NITROGEN. This gas constitutes seventy-nine hundredths, or about four-fifths, of the atmosphere. Its name is derived from the Greek words vo-pop, nitre, and yesace, I produce. It was formerly called azote, or arctic gas, from a privative, and Ca,6, life, indicating its inability to support respiration. It was first discovered by Dr. Rutherford of Edinburgh ; he gave it the name of mephitic air, and described it in his `De Are Mephitico,' published in 1772. Dr. Priestley procured it about the same period, and described several of its qualities in the Philosophical Transactions ' for 1772 ; he after wards called it phlogisticated air.
This gas may be obtained by several processes. Dr. Rutherford separated it from the oxygen gas, with which it forms atmospheric air, by repeatedly breathing the same portion of atmospheric air, and agitating it afterwards with a solution of caustic alkali ; this absorbed the carbonic acid formed during respiration, and left the nitrogen gas unacted upon.
Dr. Priestley procured nitrogen gas by exposing a given volume of atmospheric air to a mixture of sulphur and iron filings ; by this the oxygen was absorbed, and the nitrogen gas left. He also prepared it by some other means.
The readiest method of obtaining this gas is by the following process : Put a small piece of phosphorus in a little cup fixed to a cork floating on water, and set fire to it ; while burning, hold inverted over it and dipping a little into the water, a glass jar or wide-mouthed bottle containing atmospheric air. During combustion the phosphorus unites with the oxygen of the air to form phosphoric acid, but having no affinity for the nitrogen gas, that remains nearly in a pure state, when it has either stood over the water or been agitated with that liquid, until all the white vapour of phosphoric acid has disappeared.
Nitrogen may also be separated from atmospheric air by the slow combustion, as it is termed, of phosphorus; that is, by merely exposing a stick of phosphorus, supported by a wire, in a bottle of air inverted over water.
By the exposure of moisteued iron filings or iron borings to air in a bottle inverted over water ; the metal oxidises, and the nitrogen gas is left : this acts even more slowly than the phosphorus in the above experiment.
By the agitation of mercury and lead in a corked bottle for a few minutes a black powder is formed, which is probably a mixture of an oxide of lead with finely-divided mercury ; nitrogcu gas is left, The passing of atmospheric air over iron or, better, copper boriugs, heated to redness in a tube, is a useful method of preparing nitrogen when required on the large scale, but the process does not yield it in a perfectly pure state.
By exploding a mixture of 100 volumes of air and 42 of hydrogen, water is formed, and about 80 volumes of nearly pure nitrogen gas left.
Finally, nitrogen, when required absolutely pure, is best prepared by heating a mixture of chloride of ammonium aud slightly alkaline nitrite of potash [Nrraous Acm], the nitrite of ammonia thus formed being split up according to the following equation N11,0, = 4110 + N.
Nitrite of ammonia. Water. Nitrogen.
To free the gas from all traces of ammonia, it should be passed through a bottle containing dilute sulphuric acid.
Nitrogen is transparent, colourless, inodorous, and insipid. Its specific gravity is ; 100 cubic inches of it weighing grains at 60° Fehr., the barometer marking 30 inches. Its refractive power, compared with air as unity, is 1020; its specific heat, according to 'Berzelius, being Nitrogen is generally considered as an electropositive element, and in voltaic combinations is attracted to or elicited from the negative surface. Mr. Faraday has, however, found that, according to the nature of the substance decomposed, nitrogen may appear at either pole; but he concludes, that when it is evolved at the negative one it is a primary result, and when at the positive a secondary one.