The Boy's Playbook of Science Part 11

NITROGEN, OR AZOTE.

[Greek: _Nitron_], nitre; [Greek: _gennao_], I form; [Greek: _a_], privative; [Greek: _zoe_], life. Symbol, N; combining proportion, 14.

Also termed by Priestley, _phlogisticated_ air.

In the year 1772, Dr. Rutherford, Professor of Botany in the University of Edinburgh, published a thesis in Latin on fixed air, in which he says:--"_By the respiration of animals healthy air is not merely rendered mephitic_ (i.e., charged with carbonic acid gas), _but also suffers another change. For after the mephitic portion is absorbed by a caustic alkaline lixivium, the remaining portion is not rendered salubrious; and although it occasions no precipitate in lime-water, it nevertheless extinguishes flame and destroys life._" Such is the doctor's account of the discovery of nitrogen, which may be separated from the oxygen in the air in a very simple manner. The atmosphere is the great storehouse of nitrogen, and four-fifths of its prodigious volume consist of this element.

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_Composition of Atmospheric Air._

Bulk. Weight.

Oxygen 20 22.3 Nitrogen 80 77.7 --- ---- 100 100.

The usual mode of procuring nitrogen gas is to abstract or remove the oxygen from a given portion of atmospheric air, and the only point to be attended to, is to select some substance which will continue to burn as long as there is any oxygen left. Thus, if a lighted taper is placed in a bottle of air, it will only burn for a certain period, and is gradually and at last extinguished; not that the whole of the oxygen is removed or changed, because after the taper has gone out, some burning sulphur may be placed in the vessel, and will continue to burn for a limited period; and even after these two combustibles have, as it were, taken their fill of the oxygen, there is yet a little left, which is snapped up by burning phosphorus, whose voracious appet.i.te for oxygen is only appeased by taking the whole. It is for this reason that phosphorus is employed for the purpose of removing the oxygen, and also because the product (phosphoric acid) is perfectly soluble in water, and thus the oxygen is first combined, and then washed out of a given volume of air, leaving the nitrogen behind.

_First Experiment._

To prepare nitrogen gas, it is only necessary to place a little dry phosphorus in a Berlin porcelain cup on a wine gla.s.s, and to stand them in a soup plate containing water. The phosphorus is set on fire with a hot wire, and a gas jar or cylindrical jar is then carefully placed over it, so that the welt of the jar stands in the water in the soup plate.

At first, expansion takes place in consequence of the heat, but this effect is soon reversed, as the oxygen is converted into a solid by union with the phosphorus, forming a white smoke, which gradually disappears. (Fig. 106.)

[Ill.u.s.tration: Fig. 106. A. Cylindrical gla.s.s vessel, open at one end, and inverted over B, the wine-gla.s.s, supporting C, the cup containing the burning phosphorus, and the whole standing in a soup-plate, D D, containing water.]

Supposing two grains of phosphorus had been placed in a platinum tube, and just enough atmospheric air pa.s.sed over it to convert the whole into phosphoric acid, the weight of the phosphorus would be increased to 4 grains by the addition of 2 grains of oxygen; [Page 104] now one cubic inch of oxygen weighs 0.3419, or about 1/3rd of a grain, hence 7.3 cubic inches of oxygen disappear, which weigh as nearly as possible 2 grains, so that as 36.5 cubic inches of air contain 7.3 cubic inches of oxygen, that quant.i.ty of air must have pa.s.sed over the 2 grains of phosphorus to convert it into 4 grains of phosphoric acid.

For very delicate purposes, nitrogen is best prepared by pa.s.sing air over finely-divided metallic copper heated to redness; this metal absorbs the whole of the oxygen and leaves the nitrogen. The finely-divided copper is procured by pa.s.sing hydrogen gas over pure black oxide of copper.

_Second Experiment._

[Ill.u.s.tration: Fig. 107. A. Gla.s.s jar, with collar of leather, through which the stamper, C, works. B B. The tube containing the finely-divided lead, part of which falls out, and is ignited, and retained by the little tray just below, being part of the iron stand, D D, with crutches supporting the ends of the gla.s.s tube, and the whole stands in the dish of water, E E.]

