But the hollow sphere theory of the earth, that FRANKLIN had linked to an
atmospheric theory of earthquakes, could not explain volcanism. The British chemist
HUMPHREY DAVY  provided a single and uniform theory of volcanism and
earthquakes that was based on the hypothesis of large subterranean caves. DAVY was
a contemporary of ALEXANDER VON HUMBOLDT and the last major
representative of the assumption of a cold interior of the earth, that was usually linked
to the ideal that the subterranean caves were inhabitable.Although HUMBOLDT views these considerations to be results of a human obsession of filling unseen spaces with phantasy figures and creatures, they are important enough for him to summarize them in a detailed chapter. "Die Hohlkugel ist nach und nach mit Pflanzen und Thieren bevölkert worden, über die zwei kleine unterirdisch kreisende Planeten, Pluto und Proserpina, ihr mildes Licht ausgießen. Immer gleiche Wärme herrscht in diesen inneren Erdräumen, und die durch Compression selbstleuchtende Luft könnte wohl die Planeten der Unterwelt entbehrlich machen. Nahe am Nordpol, unter 82 Breite, da wo das Polarlicht ausströmt, ist eine ungeheure Oeffnung, durch die man in die Hohlkugel hinabsteigen kann."[1]
Both HUMBOLDT and the British chemist Sir HUMPHREY DAVY were
asked in public several times whether they would be ready to embark on such an
underground expedition. While such an invitation was totally absurd for
HUMBOLDT, DAVY could have accepted it. In contrast to HUMBOLDT assuming a
firy fluid core, the famous chemist, who was able to describe inflammable metallic
substances, had originally thought that volcanic activities are only local and are based
on chemical reactions that are due to the contact between metalloid bases, air and
water. Although DAVY did not assume that the earth is hollow, as HALLEY still did,
but thought that the core of the earth consists of non-oxidated masses of metalloids of
alkalis and earths, he triggered speculations about a "journey to the centre of the
earth" by assuming a cold interior of the earth. This was illustrated by the famous
book by JULES VERNES
"La terre a été échauffée par la combustion de sa surface, non autrement. Sa surface étai composée d'une grande quantité de métaux, tels que le potassium, le sodium, qui ont la propriété de s'enflammer au seul contact de l'air et de l'eau; ces métaux prirent feu quand les vapeurs atmosphériques se précipitèrent en pluie sur le sol; et peu à peu, lorsque les eaux pénétrèrent dans les fissures de l'ecorce terrestre, elles déterminèrent de nouveauc incendies avec explosions et éruptions. De là les volcans si nombreux aux premiers jours du monde. - Et qu'Humphry Davy me rendit sensible, ici même, par une expérience bien simple. Il composa une boule métallique faite principalement des métaux dont je viens de parler, et qui figurait parfaitement notre globe; lorsqu'on faisait tomber une fine rosée a surface, celle-ci se boursouflait, s'oxydait et formait une petite montagne; un cratère s'ouvrait à son sommet; l'éruption avait lieu et communiquait à toute la boule une chaleur telle qu'il devenait impossible de la tenir à la main."In his last book "Consolation in travel and last days of a Philosopher", that, according to HUMBOLDT, creates a rueful feeling, DAVY revoked this hypothesis under the pressure of empirical counter-arguments. The high average density of the whole earth (5,44) compared to the specific weight of potassium (0,865) and sodium (0,972) or the so-called earth metals (1,2) contradicted the ideas of DAVY just as the scarcity of hydrogen gas in the eruption of volcanoes. In addition the British astronomer HOPKINS [2] had doubts about the penetration of water in the hearths
of volcanoes and famous experts like GAY-LUSSAC , BERZELIUS and
WÖHLER[3] had several doubts of a chemical nature.
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