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:

1. Add a little water to some baking powder and cause the gas that forms
to pass through lime-water. What causes the biscuits to "rise"?

2. Mix flour and water in a jar, add a bit of yeast cake and a little
sugar, and let stand in a warm place. Test the gas that forms, for
carbon dioxide. What causes bread to rise?

3. Uncork a bottle of ginger ale, shake the bottle, and lead the gas
that comes off through lime-water.

4. Most portable fire extinguishers depend on the generation of carbon
dioxide.

Show the similarity between our bodies and the candle. The candle needs
oxygen; it produces heat, and yields water and carbon dioxide. Much of
our food is somewhat similar in composition to the wax of a candle; we
breathe oxygen, our bodies are warmed by