268 Natural Hijlory [Book VI. 



beneath, and the liquid matter is forced through an 

 iron pipe inferted into the top of the oven, and which 

 communicates with proper condenfmg veflels. By 

 this procefs a corrofive watery liquor and two forts of 

 oil are obtained. Six barrels of the mixed oily matter 

 produce about five barrels of oil of a thicker confif- 

 tence ; of the oil thus thickened, one part is lighter 

 than the other, which is drawn off, and is not at prefent 

 applied to any ufe *. The thicker part is ufed as an 

 inferior kind of tar. 



Coal, like moil other confiderable mafles of matter 

 found near the furface of the earth, is difpofed in beds 

 or ftrata, which are fometimes parallel with the hori- 

 zon, but generally form different angles with refpect 

 to it ; the fame ftratum uniformly preferves the fame 

 direction. The ftrata of coal are difpofed between 

 flrata of other matters, The following ftrata are 

 nfually found in thofe diftricts of country, both in 

 England and Scotland, where coal abounds: whin- 

 ftone, freeftone, fandftone or freeftone of a coarfer 

 texture, metalftone, which is ,a hard argillaceous fub- 

 ftance containing balls of iron ore, and fhiver, which 

 is the moft common of them all, and is an indurated 

 bole, ufually of a blackifh colour. 



Thefe ftrata do not bear on each other in the fame 

 order in different collieries, nor are they of any uni- 

 form thicknefs. The' ftrata of coal themfelves are 

 divided by other partings, called backs and cutters, into 

 innumerable cubic, prifmatic, and rhomboidal fi- 

 gures. 



In all places where the flrata lie regularly, they arc 

 divided and fubdivided in the manner above men- 

 tioned, and fometimes extend uninterruptedly through 



* See Bifliop Watibw's Eflays. 



