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‘the Diamond from Anthracite and Graphite. 341 
must. once have been soft, because specimens containing air- 
bells exhibit a polarising structure around those bubbles ; for, 
as the author himself points out, a similar structure may be 
developed in glass, “ by a compressing force propagated cir- 
cularly from a point ;” and a solid mass of carbon contracting 
in dimensions whilst it became transparent, would, if I mis: 
take not, have a similar structure developed in it, around any 
bubbles of gas which it might inclose. Taking for granted, 
then, that it is not necessary, in a theory of the origin of the 
diamond, to provide for its once having been soft,* the fol- 
lowing reasons may be assigned for supposing that anthra- 
cite is more likely than other compounds of carbon to yield 
the diamond. 
1st, Anthracite, as it occurs in nature, is described by the 
mineralogist, as passing, by insensible gradations, into com- 
mon coal on the one hand, and into graphite or pure carbon 
on the other; so that it may be regarded as marking the 
transition from fossilized vegetable matter to uncombined 
earbon. In speaking thus, I do not seek to affirm that, in 
every locality, anthracite can be shewn to have been derived 
from coal and to be passing into graphite; but this can be 
demonstrated for many anthracites, and may be extended to. 
all, if it be conceded that, from the less bituminous coals on- 
wards to graphite, we have a series of chemical compounds 
in which the proportion of carbon constantly increases till 
it excludes every other organic constituent, whilst the ma- 
jority are ranked by the mineralogist under the common title 
of anthracite. 
2d, Anthracite consists in greater part of carbon, some- 
times containing nearly 95 per cent. of it. 
3d, The other constituents of anthracite, with the excep= 
tion of the ash, which is often under 1 per cent., form vola- 
tile compounds with each other and with the oxygen of the 
air, There is thus a provision, in the spontaneous slow oxi- 
dation of anthracite by air, for depriving it of all its consti- 
tuents (the ash excepted) but carbon. I do not attach impor- 
* I do not wish, however, to be understood as affirming that no diamond 
was ever soft, but simply that itis not necessary to assume that all were, 
