oxygen, forming phosphorus pentoxide (P,O), which dissolves in water, whilst the residue is nitrogen.

Place a small piece of dry phosphorus on a porcelain crucible lid, and fix this on a flat piece of cork, so that the lid may be floated on the water in the pneumatic trough, or other tray containing water. Light the phosphorus, and quickly surround it with a bell-jar, the tubulure of which is closed with a cork (Fig. 5). A

portion of the air will first be expelled by the heat, and the phosphorus will continue to burn as long as any oxygen remains in the air of the bell-jar; when that is exhausted it will cease to burn. Observe that the water rises inside the bell-jar, replacing the oxygen, which is no longer gaseous, but has combined with the phosphorus to form phosphorus pentoxide. Wait a few minutes to allow the pentoxide to be completely dissolved by the water. The bell-jar then contains nitrogen.

20 Nitrogen does not support combustion, and is not combustible.

Place a lighted taper in the bell-jar, and observe

that the flame is at once extinguished, and that the gas does not burn.

21. Preparation of nitric acid.

All nitrates when heated with sulphuric acid are decomposed, nitric acid and a sulphate being formed. Thus, when sulphuric acid acts on potassium nitrate (nitre or saltpetre), nitric acid, and hydrogen potassium sulphate (bisulphate of potash) are produced:

KNO3 + H2SO4

IOI'I + 98

=

=

HNO3 + HKSO,

63 + 136.1.

Put about twenty grams of potassium nitrate (nitre) in a small stoppered retort, place in the tubulure

a small funnel, and through this pour the same weight of strong sulphuric acid. Withdraw the funnel without soiling the neck of the retort, place the stopper in the tubulure, and support the retort upon a piece of wire

gauze placed across the ring of the retort stand. Let the neck of the retort pass into a clean flask, so arranged that a stream of water may fall on it, and thus aid the condensation of the acid (Fig. 6). Now heat the retort, and observe the formation of red fumes (lower oxides of nitrogen) and soon after, the condensation of the nitric acid on the neck of the retort. When the contents of the retort cease to boil, withdraw the lamp, separate the receiving flask containing the nitric acid from the retort, and pour out the contents of the latter into a dry porcelain dish. When cold, break up the cake of hydrogen potassium sulphate thus formed, and preserve it for future examination. 22. Tests for nitric acid.

(a) Add a few drops of the nitric acid prepared as above to a solution of indigo contained in a test-tube. The blue colour will speedily disappear owing to the oxidizing action of the nitric acid.

(b) Place a few bits of copper turnings in a testtube, cover them with water, and add a little nitric acid. The copper soon begins to dissolve, forming a blue solution, and at the same time brown vapours fill the test-tube. In this case also the nitric acid acts as an oxidizing agent, forming copper nitrate, and red fumes of the oxides of nitrogen are given off. For explanation see 30 and 31.

(c) Add a few drops of nitric acid to a little water contained in a test-tube, and then add some strong sulphuric acid, and shake until the liquids are thoroughly mixed. Allow this mixture to cool completely,

and then pour gently on to the surface of the liquid a solution of ferrous sulphate prepared by dissolving a few crystals of the salt in water. This solution is lighter than the other, and if poured on gently will form a layer of liquid resting on the heavier sulphuric acid, and a black ring will form where the two liquids meet. This is caused by the liberation of nitrogen dioxide (by the action of the FeSO, on the HNO3), which forms a dark-coloured compound with the ferrous sulphate. On shaking the tube, nitrogen dioxide will escape with effervescence, and the black ring will disappear.

Retain a portion of nitric acid for 27.

23. Tests for nitrates.

The same reactions may be used to test for combined nitric acid (i.e. nitrates), but sulphuric acid must first be added to liberate it.

Repeat the tests a, b, and c (22), substituting for the nitric acid a solution of potassium nitrate in water, to which a few drops of strong sulphuric acid have been added.

24. Preparation of ammonia.

When caustic alkalies act on salts of ammonium, ammonia gas is liberated. Thus :

Ammonium chloride and caustic lime yield aminonia, calcium chloride, and water :

2 (NH4) C1 + CaO

107 + 56

= 2 NH3+ Ca Cl2 + H2O

= 34 + III + 18.

Place about ten grams of powdered ammonium chloride (sal-ammoniac) and the same weight of

powdered lime in a test tube provided with a tightlyfitting cork, through which passes a tube bent as shown in the figure. Place over this an inverted dry gas-jar, and heat the mixture in the test tube. After the air has been expelled, the ammonia gas will come over, and in virtue of its lightness be retained in the jar. When the jar is filled, remove it from the

FIG. 7.

upright tube, place the palm of the hand on the mouth, and immerse the jar (mouth downwards) in the pneumatic trough filled with water: the gas will be rapidly absorbed, and the water will rise so as to fill the jar.

C