"Because this liquid mass would be subject, like the ocean, to the
lunar attraction, and therefore twice every day there would be
internal tides, which, upheaving the terrestrial crust, would cause
periodical earthquakes!"

"Yet it is evident that the surface of the globe has been subject to
the action of fire," I replied, "and it is quite reasonable to
suppose that the external crust cooled down first, whilst the heat
took refuge down to the centre."

"Quite a mistake," my uncle answered. "The earth has been heated by
combustion on its surface, that is all. Its surface was composed of a
great number of metals, such as potassium and sodium, which have the
peculiar property of igniting at the mere contact with air and water;
these metals kindled when the atmospheric vapours fell in rain upon
the soil; and by and by, when the waters penetrated into the fissures
of the crust of the earth, they broke out into fresh combustion with
explosions and eruptions. Such was the cause of the numerous
volcanoes at the origin of the earth."

"Upon my word, this is a very clever hypothesis," I exclaimed, in
spite rather of myself.

"And which Humphry Davy demonstrated to me by a simple experiment. He
formed a small ball of the metals which I have named, and which was a
very fair representation of our globe; whenever he caused a fine dew
of rain to fall upon its surface, it heaved up into little
monticules, it became oxydized and formed miniature mountains; a
crater broke open at one of its summits; the eruption took place, and