from the Coal Measures of Torbane-hill. 
41 
in colour, somewhat resembling that of the mineral. There 
is also another variety of coal, termed the Black Methil, but 
in this the streak is black, as in all other coals. Yet the 
microscopic characters of both these varieties are very similar, 
and differ in no respect from coals generally. A curious fact, 
however, I learnt from the chemists in Edinburgh, that the 
composition of the Brown Methil came nearer to that of the 
Torbane-hill mineral than any of the other known coals did ; 
a fact which is borne out by the similarity in their external 
appearance. 
Examination of Coal by the Microscope. — If a small cubical 
block of any kind of coal be examined under a power of 
50 diameters, four of its six sides will exhibit more or less of 
a fibrous structure, precisely like that of wood ; the other two 
sides, if perfectly flat, will appear bright and polished, and 
show very little structure : these correspond to the transverse 
sections of wood. Treat the Torbane-hill mineral in the same 
way, and how very different are the results ! Nearly the 
same structure will be found on all its sides, but in none is 
there the least trace of a fibrous arrangement. 
Examination of Sections of Coal hy the Microscope, — If a 
section of any well-known coal, cannel or otherwise, be 
reduced sufficiently thin to be transparent, a work sometimes 
of considerable labour and difficulty, it will be found to ex- 
hibit one of two structures, according to the direction in which 
the section has been made. These, for the sake of distinction, 
may be called the cellular and the fibrous ; the first corre- 
sponding with a horizontal section, the second with a 
vertical section, of wood. If it so happen that a section 
taken at random from any specimen of coal should exhibit one 
of these structures above named, by cutting at right angles, 
the other will be found. Thus, for instance, if the first section 
should correspond to a horizontal section of wood, the cut at 
right angles to it will correspond with the vertical one ; and, 
of course, if the section be an oblique one, an intermediate 
structure would be observed. This remarkable fact is con- 
stant in all the coals I have examined, and a knowledge of it 
enables the observer to tell at once whether any section taken 
at random was a horizontal or a vertical one. How strangely 
different this from the Torbane-hill mineral ! Cut that mineral 
in any way you please, and there will be little or no difference 
in appearar.ce. The structure of the transverse sections of coal 
is so very peculiar and so characteristic, that I must briefly 
point out the means it affords of distinguishing coal from any 
other modification of vegetable tissue. The peculiarity con- 
sists in this, — that, in the midst of a black opaque ground, 
VOL. II. e 
