COAL. 



37 



tillation. The Forest of Dean and Gloucestershire coal generally, are also 

 valuable, and work well, though not so productive as the preceding kinds. 



The Cannel coal* from Wigan, in Lancashire, and Scotland, produces gas 

 of better quality and in greater abundance than any other variety. 



The following scale, constructed from the results of accurate experiment, 

 will give a good idea of the relative values of the different kinds of coal men- 

 tioned above. It must be borne in mind that the quantities of gas are pro- 

 duced by experiment, the coal being submitted to distillation in very thin 

 strata : in practice (especially as the coal is now generally decomposed) the 

 results will not be so large ; still the graduation of the varieties will remain 

 the same. 



The experiments were conducted upon the plan I shall now proceed to 

 explain. 



Experiments on the Products from Coal. 



I have before observed that it is frequently difficult to detect the good from 

 the bad kinds of coal, and in large operations it would not be prudent to rely 

 solely on the judgement : I have known men of the greatest practice com- 

 pletely deceived. A specimen with a perfect fracture and lustre may produce 

 inferior gas, and that which upon inspection would be rejected may yield gas 

 in abundance and of excellent quality. It is therefore essential to prove the 



* So called from its having been used in some districts as a substitute for candles. In Scotland 

 it is called Parrot coal. 



