than 125 deg. F.

We say that we burn oil in our lamps, but what we really do is to heat
the oil until it gives off gas, and then we burn the gas. To keep the
flame regular and help on the burning, we use a chimney on the lamp.
The hot air rises in the chimney and the cold air underneath rushes in
to take its place and brings oxygen to the flame. In a close, stuffy
room no lamp will give a good clear light, because there is not oxygen
enough for its flame. Let in fresh air, and the light will be
brighter. If you hold a cold plate in the flame before the chimney is
put on, soot or carbon will be deposited. A lamp gives light because
these particles of carbon become so hot that they glow. In lamps using
a "mantle," there is the glow not only of these particles, but also of
the mantle. In a wax candle, we light the wick, its heat melts the wax
and carries it to the flame. When the wax is made hot enough, it
becomes gas, and we burn the gas, not the wax. Wax alone will melt,
but not take fire even if a burning match is held to it. The reason is
that the match does not give heat enough to turn the wax into gas. But
put a bit of wax upon a bed of burning coals, where there is a good
supply of heat, and it will turn into gas and burn.

The products made from petroleum are as different in their character
and uses as paraffin and naphtha. Some of them are used for oiling
machinery; tar is used for dyes; naphtha dissolves resin to use in
varnish; benzine is the great cleanser of clothes, printers' types,
and almost everything else; gasoline runs automobiles, motors, and
many sorts of engines; paraffin makes candles, seals jelly glasses,
covers the heads of matches so that they are no longer spoiled by
being wet, and makes the ever-useful "waxed paper"; printers' ink and
waterproof roofing-paper both owe a debt to petroleum. Even in
medicine, though a little petroleum is no longer looked upon as a
cure-all, vaseline, one of its products, is of great value. It can be
mixed with drugs without changing their character, and it does not
become rancid. For these reasons, salves and other ointments can be
mixed with it and preserved for years.

XI

LITTLE GRAINS OF SALT

The most interesting mine in the world is that of Wieliczka in Poland.
In it there are some thirty miles of streets and alleys; there are
churches with pillars, shrines, and statues; there are stairs,
monuments, and restaurants; there is a ballroom three hu