demanding the sale of
safety matches only.

39. Some Unfamiliar Forms of Burning. While most of us think of
burning as a process in which flames and smoke occur, there are in
reality many modes of burning accompanied by neither flame nor smoke.
Iron, for example, burns when it rusts, because it slowly combines
with the oxygen of the air and is transformed into new substances.
When the air is dry, iron does not unite with oxygen, but when
moisture is present in the air, the iron unites with the oxygen and
turns into iron rust. The burning is slow and unaccompanied by the
fire and smoke so familiar to us, but the process is none the less
burning, or combination with oxygen. Burning which is not accompanied
by any of the appearances of ordinary burning is known as oxidation.

The tendency of iron to rust lessens its efficiency and value, and
many devices have been introduced to prevent rusting. A coating of
paint or varnish is sometimes applied to iron in order to prevent
contact with air. The galvanizing of iron is another attempt to secure
the same result; in this process iron is dipped into molten zinc,
thereby acquiring a coating of zinc, and forming what is known as
galvanized iron. Zinc does not combine with oxygen under ordinary
circumstances, and hence galvanized iron is immune from rust.

Decay is a process of oxidation; the tree which rots slowly away is
undergoing oxidation, and the result of the slow burning is the
decomposed matter which we see and the invisible gases which pass into
the atmosphere. The log which blazes on our hearth gives out
sufficient heat to warm us; the log which decays in the forest gives
out an equivalent amount of heat, but the heat is evolved so slowly
that we are not conscious of it. Burning accompanied by a blaze and
intense heat is a rapid process; burning unaccompanied by fire and
appreciable heat is a slow, gradual process, requiring days, weeks,
and even long years for its completion.

Another form of oxidation occurs daily in the human body. In Section
35 we saw that the human body is an engine whose fuel is food; the
burning of that food in the body furnishes the heat necessary for
bodily warmth and the energy required for thought and action. Oxygen
is essential to burning, and the food fires within the body are kept
alive by the oxygen taken into the body at every breath by the lungs.
We see now one reason for an abundance of fresh air in daily life.

40. How to Breathe