Though Giselle is asking about the silver, there is a second meaning in which she is unwittingly asking about the location of the palace: i.e., where is this utopia you promise? His response, "I forbid thee to ask," is like an angry demand that the utopian fantasy be kept pure and "far away."
Referring to the silver, the answer he cannot give Giselle is the answer he
tries to give Mrs Gould in the next chapter. Recall the symbolism in
which the location of the silver on the Great Isabel corresponds to the treasure
of individual character and the dream-ideal of individual fulfilment. This symbolic
secret is "too dangerous" to utter as long as the People's sham relationship
to the exploiting society must continue.