There is here a gradation which may be best understood, by comparing the
extremes.
On the one hand, we know by experiment, that oily and bituminous
substances can be melted and partly changed into vapour by heat, and
that they become harder and denser, in proportion as the more volatile
parts have evaporated from them. On the other hand, coaly substances are
destitute of fusibility and volatility, in proportion as they have
been exposed to greater degrees of heat, and to other circumstances
favourable to the dissipation of their more volatile and fluid parts.
If, therefore, in mineral bodies, we find the two extreme states of this
combustible substance, and also the intermediate states, we must either
conclude, that this particular operation of heat has been thus actually
employed in nature, or we must explain those appearances by some other
means, in as satisfactory a manner, and so as shall be consistent with
other appearances.
In this case, it will avail nothing to have recourse to the false
analogy of water dissolving and crystallising salts, which has been so
much employed for the explanation of other mineral appearances.
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