n mixing paint.

483. Sodium is a metal; chlorine is a poisonous gas; yet salt,
which is made up of these two, is a harmless food.

484. When bricklayers mix water with cement and lime, the
resulting mortar boils and steams.

485. Green plants will not grow in the dark.

486. Parts of the body are constantly uniting with oxygen.
This keeps the body warm.

487. Water will not always put out a kerosene fire.

488. Delicately colored fabrics should be hung in the shade to
dry.

489. A match glows when you rub it in the dark.

490. Candy hardens when it cools.

SECTION 52. _Explosions._

What makes a gun shoot?

What makes an automobile go?

Usually we think of explosions as harmful, and they often are, of
course. Yet without them we could no longer run automobiles; gasoline
launches would stop at once; motorcycles would no longer run; gasoline
engines for pumping water or running machinery would not be of any
use; and all aviation would immediately cease. Tunneling through
mountains, building roads in rocky places, taking up tree stumps,
and preparing hard ground for crops would all be made very much more
difficult. War would have to be carried on much as it was during
the Middle Ages; soldiers would use spears and bows and arrows;
battleships would be almost useless in attacking; modern forts would
be of little value; cannon, guns, rifles, howitzers, mortars, and
revolvers would all be so much junk.

[Illustration: FIG. 179. The explosion of 75 pounds of dynamite. A
"still" from a motion-picture film.]

[Illustration: FIG. 180. Diagram of the cylinder of an engine. The
piston is driven forward by the explosion of the gasoline in the
cylinder.]

WHAT MAKES AN AUTOMOBILE GO. In all the above cases the explosions
are caused by chemical action. When gasoline mixed with air is sprayed
into the cylinder of an automobile, an electric spark makes the
gasoline combine with the oxygen of the air; the gasoline suddenly
burns and changes to steam and carbon dioxid. As you already know,
when a liquid like gasoline turns to gases such as steam and carbon
dioxid, the gases take much more room. But that is not all that
happens. Much heat is released by the burning of the gasoline spray,
and heat causes expansion. So the gases formed by the burning gasoline
are still further expanded by the heat released by the burning.
Therefore they need a great deal more roo