2. Place a very short candle on a thin piece of cork afloat on water in a plate; light the candle, and again invert the jar over it. Note that the candle goes out and the water rises only a short distance in the jar; therefore _all_ the air has not been used up. 3. Slip the glass top of the jar under the open end and set the jar mouth upward on the table without allowing any water to escape. Now plunge a lighted splinter into the jar. The flame is extinguished. Air, therefore, contains an active part that helps the candle to burn and an inactive part that extinguishes flame. The names _oxygen_ and _nitrogen_ may be given. These gases occur in air in the proportion of about 1:4. (This method is not above criticism. Its advantage for young pupils lies in its simplicity.) OXYGEN Make two or three jars of oxygen, using potassium chlorate and manganese dioxide. (See any Chemistry text-book.) Let the pupils examine the chemicals, learn their names, and know where to obtain them. Perform the following experiments: 1. A glowing splinter relights and burns very brightly if plunged into oxygen. 2. A piece of picture wire tipped with sulphur burns with great brightness. 3. Burn phosphorus or match heads in a spoon. A spoon may be made by attaching to a wire a bit of crayon having a hollow scooped on its upper surface. A clay pipe bowl attached to a wire will answer. From these experiments pupils will learn the value of nitrogen as a diluent of the oxygen. Pure oxygen entering the lungs would be just as destructive as it would be entering the furnace. CARBON DIOXIDE 1. Make a jar of this gas. Washing soda and vinegar will answer if hydrochloric acid and marble are not obtainable. (Consult the _Science of Common Life_, Chap. XIII, and any Chemistry text-book.) 2. Lower a lighted candle first into a jar of air then into the jar of carbon dioxide. 3. Make some lime-water by stirring slaked lime with water and allowing the mixture to settle. Shake up some clear lime-water with a jar of the gas. Pupils will be made to understand that the milky colour will in future be considered the test for carbon dioxide. 4. Have one of the pupils cause his breath to bubble through some clear lime-water for a minute. Using a bicycle pump, cause some fresh air to bubble through lime-water. 5. Hold a clear jar inverted over the candle flame for a few seconds, then test with lime-water. 6. Invert a large jar over a leafy plant for a day. Keep in the dark and test the jar with lime-water. Is this gas likely to be in the air? Set a plate of lime-water in the school-room for a day or two, and then examine it. Try to pour the gas from jar to jar and use a candle as a test. Is the gas heavier than air? On account of its weight, the gas often collects in the bottoms of old wells, mines, and tunnels. It is dangerous there since it will not support life. USES: