Chapter XIX: Nitrogen

Nitrogen is an important constituent of our atmospheric air of which it makes up about eighty per cent. The other twenty per cent., as has already been stated, is oxygen. In the air the nitrogen is found in the free or uncombined state, and we may reasonably suppose that it exists here to fulfil some important offices. Unquestionably one of these is that of diluting the oxygen, the energetic constituent of air, and lessening its activities—for nitrogen itself is extremely inert. From the part it performs in the atmosphere, nitrogen derives a considerable portion of the interest with which it is invested.

Discovery of Nitrogen.

Perhaps the first clearly defined recognition of nitrogen as a constituent of the air is referable to the genius of a wonderful man, who, in obscurity and with the most imperfect appliances, obtained an insight into the constitution of substances which has rarely been surpassed. Reference is here made to the Swedish, or rather Prussian, chemist Scheele, some of whose discoveries have been briefly adverted to in earlier chapters. It has already been stated that the distinct notion of a gas dates but little more than a hundred years back; and this statement is intended to call to mind that brilliant period in the history of chemistry when among others, Black in Scotland, Cavendish and Priestley in England, Lavoisier and his worthy associates in France, and finally, the sagacious Scheele in Sweden, were engaged in a generous rivalry in chemical studies, which made the close of the eighteenth century a period in the history of chemistry that will not be forgotten so long as the science itself shall endure. At this time unstinted effort was devoted, with ingenious but imperfect appliances, to the study of gases. Of course the atmospheric air, as the gas most vast in quantity, most accessible for experiment, most important in its relation to the economy of living nature, received its full share of attention. It was at this period that Dr. Rutherford, a professor in the University of Edinburgh, demonstrated that after living animals have breathed in a confined bulk or volume of air, there remains an inert and peculiar gas behind. And Priestley showed that after the burning of charcoal in a confined volume of air there remains a gaseous material equal to about four-fifths of the amount of original air used. But it was Scheele who first clearly pointed out that the air contained a second distinct constituent that fails to support combustion and animal respiration. And Lavoisier first proved this constituent to be an elementary substance and he gave to it the name _azote_, which it still retains in the French nomenclature of chemistry.

It is not forgotten that a critical examination of the history of human knowledge respecting the atmosphere reveals the fact that a wonderfully clear, even though incomplete, account of the functions of the active constituent of the air was printed as early as the year 1669, by an English physician named John Mayow.[9] This affords another illustration of the fact, recognized by all students of history, that often in the progress of knowledge, before the clear and full dawn there seems to be a twilight; at such a time, and before the darkness has been fully dispelled, there have been found here and there men gifted with supernatural vision who have been able to read the laws of nature long before acknowledged philosophers even had found light sufficient. And so the truths learned by Mayow, though clearly stated by him, failed of recognition until they were rediscovered a hundred years later. (See p. 121.)

[9] KOPP, HERMANN: _Geschichte der Chemie_. Dritter Theil. s. 193.

Preparation of Nitrogen.

Nitrogen is usually prepared from the air by the withdrawal of oxygen from it. This withdrawal is effected by some substance which has a strong affinity for oxygen.

Thus one method frequently resorted to for the preparation of nitrogen, is to burn phosphorus in air. Phosphorus is placed in a little crucible of porcelain and then floated upon a cork on the surface of water in a pneumatic trough. A bell-glass of air is now inverted over the phosphorus, after the latter has been set on fire. The phosphorus burns at the expense of the oxygen in the bell-glass. Thus the oxygen is little by little withdrawn and as a result the nitrogen is left.

Another method for preparing nitrogen is based upon the same general principle. It is the following: Pass a current of dry air through a tube containing copper turnings heated to dull redness in a furnace. Under these circumstances the copper absorbs oxygen from the air, and leaves the nitrogen, which passes on to a receiver prepared for it.

Properties of Nitrogen.

Nitrogen prepared by these methods, or by any others, possesses the following characteristics:

It is a gas that is colorless, odorless and tasteless. It is not necessary to make any scientific demonstration of these facts, because with every breath of air drawn into the lungs of a human being a large quantity of nitrogen is inhaled, and it is easily perceived to be without odor or taste, while a glance of the eye into the atmosphere shows that, in moderate quantities at least, it is free from color. Up to a period dating but a few years back, nitrogen was spoken of as one of the permanent gases; and this word permanent was intended to convey the idea that it is not condensable to the liquid form. It is true that it was surmised that for every gas there must be a point of very low temperature and very great pressure at which the gas would assume the liquid form. Yet nitrogen, and two or three others, successfully resisted all such attempts to liquefy them until toward the close of the year 1878. Since that time, successful effort has been made to bring to a higher degree of perfection the appliances used for subjecting gases at once to intense cold and enormous pressure. With these, it is believed that small amounts of nitrogen have been liquefied. And it may even be said that there is now no permanent gas known, but that all gaseous substances may in fact be liquefied.[10]

[10] SCRUTZENBERGER, PAUL: _Traité de Chimie Générale_, Paris, 1880, i, 30.

