42 Structure of a peculiar Combustible Mineral^ 
numerous brown transparent rings, each having a black dot 
in the centre, are interspersed ; they appear like transverse 
sections of thick-walled cells or of woody fibres. In some 
coals they occur in close proximity to each other, as in woods 
generally : in other cases they are more or less separated, 
either by the black material before alluded to, or by a network 
of rather smaller rings, in which the central dot is absent. 
There are many coals, especially some of the common domes- 
tic kinds, in which it is difficult to recognise this structure in 
every part of the section ; in these coals a rich brown struc- 
tureless material — bituminous or not I cannot say — seems to 
be in excess, and so obscures the characteristic appearances of 
the rings. In longitudinal sections the woody fibres are 
generally well seen, and a tendency to split in the direction 
of their length (as shown in Plate V., fig. 5), may always be 
observed. Amongst the fibres may be noticed certain elon- 
gated cells, of a rich brown colour, having a dark line running 
down through the centre : these are constant in all coals, and 
when divided transversely, appear as the rings before noticed. 
Their size is tolerably uniform in many coals (as shown in 
Plate v., fig. 2). Mr. Witham was acquainted with the 
differences between a longitudinal and a transverse section of 
coal, as may be seen on referring to the 2nd edition of his 
work on the " Fossil Vegetables of the Carboniferous and 
Oolitic Deposits ;" both the rings and the elongated cells are 
well figured, and his remarks on the value of investigating the 
microscopic structure of coal, are very excellent. The absence 
of vascular tissue in the numerous sections of coal, made both 
by Dr. Adams and myself, would lead to the supposition that 
the wood of which it is composed must approach very near to 
that of the Coniferae. 
Examiiiation of the Powder of Coal. — When coal is reduced 
to a fine powder, and examined either in water or in Canada 
balsam, it will be found to consist principally of short opaque 
cylinders or fibres, occurring singly or in bundles, and of 
angular dark-brown plates of various sizes, probably composed 
of bituminous matter (as shown in Plate V., fig. 7) ; the 
remainder of the mass is made up of minute transparent 
particles of silica, with an occasional mixture of fragments 
of cells and fibres. Many blocks of coal have a fine dull 
black powder on two of their outer surfaces, which will make 
the fingers very black : this I call the charcoal layer, and in 
it will be found fragments of woody tissue of cells, and even 
of vessels. My investigations lead me to believe that this 
layer is derived from plants which existed at the same time 
as the coal- wood, but were not capable of being converted 
