hausted by the people and the fires in it. After the
subject has been explained, the following experiment can be prepared and
put aside till the next lesson.

(4) Fill two bottles with air from the lungs, as in (3) having previously
introduced a cutting from a plant into one of the bottles. Allow them to
stand in the sun for a day or two. Then test both bottles with a burning
match. If properly done, the result will be very striking. The end of
the cutting should be in the water of the dish. This experiment will not
succeed excepting with bottles such as are used for chemicals, which have
their mouths carefully ground. Common bottles allow the air to enter
between the bottle and the glass.[1]

[Footnote 1: See note on page 13.]

[Illustration: FIG. 3.]

4. _Fuel_.--Light a match and allow it to burn until half charred. Blow it
out gently, so as to leave a glowing spark. When this spark goes out it
will leave behind a light, gray ash. We have to consider the flame, the
charred substance, and the ash.

Flame is burning gas. In all ordinary fuels, carbon and hydrogen, in
various combinations and free, make the principal part. The first effect
of the heat is to set free the volatile compounds of carbon and hydrogen.
The hydrogen then begins to unite with the oxygen of the air, forming
water, setting free the carbon, which also unites with oxygen, forming
carbonic acid gas. The burning gases cause the flame. The following
experiment will illustrate this.

[Illustration: Fig. 4.]

(5) Fit a test-tube with a tight cork, through which a bit of glass
tubing, drawn out into a jet, is passed, the tubing within being even with
the cork. Place some bits of shaving in the tube, cork it, and make the
cork perfectly air-tight by coating it with bees wax or paraffine. Heat
the test-tube gently over an alcohol lamp. The wood turns black, and vapor
issues from the jet, which may be lighted (Fig. 4). Care should be taken
to expel all the air before lighting.

(6) That the burning hydrogen forms water by uniting with the oxygen of
the air, may be shown by holding a cold glass tumbler over the jet, or
over any flame. The glass will be dimmed by drops of moisture.

The charred part of the wood is charcoal, which is one form of carbon.
Our ordinary charcoal is made by driving off all the gases from wood, by
burning it under cover where only a little air can reach it. The volatile
gases burn more readily than the carbon, and are the