What is Humphry Davys theory

From the text:

Well, Humphry Davy did call upon me on his way through Hamburg. We were long engaged in discussing, amongst other problems, the hypothesis of the liquid structure of the terrestrial nucleus. We were agreed that it could not be in a liquid state, for a reason which science has never been able to confute."

"What is that reason?" I said, rather astonished.

"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!"