120 BERTHA'S VISIT TO HER 



He then continued, " Sir James Hall thinks 

 that peat may have been converted into coal by 

 heat acting under great compression ; and he 

 has actually succeeded in making a substance 

 very like it. * When I have more leisure I 

 will describe the ingenious process which he 

 adopted, as well as some other experiments of 

 the same nature, by which this distinguished 

 philosopher discovered the means of fusing lime- 

 stone, of imitating volcanic lava, and of forming 

 solid sandstone from loose sand. 



" But to return to our coals : the chief diffe- 

 rence between the various kinds of coal which 

 are applied to economical purposes,, arises from 

 the proportion of bitumen they contain. What is 

 called caking coal yields about 40 per cent. ; when 

 burning it swells, agglutinates, and emits much 

 smoke and gas, which inflame at a certain tempe- 

 rature. Cannel coal has only 20 per cent, of bitu- 

 men, and does not agglutinate or cake. It burns 

 with a bright flame like a candle, from which cir- 

 cumstance it takes its name, cannel being the 

 common pronunciation of candle in the North 

 of England. The third sort I shall mention is 

 called anthracite by mineralogists ; but its com- 

 mon name is blind coal, or Kilkenny coal, from 

 a district in Ireland, where there are vast beds of 

 it. It contains little or no bitumen ; it neither 

 cakes nor flames, and gives out very little smoke. 

 Eut as there are several varieties of coal between 



