to the soil. The most important of such nitrogen compounds are nitrates, which are present in every fertile soil. These may be produced by the decay in the soil of nitrogenous animal or vegetable matters existing in it or supplied to it as manure; or they may be added to it ready formed—as, for instance, in the form of sodium nitrate, which is frequently applied as a manure to soils poor in nitrates.
Many attempts have been made to discover methods by which the free nitrogen of the atmosphere could be converted on a manufacturing scale into nitrogen compounds, but as yet no workable and economical method has been discovered. There are numerous methods for preparing pure or nearly pure nitrogen. One method by which it is obtained very nearly pure is to remove the oxygen as completely as possible from atmospheric air. This can best be done by passing a current of air slowly through a red-hot tube packed with copper turnings or with spirals of copper wire gauze. The oxygen of the air combines with the copper to form an oxide of copper, whilst the nitrogen passes on. Or the oxygen can be very completely removed from a confined portion of moist air by suspending in it pieces of phosphorus until the volume of the gas ceases to diminish. Nitrogen can also be prepared by passing a current of chlorine gas into a solution of ammonia. In this method care must be taken always to use ammonia in excess, otherwise chloride of nitrogen would be produced, which is a dangerously explosive substance. A steady current of nitrogen can be very easily obtained by heating together strong solutions of ammonium chloride and of potassium nitrite. Owing to the great difficulty of forming compounds from free nitrogen, new compounds of nitrogen are generally prepared from compounds already existing. The most important of these compounds are the nitrates which are found in the soil in very considerable quantity in different parts of the world, and in some places occur in large deposits, as in the nitrate regions of South America. From these nitrates Nitric Acid (q.v.) and other compounds of nitrogen are derived. Several compounds of nitrogen with oxygen are known.
Nitrous Oxide, or laughing-gas, , was discovered in 1772 by Priestley, who obtained it by the action on nitric oxide of easily oxidisable substances. It is now prepared by the action of heat on ammonium nitrate:
It is a colourless gas, which can with moderate ease be reduced to the liquid state, a pressure of thirty atmospheres being required to liquefy it at . Davy discovered in 1800 that it possessed anaesthetic properties, and it is now used as an anaesthetic in minor surgical operations. For such purposes it is stored in the liquid state by dentists and others in wrought-iron bottles. It is a supporter of combustion, and burning materials placed in it continue to burn with almost as great brilliancy as if placed in free oxygen.
Nitric Oxide, , is produced by the action of nitric acid on copper, silver, or mercury. It is a colourless gas, which like nitrogen can only be reduced to the liquid state by pressure after it has been cooled to a very low temperature. Its most striking character is the great readiness with which it unites with free oxygen, forming dense brown fumes, which consist mainly of peroxide of nitrogen.
Nitrous Anhydride has been described as a gas easily condensable to a liquid, possessing the composition represented by the formula . It seems very questionable whether a single substance exists either in the state of gas or of liquid possessing this composition. The gas which was supposed to be nitrous anhydride was certainly a mixture. Although nitrous anhydride cannot be obtained pure, and the corresponding acid—nitrous acid—is an unstable substance at ordinary temperatures, a number of nitrates, salts corresponding to nitrous acid, are known.
Peroxide of Nitrogen, or , is produced when oxygen and nitric oxide are mixed. It is a dark-brown liquid at ordinary temperatures, which boils at , yielding a brown vapour, and solidifies at , forming colourless crystals.
Nitric Anhydride, , is the anhydride of nitric acid, and can be obtained amongst other ways from nitric acid by the action on it of phosphoric anhydride (see NITRIC ACID).
The following are some of the other more important nitrogen compounds: ammonia and the ammonium salts (see AMMONIA); the vegetable alkaloids (see ALKALOIDS); aniline and its derivatives; cyanogen and the cyanides, including hydrocyanic acid; albumenoid substances; urea and uric acid; nitro-benzene, and other nitro compounds; pyridine and allied substances, hydrazine compounds, ptomaines, &c. See AZOTISED BODIES.