SOLUTION OF GASES
Study air dissolved in water, by gently heating water in a test-tube and observing the bubbles of air that gather on the inner surface of the test-tube. Aquatic animals, such as fish, clams, crayfish, crabs, subsist on this dissolved air.
LIMESTONE
Pieces of this rock may be found in all localities. Teach pupils to recognize it by its gray colour, its effervescence with acid, and the fossils and strata that show in most cases. If exposed limestone rocks are near, visit them with the pupils and note the layers, fossils, and evidences of sea action. Compare lime with limestone as to touch, colour, and action on water and litmus. Try to make lime by putting a lump of limestone in the coals for some time; add water to this. Other forms of limestone are marble, chalk, egg-shells, clam-shells, scales in tea-kettles.
Geographically, the study of limestone is of great importance. Grind some limestone very fine, add a very little of this to water, and bubble carbon dioxide through for some time; note the disappearance of the limestone. This explains how limestone rocks are being slowly worn away and why the water of rivers, springs, and wells is so often "hard".
Catch some rain-water in the open and test it for hardness. It will be found "soft". Place a few limestone pebbles in a tumbler with this soft water and after a day or two test again. The water will be "hard".
Compare, as to hardness, the water from a concrete cistern with that from a wooden one.
CARBON
Procure specimens of hard and soft coal, coke, charcoal, graphite, peat, and petroleum. Note the distinctive characteristics of each. Discuss the uses. Try to set each on fire. Note which burns with a flame when laid on the coals or placed over the spirit-lamp. Put a bit of soft coal into a small test-tube; heat and light the gas that is produced. This gas, when purified, is one kind of illuminating gas. Note the coke left in the test-tube.
Fill the bowl of a clay pipe with soft coal and seal it up with plaster of paris. After this has hardened, place the bowl in hot coals or in the flame of a spirit-lamp and light the coal-gas at the end of the stem. After all the gas has been driven off, look for the coke inside.