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.

Heat a bit of wood in a small test-tube and light the gas that is
evolved. Note the charcoal left.

Cover a piece of wood with sand or earth; heat, and note that charcoal
is formed. This illustrates the old method of charcoal-burning. This
subject is closely related to industrial geography.

HYDROGEN

A convenient way to prepare hydrogen is to use zinc and hydrochloric
acid with a test-tube for a generator. (Consu