These waste substances,
composed chiefly of carbon and hydrogen, unite with oxygen breathed in
from the air, forming carbonic acid gas and water, which are breathed
out of the system. The action is a process of slow combustion, and it is
principally by the heat thus evolved that the body is kept warm. As we are
thus constantly taking oxygen from the air, a close room becomes unfit to
live in and a supply of fresh air is indispensable. The cycle of changes
is completed by the action of plants, which take in carbonic acid gas, use
the carbon, and return most of the oxygen to the atmosphere.
APPARATUS FOR EXPERIMENTS.[1]
[Footnote 1: The glass apparatus required, including an alcohol lamp, may
be obtained for one dollar by sending to the Educational Supply Co., No. 6
Hamilton Place, Boston.]
Two small wide-mouthed bottles. A narrow-necked bottle. A glass funnel. A
bit of bent glass-tubing. A bit of straight glass-tubing. A flat piece of
glass. A test-tube, with jet. An alcohol lamp. A bent wire with taper.
A card. A slip of a plant. A dish and pitcher of water. Beeswax or
paraffine. Shavings. Lime water. Matches.
_Gray's First Lessons. Revised edition_. Sect. XVI, 445-7, 437.
_How Plants Grow_. Chap. III, 279-288.
II.
SEEDLINGS.
1. _Directions for raising in the Schoolroom_.--The seeds should be
planted in boxes tilled with clean sand. Plates or shallow crockery pans
are also used, but the sand is apt to become caked, and the pupils are
likely to keep the seeds too wet if they are planted in vessels that
will not drain.
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