superincumbent lava, where it is only thirteen feet in thickness; nor had
the lava been originally thicker, and since reduced by degradation, as
could be told from the degree of cellularity of its surface. I have already
observed that the sea must have been shallow in which the calcareous
deposit was accumulated. In this case, therefore, the carbonic acid gas has
been retained under a pressure, insignificant compared with that (a column
of water, 1,708 feet in height) originally supposed by Sir James Hall to be
requisite for this end: but since his experiments, it has been discovered
that pressure has less to do with the retention of carbonic acid gas, than
the nature of the circumjacent atmosphere; and hence, as is stated to be
the case by Mr. Faraday, masses of limestone are sometimes fused and
crystallised even in common limekilns. (I am much indebted to Mr. E.W.
Brayley in having given me the following references to papers on this
subject: Faraday in the "Edinburgh New Philosophical Journal" volume 15
page 398; Gay-Lussac in "Annales de Chem. et Phys." tome 63 page 219
translated in the "London and Edinburgh Philosophical Magazine" volume 10
page 496.) Carbonate of lime can be heated to almost any degree, according
to Faraday, in an atmosphere of carbonic acid gas, without being
decomposed; and Gay-Lussac found that fragments of limestone, placed in a
tube and heated to a degree, not sufficient by itself to cause their
decomposition, yet immediately evolved their carbonic acid, when a stream
of common air or steam was passed over them: Gay-Lussac attributes this to
the mechanical displacement of the nascent carbonic acid gas. The
calcareous matter beneath the lava, and especially that forming the
crystalline spicula between the interstices of the scoriae, although heated
in an atmosphere probably composed chiefly of steam, could not have been
subjected to the effects of a passing stream; and hence it is, perhaps,
that they have retained their carbonic acid, under a small amount of
pressure.