2 1 6 Elements of Mineralogy. 



gravity is about 1,4, it is frequently mixed 

 with pyrites. 



III. VARIETY. 



Coal containing a moderate proportion of Petrol 

 and Bitumen. 



This burns with more or lefs flame accord- 

 ing to the proportion of petrol; in its fracture, 

 it prefents a rougher furface than Canne/coal: 

 its fpecific gravity is from 1,3 to 1,37; the 

 beft coal is of this fort, by diflillation it 

 affords firft fixed air, then an acid liquor, 

 afterwards inflammable air, and a light oil 

 of the nature of petrol, then a volatil alkali, 

 and laftly a denfe pitchy oil ; the refiduum is 

 nearly I of the whole, and being flowly 

 burnt, affords 13 percent, of aihes, which is 

 moftly argillaceous Earth, of which T ^- or 

 thereabouts is magnetic. Mem. Stock. 1781. 



j oo Parts of this coal contain about 1 7 of 

 Earth, of which 4 are martial; hence we fee 

 that coal does not confift of a fhiftus pene- 

 trated with petrol, as many have thought, 

 for then a large proportion of filex, mag- 

 nefia, ari4 calcareous earth fhould be founci 



JV. VARIETY, 



