36 COAL. 



possesses higher degrees of lustre upon its more perfect conchoidal fracture. 

 Slate Coal possesses a more or less coarse slaty structure, which, however, 

 seems to be rather a kind of lamellar composition than real fracture. 



All these varieties are objectionable for the production of gas, owing as well 

 lo the extraneous matter contained in them, as to the deficiency of their pro- 

 ducts in distillation. The following varieties may easily be distinguished by a 

 little practice, and are all ceconomical. Pitch Coal is of a velvet-black colour, 

 generally inclining to brown, with a strong lustre, and presenting in every 

 direction large and perfect conchoidal fracture. Foliated and Coarse Coal have 

 both a lustrous fracture, but approach more to a granular appearance. Cannel 

 Coal is without visible composition, and has a flat conchoidal fracture in every 

 direction, with but little lustre, by which it is distinguished from pitch coal ; it 

 is most like the moor coal, but the difference in their specific gravity is 

 greater than between almost any other two varieties, by which it is the best 

 distinguished. Sp. gr. of cannel coal T4 ; sp. gr. of moor coal I'lO. 



The transitions between the varieties of coal are hardly perceptible, and it 

 requires the nicest judgement to detect the good from the bad. They all con- 

 sist of bitumen and carbon in various proportions, are more or less easily 

 inflammable, and burn with flame and a bituminous smell ; several varieties 

 become soft, which is invariably an excellent criterion for valuable coal ; 

 others coke when kindled, and leave a more or less earthy residue. 



The varieties called slate coal, foliated coal, coarse coal, and pitch coal 

 occur chiefly in the coal formation : some varieties of pitch coal, and also the 

 moor coal, bituminous wood, and common brown coal, are met with in the 

 formations above the chalk ; the earthy coal, and some varieties of bitu- 

 minous wood and common brown coal, are often included in diluvial and 

 alluvial detritus. In the neighbourhood of Garstang and of Lancaster these 

 latter varieties are met with beneath a bed of peat thirty feet deep : trunks of 

 trees, hazel-nuts, and many kinds of bark and ferns are abundant. 



The Anthracite is a slaty, glance coal, perfectly free from bitumen, and 

 therefore totally unfit for purposes of gas manufacture. It consists, in some 

 specimens, of 95 per cent, of carbon, the remaining parts being oxide of iron, 

 silica, and alumina, in various proportions. 



The Staffordshire bed also furnishes large quantities of coal for the produc- 

 tion of gas, and the line between some varieties of these and Newcastle coal 

 can hardly be drawn. They generally require a higher temperature for dis- 



