38 Structure of a peculiar Combustible Mineral^ 
readily traced ; even a part so minute as a single woody fibre 
can easily be recognised. 
In some specimens, the line of demarcation between the 
coal and the mineral is not very decided, owing to the coal 
and the plants found in connexion with it being so intimately 
blended ; in all such cases recourse should be had to the 
streak, as the best guide to distinguish them. In every part 
of the block containing coal and coal plants, the streak is 
black ; but in the smallest portions of the mineral it is brown. 
It is a curious fact, however, that in the specimen now before 
you, three kinds of structure are visible to the naked eye : 
1st, true coal ; 2nd, a mixture of coal with a few coal plants, 
principally Stigmarice ; 3rd, the mineral. When sections are 
made through the block in two directions at right angles to 
each other, the coal and the mixture of coal and plants will 
exhibit a structure corresponding with longitudinal and trans- 
verse sections of wood, but the mineral is the same in both 
sections. The yellow particles occupy all the interstices in the 
coal, and vary in shape, according to the spaces they have to 
fill (as shown in fig. 5) ; but whether they be elongated or of 
circular figure, more or less of the radiated structure is present 
in every particle. In such sections the vegetable tissues may 
be distinguished from the earthy ingredient by their rich 
brown colour. 
Examination of the Powder. — When the Torbane-hill mineral 
is reduced to powder, and examined either in water or in 
Canada balsam, the combustible and incombustible portions 
can be well seen ; the one occurring in the form of the yellow 
or amber coloured particles before noticed, and constituting 
full two-thirds of the mass (as shown in Plate III., fig. 6), 
whilst the remainder is made up of minute opaque granules, 
having occasionally amongst them some which are quite 
transparent, and probably siliceous. 
Characters of the so-called Coke and of the Ash. — Three por- 
tions of the coke of the Torbane-hill mineral, each about 
4 inches square, obtained from a gas-retort by Mr. Gratton, 
were of a greyish colour, and when scraped became perfectly 
black. The remains of plants were very visible throughout 
the substance of each, and were even more distinctly seen in 
the specimens of coke than in the mineral itself before being 
subjected to heat, for every part, however minute, had 
assumed a silvery appearance. When a flat piece of the coke, 
about half an inch square, is examined as an opaque object 
under a power of 50 or 100 diameters, it presents a peculiar 
sponge- like structure ; and when contrasted with a portion of 
the mineral, it will be noticed that all the yellow particles 
