from the Coal Measures of Tor bane-hill. 39 
have disappeared, and a pitted appearance is produced, the 
pits being nothing more than the cavities in which the yellow 
particles were lodged, and the walls of the pits being the 
granular earthy ingredient which at one time surrounded the 
yellow particles. When small fragments of the coke are 
scraped off and subjected to a power of 250 diameters, none 
of the yellow combustible principle is present, the entire 
bulk being made up of dark granular masses. If the mineral 
be burnt in an open fire, the ash will be nearly white ; and 
when examined microscopically, no trace of the yellow com- 
bustible matter will be seen, and the granules (as shown in 
fig. 7) will be very minute, and of a light colour. These 
appearances will be constant, if care be taken to select a part 
of the mineral in which no traces of plants are visible to the 
naked eye ; but if portions of plants be present, they will be 
readily recognised by their woody and vascular tissues. The 
principal distinction, therefore, between the coke of the gas- 
works and the ash is, that in the former the granules are 
larger and blacker than they are in the latter. 
From these and numerous other observations, I con- 
clude that the mineral in question is a clayey substance, im- 
pregnated with a combustible material occurring in the form 
of rounded particles of a rich yellow or amber colour, but 
whether these particles be bituminous or not the chemists 
must decide. 
What I have already stated refers exclusively to the 
Torbane-hill mineral, and no mention has yet been made of 
the structure of coal. Under this head I could enter into a 
detailed account of most of the well-known varieties of British 
coal, my knowledge of which has been principally derived 
from a careful investigation of sections made by myself and 
by my friend Dr. James Adams, of Glasgow ; and I am 
happy in having this opportunity of bearing testimony to 
the correctness of the observations of Dr. Adams, upon which 
his opinions had been formed prior to my having the pleasure 
of his acquaintance. Were I now to describe these, I fear 
you would be kept here many hours ; but it is the intention 
of Dr. Adams and myself, at no very distant period, to read a 
paper on the minute structure of the principal kinds of British 
coal, before the Geological Society, as we deem that the most 
fitting place for such a subject. For our present purpose, 
therefore, it will be merely necessary for me to give, in as con- 
cise a manner as possible, the results of the investigations of 
Dr. Adams and myself on this point ; but I would have you 
understand that although I give you a general description of the 
structure of coal, I have with me the specimens from which 
you will be enabled to judge for yourselves whether my state- 
