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has excited much attention and surprise ; and has hitherto, 
in my opinion, been referred to a very wrong cause, viz., 
mere molecular arrangement. Diamond is twice as dense, 
it is colourless and transparent ; whereas the other is black 
and opaque. Its hardness is incomparably greater, and it is 
a non-conductor of electricity; whilst the other conducts 
tolerably well. It becomes positively electric by friction ; 
whereas anthracite becomes negatively so. 
The chief difference between diamond and coke is, that 
diamond is colourless and transparent, but coke is opaque 
and has a metallic lustre. It is a non-conductor of elec- 
tricity, but coke conducts extremely well. It is also much 
harder than coke; for though coke cuts glass, and even 
scratches topaz, it will not scratch corundum. Diamond, 
as is well known, is converted by intense heat into the coke 
form of carbon. 
Coke and graphite have both the same metallic lustre, and 
differ chiefly in hardness ; graphite, when pure, being vastly 
softer. When coke is used for the electric light, that piece 
which is attached to the anode is softened, and partly con- 
verted into graphite, and its specific gravity is thereby in- 
creased ; whereas that attached to the cathode is unaltered. 
With regard to the square prismatic form of carbon, I 
would state that anthracite, charcoal, and lamp-black belong 
to it ; and the chief difference between them and coke is, the 
want of the metallic lustre which the latter possesses. It is 
also very much harder ; for anthracite will not scratch cal- 
careous spar, whereas coke is inferior in hardness only to 
diamond and corundum. Coke is also a very much better 
conductor of electricity than they are. When exposed to a 
white heat, anthracite will cut glass like coke ; and both it, 
charcoal, and lamp-black, when thus heated, conduct elec- 
tricity vastly better than when only heated to dull redness. 
The specific heat of charcoal is also diminished. I therefore 
