§ 25. NATURE OF COAL. 699 



between the eye and the light, appears of a rich brown 

 colour, and displays a ligneous structure, resembling that 

 of deal or fir : it is, in fact, a thin section of fossil coni- 

 ferous wood ; for jet is nothing more than the wood of 

 some species of fir or pine, that has undergone the process 

 of bituminization, as I shall presently explain. When 

 viewed under a microscope, the small glands, which I have 

 mentioned as peculiar to the Coniferce (Lign. 154, Jig. 23), 

 are distinctly visible. The other specimens before us are 

 silicified woods, prepared in the same manner. A few 

 words, in explanation of the mode by which sections of such 

 extreme thinness are obtained, may not be uninteresting. 

 A slice is first cut from the fossil wood by the usual pro- 

 cess of the lapidary ; one surface is ground perfectly flat, 

 and polished, and then cemented to a piece of plate glass 

 by means of Canada balsam ; the slice thus firmly attached 

 to the glass is next ground down to the requisite degree of 

 tenuity, so as to permit its structure to be seen by the aid 

 of the microscope. It is by this ingenious process that the 

 intricate structure of any fossil plant can now be inves- 

 tigated, and the nature of the original determined, with as 

 much accuracy as if it were living.* 



25. Nature of Coal. — In this stage of our inquiry, 

 the nature of the process by which vegetables are con- 

 verted into the mineral substance termed Coal, requires 

 additional consideration. When sections of coal, obtained 

 by the method above described, are examined under the 

 microscope, the fine reticulated structure of the tree or 

 plant whence it was derived, is distinctly visible ; and often 

 the cells are filled with a light amber-coloured matter of a 

 bituminous nature, and so volatile as to be readily expelled 

 by heat before the texture of the coal is destroyed.f 



* See Observations on Fossil Vegetables, by Henry Witham, 

 Esq. F.G.S. 1 vol. 4to. With plates of the internal structure of 

 fossil plants. 



f Mr. Hutton, on " Coal." 



