in the Distillation of M'^ood, &c. 213 



'c bitumen, varying in its composition, from the fattest specimens 

 of Newcastle to the driest of Kilkenny, and owing its compact- 

 ness, as well as the other modifications w-hich it exhibits, to the 

 peculiar circumstances under which it has been formed^ the 

 changes it may subsequently have undergone, or the substances 

 with which it has accidentally been mixed. The power of yield- 

 ing naphtha on distillation is rather to be resorted to as the di- 

 stinction between the one end and the other of the series ; aisd it 

 would be surely equallv correct to call coal a compound of char- 

 (!oal and naphtha, as a compound of cliarcoal and asphaltum. 



Its several varieties will inoreover be found to vary from each 

 other bs' containing greater or less proportions of carbon, com- 

 pared with their other ingredients ; just as in asphaltum the re- 

 lative proportions of the hydrogen, azote, and oxygen,^ to the 

 carbon, are found to differ from these which constitute petro- 

 leum or naphtha. 



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

 'anthracite, which contain-^ only carbon, if we reckon the earths 

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

 distillation of asphaltum is charcoal, and the intermediate steps 

 tlirough which asphaltum passes in its progress to charcoal, re- 

 semble precisely the corresponding changes which occur in the 

 distillation of coal till coak is formed, and confirm by their che- 

 mical analogy the view here held forth of the chemical compo- 

 sition 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 hydrogen. At a particular period of this 

 distillation it wttl 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 ])roperly be said to belong to one genus or family, ot 

 which the principal chemical ingredients are carbon and hydro- 

 gen ; and it is chiefly to the difference in the relative proportions 

 of those two substances that we are to look for the differences 

 which characterize the several bitumens, from naphtha placed at 

 one extreme, to anthracite placed at the other. The chasm in 

 this series, from asphaltum to fat coal, is in fact rather ajjparent 

 than real, being more properly a mechanical or accidental than 

 a chemical or essential one I cannot here avoid taking notice 

 of the very loose experiments of Mr. Kirwan on the analysis of 

 '•(yal, (which consisted in projecting portions of coal on melted 

 Mte,) as his deductions are at war with this view of the subject, 

 1 though not more so thain with all chemical reasoning. They 

 'TO founded on an assumption, that coal was carbon improg- 

 O 3 nated 



