*Preparation and Properties of Oxygen.*—Intimately mix 3 grams (1/2 teaspoonful) of potassium chlorate with half its bulk of manganese dioxide, and place the mixture in a large test tube. Close the test tube with a tight-fitting stopper which bears a glass tube of sufficient length and of the right shape to convey the escaping gas to a small trough or pan partly filled with water, on the table. Fill four large-mouthed bottles with water and, by covering with cardboard, invert each in the trough of water. Arrange the test tube conveniently for heating, letting the end of the glass tube terminate under the mouth of one of the bottles (Fig. 58). Using an alcohol lamp or a Bunsen burner, heat over the greater portion of the tube at first, but gradually concentrate the flame upon the mixture. Do not heat too strongly, and when the gas is coming off rapidly, remove the flame entirely, putting it back as the action slows down. After all the bottles have been filled, remove the end of the glass tube from the water, but leave the bottles of oxygen inverted in the trough until they are to be used. On removing the bottles from the trough, keep the tops covered with wet cardboard. [Fig. 58] Fig. 58—*Apparatus* for generating oxygen. 1. Examine a bottle of oxygen, noting its lack of color. Insert a small burning splinter in the upper part of the bottle and observe the change in the rate of burning. The air contains free oxygen, but it is diluted with nitrogen. Compare this with the undiluted oxygen in the bottle as to effect in causing the splinter to burn. 2. In a second bottle of oxygen insert a splinter without the flame, but having a small spark on the end. As soon as the oxygen kindles the spark into a flame, withdraw from the bottle and blow out the flame, but again insert the spark. Repeat the experiment as long as the spark is kindled by the oxygen into a flame. This experiment is usually performed as a test for undiluted oxygen. 3. Make a hollow cavity in the end of a short piece of crayon. Fasten a wire to the crayon, and fill the cavity with powdered sulphur. Ignite the sulphur in the flame of an alcohol lamp or Bunsen burner, and lower it into a bottle of oxygen. Observe the change in the rate of burning, the color of the flame, and the material formed in the bottle by the burning. The gas remaining in the bottle is sulphur dioxide (SO2), formed by the _uniting_ of the sulphur and the oxygen. 4. Bend a small loop on the end of a piece of picture wire. Heat the loop in a flame and insert it in some powdered sulphur. Ignite the melted sulphur which adheres, and insert it quickly in a bottle of oxygen. Observe the dark, brittle material which is formed by the burning of the iron. It is a compound of the iron with oxygen, similar to iron rust, and formed by their uniting.