As a simple and uncombined substance, nitrogen is characterized by extreme inactivity. It does not burn; it does not support combustion; it cannot be made to enter into chemical union with other substances, except by specially devised and circuitous processes.

While on the one hand inertness is the marked characteristic of the nitrogen, on the other hand this element is a constituent of a very large number of compounds. Moreover, these compounds are themselves often characterized by a high degree of activity. Of the last two declarations the first one seems to be inconsistent with the properties of nitrogen in its elemental form. The second one seems inconsistent, but less so when it is carefully considered. Thus the activity of the compounds of nitrogen is to a certain extent referable to their instability. The meaning of instability, as used here, is that the compounds are easily decomposed; and this is because the inert nitrogen readily lets go its hold upon the other elements. Whence it appears, that the activity of the compounds, in reality referable to the energetic action of the element or elements now loosed from the nitrogen, rather than the nitrogen itself.

In nature, nitrogen is found as a constituent in some very important compounds. Thus it seems to be an essential element of some of the principal animal matters, such as muscular fibre and the material of the brain. Again, it is a constituent of ammonia gas and also of a multitude of compounds derived from it. Now these compounds are members of a group of substances which serve as most valuable kinds of food for living plants. So it may be said that both living animals and plants seem to be in a peculiar way dependent upon nitrogen or nitrogenous matters.

Compound of Nitrogen with Hydrogen.

Under favorable circumstances, nitrogen and hydrogen combine to form the stable, interesting and important compound called ammonia gas and having the formula NH₃.

While this gas may be produced by the direct union of the constituents—that is when a mixture of nitrogen gas with hydrogen gas has an electric discharge slowly passed through it—this is not a common mode of procedure. Ammonia gas is oftener produced by a natural or artificial decomposition of certain substances that contain nitrogen and hydrogen among their constituents. As it has already been stated that many animal matters contain nitrogen and hydrogen, it follows that animal matters when decomposed, afford ammonia gas; and so they do, in fact, whether the decomposition is in the course of their natural decay, or whether it is conducted artificially, as for example when _animal_ matters are heated in closed vessels to the point of decomposition. Indeed ammonia gas and its important commercial compounds were formerly produced in this last mentioned manner.

Ammonia gas—or some compound of it—is also formed, as may be readily imagined from what has already been said, from decomposition of _vegetable_ matters containing nitrogen. It is a fact that at the present day the principal supplies of ammonia gas and its compounds for the uses of commerce and the arts come from such a source, that is from the artificial decomposition of bituminous coal. It is true that in the ordinary sense coal is not vegetable matter. But careful examination of it, shows that it is very directly derived from the vegetation of ancient forests. The vegetable matter has been packed away in the earth and has been subjected to water, heat and pressure under such conditions that these agencies have changed it to the form in which we find it. Now the coal-gas industry of the present day is so conducted as to decompose coal and collect many of the products of its decomposition. One of these products is ammonia gas. To the decomposition of coal, therefore, the business world at present looks for its supply of ammonia gas and the many compounds derived from it.

The name ammonia gas, indicates that it ordinarily exists in the aeriform condition. It has a very pungent odor, well-known as that evolved from smelling-salts. It dissolves in water with very great facility and in very large quantities. It has a strong tendency to combine with acids. This last fact may be easily illustrated by simple experiments within the reach of almost any one.

Experiment with Ammonia.

Provide two wine-glasses or two shallow vessels of any sort. Into one of them pour the liquid known as spirits of hartshorn, and called by the chemist ammonic hydrate. Into the other pour some concentrated chlorohydric acid. Abundant white clouds will quickly form above the vessels and between them. These clouds are composed of minute particles of a solid, called by the chemist ammonic chloride and expressed by the formula NH₄Cl. The reason for their formation is this: from the spirits of hartshorn escapes ammonia gas (NH₃); from the acid there constantly escapes chlorohydric gas (HCl); the two gases meeting in the atmosphere combine with energy, and form the smoky product referred to.

