or four wide-mouthed bottles (250-cc.) of the gas. Before the heat is withdrawn remove the cork from the tube (why?).

Reserve the tube with its contents for study in § 19. What is the source of the oxygen? What is the function of the manganese dioxide?

16. Preparation from sodium peroxide and water. Sodium peroxide is a white solid containing 41 per cent of oxygen, and when treated with water, a part of this is set free. Arrange an apparatus according to Fig. 17. By means of a short piece of rubber tubing A connect the

funnel B with a glass

tube C, pinching the tube shut with a screw clamp. Place about 5 g. of sodium peroxide in the bottom of D and partly

fill the funnel with warm water. Very cautiously open the screw clamp so that the water will run down and fall, drop by drop, upon the peroxide. A steady current of oxygen is given off, and the gas should be collected as in the preceding exercise. (A separatory funnel may be used to advantage instead of the funnel and screw clamp.)

17. Preparation from potassium permanganate and hydrogen peroxide. Using the apparatus shown in Fig. 17, place a solution of 5 g. of potassium permanganate in 25 cc. of water in D and add 3 cc. of sulfuric acid. Fill the funnel with commercial hydrogen peroxide and allow it to flow very slowly into the solution in D.

18. Properties and conduct of oxygen. a. Note the properties of the gas. (The slight cloud often present when oxygen is prepared from potassium chlorate is due to an impurity and will disappear if the gas is allowed to stand over water.)

b. Repeatedly thrust a glowing splint into a bottle of the gas. c. Heat some sulfur in a deflagrating-spoon until ignited. Note the color and size of the sulfur flame. Now lower the burning sulfur into a bottle of oxygen and note the change.

d. Tip a piece of picture-frame wire, 12 or 15 cm. long, with sulfur by wrapping a bit of cotton about the end of the wire and dipping this into melted sulfur. Ignite the sulfur by holding it in a Bunsen flame for an instant, then thrust the wire into a bottle of oxygen.

Are the changes observed in b, c, and d examples of chemical action? What becomes of the oxygen? What is the name of the product formed in each case?

19. Separation of the compounds present in the residue left in the preparation of oxygen. Heat the tube containing the residue obtained in § 15 until no more oxygen is evolved. After the tube is cool, half fill it with water and shake the contents thoroughly. After a few minutes filter off the solid matter (what is it?). Evaporate the filtrate to a volume of 4 or 5 cc., and set it aside until crystals are deposited. Convince yourself that the substance is different from the potassium chlorate with which you started.