334 Anthracite Coat of Pennsylvania. 
_ None of these varieties of coal would, before ignition, im- 
press glass, but the Pennsylvania anthracites, after having 
~ sustained the heat of the furnace, became so hard that with 
an angular piece, I could write my name, on green window 
_glass, with a flourishing hand, much as with a piece of quartz. 
This is in accordance with what many persons have observed 
with respect to the effects of heat upon different varieties of 
carbon, as in the Surturbrand of Iceland—a variety of wood- 
charcoal, which is supposed to have acquired its power of 
scratching glass, from having been long and intensely heated 
beneath a current of melted lava. Similar results were ob- 
tained by Sir Humphrey Davy, with the great battery of the 
Royal Institution, and those observed in the use of Dr. Hare’s 
instruments have been already described in this Journal. 
Ihave often observed that well prepared charcoal, after very 
strong ignition, speedily spoils a knife, used in shaping it 
into points, and even the best files do not long withstand its 
action, as their teeth are eventually worn down by the hard 
integrant molecules of the charcoal. 
I have observed also that even the coak obtained by igniting 
the bituminous coal rapidly destroyed the polish of glass, thus 
producing upon its surface, the effects of the gritty powders, 
used in grinding this substance ; these effects do not appear 
toresult from the impurities but from the induration of the 
carbon. 
I was surprised to find that the residuum, both from the 
¢ommon bituminous Liverpool coal, and from the Cannel coal, 
allowed me to write rapidly and distinctly with the pieces upon 
the hardest green window glass, and when two or three points 
ignition, the bituminous coals should, in this respect, have been still 
more remarkably diminished. Perhaps a satisfactory solution of the 
difficulty can be found in the fact, that the anthracites, containing no 
bitumen, do not cake or grow soft in the fire, which the bituminous 
coals do, to such a degree, as to undergo almost an apparent fusion in 
their own bitumen—a little as many salts, when suffering the aqueous 
fusion, do in their own combined water of crystallization. Thus the 
coaks of the bituminous coal are in consequence of the escape of gas left 
exceedingly porous, from innumerable cavities, so minute that being, 
of course, occupied by air, which the hydrostatic power of the water can 
not displace, they are necessarily rendered very light, while the trials 
made on glass evince that their integrant particles are very hard and 
dense, and that the small specific gravity is really an illusion; pumice- 
stone is an example which illustrates this case, and appears to be per- 
fectly analogous. 