The chemical change is represented by the following equation:

=NH₃= + =HCl= = =NH₄Cl=

One molecule of One molecule of One molecule of
Ammonia gas, Chlorohydric acid, Ammonic chloride,
17 36½ 53½
parts by weight. parts by weight. parts by weight.
\____________________________________/ \______________/
| |
53½ 53⅓

The ammonic chloride thus produced is an article of commerce, well-known under the name _sal ammoniac_. As has been said, it is a solid and it belongs to the class of substances designated by chemists as salts. In fact one of the most striking characteristics of ammonia gas is its power to produce salts by union with acids. Here is a list of three well-known salts of this sort:

With Chlorohydric acid, HCl it produces Ammonic chloride, NH₄Cl
” Nitric acid, HNO₃ it produces Ammonic nitrate, NH₄NO₃
” Sulphuric acid, H₂SO₄ it produces Ammonic sulphate, (NH₄)₂SO₄

Compounds of Nitrogen and Oxygen.

Nitrogen and oxygen ordinarily manifest scarcely any affinity for each other. There are conditions however under which they unite; and moreover they unite in different proportions so as to form at least five different compounds. These may be presented in the form of the following striking series:

Nitrogen protoxide (called laughing-gas,) N₂O.
Nitrogen dioxide, N₂O₂ (or NO).
Nitrogen trioxide or nitrous anhydride, N₂O₃.
Nitrogen tetroxide (brown fumes,) N₂O₄ (or NO₂).
Nitrogen pentoxide or nitric anhydride, N₂O₅.

Of these compounds, unquestionably the most important is nitric anhydride—and this not on account of itself, for it is very rarely produced either in the arts or in the investigator’s laboratory. Its importance is referable to the fact that added to water, it produces nitric acid.

This chemical change is represented by the following equation:

=N₂O₅= + =H₂O= = =2HNO₃=

One molecule of One molecule of Two molecules of
Nitric anhydride, Water, Nitric acid,
108 18 126
parts by weight. parts by weight. parts by weight.
\____________________________________/ \_______________/
| |
126 126

Nitric Acid.

This acid has been referred to in another place as one of three principal acids of commerce. Certain of its most striking properties may be displayed in an easy and interesting manner by any one. For this purpose the following experiments are suggested:

_First experiment._—Nitric acid turns quill yellow.

Place a few fragments of white quill in a test-tube. Add a few drops of nitric acid and then some water. Now warm the mixture. The quill will be found to acquire a yellow color. Fill the tube with cold water in order both to dilute the acid and to cool it. Pour away the liquid, and wash the quill in water. The yellow color will be found to be permanent. Many other animal matters are turned to a permanent yellow color by nitric acid.

_Second experiment._—Nitric acid attacks copper with violence. There is liberated by the process a gas called nitrogen dioxide (N₂O₂), which is colorless but which becomes brown upon exposure to the atmospheric air. The chemical change gives rise to a solution sometimes green and sometimes blue, according to circumstances.

Place in a test-tube a small piece of metallic copper in the form of either wire or foil. Add some nitric acid to the copper. Then warm it gently until the copper disappears. The brown fumes will be recognized. The colored solution of cupric nitrate, Cu(NO₃)₂ should also be noticed.

_Third experiment._—Nitric acid attacks zinc with great violence.

Try another experiment quite similar to that just described, only employ zinc in place of copper. Brown fumes are evolved, and a colorless solution is produced containing zinc nitrate, Zn(NO₃)₂.

_Fourth experiment._—Nitric acid attacks iron with violence.

Try another experiment, quite similar to the second and third, only employ iron instead of the other metals mentioned. The fine iron wire used by florists is suitable for this purpose. The same brown fumes are evolved. A metallic nitrate is also produced; it is called ferric nitrate and its formula is Fe₂(NO₃)₆. The solution is yellow, or but slightly colored.

_Fifth experiment._—Nitric acid dissolves a nickel coin.

An experiment similar to those already detailed may be tried upon a nickel coin; but it is not necessary to entirely dissolve the coin. After the acid has acted for a few moments, water may be poured into the tube so as to dilute the acid, and at the same time to cool it. Then the liquid may be poured away and the coin withdrawn. In addition to the brown fumes evolved, the feature most noticeable is the decided green color of the solution. This is referable, to a considerable degree at any rate, to the nickel present. Nickel imparts a green color to most of its solutions.

These experiments suggest that nitric acid has a marked influence upon the metals. This is in fact one of its prominent characteristics; and it is largely used in the arts for the purpose of dissolving metals.