NITROGEN, a colorless, odorless gas, non inflammable under ordinary conditions and non-explosive. The density of pure nitrogen under normal temperature and pressure condi tions is 0.96737. The density of atmospheric nitrogen (containing the inert gases) is 0.97209. The atomic weight of nitrogen is 14.01. The critical pressure is 35 atmospheres, and the critical temperature C. The melting point of solid nitrogen is C., and the boiling point of liquid nitrogen —195.5° C. The nitrogen thermometer is used in modern precise thermometry, and the following constant was determined with the greatest possible care: If a given mass of nitrogen at initial temperature of 0° C. and pressure of one meter of mercury be heated at constant volume to 100° C., its pressure becomes equal to that due to 1.36745 meters of mercury.
Historical.— Nitrogen was first recognized as a distinct substance by Rutherford of the University of Edinburgh in 1772. His demon stration consisted in showing that when a small animal breathes the air in an enclosed space for a while, and the carbon dioxide produced is removed by absorption, there still remains a gas that is incapable of supporting respiration. The individuality of nitrogen and its existence in the atmosphere was first demonstrated by Lavoisier, who named it °azote," signifying °without life?' The French still call nitrogen by this name, the English name being derived from the Latin enitrum,s meaning saltpetre.
Occurrence.— Nitrogen is one of the most widely distributed elements. In the free form it makes up 79 per cent by volume of the atmos phere, or 77 per cent by weight. It also occurs in the air in chemical combination as nitric and nitrous acids as well as ammonia, but in only small quantities, it being estimated that the nitrous gases are present to the extent of about one part in 3,000,000. Nitrogen is found in volcanic gases, in the air-bladders of certain fish, the bone cavities of birds, the cavities of plants, in meteorites, in the nebula and in the atmosphere of the sun. Nitrogen is found in many forms in combination with other elements. It occurs as an essential constituent in vegetable and animal matter in the form of proteins which average about 16 per cent nitrogen. It is found in Bengal in the form of potassium nitrate (saltpetre) and in Chile and Peru in the form of sodium nitrate (Chile saltpetre). Quantities of combined nitrogen are obtained in the form of ammonia by destructive distilla tion of coal. The natural manures, such as the guanos, also contain considerable combined nitrogen.
Preparation.— There are various ways of obtaining nitrogen from the air by use of chemicals to absorb the accompanying oxygen. Such nitrogen is never more than 99 per cent pure, as the inert gases (see ARGON) are still present. Nitrogen is also obtained from com bustion gases, for example, from gas producers, and it can, of course, be obtained by factional distillation of liquid air, the average purity by this method being about 99.9 per cent nitrogen.
In order to obtain pure nitrogen the earliest experimentalists resorted to chemicals. The simplest method is to heat ammonium nitrite, but as this compound is somewhat unstable and inclined to decompose, the same result is ob tained by using the more stable form—sodium nitrite; this is mixed with ammonium chloride, and when heat is applied to the mixture, ammonium nitrite is formed and immediately decomposes into water and pure nitrogen. It is also possible to obtain pure nitrogen from air by means of metallic nitrides. Nitrogen combines directly with lithium, calcium and magnesium when heated. These nitrides, on further heating, again give up their nitrogen. Comparatively pure nitrogen is obtained in large quantities as a residue from the carbonat ing towers in the Solvay plants for making soda by the °ammonia-soda') process.
Compounds of Nitrogen.— Although there are so many chemical compounds of nitrogen, it is a very indifferent or inert element as com pared with the activity of oxygen. One of the great problems of science has been the so-called °fixation° of nitrogen and it cannot be said that the problem is at the present time by any means satisfactorily solved. Granting, however, that the nitrogen has been fixed, whether naturally or synthetically, in the form of nitric acid or ammonia or their salts, the compounds are many of them intensely active; e.g., nitric acid, and the well-known explosive, nitroglycerine, T.N.T. and picric acid. The literature of the nitro gen compounds is very extensive and new work is constantly being reported on the thousands of derivatives of nitrogen. Nitrogen does not combine with oxygen under ordinary conditions. The combination takes place under the in fluence of an electric spark, and such a combina tion always accompanies lightning. For this rea son, some oxides of nitrogen are always present in rain water but the small amounts thus formed could scarcely have supplied vegetation through out the ages. Certain classes of plants, notably legumes (alfalfa, cowpeas, soy beans, etc.), are provided with means of taking tip the free nitrogen from the atmosphere by the assistance of bacteria which flourish on the nodules of the roots of the plants. These bacteria have the power of converting the free nitrogen of the air into compounds which in turn are taken up by the nodules and assimilated by the plants, sometimes to the extent of over 5 per cent of combined nitrogen, in the form of proteins. A great deal of work has been done on design ing suitable apparatus for the combination of nitrogen and oxygen by electric means, but the process is still only commercially possible where water power is very cheap, as in the plants in Norway.