V. 



ON THE FORMATION OF COAL. 



THE lumps of coal in a coal-scuttle very often have a 

 roughly cubical form. If one of them be picked out and 

 examined with a little care, it will be found that its six 

 sides are not exactly alike. Two opposite sides are com- 

 paratively smooth and shining, while the other four 

 are much rougher, and are marked by lines which run 

 parallel with the smooth sides. The coal readily splits 

 along these lines, and the split surfaces thus formed are 

 parallel with the smooth faces. In other words, there 

 is a sort of rough and incomplete stratification in the 

 lump of coal, as if it were a book, the leaves of which 

 had stuck together very closely. 



Sometimes the faces along which the coal splits are 

 not smooth, but exhibit a thin layer of dull, charred- 

 looking substance, which is known as " mineral charcoal." 



Occasionally one of the faces of a lump of coal will 

 present impressions, which are obviously those of the 

 stem, or leaves, of a plant ; but though hard mineral 

 masses of pyrites, and even fine mud, may occur here 

 and there, neither sand nor pebbles are met with. 



When the coal burns, the chief ultimate products of 

 its combustion are carbonic acid, water, and ammoniacal 



