something with
oxygen. The flame is a real thing. It is made up of hot gases,
rising from the hot fuel, and it is usually filled with tiny glowing
particles of carbon. When you burn a piece of wood, the heat partly
separates its elements, just as heating sugar separates the carbon
from the water. Some of the hydrogen gas in the wood and some of the
carbon too are separated from the wood by the heat. These are pushed
up by the cooler air around and combine with the oxygen as they rise.
The hydrogen combines more easily than the carbon; part of the carbon
may remain behind as charcoal if your wood does not all burn up,
and many of the smaller carbon particles only glow in the burning
hydrogen, instead of burning. That is what makes the flame yellow. If
you hold anything white over a yellow flame, it will soon be covered
with black soot, which is carbon.

WHAT SMOKE IS. Smoke consists mostly of little specks of unburned
carbon. That is why it is gray or black. When you have black smoke,
you may always be sure that some of the carbon particles are not
combining properly with oxygen.

Yellow flames are usually smoky; that is, they usually are full of
unburned bits of carbon that float off above the flame. But by letting
enough air in with the flame, it is possible to make all the little
pieces of carbon burn (combine with the oxygen of the air) before they
leave the heat of the burning hydrogen. That is why kerosene lamps do
not smoke when the chimney is on. The chimney keeps all the hot gases
together, and this causes a draft of fresh air to blow up the chimney
to push the hot gases on up. The fresh air blowing up past the flame
gives plenty of oxygen to combine with the carbon. The drum part of
an oil heater acts in the same way; when the drum is open, the heater
smokes badly; when it is closed up, enough air goes past the flame
to burn up all the carbon. But if you turn either lamp or heater
too high, it will smoke anyway; you cannot get enough air through to
combine with all the carbon.

The hottest flames are the blue flames. That is because in a blue
flame all the carbon is burning up along with the hydrogen of the
fuel--both are combining with the oxygen of the air as rapidly as
possible. A gas or gasoline stove is so arranged that air is fed
into the burner with the gas. You will see this in the following
experiment:

EXPERIMENT 96. Light the Bunsen burner in the laboratory. Open
wide the little valve