ormed by the latter kind of
operation.

Instances of which, we seem to have, in some granites, on the one
hand;--and in some sorts of limestones on the other.

To this I must add also; that there appear further, to have been some
stones formed _by a sort of precipitation_: much in the same manner as
_Grew_ describes[A] the kernels, and stones of fruit to have been
hardened.

And I have met with many instances, wherein it appears unquestionably,
that all these kind of processes in nature are going on continually: and
that extraneous substances are actually inclosed, and _continually
inclosing_, which could not be _antediluvian_; but must have been
recent.

To these short premises, I must beg leave to add; that in two papers
formerly printed in the Philosophical Transactions,[B] I endeavoured, by
some very remarkable instances, to prove, that iron, wherever it comes
into combination with any substances that are tending to consolidation,
_hastens the process exceedingly_;--and also renders the hardness of the
body much greater.

And I have also endeavoured, elsewhere,[C] to shew, in consequence of
conclusions deduced from experiments of the most unquestionable
authority, that _air_, in its various shapes and modifications, is
indeed _itself_ the great consolidating fluid, out of which solid bodies
are composed; and by means of which the various attractions take place,
which form all the hard bodies, and visible substances upon earth.

From all these premises then, it was impossible for me not to be led to
conclude; that we have, in this august phaenomenon of the fall of stones
from the clouds, in Tuscany, an obvious proof, as it were before our
eyes, of the combined operation of those very powers, and processes, to
which I have been alluding.

It is well known; that pyrites, which are composed of iron, and
sulphur, and other adventitious matter, when laid in heaps, and
moistened, will take fire.

It is also well known, that a mixture of pyrites of almost any kind,
beaten small, and mixed with iron filings and water, when buried in the
ground will take fire; and produce a sort of artificial volcano. And,
surely then, wherever a vast quantity of such kind of matter should at
any time become mixed together, as flying dust, or ashes; and be by any
means condensed together, or compressed, the same effect might be
produced, even in the atmosphere and air.

Instead, therefore, of having recourse to the supposition, of