A very instructive experiment is performed by heating a good ma.s.s of tartrate of lead in a gla.s.s tube which is hermetically sealed, and being placed on an iron support, is then covered by a capped air jar with a sliding rod and stamper, the whole being arranged in a plate containing water. When the stamper is pushed down upon the gla.s.s the latter is broken (Fig. 107), and the air gradually penetrates to the finely divided lead, when ignition occurs, and the oxygen is absorbed, as demonstrated by the rise of the water in the jar. On the same principle, if a bottle is filled about one-third full with a liquid amalgam of lead and mercury, and then stopped and shaken for two hours or more, the finely divided lead absorbs the oxygen and leaves pure nitrogen. Or if a mixture of equal weights of sulphur and iron filings, is made into a paste with water in a thin iron cup, and then warmed and placed under a gas jar full of air standing on the [Page 105] shelf of the pneumatic trough, or in a dish full of water, the water gradually rises in the jar in about forty-eight hours, in consequence of the absorption of the oxygen gas.

_Third Experiment._

Nitrogen is devoid of colour, taste, smell, of alkaline or acid qualities; and, as we shall have occasion to notice presently, it forms an _acid_ when chemically united with oxygen, and an alkali in union with hydrogen. A lighted taper plunged into this gas is immediately extinguished, while its specific gravity, which is lighter than that of oxygen or air, is demonstrated by the rule of proportion.

Weight of 100 cubic Weight of 100 cubic Specific inches of air at 60 Unity. inches of nitrogen at gravity of Fahr., bar. 29.92 in. 60 Fahr., bar. 29.92 in. nitrogen.

30.829 : 1 :: 29.952 : 971

And its levity may be shown very prettily by a simple experiment. Select two gas jars of the same size, and after filling one with oxygen gas and the other with nitrogen gas, slide gla.s.s plates over the bottoms of the jars, and proceed to invert the one containing oxygen, placing the neck in a stand formed of a box open at the top; then place the jar containing nitrogen over the mouth of the first, withdrawing the gla.s.s plates carefully; and if the table is steady the top gas jar will stand nicely on the lower one. Then (having previously lighted a taper so as to have a long snuff) remove the stopper from the nitrogen jar and insert the lighted taper, which is immediately extinguished, and as quickly relighted by pushing it down to the lower jar containing the oxygen. This experiment may be repeated several times, and is a good ill.u.s.tration of the relative specific gravities of the two gases, and of the importance of the law of universal diffusion already explained at p.

6, by which these gases _mix_, not _combine_ together, and the atmosphere remains in one uniform state of composition in spite of the changes going on at the surface of the earth. Omitting the aqueous vapour, or steam, ever present in variable quant.i.ties in the atmosphere, ten thousand volumes of dry air contain, according to Graham:--

[IIl.u.s.tration: Fig. 108. A. Gas jar containing nitrogen, N, standing on B, another jar full of oxygen, O. The taper, C, is extinguished at N, and relighted at O. D D. Stand supporting the jars.]

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Nitrogen 7912 Oxygen 2080 Carbonic acid 4 Carburetted hydrogen (CH_{2}) 4 Ammonia a trace ------- 10,000

_Fourth Experiment._

It was the elegant, the accomplished, but ill-fated Lavoisier who discovered, by experimenting with quicksilver and air, the compound nature of the atmosphere; and it was the same chemist who gave the name of azote to nitrogen; it should, however, be borne in mind that it does not necessarily follow because a gas extinguishes flame that it is a _poison_. Nitrogen extinguishes flame, but we inhale enormous quant.i.ties of air without any ill effects from the nitrogen or azote that it contains; on the other hand, many gases that extinguish flame are _specific poisons_, such as carbonic acid, carbonic oxide, cyanogen, &c.

Lavoisier's experiment may be repeated by pa.s.sing into a measured jar, graduated into five equal volumes, four measures of nitrogen and one measure of oxygen; a gla.s.s plate should then be slid over the mouth of the vessel, and it may be turned up and down gently for some little time to mix the two gases, and when the mixture is tested with a lighted taper, it is found neither to increase nor diminish the illuminating power and the taper burns as it would do in atmospheric air. (Fig. 109.)

[Ill.u.s.tration: Fig. 109. A. Gas jar divided into five equal parts. B B.

Section of pneumatic trough, to show the decantation of gas from one vessel to another. The gas is being pa.s.sed from C to A, through the water.]

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HYDROGEN.

Hydrogen ([Greek: _udor_], water; [Greek: _gennao_], I give rise to), so termed by Lavoisier--called by other chemists inflammable air, and phlogiston. Symbol, H; combining properties, 1. The lightest known form of matter.

