AIR AND OXYGEN 41 



not material, but the heat, given out by the oxidation, which 

 keeps the body warm. 



The opposite of oxidation, the removal of oxygen, is spoken of 

 in chemistry as reduction. But this term, also, has been stretched 

 to cover other similar kinds of chemical change. 



Spontaneous Combustion. Sometimes a mere slow oxi- 

 dation develops into a combustion, which is then known as spon- 

 taneous combustion. To understand this, we must note the fact 

 that a given weight of material, say, iron, in combining with 

 oxygen to form a given oxide, will liberate the same total amount 

 of heat whether the union proceeds rapidly or slowly. If the 

 action proceeds slowly, and the material being oxidized is freely 

 exposed to the air, the latter will become heated and will carry 

 off the heat as fast as it is produced. Thus, no particular rise 

 in temperature will occur. If, however, the material is a poor 

 conductor of heat, like hay or rags, and there is sufficient air for 

 oxidation, but not enough to carry off the heated air, the heat may 

 accumulate and a temperature sufficient to start combustion may 

 be reached. 



Such a situation sometimes arises in hay-stacks. It occurs 

 also when rags, saturated with oils used in making paints 

 (linseed oil and turpentine) are left in a heap. These oils, 

 in " drying," combine with oxygen from the air and turn into 

 a tough, resinous material. The rags, being poor conductors 

 of heat, may finally become hot enough to burst into flame, and 

 serious conflagrations often owe their origin to causes such as 

 this. Oily rags should always be disposed of by burning, or 

 should at least be placed in a closed can of metal. Fires in coal 

 bunkers of ships arise from the same cause slow oxidation, with 

 accumulation of the resulting heat. That coal does undergo 

 slow oxidation, especially when freshly mined, is shown by the 

 fact that such coal, if left exposed to the air for months, may 

 lose from 2 to 5 per cent or more of its heating value. 



