f
chemistry. It marked the dawn of a new period in the growth of
the science.

~Combustion in the broad sense.~ According to the definition given above,
the presence of oxygen is necessary for combustion. The term is
sometimes used, however, in a broader sense to designate any chemical
change attended by the evolution of heat and light. Thus iron and
sulphur, or hydrogen and chlorine under certain conditions, will combine
so rapidly that light is evolved, and the action is called a combustion.
Whenever combustion takes place in the air, however, the process is one
of oxidation.

~Spontaneous combustion.~ The temperature reached in a given
chemical action, such as oxidation, depends upon the rate at
which the reaction takes place. This rate is usually increased
by raising the temperature of the substances taking part in the
action.

When a slow oxidation takes place under such conditions that
the heat generated is not lost by being conducted away, the
temperature of the substance undergoing oxidation is raised,
and this in turn hastens the rate of oxidation. The rise in
temperature may continue in this way until the kindling
temperature of the substance is reached, when combustion
begins. Combustion occurring in this way is called _spontaneous
combustion_.

Certain oils, such as the linseed oil used in paints, slowly
undergo oxidation at ordinary temperatures, and not
infrequently the origin of fires has been traced to the
spontaneous combustion of oily rags. The spontaneous combustion
of hay has been known to set barns on fire. Heaps of coal have
been found to be on fire when spontaneous combustion offered
the only possible explanation.

~Importance of oxygen.~ 1. Oxygen is essential to life. Among living
organisms only certain minute forms of plant life can exist without it.
In the process of respiration the air is taken into the lungs where a
certain amount of oxygen is absorbed by the blood. It is then carried to
all parts of the body, oxidizing the worn-out tissues and changing them
into substances which may readily be eliminated from the body. The heat
generated by this oxidation is the source of the heat of the body. The
small amount of oxygen which water dissolves from the air supports all
the varied forms of aquatic animals.

2. Oxygen is also essential to decay. The process of decay is really a