160 
But besides these I find that there are two other crystal- 
line species, viz., hard coke, which, like diamond, belongs to 
the regular system, though it has very different properties 
and specific gravity, viz., about 1.89; and anthracite or 
charcoal, crystallized in the square prismatic system, and 
having a specific gravity of 1.77. These various forms are 
shown in Figs. 1, 2, 3, and 4, Plate II. 
The manner in which I have been able to ascertain the 
crystalline form of coke, anthracite, and charcoal, is thus : — 
I bruise a portion in a mortar, along with soft chalk, which 
protects the fragments from injury, and then dissolve away 
the chalk with acid, and wash the powder left ; then spread- 
ing some of it on a piece of the thin glass used for micro- 
scope object covers, and drying it, I examine it through the 
glass with a magnifying power of about 400 linear, and 
measure the angles by means of the doubly-refracting gonio- 
meter, invented by Dr. Leeson. By this means, and using 
proper precautions, the angles of a fragment of i5 ! 55 th of 
an inch in diameter, or even less, can be measured within a 
small fraction of a degree. Of course by this method one 
can only ascertain the magnitude of the angles which occur 
on one plane ; but by measuring those of various fragments, 
one can, by calculation, build up from them the primary form 
from which they are derived. 
It is not every sample of coke which shows the crystalline 
structure well. Those which are highly vesicular do not 
show it at all distinctly; but if we take a dense, highly- 
metallic-looking specimen, although the particles which have 
good angles well placed for measurement are rare, yet still 
sufficient can be found by a patient examination to show that 
it belongs to the regular system. The angles which are 
good and distinct are, I may say, invariably found to be of 
90°, 60°, 30°, 45°, and 70J% all of which are related, in a 
very simple manner, to the regular system ; and I would 
