CHAPTER XVI.

NITROGEN AND THE ATMOSPHERE.

186. Existence of Nitrogen. - Nitrogen is found uncombined chiefly in air, of which it makes up about 78% by volume and 75.5% by weight. It is found. combined with many elements, as with hydrogen in ammonia, and with hydrogen and oxygen in nitric acid. Nitrogen is an essential constituent of all animals and of many plants.

187. Preparation. - Crude nitrogen may be prepared from air by the removal of the oxygen by means of phosphorus or copper. With phosphorus the operation is as follows:

A vessel (Fig. 40) of air is placed over a bit of burning phosphorus which is floated in a

small dish upon water. The phosphorus unites with the oxygen in this confined portion of air to form phosphorus pentoxide, which is a white solid easily dissolved by the water. If the experiment is carried out accurately, the water which rises into the vessel after the experiment

FIG. 40.

is a measure of the oxygen used up by the phosphorus.

The oxygen of air is removed more satisfactorily by hot copper. The apparatus is shown in Fig. 41.

Pure nitrogen may be prepared by heating ammonium nitrite, NH4NO2, or, better, a mixture of solutions of ammonium chloride

(NH4Cl) and sodium nitrite (NaNO2). When this mixture is heated, a regular stream of nitrogen is evolved.

The equations are:

(1) NH4Cl+NaNO2 →→→→→ (NH,NO2)+NaCl.

(2) (NH,NO2)→→→ N2+2 H2O.

188. Properties of Nitrogen. - Pure nitrogen is a gas without taste, odor, or color, and about 0.97 as heavy as air. 100 c.c. of water under ordinary conditions can dissolve only about 1 c.c. of the gas. One liter of nitrogen weighs about 1.25 g. under standard conditions, i. e., at 0° C. and 760 mm. pressure.

CHARACTER OF THE ATMOSPHERE.

171

Since ordinary combustion and respiration require oxygen, it naturally follows that " that "atmospheric nitrogen," i. e., air deprived of oxygen, no longer supports either combustion or respiration. Pure nitrogen, like atmospheric nitrogen, is an extremely inactive substance, combining directly with only a few elements. It does combine with magnesium, titanium, lithium, etc., at an elevated temperature, giving nitrides, e. g., Mg,N2.

Under the influence of the electric spark, nitrogen unites with hydrogen and oxygen to form nitrous and nitric acids, and with hydrogen alone to form ammonia; hence these compounds, and the substances formed from them, viz., ammonium nitrite and ammonium nitrate, are found in the atmosphere, in water, and in the soil.

Bacteria living on the roots of some plants, such as peas and clover, can take up nitrogen directly from the air, forming albumins.

189. "Atmospheric Nitrogen " a Mixture.-Careful experiment shows that a liter of atmospheric nitrogen weighs 1.2571 grams and one of pure nitrogen 1.2507 grams. The cause of this difference is that atmospheric nitrogen contains another substance, heavier than nitrogen. Thus argon was discovered in 1894.,

M

190. Character of the Atmosphere.

The atmos

phere is the gaseous mantle of the earth. Some of

Bacteria are so universally present and of such great importance to many changes taking place in the atmosphere that they may rightly be classed among its variable ingredients.

By pure air we mean a mixture of the constant constituents of the atmosphere. The proportions, by volume and by weight, of the three most abundant of these are as follows:

Hydrogen exists in small quantities in the atmosphere: 100 liters of it contain about 19 c.c. of hydrogen. Hydrogen is thus present in almost as great an amount by volume as carbon dioxide (cf. § 193).

Nitrogen and oxygen, the most abundant constituents of air, have been described already. The rare gases of the atmosphere include argon, helium, neon, krypton, and xenon. These form a natural family (cf. § 377). All are inert, and have a valence of o.

191. Argon. The discovery of argon almost took place a hundred years before this substance was actually studied by Ramsay and Rayleigh in 1894; for Cavendish, the discoverer of hydrogen, records the observation that he could not get all the nitrogen of the air to combine with oxygen by "sparking a mixture of these gases in the presence of potassium hydroxide. This was in 1785. The "residual nitrogen was argon and the other inert gases which are mixed with atmospheric nitrogen. By repeating Cavendish's experiments, Ramsay and Rayleigh obtained argon.

A second way of obtaining this substance is to pass pure air over heated copper, which takes up the oxygen, and then over magnesium, which absorbs the nitrogen as magnesium nitride, Mg3N2. The nitrogen may also be removed by lithium or calcium.

Argon may be condensed to a colorless liquid, boiling at -185° C., and at lower temperatures may even be obtained in the solid state. In gaseous form it is heavier than oxygen. It is much more soluble in water than nitrogen, hence in air which has been dissolved in water and afterward expelled from solution the proportion of argon is greater than in the atmosphere. Argon is almost without chemical activity, hence its name.

192. Helium. By means of the spectroscope, helium was discovered to be a constituent of the sun