Heat 85 



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peratures of two or more substances, such as phosphorus, 

 sulphur, wood and coal. To do so, place a sheet of wire gauze 

 about a foot square on an iron ring stand. On the gauze put a 

 small amount of each substance. Caution: Keep the phos- 

 phorus under water until required and use only a small piece. 

 Observe for five minutes. Does anything happen to any sub- 

 stance on the gauze? Apply heat gradually under the gauze 

 and notice the order in which the different substances kindle 

 as the degree of heat increases. Remembering that the kindling 

 temperature of a substance is the lowest temperature at which 

 it will burn, what do you conclude in regard to the compara- 

 tive kindling temperatures of these substances? 



'Smoke. If there is not a sufficient supply of oxygen to 

 burn combustible material completely, smoke is likely to form. 

 This often happens when starting a fire or when using damp 

 material. Smoke may occur for a little time after a large supply 

 of fuel is placed on a hot fire. It is a sign of incomplete 

 combustion. The rising gaseous products released by the heat 

 become laden with minute particles of carbon and with other 

 unconsumed bits of matter. Smoke, then, is a mixture of dif- 

 ferent forms of matter, some of which reflect the light and so 

 make the mixture visible to us. 



Spontaneous Combustion. Sometimes fires which cannot 

 easily be accounted for occur in buildings and result in great 

 loss of property. In such cases, it frequently happens that their 

 origin may finally be traced to some oily rags thrown into a 

 closet where there is very little or no ventilation. These oily 

 rags gradually become warmer as the oil oxidizes, until heat 

 enough has accumulated to raise their temperature to the kindling 

 point, and then the fire will start. This is spontaneous combus- 

 tion a nre starting as the result of oxidation without any out- 

 side aid. It sometimes occurs in hay barns where the newly cut 

 grass is packed so tightly that it receives no ventilation. It 

 also occurs in bituminous, or soft, coal piles where there is 

 poor ventilation. This becomes a serious problem where 

 much coal is piled together, as in the coal bunkers, or bins, of 

 ships. 



