Place a small quantity of mercuric oxide in a dry test tube, and heat it over a Bunsen lamp. The sub. stance darkens in colour, and a ring of minute globules of mercury soon forms on the cool part of the tube. That the tube contains oxygen may be shown by plunging a glowing chip of wood into it, and observing that the wood will be rekindled. It is advisable to keep the thumb loosely on the mouth of the test tube to prevent the escape of oxygen by diffusion.

4. Preparation of oxygen from potassium chlorate. Potassium chlorate when heated is decomposed into oxygen and potassium chloride.

Potassium chlorate yields potassium chloride and oxygen:

KCIO3 KCl + 03.

=

122'6

=

74.6 + 48.

Place a few crystals of potassium chlorate in a dry test tube, and heat gently. The salt soon fuses and then begins to effervesce, giving off oxygen, which may be recognized, as before, by its power of rekindling a glowing chip. When oxygen ceases to be evolved, the residue is a white salt called potassium chloride.

5. Tests for potassium chlorate and chloride.

A solution of potassium chlorate is not precipitated by a solution of silver nitrate; but potassium chloride is precipitated, silver chloride being formed.

Potassium chloride and silver nitrate yield potassium nitrate and silver chloride :

KCl+AgNO3 == KNO, + AgCl.

74.6 + 170 = IOII

143'5.

Dissolve a crystal of potassium chlorate in distilled water, then add solution of silver nitrate, no precipitate will be formed. Dissolve also the residue of potassium chloride (obtained in 4) in distilled water, and add silver nitrate solution. A curdy white precipitate of silver chloride will at once be formed.

6. Preparation of oxygen from potassium chlorate and manganese dioxide.

Potassium chlorate when mixed with about one-fifth of its weight of manganese dioxide, gives off its oxygen at a much lower temperature than when heated alone, although the oxide itself remains quite unaltered.

Place in a small flask (about 100 cubic centimetres capacity) a mixture of potassium chlorate and manganese dioxide in the proportions already mentioned. Then fit into the neck of the flask a cork through

which a bent conducting tube passes, the lower end of which is placed under water in the pneumatic trough. Fill some gas jars with water and invert them in the trough. Support the flask on a retort stand, and heat the mixture (Fig. 2) ; as soon as heat is applied, bubbles of gas will begin to rise through the water, consisting of air expelled from the flask by the heat. These are allowed to escape, and the oxygen, which soon begins to come off abundantly, is collected by placing the inverted bottles over the end of the conducting tube, and thus allowing the bubbles of oxygen to ascend into the bottles and displace the water. As soon as the first bottle is filled with oxygen, place a shallow earthenware tray under the neck, and remove it from the trough, taking care that the tray contains enough water to prevent the escape of the gas. When four bottles have been filled in this manner, remove the flask and conducting tube and allow the former to cool.

7. The residue from the preparation of oxygen from potassium chlorate and manganese dioxide is potassium chloride, and unaltered manganese dioxide.

Dissolve in water the residue in the flask from the preparation of oxygen, filter from the insoluble manganese dioxide, and evaporate the filtrate in a porcelain basin to a small bulk; on cooling, crystals of votassium chloride will separate out. Pour off the mother liquor, dry the crystals between filter paper, and keep them in a small bottle for future experiments.

8. Combustions in oxygen.

When a taper is burned in oxygen, the carbon which

it contains unites with the oxygen, forming carbon dioxide (carbonic acid gas), and the hydrogen also unites with the oxygen, forming hydrogen monoxide (water). Carbon and oxygen yield carbon dioxide :

Place a small piece of taper on an iron wire support, light it, and plunge it into one of the jars of oxygen (prepared in 6). Observe that it burns much more brightly than in air. Withdraw it, blow the light out, and if only a spark remain on the wick, observe that when again plunged into the oxygen it is at once rekindled. This serves as a convenient test for the presence of oxygen. When solution of calcium hydrate (lime water) is placed in contact with carbon dioxide, calcium carbonate (chalk) and water are formed, and the former, being insoluble in water, is precipitated as a white powder.

Calcium hydrate and carbon dioxide yield calcium carbonate and water :

Ca(HO)2 + CO2 = CaCO3 + H2O.

Prove that the jar in which you have burned the taper contains carbon dioxide, by adding a little clear lime water to it and shaking so as to bring the gas and liquid in contact. A white precipitate will be formed, consisting of calcium carbonate.

9. When charcoal is burned in oxygen carbon dioxide is produced. (Compare 8.)

Place a few pieces of charcoal, about the size of peas, in the deflagrating spoon, hold this in the lampflame till the charcoal is just kindled, and then plunge the spoon into a jar of oxygen. The charcoal will burn with great brilliancy, and, if enough oxygen be present, nothing will remain on the spoon but a little white ash (the inorganic matter in the charcoal).

Prove the presence of carbon dioxide (as in 8) by adding lime water.

A similar union of carbon and oxygen takes place in animals when breathing, but the combustion is slow, not rapid. A portion of the carbon of their bodies unites with the oxygen which they inhale from the air, and carbon dioxide is produced. Observe this by blowing the air from the lungs through a glass tube into some clear lime water, a white precipitate of calcium carbonate soon forms, caused by the union of the carbon dioxide of the breath with the lime. (See Roscoe, p. 11.)

10. When sulphur is burned in oxygen, sulphur dioxide (sulphurous anhydride) is produced.

Sulphur and oxygen yield sulphur dioxide :

S+ O2 32+32

Place a few pieces of sulphur in the deflagrating spoon, heat until the sulphur is melted and takes fire, and then plunge it into a jar of oxygen. Observe that the sulphur, which only burned feebly in air, burns