COAL: ITS COMPOSITION 119 



built up or broken down from the material of which, coal 

 is really formed, and, for anything known to the contrary, 

 a piece of bituminous coal is homogeneous throughout in 

 chemical composition and only splits up into many and 

 various bodies when heated. It may be inferred that if 

 the coal begins to split up as soon as heated so it will con- 

 tinue to split up as more heat is applied, the material split- 

 ting up more and more into lighter and heavier portions 

 so that nothing but pitch remains in the still, and after a 

 little further heating, even this is resolved into coke and 

 vapor. 



When coal is burned in a fire exposed to air, there is a 

 perhaps more complicated set of reactions put into oper- 

 ation. These are operations both of distillation and com- 

 bustion. An experiment first shown by Horace Allen was 

 the sprinkling upon a red-hot plate of porcelain of some 

 finely divided bituminous coal. At once vapor commences 

 to be given off and a dark spot surrounds each bit of coal. 

 The coal does not glow so long as the vapor is coming away 

 from it. When the vapor ceases to escape the coal begins 

 to get hot and the dark spots on the plate disappear. The 

 coal now begins to glow, to sparkle ; in fact, to oxidize and 

 disappear. 



Now, from this experiment much may be learned. First, 

 that the primary effect of heating coal is to drive off its 

 volatile portions. Actually, of course, heat renders the 

 coal partly volatile and drives this part away. The vapor- 

 izing of this demands heat and the vapor renders so much 

 heat latent that it dulls the surface of the plate. When 

 this chilling effect is finished by the escape of all vapor, 

 the remaining bit of coke gradually becomes hotter. But 

 it does not oxidize brightly until it has attained a high 

 temperature. These actions teach that coal upon a grate 

 will be very seriously cooled if fresh coal is thrown upon 

 it, and that the volatile matter must be thrown off any 

 piece of coal before its carbon skeleton will begin to burn. 

 In a thick bed of coked coal on a grate the chilling effect 



