obtained in the distillation of Wood ^ 'bfc. 15 



gone, or the substances with which it has accidentally been mixed. 

 The power of yielding naphtha on distillation, is rather to be re- 

 sorted to as the distinction between the one end and the other of the 

 series ; and it would be surely equally correct to call coal a com- 

 pound of charcoal and naphtha, as a compound of charcoal and 

 asphaltum. 



Its several varieties will moreover be found to vary from each 

 other by containing greater or less proportions of carbon, compared 

 with their other ingredients ; just as in asphaltum the relative pro- 

 portions of the hydrogen, azote, and oxygen, to the carbon, are 

 found to differ from those which constitute petroleum or naphtha. 



The last link of the chain of coal (chemically considered) Is 

 anthracite, which contains only carbon, if we reckon the earths 

 mixed with it as adventitious matter. So the last result of the dis- 

 tillation of asphaltum is charcoal, and the intermediate steps through 

 which asphaltum passes in its progress to charcoal, resemble pre- 

 cisely the corresponding changes which occur in the distillation 

 of coal till coak is formed, and confirm by their chemical analogy 

 the view here held forth of the chemical composition of coal, and 

 the gradation to be traced in nature from fat coal, to anthracite. 

 If asphaltum be subjected to distillation it gives petroleum. By 

 degrees its solubility in naphtha diminishes, in consequence of its 

 carbonaceous ingredient becoming more disproportioned to its hy- 

 drogen. At a particular period of this distillation it will be found 

 to resemble fat coal ; by and by, it resembles blind coal, and gives 

 no stain to naphtha ; ultimately, pure charcoal remains. All these 

 bituminous compounds may therefore properly be said to belong to 

 one genus or family, of which the principal chemical ingredients are 

 carbon and hydrogen, and it is chiefly to the difference in the 

 relative proportions of those two substances that we are to look 



