COMBUSTIBLES 61 



There is always a waste in the burning of fuels, even 

 when the combustion is complete, for all fuels, except oil 

 and gas, contain substances which do not burn. These 

 substances produce the ashes which we always find under 

 our fires. These ashes often contain particles of fuel 

 which would have burned if the fire had been hotter, so 

 it is seen that there are two losses from incomplete com- 

 bustion. What are they? 



All combustibles, except some oils and some gases, 

 come directly from plants or animals. The following 

 experiment will show the difference between the two 

 kinds of combustion. 



Experiment 31. The Combustion of Different Ma- 

 terials. 



Apparatus: Alcohol lamp or Bunsen burner. 



Materials : Wood, charcoal, coal, coke, paper, straw, 

 feathers, piece of wool, piece of silk, piece of cotton, piece 

 of linen, whalebone, piece of leather. 



a. Hold a piece of each material in the flame and 

 note how it burns. Arrange all of those materials which 

 burn easily in one list and those that burn slowly in 

 another list. Which ones come from animals and which 

 from plants? Which is the safest clothing to wear when 

 near flames, cotton or woolen? 



Goods may be prepared so that they will not burn 

 readily. This is called fireproofing. See Section 73. 



