28 



MINERALOGY. 



and has less lustre. Its chief varieties are the pitching 

 or caking coal, which in burning is apt to cake or unite 

 into solid masses, and the cannel coal, which burns with 

 so clear a flame that it has sometimes been used as can- 

 dles. Inkstands, boxes, etc., are made from it. The min- 

 eral called jet, so much used in jewelry, is allied to can- 

 nel coal, but has a much deeper color, and is capable of 

 a brilliant polish. Brown coal, or lignite, is not so per- 

 fectly formed coal as the other varieties, and the struc- 

 ture of the original wood is often apparent in it. 



41. Coal of Vegetable Origin. That coal is made from 

 plants might be properly inferred from the remains and 

 impressions of the plants found in the layers of rocks be- 

 tween which the coal is packed, and sometimes found 

 even in the coal itself. But the proof is more positive 

 than this. Vegetable structure is found in the coal by 

 microscopical examination. A piece of anthracite coal 

 which has been partially burnt is used, because the vege- 

 table cells, being somewhat silicious, remain unaltered 



in the burning out 

 of a portion of the 

 carbon. In Fig. 18, 

 a, we have repre- 

 sented a small bit 

 of such a piece of 

 anthracite as seen 

 through the micro- 

 scope. The ducts 

 lie side by side, as 

 they did ages ago 

 in the woody fibre 

 of the plant. In b 

 two of the ducts are 

 very highly magnified, the white spaces showing us the 

 silica, and the black lines the carbon which was not 

 burned out. The evidence from such an examination is 

 not as clear in soft bituminous coal, because the original 

 texture is not so well preserved. 



Fig. IS. 



