s ever destroyed. There is exactly as much
carbon in the carbon dioxid that floats off from a fire as there was
in the wood that was burned up; and there is exactly as much hydrogen
in the water vapor that floats off from the fire as there was in the
wood. Chemists have caught all the carbon dioxid and the water vapor
and weighed them and added their weight to the weight of the ashes;
and they have found them to weigh even more than the original piece of
wood, because of the presence of the oxygen that combined with them in
the burning.

If everything in the world were to burn up, using the oxygen that is
already here, the world would not weigh one ounce more or less than it
does now. All the elements that were here before would still be here;
but they would be combined in different compounds. Instead of wood
and coal and oxygen we should have water and carbon dioxid; instead of
diamonds, we should have just carbon dioxid; and so on with everything
that can burn.

WHY WATER PUTS OUT A FIRE. Water puts out a fire because it will not
let enough free oxygen get to the wood, or whatever is burning, to
combine with it. The oxygen that is locked up in a compound, like
water, you remember, has lost its ability to combine with other
things. Sand puts out a fire in the same way that water does. Most
fire extinguishers make a foam of carbon dioxid (CO_2) which covers
the burning material and keeps the free oxygen in the air from coming
near enough to combine with it.

Water will not put out burning oil, however, as the oil floats up on
top of the water and still combines with the oxygen in the air.

WHY ELECTRIC LAMPS ARE USUALLY VACUUMS. Electric lamps usually have
vacuums inside because the filament gets so hot that it would burn up
if there were any oxygen to combine with it. But in a globe containing
no oxygen the filament may be made ever so hot and it cannot possibly
burn.

High-power electric lamps are not made with vacuums but are
"gas-filled." The gas that is oftenest put into lamps is
nitrogen,--the same gas that is mixed with the oxygen in air. By
taking all the oxygen out of a quantity of air, the lamp manufacturers
can use in perfect safety the nitrogen that is left. It will not
combine with the glowing filament. There is no oxygen to combine with
the filament; so the lamp does not burn out.

WHAT FLAMES ARE. When you look at a flame, it seems as if fire were
a real thing and not merely a process of combining