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. Air, which is essential to life and health,
should enter the body through the nose and _not through the mouth_.
The peculiar nature and arrangement of the membranes of the nose
enable the nostrils to clean, and warm, and moisten the air which
passes through them to the lungs. Floating around in the atmosphere
are dust particles which ought not to get into the lungs. The nose is
provided with small hairs and a moist inner membrane which serve as