Man's physical universe

xanabras

COMBUSTION 645

heat is dispersed nearly as fast as it is produced. Regardless of the

rate of the reaction, the same amount of heat is evolved for the same

amounts of reactants.

A log of wood will gradually decay as it lies on the ground in the

forest, and it is eventually consumed, giving the same products and

the same amount of heat just as truly as if it had been burned. All

forms of decay are oxidation reactions which are caused by living

microorganisms.

Combustion Sometimes Starts Spontaneously.

Sometimes the heat produced by decay accumulates until

a temperature

is reached at which combustion takes place; this temperature

is called the kindling temperature. For example, an oily rag containing

some oil such as linseed oil which is easily oxidized may start a fire.

Linseed oil is used in paints because it will slowly combine with oxygen

to form a tough, resistant coating. Such paints dry best when

plenty of oxygen is available. If an oily rag is placed in a closet or

some other place where the heat produced by the oxidation of the oil

will not diffuse away as rapidly as it is produced, the kindling temperature

may be reached and a fire started; such a process is called

spontaneous combustion. Many fires are started by the spontaneous

combustion of damp hay, paper, coal, and other organic materials.

An important factor in spontaneous combustion is that the speed of

a chemical reaction is roughly doubled or trebled for each ten-degree rise

in the centigrade temperature. Thus, as the temperature rises, heat is

given off more and more rapidly until the slow oxidation becomes

rapid combustion.

Some substances are so active with oxygen that spontaneous combustion

takes place in even the most exposed places; thus a lump of white

phosphorus left on the table will start burning spontaneously. Certain

other substances having a higher kindling temperature will ignite

spontaneously only on unusually hot days. A hot piece of iron placed

in the vapor of carbon disulfide will cause it to ignite. When grease

is spilled on a hot stove lid, it ignites because its kindling temperature

has been reached. The purpose of a match used in lighting a fire is

to raise the temperature of a portion of a combustible material above

the kindling temperature.

Incendiary leaflets, dropped from airplanes during World War II,

consisted of sheets of wet guncotton (nitrocellulose)

containing finely

divided phosphorus between them. After the guncotton dried, the

phosphorus reacted with the oxygen of the air and ignited the guncotton,

which burned with a hot flame.

More magazines by this user
Similar magazines