Full Idea
The reference of 'Etna' cannot be Mount Etna itself, because each piece of frozen lava which is part of Mount Etna would then also be part of the thought that Etna is higher than Vesuvius.
Gist of Idea
In 'Etna is higher than Vesuvius' the whole of Etna, including all the lava, can't be the reference
Source
Gottlob Frege (Letters to Jourdain [1910], p.43)
Book Reference
'Meaning and Reference', ed/tr. Moore,A.W. [OUP 1993], p.43
A Reaction
This seems to be a straight challenge to Kripke's baptismal account of reference. I think I side with Kripke. Frege is allergic to psychological accounts, but the mind only has the capacity to think of the aspect of Etna that is relevant.