Every 100 parts by weight of water contain 11 parts of hydrogen gas; and as the quant.i.ty of water on the surface of the earth represents at least two-thirds of the whole area, the source of this gas, like that of oxygen or nitrogen, is inexhaustible. Van Helmont, Mayow, and Hales had shown that certain inflammable and peculiar gases could be obtained, but it was reserved for the rigidly philosophic mind of Cavendish to determine the nature of the elements contained in, and giving a speciality to, the inflammable gases of the older chemists. By acting with dilute acids upon iron, zinc, and tin, Cavendish liberated an inflammable elastic gas; and he discovered nearly all the properties we shall notice in the succeeding experiments, and especially demonstrated the composition of water in his paper read before the Royal Society in the year 1784.

_First Experiment._

Hydrogen is prepared in a very simple manner, by placing some zinc cuttings in a bottle, to which is attached a cork and pewter or bent gla.s.s tube, and pouring upon the metal some dilute sulphuric or hydrochloric acid. Effervescence and ebullition take place, and the gas escapes in large quant.i.ties, water being decomposed; the oxygen pa.s.ses to the zinc, and forms oxide of zinc, and this uniting with the sulphuric acid forms sulphate of zinc, which may be obtained after the escape of the hydrogen by evaporation and crystallization. (Fig. 110.)

Zn + HO.SO_{3} = ZnO.SO_{3} + H; or, Zn + HCl = ZnCl + H.

In nearly all the processes employed for the generation of hydrogen gas, a metal is usually employed, and this fact has suggested the notion that hydrogen may possibly be a metal, although it is the lightest known form of matter; and it will be observed in all the succeeding experiments that a metallic substance will be employed to take away the oxygen and displace the hydrogen.

[Ill.u.s.tration: Fig. 110. A. Bottle containing zinc cuttings and water and fitted with a cap and two tubes, the one marked B, containing a funnel, conveys the sulphuric acid to the zinc and water, whilst the gas escapes through the pipe C.]

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Whenever hydrogen is prepared it should be allowed to escape from the generating vessel for a few minutes before any flame is applied, in order that the atmospheric air may be expelled. The most serious accidents have occurred from carelessness in this respect, as a mixture of hydrogen and air is explosive, and the more dangerous when it takes fire in any closed gla.s.s bottle.

_Second Experiment._

If a piece of pota.s.sium is confined in a little coa.r.s.e wire gauze cage, attached to a rod, and thrust under a small jar full of water, placed on the shelf of the pneumatic trough, hydrogen gas is produced with great rapidity, and is received into the gas jar. The bit of pota.s.sium being surrounded with water, is kept cool, whilst the hydrogen escaping under the water is not of course burnt away, as it is whenever the metal is thrown on the _surface_ of water.

_Third Experiment._

[Ill.u.s.tration: Fig. 111. A. Flask containing water, and producing steam, which pa.s.ses to the iron tube, B B, containing the iron borings heated red hot in the charcoal stove C. The hydrogen pa.s.ses to the jar D, standing on the shelf of the pneumatic trough.]

Across a small iron table-furnace is placed about eighteen inches of 1-inch gas-pipe containing iron borings, the whole being red-hot; and attached to one end is a pipe conveying steam from a boiler, or flask, or retort, whilst another pipe is fitted to the opposite end, and pa.s.ses to the pneumatic trough. Directly the steam pa.s.ses over the red hot iron borings it is deprived of oxygen, which remains with the iron, forming the rust or oxide of iron, whilst the hydrogen, called in this case _water gas_, escapes with great rapidity. When steam is pa.s.sed over red-hot charcoal, hydrogen is also produced with carbonic oxide gas, and this in fact is the ordinary process of making _water gas_, which being purified is afterwards saturated with some volatile hydrocarbon and burnt. At first sight, such a mode of making gas would be thought extremely profitable, and in spite of the numerous failures the _discovery_ (so called) of _water gas_ is reproduced as a sort of _chronic wonder_; but experience and practice have clearly demonstrated that _water gas_ is a fallacy, and as long as we can get coal it is not worth while going through the round-about processes of first burning coal to produce steam; secondly, [Page 109] of burning coal to heat charcoal, over which the steam is pa.s.sed to be converted into gas, which has then to be purified and saturated with a cheap hydrocarbon obtained from coal or mineral naphtha; whilst ordinary coal gas is obtained at once by heating coal in iron retorts. (Fig. 111.)

Thus, by the metals zinc, tin, pota.s.sium, red-hot iron (and we might add several others), the oxygen of water is removed and hydrogen gas liberated.